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Check out our DIY Raman Platform and other OEM modules for your research.

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The installed slit will ultimately determine the throughput and optical resolution of a spectrometer. Light that enters the optical bench of a spectrometer through lens or a fiber is focused onto the pre-mounted and aligned slit. The angle of the light that enters the optical bench is controlled by the slit.

Slit widths are available in several different sizes from 5 µm to as big as 800 µm with a 1 mm (standard) to 2 mm height. As slits are aligned and permanently mounted into a spectrometer and should only be changed by a trained technician, it is essential to choose the appropriate slit for an application.

The most commonly used slits in spectrometers are 10, 25, 50, 100 and 200 µm. In systems that use optical fibers for input light coupling, a fiber bundle matched with the shape of the entrance slit (stacked fiber) may be used to increase the system throughput and coupling efficiency.

Technical Details

The function of the entrance slit is to define a clear-cut object for the optical bench. One of the key factors that it impacts the throughput of the spectrograph is the size (height (H_s) and width (W_s)). The image width of the entrance slit is an important factor in finding the spectral resolution of the spectrometer when it is greater than the pixel width of the detector array. An appropriate entrance slit width should be selected to balance the resolution and throughput of a system.

The image width of the entrance slit (W_i) can be estimated as:

    W_i = (M²×W_s²+W_o2) ^1/2

Where M is the magnification of the optical bench that is set by the ratio of the focal length of the focusing mirror (lens) to the collimating mirror (lens); W_o is the image broadening caused by the optical bench; and W_s is the width of the entrance slit. W_o is on the order of a few tens of µm for a CZ optical bench.

So, reducing the width of the entrance slit below this value will not significantly improve the resolution of the system. The axial transmissive optical bench offers a considerably smaller W_o and so can achieve a considerably higher spectral resolution. The pixel width (W_p ) of the array detector sets another limit on spectral resolution. Reducing W_i below W_p will not help increase the resolution of the spectrometer.

If the resolution requirement is satisfied, the slit width should be as wide as possible to enhance the throughput of the spectrograph.

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Raman spectroscopy, a molecular spectroscopy which is observed as inelastically scattered light, allows for the interrogation and identification of vibrational (phonon) states of molecules. As a result, Raman spectroscopy provides an invaluable analytical tool for molecular fingerprinting as well as monitoring changes in molecular bond structure (e.g. product formation; state changes and stresses & strains; crystalline form and crystallinity).

In comparison to other vibrational spectroscopy methods, such as FTIR and NIR, Raman has several major advantages. These advantages stem from the fact that the Raman effect manifests itself in the light scattered off of a sample as opposed to the light absorbed by a sample. As a result, Raman spectroscopy requires little to no sample preparation and is insensitive to aqueous absorption bands. This property of Raman facilitates the measurement of solids, liquids, and gases not only directly, but also through transparent containers such as glass, quartz, and plastic.

Raman spectroscopy is highly selective, as is the complementary method of FTIR , which allows it to identify and differentiate molecules and chemical species that are very similar, and measure small changes in samples. Figure R-1 shows an example of five molecules – Acetone, Ethanol, Dimethyl Sulfoxide, Ethyl Acetate, and Toluene, with peaks from specific functional groups marked. Although these organic solvents have similar molecular structure, their Raman spectra are clearly differentiable, even to the untrained eye. Using Raman spectral libraries, it is easy to see how easily Raman spectra can be used for material identification and verification.

Basic Function of Spectrometers

The basic function of any spectrometer is to take in light, break it into its spectral components, digitize the signal as a function of wavelength, and read it out and display it via a computer. In the first step of this process, light is directed through a fiber optic cable into the spectrometer through an entrance slit, which is a narrow aperture.

The slit vignettes the light as it enters the spectrometer. Then, in most spectrometers, the divergent light is collimated by a concave mirror and directed onto a grating. Following this, the grating disperses the spectral components of the light at slightly varying angles.

