Research Journal

Friday, August 25, 2006

So I FINALLY got the LG modes to be perfectly colinear. What was really cool was that the interference pattern rotates around its center point! Here are some pictures taken with the computer camera.


These interferograms might look like an HG10 mode but when one pulls on the mirror via a rubber band, the mirror displacement causes the two points of light to revolve around. Each picture seen above corresponds to a different rubber band pulling position. Getting this pattern to show up in the first place required lots of fine tuning and adjusting of mirrors. Now what was weird was that after coming back from lunch, without turning off the laser, the beam being produced from the open cavity He-Ne was no longer a TEM00 but instead an HG10 mode! Dr. Noe speculated that there might have been a dust particle that just happened to land on the Brewster Window at the exact right position to create such a mode. He blew on the window and sure enough, the TEM00 mode reappeared! 'Til Monday!

Thursday, August 24, 2006

Didn't go in to lab today (school schedule changes) but yesterday was exciting. Two LG modes of opposite azimuthal indices were interfered and there was a fork visible in the fringes! I'll put up photos from the CCD camera ASAP (forgot to bring floppy home). What was really neat was how I got to see the LG l=1,-1 turn into two intensity lines in the fork which was expected (1-(-1)=2). When the two LG modes are perfectly colinear they will produce spirals, and to see this is my goal for tomorrow.

Tuesday, August 22, 2006

Spent past two days trying to interfere LG modes of opposite azimuthal indices. Inserted a dove prism in one arm of the Mach-Zender to produce a mirror beam that then interferes with the original one so that ultimately, it negates the l number of the LG mode. To create the LG mode, I want to use a stressed optical fiber. However, to test that my apparatus works, optical vortices created from a forked diffraction grating was used. Had to catch the train so couldn't align it today but that is what I plan to do for tomorrow.

Saturday, August 19, 2006

I made an interferometer yesterday. It's not quite as big as the one that is already set up in the lab, but it was cool nonetheless. There seems to be an intensity difference between the split beams, which makes sense because one beam goes straight through, while the other reflects off of two mirrors before it recombines with the other. Circular fringes were then achieved by inserting a converging lens (f = 5 cm ) in one of the beam paths. The coolest part has to be when one uses a rubber band to change the spacings between the fringes. The next step is to insert a dove prism in one of the beam paths to have light interfere with its mirror image. I think ultimately, an LG mode hopefully created by an optical fiber will be sent through everything.

Monday, August 14, 2006

That's it. Seven weeks went by just like that and Senior year has just begun. We made powerpoint presentations last week and so I'll probably be putting that up soon. Will be in lab on Wednesday. Want to start building stuff.

Wednesday, August 2, 2006

Sam Goldwasser came to our lab and will be here today and tomorrow. In the afternoon, he set up our open cavity He-Ne laser and after some time aligning the output coupler, lasing was achieved! We even got some very clean HG modes to show up too:


HG10, HG20 and HG30 modes produced by the open cavity HeNe laser.

Unfortunately, do to my idiocy, I inadvertently moved the laser which caused the lasing to stop. I continued to spend the next two hours trying to get the alignment back but to no avail. Now, I have no idea how it was done in the first place because it actually is quite difficult adjusting the mirrors so that it reflects at just the right angle.

Anyway, if I ever do get the laser operational again, there are a number of different routes I could go. We actually tried inserting an adjustable iris inbetween the Brewster window of the tube and the output coupler and at a certain diameter, a very faint, possible LG10 mode was produced. One would have to interfere it with a plane wave in order to confirm whether or not it really is an LG mode. I think I would like to perhaps setup an interferometer in the near future...

Monday, July 31, 2006

Ah I haven't updated in a while so I'll try to make this entry a bit longer to make up for it. Last week, what happened.. oh yeah. It turns out that when one measures the width of a laser beam based on the razor blade intensity measurements at points 1/e^2 and 1-1/e^2 (see Mini-project page for details), the calculatd beam width is smaller than what the theory predicts, by a factor of about 1.5 (actually radical pi works real well). This can be explained from the fact that the intensity measured from the razor blade method is the integral of the Gaussian equation, which is incomputable by hand. So in attempt to find out the relationship between identical points on the Gaussian curve and its integral, I bought a copy of Mathematica for $5 at the SAC. It's actually pretty cool program and I spent most of today messing around with it.

