A method for performing incoherent-to-coherent conversion in photorefractive crystals is presented. The technique is experimentally demonstrated in Bil2Si020 and a theoretical framework is established to analyze the performance of the device.
High-speed one- and two-dimensional spatial light modulation may be carried out using the electroabsorption effect in a GaAs buried-channel charge-coupled device (CCD). For photon energies slightly lower than the energy gap, the transmission through or along the surface of a CCD structure may be controlled by the signal charge in the wells, through the change in electric field with charge. The modulator is thus electrically addressable and it promises optical quality sufficient for coherent applications. The predicted performance has been determined for both Fourier transformation or spectral analysis and correlation/convolution. Although the modulation depth for normal-incidence illumination is only 10 to 20%, the calculated dynamic range is more than adequate for optical signal processing. In one-dimensional structures using a guided wave beneath the gate electrodes, the modulation depth approaches 100%, making the device an attractive alternative to acousto-optic modulators.
The architecture, operation, and performance of a new two-dimensional, optically addressed, membrane spatial light modulator are described. The modulator, the Optical-to-Optical Deformable Mirror Device (OTO-DMD), can be used to convert incoherent images to coherent images for optical information processing applications.
A vacuum-demountable prototype electron-beam-addressed spatial light modulator that consists essentially of an electron gun, a microchannel plate and an electro-optic crystal is being investigated. This paper discusses the underlying principles of operation of the device which center around the interaction of an electron beam with a microchannel plate and the addressing of an e-lectro-optic crystal with a flux of nonmonoenergetic electrons.
Image correlation experiments are described using the magneto-optic spatial light modulator as an input device and as a spatial filter on which a computer generatedfFourier transform holograms are recorded. Algorithms for processing optically digitized images using the same device are also discussed.
A potentially high-performance, optically-addressed, spatial light modulator, called the photo-emitter membrane light modulator (PEMLM) is being developed. The operational theory of the PEMLM is presented, accompanied by a discussion of its ultimate performance objectives and limitations. The PEMLM offers the potential for framing rates in excess of 1kHz, 50 1p/mm resolution, storage times of days, extreme sensitivities of less than 1 nJ/cm2, and an intrinsic ability to perform such image processing operations as thresholding and hardclippiny, contrast reversal and enhancement, image addition and subtraction, and optical binary logic. A developmental laboratory prototype has been constructed.
Electromechanical devices can be used to rapidly modulate light if their mechanical inertias can be made small enough. Their usefulness depends on being able to produce them in large arrays with high yield and at low cost, to integrate them with their drive circuitry, and to achieve a high level of reliability and reproducibility. In recent years micromechanical fabrication techniques have been used to produce arrays of miniature optical modulators on silicon that can operate at frequencies up to ~1 MHz with negligible drive power. They are completely compatible with IC technology and can be integrated with elec-tronic circuits. In large arrays they can transfer data at a high rate for parallel optical readout of microelectronic devices and sensor arrays. Once in optical form the data are ameanable to optical processing. This paper discusses the development of the micromechanical modulator, its features, performance characteristics and limitations and a number of potential applications to data transfer and processing.
This paper discusses the material issues of acousto-optic (AO) devices. Criteria for the selection of materials are presented for the three basic types of AO devices that include deflectors (Bragg cells), modulators, and tunable filters. Comparison of physical properties of the various AO materials shows tradeoffs are required to suit specific applications. As an example of how to search for new materials, the potential of a chalcopyrite compound (ZnGeP2) for AO device applications is discussed.
Birefringence and rotatory power are two important parameters for optical modulation by liquid crystals. A new technique has been developed for birefringence measurements at any wavelength in the ultraviolet, visible or infrared spectral regions. The enhanced birefringence of some liquid crystals in the infrared region indicates that these materials will be useful for electro-optic applications in this region. Rotatory power of 90° twisted nematic liquid crystals was measured and found to be in very good agreement with theoretical calculations for a wide spectral range and for different liquid crystal thicknesses. A quantitative relationship between liquid crystal birefringence, wavelength and cell thickness was derived for use in the design of IR liquid crystal light valves with high contrast ratios and minimum response times.