The light is then focused by a second concave mirror and imaged onto the detector. Alternatively, all of the three functions can be simultaneously performed using a concave holographic grating. There are various pros and cons to this alternative, which are discussed in the article titled, “An Introduction to a Spectrometer: Diffraction Grating”

Once the light is imaged onto the detector, the photons are converted into electrons. These electrons are digitized and read out through a USB (or serial port) to a computer.

Based on the number of pixels in the detector and the linear dispersion of the diffraction grating, the software interpolates the signal to generate a calibration that enables the data to be plotted as a function of wavelength over the given spectral range. This data can be subsequently used and manipulated for many spectroscopic applications.

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When considering the quantum particle interpretation, light is thought of as a photon which strikes the molecule and then inelasticaly scatters. In this interpretation the number of scattered photons is proportional to the size of the bond. For example, molecules with large Pi bonds such as benzene tend to scatter lots of photons, while water with small single bonds tends to be a very weak Raman scatterer. Figure R-2 shows a visual comparison of the two methods.

When deriving the Raman effect, it is generally easiest to start with the classical interpretation by considering a simple diatomic molecule as a mass on a spring (as shown in figure R-3) where m represents the atomic mass, x represents the displacement, and K represents the bond strength.

When using this approximation, the displacement of the molecule can be expressed by using Hooke’s law as,

By replacing the reduced mass (m1m2/[m1+m2]) with μ and the total displacement (x1+x2) with q, the equation can be simplified to,

By solving this equation for q we get,

where ν_m is the molecular vibration and is defined as,

From equations R-3 and R-4, it is apparent that the molecule vibrates in a cosine pattern with a frequency proportional to the bond strength and inversely proportional to the reduced mass. From this we can see that each molecule will have its own unique vibrational signatures which are determined not only by the atoms in the molecule, but also the characteristics of the individual bonds. Through the Raman effect, these vibrational frequencies can be measured due to the fact that the polorizability of a molecule, α, is a function of displacement, q. When incident light interacts with a molecule, it induces a dipole moment, P, equal to that of the product of the polorizability of the molecule and the electric field of the incident light source. This can be expressed as,

where E_o is the intensity and ν_o is the frequency of the electric field. Using the small amplitude approximation, the polorizability can be described as a linear function of displacement,

which when combined with equations R-3 and R-5 results in,

In Equation R-7 we see that there are two resultant effects from the interaction of the molecule and the incident light. The first effect is called Rayleigh scattering, which is the dominate effect and results in no change in the frequency of the incident light. The second effect is the Raman scattered component and when expanded to,

can be shown to shift the frequency of the incident light by plus or minus the frequency of the molecular vibration. The increase in frequency is known as an Anti-Stokes shift and the decrease in frequency is known as a Stokes shift. By measuring the change in frequency from the incident light (typically only the Stokes shift is used for this measurement) the Raman effect now gives spectroscopists a means of directly measuring the vibrational frequency of a molecular bond.

Now that we have derived the Raman effect using the classical wave interpretation, we can use the quantum particle interpretation to better visualize the process and determine additional information. As discussed earlier in the quantum interpretation, the Raman effect is described as inelastic scattering of a photon off of a molecular bond. From the Jablonski diagram shown in figure R-4, we can see that this results from the incident photon exciting the molecule into a virtual energy state.

When this occurs, there are three different potential outcomes. First, the molecule can relax back down to the ground state and emit a photon of equal energy to that of the incident photon; this is an elastic process and is again referred to as Rayleigh scattering. Second, the molecule can relax to a real phonon state and emit a photon with less energy than the incident photon; this is called Stokes shifted Raman scattering. The third potential outcome is that the molecule is already in an excited phonon state, is excited to a higher virtual state, and then relaxes back down to the ground state emitting a photon with more energy than the incident photon; this is called Anti-Stokes Raman scattering. Due to the fact that most molecules will be found in the ground state at room temperature, there is a much lower probability that a photon will be Anti-Stokes scattered. As a result, most Raman measurements are performed considering only the Stokes shifted light.

