Scott Campbell

Optical pattern classifier with Perceptron learning

An optical realization of a single layer pattern classifier is described in which Perceptron learning is implemented to train the system weights. Novel use of the Stokess principle of reversability for light is made to realize both additive and subtractive weight modifications necessary for true Perceptron learning. This is achieved by using a double Mach-Zehnder interferometer in conjunction with photorefractive hologram recording. Experimental results are given which show the high quality subtractive changes that can be made.

2-D shift-invariant volume holographic correlator

Abstract We describe a shift-invariant optical correlator with a volume hologram as a matched filter. Shift-invariance is achieved by the combination of degeneracy in one direction and image shifting in the perpendicular direction. The amount of shift-invariance along the degeneracy direction is evaluated and various methods to achieve image shifting are proposed. Both theoretical and experimental results are presented and discussed.

Crosstalk in volume holographic memory

Crosstalk noise, due to the finite dimension of the recording medium, is a fundamental limitation to storage capacity in volume holographic memory. We present a general theoretical analysis on the crosstalk noise in angle- and wavelength-multiplexed volume holographic memory systems. Results on storage capacity and its dependence on hologram separation and required signal-to-noise ratio are obtained and discussed. Crosstalk noise may also arise from the expansion or shrinkage of the storage medium during and after the hologram recording process. Its effects, including Bragg mismatch, pixel displacement and impulse broadening, are analyzed for Fourier-plane holographic memories. Complete compensation and optimization to eliminate or reduce these effects are presented and discussed.

Optical thresholding and maximum operations.

Self-oscillations in nonlinear optical four-wave mixing and resonators are considered. Some unique properties of these oscillations can be employed for implementing parallel optical thresholding, comparing, and maximum operations. Both theoretical and experimental results that are obtained by utilizing the photorefractive nonlinearity are presented and discussed.

Optical image processing by matched amplification.

An image-processing technique, which we call matched amplification, is proposed and demonstrated. Analogous to the well-known technique of matched filtering, matched amplification manipulates an input image at its Fourier plane by using a Fourier transform of a reference image. In matched amplification, however, the Fourier components of the input image that matchthose of the reference image are amplified rather than attenuated as in the case of matched filtering. Owing to this amplification process, matched amplification is advantageous for applications that require cascadability, high energy efficiency of the processed image, or large intensity dynamic range of the input image. We show experimental results using photorefractive two-beam coupling in BaTiO(3) as the coherent amplification stage.

Photorefractive square-law converter

We demonstrate a new type of optically addressed spatial light modulator. By imaging an external grating into a photorefractive material, we write holographic gratings without the need for interference between our input image and a coherent reference beam. This technique allows us to obtain the high contrast and resolution of photorefractive holography while permitting writing with incoherent input images. We evaluate and discuss various performance parameters such as resolution, contrast, and sensitivity that characterize the device.

Stable photorefractive square-law conversion using moving grating techniques.

We demonstrate a technique by which the temporal oscillations in the response of a photorefractive square-law converter, a recently developed high-contrast spatial light modulator, may be removed. This technique uses the translation of an amplitude grating incoherently imaged upon a photorefractive crystal to obtain a moving intensity pattern within the material. We thus obtain running-wave effects similar to those seen by previous investigators writing photorefractive gratings by using the interference of two coherent frequency-shifted beams. We show both theoretically and experimentally that the criteria for removing temporal oscillations simultaneously yields a significant improvement in the diffractionefficiency of the square-law converter.

Reconfigurable Optical Interconnection Using Photorefractive Holograms

The use of photorefractive holograms in conjunction with a spatial light modulator (SLM) to realize a reconfigurable optical interconnection with very high energy efficiency is briefly discussed. In this approach, the SLM is used as a programmable binary matrix mask to encode the interconnection pattern to a coherent laser beam whereas the photorefractive crystal is used as a dynamic holographic medium to store the pattern and to efficiently diffract the readout beam into the selected channels at high speed. We report recent experimental results on issues such as energy efficiency, reconfiguration time, contrast, and uniformity.