Micro-Optics: Fibres, Spheres, Disks and Rings

Really Definitely Finished Now

2008-03-28T23:04:00.003+00:00

46 pages containing 10,044 words and 44 images. 5 days of 24/7 work (yes, even while I was asleep). I could have written double the amount!

And Now For The Report

2008-03-23T00:29:00.002+00:00

I now have to write a report on this project. It's got to be ~40 pages long, so it resembles an uber-lab report. I have until next Friday to get it done... Which is why I'm still awake, typing it...

Finished!

2008-03-20T18:36:00.004+00:00

I did my final presentation today - I'm finished my project! I still have to write my report for next Friday, but I have no more classes this term. I think my talk went pretty well overall. I wish I had been asked more questions - I had so much to say!

I did run the 980nm laser through the taper yesterday. I observed the exact same effects as with the 1460nm laser, but the fibre was not jumping (which is good). An example video is shown below with a modulated field and oscillating clump of beads. The camera can sense the 980nm light so we can see visually when the light field switches on and off.

How Very Strange!

2008-03-18T19:43:00.007+00:00

I went in to Tyndall today with full expectations of another failure at this optical tweezers malarky. I commandeered Jonathan's lab bench, pulled a micron taper and started again. This time, I used a different glass slide stage, glycerol in my water-bead suspension and a 20X objective. And, I saw some very strange effects. I mean, REALLY strange. And quite funny.

So I skipped lunch and at about 1pm, I switched on the laser, increased the pump current to well above threshold. I watched the beads scatter diagonally from the taper as I increased the current! Deacreasing the current brought them back towards the taper, and with no power they just drifted very slowly, or stayed put and resumed Brownian motion. Jonathan came back and he attached the CCD camera to the microscope so I could take some videos. The video below is of us manually adjusting the pump current and thereby increasing and decreasing the intensity.

Vibrations and movement of the fibre is because I kept knocking into the lab bench while I was dialling up and down the pump current. What is weird is that the evanescent field does not range that far into the liquid. Some other forces could be at work here - some sort of effect from the static charging of the beads or water-glycerol absorption? I'm not really sure.

Jonathan got an intensity modulator to plug into the current driver so that we could oscillate the field strength in a systematic way. We found some even stranger stuff this way. By now, many of the beads were clumping, so we ended up with fantasic looking bead formations... See the video below! (Click here to see it on youtube.com.) The input signal was a square wave and we were in the kilohertz range for the fastest oscillations.

Possible explanations are coherence between the fibre and beads in terms of fluid motion and the static charge effects as mentioned already. We also have the absorbtion of water near my laser wavelength of 1460nm. I'm going to try it once more tomorrow morning using no glycerol and with a 980nm laser.

There is also the big question of why the fibre is vibrating in a uniform way when using the modulator. If it is generating fluid motion to push and pull the beads, then why can I still control them without the modulator where the fibre is not vibrating in accordance with the beads.

I have GOT To Stop Dreaming About This Project

2008-03-16T10:55:00.003+00:00

I've dreamt of this project solidly for a week. It's a bit mad. Or, well, I'm a bit mad. I have an idea though - two 'modifications' to the experiment which I think will work. I dreamt about them last night and it worked beautifully in dreamland (a simple translation to reality isn't too much to ask, right?). I saw all the beads being propelled towards and along the fibre. I'm so optimistic about this that I have included the following slide in my presentation:

There will be a second movie of trapped beads in there by Wednesday!

Second Last Chance

2008-03-14T21:33:00.003+00:00

Today was my second last chance to get this experiment working - I am going to try once again next Tuesday using a few ideas I've been thinking about as well as Sile's idea of enhancing the electric field by applying a voltage along the fibre.
Here's a video of the spheres next to the fibre not being trapped but undergoing Brownian motion. Damn beads. Even at ~90% power they wouldn't budge!

Argh

2008-03-13T19:42:00.002+00:00

I spent the day using my final set-up to try and trap the polystyrene beads. No such luck. I don't think my laser is powerful enough. And of course, there are all those problems I explained in the post below. I did come across one very strange thing during my first attempt at trapping. I was scrolling along the taper looking for beads and I came across two very large spheres stuck onto the side of the taper. I thought they were bubbles at first so I lowered to objective onto them to burst or move them, but they stayed put. I tried moving them with a micropipette and they didn't move. Johnathan thought they were two microspheres but we couldn't see why they would end up in my water.
While I was making a new sample and fibred slide to see if these spheres were an optical effect, I broke the taper in half - and I never got a photo of them! So then I tried trapping with half tapers. I also had no luck with this. I tried to trap the beads with a looped section of the half taper (inspired by Yuqiang 'lassooing' a microsphere with his taper) but I couldn't focus on the loop as the objective kept pushing it around in the water.

