Monday, May 01, 2006

Senior Thesis Paper

Below is the link to my final senior thesis paper, including a final bill of materials as well as some raw initial data.

Monday, April 24, 2006

2006 Technical Open House

Final Project Presentation presented on March 30, 2006. Below is a link to a PDF of the final delivered presentation

Monday, March 20, 2006

Week of 3-6-2006

The following specific project tasks were completed this week:

-At the suggestion of Dr. Blair, the addition of a 1/4 waveplate into the LC modulator/cross polarization device was investigated. Such a device would initially circularly polarize incident light when placed before the first polarizer. This would allow for the LC cell control voltage to create a 50% attentuation level corresponding to zero drive voltage.

-Below are links ound for possible 2" square 1/4 waveplates. Two options are available, zero order and multi order. Zero order waveplates have a retardance that is less dependant on wavelength than multi order. Since the system currently in the setup operates at specific wavelengths, a zero order waveplate is not a necessity. Polymer film waveplates were also investigated with similar, and are likely to be used due to their relativly cheap cost. These are available from a mail order vendor.

- The oritentation of the current cross polarizer/lc cell setup was optimized resulting in the configuration with polarizers oriented perpendicularly to LC cell. This configuration provides the highest modulation contrast between off and on states as well as best sinusoidal response.

- Optics in the final system were cleaned and room was left for the later addition of a 1/4 waveplate device. Testing began using a solid state ruby rod to try to achieve slow light propagation behaviours. Proper stable modulation was achieved, however, the laser used was operating at approximately 200mW, slightly less than earlier testing, and slightly too low to accurately achieve propagation delay. To achieve higher output power, the laser with need tuning.

1/4 Waveplate links:

Zero Order
Multi Oder:
Melles Griot:
Mica Plate
Quartz Plate:

Monday, March 06, 2006

Week of 2-27.06

The following specific project tasks were completed this week:

After further characterization of the LC cross polarizer variable attenuator device, the following observations were made this week:
- The device works best at around 40 - 60 Hz, and exhibits a very close sinusoidal approximation at this frequency. There is however, a moderate DC shift in the dectector levels at this frequency.
- A DC level shift in the detector signal is present regardless of the driving signal frequency. This shift however increases with increasing modulation frequency. The amplitude modulation riding this DC shift also decreases with increasing frequency, however, it remails sinusoidal.
- At low frequencies (5-10Hz), the device does not produce a very accurate sinusoidal approximation. The output exhibits a signal which has peaks which are roughly sinusoidal with some high frequency nosie, and valleys which are roughly flat. Basically a square signal with a rounded response for the high level of the square wave signal. This is essentially the modulation response seen last week. This response (especially the high frequency noise) can be improved by using a filter and avoiding detector saturation.
From looking at the Bigelow, Lepeshkin, and Boyd papers, for ultraslow propagation, the spectrial hole required in ruby is 36 Hz and for alexandrite is 8.4 Hz. Ultra-fast propagation in alexandrite requries a spectrail antihole of 612Hz. To achieve both superluminal and ultraslow propagation requires frequencies of around 250Hz and less, with ultraslow propagation possible at the lower frequencies in this range (50Hz) Looking at what I've been able to do with the LC cell and what the papers indicate is needed, it looks like for ultraslow propagation in ruby, the LC cells will work well. Something faster may be needed for alexandrite and ultrafast propagation.

Week of 2-20-06

The following project tasks were completed this week:

Using the results obtained last week, further testing of whether the LC retarder/cross polarizer modulation device was performed. Due to the sensitivity of achiving superluminal and ultraslow propagation phenomina on precise modulation frequency control, and on the advice of Dr. Blair, further characterization the LC cell was performed. While the LC cell provided accurate low frequency modulation, it was not necissarily the best approximation of a sinusoid at low frequencies (<10Hz). These frequencies prove necessary to obtain accurately, as they correspond with the spectral hole in alexandrite allowing for superluminal propagation.

The LC cell/cross polarization device was taken out of the final setup and placed into a new characterization setup utilizing a Gre:Ne laser, the same Newport Detector, a precise function generator, a digital oscillscope, and a variable filter wheel to avoid detector saturation. This move was necessary to precisely characterize the LC retarder using equiptment not available in the teaching laboratory.


