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.