By further investigating the quantum interpretation of the Raman effect, it can be shown that the power of the scattered light, P_s, is equal to the product of the intensity of the incident photons, I_o, and a value known as the Raman cross-section, σ_R. It can be shown that,

where λ equals the wavelength of the incident photon. Therefore,

From equation R-10 it is clear that there is a linear relationship between the power of the scattered light and the intensity of the incident light as well as a relationship between the power of the scattered light and the inverse of the wavelength to the fourth power. Therefore, it would appear that it is always desirable to use a short excitation wavelength and a high power excitation source based on these relationships. However, as we will see in the next section, this is not always the case.

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Gratings will have an impact on optical resolution and the maximum efficiency for a particular wavelength range. There are two parts in the grating, the blaze angle and groove frequency, both of which are explained in detail in the following sections.

There are two types of diffraction gratings – holographic gratings and ruled gratings. Ruled gratings are developed by etching several parallel grooves onto the surface of a substrate and then coating it with a highly reflective material.

Holographic gratings are developed by interfering two UV beams to create a sinusoidal index of refraction variation in a piece of optical glass. This technique provides a more uniform spectral response, but a considerably lower overall efficiency.

Although ruled gratings are the simplest and cheapest gratings to manufacture, they exhibit much more stray light, which is caused by surface imperfections and other errors in the groove period. So, holographic gratings are usually chosen to improve the stray light performance in spectroscopic applications (such as UV spectroscopy) where the detector response is poorer and the optics is suffering severe loss.

Another benefit of holographic gratings is that they are easily formed on concave surfaces, allowing them to operate as both the focusing optic and the dispersive element simultaneously.

Groove Frequency

The amount of grooves per mm ruled into the grating determines the amount of dispersion. This is commonly known as groove frequency or groove density. The spectrometer’s wavelength coverage is determined by the groove frequency of the grating, which is also a key factor in the spectral resolution.

The wavelength coverage of a spectrometer is inversely proportional to the dispersion of the grating due to its fixed geometry. However, the higher the dispersion, the higher the resolving power of the spectrometer. On the other hand, decreasing the groove frequency decreases the dispersion and increases wavelength coverage at the cost of spectral resolution.

For instance, if an Exemplar spectrometer with a 900 g/mm was selected, it would provide a wavelength range of 370 nm, with an optical resolution as low as 0.5 nm. Similarly, if an Exemplar with a 600 g/mm grating were to be chosen, it would provide up to 700 nm of wavelength coverage with an optical resolution as low as 1.0 nm.

As can be seen from this example, the wavelength coverage can be increased at the sacrifice of optical resolution.

Optical signals in wavelengths from different diffraction orders may end up at the same spatial position on the detector plane when the required wavelength coverage is broad, i.e. λ_max > 2λ_min. This will become evident when looking at the grating equation. In this case, a linear variable filter (LVF) is needed to perform “order sorting” or eliminating any unwanted higher order contributions.

For fixed grating spectrometers, it can be shown that the angular dispersion from the grating is described by

where d is the groove period (which is equal to the inverse of the groove density), Beta is the diffraction angle, m is the diffraction order, and λ is the wavelength of light as can be seen in Figure 1.

Considering the focal length (F) of the focusing mirror and by assuming the small angle approximation, Equation 1 can be rewritten as

which provides the linear dispersion in terms of nm/mm. From the linear dispersion, the maximum spectral range (λ_max – λ_min) can be calculated based on the detector length (L_D). This detector length can be calculated by multiplying the total numbers of pixels on the detector (n) and the pixel width (W_p) resulting in the expression

From Equation 3, it is clear that the maximum spectral range of a spectrometer is determined by the groove density (1/d), the focal length (F), and the detector length (L_D). The minimum wavelength difference that can be resolved by the diffraction grating is provided by the following equation:

where N represents the total number of grooves on the diffraction grating. This is consistent with the transform limit theory which states that the smallest resolvable unit of any transform is inversely proportional to the number of samples. Typically, the resolving power of the grating is considerably higher than the overall resolving power of the spectrometer, demonstrating that dispersion is only one of the several factors that determine the overall spectral resolution.

It must be noted that the longest wavelength to be diffracted by a grating is 2d, placing an upper limit on the spectral range of the grating. This long wavelength limitation may restrict the maximum groove density allowed in a spectrometer, for near-infrared (NIR) applications.