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.

Tuesday, January 24, 2006

Week of 1-16-2006

The following project tasks were completed this week:

Followed up with Boulder Nonlinear Systems regarding thier XT and TS shutter systems. Looking at the Bigelow paper, it was found that a maximum carrier modulation frequency of 250Hz is necessary for super luminal propagation. Was put in contact with Kipp Buchart, a solutions engineer at BNS. According to Kipp, BNS shutters elicit a nonlinear response with a linear drive voltage, so to properly operate the shutter for our applications a normal sinusoidal function generator will need to integrate a nonlinear transfer function before driving the shutter. The main issue with using the exisitng BNS shutters is that they elicit a 25 ms total response time corresponding to a modulation frequency of 40 Hz. This will however be sufficient for a signal frequency to be modulated on top of the 250Hz carrier frequency. It was decided between Dr. Blair and myself that this will be used for our low frequency signal modulation frequencies, and that Dr. Blair will speak to the BNS representatives the week of 11-23 at a conference he will be attending in San Jose California.

I also discovered that sinusoidal modulation can be achieved using the original square chopper wheel. This is achieved by taking the chopper wheel with chopping slits just wider than the beam diameter and setting the wheel at an angle such that the edges of the beam are barely clipped by the sides of the chopper wheel. When the wheel spins, an approximation of a sinusoid is achieved. This simple method will be very sufficient to achieve our modulation needs.

This method was implmented in the setup late Friday, and will be tested the following week.

Monday, January 09, 2006

Christmas Break Senior Project Progress

Happy Holidays!

The following is a summary of the main tasks and project progress completed over the break:

- Following coversation with Colorlink Engineering Gary Sharp I spoke with John Korah again. After several back and forth coversations, we discoverd that for the particular application required in my project, the Colorlink LC modulator would not be sufficient. The modulator is a non-linear component, and its retardance level is based on a preset input voltage based off a device characterization table. This would make interfacing the device with the experiement extremely difficult as smooth sinusoidal (analog or fast digital) operatation is essential. Unfortunatley, a good amount of time was spent communicating and planning with Colorlink before this discovery was made. A ColorLink engineer I spoke with recommended looking at a spatial light modulator as a possible solution, and was refered to the vendor DisplayTech.

- DisplayTech's spatial light modulator (SLM) was investigated as a possible solution to achieve sinusoidal intensity modulation without using an expensive electro-optical modulator system (well over $10k). However, upon investigation of the product and corresponence with Dr. Blair, it was found that the SLM offered by Display Tech was a multi-pixel modulation device, and although would work sufficiently for my application, was somewhat of an overkill, as only one large pixel was needed to modulate. Whlie a possible solution, it was determined that a better less expensive method could be used. Product information for this SLM is located at the link below:

- At Dr. Blair's prompting, I investigated extensively the use of variable optical shutters in analog modulation. I contacted Kelly Gregorak of Boulder Nonlinear Systems and found information regarding their TS and XT optical shutters. A link to their product information website is included below. The TS shutter offered by BNS seemed ideal, however the driver required must be at least 100V, and if a cheap driver does not exist, a a regular e-o modulator is just as reasonable a solution despite the price. The XT shutter however can use a 20V driver and can operate around 200Hz (which should be sufficient - this will be investigated early in the week of 1 - 09 -06). If BNS can identify a cheap driver, the TS will be used, however if one is not available and the modulation frequencies will work for slow and fast light propagation, the XT shutter will be utilized.

- The majority of the break was spent further investigating exisiting solutions to the problem of achieving the neccessary modulation working around cost effective technologies. This proved a fairly formidable task (much more so than I had assumed) however, it appears now that a reasonable solution has been found that fits the necessary specifications and pricing. This task was further complicated by working around the schedules of sales and engineering representatives at vendor companies and their holiday plans. Overall though, I feel that the project has made a significant and necessary step ahead, and with further work, the main goals of creating a robust and operating experiement setup will be near completion soon, allowing time for preparing the experiment for exhibition.

- A link to the main reference for the expermental conditions necessary to achieve slow an superluminal propagation in room temperature Alexandrite crystal is below: