The use of a rectangular aperture for the coupling of a microstrip line to a patch antenna, a dipole antenna, or another microstrip antenna is explored. The relevant theoretical and experimental results are reported. It is noted that the circuits considered here should have many applications for phase arrays, MMIC and MIC packaging, and microwave circuit interconnects.

A method for investigation of breakdown in thin films and on surfaces of semiconductors under the influence of a strong electromagnetic field is suggested. The field causes drastic nonlinear chances in conductivity of a semiconductor leading to dielectric-metall transition on the surface of the sample. Simultaneously to an impulse of microwave radiation a beam of laser light is acting on the sample to detect changes in its electrophysical state. Since exciting (microwave) and analyzing (oPtics) channels are frequency separated the method is contact less, free from additional noises and stray coupling. Deendance of the critical value of the external field on semiconductor Parameters is being discussed

Acousto-optic deflectors play an important role in optical signal processing systems due to their real time processing capabilities, as well as their conversion capabilities of a function of time to a function of space and time. In this work Brimrose investigated the design and fabrication of state-of-the-art, very large time-bandwidth acousto-optic devices from TeO₂ single crystals.

This paper discusses the fundamental design issues associated with wideband, high diffraction efficiency phased-array Bragg Cells. Also discussed are the key fabrication steps necessary to obtain optimal results. TRW has fabricated and tested 1 GHz bandwidth (fc=2.5 GHz) and 2 GHz bandwidth (fc=3.0 GHz) GaP longitudinal mode Bragg Cells. Results presented include diffraction efficiency, two-tone 3rd-order intermodulation products, and frequency response.

This paper reviews the operating principle of the acousto-optic tunable filter (AOTF) and describes some of the recent progress. A ray tracing algorithm was derived that was used to determine the image aberration due to the AOTF. The calculated result is compared with the experimental result of the AOTF configured as an electronically tuned imaging spectrometer. Due to the recent progress of AOTF technology, a wideband (10 nm) visible AOTF with a one inch aperture is now realizable.

Design and fabrication considerations for 32-channel modulation in a single acousto-optic device are presented. Preliminary research work was focused on reducing acoustic and electrical crosstalk between channels. The Bragg cell attains diffraction efficiency of 40% per channel when the RF power is 150 mW and a channel to channel isolation of 24 dB.

This paper presents methods of microwave frequency offsets on the output of a laser beam by acousto-optic Bragg interactions. The Bragg cells operate at a subharmonic of the final microwave frequency and yield a tunable, single sideband offset. The optical system performing this operation is called a microwave acousto-optic frequency translator (MAOFT). The concept of the MAOFT and experimental arrangements are reported in this paper.

transfer characteristic, thereby increasing the dynamic range to more than 85dB. This inexpensive detector features a variable array length, compact drive electronics housed in a customized module, low-light level performance, a low dark current, and can operate at a data rate of up to 20 MHz. A commercially available linear sensor array has been operated in an unconventional way to achieve a compressive transfer characteristic, thereby increasing the dynamic range to more than 85 dB. This inexpensive detector features a variable

Preliminary results of a study aimed at the development of a simultaneous signal instantaneous frequency measurement (IFM) receiver using optical implementation methods are reported. A method for detecting simultaneous signals in an IFM receiver is briefly reviewed. Two architectures (coherent and incoherent) for optical phase correlators are then described and compared. Some considerations concerning the practical implementation of the two architectures are discussed.

Various forms of coherent optical signal processors require an input reference signal that is composed of a comb of frequencies equally spaced over a wide operating bandwidth. The principle function of this waveform is to provide an array of local oscillators to simultaneously downconvert an array of contiguous subbands over a wide bandwidth to a common IF or baseband. An additional requirement of this reference signal for optical processors is that the power be uniformly distributed in time to maximize dynamic range. A periodic chirp is an example of an efficient reference waveform. This paper presents an analog RF feedback technique that requires only two CW inputs to generate a uniform comb of reference frequencies such that the time waveform is a periodic chirp. An analysis is presented that defines the relationship between the generator design and output signal parameters, and discusses factors that impact signal quality and stability. A simulation is used to illustrate the behavior of the generator. An acousto-optic implementation is also discussed, and an example of an architecture that incorporates the signal generator within a coherent optical Fourier analyzer is described. The results of an experimental demonstration of a feedback chirp generator are reported.

Multi-channel acousto-optic processors have been proposed for analysis of microwave signal waveforms. Such architecture provides the added signal processing power for future microwave environments. Frequency range up to 100 GHz may be processed using a wedged Fabry-Perot cavity as channelizer and subsequently a time integrating spectrum analyzer. Modulation parameters of complex modulated waveforms may be analyzed by means of a multi-channel CAF processor. Potential capability and developmental needs are described.