Blaze Angle

As a grating diffracts incident polychromatic light, it doesn’t do so with uniform efficiency. The groove facet angle, also known as the blaze angle, determines the overall shape of the diffraction curve. With this property, it is possible to calculate which blaze angle will correspond to which peak efficiency; this is known as the blaze wavelength. Figure 2 illustrates this concept and compares three different 150 g/mm gratings blazed at 500 nm, 1250 nm, and 2000 nm.

Gratings can be blazed to provide high diffraction efficiency (>85%) at a specific wavelength, i.e. a blaze wavelength (λ_B). As a rule of thumb, the grating efficiency will decrease by 50% at 0.6 xλ_B and 1.8 xλ_B. This sets a limit on the spectral coverage of the spectrometer. Typically, to improve the overall signal to noise ratio (SNR) of the spectrometer, the blaze wavelength of the diffraction grating is biased toward the weak side of the spectral range.

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Raman spectroscopy is predicated on the ability to measure a shift in wavelength (or frequency) and, therefore, a monochromatic excitation source should be used. While a laser is usually the best excitation source, not all lasers can be used for Raman spectroscopy, so the laser frequency should be very stable and should not mode hop, as this will lead to errors in the Raman shift.

Also, a clean, narrow bandwidth laser should be used because the quality of the Raman peaks is directly influenced by the stability and sharpness of the excitation light source.

Wavelength is the final consideration when deciding which laser to use for a Raman spectrometer. From the previous section, it is clear that when the wavelength is shorter, the Raman signal becomes more powerful. However, as stated before, this is not the only consideration, particularly when it comes to dealing with organic molecules.

When excited by high energy (short wavelength) photons, most organic molecules will tend to fluoresce. Although fluorescence is usually considered to be a low light level process, it can still overwhelm the signal in the Raman spectrum (Figure 5). This is because the Raman effect contains a very small fraction (approximately 1 in 10⁷) of the incident photons. Therefore, only visible lasers are used for inorganic materials such as carbon nanotubes.

In the case of organic molecules, laser wavelength should be shifted into the near infrared to reduce fluorescence without exceeding the CCD spectral detection limits. 785 nm diode lasers have become the industry standard because of their availability and the fact that they enable maximum fluorescence reduction without affecting the spectral range or resolution. A 532 nm laser is the best choice for increased sensitivity with inorganic molecules, as fluorescence is no longer an issue.

As discussed before, Raman scattering is extremely weak and needs long integration times to collect enough photons to measure a discernible signal. This makes it essential to use a TE cooled spectrometer to reduce the dark noise.

For weak Raman scatters or very low concentrations, a back-thinned CCD may be required to further increase the spectrometer‘s sensitivity. By etching the detector to just a few microns thick, the probability of an electron being reabsorbed as it travels through the detector based on Beer’s law is considerably reduced. This boosts the detector’s sensitivity from a maximum quantum efficiency of 35% to more than 90%.

Due to the highly selective nature of Raman spectra, they may include closely spaced peaks that may need to be resolved depending on the application. This can be achieved using a high resolution spectrometer. Usually, standard spectrometer configurations are meant for 785 nm and 532 nm laser excitation wavelengths, but custom excitation wavelengths are also available.

These spectrometers can provide a range of configurations that are exclusively designed for high resolution and wide spectral range. Standard spectral ranges are available from as low as 65 cm^-1 (filter dependent) to as high as 4000 cm^-1, with a spectral resolution as fine as 3 cm^-1.

Whenever a sample is measured, the only effective way of directing the laser light to the sample, collecting the Raman scatter, and directing it to a spectrometer is to use a fiber optic probe. A Raman probe must be able to direct and focus the monochromatic excitation source (usually a laser) to the sample, collecting the scattered light, and then directing it to the spectrometer. A typical design for a Raman probe is shown in Figure 6.

Given that a pure signal is very important to Raman spectroscopy, a narrow band-pass filter is placed in the optical path of the excitation source before it reaches the sample. As the Raman effect is very weak, the signal should be collected at a 0° angle normal to the sample. This results in interference from Rayleigh scattering, and therefore the collected signal should be filtered through a long pass filter before it is directed to the spectrometer.

