Cross-polarization beam coupling in photorefractive GaAs crystals

Optical competitive learning with, VLSI/liquid-crystal winner-take-all modulators

A new approach to self-aligning unsupervised optical learning based on the competitive learning algorithm and adaptive holographic interconnectións is introduced. A volume hologram is used to diffract the light from an input spatial light modulator so that it focuses upon a custom winner-take-all very-large-scale-integrated circuit liquid-crystal spatial light modulator. The units that receive the most light switch to a reflective state, and the light reflected from the winning pixels interferes in the volume of the dynamic hologram with the phase conjugate of the input pattern in order to add an outer-product perturbation to the current holographic interconnection. As a set of input patterns is cycled through many times, the system learns to diffract the light from a particular input class upon a self-organized set of detectors that recognize similar input patterns without the aid of a teacher or any required alignment. Beam-propagation simulations are used to show that the holographic optical learning network faithfully reproduces the behavior of the ideal competitive learning algorithm. A winner-takeall detector/modulator device containing a total of 576 optical neurons grouped into 31 separate competitive patches in the required sparse-grid topology has been successfully fabricated, and experimental results are presented.

Orientational dependence of photorefractive two-beam coupling in InP:Fe

We present the theory for photorefractive two-beam coupling without optical activity in cubic crystals rotated around the [110] axis. We also describe two-beam coupling experiments in InP:Fe that fit the theory closely. The 15.5% gain enhancement for grating wave vectors aligned along the <111> directions is demonstrated explicitly.

Polarization-rotation switch using picosecond pulses in GaAs

Photorefractive and free-carrier nonlinearities in GaAs transfer energy from a strong picosecond pump, which is linearly polarized at an arbitrary angle to a weak s-polarized probe, into a p-polarized beam propagating in the probe direction. By placing the GaAs between a crossed pair of high-quality polarizers that are set to extinguish the probe in the absence of the pump, an efficient, high-contrast, high-speed optical switch is realized. By varying the pumpprobe ratio, delay, fluence, and polarization, we can maximize the intensity of the p-polarized component. This intensity with the pump present divided by that without the pump, the on-off ratio, is approximately 2 at fluences as low as 0.03 mJ/cm(2) and approaches 10,000 at fluences of 15 mJ/cm(2) and pump polarizations of 45 degrees .

Matrix-vector multiplication in thin photorefractive GaAs crystals

Optical matrix-vector multiplication using four-wave mixing in a thin photorefractive GaAs crystal is demonstrated. Using a thin wafer of GaAs offers the potential to integrate the encoding spatial light modulators directly on the wave-mixing medium.