We propose a new type of linear optical processor generating the convolutional ambiguity function of a spatial signal and a temporal signal. The core of the device is an anisotropically-dispersive (or alternatively a dispersively-anisotropic) homogeneous medium, and its principle of operation is based on the interaction between spatial diffraction and temporal dispersion.
Volume holographic storage in photorefractive Bi12Si020 (BSO) crystals is utilized to perform dynamic incoherent-to-coherent image conversion by means of selective spatial erasure of a uniform grating with white (or quasi-monochromatic) light. Preliminary results with binary and grey level transparencies are presented, and the conversion process is described in terms of a simple model which relates the diffracted intensity to the space-variant effective modulation ratio.
Ferroelectric tetragonal tungsten bronze crystals, e.g., SBN:60, SBN:50, BSKNN, KLN and SKN have been grown and are shown to possess large electro-optic r33 coefficients. Striation-free SBN:60 single crystals show a strong enchancement in photorefractive characteristics, indicating a major interest for photorefractive and spatial light modulator device applications.
The number of multiplications per second and fabrication issues associated with several different acousto-optic systolic processors are discussed and the flexibility in the operations achievable by format control are briefly reviewed. Emphasis is given to the effects of divergence of the optical input beam. Various input sources and interconnection schemes are considered. These include: fiber and GRIN optics, multi-channel acousto-optic cells and individually collimated laser diodes. Quantitative theoretical and experimental data are provided. A new architecture using spatial-multiplexing of the input sources and frequency-multiplexing of the acousto-optic cell data is described and used for handling bipolar and complex-valued matrix and vector elements.
Applications of an electrooptic-grating-based integrated optical spatial light modulator (IOSLM) to analog numerical computation are discussed. Characterization of a grating array for use in an engagement-type matrix-vector multiplier are presented and discussed.
Optical information processing architectures for performing matrix-matrix multiplication may also be used for performing other matrix operations as well. In this paper we will illustrate this concept by way of two examples; solution of simultaneous algebraic equations using the Gauss elimination algorithm and QR factorization using the Gram-Schmidt orthogonalization algorithm.
This paper describes initial work on the design and application of a versatile optical image processor based on the combination of degenerate four wave mixing (DFWM) and an optically addressed spatial light modulator. This has demonstrated for the first time a processor which can be both programmed and addressed in real-time and which through the use of a low power HeNe laser, has enabled an appreciable reduction in the size and power requirements of such a system. The combination of SLM and DFWM technologies enables a number of processing operations to be implemented. The light valve can be used both as a means of implementing incoherent to coherent image conversion and for certain preprocessing operations such as contrast modification and image subtraction. Degenerate four wave mixing is an all-optical phenomenon whereby three input waves mix within a non-linear medium to generate a fourth wave. By spatially modulating two of the input waves, and including Fourier transforming elements, the output wave can, under certain conditions, represent the correlation product of the two inputs. The design specification for the processor and progress towards its practical implementation will be considered. An assessment of the performance of the system will be described and initial results for both phase conjugate imaging and pattern recognition (for simple test images) will be given.
A real time integrated optical autocorrelation device is proposed which offers the potential for improved performance and ease of fabrication over existing alternatives. An analysis of the signal processing capabilities of this guided wave device is presented and a comparison is made to other integrated optical approaches.
A dynamically programmable real-time integrated optical matched spatial filter and correlating device for pattern recognition applications is proposed. Both one and two dimensional signal processing capabilities are considered and extensions to pattern generation are discussed.
Experimental results for a correlator with unique features for pattern recognition based on both the color and shape of an input object are described. The unique features are that the object may be real, remote and naturally illuminated with real time correlation. The apparatus used is a joint spectral-spatial matched filter used with a real time Spatial Light Modulator (SLM) and a coherent correlator. Input color images were transformed into red andgreen color coded coherent (Argon laser 5145Å) images by a color coded grid with the SLM and processed by the correlator. Several colored (red, yellow, purple, white) versions of the same object were processed and found readily distinguishable. Applications in areas such as Robotics and quality control are discussed.
