Because the Internet is a vast collection of resources, it is clear that sharing your information with other participants can help you prosper. It is not clear, however, at what risk you place yourself when you actually connect. It is our opinion that some companies see the Net as a huge untapped marketplace, full of consumers and advertising opportunities, but don’t realize that the Internet has its own version of an “underworld” as well. It is this, above all else, that compels us to protect our data, and where the emergence of the virtual private network presents itself is a stepping stone into the 21st century. The protection of private data is the core of the virtual private network, and the two most relevant technologies (encryption and firewalls) are what make it all possible.

In this chapter we will present an overview and background of the technologies used to build a VPN, and how they are incorporated into the products and services covered in this book. We will start with a discussion of how firewall techniques are used to protect an entire network at its gateway routers. Next, we will present you with a general background on encryption: how it is used in a traditional sense, plus how it will be deployed using a VPN. Following this, we will discuss authentication techniques and how they are used in conjunction with the encryption algorithms with VPNs. Also, we will delve into the protocols that have arisen from the growth of the VPN industry. Lastly, we will briefly cover various compromise methodologies that a potential assailant may use to try to gain access to your private network or data.

2.1 Firewall Deployment

The first of the security-related technologies that we cover in this book is the firewall. A firewall is a system that stands between your internal network and the world outside. Firewalls have been employed on large public networks for many years and are a great starting place in the development of a security strategy. The reason to start with firewalls is that they are generally placed at the point at which your network interconnects with a public network, like the Internet. Although not a perfect strategy, a firewall is easy to configure; it requires only the modification of one gateway router. Of course, if you have a large, multiply- connected WAN, with many paths to the Internet, then it should be noted that you will need to create a firewall for each interconnection point. The complexity of this process increases dramatically from the single point gateway to the multiple point gateway.

2.1.1 What Is a Firewall?

The U.S. Department of Defense, probably the world’s authority on data sensitivity and security controls, used a system of confidences defined as security levels to restrict access to classified documents. The criteria for determining how a governmental computer should be protected were detailed in the fabled “Orange Book.” It stated that to secure highly sensitive data, one must never connect the computer to an exterior network. This is of course the best firewall strategy that exists, but it is too restrictive to be practical. We know the value of interconnection like the rest of you; we just want you to realize that the best firewall for extremely sensitive materials is to isolate them on a computer without a network connection at all.

Firewalls usually serve two main functions for a network administrator. The first is to control which machines an outsider can see and the services on those machines with which he can converse. The second controls what machines on the Internet an internal user can see, as well as what services he can use. A firewall is much like a traffic cop, organizing which paths network traffic can take, and stopping some altogether. Internet firewalls usually do this by inspecting every packet that tranverses the gateway router, which is why they are usually referred to as “packet filtration” systems.

For this chapter, we will use our large branch network as an example. We will further assume that we have a Cisco 2500 series router and 40 workstations. Of the 40 computers, three are servers: one FTP server, one mail server, and one web server. We have a full class C address (2.48.29.0/24) allocated to us from the NIC (Network Information Center); we will be presenting examples throughout this section on how to set up different firewall topologies using our 40 machines and the network provided earlier. Figure 2-1 illustrates what the firewall will be doing in a basic sense for both our large branch as well as our main corporate network (at the top).

2.1.2 What Types of Firewalls Are There?

Since almost all firewalling techniques are designed around a similar model, a centralized point of control, there are only a few variations at the top level that need to be explored. You are probably already familiar with the packet filtration firewall; most people are these days, given the recent attention paid to it by the news media. In this section we will discuss the operation and configuration of four architectures of firewall design. There are many variations of the four that you may have seen implemented, and certainly we are omitting several of the most complex and advanced architectures. But we hope to familiarize you with what a firewall is, how it works, how to set one up, and, most relevant to this book, how it fits into the world of the virtual private network.

2.1.2.1 Packet restriction or packet filtering routers

Routers and computers that conduct packet filtration choose to send traffic to a network based on a predefined table of rules. The router does not make decisions based on what’s inside the packet’s payload, but rather on where it is coming from and where it is destined. It only considers that if the packet matches a set of parameters, it should take appropriate action to either allow or deny the transit. These allow and deny tables are set up to conform to the overall network security policies put in place by the network administrator or security coordinator.

A peek into the operation of a packet filter shows us that the router never even looks at any of the packet’s payload, but only at the TCP/IP header information, to make its screening decisions. Thus, as shown in Figure 2-2, if a router were asked to allow all traffic from network 1.34.21.0/24, it would check all packets for a matching source address and pass them across. Should a packet be received from another network, the filter would disallow the transit, and the packet would be thrown away. So, in essence, this is how the entire operation of this firewall affords security to the site.

Packet filtering can take on two basic forms. First is an open network with selective filtering of unwanted traffic. For each type of network attack, an appropriate filter must be put in place on the router. Second is the closed network with selective filtering of desired traffic. Although affording greater security, even for those attacks that haven’t been thought of yet, the drawback for the network administrator is having to update the firewall as new computers or services are added or changed.

As you can guess, a packet filter suffers from several inadequacies. First off, there’s no way to do user authentication; either a peer pair is allowed, or it’s not. For example, either machine 1.34.21.44 can pass mail traffic (ports 25 and 110) to our mail server on our large network (2.48.29.4), or it can’t. There’s no provision for who is trying to send the mail. Shouldn’t it be possible for Bob, one of our employees who is visiting the ZZZ Cyber Coffee Shop (the owners of network 1.34.21.44), to be able to check his email and have a coffee?

Further, be glad for performance reasons that the router doesn’t actually open all the packets it gets. Routers these days are asked to perform miracles, especially with the race for more and more bandwidth. The router’s job is to decide where to send the traffic, not really to catch and throw away packets that are security risks.

What we’re suggesting, of course, is that there will be a marked change in what gateway networks will look like in the future. We believe that there will be a decoupling of routing equipment and packet filtration (or even security equipment, for that matter) in the very near term. Actually, this may already be the case. New products are already coming out that support dynamic authentication through a packet filtering router directly to the user level, even across an encrypted link.

A last impediment is that frequent changes to the network may require wholesale reconfiguration of the gateway router and the packet filtration firewall that lives on it. This can be time-consuming and disaster-prone if either an uncaught mistake leaves most of the network wide open, or a subtle change leaves the router crippled and unable to perform its first duty as a network traffic director.

2.1.2.2 Bastion host

A bastion host or screening host, as it is sometimes called, uses both a packet filtering mechanism provided by the router plus a secured host. A secured host is one that has had its operating system and major services combed over by a security expert. The primary security is provided by a packet filtering router, and the secured host is used to stage information flow in either direction. The bastion host is a security-checked machine that is connected to the Internet with the same method as other machines. The gateway allows traffic to pass to it in a less restricted fashion. Bastion hosts are typically used in combination with filtering routers because simple packet filtration systems can’t filter on the protocol or the application layer. (See Figure 2-3 for a sample configuration.)

A bastion host is much easier to configure than a distributed server and tons easier to maintain, because the bulk of the traffic is being sent to one system. Since the bastion host is situated on the internal wire, it needs no special exemptions from other locally connected equipment. The site’s security policy will dictate what needs to be configured on the packet filtering router, which will be as restrictive as necessary. It’s not uncommon at all for an administrator to use a combination of strategies, employing both the packet filtering router and a bastion host.One of the great things about the configuration of a bastion host for security measures is that configuration of the packet filter becomes a generic “deny everything” statement, preceded by some very specific allow statements that pertain only to the bastion host. For large and quickly changing networks, you can see that this reduces the load of the security personnel. Adding new machines or having users install poorly secured equipment does not affect the firewall or the protection afforded by the bastion host.

Of course, having a centralized point of control does have its disadvantages. For one, a large, busy network would need several machines acting as bastion hosts (making the administration of them more time-consuming), or even better, a perimeter network of bastion hosts might be required (see the next section). Each machine needs its own section in the packet filtration firewall, piling on complexity, and with each machine comes the headache of having to test and double test it for purity. Along with the need for multiple hosts to prevent network congestion, the centralization of information at the bastion will tend to draw attack attention there, making it ever more important to secure and monitor it around the clock. It should go without saying that a major drawback to this type of firewall configuration is that it can lead to a tragic security hazard should an assailant get system operator privileges on the bastion host. Thus, a single point of control equals a single point of failure.

2.1.2.3 DMZ or perimeter zone network

A popular ploy to separate large corporate internal networks from the hostile environment of the Net is to erect a “routing network” on which all inbound and outbound traffic must travel. Huge installations normally have such networks already set up so that they can effectively separate the local traffic from the metropolitan traffic from the wide-area or worldwide traffic. As you might have guessed, a routing network consists of only routers, including those both internally and externally connected, and usually goes by the term “backbone.” A sample configuration is shown in Figure 2-4.

You might be wondering why the term DMZ is sometimes used interchangeably for a perimeter zone network. DMZ stands for “demilitarized zone” and serves the same purpose as it does in areas of geographical conflict: it’s a buffer zone between two hostile parties that must coexist in close proximity. In creating a perimeter zone network, the added security you get is multifold. First, there are at least two routers involved in protecting your internal network. One router sits as the gateway to the Internet, and one sits as the gateway to your internal network. The network the two routers share should not have any other host equipment on it other than routing equipment and trusted host equipment (used as a bastion host, detailed earlier).

The second security feature inherent in the DMZ architecture involves a security breach at the outside perimeter router level or at any host on the perimeter network; intruders can sniff only packets transiting through, and nothing else. To gain access to the internal network, they would then have to crack the internal perimeter router, which should dishearten them enough to make them disappear. Plus, a VPN solution from the internal network would almost certainly involve encrypting packets, further complicating a compromise attempt.

In a standard perimeter zone construction, the most complex and careful controls are placed on the internal router, which is the one that separates the internal network from both the perimeter network and the external network. It is a very common practice to erect the DMZ network in this fashion, because this configuration can be likened to tiers of concentric circles—each one further out provides less security. Also, it is becoming common practice to use Network Address Translation (NAT) at the internal router to further complicate locating and hijacking internal communications. NAT provides security by translating non-routable addresses (like the 192.168.0.0 range) into real Internet addresses in a dynamic fashion. There is no easy way to exchange traffic with internal hosts except by circumventing the machine doing the NAT translation.

The tightest security you can make with a DMZ would be to disallow all traffic outbound from the internal network from the exterior router, and to disallow all traffic inbound to the internal network from the Internet. In essence, this makes all traffic a two-step process. Clients on the Internet can peer only with machines that are located on your perimeter network, and clients that are deep inside the internal network can’t see the Internet directly; they too need to use a middleman through a bastion host on the DMZ. You can see why this can really ruin an attacker’s day. As we stated earlier, most acts of compromise are done by convenience. The harder you make it for the snoops to snoop, the harder you make it for them even to assess the steps required in their warfare, and the more difficult you make their ultimate goal, the faster they are going to evaporate.

2.1.2.4 Proxy servers

Proxies act much like bastion hosts, and in some firewall texts, the two overlap almost completely. We use the term “bastion host” to refer to a computer that acts as a staging area for information that is in transit either to or from the Internet. We use the term “proxy server” to refer to a type of bastion host that is running specialized software that masquerades as an internal machine to an external one. In the following example, we contrast a typical bastion host and typical proxy server.

A good illustration of an application for a bastion host is email. A bastion host is typically set up to act as the “delivery point” for email inbound from the Internet. Hence a DNS mail exchanger record (MX) is traditionally set up to point traffic to the bastion for delivery. From there, the bastion may re-deliver the mail to an interior mail host (which it can see due to its position in the firewall), or it could hold onto the mail, waiting for the client to read it with a POP mail client. A whole selection of different firewalls can be constructed in this manner.

By contrast, a proxy service is more of an “in-transit” checkpoint than an information staging area. The proxy pretends to be one end of a connection, but protects the true sender or recipient from unwanted traffic. The service that presents the greatest trouble to a security manager’s life is the standard file transfer protocol (FTP). It’s insecure because it uses random, high-numbered ports to establish a peer-to-peer session with the client. Having a service that operates on more than one port, and especially one that operates on most any port greater than 1023, provides a real nightmare to the security administrator. To address this, a “passive” FTP session can be established (using the control and data ports [20 and 21] for actual data transit rather than one greater than 1023), but not all clients support it.

Using a proxy, as shown in Figure 2-5, is another option for establishing FTP across a firewall. After you set up a host machine on a perimeter network that acts for the client, which is located on the internal network, a full connection can be made with little security to give up. The FTP proxy lives on the perimeter network and is granted access through the exterior firewall to conduct FTP sessions. Special software must be installed on the proxy so that it can accept incoming requests from an FTP client beyond the interior gateway and masquerade as the client in talking to the outside world.

The same security model using proxy servers can be tooled using a dynamic firewall filtration router such as the Cisco PIX or the Firewall-1 system. A more complete description of the PIX’s abilities can be found in Chapter 9.

Because a proxy service is more like a host computer than any sort of firewall, special care must be given to ensure that the proxy server is well protected by the site’s security policy. Plus, it is important to note that a proxy service is an additional measure of protection and certainly should not be considered a total solution. The shield of a packet filtration firewall can help keep things segregated, and/or the network can be segmented in different subnets, isolating high-risk units from low-risk ones.

2.1.3 Use of Firewalling in a VPN

The importance of firewalling to a virtual private network is straightforward and to the point. Since a VPN is an interconnection of two or more disconnected networks utilizing public resources (such as the Internet) for transit, it follows that these networks individually must be protected in and of themselves. Imagine each network that needs to be placed in a VPN as a separate bubble, with its own connections and users.

Viewed this way, each separate bubble needs a protective wall around it to make it safe from invasion. The concept behind using firewalls with a VPN is to secure the networks as if they were isolated; then the system administrator opens specific ports in the packet filtering router to allow the encrypted data to stream from one bubble to the next. Thus, a private and secure communication (based on the type and implementation of the cryptographic routines used) is set up in a channel between two sites. The VPN software provides the security and the application layer routing, so that the networks in question will appear to be as one when presented to users at either end.