Explanation Of The Balls Post

2008-03-12T20:24:00.003+00:00

I spent most of the day fiddling with the imaging set-up and water-bead suspension. I came across a few problems which will probably prevent me from getting any real results from this experiment.

The working distance of the objective is 0.17mm therefore it can only focus on the upper surface of the glass slide and I cannot resolve the water, beads or fibre below. I removed the upper glass slide acting as a cover slip, and...

I used water as the "index-matching" substance rather than the low-viscosity oil. I was able to obtain very clear images of the fibre in water.

The beads are therefore going directly into the same water which the objective is moving around in. The beads do not sink (or maybe they do, but my water sample evaporates before I can see this), they just rest on top of the water blob. So, when I put the objective lens into the water for imaging, the beads are pushed away. Only the ones 'stuck' to the fibre stay put. This explains why I could not focus on the beads perfectly - they were pushed away. The first image below shows an unfocused bead in the bottom right corner - I have a video of this undergoing Brownian motion. When I tried to focus on this bead, I just pushed it away with the microscope.

The lifetime of my sample is not very long because the halogen lamp strongly encourages evaporation!

A simple solution to the bead problem is maybe placing them on the glass slide first and then dropping the water sample over them. I will have to minimise objective movement tomorrow by finding a nice area of the taper with a good bead selection and just staying at that point.
Another option to try and stabilise the beads is to use a water-glycerol solution (glycerol is soluble in water). I will ask around tomorrow to see if I can get some.

Balls

2008-03-12T19:37:00.004+00:00

Photos of fibre and polystyrene balls!

Needle In A Haystack

2008-03-11T22:36:00.002+00:00

Today was stinky. My lab journal reads 'argh!'. I came in at about 11:15 after my two classes and began trying to obtain an image of the fibre and beads with the camera.

It was not happening.

I started with the camera lens fitted thinking that I could catch the microscope image and focus it onto the CCD chip. I just got a lovely top-down view of my microscope set-up. Michael told me to take the lens off and work with the chip only. This proved slightly better because I could pick out the circular image from the objective only. But, I spent the next 4 hours trying to focus the image. That was also just not happening. Jonathan gave me another translation stage so I could move the glass slides precisely rather than coarsely move the objective. I won't go in to detail about how many configurations of camera, objective, slide and sample I used.

I got fed up at about 5pm and went to the shop to get chocolate and apple juice. By chance, I saw Sile and ran after her to ask her for some help. She went through in 20 minutes what I had spent the day doing and managed to focus an image of the many random blobs of water and god knows what on the slide. Extreme relief! I then finished my apple juice and chocolate and came home. I was seriously worried that my project had come to a dead end today. Thank god for PI's.

Imaging Again

2008-03-10T20:11:00.001+00:00

On Saturday night I had a dream about the experiment set-up and as soon as I woke, I scribbled it down. I built it this morning and seems pretty good.
I also finished soldering the BNC connector to the CCD camera. I got Michael to check it, then I hooked it up to the power supply and it worked! I have to shrink wrap the soldered joints tomorrow because they're quite fragile. I'm afraid I'll break them. Tomorrow, I have to image the spheres and fibre.

A Busy Few Days

2008-03-07T19:39:00.005+00:00

I had a busy few days and didn't get a chance to update this blog until now. I have everything written down in my lab journal thankfully.

Wednesday
Sile ordered my CCD camera in the morning so during the afternoon, Yuqiang let me use his stage for fibre-pulling. The fibre I pulled was okay except for a slight bend in the taper (sorted out with slow pulling). It had a diameter of about 1 micrometre. I moved it over to my bench so I could splice it to my laser fibre and photodetector. I removed the U-bench and filter I was using to reduce the power from the laser before it hit the detector. I connected the other end of the taper up to the DSO and increased the pump current above the lasing threshold... And I got no signal.
Jonathan pointed out that I'd need a much bigger pump current to overcome the transmission loss across the taper. I had the current driver limited to about 35mA to save the precious photodetector - I was definitely going to get nothing through the taper at that current! Also, I didn't really need the photodetector here - I just wanted to see that there was light coming out the other end. Duh. I dunno, I feel like the lab blonde.
I disconnected the detector, delimited the current driver and set the current to 300mA. Lots of light out the other end! I then began reconnecting the photodetector to the fibre and DSO. And, what did I do? I pulled the taper to damn hard and broke it. I deservedly got an 'I told you so' from Jonathan. I called it a day and went home.

