Recent advances in high speed semiconductor lasers[l] have made possible the use of optical transport and control in microwave systems such as phased arrayed radar[21, various microwave subcarrier multiplexed networks scheines[3J as well as cable television distribution[4). For a typical laser with a differential efficiency of 0.4mW/mA and emitting xmw of CW power, a microwave power of (-8+2Olog10x)dBm is required in a 50k) system for full optical modulation. This is much higher than typical received signal powers from microwave antennae, and considerable cost is incurred in providing the necessary amplification at the undersirable location of an antenna horn.

For the first time, the rising low frequency relative intensity noise characteristic of semiconductor lasers is explained. Using a multi-mode rate equation analysis, it is shown that the magnitude and shape of the low frequency relative intensity noise is strongly dependent on the values of the differential carrier lifetime at threshold and other laser's parameters. It is further shown that a single mode laser will also exhibit enhanced low-frequency noise unless the side-mode suppression ratio is very high. The rate of spontaneous emission into the guided modes, the nonlinear gain coefficient and the carrier lifetime at lasing threshold are measured using an optical modulation technique. These results are used for understanding the noise and modulation properties of diode lasers.

Reconfigurable beam-forming networks may be implemented successfully with optical technologies. To exploit the advantages fully the properties of optics have to be considered in the design of the beam-forming network. The benefit may be significant when the proper technique and technology is selected. An example is given for a multi-beam application of a phased-array antenna for an ISDN network in the sky.

The required output power of a laser source and suppression of antenna sidelobe level are discussed for an optically controlled array antenna in which microwave amplitude and phase distributions are generated and controlled using optical Fourier transform and heterodyne/homodyne detection. A heterodyne detection model using two laser diodes, the frequency difference of which is set to desired microwave frequency, is compared with an original homodyne detection model, using an external frequency modulator. The heterodyne detection model may be favorable from the viewpoint of available laser diode output power. For example, the required laser output power in the heterodyne detection model can be about 10-15 dB less than that in the homodyne detection model in order to obtain the same C/N0 value of microwave signal. It is also shown that the antenna sidelobe level can be suppressed by about 20 dB without changing beamwidth by using a reference beam with a tapered distribution.

An electrooptic phase modulator which can be used to provide tapered amplitude and phase to drive phased array antenna is proposed. The beam steering can be achieved by modulating it with one signal. Computer aided design and layout tools have been developed to facilitate the development of waveguide modulators. Low-loss waveguide structures have been found using simulation programs.

The performance of the high speed InGaAsP ridge waveguide lasers and the GaAlAs single-mode lasers operating as a micromixer have been investigated by using a circuit model derived from the iterative method and device rate equations. The laser mixer has many advantages over the conventional microwave mixer, including: high conversion gain, wide dynamic range, low noise figure and high local oscillator/intermediate frequency (LO/IF) isolation. The analytic results also indicate that the heterodyne microwave fiber optic link using a laser mixer provides the advantages of system simplicity, cost-effectiveness and improved system signal-to-noise ratio (SNR) than the conventional microwave fiber optic links. System SNR of two fiber optic links were measured: a 12 GHz link with 70 MHz IF has a SNR of better than 120 dB/Hz and a 26.5 GHz up-converted link has a SNR of 100 dB/Hz, without laser diode RF impedance matching. Other system applications of the laser mixer are also presented.

Designs for traveling wave photodetectors in semiconductor materials are presented, and advantages over conventional photodetectors are discussed.

The performance of the high speed InGaAsP ridge waveguide lasers and the GaAlAs single-mode lasers operating as a micromixer have been investigated by using a circuit model derived from the iterative method and device rate equations. The laser mixer has many advantages over the conventional microwave mixer, including: high conversion gain, wide dynamic range, low noise figure and high local oscillator/intermediate frequency (LO/IF) isolation. The analytic results also indicate that the heterodyne microwave fiber optic link using a laser mixer provides the advantages of system simplicity, cost-effectiveness and improved system signal-to-noise ratio (SNR) than the conventional microwave fiber optic links. System SNR of two fiber optic links were measured: a 12 GHz link with 70 MHz IF has a SNR of better than 120 dB/Hz and a 26.5 GHz up-converted link has a SNR of 100 dB/Hz, without laser diode RF impedance matching. Other system applications of the laser mixer are also presented.