Firewall techniques are the first line of protection in the fabric of a VPN, and they must be developed and tested before the benefits of the VPN can be fully harvested. Even if the VPN software or hardware you deploy has built-in firewalling that seems to be everything you would ever need, chances are that you will need to follow some security guidelines on your network anyway, just to stay on the safe side.

2.2 Encryption and Authentication

The configuration and deployment of a virtual private network obviously involves more than just a packet filtration router. Otherwise, all you would have is a smoked glass window hiding your data from the rest of the world. The real concept of this book, and that of the VPN, is the secure communication between two distinct networks over a public medium, done in such a way that they seem to be sharing a LAN from either end. Thus far, our discussion of firewalling techniques only covers half of the equation. Firewalls either allow or deny traffic based on the source and destination, but once the traffic makes it into your network, the disciplines of authentication and encryption add further protection by securing the conversation.

Encryption can be regarded as a method for altering data into a form that is unusable by anyone other than the intended recipient, who has the means necessary to decrypt it. The input to an encryption algorithm is typically called clear text, while the output is referred to as ciphertext or crypt text. The encryption process protects the data by making the assailant work too hard or too long to get at what’s being hidden. As we will discover, cryptographic routines use mathematics to alter the data in such a way that the process is difficult and expensive to reverse. As with all things, there are sometimes several ways to peel a banana.

Another important topic that we will discuss in this section—a topic that is closely linked with cryptography—is the art and science of authentication. Where encryption and cryptography deal with the conversion of data into a protected form for transmission to a trusted party in a hostile environment, authentication is the identity checking and confirmation of that entity, which guarantees their claim with a great degree of certainty. The notion of authentication is very important to the concepts employed by creating a VPN. Without knowing with certainty the identity of a participant, how could you entrust a data communication channel to them? It would be like inviting them over to your office and giving them the keys to the filing cabinet and access to a photocopier.

2.2.1 A Brief History of Cryptography

A major tenet of the art and science of cryptography is that the transformation process must be a fairly quick one for the owner of the data (the encryptor)—otherwise it would be too slow to be useful—yet computationally difficult (if intercepted) for a hostile third party to reverse. Hence, most algorithms that morph data for security purposes do so in a way that is programmatically complex. In this section, we will explore the world of ciphers from about five thousand feet up. We will cover some of the nastier mathematics that make encryption work, but we aim to do so in a fashion that won’t leave you wanting a degree in higher math.

The algorithms discussed here fall into three basic categories.

The first category of algorithms uses a one-way transformation process to alter the clear text into ciphertext. These transformation programs are typically referred to as hash algorithms. The value of hashes and message digests is that they are easy to compute but hard to reverse, and rarely repeat. Hashes don’t normally have keys associated with them, as do the next two types of encryption techniques.

The second and third types of encryption algorithms are the private key and public key cryptosystems. There are other common names for these encryption procedures, including asymmetric and symmetric algorithms, or one-key and two-key systems. All these terms refer to the same processes. The hash algorithms briefly discussed in the previous paragraph are sometimes referred to as no-key or zero-key encryption operations because, as the name would suggest, hash algorithms do not use a key.

This brings us to the topic of randomness and why pure random numbers are extremely important to the application of these cryptographic concepts. The transmission of encrypted data over a network in a VPN typically requires a key exchange. This means that for each separate transaction between a client and a server, a new set of keys would be produced. Although this may seem unnecessary, it would be disastrous if the same fixed keys were always used and a third party were to gain access to them without the knowledge of either party, or if the message was recorded, cracked, and the keys reused. In essence, the key snoop would be able to decrypt all conversations until the key files were changed, which wouldn’t happen unless the parties recognized the attack.

To produce a “cryptographically strong key” on the fly, a computer must have access to a good pool of random numbers. Using something seemingly random, like transformations based on clock time, seconds past a certain fixed date, or other easily obtainable environmental conditions, proves to be an inadequate solution. If the attacker knows that the key generator uses the time of day for the key, it is highly likely that a constrained brute force approach could be used to help narrow the scope of the problem to one that is not computationally infeasible.

Now let’s discuss network security and the use of encryption with networking protocols to secure a data transit stream. We know that firewalls aren’t 100% airtight: attackers can still use social engineering like password guessing to gain access, circumvent your routers altogether by dialing in directly, or just stubbornly probe all avenues for entry in an  exhaustive fashion. So it’s wise to add an additional layer of protection: encrypt the data transfer so that even if a snoop were tapping the line, all they would see is “garbage.”

One serious flaw (or design element) in using cryptography to seal up data is that it is only a temporary fix. The real comparison should be to use a suitable key length or encryption algorithm that outpaces the ever-increasing advance in technological capabilities. Also, the lifetime of that data itself should be compared in a similar fashion. Using small keys and weak (but fast) encryption techniques is fine for data that will be worthless in 24 hours, especially since it will take a would-be cipher hack more time than that to crack it.

A private network is composed of computers owned by a single organization that share information specifically with each other. They’re assured that they are going to be the only ones using the network, and that information sent between them will (at worst) only be seen by others in the group. The typical corporate Local Area Network (LAN) or Wide Area Network (WAN) is an example of a private network. The line between a private and public network has always been drawn at the gateway router, where a company will erect a firewall to keep intruders from the public network out of their private network, or to keep their own internal users from perusing the public network.

There also was a time, not too long ago, when companies could allow their LANs to operate as separate, isolated islands. Each branch office might have its own LAN, with its own naming scheme, email system, and even its own favorite network protocol—none of which might be compatible with other offices’ setups. As more company resources moved to computers, however, there came a need for these offices to interconnect. This was traditionally done using leased phone lines of varying speeds. By using leased lines, a company can be assured that the connection is always available, and private. Leased phone lines, however, can be expensive. They’re typically billed based upon a flat monthly fee, plus mileage expenses. If a company has offices across the country, this cost can be prohibitive.

Private networks also have trouble handling roving users, such as traveling salespeople. If the salesperson doesn’t happen to be near one of the corporate computers, he or she has to dial into a corporation’s modem long-distance, which is an extremely expensive proposition.

This book is about the virtual private network (VPN), a concept that blurs the line between a public and private network. VPNs allow you to create a secure, private network over a public network such as the Internet. They can be created using software, hardware, or a combination of the two that creates a secure link between peers over a public network. This is done through encryption, authentication, packet tunneling, and firewalls. In this chapter we’ll go over exactly what is meant by each of these and what roles they play in a VPN; we’ll touch upon them again and again throughout the book. Because they skirt leased line costs by using the Internet as a WAN, VPNs are more cost-effective for large companies, and well within the reach of smaller ones.

In this chapter, we’ll also talk about Intranets as the latest trend in corporate information systems, and how they were the impetus for VPNs.

1.1 What Does a VPN Do?

A virtual private network is a way to simulate a private network over a public network, such as the Internet. It is called “virtual” because it depends on the use of virtual connections—that is, temporary connections that have no real physical presence, but consist of packets routed over various machines on the Internet on an ad hoc basis. Secure virtual connections are created between two machines, a machine and a network, or two networks.

Using the Internet for remote access saves a lot of money. You’ll be able to dial in wherever your Internet service provider (ISP) has a point-of-presence (POP). If you choose an ISP with nationwide POPs, there’s a good chance your LAN will be a local phone call away. Some ISPs have expanded internationally as well, or have alliances with ISPs overseas. Even many of the smaller ISPs have toll-free numbers for their roaming users. At the time of this writing, unlimited access dial-up PPP accounts, suitable for business use, are around $25 per month per user. At any rate, well-chosen ISP accounts should be cheaper than setting up a modem pool for remote users and paying the long-distance bill for roaming users. Even toll-free access from an ISP is typically cheaper than having your own toll-free number, because ISPs purchase hours in bulk from the long-distance companies.

In many cases, long-haul connections of networks are done with a leased line, a connection to a frame relay network, or ISDN. We’ve already mentioned the costs of leasing a “high cap” leased line such as a T1. Frame relay lines can also give you high speeds without the mileage charges. You purchase a connection to a frame cloud, which connects you through switches to your destination. Unlike a leased line, the amount you pay is based more on the bandwidth that’s committed to your circuit than distance. Frame connections are still somewhat expensive, however. ISDN, like the plain old telephone system, incurs long-distance charges. In many locations, the local telephone company charges per minute even for local calls, which again runs expenses up. For situations where corporate office networks are in separate cities, having each office get a T1, frame relay, or ISDN line to an ISP’s local POP would be much cheaper than connecting the two offices using these technologies. A VPN could then be instituted between the routers at the two offices, over the Internet. In addition, a VPN will allow you to consolidate your Internet and WAN connections into a single router and single line, saving you money on equipment and telecommunications infrastructure.

1.1.1 The Rise of Intranets

By now you’ve probably heard of Intranets and the stir they’ve caused at many businesses. Companies are running TCP/IP networks, posting information to their internal web sites, and using web browsers as a common collaborative tool. An example of an Intranet application is a customer database accessible via the Web. Salespeople could use this database to contact current customers about new product offerings and send them quotes. The database could have a HyperText Mark- Up Language (HTML) front end, so that it would be accessible from any web browser.

The rise of Intranets was spurred on by the growth of the Internet and its popular information services, commonly known as the World Wide Web. It was as if the corporate sector had finally caught on to what the Internet community had been doing for years: using simple, platform-independent protocols to communicate more effectively. No matter how muchmarketing hype you hear, an Intranet is simply Internet technology put to use on a private network.

1.1.1.1 How VPNs relate to Intranets

Virtual private networks can be used to expand the reach of an Intranet. Since Intranets are typically used to communicate proprietary information, you don’t want them accessible from  the Internet. There may be cases, however, where you’ll want far-flung offices to share data or remote users to connect to your Intranet, and these users may be using the Internet as their means of connection. A VPN will allow them to connect to the Intranet securely, so there are no fears of sensitive information leaving the network unprotected. You might see this type of connection also referred to as an “Extranet.”

Using our previous example of the customer database, it’s easy to see how a VPN could expand the Intranet application’s functionality. Suppose most of your salespeople are on the road, or work from home. There’s no reason why they shouldn’t be able to use the Internet to access the web server that houses the customer database application. You don’t want just anyone to be able to access the information, however, and you’re also worried about the information itself flowing unencrypted over the Internet. A VPN can provide a secure link between the salesperson’s laptop and the Intranet web server running the database, and encrypt the data going between them. VPNs give you flexibility, and allow practically any corporate network service to be used securely across the Internet.

1.2 Security Risks of the Internet

The risks associated with the Internet are advertised every day by the trade and mainstream media. Whether it’s someone accessing your credit card numbers, prying into your legal troubles, or erasing your files, there’s a new scare every month about the (supposedly) private information someone can find out about you on the Internet. (Not to mention the perceived risk that you might happen upon some information that you find offensive, or that you might not want your children to see.)

For corporations, the risks are even more real and apparent. Stolen or deleted corporate data can adversely affect people’s livelihoods, and cost the company money. If a small company is robbed of its project files or customer database, it could put them out of business.

Since the Internet is a public network, you always risk having someone access any system you connect to it. It used to be that a system intruder would have to dial into your network to crack a system. This meant that they would have to find a phone number connected to a modem bank that would give them access, and risk the possibility of the line being traced. But if your corporate network is connected over the Internet and your security is lax, the system cracker might be able to access your network using any standard dial-up account from any ISP in the world. Even unsophisticated users can obtain and use automated “security check” tools to seek out holes in a company’s network. What’s worse is that, chances are, you’ll never know that it’s happening.

Before we put our private data out on the Internet, we’d better make sure a VPN is robust enough to protect it.

1.2.1 What Are We Protecting with Our VPN?

The first things that come to mind when you think of protection are the files on your networked computers: documents that contain your company’s future plans, spreadsheets that detail the financial analysis of a new product introduction, databases of your payroll and tax records, or even a security assessment of your network pointing out holes and problematic machinery. These files are a good starting point, but don’t forget about the other, less tangible assets that you connect to the Internet when you go online. These include the services that you grant your employees and customers, the computing resources that are available for use, and even your reputation. For instance, a security failure can cause your vendors’ email to bounce back to them, or prevent your users from making connections to other sites.

The easiest thing would be to isolate, tabulate, and lock down your private data. Well over half the data you manage and distribute might call for some sort of security. Just think, even something as innocuous as customer records and addresses could be used against you in a negative advertising campaign; this might hurt you far worse than a negative campaign aimed at a random slice of the population.

Unfortunately, in the client-server world of telecommuters, field sales agents, and home offices, it’s not so easy to keep all private data locked down in a single, protected area. The chief financial officer of a company may need to access financial information on the road, or a programmer working from home may need to access source code. VPNs help alleviate some of the worry of transmitting secure files outside of your network. In Chapter 2, we will examine possible threats to your network and data, and explore the technologies that VPNs use to avoid them.

1.3 How VPNs Solve Internet Security Issues

There are several technologies that VPNs use to protect data travelling across the Internet. The most important concepts are firewalls, authentication, encryption, and tunneling. Here we will give them a cursory rundown, then go into more detail in Chapter 2.

1.3.1 Firewalls

An Internet firewall serves the same purpose as firewalls in buildings and cars: to protect a certain area from the spread of fire and a potentially catastrophic explosion. The spread of a fire from one part of a building is controlled by putting up retaining walls, which help to contain the damage and minimize the overall loss and exposure. An Internet firewall is no different. It uses such techniques as examining Internet addresses on packets or ports requested on incoming connections to decide what traffic is allowed into a network.

Although most VPN packages themselves don’t implement firewalls directly, they are an integral part of a VPN. The idea is to use the firewall to keep unwanted visitors from entering your network, while allowing VPN users through. If you don’t have a firewall protecting your network, don’t bother with a VPN until you get one—you’re already exposing yourself to considerable risk.

The most common firewall is a packet filtration firewall, which will block specified IP services (run on specific port numbers) from crossing the gateway router. Many routers that support VPN technologies, such as the Cisco Private Internet Exchange (PIX) and the 3Com/U.S. Robotics Total Control, also support packet filtration. Proxies are also a common method of protecting a network while allowing VPN services to enter. Proxy servers are typically a software solution run on top of a network operating system, such as Unix, Windows NT, or Novell Netware.

