An integrated semiconductor interconnect chip is being developed for the distribution of microwave signals to the subarrays and/or radiating elements of a phased array antenna (PAA). The baseline chip design calls for a 1 X 4 distribution fanout, and consists of both waveguide splitters and integrated optical amplifiers to compensate for inherent splitting as well as excess waveguide and coupling losses. Analysis is being performed to evaluate the RF performance of photonic links containing semiconductor optical amplifiers, and to determine the most judicious use of such an interconnect in the photonic feed network of a PAA.

We present a novel concept, the fiber optic Bragg grating true- time-delay (TTD) element, for implementing true time delay in the distribution network of an optically fed phased array antenna. the device utilizes narrowband optical Bragg reflection gratings written holographically into the core of a single-mode fiber at various positions along its length. An optical carrier is modulated by the RF signal of interest and launched into this delay-line fiber. The desired RF time delay may be realized by wavelength-selectable nature of the TTD device offers the possibility for simplified beamsteering control and channel multiplexing.

An optical GaAs/AlGaAs heterojunction bipolar transistor technology has been developed using transparent indium-tin-oxide emitter contacts for unobstructed through-the-top optical access. Optical tuning and injection locking has been observed in 6 GHz microwave oscillators made with this device. Optical tuning ranges up to 25 MHz and locking ranges up to 2.5 MHz have been demonstrated with the optical injection of RF power at 30 dB below the oscillator power level.

High-speed high-performance optical phase modulators are being developed for use in a coherent Photonic Integrated Circuit(PIC) technology. These phase modulators are the critical component of a PIC program at Sandia National Laboratories targeted for microwave/millimeter-wave signal processing and control including phased-array antenna control. The primary design goals for these modulators are amenability for integration into PICs, high figure of merit (FOM) and large bandwidths allowing for operation at MMW frequencies. Depletion-edge-translation optical phase modulators have been selected as the device technology of choice due to their high FOM. These modulators unfortunately suffer from a large terminal capacitance which greatly limits speed. To overcome this problem, a distributed electrode design based on the use of slow-wave coplanar strips has been developed. Device design and measurements are presented in this paper.

The spectral sub-structures of XPM were measured using a high resolution optical spectral analysis system. The observation is in good agreement with the theoretical prediction. The oscillatory behavior of the XPM spectrum is suggested to be used for a high-bit all-optical XPM A/D converter. With the advent of ultrashort laser sources, it has been established that a light pulse can be used to control the properties of its own or another light pulse by using optical nonlinear processes in a material. This important property can be utilized for the future generation of high-speed optical computations and communications. Over the past several years, the operation of ultrafast optical logic gated using picosecond Sagnac Kerr Interferometer switch and phase conjugation gates have been successfully demonstrated. The third order nonlinear process cross-phase-modulation (XPM), in which the phase of a weak signal pulse is modulated by the index change induced by a strong pump pulse to produce the spectral broadening, provides a novel method to modulated the frequency of optical pulses at THz speed. We propose the ultrafast all optical A/D converter using XPM and report on measurements on the fine spectral structures of the XPM spectra of picosecond laser pulses using high resolution grating spectral analysis system. The observed substructure of the oscillatory XPM spectrum is suggested to be used for increasing the accuracy of all optical XPM A/D converters.

Optical heterodyne has been proposed to generate the microwave signals to be used in phased array antenna systems. Optical injection-locking of secondary lasers to distinct FM-sidebands of a current-modulated laser diode has been used to improve the spectral purity of the microwave signal generated by heterodyne. A very narrow linewidth microwave signal superimposed over a Lorentzian shaped noise floor has been obtained. The spectral purity of the produced microwave signal has been characterized by three distinct means: power spectrum, phase fluctuations, and time-domain frequency stability (Allan variance). The power spectrum shows a linewidth smaller than 25 mHz (FWHM) and the Allan standard deviation gives a level 1 X 10^-11 for an averaging time of 1 sec.

A free space coupled intensity modulator is designed at 20GHz. Slot antennas are printed on LINbO₃ substrate and are individually connected to short length of electrodes, intensity modulation occurs by radiation coupling. Measurement of a different pattern of slot antennas are given, it is suggested to couple linear slot antennas in order to increase the bandwidth. Characterization of three designs of intensity modulators are presented.