The flexibility of fiber optics allows the probe to be taken to a solid sample and also enables it to be immersed in slurries or liquids in both process and lab environments (for kinetic measurements in real time). Fiber optics can also be coupled to cuvette holders, microscopes, and a host of sampling accessories.

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Fiber optic probes are the ideal solution for monitoring real-time kinetic reactions, analyzing large or awkwardly shaped samples, sampling in vivo, and any other application where bringing the sample to the spectrometer is difficult. Due to their flexibility and user-friendliness, fiber optic probes have become one of the most widespread tools in modern spectroscopy.

This article will discuss four of the most common fiber optic probes – reflectance probes, dark-field reflection probes, transflectance dip probes, and Raman probes.

Reflectance Probes

A reflectance probe is the most basic fiber optic probe. In its simplest form, the reflectance probe has a bifurcated fiber where the bundled or distal end is placed in a metal sheath rather than in a SMA connector (Figure 1). In this setup, the bifurcated ends can be connected to a light source, such as a fiber coupled Tungsten Halogen lamp, while the other is connected to a spectrometer.

The light from the lamp travels via the first bifurcated end to the distal end of the probe and reflects off of the sample. The reflected light will then travel from the distal end to the second bifurcated end, where it will then travel into the spectrometer for analysis.

The system has to be calibrated by taking a reference scan before reflection data can be collected by the spectrometer. To take this reference scan, a white light reflectance standard, such as PTFE, is placed at the same geometry from the probe as will be used in the actual measurement.

This will enable the spectrometer to determine which wavelengths of light are reflected and which are absorbed by measuring the ratio between a “perfect” white light reflector and the sample of interest.

There are two standard geometries that are employed when measuring reflection: 0° and 45° normal to the sample. The probe will pick up the specular (mirror like) component of the reflected light and the diffuse component when measuring at 0°.

However, most of the specular light is not collected by the probe when measured at 45°. This is a significant consideration in applications such as colorimetry and NIR spectroscopy, where the specular component can distort the spectrum and skew the results.

Employing a round-to-slit fiber optic bundle is a more complex method of designing reflectance probes. This is a common method to overcome the issue of weak photon energy in the NIR. This method is applied in several reflection probes designed to work in the NIR; a 6-around-1 configuration is employed on the distal end and 6 fibers are stacked on the bifurcated end attached to the spectrometer.

As shown in Figure 2, the 6 outer fibers are going to the slit configuration on the spectrometer, while the center fiber connects to the light source in the other bifurcated end.

To increase the spectral range over which the reflection data is collected, reflectance probes can also be scaled up to trifurcated and quadfurcated designs.

Dark Field Reflectance Probes

Specular reflection does not contain any useful information for NIR spectroscopy, but it can typically be removed by measuring the sample at a 45° angle. However, dark-field illumination – a method borrowed from microscopy – can be employed if the sample cannot be measured at a 45° angle, such as when working in a production or field setting.

The diffusely reflected light is subsequently collected by a bundle of 7 fibers in the center of the probe, which directs the light to the spectrometer in a slit configuration (Figure 3). To redirect the light away from the center fiber bundle, a lens is used at the distal end of the probe, further reducing the specular components of the light.

Transflectance Dip Probes

Reflection probes can be used to measure liquids, even though they are primarily designed to measure solids. A dip probe is generally the probe of choice while measuring liquid samples, as it can be submerged into the sample, allowing for kinetic data to be collected.

A fiber dip probe is similar to a reflection probe in design, although special effort is taken to guarantee that it is inert and liquid tight. The main functional difference is the presence of a cavity, which fills with the liquid sample when immersed.

To reflect the transmitted light back through the sample and into the collection fiber, this cavity is equipped with an optically transparent window placed at the distal end of the fiber and a small mirror placed at the bottom of the cavity (Figure 4).

This setup is commonly known as a transflectance, as it combines transmission and reflection, doubling the optical path length.

It must be noted that a dark-field reflectance probe configuration can also be used to make transflectance measurements. An adaptor can be placed over the dark-field probe to allow for transflectance measurements in slurries and liquids (Figure 3).