1.3.2 Authentication

Authentication techniques are essential to VPNs, as they ensure the communicating parties that they are exchanging data with the correct user or host. Authentication is analogous to “logging in” to a system with a username and password. VPNs, however, require more stringent authentication methods to validate identities. Most VPN authentication systems are based on a shared key system. The keys are run through a hashing algorithm, which generates a hash value. The other party holding the keys will generate its own hash value and compare it to the one it received from the other end. The hash value sent across the Internet is meaningless to an observer, so someone sniffing the network wouldn’t be able to glean a password. The Challenge Handshake Authentication Protocol (CHAP) is a good example of an authentication method that uses this scheme. Another common authentication system is RSA.

Authentication is typically performed at the beginning of a session, and then at random during the course of a session to ensure that an impostor didn’t “slip into” the conversation. Authentication can also be used to ensure data integrity. The data itself can be sent through a hashing algorithm to derive a value that is included as a checksum on the message. Any deviation in the checksum sent from one peer to the next means the data was corrupted during transmission, or intercepted and modified along the way.

1.3.3 Encryption

All VPNs support some type of encryption technology, which essentially packages data into a secure envelope. Encryption is often considered as essential as authentication, for it protects the transported data from packet sniffing. There are two popular encryption techniques employed in VPNs: secret (or private) key encryption and public key encryption.

In secret key encryption, there is a shared secret password or passphrase known to all parties that need access to the encrypted information. This single key is used to both encrypt and decrypt the information. The data encryption standard (DES), which the Unix crypt system call uses to encrypt passwords, is an example of a private key encryption method.

One problem with using secret key encryption for shared data is that all parties needing access to the encrypted data must know the secret key. While this is fine for a small workgroup of people, it can become unmanageable for a large network. What if one of the people leaves the company? Then you’re going to have to revoke the old shared key, institute a new one, and somehow securely notify all the users that it has changed.

Public key encryption involves a public key and a private key. You publish your public key to everyone, while only you know your private key. If you want to send someone sensitive data, you encrypt it with a combination of your private key and their public key. When they receive it, they’ll decrypt it using your public key and their private key. Depending on the software, public and private keys can be large—too large for anyone to remember. Therefore, they’re often stored on the machine of the person using the encryption scheme. Because of this, private keys are typically stored using a secret key encryption method, such as DES, and a password or passphrase you can remember, so that even if someone gets on your system, they won’t be able to see what your private key looks like. Pretty Good Privacy (PGP) is a well-known data security program that uses public key encryption; RSA is another public key system that is particularly popular in commercial products. The main disadvantage of public key encryption is that, for an equal amount of data, the encryption process is typically slower than with secret key encryption.

VPNs, however, need to encrypt data in real time, rather than storing the data as a file like you would with PGP. Because of this, encrypted streams over a network, such as VPNs, are encrypted using secret key encryption with a key that’s good only for that streaming session. The session secret itself (typically smaller than the data) is encrypted using public key encryption and is sent over the link. The secret keys are often negotiated using a key management protocol.

The next step for VPNs is secure IP, or IPSec. IPSec is a series of proposals from the IETF outlining a secure IP protocol for IPv4 and IPv6. These extensions would provide encryption at the IP level, rather than at the higher levels that SSL and most VPN packages provide.

IPSec creates an open standard for VPNs. Currently, some of the primary VPN contenders use proprietary encryption, or open standards that only a few vendors adhere to. Rather than seeing IPSec as a threat to their current products, most vendors see it as a way to augment their own security, essentially adding another interoperable level to their current tunneling and encryption methods.

1.3.4 Tunneling

Many VPN packages use tunneling to create a private network, including several that we review in this book: the AltaVista Tunnel, the Point-to-Point Tunneling Protocol (PPTP), the Layer 2 Forwarding Protocol, and IPSec’s tunnel mode. VPNs allow you to connect to a remote network over the Internet, which is an IP network. The fact is, though, that many corporate LANs don’t exclusively use IP (although the trend is moving in that direction). Networks with Windows NT servers, for instance, might use NetBEUI, while Novell servers use IPX. Tunneling allows you to encapsulate a packet within a packet to accommodate incompatible protocols. The packet within the packet could be of the same protocol or of a completely foreign one. For example, tunneling can be used to send IPX packets over the Internet so that a user can connect to an IPX-only Novell server remotely.

With tunneling you can also encapsulate an IP packet within another IP packet. This means you can send packets with arbitrary source and destination addresses across the Internet within a packet that has Internet-routable source and destination addresses. The practical upshot of this is that you can use the reserved (not Internet-routable) IP address space set aside by the Internet Assigned Numbers Authority (IANA) for private networks on your LAN, and still access your hosts across the Internet. We will look at how and why you would do this in later chapters.

Other standards that many VPN devices use are X.509 certificates, the Lightweight Directory Access Protocol (LDAP), and RADIUS for authentication.

1.4 VPN Solutions

A VPN is a conglomerate of useful technologies that originally were assembled by hand. Now the networking companies and ISPs have realized the value of a VPN and are offering products that do the hard work for you. In addition, there is an assortment of free software available on the Internet (usually for Unix systems) that can be used to create a VPN. In this book, we’re going to look at some of the commercial and free solutions in detail. Which one you choose for your network will depend on the resources available to you, the platforms you run, your network topology, the time you wish to spend installing and configuring the software, and whether or not you want commercial-level support. We can’t cover every vendor and product in this book; they change too quickly. Instead, we offer guidelines you can use on all networks and details on a few stable products that were available when we were writing this edition—we don’t mean to imply that there’s anything less valuable about competing products.

VPN packages range from software solutions that run on or integrate with a network operating system (such as the AltaVista Tunnel or CheckPoint Firewall-1 on Windows NT or Unix), to hardware routers/firewalls (such as those from Cisco and Ascend), to integrated hardware solutions designed specifically for VPN functions (such as VPNet and the Bay Networks Extranet Switch). Some VPN protocols, like SSH or SSL, gained popularity for performing other functions, but have since become used for VPNs as well.

In addition to products, ISPs are also offering VPN services to their customers. The tunneling usually takes place on the ISP’s equipment. If both ends of the connection are through the same ISP, that ISP might offer a Service Level Agreement (SLA) guaranteeing a certain maximum amount of latency and up time.

1.4.1 Quality of Service Issues

Running a virtual private network over the Internet raises an easily forgotten issue of reliability. Let’s face it: the Internet isn’t always the most reliable network, by nature. Tracing a packet from one point to another, you may pass through a half-dozen different networks of varying speeds, reliability, and utilization—each run by a different company. Any one of these networks could cause problems for a VPN.

The lack of reliability of the Internet, and the fact that no one entity controls it, makes troubleshooting VPN problems difficult for a network administrator. If a user can’t dial into a remote access server at the corporate headquarters, or there’s a problem with a leased line connection, the network administrator knows there are a limited number of possibilities for where the problem may occur: the machine or router on the far end, the telecommunications company providing the link, or the machine or router at the corporate headquarters. For a VPN over the Internet, the problem could be with the machine on the far end, with the ISP on the far end, with one of the networks in between, with the corporate headquarters’ ISP, or with the machine or router at the corporate headquarters itself. Although a few large ISPs are offering quality of service guarantees with their VPN service (if all parties involved are connected to their network), smaller ISPs can’t make such a guarantee—and there will always be times when the network administrator is left to her own resources. This book will help you isolate and identify the problem when something goes wrong on your VPN.

1.5 A Note on IP Address and Domain Name Conventions Used in This Book

The notation 1.0.0.0/24 is commonly used in describing IP address ranges. It means “start with the address 1.0.0.0 and allow the right-most 8 bits to vary.” The 8 is calculated by using 32 bits (the maximum for an IP address) minus 24 (the size specified after the “/”). So 1.0.0.0/24 means all addresses from 1.0.0.0 to 1.0.0.255.

We’ve elected to use the same IP address ranges and domain name throughout this book. For Internet-routable IP address ranges, we’re using the blocks 1.0.0.0-1.255.255.255 (or 1.0.0.0/8) and 2.0.0.0-2.255.255.255 (2.0.0.0/8), which we subnet to suit our needs. These ranges were chosen because they are designated as Internet routable, but are reserved by the IANA and aren’t currently being used. We hope that using these ranges, rather than randomly picking some or choosing them from “active” registered networks, will makes examples and figures easier to understand while protecting the innocent. We found that this helped us maintain our own sanity while writing the book.

For internal networks, we use the IP ranges set aside in RFC 1918 for use on private networks. These ranges are 10.0.0.0-10.255.255.255 (or 10.0.0.0/8), 172.16.0.0- 172.31.255.255 (or 172.16.0.0/12), and 192.168.0.0-192.168.255.255 (or 192.168.0.0/16). We also subnet these as we deem necessary for an example.

The domain name we use for our examples is ora-vpn.com. Within this domain, however, we don’t have a hostname convention, because we typically create a hostname to match whatever solution we are writing about in a given chapter.

With the drive of ever-increasing Internet traffic, efficient use of signal bandwidth has become a key technology to increase the transmission capacity over already installed optical fibres and amplifiers. However, a binary signal beyond 40Gbit/s is severely limited by the operating speed of electrical and optical components, and also by the rapidly reducing chromatic dispersion (CD) and the polarization mode dispersion (PMD) tolerance [1]. On the other hand, multi-level modulation formats combined with coherent detection have become a promising technology to increase the capacity of optical fibre transmission and also to extend transmission distance [1-4]. Among multi-level modulation formats, 16 quadrature amplitude modulation (QAM), which carries four bits per symbol, is an attractive candidate. With respect to the constellation distributions, 16QAM can be categorized to three groups: square-16QAM, star-16QAM and 16APSK (amplitude phase shift key). Recently, square-16QAM has been extensively investigated and multiple schemes have been proposed and experimentally demonstrated [5][6]. Compared with square-16QAM, few research works are on the generations of star-16QAM and 16APSK signal. Moreover, the previously proposed transmitters of 16QAM could be complicated and output only one format.

In this paper, for the first time to the best of our knowledge, we propose a novel scheme for a 40Gbit/s 16QAM transmitter, which can generate various 16QAM signals. The feasibility of  proposed transmitter is verified by simulations. Furthermore, the generated 16QAM signals are transmitted through an 80- km transmission line. Coherent detection is performed afterthe transmission link. Clear constellation diagrams and error free performance are achieved.

II. PRINCIPLE

A. Star-16QAM

The schematic diagram of the proposed transmitter is depicted in Fig.1, which consists of a DPMZM followed by a PM. The DPMZM comprises a pair of x-cut LiNbO3 MZMs (MZMA, MZMB) embedded in the two arms of a main MZM structure. The two sub-MZMs have the same structure and performance, and the main MZM superimposes the outputs of the two sub-MZMs. The structure of the transmitter can be divided into two stages. In the first stage, the continuous wave is 4APSK modulated by a DPMZM. In particular, two sub-modulators of the DPMZM are biased at null point and driven by 10-Gbit/s Data1 and Data2, respectively, to generate two BPSK signals with unequal amplitudes. The two BPSK signals are then constructively interfered by adjusting Bias-c such that a 4APSK signal can be obtained, as depicted in Fig. 1. In the second stage, the generated 4APSK signal is further phase-modulated by a 4-level electrical signal, which is obtained by combining Data3 and Data4 at 10Gbit/s, to realize 4-PSK (0,π/4, π/2, 3π/4) modulation .In that case, a star-16QAM signal is achieved.

B. Square-16QAM

To obtain a square-16QAM signal, we only need to adjust the amplitudes of electrical signals and adjust the biases of modulators, while the stucture of the transmitter remains unchanged. The schematic diagram is shown in Fig.2. In the first stage, both sub-MZMs of the DPMZM are biased at quadrature point and driven by Data1 and Data2 with the same amplitude to generate two 2ASK signals with equal extinction ratio (ER). By adjusting the bias of main MZM, the two ASK signals achieve a 90°phase difference and then are combined to obtain an offset QPSK signal with its origin biased at first quadrant, as depicted in Fig. 2. In the second stage, the generated offset QPSK signal is further QPSK modulated by a PM, which is driven by 4-level electrical signal to realize a star-16QAM signal.

C. 16APSK

Some adjustments on the amplitudes of electrical signals and the biases of DPMZM are required to achieve 16APSK modulation. In particular, MZM-a of the DPMZM is biased at quadrature point and driven by Data1 at 10Gbit/s to generate 2ASK with finite ER. MZM-b of the DPMZM is also biased at quadrature point and driven by Data2 at 10Gbit/s to produce 2ASK with infinite ER. The two optical signals are then interfered constructively by adjusting the bias of the main MZM, a 4-ASK signal is realized. The PM, which is driven by a 4-level electrical signal generated by combining Data3 and Data4 at 10Gbit/s, further QPSK-modulates the generated 4- ASK signal to obtain a 16APSK signal.

III. SIMULATION AND RESULTS

To show the feasibility of proposed transmitter and investigate the transmission performance, we perform a simulation with its setup depicted in Fig. 4, using VPI TransmissionMaker. At the transmitter, a distributed feedback (DFB) laser with a linewidth of 1 MHz is used as continuous wave (CW) light source, which is modulated by the transmitter to produce 40-Gbit/s star-16QAM signal. The output of the transmitter is 0dBm after being amplified by an erbium-doped fibre amplifier (EDFA) and filtered by a tunable bandpass filter (BPF) with a bandwidth of 1.6 nm. At the receiver, coherent detection is performed. Another DFB with the same frequency and phase is used as local oscillator (LO), which is mixed with the received star-16QAM signal in an optical 90-degree hybrid. The outputs of hybrid are detected by two balanced detectors (BD) with the same performance. The real and imaginary parts of the star-16QAM signal are obtained by simultaneously sampling the outputs of the receivers. The sampled data are then processed off-line, which is realized using MATLAB program, including resampling, carrier phase estimation, and constellation recovery. 16384 bits are sampled in the system. The back-to-back (BTB) eye diagram and constellation map of star-16QAM are shown in Fig.5 (a) and (b), respectively. After transmission through an 80-km standard single-mode fiber (SSMF), the signal is boosted by another EDFA to 2 dBm. A 16-km dispersion compensating fiber (DCF) is used to compensate CD accumulated through the transmission link. TheSSMFhasa dispersion Dψ= 16ps/(nm≤km), a dispersion slopeSψ=006 ps/(nm21/km, and a lossα ψ=02dB/km. The DCF parameters areDψ=• 80ps/(nm≤km),Sψ=• 018ps/(nm2γ = 264W1/km, and α = 06dB/km, respectively.The signal is boosted to 0 dBm and then coherently detected in the receiver. It is shown that after 80-km transmission, the eye diagram and constellation map of star-16QAM are still clear enough to enable error-free operation,asdepictedinFig.6(a)and(b),respectively. ≤km), a nonlinear index γ ψ= 131W≤km),ψ/km, and α = 06dB/km, respectively.

