T. C. Strand

Architectural implications of a digital optical processor

A general technique is described for implementing sequential logic circuits optically. In contrast with semiconductor integrated circuitry, optical logic systems allow very flexible interconnections between gates and between subsystems. Because of this, certain processing algorithms which do not map well onto semiconductor architectures can be implemented on the optical structure. The algorithms and processor architectures which can be implemented on the optical system depend on the interconnection technique. We describe three interconnection methods and analyze their advantages and limitations.

Adaptive Restoration Of Images With Speckle

Speckle is a granular noise that inherently exists in all types of coherent imaging systems. The presence of speckle in an image reduces the resolution of the image and the detectability of the target. Many speckle reduction algorithms assume speckle noise is multiplicative. We instead model the speckle according to the exact physical process of coherent image formation. Thus, the model includes signal-dependent effects and accurately represents the higher order statistical properties of speckle that are important to the restoration procedure. Various adaptive restoration filters for intensity speckle images are derived based on different speckle model assumptions and a nonstationary image model. These filters respond adaptively to the signal-dependent speckle noise and the nonstationary statistics of the original image.

Sequential optical logic implementation

An optical system that performs sequential binary logic operations is described. The system consists of a spatial light modulator (SLM) used to provide a nonlinear threshold response and a computer-generated hologram to provide interconnections between logic gates. A 2-D array of logic gates with binary inputs and outputs is formed on the active surface of the SLM. These gates are interconnected by a 2-D array of subholograms, one for each gate. Arbitrary logic circuits consisting of NOR gates and inverters can be implemented, and the system can be reconfigured by changing a single holographic element. The system is demonstrated using a twisted-nematic liquid crystal light valve as the SLM. A test circuit is implemented that includes a synchronous master–slave flip-flop and an oscillator consisting of five inverters in a feedback loop. Experimental results of this test circuit are presented.

Optical pseudocolor encoding of spatial frequency information

An optical spatial filtering system is described which color encodes the local spatial frequency content of an image. It is shown that partially coherent illumination has advantages over coherent or incoherent illumination in this system. Experimental results are shown which indicate that the system can be used to perform a simple type of texture-to-color conversion. The system could be used to enhance textural differences for a human observer or as a preprocessor which provides texture related information to a second image processing device.

Range measurement using Talbot diffraction imaging of gratings

A range measurement technique is presented which allows real-time range data acquisition for all points in an image. The technique relies upon the systematic variation in the Fresnel diffraction pattern of a grating as a function of propagation distance. The technique uses a single view of the volume in question so there are no hidden points. Furthermore, there is no need for beam scanning or is any off-line processing required. An extensive theory of operation is presented along with grating and illumination design methods which allow one to tailor the technique to specific measurement requirements. Qualitative experimental results are given along with quantitative results.

Variable Grating Mode Liquid Crystal Device for Optical Processing Computing

Abstract Certain nematic liquid crystal mixtures are observed to form a “variable grating mode” (VGM) for appropriate choices of cell design and applied voltage. In this mode of operation, a phase grating in the plane of the cell arises from a periodic variation in the orientation of the liquid crystal director. The grating spatial frequency is observed to vary linearly as a function of the applied voltage above the formation threshold. Liquid crystal and device parameters characteristic of the observed variable grating mode are presented. Utilization of the VGM effect in a photo-conductively-addressed device is shown to provide an intensity-to-spatial frequency conversion. Applications of this unique type of optical transducer to arbitrary nonlinear optical processing problems are described. Results of level slicing experiments and implementation of optical logic functions are presented.

Optical logic with variable-grating-mode liquid-crystal devices

A liquid-crystal device that performs a two-dimensional intensity-to-spatial-frequency conversion has been investigated for use as an optical transducer. When such a device is used as the input transducer in an optical filtering arrangement, image-intensity levels can be easily manipulated by using appropriate Fourier plane filters. The variable-grating-mode liquid-crystal device is well adapted to performing binary logic operations on images. Results demonstrating the implementation of common logic operations are presented.

Polarization properties of the variable-grating-mode liquid-crystal device.

The principal features of the liquid-crystal molecular orientation within the variable-grating-mode liquid-crystal device have been determined as a function of the applied voltage across the cell by measurement of the polarization properties of light diffracted by the liquid-crystal birefringent phase grating.

Real-time parallel optical analog-to-digital conversion

A technique for optically performing parallel analog-to-digital conversion on incoherent two-dimensional inputs at real-time rates is described. The system uses a birefringent optical real-time input transducer combined with a detector array and optical thresholding. Quantization and bit plane outputs are produced in parallel without scanning. An experimental system with three-bit accuracy is described.

Optical Computing With Variable Grating Mode Liquid Crystal Devices

A liquid crystal device that performs a two-dimensional intensity-to-spatial frequency conversion has been developed for use as an optical transducer. When such a device is used as the input transducer in an optical filtering arrangement, image intensity levels can be easily manipulated via Fourier plane filters. This variable input intensity-output intensity transfer function has numerous potential applications in image processing. Implementation of a variable level slice operation is discussed and experimental results presented. The VGM LCLV device is also particularly well adapted to performing binary logic operations on 2-D images. This application is discussed, and results demonstrating the implementation of common logic operations are presented.