Monday, February 20, 2006

Week of 2-13-06

The following project tasks were completed this week:

The LC retarder was obtained at the end of last week, and initial testing was started this week. To first confirm proper device operation, the LC device was placed between two polymer sheet polarizers, and a sinusoidal voltage was applied to the wires attached to the control electrodes, which I soldered to the device. A sinusoidal response in transmission intensity through the device was seen. It was found that to achieve maximum contrast, the device required approximately a +/- 4 V drive voltage.

Testing of the device was then performed in the actual setup using the Argon laser. This utilized the LC retarder/cross polarizer device, the Argon laser, a function generator, a Newport optical power detector, and an oscilloscope. It was seen that with a sinusoidal drive voltage, the transmission intensity was approximately sinusoidal, however, the duration of the sinusoid peaks was slightly shorter than the valleys. I spoke with Gary Sharp at Colorlink about this problem, and we deduced that it was because the LC device has a faster ‘on’ time than ‘off’ time and this would cause this sort of skewed response. To fix this problem, a sawtooth wave was applied with the same voltage levels. It was observed that this fixed the skewed sinusoid that was observed before. Using this drive signal, modulation frequencies as low as 3 Hz and as high as 100Hz were accurately achieved.

This modulation setup was integrated into the final experimental setup, and final testing of super-luminal and slow light propagation is now possible.

Week of 2-6-06

The following project tasks were completed this week:

LC device ordered from Colorlink. After speaking with Dr. Blair upon his return from PW Conference, I found that the device John was describing was a variable liquid crystal retarder capable of varying polarization angles over a 90 degree range based on a linear input voltage. When placed in between a set of crossed polarizers (polarizers oriented at 90 degrees) at a 45 degree angle, and the control voltage of the LC device is driven by a function generator outputing a sinusoidal voltage signal of the desired frequency, the optical intensity transmission through the LC retarder/cross polarizer will also vary sinusoidally.

During the wait before the LC device was received, basic information was obtained regarding response times and transmission properties of LC retarders through web searches and a literature review of the subject.

Week of 1-30-06

The following project tasks were completed this week:

Dr. Blair met with John Korah and Gary Sharp at PhontonicsWest Conference in San Jose. After looking at the LC device that John had described, Dr. Blair confirmed it will work well for our application. I contacted John to ask what was needed to obtain the LC device, and was informed that since this was for a senior project that device could be donated at no cost, and that it could be shipped next Monday.

I began to setup a cross polarization device as well as a fixed holder for the LC device. Power measurements of the laser were also taken to check that it was still operating with the same output parameters as previously taken power stability measurements.

Week of 1-23-06

The following project tasks were completed this week:

Using the method discovered the previous week, high frequency sinusoidal modulation was achieved using an optical chopper wheel. It should be noted however, that a true sinusoid was not obtained, rather, a close approximation to ideal modulation was achieved. The sinusoidal approximation was sufficient between approximately 30 – 100Hz. Testing was done by using an argon laser operating at 488nm and picking up the modulated beam signal using a Newport detector and an oscilloscope. The results of these initial modulation tests indicated that using the chopper wheel is a viable option for high frequency modulation, but was not feasible for lower frequency modulation of approximately 10Hz.

Methods for low frequency modulation were further investigated. John Korah of Colorlink was contacted to follow up on a question previously asked regarding their liquid crystal (LC) attenuators. John mentioned that for our application, a device could be built specifically to achieve our modulation needs using a LC retarder device, a component of many of their optical attenuators. I suggested that John and Colorlink’s product engineer Gary Sharp meet with Dr. Blair at a conference in San Jose that they would all be attending the week of the 30th so Dr. Blair could see the device John was describing and be able to confirm that it would work in our application.