By adjusting the amplitudes of electrical signals and biases of DPMZM in the transmitter, we obtain square- 16QAM signal, which is also transmitted through the mentioned link. After coherent detection and off-line processing, clear eye diagrams and constellation the conditions of BTB and after 80-km transmission are achieved,showing Fig.7a nd Fig.8,respectively.

16APSK can also be obtained with some modifications on the transmitter. Coherent detection and off-line processing are performed on the received 16APSK signal and clear eyes and constellation are obtained,with their diagramming Fig.9 and Fig.10.

IV. CONCLUSION

We have proposed a novel transmitter based on DPMZM and a following PM. Star-16QAM, square-16QAM and 16APSK are realized by VPI TransmissionMaker and thus the feasibility of the proposed transmitter is proved. The signals are also transmitted through an 80-km SSMF and detected by a coherent receiver. Clear eye diagrams and constellation maps are obtained and error performance is verified.

Single Mode Fibers

The core size of single mode fibers is small. The core size (diameter) is typically around 8 to 10 micrometers (μm). A fiber core of this size allows only the fundamental or lowest order mode to propagate around a 1300 nanometer (nm) wavelength. Single mode fibers propagate only one mode, because the core size approaches the operational wavelength (λ). The value of the normalized frequency parameter (V) relates core size with mode propagation.

In single mode fibers, V is less than or equal to 2.405. When V is less than or equal to 2.405, single mode fibers propagate the fundamental mode down the fiber core, while high-order modes are lost in the cladding. For low V values (<= 1.0), most of the power is propagated in the cladding material. Power transmitted by the cladding is easily lost at fiber bends. The value of V should remain near the 2.405 level.

Single mode fibers have a lower signal loss and a higher information capacity (bandwidth) than multimode fibers. Single mode fibers are capable of transferring higher amounts of data due to low fiber dispersion. Basically, dispersion is the spreading of light as light propagates along a fiber. Dispersion mechanisms in single mode fibers are discussed in more detail later in this chapter. Signal loss depends on the operational wavelength (λ). In single mode fibers, the wavelength can increase or decrease the losses caused by fiber bending. Single mode fibers operating at wavelengths larger than the cutoff wavelength lose more power at fiber bends. They lose power because light radiates into the cladding, which is lost at fiber bends. In general, single mode fibers are considered to be low-loss fibers, which increase system bandwidth and length.

Multimode Fibers

As their name implies, multimode fibers propagate more than one mode. Multimode fibers can propagate over 100 modes. The number of modes propagated depends on the core size and numerical aperture (NA). As the core size and NA increase, the number of modes increases. Typical values of fiber core size and NA are 50 to 100 μm and 0.20 to 0.29, respectively.

A large core size and a higher NA have several advantages. Light is launched into a multimode fiber with more ease. The higher NA and the larger core size make it easier to make fiber connections. During fiber splicing, core-to-core alignment becomes less critical. Another advantage is that multimode fibers permit the use of light-emitting diodes (LEDs). Single mode fibers typically must use laser diodes. LEDs are cheaper, less complex, and last longer. LEDs are preferred for most applications.

Multimode fibers also have some disadvantages. As the number of modes increases, the effect of modal dispersion increases. Modal dispersion (intermodal dispersion) means that modes arrive at the fiber end at slightly different times. This time difference causes the light pulse to spread. Modal dispersion affects system bandwidth. Fiber manufacturers adjust the core diameter, NA, and index profile properties of multimode fibers to maximize system bandwidth.

Properties of Optical Fiber Transmission

The principles behind the transfer of light along an optical fiber were discussed earlier in this chapter. We learned that propagation of light depended on the nature of light and the structure of the optical fiber. However, our discussion did not describe how optical fibers affect system performance.

In this case, system performance deals with signal loss and bandwidth.

Signal loss and system bandwidth describe the amount of data transmitted over a specified length of fiber. Many optical fiber properties increase signal loss and reduce system bandwidth. The most important properties that affect system performance are fiber attenuation and dispersion.

Attenuation reduces the amount of optical power transmitted by the fiber. Attenuation controls the distance an optical signal (pulse) can travel as shown in Figure 20. Once the power of an optical pulse is reduced to a point where the receiver is unable to detect the pulse, an error occurs. Attenuation is mainly a result of light absorption, scattering, and bending losses. Dispersion spreads the optical pulse as it travels along the fiber. This spreading of the signal pulse reduces the system bandwidth or the information-carrying capacity of the fiber. Dispersion limits how fast information is transferred as shown in Figure 20. An error occurs when the receiver is unable to distinguish between input pulses caused by the spreading of each pulse. The effects of attenuation and dispersion increase as the pulse travels the length of the fiber as shown in Figure 21.

In addition to fiber attenuation and dispersion, other optical fiber properties affect system performance. Fiber properties, such as modal noise, pulse broadening, and polarization, can reduce system performance.

Modal noise, pulse broadening, and polarization are too complex to discuss as introductory level material. However, be aware that attenuation and dispersion are not the only fiber properties that affect performance.

Attenuation

Attenuation in an optical fiber is caused by absorption, scattering, and bending losses. Attenuation is the loss of optical power as light travels along the fiber. Signal attenuation is defined as the ratio of optical input power (Pi) to the optical output power (Po). Optical input power is the power injected into the fiber from an optical source. Optical output power is the power received at the fiber end or optical detector. The following equation defines signal attenuation as a unit of length,

Where,
L = Length of the cable, kilometers.
Pi = Power input, watts.
P0 = Power output, watts.

Signal attenuation is a log relationship. Length (L) is expressed in kilometers. Therefore, the unit of attenuation is decibels/kilometer (dB/km). As previously stated, attenuation is caused by absorption, scattering, and bending losses. Each mechanism of loss is influenced by fibermaterial properties and fiber structure. However, loss is also present at fiber connections. The present discussion remains relative to optical fiber attenuation properties.

Absorption

Absorption is a major cause of signal loss in an optical fiber. Absorption is defined as the portion of attenuation resulting from the conversion of optical power into another energy form, such as heat. Absorption in optical fibers is explained by three factors:
• Imperfections in the atomic structure of the fiber material
• The intrinsic or basic fiber-material properties
• The extrinsic (presence of impurities) fiber-material properties

Imperfections in the atomic structure induce absorption by the presence of missing molecules or oxygen defects. Absorption is also induced by the diffusion of hydrogen molecules into the glass fiber. Since intrinsic and extrinsic material properties are the main cause of absorption, they are discussed further.

Intrinsic Absorption. – Intrinsic absorption is caused by basic fiber-material properties. If an optical fiber were absolutely pure, with no imperfections or impurities, then all absorption would be intrinsic. Intrinsic absorption sets the minimal level of absorption.

In fiber optics, silica (pure glass) fibers are used predominately. Silica fibers are used because of their low intrinsic material absorption at the wavelengths of operation.

In silica glass, the wavelengths of operation range from 700 nanometers (nm) to 1600 nm. Figure22 shows the level of attenuation at the wavelengths of operation. This wavelength of operation is between two intrinsic absorption regions. The first region is the ultraviolet region (below 400-nm wavelength). The second region is the infrared region (above 2000-nm wavelength).

Intrinsic absorption in the ultraviolet region is caused by electronic absorption bands. Basically, absorption occurs when a light particle (photon) interacts with an electron and excites it to a higher energy level. The tail of the ultraviolet absorption band is shown in Figure 22.

The main cause of intrinsic absorption in the infrared region is the characteristic vibration frequency of atomic bonds. In silica glass, absorption is caused by the vibration of siliconoxygen (Si-O) bonds. The interaction between the vibrating bond and the electromagnetic field of the optical signal causes intrinsic absorption. Light energy is transferred from the electromagnetic field to the bond. The tail of the infrared absorption band is shown in Figure 22.

Extrinsic Absorption. – Extrinsic absorption is caused by impurities introduced into the fiber material. Trace metal impurities, such as iron, nickel, and chromium, are introduced into the fiber during fabrication. Extrinsic absorption is caused by the electronic transition of these metal ions from one energy level to another.

Extrinsic absorption also occurs when hydroxyl ions (OH-) are introduced into the fiber. Water in silica glass forms a silicon-hydroxyl (Si-OH) bond. This bond has a fundamental absorption at 2700 nm. However, the harmonics or overtones of the fundamental absorption occur in the region of operation. These harmonics increase extrinsic absorption at 1383 nm, 1250 nm, and 950 nm. Figure 22 shows the presence of the three OH- harmonics. The level of the OHharmonic absorption is also indicated.

These absorption peaks define three regions or windows of preferred operation. The first window is centered at 850 nm. The second window is centered at 1300 nm. The third window is centered at 1550 nm. Fiber optic systems operate at wavelengths defined by one of these windows.

The amount of water (OH-) impurities present in a fiber should be less than a few parts per billion. Fiber attenuation caused by extrinsic absorption is affected by the level of impurities (OH-) present in the fiber. If the amount of impurities in a fiber is reduced, then fiber attenuation is reduced.

Scattering

Basically, scattering losses are caused by the interaction of light with density fluctuations within a fiber. Density changes are produced when optical fibers are manufactured.

During manufacturing, regions of higher and lower molecular density areas, relative to the average density of the fiber, are created. Light traveling through the fiber interacts with the density areas as shown in Figure 23. Light is then partially scattered in all directions.

In commercial fibers operating between 700-nm and 1600-nm wavelength, the main source of loss is called Rayleigh scattering. Rayleigh scattering is the main loss mechanism between the ultraviolet and infrared regions as shown in Figure 22. Rayleigh scattering occurs when the size of the density fluctuation (fiber defect) is less than one-tenth of the operating wavelength of light. Loss caused by Rayleigh scattering is proportional to the fourth power of the wavelength (1/λ4). As the wavelength increases, the loss caused by Rayleigh scattering decreases.

If the size of the defect is greater than one-tenth of the wavelength of light, the scattering mechanism is called Mie scattering. Mie scattering, caused by these large defects in the fiber core, scatters light out of the fiber core. However, in commercial fibers, the effects of Mie scattering are insignificant. Optical fibers are manufactured with very few large defects.

Bending the fiber also causes attenuation. Bending loss is classified according to the bend radius of curvature: microbend loss or macrobend loss.

Microbends are small microscopic bends of the fiber axis that occur mainly when a fiber is cabled. Macrobends are bends having a large radius of curvature relative to the fiber diameter. Microbend and macrobend losses are very important loss mechanisms. Fiber loss caused by microbending can still occur even if the fiber is cabled correctly. During installation, if fibers are bent too sharply, macrobend losses will occur.

Microbend losses are caused by small discontinuities or imperfections in the fiber. Uneven coating applications and improper cabling procedures increase microbend loss. External forces are also a source of microbends. An external force deforms the cabled jacket surrounding the fiber but causes only a small bend in the fiber. Microbends change the path that propagating modes take, as shown in Figure 24. Microbend loss increases attenuation because low-order modes become coupled with high-order modes that are naturally lossy.

Macrobend losses are observed when a fiber bend’s radius of curvature is large compared to the fiber diameter.

These bends become a great source of loss when the radius of curvature is less than several centimeters. Light propagating at the inner side of the bend travels a shorter distance than that on the outer side. To maintain the phase of the light wave, the mode phase velocity must increase.

When the fiber bend is less than some critical radius, the mode phase velocity must increase to a speed greater than the speed of light. However, it is impossible to exceed the speed of light. This condition causes some of the light within the fiber to be converted to high-order modes. These high-order modes are then lost or radiated out of the fiber.

Fiber sensitivity to bending losses can be reduced. If the refractive index of the core is increased, then fiber sensitivity decreases. Sensitivity also decreases as the diameter of the overall fiber increases. However, increases in the fiber core diameter increase fiber sensitivity. Fibers with larger core size propagate more modes. These additional modes tend to be more lossy.

Dispersion

There are two different types of dispersion in optical fibers.

The types are intramodal and intermodal dispersion. Intramodal, or chromatic, dispersion occurs in all types of fibers. Intermodal, or modal, dispersion occurs only in multimode fibers. Each type of dispersion mechanism leads to pulse spreading. As a pulse spreads, energy is overlapped. This condition is shown in Figure 25. The spreading of the optical pulse as it travels along the fiber limits the information capacity of the fiber.

Intramodal, or chromatic, dispersion depends primarily on fiber materials. There are two types of intramodal dispersion. The first type is material dispersion. The second type is waveguide dispersion.

Intramodal dispersion occurs because different colors of light travel through different materials and different waveguide structures at different speeds.

Material dispersion occurs because the spreading of a light pulse is dependent on the wavelengths’ interaction with the refractive index of the fiber core. Different wavelengths travel at different speeds in the fiber material. Different wavelengths of a light pulse that enter a fiber at one time exit the fiber at different times. Material dispersion is a function of the source spectral width. The spectral width specifies the range of wavelengths that can propagate in the fiber. Material dispersion is less at longer wavelengths.

Waveguide dispersion occurs because the mode propagation constant (β) is a function of the size of the fiber’s core relative to the wavelength of operation. Waveguide dispersion also occurs because light propagates differently in the core than in the cladding.

In multimode fibers, waveguide dispersion and material dispersion are basically separate properties. Multimode waveguide dispersion is generally small compared to material dispersion. Waveguide dispersion is usually neglected.

However, in single mode fibers, material and waveguide dispersion are interrelated.

The total dispersion present in single mode fibers may be minimized by trading material and waveguide properties depending on the wavelength of operation.