Metal-semiconductor-metal photo-diodes with interdigitated Schottky barrier fingers have been developed for applications in monolithic optical receiver circuits with the purpose of detecting millimeter wave modulation signals being transmitted via an optical carrier. The devices are planar and incorporated submicron finger spacings and a thin absorption region for speed with a buried stack of tuned Bragg reflectors for enhanced sensitivity at the carrier wavelength. These devices have been integrated with short-gate MODFET amplifiers to form the complete monolithic integrated optical receiver circuit. The circuits comprise of the active devices and coplanar waveguide matching/filter networks for optimal MMW performance. DC bias networks have also been integrated with the circuits.

Abstract not available.

High-frequency and high-responsivity photodetectors which possess a high saturation intensity are needed for high performance analog fiber optic links. We report the results of a high saturation intensity InGaAs/InP PIN waveguide photodetector. At an optical wavelength of 1.3 micrometers , the detector has a responsivity of 0.5 A/W and a flat frequency response from 800 MHz to 20 GHz. With a bias of -4 V, the normalized frequency response of the detector remains unchanged when optical powers up to 10 mW are incident on the detector. An initial linearity measurement is made by measuring the linearity of a fiber optic link which uses the detector with a DC photocurrent of 140 (mu) A as the receiver. The linearity of the link is limited by the modulator of the link and there is no detectable nonlinearity introduced by the detector.

A fully packaged and connectorized 1.32 micrometers InGaAsP electroabsorption modulator for analog fiber optic link applications is described. This Franz-Keldysh effect modulator has a 3-dBe bandwidth exceeding 20 GHz, a fiber-to-fiber optical insertion loss of less than 9 dB, and a high modulation efficiency (equivalent switching voltage less than 10 V). A fiber link spurious free dynamic range of greater than 100 dB in a 1 Hz bandwidth is achievable using this modulator, which makes it attractive for Navy shipboard applications.

Photonic ON/OFF and routing switches can be made using alterations of the modal effective index caused by changing the cladding on a dielectric waveguide. Drumhead-like aluminum membranes are formed electrostatically pulled into contact with the waveguides, with operating times of tens to hundreds of microseconds. An ON/OFF switch (or digital modulator) with an extinction ratio of 19 dB has been demonstrated using the change in the imaginary part of the effective index of a silicon nitride rib waveguide. Routing switches based on directional couplers are under development, and their feasibility has been demonstrated with a deposited (fixed) aluminum film. The switches are fabricated on silicon using VLSI-compatible processes, and can be used to select optical time delays for phased-array signal processing.

We compare the GaAs and Silica-based approaches for realizing integrated time-shift networks. The performance of a fully functional 2-cm X 2-cm monolithic GaAs circuit is reviewed in detail. In addition, we describe the design of an optoelectronic- switched network that uses Silica-based star-couplers and waveguide arrays.

Diode Laser Logic is a complete, new logic family comprised of Q- switched lasers and photodetectors. The lasers and detectors are monolithically integrated on multiple quantum well laser heterostructure with semi-insulating substrates. Preliminary results indicate an ON/OFF contrast ratio better than 500:1 and a gain in excess of 20. As a result, the devices can be cascaded to form more complex optical circuits. In addition to introducing the Diode Laser Logic family, a basic model is presented and the fabrication process is discussed.

A high-speed distributed electrode phase modulator has been designed and fabricated. The processing of this device presented many challenges: (1) smooth rib waveguides etched to an accuracy of +/- 200angstroms; (2) polyimide planarization of 5micrometers step heights, with vias patterned as small as 1.6 micrometers ; (3) contact metalization with resistivities as low as 1 X 10^-6(Omega) -cm²; (4) coplanar n and p gold contacts 2.5 micrometers thick, with a 0.5 micrometers gap between contacts; (5) ion- implantation to achieve both electrical and optical isolation. A brief description of how each of these processes have been accomplished will be presented. The epitaxial growth structure of this device will be discussed, including SEM cross-sections of the completed device.