Dr. Noe had given me a problem to work on over the weekend which was to predict what happens when two LG modes of opposite "spin" interfere with each other. Having just learned Mathematica, I think this is a good time to show off my very basic but newly acquired Mathematica skills. Here is the command and graph for this problem (left) and for a normal Gaussian laser beam (right). The height is proportional to the intensity of the laser beam.

Ok, I think I've run out of things to extend this entry... oh yeah. Apparently on Wednesday, we are going to have a visitor who sells laser pointers that produce LG modes (I think) and so one can do an experiment like the one mentioned above to see the phases of the laser.

Tuesday, July 25, 2006

So I finished data collecting today for my mini-project. I added a 15 cm focal length convex lens and took razor blade measurements of the laser beam intensity at various 1 inch increment distances after the lens. When one puts a piece of paper to see the spot size after going through the lens, by eye, the laser beam does not appear to focus. The beam just looks the same as it was before and actually apears to only diverge, despite being a convex lens. However, when intensity measurements were made, the data showed that the intensity did change as originally expected. At around the focal point (calculated to be around 8.4 inches away from the lense using the ray optics thin lens equation), the width of the laser beam was the smallest (found to be around 6 inches). This was actually unexpected since by eye the beam seemed to not converge at all. I'll try and put up pictures along with my data.

Monday, July 24, 2006

Last Friday, I start data collection for my mini-project on measuring a Gaussian beam size. By using a razor blade mounted onto a very, very precise translator, intensity points were obtained at various positions where the razor blade blocked out the laser beam. Theoretical Gaussian and integral Gaussian equations were calculated and graphed to help compare with the collected data. The razor blade data did indeed match the integral Gaussian curve, although some of the parameters had to be altered in order to show the fit.

Today, I used a pinhole to measure the intensity distribution across the laser beam and I got a Gaussian curve, which was very nice. I didn't get the chance to play around with the parameters so that will be the first thing I do tomorrow.

Thursday, July 20, 2006

ahhhhhhh another week has come and gone. Why is it that this summer specifically feels like it's going by so fast? What is going on ... ?

Went to Brookhaven National Laboratory today. Saw RHIC (Relativistic Heavy Ion Collider). Was huge. Saw STAR (Solenoidal Tracker At RHIC). Lots of cables... and wires. Never seen anything like it. Bus was an hour late. Took naps on the bus. Went back to lab. Read some papers on astigmatic mode converters.

Wednesday, July 19, 2006

I started setting up the apparatus to my mini-project today. The initial setup consisted of a HeNe laser, two lens system, and photodetector. I first took a convex lens (I first made the mistake of using a concave lense, thinking it made light converge) of focal length 15 cm and placed it in front of the HeNe laser. Then, I took the second lens of 5 cm focal length and placed it 30 cm infront of the first lens, since it was double the focal distance of the first lens. At this point, I decided to check if the light did converge and when I held up a white paper plate in between the two lenses, I found that the laser beam did not get smaller. By moving the plate back and forth near the focal point, one would expect to see the spot size to reach a minimum and then expand again. This was not the case. After some discussion with Scott and Matt, and with the help of Dr. Noe, we figured that this was because the beam size was very small and only passed through a very small portion of the lens and so in reality, it wouln't really curve that much. To solve this, a third lens was inserted to expand the light ray so that the laser would pass through more of the lenses. Mathematically, this works out too with the thin lens equation, where the distance of the image will increase when the distance of the object is a lot closer.

Unfortunately, I found out today that the exact experiment I had in mind of doing has already been done. And it was done in 2002 too. Ahh I guess its back to the drawing board- or papers in this case.

