Weekly Progress Reports
Week 1: Feb. 4-8 2002
I set up the HeNe laser with the mirrors, PAF Fiber Port, the single mode fiber, and the detector. The signal coming out of the single mode fiber is very weak, creating a very faint red dot on a piece of paper. I need to continue aligning the mirrors until I get a satisfactory amount of the beam transmitted through the fiber (based on previous web reports, 30-60%). With the HeNe shining directly into the detector, the signal is about 3 V. The signal emerging from the single mode fiber is only 1 or 2 mV.
Week 2: Feb. 11-15
I have successfully coupled the laser to the single mode fiber for the first time! I had to try several different strategies before getting this to work. I tried using a multi-mode fiber, which is much easier to couple. Then I made sure the laser was traveling straight through the coupler by taking out the fiber and projecting the laser beam onto a card with a cross on it to make sure the laser beam did not travel appreciably in the x and y directions. Then I put the fiber back on the coupler, but with it completely unscrewed. I then iteratively moved the 2 mirrors until I got maximum beam output. Next I pushed the fiber slightly closer and repeated the process. Eventually I moved the fiber closer and closer until it was all the way on the coupler, then progressively screwed the fiber on; each time looking for a maximum with the mirrors. Finally the cable was screwed on tightly and I had a fairly bright image out of the other end of the fiber. With the meter I measured a maximum signal of 38.2 mV coming out of the fiber, with a signal of 3.212 V of the full laser before the mirrors. Therefore I am only coupling about 1.2% of the laser light, but this is a good start (much better than I had previously gotten). Now I just need to continue iteratively tuning the mirror, until I can hopefully get up to 60% coupling.
I was not able to improve on the result from 2/13 by adjusting the mirrors. Instead, I realized that I had to move the (unscrewed) fiber in and out of the fiber port until I found a maximum in the output, and then screw the fiber all the way in and change the z-position of the fiber port's lens until I found the maximum again. This is accomplished by very slowly turning three screws on the fiber port. The net change in the z-position of the lens was a couple of millimeters. With 3.2 V input and 2.1 V output from the fiber, about 66% of the beam is being coupled to the fiber. This is right on par with other previous projects with the same setup. On Monday I will play with it some more to see if I can get even better couple. Then I will take everything apart and start over again (hopefully it will go more quickly the second time around).
Week 3: Feb. 18-22
I was able to sucesfully couple the laser to the fiber again within about an hour. This time I was able to get about 63% coupling. From comparing with other students' experiences, it seems this is about the best I can expect (both in coupling percentage, and the time it takes), so now I will move on to the next part of my project. I will measure the properties of a laser in one room with a wavemeter in another room, with an optical fiber carrying the beam between rooms. An older student, Oleg Kritsun, has worked with this set up before, so he will help me get started. I will use a web cam connected to a laptop computer and an internet connection so I have a real-time view of the wavemeter as I play with the laser in the other room. I have also found some information on the web about harmonic generation with tapered fibers. The main website is Blaze Photonics , a private company that will eventually manufacture various exotic types of fibers. Another website is here , not much content, but lots of pretty pictures.
Week 4: Feb 25 - March 1
I now have permission to use the wavemeter in Orozco's lab, but I have to wait until next monday because of room maintenance. I did some research on Web of Science (which proved to be a very user-friendly and helpful resource), and found the main paper on frequency combs generated using tapered fibers: T. A. Birks et al, "Supercontinuum generation in tapered fibers" Optics Letters 2000, vol. 25, pp 1415-1417. This reports the main result of their experiment and describes the equipment, but is short on details about how the frequency comb is actually generated in the fiber. I will have to look up some more articles to get this information.
Week 5: March 4 - 8
This week I was able to get into the lab and align Metcalf's Ti-Saph laser into a single mode fiber. There was a video camera set up at the other end of the fiber, trained on a power meter that measured the output of the fiber. There was only one mirror in this setup, so I had to use the screws on the fiber coupler to align the beam in the X-Y direction. I think I was able to get up to about 10% of the beam coupled, but this was unclear because the power meter at the other end had some range issues (this just further brings home the point that you should use the same power meter to measure the input and output beam). I discovered one difficulty with working in two different rooms at once: since the power meter was not self-ranging, I had to keep running back and forth between the rooms to adjust the range of the meter as the output power increased. The input end of the fiber had a home-made connection on it, so the alignment was unreliable (it would uncouple if I wasn't careful about where I touched the coupler). Oleg told me he fixed this problem by cutting off the end of the fiber and repolishing it. I didn't do any extensive test with the output light because of the above-mentined instability, but I proved to myself that I could couple a laser to a fiber after learning how in the easier environment of the laser teaching center.
Week 6: March 12 - 15
I have completed my experimental goals for this project, so this week I have just been doing some more research on tapered fibers and frequency combs in preparation for my presentation next week. I have found a paper that gives more theoretical background on frequency combs (I will place the reference here soon). This weekend I will see if I can find one or two more papers, I would like to find out more about the possible applications for frequency combs.