A self-aligned process is used, for the first time, to fabricate triangular semiconductor ridge ring lasers. Very smooth dry- etched facets have been achieved using chemically assisted ion beam etching, with SiO₂ as the etch-mask. The dependence of the threshold current on the ring structure angle is presented. The ring lasers show stronger tendency toward single- longitudinal-mode operation than the Fabry-Perot lasers fabricated on the same wafer. The introduction of an external mirror in the ring laser structure results in highly asymmetric output, which further enhances the side-mode-suppression-ratio.

Interelement optical coupling introduces microwave frequency time constants to semiconductor laser array systems. The phenomenon is related to beating between lateral array modes and can take place at frequencies from 10 to 50 GHz for typical array designs employing index guided elements. Linearized coupled rate equation analysis has previously found small-signal modulation resonances at those frequencies. Here, the full rate equation theory is used for large signal analysis and novel schemes for utilizing those microwave frequency resonances for high speed optical signal transmission are theoretically demonstrated. We show, for example, that digital signals with bit rates many times the relaxation oscillation frequency are possible in principle.

An ultrafast optical deflector based on the induced nonlinear refractive index change of an area-modulation laser pulse for low-signal level optical A/D waveform converter has been designed. Using resonance enhanced Kerr nonlinear media, an optical deflector with the deflection time of approximately 1 ns to 100 ns with a 30-ps time resolution may be achieved for low- level input optical signals.

The beam pattern of a linear phased-array antenna system employing a photonic feed network is analyzed using a model for the individual feed element noise including both additive and multiplicative equivalent noise generators. It is shown that uncorrelated multiplicative noise power of the individual feeds is reduced by a factor of N in the output of an N-element linear array. However, the uncorrelated additive noise of the individual feed paths is not mitigated, and therefore will determine the minimum noise floor of a large phased-array antenna.

A 25-channel, 1-bit, free-space, nematic liquid crystal (NLC) spatial light modulator-based optical time delay unit is demonstrated with worst case rf SNRs ranging from 50 to 60 dB for various delay/no-delay modes of operation. Single channel performance demonstrating essentially no crosstalk gives rf SNRs from 74 to 85 dB (all other channels are not fed by optical beams in this mode). A frequency up-conversion method for the phase- based acousto-optic NLC analog optical beamformer is demonstrated at 1015 MHz, showing a record -72.1 dB rf signal power nulling using a parallel-rub NLC device. A novel in-phase and quadrature optical beamformer is demonstrated.

This paper addresses the problem of dynamic optical processing for the control of phased array antennas. The significant result presented is the demonstration of a continuously variable photonic RF/microwave delay line. Specifically, it is shown that by applying spatial frequency dependent optical phase compensation in an optical heterodyne process, variable RF delay can be achieved over a prescribed frequency band. Experimental results which demonstrate the performance of the delay line with regard to both maximum delay and resolution over a broad bandwidth are presented. Additionally, a spatially integrated optical system is proposed for control of phased array antennas. The integrated system provides mechanical stability, essentially eliminates the drift problems associated with free space optical systems, and can provide high packing density. This approach uses a class of spatial light modulator known as a deformable mirror device and leads to a steerable arbitrary antenna radiation pattern of the true time delay type. Also considered is the ability to utilize the delay line as a general photonic signal processing element in an adaptive (reconfigurable) transversal frequency filter configuration. Such systems are widely applicable in jammer/noise canceling systems, broadband ISDN, spread spectrum secure communications and the like.

This paper describes an acousto-optic signal processor that generates and controls the set of time delays required for RF antenna array true time delay beamsteering. The time delay controller can scan an RF array beam rapidly over a continuum of angles with a tone whose frequency determines the pointing angle of the RF array beam. The time-delay controller also provides the proper relative phases between the carriers of the time delayed signals at the RF of the antenna array. A proof-of-principle breadboard of the time delay controller was assembled. Results of the demonstration of the breadboard are presented.

This paper presents recent advances toward the experimental demonstration and the far field pattern characterization of an optically controlled phased array antenna operating at 3 GHz. The 2D optical architecture is based on polarization switching by N (N equals 6) spatial light modulators of pxp pixels (p equals 4). It provides 2^N-1 time delays values and an analog control of the 0 to 2(pi) phase for each of the pxp microwave signals actuating an active antenna. These signals originate from the coherent detection of a dual frequency laser beam obtained with an acousto-optic frequency shifter. Furthermore, we also present the principle of a new photodetector based on the synchronous drift of photogenerated carriers with a moving interference pattern. This moving pattern results from the interference of the two components of the dual frequency optical carrier of the microwave signal used in the above architecture. It provides a large detection volume which could yield a high saturation intensity. A theoretical analysis is detailed and followed by an experimental demonstration in GaAs at a frequency f equals 210 MHz.

A universally accepted figure of merit for analog microwave transmission links is the spur-free dynamic range (SFDR), which is the ratio (usually expressed in dB) of the largest to smallest signal a link can transmit and receive without introducing any measurable distortion. This paper presents the result of a study of broadband, microwave fiber optic links that contain high- linearity integrated-optic modulators. This study focused on two distinct modulator forms, the dual-parallel Mach-Zehnder modulator and the linearized directional-coupler modulator. Computer simulations were performed to determine how the SFDR was affected by variations of modulator parameters. In addition, the dynamic range and noise figure of links that included preamplifiers were calculated. The results of the link analysis was that the linearized directional-coupler modulator provided the highest SFDR for a broadband microwave fiber optic link.

We investigate, theoretically and experimentally, three analog optical links: (a) a direct detection link with an optical preamplifier, (b) a coherent AM (Amplitude-Modulated) link and (c) a coherent AM link with an optical preamplifier, and compare their performance with a conventional direct detection link. The theoretical analyses and the experimental measurements present cross-over points between the four links' dynamic range performance, and agree that these are due to certain noise terms like thermal noise, shot noise, relative intensity noise (RIN), signal-ASE (amplified spontaneous emission) beat noise, ASE-ASE beat noise, and LO (local oscillator) -ASE beat noise becoming dominant as the received optical power changes. For low received optical power, the use of an optical amplifier, coherent detection and the combination of both techniques can be used to improve the dynamic range over the conventional direct detection link. However, in the shot noise-limited regime, the conventional direct detection link gives the best performance. We also show that coherent AM links are more sensitive to RIN than the direct detection links. The RIN-limited spurious-free dynamic range of the coherent AM links is 4 dB worse than that of the direct detection links.

We analyze a true-time delay, optically controlled phased array antenna system whereby beam forming is accomplished using a large number of antenna elements that can receive any of several different microwave true-time-delays via use of coherent optical carriers through a single transmission fiber. We calculate the SNR and the signal-to-interchannel interference ratio (SIR) of one channel for various modulation-demodulation schemes. We show that for AM modulation, with a bandwidth of 1 MHz, a transmitter power of -10 dBm and a laser linewidth of 1 MHz, we can obtain a dynamic range (DR) of approximately 46 dB, with a channel separation six times of that of the microwave oscillation frequency. For the FM scheme, DR is approximately equal to 60 dB for a laser linewidth of 1 MHz, a FM modulation index of 2 and a channel spacing of 12f_m. It is found that coherent optically controlled phased array antennas provide improved controllability, and can have the SNR and DR that meet the stringent requirements of high resolution microwave antenna systems.

Nulling is a technique to reduce unwanted interference by selecting specifically against some characteristic of the interference. In the case of an antenna with several subapertures or beams, spatial discrimination against the interference is implemented through appropriate complex weighting of the antenna outputs prior to combining to make the net antenna sensitivity orders of magnitude lower in the direction of the interference. This paper describes how optics has been used to achieve a wide- bandwidth (2 GHz) null centered at 6 GHz for the case of two antenna outputs. The broadband null-depth is approximately 40 dB over the 5 to 7 GHz RF band. Such a deep null over this wide band could not be achieved using microwave technology alone.

The inherent low loss, wide instantaneous bandwidth, and light weight over large distances of the optical fiber make it an attractive option for a coaxial-based link system. Fiber optic links have been demonstrated for operation at rf frequency and into the mm-wave range within the last few years. In order to extend the dynamic range even more, either the modulation approach has to be modified or an rf linearization technique has to be used that will have an impact on the instantaneous frequency bandwidth. This papers describes a novel approach for extending the dynamic range of a fiber otic link by utilizing a double-sideband, suppressed-carrier modulation approach in conjunction with an optical amplifier. The system will be an inherently wide bandwidth system limited at this time to operation at 1550 nm due to the current availability of optical amplifiers. Before describing the linearized system, a brief discussion of the operational dynamic range as related to externally modulated systems is described.