Two dimensional images are converted into a very long 1-dimensional data stream by means of raster scan. It is shown that the 1-dimensional correlation function of such long data streams is equivalent to the raster scan converted data of 2-dimensional correlation function of images. Real time correlation of high resolution two-dimensional images has been demonstrated using commercially available components. The advantages of this approach includes programmable electronics reference images, easy interface to objects of interest using conventional image collection optics, real time operation with high resolution images using off-the shelf components, and usefulness in the form of either black and white or full colored images. Such system would be versatile enough for robotics vision, optical inspection, and other pattern recognition and identification applications.

A key element in any optical system is the light source and semi-conductor lasers have become the most attractive candidates for many applications due to their small size, low drive voltages and high output powers. However, their linewidths need to be reduced for operation at ever-increasing modulation rates. Monolithic laser designs such as the distributed feedback (DFE) and distributed Bragg reflector (DER) produce line-narrowed outputs by including an internal grating within the laser cavity [1). If the cavity is extended beyond the active semi-conductor chip then this long external cavity arrangement, with a reflector at the end, can produce far greater line-width reductions provided that a mechanism is included for selection of a single longitudinal lasing mode [2). Applications for tunable LEC lasers have increased dramatically in recent years and now extend from local oscillators in coherent detection schemes to testing of optical components such as filters and amplifiers and they might in future also be employed in sophisticated fibre optic sensor systems. Various designs have been proposed for mechanical arrangements which would enable wide-range tuning whilst maintaining a single longitudinal mode [3-5) but, in practice, suffer from backlash, hysteresis and slow operation which make them non—ideal for field operation. In the present work we seek an improved control system which will enable wavelength tuning to be performed with the ease and consistency of the electronic tuning schemes.

The fabrication, performance characteristics, and applications of monolithic in-plane surface-emitting lasers (IPSELs) with dry-etched 45-degree micromirrors are reviewed. Several types of such laser diode structures in both junction-up and junction-down configurations are considered. The performance goals for IPSELs with 45-degree micromirrors are high power and efficiency, high duty cycle and CW operation, good reliability, and high fabrication yields. The proposed approach for achieving these goals includes uniform quantum well material growth and dry etching of the laser micromirrors with tight fabrication tolerances.

Using a KTP crystal in the size of 6 .5x9x20 mm (length) , cut at 0 = 57° in X-Z plane, and with the second harmonic (532 nm) of the repetition rate mode-locked Nd:YAG laser light as the pumping source (pulse duration of 40 ps), tunable parametric laser emission from 630 nm to 3420 nm (in addition to those in the vicinity of degeneracy point) has been obtained by angle tuning for phase-matched type II OPG process. The maximum energy conversion efficiency is 30%, the peak power at the signal wave of 1300 nm is 20 NW.

A method of high frequency lightwave modulation based on the self-sustained pulsation (SSP) of laser diodes is presented. The microwave/millimeter wave frequency subcarrier is generated by the laser diode itself and can be tuned in a wide range by simply changing the DC bias. A theoretical mode has been developed and revealed the Hopf bifurcation is the origin of the SSP. The theoretical analysis also shows that the SSP may well exceed 100 GHz by properly controlling the device parameters. The SSP characteristics of the V-channeled substrate inner-stripe (VSIS) GaAlAs laser diodes have been studied both in the optical and microwave domain. The SSP frequency of the diodes can be tuned between 1-7 GHz. The modulation of the SSP generated subcarrier up to 1 GHz has also been demonstrated.

Microwave phase shift and attenuation of ridged hetero-structure GaAs/AlGaAs coplanar waveguide under optical diode laser illumination have been investigated. Large phase shift and change of attenuation has been observed with very low optical power. Their magnitudes are very sensitive to variations of doping in the GaAs.

The objective of this paper is to make a comparison between some microwave transmission lines of interest for travelling wave electrooptic modulators. It concerns the structures using III-V semiconductors and in particular GaAs/GaAlAs heterostructures. We study the effects of different electrode geometries and epitaxial compositions on the performance of high speed electrooptic modulators. We point out show wave phenomena and their effects on the microwave characteristics. It appears that CoPlanar Strips (CPS), under microwave odd mode excitement, are the most interesting for high speed electrooptic modulation.

In this communication we report a novel structure for the electrooptic intensity modulation. To transfer the phase modulation into intensity modulation, the structure proposed uses the the interference between a guided mode and a group of radiation modes, generated at an optical discontinuity. The structure is studied using the standard BPM as well as a modal technique based on the analysis of the radiation mode spectrum of the optical field. An extinction ratio in the order of 7 dB is obtained and a higher ratio is expected for an optimized structure. This structure seems to be promising for waveguide modulators as it shows several practical advantages compared to the classical Mach-Zehnder or directional coupler modulators.

Laser diagnostics of microwave radiation alreadY presented in some articles means interaction of weak monochromatic laser light with powerful microwave radiation in a semiconductor laYer. This interaction maY be described bY known dependencies that determine changing in reflection index (for optical and microwave radiation) caused bY changing phYsical Properties of the semiconductor under the influence of microwave radiation. One of those dependencies is varying mobility of free charge carriers because of shifting its electron temperature.

The research and development of electronic and electro-optic components for geosynchronous and low earth orbiting satellite optical high bandwidth communications at the NASA-Goddard Space Flight Center is reviewed. Intersatellite optical communications retains a strong reliance on microwave circuit technology in several areas - the microwave to optical interface, the laser transmitter modulation driver and the optical receiver. A microwave to optical interface is described requiring high bandwidth electronic downconverters and demodulators. Electrical bandwidth and current drive requirements for the laser modulation driver for three laser alternatives are discussed. Bandwidth and noise requirements are presented for optical receiver architectures.

A wavelength-multiplexed fiber optic link architecture has been designed for transmitting wideband IF and LO signals at microwave frequencies over long distances. The architecture involves downconverting the input RF signal, directly amplitude modulating a laser diode, transmitting the resulting RF-modulated optical carrier through single mode fiber and subsequently upconverting the recovered RF signal to the original band. Wavelength division multiplexing and demultiplexing is used to transmit the local oscillation signal (required for down- and up-conversions) from the transmitter to the receiver through the same fiber. By using a frequency-divided LO signal in conjunction with frequency multipliers, we can down/up convert the wideband RF signal anywhere in the RF spectrum up to 100 GHz. A demonstration link has been implemented using this architecture and operates from 6.3 to 16 GHz. This link uses a 1.3 micrometers , 10 GHz RF bandwidth laser diode for IF transmission and a 1.55 micrometers , 1.5 GHz RF bandwidth laser diode for LO transmission.

The use of fiber optic (F.O.) systems has flourished with the advent of high quality and reliable low-loss fiber and opto-electronic devices such as semiconductor lasers and photodiodes. Moreover, extension of F.O. communication into the microwave and millimeter wave frequencies has been of particular interest because of system miniaturization and the potential of low-loss bandwidth available. With the diversity of F.O. systems now being designed and deployed, it has become increasingly difficult to classify and evaluate the various microwave and millimeter wave link architectures. This paper summarizes and reviews twelve F.O. link architectures with respect to design, performance and high-frequency potential.

Optical techniques for both microwave device and circuit characterization, and phase-locking of voltage controlled oscillator (VCO) are presented. The optical approach to on-wafer testing of GaAs microwave/millimeter-wave monolithic integrated circuits (MMICs) with built-in test structures has been demonstrated at Ku-band. A MMIC VCO has also been successfully phased-locked to a picosecond optical source based on GaAs optical/microwave intermixing effect. These accomplishments have resulted in the potential applications of these techniques to MMIC manufacturing and phased-array system applications.

The simultaneous effect o pulse-modulated microwave and optical infrared radiations on a laYer of crYstalline silicon Placed in a waveguide is investigated. The influence of these radiations through changing the mobility of free charge carriers and altering the concentration of photocarriers is studied, The experimental method for investigating the processes of photocarrier diffusion caused bY laser stimulation of microwave nonstationary absorption in a semiconductor is discussed. Processes which take Place in semiconductor under the influence of a strong impulse electromagnetic field are taken into consideration, for instance, laser infrared and microwave radiation mutual modulation, phYsical properties of a semiconductor dependance on temperature, the influence of electrical component of microwave radiation under conductivity of a semiconductor1. Besides these traditionally investigated processes, the spatial-nonuniform distribution of excessive charge carriers in a semiconductor is discussed. In recent Years high power levels of microwave radiation have been achieved using generators of various types including those on relativistic electron beams operating both in the impulse and continuous modes2. This paper Presents a studY of method of electromagnetic radiation diagnostic based on the jnteraction of powerful Pulse-modulated microwave and laser infrared radiations with a semiconductor.

An efficient implementation of the harmonic balance method, using novel numerical algorithms, that are both robust and efficient, coupled with analytical expressions developed for the elements of the Jacobian matrix, is presented. The approach possesses excellent convergence property and speed. Simulated performances using the developed approach for a 10-GHz GaAs MESFET amplifier are found in good agreement with the measured results. Developed techniques should be very useful for analyzing nonlinear microwave as well as optical components.

The design of an integrated monolithic microwave and photonic frozen-wave generator is described. The photonic circuit consist mainly of a number of semiconductor photoconductive switches, actuated by picosecond laser pulses. A pig-tail fiber optic cable bonded to the switches helps to direct the laser pulses to their desired locations. Each switch can hold 10kV, with an on-resistance of less than 1ohm and has a rise time of 50 picoseconds. The switches are designed as ZnS/CdTe double heterostructure photoconductive devices on a semi-insulating CdTe substrate. The microwave circuit components are also designed on the same substrate thereby allowing for monolithic integration of the microwave and photonic circuits. The microwave subsystem uses mainly lumped components and it is designed to support the propagation of transverse electromagnetic (TEM) waves at 20GHz.

The frequency response of integrated optic modulators with traveling wave electrodes is limited by the phase velocity mismatch between the optical and RF signals. At millimeter wave frequencies the short electrodes that must be used to avoid this phase mismatch severely reduce the modulator sensitivity. One approach that provides high frequency modulators with increased sensitivity is the use of periodic intermittent interaction (PII) electrodes and enhanced periodic phase reversal (EPPR) electrodes. In this paper, the RF signal on the electrode is represented as a summation of traveling wave space harmonics. The electrode geometry is adjusted so that the phase velocity of the dominant space harmonic equals the phase velocity of the optical signal at the design frequency. General velocity-matching conditions are derived for both the PII and the EPPR modulators; the bandpass frequency response is then calculated by adding the individual responses of all space harmonics traveling down the electrode. Design examples are provided of periodic electrodes for LiNbO3 and GaAs modulators for operation at 35 GHz. Finally, a practical method for velocity-matching is described that requires only the two-port characterization of an individual electrode unit section.

Optical modulators based on electrooptically-induced lateral mode interference in multimode AlGaAs/GaAs waveguides at lambda = 1.32 micron are investigated for high-frequency linear applications. The key design issue is achieving higher slope efficiency while maintaining significant linearity improvements over sinusoidal interference-based modulator designs. We report experimental results which demonstrate indirectly a reduction in intermodulation distortion of 48 dB compared to an ideal Mach-Zehnder interferometer.

Integrated optical time-shift networks consisting of cascaded pairs of 2 x 2 linear electrooptic switches and optical delay lines in GaAs waveguides at 1.32 micron are investigated for true-time optical beam forming in phased array antennas. We report new state-of-the-art results in curved waveguide and corner bend insertion loss, and preliminary results from 2-bit time delay generators (TDGs) constructed in the form of GaAs-based photonic integrated circuits utilizing these components. These results represent significant progress in our longer-term goal of demonstrating a 7-bit TDG with a loss matching monolithic microwave integrated circuit (MMIC) delay line techniques, while providing very wide bandwidth unmatched by MMIC technology.

Special device modeling, design and layout, and precision processing controls were employed to fabricate new balanced-bridge 2x2 and 4x4 switches on X-cut, Y-propagation LiNbO3 substrate using Ti indiffused optical waveguides. The best of these devices achieved extinction ratio and crosstalk isolation of better than 93 dB electrically (46.5 dB optically). The new switches demonstrate good reproducibility with electrical crosstalk less than -80 dB.

New field-effect electro-optic modulators (EOM) have been investigated and demonstrated. The new EOM has inherently attractive characteristics of broad spectral bandwidth, insensitivity of polarization, controllable waveguide profile, high power-handling capability, low loss, and a simplified fabrication process which can lead to mass producibility and low cost. A LiNbO3 device has experimentally demonstrated an extinction ratio of 25 dB.

By introducing strain into semiconductor lasers using GaAs/Ga_(1-x)In_(x)As quantum wells, their modulation bandwidth has been increased to beyond 20 GHz. Our approach to high modulation bandwidth strained layer quantum well lasers has been to fabricate short cavity (less than l00-micron) multiple quantum well (MQW) structures. In order to fabricate lasers of this length, the facets must be etched by chemically assisted ion beam etching (CAIBE) rather than cleaving. In such short cavity MQW lasers fabricated by CAIBE using two different layer structures, a 3-dB modulation bandwidth of 28 GHz is obtainable under CW conditions. The current study investigates (1) the growth of different quantum well and barrier materials and their effects on device performance, (2) the dependence of differential gain and damping on quantum well width, depth, and number, and (3) the relationship between threshold current and high speed performance. Design criteria for optimizing high-speed performance are presented.

InGaAs/InP multiple quantum well electroabsorption modulators grown by MOCVD are designed and fabricated for 1.5 micrometers wavelength operation. For rib loaded waveguide modulators fabricated on n-InP substrate with a 3 micrometers rib width and a 2 micrometers intrinsic waveguide layer, a capacitance of 0.2 to 0.3 pF and a reverse breakdown voltage > 20 V are obtained. The extinction ratio of the modulators is more than 14 dB and the 3 dB optical bandwidth is 18 GHz. The modulator's RF efficiency and optical insertion loss still need to be improved. For modulators made on semi-insulating InP substrate, a capacitance in the range 0.1 to 0.2 pF is measured.

This presentation deals with MSM photodetectors and post-detection amplifiers capable of responding at millimeter-wave frequencies. Monolithic integration, incorporating pseudomorphic MODFETs (SMODFETs) as the amplifiers, is used with coplanar waveguide resonant impedance transformers. Interdigitated Schottky barrier metal fingers, with submicron spacing and unique Bragg reflector layers to maximize light absorption, are used for the photodetector. For a 0.5-micron gap detector, a measurement shows a flat frequency response at least up to 40 GHz. Quantum efficiency is expected to be more than 60 percent for a 0.75 micron gap and 0.25 micron finger detector with 0.375 micron absorption layer.

Progress will be presented regarding circuit elements to be eventually incorporated into a Photonic Integrated Circuit (PIC). Data from elements such as phase modulators and couplers will be reviewed. They have been developed employing ridge waveguides in MBE grown GaAs/AlGaAs material structures. A general approach to PICs will be discussed as well as their incorporation into optical frequency translation (OFT) modulation circuits for coherent optical systems. Detailed theoretical discussion of one, two, and four-arm phase-modulator-based OFT circuits is included.

The frequency detuning-linewidth characteristics of the mode-locked frequency of a monolithic, passively mode-locked laser at millimeter wave frequency have been investigated. By changing the applied biases to the gain and absorber sections, a tuning range of about 20 MHz was observed at a mode-locked frequency of 47.61 GHz, while keeping the mode-locked linewidth to below 20MHz. The gain current variation needed for this tuning range is about 20 mA, while the absorber bias voltage affects the tuning only weakly.

This paper describes laser measurement and modelling techniques pursuant to developing impedance matching networks for bandwidths over an octave. Models presented include a novel large signal model that has a measurement based nonlinear photon resistance term to induce curvature into the L-I characteristic of the laser. It is found that predicted intermodulation performance varied about 1 dB between the linear and nonlinear photon resistance models implying the dominate nonlinearity is due to the I-V characteristic of the laser. Finally, the models are used with microwave design tools to synthesize and optimize a Chebyshev impedance matching network employing a Norton equivalent transformer. The impedance matching circuit results in a simulated 10 dB improvement over resistive matching with a VSWR of less than 1.5 over the band of interest.

Vertical cavity surface emitting lasers (VCSELs) with a low threshold current and a large modulation bandwidth are desirable for applications in digital and analog optoelectronic signal processing and optical interconnection. These lasers emit light perpendicular to the surface of the semiconductor wafer and allow processing and optoelectronic integration using planar techniques. They also have a short optical cavity and small active volume that promise high speed and low threshold current operation. In this paper we describe the high frequency response characteristics of low threshold current surface emitting lasers we have developed. The laser is a GaAs vertical cavity surface emitting laser with a single quantum well active region and a short Bragg reflector cavity. We describe the key dc operation characteristics of the laser, such as threshold current, device resistance, and differential quantum efficiency that are important to laser high speed operation. The investigation of the dynamic response of the lasers under various direct current modulation conditions is discussed, and experimental results on modulation response and second harmonic distortion are presented. The limitations on high speed response and the effect of parasitic impedance on dynamic response of the laser are described.

A novel GaAs/GaAlAs quantum well optoelectronic switching device, which exhibits an s-type negative differential resistance at room temperature and emits high performance stimulated emission, was demonstrated. The device structure is similar to the conventional separate confinement heterojunction quantum well laser, and thus can be easily integrated with state-of-the-art optoelectronic integrated circuits. The devices can be switched optically and/or electrically. Threshold current densities of 1.2 kA/sq cm, differential quantum efficiencies as high as 67 percent (0.5 W/A slope efficiency per facet) and output power in excess of 50 mW per facet were obtained.

The design, fabrication, and performance characteristics of passive waveguides and micromechanical switches for use as time delays for phased-array antennas are discussed. The approach considered here is to fabricate the optical waveguides and switches on silicon substrates using VLSI-compatible technology. This approach permits the use of a single off-chip emitter and detector to perform RF-optical and optical-RF conversion for each delay network. It also allows precise lithographic definition of the shorter delay waveguides. Micromechanical waveguide routing switches of two types designed to perform the path selection functions are considered.

A monolithic optical time-shift network is described which is designed to steer a dual-band microwave phased array antenna at 2 and 10 GHz. The advantages of using cascade type network architectures to achieve the desired resolution are demonstrated. The implementation of different delay times in this optical time-shifter via bias control of detectors integrated monolithically on a GaAs wafer is described.

The acousto-optic null steering processor (AONSP) implementation of an acousto-optic adaptive processor (AOAP), a specific implementation of the least mean square algorithm in a correlation canceler loop configuration for radar sidelobe jammer rejection, is described. The expected performance of a jamming canceler processor is shown to depend on specific radar and jammer characteristics and on the performance limits of the implemented algorithm. The current status of the AONSP program is summarized.