Thursday
I was wondering whether I should put the camera and microscope in the horizontal or vertical plane. The amount of water-bead suspension was a limiting factor for the horizontal configuration - the surface tension of the water is just not enough to stop it from running down away from the taper. Jonathan suggested using a plastic drawer divider for the glass slide platform. So he drilled a 15mm hole in the center of the divider and I filed it down until it was 2cm in diameter. I then drilled two holes for some M4 screws at either end - I could then screw it onto some posts and secure it to the bench. The end result is the very modest-looking platform shown below.

The halogen lamp I'm using to illuminate the slide gets really hot and the platform absorbs quite a bit of heat from it. I hope that thermal excitation doesn't perturb the beads in the water.
I had to go home relatively early because I had to finish my talk for the Journal Club next day.

Friday
I spent the morning putting the finishing touches on my talk and arrived into Tyndall at about 2pm. My CCD camera had arrived yesterday evening so I went down to Michael in the lab to see how he had connected his up. He showed me which wires I need to connect and where (another case of lab-blondeness).
I gave my talk at 3pm and it went alright I think. I have given one talk before now and that went pretty badly so I was hoping I wouldn't be nervous for this one. The atmosphere is very informal so I was grand. In hindsight, I maybe should have picked a more controversial paper.

After the talk, I went down to the lab to solder the camera wires onto the BNC connector. Like a twat, I broke the coppers while soldering and the wire ended up alot shorter on one side by time I finished fixing it. I'll finish this on Monday and make a new taper if the rig is free.

Panic Begins

2008-03-04T18:19:00.004+00:00

At my timetabled general seminar yesterday afternoon, we were told that our project presentations should be ready for the 18th of this month. So I have fallen into the trap of this unusually short term and I'm in exactly the position that I didn't want to be in. I'm running out of time - fast! I could complain about the disadvantages of having lectures during this second term and having to spend four months on first term lab reports which are ultimately worth a petty fraction of this project... But I won't.

I got back my original photodetector today so I hooked it up to the laser and DSO immediately. It worked and I hopped around with a grin and then I plotted the results and then I went back to thinking about the imaging parts of this set-up. Sile has ordered a CCD camera for me and tomorrow I must get a halogen lamp to provide a backlight for my beads and spheres. It should all look something like this:

Something that I know is going to be a nightmare is getting an image of the beads on screen. The illumination from below the spheres needs to be done with collimated light from the halogen lamp (or similar). I have to get the distances correct to ensure the CCD camera picks up the clear image from the microscope's back focus.

I need to get clamps or a stage with a cut-out from the center so that I can illuminate the sample slide from underneath. Also, there is the problem of the oil immersion microscope - I need a cover slip over the sample to put the oil drop on. That means that I have to get the water-bead suspension and fibre taper onto the bottom slide without any problems or mess.

Nail Varnish Ain't Just For Nails

2008-02-28T15:47:00.002+00:00

I tried using the extremely advanced and sophisticated technology that is clear nail varnish to fix the fibre onto the glass slide - it works quite nicely. A bit of nail varnish remover recovers the glass slide and fibre too so nothing has to go to waste.
I did some reading up about the experiment this afternoon. I wonder if I can calculate the viscous drag and 'terminal velocity' of the beads when they are trapped and propelled. I have to sort out the oil immersion objective and CCD camera first though.
I am beginning to feel like this experiment will not be done within the next few weeks...

The Black Arts

2008-02-26T18:55:00.003+00:00

So, I'm waiting for the battery cap for the detector. While it wings it's way to me, I am concentrating on fibers. Yuqiang spent this afternoon showing me how to pull fibers with the heat rig and then glue the tapers to glass slides.

Total number of fibers broken: 6

Number of fibers broken during pulling: 3

Number of successful tapers: 2

Number of fibers that caught fire: 1

I don't think these are very encouraging statistics. Jonathan described fiber-pulling as a black art. It seems very unpredictable - the same set of parameters giving you different results each time. After I had broken 3 fibers on the pulling rig, Jonathan suggested reducing the gas flow. This worked perfectly - my last taper was a beauty. Had to break it and throw it away though because tapers are basically poisonous needles.
I've noted down the frequency and time settings for pulling and reversing the rig for next time. I have to pull a shorter taper tomorrow or next day so that the cladding can be glued (or nail-varnished) onto the glass slide.