The Ge3As12Se55 glass is proven to be an excellent acousto—optic material for low power applications. High diffraction efficiency, low acoustic attenuation, low optical insertion loss, and isotropic properties make it an ideal candidate for signal processing and fiber—optic applications.

A high-quality microwave signal is generated by heterodyning two diode-laser-pumped Nd:YAG lasers. A III-V semiconductor optical waveguide containing a doping superlattice is used to manipulate the phase and amplitude of one of the laser outputs before they are mixed. This manipulation appears directly as a corresponding change in the phase and amplitude of the heterodyne microwave signal. Results are presented from near dc to 52 GHz. Phase changes as large as 8 pi, and amplitude changes as large as 42 dB have been induced by means of a 1.2-mm-long optical waveguide and less than 3 V of control voltage.

A lithium niobate integrated-optics power-splitter waveguide with electro-optic phase shifters is used in a heterodyne interferometer with a laser diode source emitting at 830 nm. An acousto-optic Bragg cell is used for frequency shifting. Multichannel 100-MHz signals with individually controlled phase information are generated by mixing the optically phase-shifted reference beams and frequency-shifted Bragg cell signal in fiber-optic receivers.

A millimeter wave integrated lithium niobate modulator, consisting of a titanium diffused optical waveguide Mach-Zender interferometer and a traveling wave coplanar waveguide electrode with periodic series stubs, is analyzed through the application of Floquet's theorem. First, a design equation for the modulator is derived by expanding the RF signal along the optical waveguide into space harmonics and then matching the velocity of the dominant space harmonic to the velocity of the optical signal. Then, the frequency response of the modulator is found by integrating, over the modulator's length, the local optical phase shifts that are electro-optically induced by all of the RF space harmonics. Finally, it is shown how the concepts developed here for an ideal (no reflections) periodic structure can be applied to the experimental determination of the modulator response by characterizing isolated unit sections of a real electrode. This approach has the added advantage of facilitating RF inmpedance matching to the modulator.

Microwave phased-array antennas using fiber optic technology demand high dynamic range (DR) electro-optic modulators (EOM) to enhance system performance. A wide DR EOM must have high sensitivity, low nonlinearity and an optimized extinction ratio. This paper describes some techniques to improve of EOM sensitivity and extinction ratio.

A vertical-cavity surface-emitting laser diode capable of narrow-line emission at injection currents below 1 mA during room temperature dc operation is described. It is estimated that surface-emitting laser diodes can potentially be modulated to speeds as high as 30 GHz. It is argued that phased array operation at high output powers should be possible, but that problems with thermal performance, wafer uniformity, and circuit layout need to be resolved.

A fiber-optic delay network consisting of eight different and selectable fiber delays has been demonstrated over a 1 to 11 GHz frequency range. The delay networks can be used as a three-bit resolution beam steerer for an electronically steered antenna. Discrete delay increments were selected by switching the bias current of high speed 1.3-micron laser diodes pigtailed to the delay lines.

The results of investigations of scaling operations in the opticallyimplemented adaptive processor are presented. It is shown that blind scaling can be employed to reduce the necessary numerical range of the system but that protection again pathological situations diminishes its allowable extent. Extended-accuracy studies have shown that much greater reduction of range could result. The applicability of algebraic integers to achievement of extended accuracy in RNS computations is investigated and promising results are reported.