This paper reports on the continuation of work on photorefractive correlation and tapped delay line structures for the implementation of adaptive signal processors. A six channel implementation of a sidelobe canceling system will be described. The laboratory model is approximately 30 cm in length. It uses indium phosphide as the photorefractive material and a six channel tellurium dioxide Bragg cell delay line. A time integrating correlator uses a 1.2 micron 'write' laser. The correlation coefficients are applied to tap the delay line with a 1.3 micron read laser. A combination of polarization and wavelength multiplexing are used to obtain greater than 60 dB read/write isolation. The system is designed to achieve greater than 40 dB of suppression of CW interference over a 20 MHz band centered at 70 MHz. The adaptation time for pulsed CW interferers is designed to achieve greater than 30 dB suppression within 100 microseconds.

An optical outer product architecture is presented which performs residue arithmetic operations via position-coded look-up tables. The architecture can implement arbitrary integer- valued functions of two independent variables in a single gate delay. The outer product configuration possesses spatial complexity (gate count) which grows linearly with the size of the modulus, and therefore with the system dynamic range, in contrast to traditional residue look-up tables which have quadratic growth in spatial complexity. The use of linear arrays of sources and modulators leads to power requirements that also grew linearly with the size of the modulus. Design and demonstration of a proof-of-concept experiment are also presented.

Two techniques developed during recent years have significantly enhanced the capabilities of residue-number-system (RNS) based processors to meet the demanding requirements of large- order systems. First, a method of factoring the prime modular subprocessor allows a major reduction in the componentry required to implement a look-up-table (LUT) based processor. Secondly, the use of CORE functions permits a drastic reduction in the number and magnitude of prime moduli required when the processor is applied in the solution of problems with many degrees of freedom. This paper will discuss the use of RNS-based processors in the solution of adaptive antenna problems, the basic drawbacks and the manner in which factored LUTs and CORE functions circumvent these drawbacks.

An optical approach to the implementation of RF synthesis is described which utilizes the single-sideband feature of acousto-optic modulation to achieve synthesis via the mix-and-divide approach without the use of filters. Alternative approaches to reference signal switching are examined, and their relative advantages evaluated. Preliminary experimental results are presented which demonstrate that the required signal purity and spurious signal suppression can be achieved in the mixing and detection of two AO-modulated beams.

Aperture amplitude and phase distributions in a planar array antenna with a beam forming network using Fourier optics and photomixing have been successfully measured. In this measurement, we used two laser diode pumped Nd:YAG lasers with oscillating wavelength of 1.3 micron region and obtained an L-band microwave signal by optical heterodyne detection. We estimated beam scanning characteristics by shifting transversally the position of the pinhole image mask. Furthermore, the diameters of the pinhole image mask were changed and the far field patterns were evaluated on the basis of the measured microwave aperture amplitude and phase distributions. As a result, the beamwidth of the far field pattern was proportional to the aperture diameter of the image mask.

Digital communication systems use complex modulation schemes that require powerful signal- processing techniques to correct chemical distortions and to minimize bit error rates. In this paper, acousto-optic processors are used, in conjunction with adaptive digital equalizers, to reduce the bit error rate of a microwave digital radio receiver. The acousto-optic processor implements an inverse channel filter that is rapidly adaptable to time varying distortions. A specific architecture is identified and a laboratory system is tested to very the ability of the processor to track and correct time-varying channels. Computer simulations are used to show that the hybrid acousto-optic and digital equalizer allows a four-fold increase in the modulation capacity of radio, relative to all digital equalization, while improving the bit error rate performance.

A novel in-line additive acoustooptic architecture for planar phased arrays is described for two dimensional beam scanning. A concept for an acoustooptic processor is demonstrated in the laboratory, and the required modulo-2 pi phase control is achieved using two control signals. Up to Ku-band operation is possible with this processor.

The benefits and limitations of photonic interconnections in solid state phased arrays are discussed. Direct and indirect modulated links are analyzed with respect to SNR, intermodulation distortion, noise figure, and clutter spreading due to nonlinearities. An advanced radar architecture having a number of fiber optic links is described.

We propose and experimentally demonstrate a two-dimensional optical architecture, based on polarization switching by N spatial light modulators of p x p pixels, which provides 2 exp N-1 time delays values and an analog control of the 0- to 2-pi phase for each of the p x p microwave signals actuating an active antenna. These signals originate from the coherent detection of a dual frequency laser beam obtained with an acoustooptic frequency shifter. Experimental results obtained at a microwave frequency of 1.8 GHz are presented.