Gluing the fiber to the slide isn't as horrible as I thought it would be. After clamping the slide to a movable stage, it's purely an eyeball alignment process. After raising the slide up to a fiber mounted across two posts, you finely adjust it until the fiber and slide are within a millimetre of each other. If I wanted to be more accurate, I could use the focus dial of the microscope and see how many rotations are needed to focus the fiber and slide, in turn, at either end. Each rotation is calibrated in terms of distances. But, I'm only fixing a fiber to a slide which doesn't require extreme precision so that detail is unecessary.

Cleaving & Splicing

2008-02-25T21:03:00.002+00:00

This evening, Yuqiang showed me how to cleave and splice fibers. I wanted to add a filter before the photodetector to reduce the laser beam power. The Thorlabs tech support guy told me that, although the manual says the damage threshold is 70mW, the detector gets fried with 60mW. Michael got me a small double lens stage with fibered connections. I had to splice my laser fiber onto the stage. The first splice didn't work so well but my second go was perfect.
I received the new photodetector before I went down to Tyndall (I convinced the Thorlabs tech support that the detector was broken before I got at it!). I had a suspicion that the laser power had killed the ball lens in the detector, but it didn't really make sense because I wasn't getting the correct voltage response on the oscilloscope even below the current (lasing) threshold. I hope the new detector does its job.

Detector Woes

2008-02-21T18:03:00.000+00:00

Well, today sucked. There's no better way to put it. My detector doesn't work and the guy from Thorlabs thinks I broke it - which seems to be a very popular notion in the lab this week despite the fact that I have not actually broken anything yet. Yeah. Take note guys. I guess my best bet for now is to move onto the fibre pulling and imaging side of things. I have to sort out a proper CCD camera and set up the oil imersion microscope. Maybe tomorrow will be better.

Filling In The Blanks

2008-02-19T20:32:00.004+00:00

Right - posting here has been a bit... Irregular. Although I was under the impression that my IRCSET application had floated off nicely into cyberspace to be picked up by referees and administrators and judging commitees, it was, in fact, lost and alone in computer-land. This year, IRCSET have attempted a software upgrade but seem to have left out the part where they debug and test it, resulting in many lost and faulty application forms. I have created a second application and Sile is going to submit it if they cannot find my original submission.

I characterised the laser today using both a diode and a spectrum analyser. The diode gave me a nice neat curve while the spectrum analyser allowed me to see some 'messy' data because of mode hopping and losses due to fibre connectors. The spectral power given by the spectrum analyser is essentially what is displayed in the voltage-current curve. The lasing threshold occurs between 20-25mA. I'm going to have a look at this again using my new detector and an oscilloscope either tomorrow or Thursday. Meanwhile, I have some rotten condensed state physics homework to finish.

Happiness Is...

2008-02-15T20:05:00.002+00:00

...A working laser!

I took some brief data with a power meter just to get an idea where the lasing threshold was - see the graph below.

Hide and Seek

2008-02-14T18:02:00.004+00:00

I'm not updating because my physics project has turned into an electronics project. I haven't even got the laser turned on yet - what will it be like when I'm actually attempting to trap with the evanescent field?!

New temperature and current controllers were ordered for me last week, and eventually the current controller turned up today. After various combinations of fuses, Kieran and me finally got it working. But we need the (still missing) temperature driver to check whether it does, in fact, work with the laser.

In other, much nicer, news, my IRCSET application is 98% complete. I hope to finish the last 2% off tomorrow.

Stupid Laser

2008-02-11T19:17:00.000+00:00

Well, it is more likely "stupid me".
The laser didn't work. Which meant that I had to check each pin in the current and temperature drivers. But I still couldn't find the missing connection.

*grumble grumble*

I won't leave the lab tomorrow until I have it sorted!

15 Seconds Of Fame

2008-02-08T19:47:00.000+00:00

After a very, well... eventful lunch this afternoon (thank you An Tuath Nua, TG4!), I finally got to practise some soldering in preparation for the connecting of laser and drivers. I soldered absolutely every wire onto every breadboard within reach. I'm pretty happy to do the real job now of connecting the 15-pin jack to 9-pin jack for the driver (with Michael's verification that I've matched the correct pins together in case of laser-explody issues).