Intermodal Dispersion
Intermodal or modal dispersion causes the input light pulse to spread. The input light pulse is made up of a group of modes. As the modes propagate along the fiber, light energy distributed among the modes is delayed by different amounts. The pulse spreads because each mode propagates along the fiber at different speeds. Since modes travel in different directions, some modes travel longer distances. Modal dispersion occurs because each mode travels a different distance over the same time span, as shown in Figure 26. The modes of a light pulse that enter the fiber at one time exit the fiber at different times. This condition causes the light pulse to spread. As the length of the fiber increases, modal dispersion increases.

Modal dispersion is the dominant source of dispersion in multimode fibers. Modal dispersion does not exist in single mode fibers. Single mode fibers propagate only the fundamental mode. Therefore, single mode fibers exhibit the lowest amount of total dispersion. Single mode fibers also exhibit the highest possible bandwidth.

The mode theory, along with the ray theory, is used to describe the propagation of light along an optical fiber. The mode theory is used to describe the properties of light that ray theory is unable to explain. The mode theory uses electromagnetic wave behavior to describe the propagation of light along a fiber. A set of guided electromagnetic waves is called the modes of the fiber.

Plane Waves
The mode theory suggests that a light wave can be represented as a plane wave. A plane wave is described by its direction, amplitude,and wavelength of propagation. A plane wave is a wave whose surfaces of constant phase are infinite parallel planes normal to the direction of propagation.

The planes having the same phase are called the wavefronts. The wavelength (λ) of the plane wave is given by,

Where,
λ = Wavelength.
c = Speed of light in a vacuum, 3 x 108 meters per second (m/s).
f = Frequency of light.
n = Index of refraction of the medium.

Figure 15 shows the direction and wavefronts of plane-wave propagation. Plane waves, or wavefronts, propagate along the fiber similar to light rays. However, not all wavefronts incident on the fiber at angles less than or equal to the critical angle of light acceptance propagate along the fiber. Wavefronts may undergo a change in phase that prevents the successful transfer of light along the fiber.

Wavefronts are required to remain in phase for light to be transmitted along the fiber. Consider the wavefront incident on the core of an optical fiber as shown in Figure 16. Only those wavefronts incident on the fiber at angles less than or equal to the critical angle may propagate along the fiber. The wavefront undergoes a gradual phase change as it travels down the fiber. Phase changes also occur when the wavefront is reflected. The wavefront must remain in phase after the wavefront transverses the fiber twice and is reflected twice. The distance transversed is shown between point A and point B on Figure 16. The reflected waves at point A and point B are in phase if the total amount of phase collected is an integer multiple of 2π radian. If propagating wavefronts are not in phase, they eventually disappear. Wavefronts disappear because of destructive interference. The wavefronts that are in phase interfere with the wavefronts that are out of phase. This interference is the reason why only a finite number of modes can propagate along the fiber.

The plane waves repeat as they travel along the fiber axis. The direction the plane wave travels is assumed to be the z-direction as shown in Figure 16. The plane waves repeat at a distance equal to λ/sin(Θ). Plane waves also repeat at a periodic frequency β = 2π sin(Θ)/λ. The quantity β is defined as the propagation constant along the fiber axis. As the wavelength (λ) changes, the value of the propagation constant must also change.

For a given mode, a change in wavelength can prevent the mode from propagating along the fiber. The mode is no longer bound to the fiber. The mode is said to be cut off. Modes that are bound at one wavelength may not exist at longer wavelengths. The wavelength at which a mode ceases to be bound is called the cutoff wavelength for that mode. However, an optical fiber is always able to propagate at least one mode. This mode is referred to as the fundamental mode of the fiber. The fundamental mode can never be cut off.

The wavelength that prevents the next higher mode from propagating is called the cutoff wavelength of the fiber. An optical fiber that operates above the cutoff wavelength (at a longer wavelength) is called a single mode fiber. An optical fiber that operates below the cutoff wavelength is called a multimode fiber.

In a fiber, the propagation constant of a plane wave is a function of the wave’s wavelength and mode. The change in the propagation constant for different waves is called dispersion. The change in the propagation constant for different wavelengths is called chromatic dispersion. The change in propagation constant for different modes is called modal dispersion.

These dispersions cause the light pulse to spread as it goes down the fiber (See Figure 17). Some dispersion occurs in all types of fibers.

Modes – A set of guided electromagnetic waves is called the modes of an optical fiber.

Maxwell’s equations describe electromagnetic waves or modes as having two components. The two components are the electric field, E(x, y, z), and the magnetic field, H(x, y, z). The electric field, E, and the magnetic field, H, are at right angles to each other. Modes traveling in an optical fiber are said to be transverse. The transverse modes, shown in Figure 18, propagate along the axis of the fiber. The mode field patterns shown in Figure 18 are said to be transverse electric (TE). In TE modes, the electric field is perpendicular to the direction of propagation.

The magnetic field is in the direction of propagation. Another type of transverse mode is the transverse magnetic (TM) mode. TM modes are opposite to TE modes. In TM modes, the magnetic field is perpendicular to the direction of propagation. The electric field is in the direction of propagation. Figure 18 shows only TE modes.

The TE mode field patterns shown in Figure 18 indicate the order of each mode. The order of each mode is indicated by the number of field maxima within the core of the fiber. For example, TE0 has one field maxima. The electric field is a maximum at the center of the waveguide and decays toward the core-cladding boundary. TE0 is considered the fundamental mode or the lowest order standing wave. As the number of field maxima increases, the order of the mode is higher. Generally, modes with more than a few (5-10) field maxima are referred to as high-order modes.

The order of the mode is also determined by the angle the wavefront makes with the axis of the fiber. Figure 19 illustrates light rays as they travel down the fiber. These light rays indicate the direction of the wavefronts. High-order modes cross the axis of the fiber at steeper angles. Loworder and high-order modes are shown in Figure 19.

Before we progress, let us refer back to Figure 18.

Notice that the modes are not confined to the core of the fiber. The modes extend partially into the cladding material. Low-order modes penetrate the cladding only slightly. In low-order modes, the electric and magnetic fields are concentrated near the center of the fiber. However, high-order modes penetrate further into the cladding material. In high-order modes, the electrical and magnetic fields are distributed more toward the outer edges of the fiber.

This penetration of low-order and high-order modes into the cladding region indicates that some portion is refracted out of the core. The refracted modes may become trapped in the cladding due to the dimension of the cladding region. The modes trapped in the cladding region are called cladding modes. As the core and the cladding modes travel along the fiber, mode coupling occurs. Mode coupling is the exchange of power between two modes. Mode coupling to the cladding results in the loss of power from the core modes.

In addition to bound and refracted modes, there are leaky modes.

Leaky modes are similar to leaky rays. Leaky modes lose power as they propagate along the fiber. For a mode to remain within the core, the mode must meet certain boundary conditions. A mode remains bound if the propagation constant beta (β) meets the following boundary condition,

Where,
λ = Wavelength.
β = Propagation constant.
n1 = Index of refraction for the core.
n2 = Index of refraction for the cladding.

When the propagation constant becomes smaller than 2πn2/λ, power leaks out of the core and into the cladding. Generally, modes leaked into the cladding are lost in a few centimeters. However, leaky modes can carry a large amount of power in short fibers.

Normalized Frequency
Electromagnetic waves bound to an optical fiber are described by the fiber’s normalized frequency.

The normalized frequency determines how many modes a fiber can support. Normalized frequency is a dimensionless quantity.

Normalized frequency is also related to the fiber’s cutoff wavelength. Normalized frequency (V) is defined as,

Where,
V = Normalized Frequency.
a = Core diameter.
λ = Wavelength of light in air.
n1 = Core index of refraction.
n2 = Cladding index of refraction.
l = Wavelength of light in air.

The number of modes that can exist in a fiber is a function of V. As the value of V increases, the number of modes supported by the fiber increases. Optical fibers, single mode and multimode, can support a different number of modes.

Optical Material Properties

The basic optical property of a material, relevant to optical fibers, is the index of refraction. The index of refraction (n) measures the speed of light in an optical medium. The index of refraction of a material is the ratio of the speed of light in a vacuum to the speed of light in the material itself. The speed of light (c) in free space (vacuum) is 3 x 108 meters per second (m/s). The speed of light is the frequency (f) of light multiplied by the wavelength of light (λ). When light enters the fiber material, the light travels slower at a speed (v). Light will always travel slower in the fiber material than in air. The index of refraction is given by,

Where,
n = Index of refraction.
c = Speed of light, 3 x 108 meters per second (m/s).
v = Speed of light in the fiber material, meters/sec.

A light ray is reflected and refracted when it encounters the boundary between two different transparent mediums. For example, Figure 8 shows what happens to the light ray when it encounters the interface between glass and air. The index of refraction for glass (n1) is 1.50. The index of refraction for air (n2) is 1.00.

Let’s assume the light ray or incident ray is traveling through the glass. When the light ray encounters the glass-air boundary, there are two results. The first result is that part of the ray is reflected back into the glass. The second result is that part of the ray is refracted (bent) as it enters the air. The bending of the light at the glass-air interface is the result of the difference between the index of refractions. Since n1 is greater than n2, the angle of refraction (Θ2) will be greater than the angle of incidence (Θ1). Snell’s law of refraction is used to describe the relationship between the incident and the refracted rays at the boundary. Snell’s Law is given by,

As the angle of incidence (Θ1) becomes larger, the angle of refraction (Θ2) approaches 90 degrees. At this point, no refraction is possible. The light ray is totally reflected back into the glass medium. No light escapes into the air. This condition is called total internal reflection.The angle at which total internal reflection occurs is called the critical angle of incidence. The critical angle of incidence (Θc) is shown in Figure 9. At any angle of incidence (Θ1) greater than the critical angle, light is totally reflected back into the glass medium. The critical angle of incidence is determined by using Snell’s Law. The critical angle is given by,

The condition of total internal reflection is an ideal situation.

However, in reality, there is always some light energy that penetrates the boundary. This situation is explained by the mode theory, or the electromagnetic wave theory, of light.

Optical Fiber Structure

The basic structure of an optical fiber consists of three parts; the core, the cladding, and the coating or buffer. The basic structure of an optical fiber is shown in Figure 10. The core is a cylindrical rod of dielectric material. Dielectric material conducts no electricity. Light propagates mainly along the core of the fiber. The core is generally made of glass. The core is described as having a radius of (a) and an index of refraction n1. The core is surrounded by a layer of material called the cladding. Even though light will propagate along the fiber core without the layer of cladding material, the cladding does perform some necessary functions.

The cladding layer is made of a dielectric material with an index of refraction n2. The index of refraction of the cladding material is less than that of the core material. The cladding is generally
made of glass or plastic. The cladding performs the following functions:

• Reduces loss of light from the core into the surrounding air
• Reduces scattering loss at the surface of the core
• Protects the fiber from absorbing surface contaminants
• Adds mechanical strength

For extra protection, the cladding is enclosed in an additional layer called the coating or buffer. The coating or buffer is a layer of material used to protect an optical fiber from physical damage. The material used for a buffer is a type of plastic.

The buffer is elastic in nature and prevents abrasions. The buffer also prevents the optical fiber from scattering losses caused by microbends. Microbends occur when an optical fiber is placed on a rough and distorted surface.

Propagation of Light along a Fiber

The concept of light propagation, the transmission of light along an optical fiber, can be described by two theories. According to the first theory, light is described as a simple ray. This theory is the ray theory, or geometrical optics, approach. The advantage of the ray approach is that you get a clearer picture of the propagation of light along a fiber. The ray theory is used to approximate the light acceptance and guiding properties of optical fibers. According to the second theory, light is described as an electromagnetic wave. This theory is the mode theory, or wave representation, approach. The mode theory describes the behavior of light within an optical fiber. The mode theory is useful in describing the optical fiber properties of absorption, attenuation, and dispersion.

Ray Theory

Two types of rays can propagate along an optical fiber. The first type is called meridional rays. Meridional rays are rays that pass through the axis of the optical fiber. Meridional rays are used to illustrate the basic transmission properties of optical fibers.

The second type is called skew rays. Skew rays are rays that travel through an optical fiber without passing through its axis.

Meridional Rays
Meridional rays can be classified as bound or unbound rays. Bound rays remain in the core and propagate along the axis of the fiber. Bound rays propagate through the fiber by total internal reflection. Unbound rays are refracted out of the fiber core. Figure 11 shows a possible path taken by bound and unbound rays in a step-index fiber. The core of the step-index fiber has an index of refraction n1. The cladding of a step-index has an index of refraction n2, which is lower than n1. Figure 11 assumes the core-cladding interface is perfect. However, imperfections at the core-cladding interface will cause part of the bound rays to be refracted out of the core into the cladding. The light rays refracted into the cladding will eventually escape from the fiber. In general, meridional rays follow the laws of reflection and refraction.

It is known that bound rays propagate in fibers due to total internal reflection, but how do these light rays enter the fiber? Rays that enter the fiber must intersect the core-cladding interface at an angle greater than the critical angle (Θc). Only those rays that enter the fiber and strike the interface at these angles will propagate along the fiber.

How a light ray is launched into a fiber is shown in Figure 12. The incident ray I1 enters the fiber at the angle Θa. I1 is refracted upon entering the fiber and is transmitted to the corecladding interface. The ray then strikes the core-cladding interface at the critical angle (Θc). I1 is totally reflected back into the core and continues to propagate along the fiber. The incident ray I2 enters the fiber at an angle greater than Θa. Again, I2 is refracted upon entering the fiber and is transmitted to the core-cladding interface. I2 strikes the core-cladding interface at an angle less than the critical angle (Θc). I2 is refracted into the cladding and is eventually lost. The light ray incident on the fiber core must be within the acceptance cone defined by the angle Θa shown in Figure 13.

Angle Θa is defined as the acceptance angle. The acceptance angle (Θa) is the maximum angle to the axis of the fiber that light entering the fiber is propagated. The value of the angle of acceptance (Θa) depends on fiber properties and transmission conditions.

The acceptance angle is related to the refractive indices of the core, cladding, and medium surrounding the fiber. This relationship is called the numerical aperture of the fiber. The numerical aperture (NA) is a measurement of the ability of an optical fiber to capture light. The NA is also used to define the acceptance cone of an optical fiber.