The possibilities for using photonic technology to enhance the performance of phased array systems are examined. The principal advantage of photonics is that a photonic array antenna can be partitioned in two halves, the beam-forming manifold consisting of photonic components and the antenna front end consisting of RF components only. The two parts are linked by optical fibers, with the transmitter, receiver, and the beam steering unit conveniently located at a remote site. Several possible architectures of such a system are discussed. It is estimated that, for array applications, a system noise figure of less than 5 dB with a dynamic range of about 60 dB can be achieved with state-of-the-art photonic components and technology.

Coherent optical analog links offer several important advantages over conventional direct detection links including the ability to separate wavelength-division multiplexed signals, frequency translation, and utilization of angle modulation formats. However, the wide linewidth of semiconductor lasers can cause substantial performance degradation of these links. This paper analyzes the signal-to-noise ratio and the dynamic range of amplitude modulated (AM) and frequency modulated (FM) coherent links, and compares them to direct detection links. It is concluded that a properly designed AM system is insensitive to laser linewidth. For optical powers less than 1mW, the performance of coherent AM links is better than that of direct detection links; for optical powers greater than 1mW, the performance of the two links is nearly the same. Coherent FM links have the potential to increase the SNR and the dynamic range by more than 10 dB as compared to direct detection and coherent AM links. However, FM links are potentially sensitive to laser linewidth, and require elaborate phase noise cancellation techniques when semiconductor lasers are used.

A high linearity, high dynamic range analog fiber optic link is described which allows high fidelity distortion-free communications transmission from 2 to 500 MHz and provides an alternative to conventional coaxial cables used to remote RF receivers from their antennas. All signals within four frequency bands (2-30, 30-90, 90-180, and 180-500 MHz) and within specified voltage ranges are detected and transmitted via fiber optics. This function is performed by a system of four separate analog fiber-optic links, one for each of the four bands. The discussion covers the electro-optic, mechanical, and thermal design of the fiber optic link, the performance model, reliability analysis, and performance tests.

A design analysis is presented for a microwave frequency fiber optic link with minimum figure and large dynamic range for transmission of a satellite downlink signal. The optic link components are briefly characterized, with attention given to optic sources, modulators, optical fiber, and optical detector. The principal parameters of the optic link are derived. These parameters are then cascaded with the parameters of the electrical components (preamplifier and postamplifier) to derive the noise figure and dynamic range of the complete communication system. A schematic diagram of the optic link is included.

We have fabricated a programmable fiber optic delay line operating in the 1550 nm band capable of providing a total delay of 1 ms in increments of 83.6 ns for RF signals up to 2.5 GHz. Electromechanical switches with 6 ms switching time are employed for programming the delay line. To overcome the switching speed limitation, we used a dual parallel delay line. Signal selection is via a high speed RF switch. For radar test application, the approach enables continuous tracking (to within a range cell of 42 ft) of a moving target with negligible loss of data. Three erbium-doped fiber optical amplifiers excited by a single 1480 nm pump laser are used to compensate for losses in each of the delay lines.

A 1.3-micron wavelength optical transmitter system with a 2-GHz bandwidth centered at 20 GHz has been implemented by actively mode-locking a semiconductor laser diode and then modulating the signal externally. The performance of a link using this transmitter over a 2-km fiber is evaluated. A hybrid package is developed for a mode-locked laser submodule operating at 15 GHz.

An optical time delay network (OTDN) for time delay steering of phased arrays of various sizes is projected. The approach involves integrated optic switches and sections of optical waveguide for time delays. An example phased array radar is defined. The impact of time delay network performance parameters such as insertion loss, network switch isolation and triple transit effects on radar system performance such as noise figure, dynamic range, phase and amplitude linearity and sidelobe levels is analyzed. Performance levels needed within the optical time delay network to meet projected radar system requirements are identified.

We describe three real-time optical processing systems, for imaging radar, that form the array phase function and produce the radar image from the incoming signals. The first is an implementation of the Steinberg algorithm where we have modified the data format by frequency multiplexing the range information to allow for the data from all range bins to be present simultaneously. The second implementation uses this same input format, but provides hysteresis on the point scatterer used to calibrate the array. This allows for moving radar arrays where the point used to calibrate the array may be time varying. Finally we describe a self oscillating phase conjugate resonator, with a coherent optical to RF converter inside the cavity. This is a hybrid optical-microwave oscillator that adapts to the incoming signal.

A new architecture is proposed for the optical beam forming in Phased Array Active Antennas (PAAA) in which the integrated optical technologies allow to realize a simple and effective coherent optical processor. The system proposed here combines the advantages of the microoptics systems (all optical processing) with the ruggedness and small size of the integrated optical circuits. These results are obtained by the use of optical multifunctional circuits integrated on LiNbO₃ performing all the necessary processing. The resulting network consists of a single integrated optical chip for each antenna row and the optical fibers for the distribution of the signals (frequency, phase) to the active radiating elements. The system architecture will be discussed and technological results in the fabrication of the integrated optical circuit will be presented.