This week I have focused on my IRCSET application more than anything (and my homework due for next week has suffered - but it's all about priorities, isn't it?). The application form takes a healthy chunk of time to complete mainly due to the personal proposal and project proposal. Writing 2000 words on why I am so wonderful and perfect for an MSc is tougher than you think!

Since the beginning of this project and making the decision to apply for an MSc with the group, I have realised there are many 'hidden extras' involved in working in Tyndall. Some extras are nice, like the French summer school for newbies. Some extras are surprising - my participation in the Cork City Marathon comes to mind... But then, there's the Journal Club. Public talking is just one of those things, isn't it? I'm scheduled to do a Journal Club talk on March 14th. Originally, I choose the paper written by Brambilla, Murugan, Wilkinson, Richardson dealing with bead velocities in evanescent fields. But then Michael gave a talk today on dark matter and the accelerating universe, so I was inspired to pick a completely random, off-the-wall paper rather than a quantum optics one. And I found an absolute cracker about spiderman suits and gecko gloves! My next aim is to mention Bose-Einstein Condensation somewhere during the presentation...

Oh and, the title of the post was a reference to our lab being featured in the TG4 programme 'An Tuath Nua'. Except, it actually wasn't featured at all despite Sile, Michael and Thejesh's efforts. We still got the free lunch though.

It Was Mentioned...

2008-02-04T18:29:00.000+00:00

It was mentioned to me today that I hadn't updated in a week... But I have really been working! On Thursday, I accidentally volunteered myself for some programming courtesy of the guys in the lab. They wanted a Labview program that took pressure readings from a vacuum monitor every x seconds - the only problem being that the manual for the monitor appeared to be non-existent. I searched online but there was nothing except a horibly brief information sheet for it. No manual means no idea how to install a LabView driver. On the plus side, I got to learn LabView which I find quite delicate, and after a summer full of raw, unadulterated MS Visual C++, much easier on the brain.

I'm going to do some practise soldering before I connect up the laser drivers and laser tomorrow. Don't want it being messy!

Imaging & Soldering

2008-01-29T22:23:00.000+00:00

On my lab bench today was:

A minature CCD camera

A 100x oil imersion microscope

A tube of oil

Clear nail varnish

Nail varnish remover

The camera and microscope form the basics of the imaging side of my experiment. I had a shot at setting these up... With quite a bit of help from Michael... The main difficulty of being a "newbie" in Tyndall is just the fact that I don't know where anything is. Secondary to that is my unfamiliarity with the various wires and cables and plugs and fittings and components. So, if I need something in the lab I either wander around blindly looking for it - not knowing what it looks like or where its found - or I ask one of the guys who are only too happy to help me which leads to me feeling extra newbie-ish and slightly guilty from bothering them. It's one of those lovely learning curves we all know, love and acts logarithmically if plotted.

Yes, so, after we discovered that the connector we were using to wire the little camera wasn't working, Michael decided that soldering was the way to go. He soldered two pins for me and I did the last. I love soldering! At least, I think I do. I'm sure I did the simplest soldering exercise possible but it's so cool!

Oh, I am so sad...

We connected the finished product to the TV and the power supply and, as expected, the camera didn't work. Hmmm.

Optical Trapping Of PolyBalls

2008-01-25T21:05:00.000+00:00

Above is a very basic outline of the apparatus I will eventually be working with towards the end of term. My experiment will attempt to optically trap a suspension of polystyrene beads using a evanescent field produced by propagation of 1480nm light through a fibre. In a previous experiment credited to Brambilla, Murugan, Wilkinson, Richardson, some of the beads were trapped not only by the field, but also through resonance with the field in the form of a whispering gallery mode. The set-up is essentially entirely foreign to me which makes it doubly exciting - getting to grips with each component (mostly through extensive googling of their model numbers).

Sile ordered me a photodetector which I had a look at today. I have a diode laser and I'm waiting for Kieran and Michael to finish with the current and temperature drivers. I'm also waiting on an SMA adaptor because of the non-standard fitments on the detector... I was very boggled trying to locate this adaptor - blissfully unaware of the sheer amount of adaptors and cables and fittings!

While I wait for the temperature and current drivers to become available, I'm going to try and use the detector with another laser. Then, my first task is to characterise the laser as a function of driving current.