The problem of determining complex weights appropriate for adaptive beam nulling is discussed in the setting of a residue number system. Algorithms available for residue number system processing that allow scaling and are extendable to large order systems (i.e., as large as 64) include Gauss elimination, the modified Cholesky method and the Gram-Schmidt method. The most appropriate of these from the standpoint of complexity is the modified Cholesky method.

An analog optical signal processor has been developed to implement the function of antenna sidelobe interference cancellation. The architecture, which includes optical correlators with Bragg cell input, an optical time modulator, and an optical spatial integrator, performs an operation which is equivalent to the multiple-loop Howells-Applebaum Least Mean Square (LMS) algorithm. This implementation offers the potential for cancelling multiple correlated interference signals (i.e., direct path and multipath interference signals) with an appropriate number of auxiliary antennas. A previous report described single frequency CW tests of such a processor. Preliminary tests with wide-band waveforms and the special test instrumentation developed to perform these tests are described in this paper.

An analog optical signal processor has been developed to implement the function of antenna sidelobe interference cancellation. The architecture, which includes optical correlators with Bragg cell input, an optical time modulator, and an optical spatial integrator, performs an operation which is equivalent to the multiple-loop Howells-Applebaum Least Mean Square (LMS) algorithm. This implementation offers the potential for cancelling multiple correlated interference signals (i.e. direct path and multipath interference signals) with an appropriate number of auxiliary antennas. A previous report described single frequency CW tests of such a processor. Preliminary tests with wide-band waveforms and the special test instrumentation developed to perform these tests are described in this paper.

An optical processor for phased array antenna beam forming has been demonstrated. It is capable of providing real-time control of both antenna beam steering and beam shaping in one or two dimensions, with an RMS phase linearity of five bits. The functions of beam shaping and steering are combined through a magnetooptic spatial light modulator (SLM) which is fast, small, and simple to use.

Butler matrices, consisting of 3 dB directional couplers and phase shifters, can be used to obtain specific phase relationships between the signals on the feedlines of the various antenna elements in a phased array system. Microwave Butler matrices consisting of microwave phase shifters and directional couplers are difficult to fabricate because of crosstalk and EMI. Integrated optical Butler matrices, in addition to being immune to EMI and having negligible cross talk, are small in size, lightweight and can be fabricated on a single substrate. Some of the advantages of using an integrated optical Butler matrix for beam steering and detection in a phased array system are discussed. Integrated optical directional couplers are fabricated on LiNbO3 substrates by a proton exchange and thermal annealing technique. A method of obtaining accurate 3 dB directional couplers is presented. Statistics based on the experimental measurements of these directional couplers are used to demonstrate the relative insensitivity of the Butler matrix performance to variations in fabrication parameters. The S/N and dynamic range of a sysem employing a Butler matrix with an optical local oscillator are calculated and compared to present-day phased-array antenna systems.

A simple, robust architecture is proposed for adaptive antenna array processing using an acousto-optic Bragg cell array and a photorefractive material in a time integrating correlator configuration. A Bragg cell array is used for computing correlation coefficients and for forming the tapped delay line filter. The resulting processor is very compact since it can be implemented as a monolithic structure by using the same material for the acousto-optic interaction and the photorefractive implementation of adaptive tap weights. Equations are derived for expected dynamic range, and time response.

Novel concepts using nonlinear optical techniques to solve some accuracy problems in optical beamforming networks for phased arrays are presented.

The true time-delay behavior of a fixed-scan fiber-optic beamforming network is analyzed using the microwave scattering parameters of the fiber-optic components within the network. Pseudooptical domain-scattering parameters are measured with respect to the microwave modulation of the optical carrier for signals in various paths of the optical beamforming network. The measurement technique is described, which uses a vector network analyzer outfitted with an optical test set, and results show microwave reciprocity and RF phase linearity over the measured frequency range from 2.0 to 3.0 GHz. Optical standing wave ratio and isolation between delay lines are also determined. The electrical scattering parameters of the beamformer were measured, and far-field radiation patterns measured in an anechoic chamber demonstrate ideal true-time delay behavior, with nonsquinted main beam radiation over a 50 percent instantaneous bandwidth at S-band.