In some applications,the performance of multichannel Bragg cells is compromised by the spreading of the acoustic waves as they propagate; the spreading causes the signals in the channels to overlap. The overlapping can be significantly reduced by a spatial filter in a Fourier/image plane; the spatial filter is shown to be equivalent to a cylindrical lens whose power is .a function of the distance from the transducer.
An incoherent imaging system based on frequency multiplexing is presented. The concept was realized by using piezoelectric-elastoopticspace to frequency converter. By modulating each image pixel at a different temporal frequency, the entire image is converted into,an electrical signal by using a single detector. The spatial information of the original image is thus mapped into the frequency domain of an electrical signal. A one to one relation exists between the spatial frequency domain and the time domain, as well as between the spatial domain and the temporal frequency domains. This mapping allows one to perform spatial frequency manipulations in real time by processing the electrical signal. This multiplexing is obtained by using a two-dimensional array of piezoelectric-elastooptic light modulators. By using these two physical effects jointly, one gains the resonance characteristics of the piezoelectric crystal oscillator, together with the modulating properties of the elastooptic effect. Two schemes were studied and presented. The first is composed of two one-dimensional arrays, each composed of discrete crystals. The second one is based on a monolithic one-dimensional array, where a monolithic array is a piezoelectric wafer in which localized resonant domains are constructed.
The Scophony scanner uses the same optical elements as the more familiar flying spot scanner: a rotating polygon mirror, an acoustooptic (A/0) modulator, and a laser light source. The flying spot scanner is designed to construct its image a pixel at a time; no more than one pixel is illuminated at any given instant. The Scophony scanner is designed to image a broad swath of the A/0 modulator's acoustic pulses onto the photoreceptor. Many pixels are illuminated at any given instant in the Scophony scanner. The acoustic pulse image motion is frozen in place by a compensating scanning action. The result is a scanner with a coherent imaging response. This coherent response implies that the optical phase of a given pixel profoundly influences the formation of neighboring pixels. The optical phase at the scanner image plane is driven by the electronic phase of the video signal applied to the A/0 modulator. This coherent response enables electronic manipulation of the video drive signal to have significant impact on the optical imaging performance of the scanner. Two electronic manipulation schemes are proposed for doubling the resolution of the Scophony scanner, one scheme for analog video signals, and one scheme for binary digital video signals. Each scheme gives superior contrast ratio performance compared with the flying spot scanner.
The acousto-electro-optic (AEO) interaction that is presented in this paper is based on the combined Acousto-optic (AO) and Electro-optic (EO) effects. The effect is analyzed and demonstrated experimentally. The application of AEO interaction to light modulation and deflection are discussed.
Coherent combination of the power of several semiconductor lasers fabricated on the same substrate has been the subject of an intense research effort in recent years, the main motivation being to obtain higher power levels than those available from a single laser in a stable radiation pattern. Best results reported so far include 2.6 Watts cw emitted power and less than 10 far-field angle (in the array plane) in arrays where all the lasers are electrically connected in parallel. A different type of coherent array, where each element has a separate contact, has been recently demonstrated. While requiring the more complex two-level metallization technology, applying a separate contact to each laser provides an additional degree of freedom in the design and the operation of monolithic arrays. The separate contacts can be employed to tailor the near-field and far-field distributions and to compensate for device-to-device nonuniformities. Furthermore, the control of the currents of the array elements allows the performance of a variety of other functions, such as beam scanning, spectral mode control, wavelength tuning and control of the mutual coherence between array elements.
The development of a liquid crystal-based visible-to-IR dynamic image converter is described. Liquid crystal studies in the IR, as well as the structure, operation and preliminary performance results of a 10.6 μm VIDIC are reported.
This paper describes a cinematic infrared scene projector based on the use of a thin film of vanadium dioxide as a spatial/temporal modulator in the 8-12 μm wavelength band. Key features of the concept are discussed, including image writing by a scanned laser beam, image erasure by a novel fast cooling and reheating process, and the avoidance of flicker in the output by the use of two optical channels coupled by a rotating framing mirror. Experimental data are given demonstrating the feasibility of operation at (512)2 pixels per frame and 30 frames per second.