Figure 14 illustrates the relationship between the acceptance angle and the refractive indices. The index of refraction of the fiber core is n1. The index of refraction of the fiber cladding is n2. The index of refraction of the surrounding medium is n0. By using Snell’s law and basic trigonometric relationships, the NA of the fiber is given by,

Since the medium next to the fiber at the launching point is normally air, n0 is equal to 1.00. The NA is then simply equal to sin(Θa).

The NA is a convenient way to measure the light-gathering ability of an optical fiber. It is used to measure source-to-fiber power-coupling efficiencies. A high NA indicates a high source-tofiber coupling efficiency.

Typical values of NA range from 0.20 to 0.29 for glass fibers. Plastic fibers generally have a higher NA. An NA for plastic fibers can be higher than 0.50. In addition, the NA is commonly used to specify multimode fibers.

However, for small core diameters, such as in single mode fibers, the ray theory breaks down. Ray theory describes only the direction a plane wave takes in a fiber. Ray theory eliminates any properties of the plane wave that interfere with the transmission of light along a fiber. In reality, plane waves interfere with each other. Therefore, only certain types of rays are able to propagate in an optical fiber. Optical fibers can support only a specific number of guided modes. In small core fibers, the number of modes supported is one or only a few modes. Mode theory is used to describe the types of plane waves able to propagate along an optical fiber.

 Skew Rays

A possible path of propagation of skew rays is shown in Figure 14. Figure 14, the first view provides an angled view and the second view provides a front view. Skew rays propagate without passing through the center axis of the fiber.

The acceptance angle for skew rays is larger than the acceptance angle of meridional rays. This condition explains why skew rays outnumber meridional rays. Skew rays are often used in the calculation of light acceptance in an optical fiber. The addition of skew rays increases the amount of light capacity of a fiber. In large NA fibers, the increase may be significant.

The addition of skew rays also increases the amount of loss in a fiber. Skew rays tend to propagate near the edge of the fiber core. A large portion of the number of skew rays that are trapped in the fiber core are considered to be leaky rays. Leaky rays are predicted to be totally reflected at the core-cladding boundary. However, these rays are partially refracted because of the curved nature of the fiber boundary. Mode theory is also used to describe this type of leaky ray loss.

Before we introduce the subject of light transmission through optical fibers, we must first understand the nature of light and the properties of light waves.

Light Propagation

The exact nature of light is not fully understood, although people have been studying the subject for many centuries. In the 1700s and before, experiments seemed to indicate that light was
composed of particles. In the early 1800s, a physicist Thomas Young showed that light exhibited wave characteristics.

Further experiments by other physicists culminated in James Maxwell collecting the four fundamental equations that completely describe the behavior of the electromagnetic fields. James Maxwell deduced that light was simply a component of the electromagnetic spectrum. This seems to firmly establish that light is a wave. Yet, in the early 1900s, the interaction of light with
semiconductor materials, called the photoelectric effect, could not be explained with electromagnetic-wave theory.

The advent of quantum physics successfully explained the photoelectric effect in terms of fundamental particles of energy called quanta. Quanta are known as photons when referring to light energy.

Today, when studying light that consists of many photons, as in propagation, that light behaves as a continuum – an electromagnetic wave. On the other hand, when studying the interaction of
light with semiconductors, as in sources and detectors, the quantum physics approach is taken. In this course we use both the electromagnetic wave and photon concepts, each in the places
where it best matches the phenomenon we are studying.

The electromagnetic energy of light is a form of electromagnetic radiation.

Light and similar forms of radiation are made up of moving electric and magnetic forces. A simple example of motion similar to these radiation waves can be made by dropping a pebble into a pool of water. In this example, the water is not actually being moved by the outward motion of the wave, but rather by the up-and-down motion of the water. The up-and-down motion is transverse, or at right angles, to the outward motion of the waves. This type of wave motion is called transverse-wave motion. The transverse waves spread out in expanding circles until they reach the edge of the pool, in much the same manner as the transverse waves of light spread from the sun. However, the waves in the pool are very slow and clumsy in comparison with light, which travels approximately 186,000 miles per second.

Light radiates from its source in all directions until it is absorbed or diverted by some substance See Figure 2, the lines drawn from the light source to any point on one of the transverse waves
indicate the direction that the wavefronts are moving. These lines are called light rays.

Although single rays of light typically do not exist, light rays shown in illustrations are a convenientmethod used to show the direction in which light is traveling at any point. A ray of light can be illustrated as a straight line.

Light Properties

When light waves, which travel in straight lines, encounter any substance, they are can be reflected, absorbed, transmitted, or refracted. This is illustrated in Figure 3. Those substances that transmit almost all the light waves falling upon them are said to be transparent. A transparent substance is one through which you can see clearly.

Clear glass is transparent because it transmits light rays without diffusingthem, such as shown in the first image in Figure 4. There is no substance known that is perfectly transparent, but many
substances are nearly so. Substances through which some light rays can pass, but through which objects cannot be seen clearly because the rays are diffused, are called translucent (See the second image in Figure 4). The frosted glass of a light bulb and a piece of oiled paper are examples of translucent materials. Those substances that are unable to transmit any light rays are
called opaque (third image in Figure 4). Opaque substances either reflect or absorb all the light rays that fall upon them.

All substances that are not light sources are visible only because they reflect all or some part of the light reaching them from some luminous source.
Examples of luminous sources include the sun, a gas flame, and an electric light filament, because they are sources of light energy. If light is neither transmitted nor reflected, it is absorbed or taken up by the medium. When light strikes a substance, some absorption and some reflection always take place. No substance completely transmits, reflects, or absorbs all the light rays that reach its surface.

Light Reflection

When a light wave passes from one medium into a medium having a different velocity of propagation, a change in the direction of the wave will occur. This change of direction as the wave enters the second medium is called refraction. As in the discussion of reflection, the wave striking the boundary (surface) is called the incident wave, and the imaginary line perpendicular to the boundary is called the normal. The angle between the incident wave and the normal is called the angle of incidence. As the wave passes through the boundary, it is bent either toward or away from the normal. The angle between the normal and the path of the wave through the second medium is the angle of refraction.

A light wave passing through a block of glass is shown in Figure 6. The wave moves from point A to point B at a constant speed. This is the incident wave. As the wave penetrates the glass boundary at point B, the velocity of the wave is slowed down. This causes the wave to bend toward the normal. The wave then takes the path from point B to point C through the glass and becomes both the refracted wave from the top surface and the incident wave to the lower surface. As the wave passes from the glass to the air (the second boundary), it is again refracted, this time away from the normal, and takes the path from point C to point D. After passing through the last boundary, the velocity increases to the original velocity of the wave. As illustrated, refracted waves can bend toward or away from the normal.

This bending depends on the velocity of the wave through different mediums.

The broken line between points B and E is the path that the wave would travel if the two mediums (air and glass) had the same density.

Another interesting condition can be shown using Figure 6. If the wave passes from a less dense to a denser medium, it is bent toward the normal, and the angle of refraction (r) is less than the
angle of incidence (i). Likewise, if the wave passes from a denser to a less dense medium, it is bent away from the normal, and the angle of refraction (r1) is greater than the angle of incidence (i1).

An example of refraction is the apparent bending of a spoon when it is immersed in a cup of water. The bending seems to take place at the surface of the water, or exactly at the point where there is a change of density.

Obviously, the spoon does not bend from the pressure of the water. The light forming the image of the spoon is bent as it passes from the water (a medium of high density) to the air (a medium
of comparatively low density).

Without refraction, light waves would pass in straight lines through transparent substances without any change of direction. Figure 6 shows that rays striking the glass at any angle other than perpendicular are refracted. However, perpendicular rays, which enter the glass normal to the surface, continue through the glass and into the air in a straight line – no refraction takes
place.

Light Diffusion

When light is reflected from a mirror, the angle of reflection equals the angle of incidence. When light is reflected from a piece of plain white paper; however, the reflected beam is scattered, or
diffused, as shown in Figure 7. Because the surface of the paper is not smooth, the reflected light is broken up into many light beams that are reflected in all directions.

Light Absorption

We have just seen that a light beam is reflected and diffused when it falls onto a piece of white paper. If the light beam falls onto a piece of black paper, the black paper absorbs most of the light rays and very little light is reflected from the paper.
If the surface upon which the light beam falls is perfectly black, there is no reflection; that is, the light is totally absorbed. No matter what kind of surface light falls upon, some of the light is absorbed.

Fiber-optic cable is jacketed glass fiber. In order to be usable in fiber-optic systems, the somewhat fragile optical fibers are packaged inside cables for strength and protection against breakage, as well as against such environmental hazards as moisture, abrasion, and high temperatures.

Packaging of fiber in cable also protects the fibers from bending at too sharp an angle, which could result in breakage and a consequent loss of signal.

Multiconductor cable is available for all designs and can have as many as 144 fibers per cable. It is noteworthy that a cable containing 144 fibers can be as small as .75 inches in diameter.

In addition to the superior transmission capabilities, small size, and weight advantages of fiber-optic cables, another advantage is found in the absence of electromechanical interference. There are no metallic conductors to induce crosstalk into the system. Power influence is nonaffecting, and security breaches of communications are (at this time)
very difficult due to the complexities of tapping optical fiber.

MAIN PARTS OF A FIBER-OPTIC CABLE

The creation of fiber-optic cables involves placing several fibers together in a process that involves use of strength members and insulated (buffered) conductors. When a number of optical fibers are placed into a single cable, they are frequently twisted around a central passive support (strength member) which serves to strengthen the cable.

Although fiber-optic cable comes in many varieties, most have the following elements in common:

• Optical fiber (core and cladding, plus coating).
• Buffer.
• Strength member.
• Jacket.

Previous sections have dealt with fiber, so only the remaining three items will be dealt with now.

Buffer

Fiber coating, or the buffer, serves three purposes: (1) Protection of the fiber surface from mechanical damage; (2) isolation of the fiber from the effects of microbends; and (3) as a moisture barrier.

The outer layer, or secondary coating, is the tough material that protects the fiber surface from mechanical damage during handling and cabling operations. The inner, or primary coating, is a material designed to isolate the fiber from damage from microbending. Both layer obviously serve as moisture barriers. With the exception of abrasion, uncoated fiber is virtually unaffected by many environments. Because of this, most environmental tests are designed to  evaluate coating performance over time.

The simplest buffer is the plastic coating applied by the fiber manufacturer to the cladding. An additional buffer is added by the cable manufacturer. The cable buffer is one of two types: loose buffer or tight buffer.

The tight buffer design features one or two layers of protective coating placed over the initial fiber  coating which may be on an individual fiber basis, or in a ribbon structure. The ribbon design typically  features 12 fibers placed parallel between two layers of tape with the ribbons lying loosely within the cable core.

An advantage to the tight buffer is that it is more flexible than loose and allows tighter turn radii. This can make tight-tube buffers useful for indoor applications where temperature variations are minimal and the ability to make tight turns inside walls is a desirable feature.

The loose buffer design features fibers placed into a cavity which is much larger than the fiber with its initial coating, such as a buffer tube, envelope, or slotted core. This allows the fiber to be slightly longer than its confining cavity, and allows movement of the fiber within the cable to relieve strain during cabling and field-placing operations.

Individual tight-buffered fiber cables are not generally used in applications subjected to temperature changes due to the added attenuation caused by the strain that is placed on fiber during the cabling process and the contraction differences of the coating material and glass fibers when subjected to these changes.

In loose-buffer tube designs, the fiber tube is usually filled with a viscous gel compound which repels water. Slotted, or envelope designs are usually filled with a water-repellent powder. Although water does not affect the transmission properties of optical fiber, the formation of ice within the cable will cause severe microbending and added dB loss to the
system.

A comparison of loose tube features to tight tube is provided in section 3, Table C.

Strength Member

Strength members add mechanical strength to the fiber. During and after installation, the strength members handle the tensile stresses applied to the cable so that the fiber is not damaged.

The most common strength members are of Kevlar aramid yarn, steel, and fiberglass epoxy rods. Kevlar is most commonly used when individual fibers are placed within their own jackets. Steel and fiberglass members are frequently used in multifiber cables.

Jacket

The jacket, like wire insulation, provides protection from the effects of abrasion, oil, ozone, acids, alkali, solvents, and so forth. The choice of the jacket material depends on the degree of resistance required for different influences and on cost.

A comparison of the relative properties of various popular jacket materials is provided in section 3, Table D.

ADDITIONAL CABLE CHARACTERISTICS

Cables come reeled in various lengths, typically 1 or 2 km, although lengths of 5 or 6 km are available for single-mode fibers. Long lengths are desirable for long-distance applications since cables must be spliced end-to-end over the length of the run, hence the longer the cable, the fewer the splices that will be required.

Fiber coatings or buffer tubes or both are often coded to make identification of each fiber easier. In the long-distance link it’s necessary to be able to ensure that fiber A in the first cable is spliced to fiber A in the second cable, and fiber B to fiber B, and so on.

In addition to knowing the maximum tensile loads that can be applied to a cable, it’s necessary to know the installation load. This is the short-term load that the fiber can withstand during the actual process of installation. This figure includes the additional load that is exerted by pulling the fiber through ducts or conduits, around corners, etc. The maximum specified installation load will establish the limits on the length of the cable that can be installed at one time, given the particular application.

The second load specified is the operating load. During its installed life, the cable cannot withstand loads as heavy as it withstood during installation. The specified operating load is therefore less than the installation load. The operating load is also called the static load. For the purposes of this discussion we have divided the discussion on cables by indoor or outdoor.

Indoor Cable

Cables for indoor applications see Figure 2-13 below) include:
• Simplex
• Duplex
• Multifiber
• Undercarpet
• Heavy- and light-duty
• Plenum

Simplex is a term used to indicate a single fiber. Duplex refers to two optical fibers. One fiber may carry the signals in one direction; the other fiber may carry the signals in the opposite direction. (Duplex operation is possible with two simplex cables.)

