A degenerate four-wave mixing arrangement employs a photorefractive crystal as a real time holographic recording medium to record a hologram of the noisy optical signal. The signal passes through the crystal and is phase conjugated by a self pumped barium titanate phase conjugator, and the resulting phase conjugate beam reads out the hologram and multiplicative noise is substantially reduced in the optical output signal.
An all optical photorefractive holographic joint transform correlator provides a first correlation plane output port using two-beam coupling, and a second correlation plane output port employing four-wave mixing. With appropriate input beam intensity ratios, the first port output is capable of high discrimination, while simultaneously the second port offers a low discrimination output.
A holographic grating is written into a photorefractive erasable holographic member and a light beam having a first wavelength which includes the joint power spectrum of a pair of joint images to be correlated is directed at the photorefractive member to partially erase the grating. A phase conjugate signal from the partially erased grating is then readout and Fourier transformed to produce the correlation output spots.
Limiting quadratic processing and compansion in photorefractive two beam coupling is disclosed. Two-beam coupling in photorefractive barium titanate employs the imaged intensity of the signal to amplify the reference beam while maintaining the phase of the reference beam. The phase distorted signal beam is converted to that of the controlled phase of the reference beam. The high pump limit of amplification in this two-beam coupling device produces an amplitude compressed output to reduce multiplicative noise. Lost contrast of the image is thereafter restored. Beam clean-up of a non-intelligence bearing beam can be carried out by a similar process; a low pass filter consisting of a pinhole plate can be used in place of the second photorefractive crystal in the Fourier plane and only the planar wavefront portion will pass through the pinhole and may be collimated by a lens to provide a cleaned planar output beam.
A method for improving the quality of holographic images by reducing local noise in a holographic memory system is disclosed. The method is characterized by: storing a white reference image and a black reference image in the memory system, storing a series of data images in the memory system, applying a simple model based on a point-to-point of linear interpolation to the series of data images, the white reference image and the black reference image to provide a series of corrected data images having reduced noise. The simple model is preferably in the form of ##EQU1## which is characterized by temporal stability for the series of data images for each set of the black and white reference images.
An all-optical nonlinear joint transform correlator has been implemented for the first time without using a spatial light modulator and digital processing in the Fourier plane. The correlator utilizes energy transfer from two-beam coupling in the Fourier plane. A compressional nonlinearity in the hard-clipped regime is implemented by pumping a weak plane wave with the intense joint spectrum of the reference and signal images. Operation of this device rivals or exceeds that of the phase-only filter for detecting objects in cluttered noise. Experimental results are compared with both plane wave and beam propagation simulations.