Raman Probes

The Raman probe is used to measure the inelastic scattering of light off of a sample. Raman scattering is a nonlinear effect that results in the shift in wavelength from a known monochromatic source. This shift is equal to the vibrational frequency of the molecular bonds in the material.

Consequently, a Raman probe must have the capacity to direct and focus the monochromatic excitation source (typically a laser) to the sample, collecting the scattered light and then directing it to the spectrometer. A typical design for a Raman probe can be seen in Figure 5.

A narrow band-pass filter is placed in the optical path of the excitation source before it reaches the sample as a pure signal is highly critical to Raman spectroscopy. It must be noted that the signal must be collected at a 0° angle normal to the sample as the Raman Effect is extremely weak.

This leads to interference from specular reflections, which is referred to as Rayleigh scattering in this case. Therefore, it is important to filter the collected signal using a long pass filter before it is directed to the spectrometer.

The Raman probe is an perfect example of how fiber optics can be coupled with other optical components, enabling simple and flexible measurement of even the most difficult spectroscopy.

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Fiber Optic Bundles

Any fiber optic assembly that consists of more than one fiber optic in a single cable is defined as a fiber optic bundle. A bifurcated fiber assembly is the most common example of a fiber optic bundle. A bifurcated fiber assembly is used to either combine signals or to split a signal. An example of a typical bifurcated fiber assembly is shown in Figure 1.

In some of the most common applications for bifurcated fiber assemblies, light is directed from a sample into two different spectrometers. Typically, this is done to extend the spectral coverage of the measurement, either to cover an extended range or to maintain higher resolution.

For instance, if a broadband measurement from 350 to 1700 nm needs to be made, both a Si and an InGaAs detector array must be used. A simultaneous measurement can be made using a bifurcated fiber assembly with one NIR fiber and one UV fiber to direct light into each spectrometer. An example spectrum of this type of measurement is shown in Figure 2.

The signal from multiple samples can also be coupled into the same spectrometer using a bifurcated fiber. Only one sample can emit light at a time when a bifurcated fiber is used in this fashion, or additional care must be taken to ensure that the signals do not have spectral overlap.

The same basic applications and principal can also be scaled up to trifurcated and quadfurcated fiber assemblies. Figure 3 shows an example of a trifurcated fiber assembly.

A “round to slit” configuration is another common bundled fiber optic assembly. There are multiple small core fibers (typically 100 µm) in this configuration that are put into one fiber assembly with fibers stacked linearly on top of each other in one end and bundled tightly in a circular fashion on the other end.

As can be seen in Figure 4, the end where the fibers are stacked linearly on top of one another form a pattern to match the entrance slit of the spectrometer.

Instead of simply using a larger core fiber, this configuration enables much higher throughput into the spectrometer. As can be seen in Figure 5, when a large core fiber is placed in front of the entrance slit of a spectrometer, most of the light is vignetted and does not enter the spectrometer.

In contrast, more light enters into the spectrometer when the smaller fibers are stacked along the entrance slit. As the slit can remain relatively narrow, this enables much higher signal to noise and sensitivity, while also maintaining resolution.

It is important to remember two important details while using a fiber optic assembly with a slit configuration. First, to obtain any benefit from the fiber stacking, a cylindrical lens must be used to prevent most of the light to be imaged above and below the detector.

Second, it is essential to properly align the fiber stack to the entrance slit of the spectrometer. This can be done by shining light into the round end of the assembly and observing the signal as the fiber is rotated in the SMA905 connection port. The fiber can be screwed down to lock the position when the peak signal is achieved.

This type of fiber optic assembly is used in NIR transmission spectroscopy, where there are very few photons and photon energy is very low. Figure 6 shows an example of a transmittance setup.

Conclusion

There are countless options available to suit any application, by combining various combinations of round, single, and stacked configurations with regular, bifurcated, trifurcated, and quadfurcated fiber assemblies. ‘An Introduction to a Spectrometer: Fiber Optic Probes’ describes how fiber bundles can be combined with other various opto-mechanical components to create more specific applications.

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