Tuesday, July 18, 2006

I finally can get started on my mini project. Dr. Noe explained to me an experiment that will figure out the beam size of a laser beam at various points within a system of lenses by measuring the intensity of the laser at various cross-sectional points blocked out by a razor blade. I think I will learn a lot in the process since now that I think about it, it is quite an involved project. Not only is there mechanical, hands on work but also work in Mathematica and this old spreadsheet system used here in the lab. Got lots of reading to do since I want to be able to know exactly what is going on at everypoint of the way.

Monday, July 17, 2006

I think I have possible mini-project and project. I originally wanted to convert LG modes into HG modes and Dr. Noe said that I could do that as a mini-project on Mathematica to show that it is possible. For my main project, I think I'm going to do what Alex Ellis did, except use a different method of creating the HG mode to put through an astigmatic mode converter and attempt to create LG modes. I think I got very lucky because Dr. Noe had just returned from visiting Kiko Galvez, a researcher in optical vortices who introduced this new method of creating HG modes. I'm quite excited to get started now although the hard part is just beginning now.

Friday, July 14, 2006

On Wikipedia (heh), it says that optical vortices (LG modes) have applications in optical tweezers, Quantum computing, and cryptography communications. It also said that the twists of optical vortices are changed when traveling in bent or stressed optical fibers. Hmm.. interesting.

Thursday, July 13, 2006

I downloaded Alex Ellis's paper today and read through it. There are so many projects here done by previous students that are amazing and I wish I could do them but can't because it's been done already; this is one of those projects. Anyways,to create HG modes from LG modes I think is a pretty interesting topic by itself because it would be proving that the physics formulas work out. However, I don't really know how practical it would be to do so. LG modes are desirable because they are useful as optical vortices and in optical tweezers, but I don't really know how useful HG modes would be. I'm not sure how practical it would be to create the HG modes in such a roundabout way too if one were to use it in an application. At the end of the paper, there was a future studies section which said that interferometry can be used to explore the phase relationship of LG beams to a plane wave reference. Maybe I could build on that.

Wednesday, July 12, 2006

I think I have possible project topic. Dr. Noe showed me the project of Alex Ellis who created Laguerre-Gaussian (LG) modes by interfering Hermite-Gaussian (HG) modes. The opposite, however, hasn't been done before in the LTC and mathematically, interfering LG modes should also produce HG modes. So one would have to find an alternative way of producing LG, possibly by diffraction or something. I also would like to find out what are the applications of such a project, besides just proving that the mathematics is indeed true. I will do further reading on this when I enter the lab later.

Monday, July 10, 2006

I started reading more indeptly about Gaussian beams and I found a possible mini-project topic. By measuring various dimensions of a laser beam, I can figure out the beam parameter product (BPP), which is the product of the beam's divergence and waist size. What I would like to do with this mini-project is to get acquainted with the various lab equipment available and also get used to this type of research and data collection. I became interested in Gaussian beams because it involves math equations that I have basically no familiarity with. Last year at the Garcia Center, the level of actual math involved in my research was very limited and now that I am in a physics lab, I feel that doing such a mini-project would be a great chance for me to become more comfortable in interpreting and understanding physics equations in text and in experimental applications.

Thursday, July 6, 2006

Wow its been a week already since my last entry. It was a long weekend (because of July 4th) so technically it has only been two lab days. I think I have a topic at hand. I spent most of yesterday reading first about laser cooling and then on optical tweezers. In terms of a mini-project, Dr. Noe said that if I were to pursue optical tweezers, I would need to learn how to focus light using a system of lenses, so maybe I'll do something along those lines.

Got to see some total internal reflection today. Matt setup a fishtank full of water and shined a laser beam through it at various angles. At one time, I think there were four, possibly five total internal reflections. The interfaces included both glass-to-air and water-to-air. Thought it was really neat.

Thursday, June 29, 2006

My first week at the Laser Teaching Center is near its end and so far I have been learning a lot. I spend most of my days reading textbooks, trying to build up a fundamental knowledge of optics. The REUs here also give lessons on various optics related topics, such as complex numbers, and the various properties of waves including interference and diffraction. I also have picked up a lot of non-optics related knowledge and they include the basic languages of Linux and HTML.

Victor Wang
Summer 2006
Laser Teaching Center