This paper presents functional architectures providing transmission link, and beamforming for multiple element microwave antennas. System performance parameters including dynamic range and bandwidth are considered from the standpoint of electrooptical component performance. Measured performance of direct modulated semiconductor laser sources, link subsystems, and a three-bit fiber-optic time-delay network are presented.

The measured bit-error-rate (BER) performance of a fiber optic data link to be used in satellite communications systems is presented and discussed. In the testing, the link was measured for its ability to carry high burst rate, serial-minimum shift keyed (SMSK) digital data similar to those used in actual space communications systems. The fiber optic data link, as part of a dual-segment injection-locked RF fiber optic link system, offers a means to distribute these signals to the many radiating elements of a phased array antenna. Test procedures, experimental arrangements, and test results are presented.

Phased array antennas long were investigated to support the agile, multibeam radiating apertures with rapid reconfigurability needs of radar and communications. With the development of the Monolithic Microwave Integrated Circuit (MMIC), phased array antennas having the stated characteristics are becoming realizable. However, at K-band frequencies (20 to 40 GHz) and higher, the problem of controlling the MMICs using conventional techniques either severely limits the array size or becomes insurmountable due to the close spacing of the radiating elements necessary to achieve the desired antenna performance. Investigations were made that indicate using fiber optics as a transmission line for control information for the MMICs provides a potential solution. By adding an optical interface circuit to pre-existing MMIC designs, it is possible to take advantage of the small size, lightweight, mechanical flexibility and RFI/EMI resistant characteristics of fiber optics to distribute MMIC control signals. The architecture, circuit development, testing and integration of optically controlled K-band MMIC phased array antennas are described.

Null shifting over a 1 GHz bandwidth is demonstrated using a fiber-optic and integrated-optic transversal filter. The null depth for this system is greater than 70 dB, measured with a 10-KHz IF bandwidth. The null shifting is achieved by varying the tap weight values. Null shifting in the response of a transversal filter is directly related to null steering for a radar direction finder. The fixed delays consist of optical fibers cut at 10-cm increments. Integrated optical 2 x 2 couplers are used as the tap weights. The weighting is controlled by an applied voltage. The depth of the null is limited by the dynamic range of the source/detector combination. Ordinary fixed weight fiber-optic transversal filters are subject to dynamic range and bandwidth limitations due to errors in cutting the fiber lengths. This problem can be remedied by using variable taps to shift the null frequency. The null depth and resolution is not detrimentally affected by using this approach.

This paper describes a model for noise figure and dynamic range in a fiber-optic link incorporating a synchronous directional coupler (SDC) as an external modulator. These RF parameters are derived in terms of laser power, modulator Vpi, transmission loss, and optical receiver characteristics.

The requirements for true time-steering of phased array antennas are reviewed and the resulting delay line hardware requirements are discussed. Fiber-optic delay-compressive 1-D delay line architectures are then described and quantitatively compared. The basics of array antenna partition are then presented. Based on these principles a delay-compressive and element-compressive 2-D fiber-optic delay line architecture is described and its basics characteristics and expected performance are discussed.

An unconstrained, adaptive, null-steering phased-array optical processor that utilizes a photorefractive crystal to time-integrate the adaptive weights, and null out correlated jammers is described. A passive processor is presented, where it is assumed that it is only known a priori that the signal is broad-band and the jammers are narrow-band. The passive processor computes the angle(s) of arrival of the jammers and extinguishes them. Also presented is an active processor in which the temporal waveform of the desired signal is known, but the look direction is not. The processor computes the angle(s) of arrival of the desired signal and steers the array to look in that direction while nulling any narrow-band jammers. These are two embodiments of a class of processors which use the angular selectivity of volume holograms to form the nulls and look directions in an adaptive phased array radar antenna pattern