Based on previous experience with air deployable fiber-optic systems, a ruggedized single- fiber cable has been developed to meet new requirements presented by such applications as robotics and radar remoting. Both multimode and single-mode versions of the cable were developed. They are terminated with a militarized connector whose design built upon previous commercial developments. To minimize the size of the termination, an adaptation of the commercial ST^TM connector was inserted into a protective housing that provides mechanical and environmental performance stability dictated by system designers. In addition, the challenging requirement of hermaphrodicity was attained. Assemblies were fully tested in accordance with an approved test plan.

A development effort aimed at the design of single mode fiber optic cables capable of withstanding the radiation and environmental effects of a space deployment is described. The program is structured in two parallel phases. The first addresses the fiber and related material optimization. The second phase emphasizes the cable design and related material to ensure a twelve-year uninterrupted service. The proposed radiation hard fiber contains H2, with the concentration of H2 optimized for enhanced radiation hardness during the expected service life of the fiber. The optical fiber is packaged in an all dielectric cable structure incorporating materials which ensure fiber stability at temperatures from -150 to +150 C.

Wide aperture arrays for transmitting and receiving wideband signals suffer beam broadening when steered away from array normal with phase delays between the array elements. This beam dispersion is eliminated if the signal is time delayed between antenna array elements. This paper describes an acoustooptic signal processing architecture that generates and controls, with the frequency of a single signal, a set of the time delays required for linear antenna array beam forming. This acousto-optic architecture allows an array to scan a beam rapidly over a continuum of angles. Results of analysis are presented showing that a narrow beam can be formed and maintained as it is scanned over a wide range of angles.

We describe the extension of the 1-D hardware-compressive wavelength-multiplexed (lambda-MUX) true time delay (TTD) steering architecture described earlier to 2-D array or subarray antennas, and we present a technique for achieving multiple beams at different angles and frequencies. In addition to its enormous hardware saving, this 2-D architecture is reversible (i.e., the same hardware can be used for both transmit and receive modes), and requires very simple digital control. The limitation of the size of the lambda-MUX TTD system imposed by the finite wavelength spacing is also discussed.

Results of an experimental investigation of the dynamic range of the photorefractive effect in several ferroelectric crystals are described in the context of adaptive filtering applications. The results indicate a very large (greater than 80 dB) dynamic range for several samples; the dynamic range is shown to be limited by noise processes inherent in the crystals. The large photorefractive dynamic range of the ferroelectric signals investigated should prove useful for signal processing and computing applications.

A preliminary design is presented for an Optical Interface Message Processor (OPTIMP) which exploits the high bandwidth, parallelism, multidimensional capability, and high storage density offered by optics. In the proposed system, the time-consuming operations in parallel processing networks, such as switching and routing, are fully implemented in optics. The design is free from optical/electrical conversion bottlenecks and can perform switching and routing in the Gigabits/s range. The design can be adapted for interconnection networks for massively parallel computers.

A parallel optical technique for use in the adaptive processing of radar signals is described. We have demonstrated such a technique in an acousto-optic adaptive signal processing system designed to null the jamming of radar signals. This system features a time integrating correlator using a new type of photorefractive spatial light modulator exhibiting high levels of contrast, resolution, uniformity and sensitivity. Using this system we have demonstrated nulling of both narrow and wide band RF signals. The operation of the photorefractive integrating device, its performance and that of the overall system are discussed.

A photonic in-phase/quadrature (I-Q) RF phase shifter utilizing the signals from two integrated optic Mach-Zehnder modulators has been tested. This system demonstrates the capability of 360-deg phase control, less than 1-deg RF errors at constant amplitude, and less than 0.5-degree errors over a 30 dB RF amplitude taper. Using this technique, a four-channel beam forming network was also developed and integrated with a printed patch, lightweight phased array antenna. High accuracy linear beam control was achieved. Null patterns of more than 30 dB were produced on boresight. The null was steered to greater than 30-deg from boresight with a linear accuracy of about 1 deg.

Optoelectronic techniques offer major benefits in the construction of space-borne phased arrays with stringent performance requirements, not only in signal distribution functions but also in the beam forming processing itself. In the course of a current project for European Space Agency we have designed the constructed proof of concept hardware to demonstrate the use of coherent optical techniques in the formation of antenna beams. The scenario at the basis of our study is for a telecommunication satellite with a 160-element phased array antenna, forming 15 beams, each of which can carry up to 20 QPSK modulated carriers, with up to 6 operating simultaneously. Operation is in the 12/14 GHz bands for downlink and uplink operation respectively. In our beamforming concept we have employed an optical heterodyne approach in which the phase shift and amplitude required for beamforming is imposed upon an optical carrier, the RF waveform required for transmission being derived subsequently by mixing with an optical local oscillator signal. Phase encoding for data modulation is also carried out in the optical domain.