Physically, duplex cables resemble two simplex cables whose jackets have been bonded together, similar to the jacket of common lamp cords. This type of cable is used instead of two simplex cables for aesthetic reasons and for convenience. It’s easier to handle, there’s less chance of the two channels becoming confused, and the appearance is more pleasing.

Multifiber cable, as the name would imply, contain more than two fibers. They allow signals to be distributed throughout a building. Multifiber cables often contain several loose-buffer tubes, each containing one or more fibers. The use of several tubes allows identification of fibers by tube, since both tubes and fibers can be color coded.

Undercarpet cable,as this name implies, is run across a floor under carpeting. It is frequently found in open-space office or work areas that are defined by movable walls, partitions. A key feature of this cable is its ability to be rearranged or

reconfigured as space needs change. One problem, however, is making turns without stressing the fibers. Unfortunately, the fiber on the outside of the turn must always take a longer path than the fiber on the inside. This unequal path length places differing stresses on the fibers. (Refer to Figure 2-14 below.)

Heavy- and light-duty cables refer to the ruggedness of the cable, one being able to withstand rougher handling than the other, especially during installation.

A plenum is the return or air-handling space located between a roof and a dropped ceiling. The National Electrical Code (NEC) has designated strict requirements for cables used in these areas.

Because certain jacket materials give off noxious fumes when burned, the NEC states that cables run in plenum must either be enclosed in fireproof conduits or be insulated and jacketed with low-smoke and fire-retardant materials.

Thus plenum cables are those whose materials allow them to be used without conduit. Because no conduit is used for these cables, they are easier to route. So, while plenum cables initially are more expensive, there are savings inherent in installation.

Other benefits are reduced weights on ceilings or fixtures and easier reconfigurations and flexibility for local area networks and computer data systems.

Outdoor Cable

Cables for outdoor applications include:

• Aerial or overhead (as found strung between buildings or telephone poles).
• Direct burial cables that are placed directly in a trench dug in the ground and then covered.
• Indirect burial, similar to direct burial, but the cable is inside a duct or conduit.
• Submarine cable is underwater, including transoceanic application.

All of the foregoing cables must be rugged and durable since their applications subject them to a variety of extremes. Typically, the internal glass fiber is the same for all types of fiber cable with some small exceptions.

Cables designed for underground use may contain one or more fibers encased in metal jackets and flooded with a moisture-proofing gel.

Hybrid Cable

This is a unique type of cable generally available on special order only. It is designed for multipurpose applications where both optical fiber and twisted pair wires are jacketed together in those situations where both technologies are called for. This style cable is also useful when future expansion plans call for optical fiber.

Hybrid cable (Figure 2- 15) allows for existing copper networks to be upgraded to fiber without the requirement for new cable. With hybrid cable, this can be accomplished without disrupting the existing service.

This cable style is also useful in applications such as local area networks (LANs) and integrated digital services networks (ISDNs) where easy or smooth transition from copper to fiber is possible at a future time, basically because the hybrid cable permits the end user to be “fiber ready.”

Cable designs are available with multiple elements including the specific wire or fiber types (single- or multimode). Fibers are color coded for ready identification. As with conventional cable, hybrids can be manufactured to specific requirements.

Breakout Cable

A breakout cable is one which offers a rugged cable design for shorter network designs. This may include LANs, data communications, video systems, and process control environments.

A tight buffer design is used along with individual strength members for each fiber. This permits direct termination to the cable without using breakout kits or splice panels. Due to the increased strength of Kevlar members, cables are usually heavier and physically larger than the telecom types with equal fiber counts.

The term breakout defines the key purpose of the cable. That is, one can “break out” several fibers at any location, routing other fibers elsewhere. For this reason breakout cables are, or should be, coded for ease of identification.

Because this type of cable is found in many building environments where codes may require plenum cables, most breakout cables meet the NEC’s requirements. The cable is available in a variety of designs that will accommodate the topology requirements found in rugged environments. Fiber counts from simplex to 256 are available.

Optical fiber consists of a thin strand (or core) of optically pure glass surrounded by another layer of less pure glass (the cladding). The inner core is the light-carrying part. The surrounding cladding provides the difference in refractive index that allows total internal reflection of light through the core. The index of the cladding is less than 1 percent lower than that of the core.

Most fibers have an additional coating around the cladding. This coating, usually one or more layers of polymer, protects the core and cladding from shocks that might affect their optical or physical properties. The coating has no optical properties affecting the propagation of light within the fiber. Thus the buffer coating serves as a shock absorber.

Figure 2-1 shows the idea of light traveling through a fiber. Light injected into the fiber and striking the core-to-cladding interface at greater than the critical angle reflects back into the core. Since the angles of incidence and reflection are equal, the reflected light will again be reflected. The light will continue zig zagging down the length of the fiber.

Light, however, striking the interface at less than the critical angle passes into the cladding where it is lost over distance. The cladding is usually inefficientas a light carrier, and light in the cladding becomes attenuated fairly rapidly.

Notice also in Figure 2-1 that the light is refracted as it passes from air into the fiber. Thereafter, its  propagation is governed by the indices of the core and cladding (and by Snell’s law.) Refer to section 3, Glossary of Terms, for a definition of Snell’s Law.

The specific characteristics of light propagation through a fiber depends on many factors including: The size of the fiber; the composition of the fiber; and the light injected into this fiber. An understanding of the interplay between these properties will clarify many aspects of fiber optics.

Fiber is basically classified into three groups:

• Glass (silica) which includes single-mode step index fibers, multimode graded index, and multimode step index.
• Plastic clad silica (PCS).
• Plastic.

Most optical fibers for telecommunications are made 99 percent of silica glass, the material from which quartz and sand are formed. Figure 2-1 on the previous page shows a fiber, which consists of an inner core (about 8 to 100 micrometers, or 0.0003 to 0.004 inches, in diameter), a cladding (125 to 140 micrometers outer diameter) and a buffer jacket for protection.

The clad is made of glass of a slightly different formula. This causes light entering the core at one end of the fiber to be trapped inside, a phenomenon called internal reflection. The light hits the boundary between the core and the cladding bouncing off the cladding much like a billiard ball and at the same angle as it travels down the fiber.

Plastic fibers are much larger in diameter and can only be used for slow-speed, short-distance transmission. Plastic-clad silica (PCS) fibers, featuring a glass core with a plastic cladding, come between glass and plastic fibers in size and performance. Plastic and PCS fibers cost less than silica glass fibers, but they are also less efficient at transmitting  light. For this reason, they are being used in cars, sensors, and short-distance data-communications applications.

There are other types of fiber emerging on the marketplace, particularly suited for specialized uses. An example would be fluoride fibers which are being developed for medical and long-haul telecommunications. Medical applications for fiber include transmitting power from a laser to destroy arterial blockages or cancer masses. Since fibers are extremely narrow and flexible, they can be threaded through human arteries to locate precise trouble areas, and in some cases may eliminate the need for surgery.

MODE

James Clerk Maxwell, a Scottish physicist in the last century, first gave mathematical expression to the relationship between electric and magnetic energy. Mode is a mathematical and physical concept describing the propagation of electromagnetic waves through media. In its mathematical form, mode theory derives from Maxwell’s equations. He showed that they were both a single form of electromagnetic energy, not two different forms as was then believed. His equations also showed that the propagation of this energy followed strict rules.

A mode is simply a path that a light ray can follow in traveling down a fiber. The number of modes supported by a fiber ranges from one to over 100,000. Thus a fiber provides a path of travels for one or thousands of light rays, depending on its size and properties.

REFRACTIVE INDEX PROFILE

This term describes the relationship between the indices of the core and the cladding. Two main relationships exist: Step index and graded index. The step-index fiber has a core with a uniform index throughout. The profile shows a sharp step at the junction of the core and cladding. In contrast, the graded index has a nonuniform core. The index is highest at the center and gradually decreases until it matches that of the cladding. There is no sharp break between the core and the cladding.

Step Index

The multimode step-index fiber is the simplest type. It has a core diameter from 100 to 970 μm, and it includes glass, PCS, and plastic constructions. As such, the step-index fiber is the most wide ranging, although not the most efficient in having high bandwidth and low losses.

Graded Index

A graded-index fiber is one where the refractive index of the fiber decreases radically towards the outside of the core. During the manufacturing process, multiple layers of glass are deposited on the preform in a method where the optical index change occurs. (Refer Figure 2-3 next page.)

As the light ray travels through the core, the fastest index is the higher or outer area in a graded-index core. (Refer Figure 2-4 next page.)

The center, or axial mode would be the slowest mode in the graded-index fiber Figure 2-5). In this circumstance, a mode would slow down when passing through the center of the fiber and accelerate when passing through the outer areas of the core. This is designed to allow the higher order modes to arrive at approximately the same time as an axial or lower order mode. This allows the multimode gradedindex fibers to transmit as far as 15-20 kilometers without great pulse spreading. Within these classifications there are three types of fiber:

• Multimode step-index.
• Multimode graded-index.
• Single-mode step-index.

STEP INDEX

Multimode Step-Index Fiber
• Bandwidth of 10 MHz/km
• Loss of 5-20 dB/km.
• Large cores of 200 to 1000 microns.
• Cladding OD up to 1035 microns.
• Is effective with low-cost LEDs
• Limited transmission distances.
• Transmits at 660-1060 wavelengths.

Single-Mode Step Index Fiber
• High bandwidth applications (4 GHz).
• Low losses, typically .3 dB to .5 dB/km.
• Core area of 8 to 10 microns.
• Cladding OD of 125 microns.
• Transmits at 1300 nm and 1550 nm wavelengths.
• Higher costs for connectors, splices, and test equipment, and transmitters/receivers.

Plastic Step-Index Fiber
• Lower bandwidth 5 MHz over distances of 200 feet.
• Losses of 150-250 dB/km.
• Core area from 1000-3000 microns.
• Cladding up to 3000 microns.
• Uses LEDs to transmit data very well.
• Very easy to connectorize.
• Inexpensive.
• Operates best at 660 nm red wavelength.

Plastic-Clad Silica Step-Index Fiber
• Bandwidth up to 25 MHz/km
• Losses of 6-10 dB/km.
• Glass core from 200-600 microns.
• Plastic cladding OD to 1000 microns.
• LEDs used to transmit data. Difficult to connectorize and unstable.
• Very resistant to radiation.
• Operates at 660-1060 wavelengths.

GRADED INDEX

Multimode Graded-Index Fiber
• Bandwidths up to 600 MHz/km.
• Losses of 2 to 10 dB/km.
• Cores of 50/62.5/85/100 microns.
• Cladding OD of 125 and 140 microns.
• Is effective with laser or LED sources.
• Medium- to low-cost for components, test equipment, and transmitters and receivers.

• Has distance limitations due to higher loss and pulse spreading.
• Transmits at 820-850 nm, 1300 nm, and 1550 nm wavelengths.
• Easy to splice and connectorize.

MULTIMODE AND SINGLE-MODE FIBER

Two general types of fiber have emerged to meet user requirements: multimode and single mode.In optical terminology, “mode” can be thought of as a ray of light.

In multimode fiber many modes, or rays, are transmitted, whereas in single-mode fiber only one mode of light can travel in the core. Refer to Figure 2-6 where the core diameters of these two types of fiber have been compared to the diameter of a single human hair.

Multimode

Multimode fiber’s larger core (diameter in the 50-μm to more than 1000-μm range) captures hundreds of rays from the light source, entering the core at many different angles. Some of these rays exceed the critical angle of incidence and are lost without penetrating the fiber.

Of the rays that are captured by the core, some travel a direct path parallel to the length of the fiber. Modes that enter at a steeper angle travel a longer, circuitous route, crisscrossing the core’s diameter as they travel down the fiber. Because of these different routes, some parts of the light pulse reach the far end sooner than other parts of the same light pulse.

These differences result in pulse broadening (or spreading) which requires more space between pulses, thereby limiting the speed at which pulses can be introduced into the fiber, and limiting the bandwidth or information-carrying capacity of multimode fiber.

Multimode fibers were developed first, and they have been installed in many long-distance telecommunications systems. In the past few years, however, single-mode technology has improved to the point where these smaller fibers are made as easily and as cheaply as multimode fibers.

Multimode fiber’s significantly larger core (more than five times the diameter of a single-mode core) has certain advantages. It is easier to align core regions for splicing and for attaching connectors, and it captures more light from lower cost sources, such as from LEDs rather than lasers. Thus multimode is usually preferred for systems that have many connectors or joints, and where distance or capacity is not a factor.

Further, methods can be devised for increasing multimode fiber’s information-carrying capacity, such as transmitting on multiple wavelengths of light. This technique is known as wavelength division multiplexing or WDM.

Single-Mode

Single-mode fiber overcomes the bandwidth shortcomings of multimode. Single-mode fiber has a much smaller core diameter (typically 8 μm to 10 μm) allowing a very narrow beam from a single source to pass through it with a minimum of pulse dispersion. The cladding diameter, however, remains at the industry standard of 125 microns for purposes of connecting and splicing.

With only one mode it is easier to maintain the integrity of each light pulse. The pulses can be packed much more closely together in time, giving single-mode fiber much larger channel capacity.

Refer to section 3, References, Tables A and B for charts offering fiber comparisons.

DISPERSION

Dispersion is the spreading of a light pulse as it travels down the length of an optical fiber. Dispersion limits the bandwidth (or information-carrying capacity) of a fiber. There are three main types of dispersion: Modal, material, and waveguide.

Modal Dispersion

Modal dispersion occurs only in multimode fiber. Multimode fiber has a core diameter in the 50-μm to more than 1000-μm range. The large core allows many modes of light propagation. Since light reflects differently for different modes,  somerays follow longer paths than others. (Refer to page 2-3, Figures 2-3, 2-4 and 2-5.)

The lowest order mode, the axial ray traveling down the center of the fiber without reflecting, arrives at the end of the fiber before the higher order modes that strike the core-to-cladding interface at close to the critical angle and, therefore, follow longer paths.

Thus, a narrow pulse of light spreads out as it travels through the fiber. This spreading of a light pulse is called modal dispersion. There are three ways to limit modal dispersion:

• Use single-mode fiber since its core diameter is small enough that the fiber propagates only one mode efficiently.
• Use a graded-index fiber so that the light rays that follow longer paths also travel at a faster average velocity and thereby arrive at the other end of the fiber at nearly the same time as rays that follow shorter paths.
• Use a smaller core diameter, which allows fewer modes.

