Noise reduction using adaptive spatial filtering in photorefractive two-beam coupling

All-optical image processing with cavity type II second-harmonic generation

The possibilities offered by type II intracavity second-harmonic generation for all-optical parallel processing of images are investigated. By injecting an image in a linearly polarized pump beam and a homogeneous field with orthogonal polarization, we obtain, according to the value of the latter, either frequency and polarization transfer or contrast enhancement and contour recognition. Noise-filtering effects are also predicted.

Wiener-like correlation filters

We introduce a new, to our knowledge, design for a Wiener-like correlation filter, which consists of cascading a phase-only filter (POF) with a photorefractive Wiener-like filter. Its performance is compared with that of the POF and the Wiener correlation filter (WCF). Correlation results show that for intermediate and higher levels of noise this correlation filter has a peak-to-noise ratio that is larger than that of either the POF or the WCF while still preserving a correlation peak that is almost as high as that of the POF.

Photorefractive two-beam coupling optimal thresholding filter for additive signal-dependent noise reduction

Computer simulations of photorefractive thresholding filters for the reduction of artifact or dust noise demonstrate an increase in signal-to-noise ratio (SNR) of 70% to 95%, respectively, of that provided by the Wiener filter for inputs with a SNR of approximately 3. These simple, nearly optimal filters use a spectral thresholding profile that is proportional to the envelope of the noise spectrum. Alternative nonlinear filters with either 1/ν or constant thresholding profiles increase the SNR almost as much as the noise-envelope thresholding filter.

Phase-coding technique for one-way image transmission through aberrating media

We propose and demonstrate one-way image transmission through an aberrating medium. In contrast to previous techniques, which usually use exact phase compensation for correcting phase distortion, our technique uses the phase- or amplitude-coded form of the exact phase. This technique allows us to utilize a wide variety of spatial light modulators for image correction in one-way image transmission through aberrating media.

Adaptive methods and system for image processing

We present results obtained in the implementation of adaptive methods and a system for image processing. The light-developing nonlinear elements are used in the filtering block of the mirror optical configuration. The processing of gray-scale and of binary images with the help of adaptive optical elements is fulfilled for realization of ground-suppressing, contouring, differentiating, and skeletonizing adaptive filters. A comparison is made between optical and digital methods applied in the pattern processing.

Fanning noise reduction in photorefractive amplifiers using incoherent erasures

We propose and demonstrate a new scheme of beam fanning reduction in photorefractive amplifiers by using incoherent erasure. This simple and effective approach employs a pump beam with a mixed polarization state or introduces a third incoherent beam for selective erasure. The approach is analyzed, and the experimental results are presented and discussed.

Speckle-free image amplification by two-wave coupling in a photorefractive crystal

A technique of speckle-noise reduction for an image amplified by two-wave coupling with a photorefractive crystal is proposed. The spatial coherence for the axis along the fringe formed in the photorefractive crystal is reduced to remove the speckle, but the spatial coherence for the axis vertical to the fringe is unaltered. This is obtained by rotating a mirror to deflect the illumination beams one dimensionally for both the signal and reference patterns during the exposure of the hologram. Because of the imageholography configuration, the images of the signal and the reference do not move on the photorefractive crystal as the two beams deflect. The beam deflection can be faster than the response of the photorefractive crystal. Some experimental results for the image amplification with the technique developed are presented, and they are compared with results obtained without speckle-noise reduction. Fringe deformation caused by deflection of the signal and reference beams is analyzed.