Material Dispersion

Different wavelengths (colors) also travel at different velocities through a fiber, even in the same mode (refer to earlier discussions on Index of Refraction). Each wavelength, however, travels at a different speed through a material, so the index of refraction changes according to wavelength. This phenomenon is called material dispersion since it arises from the material properties of the fiber.

Material dispersion is of greater concern in singlemode systems. In multimode systems, modal dispersion is usually significant enough that material dispersion is not a problem.

Waveguide Dispersion

Waveguide dispersion, most significant in a singlemode fiber, occurs because optical energy travels at slightly different speeds in the core and cladding. This is because of the slightly different refractive indices of the materials.

Altering the internal structure of the fiber allows waveguide dispersion to be substantially changed, thus changing the specified overall dispersion of the fiber.

BANDWIDTH VS. DISPERSION

Manufacturers of multimode offerings frequently do not specify dispersion, rather they specify a measurement called bandwidth (which is given in megahertz/kilometers).

For example, a bandwidth of 400 MHz/km means that a 400-MHz signal can be transmitted for 1 km. It also means that the product of the frequency and the length must be 400 or less (BW x L =400). In other words, you can send a lower frequency a longer distance: 200 MHz for 2 km; 100 MHz for 4 km; or 50 MHz for 8 km.

Conversely, a higher frequency can be sent a shorter distance: 600 MHz for 0.66 km; 800 MHz for 0.50 km; or 1000 MHz for 0.25 km.

Single-mode fibers, on the other hand, are specified by dispersion. This measurement is expressed in picoseconds per kilometer per nanometer of source spectral width (ps/km/nm).

In other words, for single-mode fiber dispersion is  most affected by the source’s spectral width; the wider the source width (the more wavelengths injected into the fiber), the greater the dispersion.

ATTENUATION

Attenuation is the loss of optical power as light travels through fiber. Measured in decibels per kilometer, it ranges from over 300 dB/km for plastic fibers to around 0.21 dB/km for singlemode fiber.

Attenuation varies with the wavelength of light. In fiber there are two main causes: Scattering and Absorption.

Scattering

Scattering (Figure 2-7), the more common source of attenuation in optical fibers, is the loss of optical energy due to molecular imperfections or lack of optical purity in the fiber and from the basic structure of the fiber.

Scattering, does just what its name implies. It scatters the light in all directions including back to the optical source. This light reflected back is what allows optical time domain reflectometers (OTDRs) to measure attenuation levels and optical breaks.

Absorption

Absorption (Figure 2-8) is the process by which impurities in the fiber absorb optical energy and dissipate it as a small amount of heat, causing the light to become “dimmer.” The amount converted to heat, however, is very minor.

Microbend Loss

Microbend loss (Figure 2-9) results from small variations or “bumps” in the core-to-cladding interface. Transmission losses increase due to the fiber radius decreasing to the point where light rays begin to pass through the cladding boundary. This causes the fiber rays to reflect at a different angle, therefore creating a circumstance where higher order modes are refracted into the cladding to escape. As the radius decreases, the attenuation increases.

Fibers with a graded index profile are less sensitive to microbending than step-index types. Fibers with larger cores and different wavelengths can exhibit different attenuation values.

Macrobend Loss

Macrobend losses (Figure 2-10) are caused by deviations of the core as measured from the axis of the fiber. These irregularities are caused during the manufacturing procedures and should not be confused with microbends.

NUMERICAL APERTURE

The numerical aperture (NA), or light-gathering ability of a fiber, is the description of the maximum angle in which light will be accepted and propagated within the core of the fiber. This angle of acceptance can vary depending upon the optical characteristics of the indices of refraction of the core and the cladding.

If a light ray enters the fiber at an angle which is greater than the NA or critical angle, the ray will not be reflected back into the core. The ray will then pass into the cladding becoming a cladding mode, eventually to exit through the fiber surface. The NA of a fiber is important because it gives an indication of how the fiber accepts and propagates light. A fiber with a large NA accepts light well; a fiber with a low NA requires highly directional light.

Fibers with a large NA allow rays to propagate at higher or greater angles. These rays are called higher order modes. Because these modes take longer to reach the receiver, they decrease the bandwidth capability of the fiber and will have higher attenuation.

Fibers with a lower NA, therefore, transmit lower order modes with greater bandwidth rates and lower attenuation.

Manufacturers do not normally specify NA for singlemode fibers because NA is not a critical parameter for the system designer or user. Light in a singlemode fiber is not reflected or refracted, so it does not exit the fiber at angles. Similarly, the fiber does not accept light rays at angles within the NA and propagate them by total internal reflection. Thus NA, although it can be defined for a single-mode, is essentially meaningless as a practical characteristic.

FIBER STRENGTH

One expects glass to be brittle. Yet, a fiber can be looped into tight circles without breaking. It can also be tied into loose knots (pulling the knot tight will break the fiber). Tensile strength is the ability of a fiber to be stretched or pulled without breaking.

The tensile strength of a fiber exceeds that of a steel filament of the same size. Further, a copper wire must have twice the diameter to have the same tensile strength as fiber.

As discussed under “Microbend Loss,” the main cause of weakness in a fiber is microscopic cracks on the surface, or flaws within the fiber. Defects can grow, eventually causing the fiber to break.

BEND RADIUS

Even though fibers can be wrapped in circles, they have a minimum bend radius. A sharp bend will snap the glass. Bends have two other effects:

• They increase attenuation slightly. This effect should be intuitively clear. Bends change the angles of incidence and reflection enough that some high-order modes are lost (similarly to microbends).

• Bends decrease the tensile strength of the fiber. If pull is exerted across a bend, the fiber will fail at a lower tensile strength than if no bend were present.

The use of light for the transmission of information is far from a new idea. Paul Revere’s lanterns were used to signal the approach of the British. And it was Alexander Graham Bell’s experiments over a century ago that led to his development of the photophone, a device that carried speech from one point to another by means of vibrating mirrors and a beam of sunlight.

Although never a commercial success, it nevertheless demonstrated the feasibility of lightwave communications. However the technique was shunted aside and virtually forgotten for almost another hundred years.

It probably would have remained in limbo had it not been for the appearance of a device called the laser.

Laser is an acronym for Light Amplification by Stimulated Emission of Radiation.

Simply described, the laser is a device that produces a unique kind of radiation — an intensely bright light which can be focused into a narrow beam of precise wavelength. The tremendous energies of the laser stem from the fact that it produces what is called coherent light .

The light that comes from a candle or an incandescent bulb is called incoherent light. It’s made up of many different, relatively short wavelengths (colors) which together appear white. They are sent out in brief bursts of energy at different times and in different directions. These incoherent light waves interfere with each other, thus their energy is weakened, distorted, and diffused.

The laser, on the other hand, emits light waves that all have the same wavelength, are in phase, and can be sharply focused to travel in the same direction over long distances with almost no dispersion or loss of power.

Lasers provide radiation at optical and infrared frequencies. With lasers (and associated electronics) it became possible to perform at optical frequencies the electronics functions that engineers were accustomed to performing at conventional radio and microwave frequencies. Thus lasers promised the ability to channel signals with very high information rates along an extremely narrow path.

INFORMATION TRANSMISSION

Fiber optics is a relatively new technology that uses rays of light to send information over hair-thin fibers at blinding speeds. These fibers are used as an alternative to conventional copper wire in a variety of applications such as those associated with security, telecommunications, instrumentation and control, broadcast or audio/visual systems.

Today the ability to transmit huge amounts of information along slender strands of high-purity glass optical fiber with the speed of light has revolutionized communications.

The large signal-carrying capacity of optical fibers makes it possible to provide not only many more, but much more sophisticated signals than could ever be handled by a comparable amount of copper wire.

ADVANTAGES/DISADVANTAGES

The advantages of fiber over copper include:

• Small Size: A 3/8-inch (12 pair) fiber/cable operating at 140 mb/s can handle as many voice channels as a 3-inch diameter copper (900) twisted-pair cable.

• Light Weight: The same fiber-optic cable weighs approximately 132 lbs per kilometer.The twisted pair cable weighs approximately 16,000 lbs.

• High Bandwidth: Fiber optics has been bandwidth tested at over 4-billion bits per second over a 100 km (60 miles) distance. Theoretical rates of 50-billion bits are obtainable.

• Low Loss: Current single-mode fibers have losses as low as .2 dB per km. Multimode losses are down to 1 dB (at 850 or 1300 nm). This creates opportunities for longer distances without costly repeaters.

• Noise Immunity: Unlike wire systems, which require shielding to prevent electromagnetic radiation or pick-up, fiber-optic cable is a dielectric and is not affected by electromagnetic or radio frequency interference. The potential for lower bit error rates can increase circuit efficiency.

• Transmission Security: Because the fiber is a dielectric the fiber does not radiate electromagnetic pulses, radiation, or other energy that can be detected. This makes the fiber/cable difficult to find and methods to tap into fiber create a substantial system signal loss.

• No Short Circuits: Since the fiber is glass and does not carry electrical current, radiate energy, or produce heat or sparks, the data is kept within the fiber medium.

• Wide Temperature Range: Fibers and cables can be manufactured to meet temperatures from -40°F to +200°F. Resistance to temperatures of 1,000°F have been recorded.

• No Spark or Fire Hazard: Fiber optics provides a path for data without transmitting electrical current. For applications in dangerous or explosive environments, fiber provides a safe transmission medium.

• Fewer Repeaters: Few repeaters, if any, are required because of increased performance of light sources and continuing increases in fiber performance.

• Stable Performance: Fiber optics is affected less by moisture which means less corrosion and degradation. Therefore, no scheduled maintenance is required. Fiber also has greater temperature stability than copper systems.

• Topology Compatibility: Fiber is suitable to meet the changing topologies and configurations necessary to meet operation growth and expansions. Technologies such as wavelength division multiplexing (WDM), optical multiplexing, and drop and insert technologies are available to upgrade and reconfigure system designs.

• Decreasing Costs: Costs are decreasing, larger manufacturing volumes, standardization of common products, greater repeater spacing, and proven effectiveness of older “paid for” technologies such as multimode.

• Nonobsolescence: Expansion capabilities beyond current technologies using common fibers and transmission techniques.

• Material Availability: Material (silica glass) required for the production of fiber is readily available in a virtually unending supply.

The few disadvantages of fiber include:

• Cost: Individual components, such as connectors, light sources, detectors, cable and test equipment, may be relatively expensive when compared directly to equivalent items in a copper system.

• Taps: Drop points must be planned because optical splitters or couplers are much more difficult to install after the system is in.

• Fear of New Technologies: Because the technology is considered to be new, people are reluctant to change and use these methods. The use of metric and physics is still an unfamiliar area to may established users.

LIGHT

Light is electromagnetic energy, as are radio waves, radar, television and radio signals, x-rays, and electronic digital pulses. Electromagnetic energy is radiant energy that travels through free space at about 300,000/km/s or 186,000 miles/s.

An electromagnetic wave consists of oscillating electric and magnetic fields at right angles to each other and to the direction of propagation. Thus, an electromagnetic wave is usually depicted as a sine wave. The main distinction then between different waves lies in their frequency or wavelength. In electronics we customarily talk in terms of frequency.
In fiber optics, however, light is described by wavelength. Frequency and wavelength are inversely related.

Electromagnetic energy exists in a continuous range from subsonic energy through radio waves, microwaves, gamma rays, and beyond. This range is known as the electromagnetic spectrum. It seems to be well understood that glass optical fiber does not conduct electrons as wire does, or channel radio-frequency signals as coaxial cable does. However, many are unclear about how the light signals are transmitted and how light acts as a messenger for video, audio, and data over fiber.

REFLECTION AND REFRACTION

Optical fiber transmits light by a law of physics known as the principle of total internal reflection. This principle was discovered by a British scientist named John Tyndall in the mid-1800s. He used it to demonstrate a way to confine light and actually bend it around corners. His experiments directed a beam of light out through a hole in the side of a bucket of water. He was able to demonstrate how the light was confined to the curved stream of water, and how the water’s changing path redirected the path of light.

Total internal reflection is even more efficient than mirrored reflection; it reflects more than 99.9 percent of the light.

The quantifiable physical property of a transparent material that relates to total internal reflection is its refractive index. Refractive index is defined as the ratio of the speed of light in a vacuum to the speed of light in a specific material.

Light travels fastest through a vacuum. As it starts to travel through denser material, it slows down a little. What is commonly called the speed of light is actually the velocity of electromagnetic energy in a vacuum such as space. Light travels at slower velocities in other materials such as glass.

Light traveling from one material to another changes speed, which results in light changing its direction of travel. This deflection of light is called refraction. In addition, different wavelengths of light travel at different speeds in the same material. The variation of velocity with wavelength plays an important role in fiber optics.

White light entering a prism contains all colors. The prism refracts the light and it changes speed as it enters. Because each wave changes speed differently, each is refracted differently. Red light deviates the least and travels the fastest. Violet light deviates the most and travels the slowest.

The light emerges from the prism divided into the colors of the rainbow. As can be seen in Figure 1-1 refraction occurs at the entrance and at the exit of the prism. The amount that a ray of light is refracted depends on the refractive indices of the two materials. Figure 1-2 illustrates several important terms required to understand light and its refraction.

• The normal is an imaginary line perpendicular to the interface of the two materials.
• The angle of incidence is the angle between the incident ray and the normal.
• The angle of refraction is the angle between the refracted ray and the normal.

Light passing from a lower refractive index to a higher one is bent toward the normal. But light going from a higher index to a lower one refracts away from the normal, as shown in Figure 1-3.

As the angle of incidence increases, the angle of refraction of 90° is the critical angle. If the angle of incidence increases past the critical angle, the light is totally reflected back into the first material  so that it doesn’t enter the second material. The angles of incidence and reflection are equal.

Thus:
• Light is electromagnetic energy with a higher frequency and shorter wavelength than radio waves.

• Light has both wave-like and particle-like characteristics.

• When light meets a boundary separating materials of different refractive indices, it is either refracted or reflected.