We describe the development and antenna range demonstration of the Westinghouse fiber optic wavelength-multiplexed true time delay steering system for use with broadband phased array antennas. The prototype system is based on a unique hardware compressive architecture and can drive 16 antenna elements over the 0.32 to 2.1 GHz band with 6-bit angular resolution over a +/- 45 degree(s) scan angle.

Presented in this paper is an overview of the development of a wideband photonic array antenna. The presentation will focus on the performance of a unique L-band 24 X 4 element conformal array, supported by a photonic true-time-delay beamforming network. A 2-ns pulse was injected into the system and the round trip impulse response was measured to demonstrate the array's 550 MHz instantaneous bandwidth.

Fiber optic RF distribution of signals has many attractive features but can be problematic when the combining function of an array is attempted at optical frequencies. A photodiode array is described that permits the simultaneous detection of RF modulation in optical links and the combining of the resultant RF signals in a coherent fashion. A small photodiode array was built and tested in a phased array to demonstrate beam formation.

This paper describes an externally modulated fiber optic link designed to transmit signals from the face of a phased array antenna to a receiver/signal processor. The architecture used modulator pods and remotely located optical sources which were interconnected by fiber. The size of the modulator pod and its power dissipation were minimized, and operation over a range of environmental conditions was achieved. Perturbations in the operating characteristics of the link due to environmental stressing was minimized through careful design. The RF design considerations were dominated by issues related to beamsteering and null formation, and cancellation ratio data is presented for a pair of links showing a null depth in excess of 60 dB.

This paper provides a brief description of several techniques used to efficiently transport millimeter-wave signals over optical fiber. Transmission of millimeter-wave signals is demonstrated using resonant modulation of monolithic semiconductor lasers, feedforward optical modulation, and by stabilization of the mode-locked frequency of a passively mode- locked laser using an optoelectronic phase-locked loop.

An advanced optically controlled multiple beam forming and steering network for an active phased array antenna is proposed in this paper. Simultaneous multiple beam forming and steering can be achieved by optical-frequency multiplexing and optical microwave control. The proposed active phased array antenna system also has the capability of frequency re-use and the co-use of active array elements.

The utilization of three dimensions of parallelism in photorefractive data processors is extended to parallel three- dimensional readout for the two radar scenarios of radar doppler and ranging processing, and 3D synthetic aperture radar. These are scenarios in which the data processing has full parallelism in all of three dimensions, making the volume holographic approach attractive. The result of this processing gives us a surface with the third dimension coded with the wavelength and the value represented by the intensity so that the three dimensions of data may be read out in parallel with the use of a three-color CCD.

A demonstration hardware with high-speed, multiple optical data links was built for board-to-board optical interconnect operations. There are two types of optical interconnect paths in this hardware: interboard and backplane interconnects. It was demonstrated that multiple 500/550 MHz signals can be simultaneously transmitted in both the interboard and the backplane configurations.

An optoelectronic switching technology based on the monolithic integration of arrays of binary optical switch nodes containing vertical-cavity surface-emitting lasers (VCSELs) and heterojunction phototransistors (HPTs) has been developed. It can be used in multi-stage optical switching networks, and provides a reconfigurable 2D architecture for parallel optical interconnections. The VCSELs can also be integrated with heterojunction bipolar transistor (HBT) technology on the same wafer to provide a high speed optoelectronic interface between the optical switch fabric and the external electronic processing elements. Since these switches can be optically or electrically addressed and detected, they permit the direct communication and transmission of data between distributed electronic processors through a reconfigurable optical switching fabric and optical data link. The experimental demonstration of integrated photonic HPT/VCSEL routing switches and optoelectronic HBT/VCSEL switches that combine GaAs/AlGaAs VCSELs with HPTs and HBTs, respectively, are described below.

An optical delay line rf filter using a 16-element laser diode array to tap an acousto-optic cell is demonstrated. The optical architecture uses heterodyne detection of multiple laser taps to sum delayed versions of an input RF signal. Experimental results are presented that verify performance as a variable delay line or RF filter. Application for the delay line filter includes analog signal processing in the RF or IF portion of a radar receiver.

We are developing a class of optical phased-array-radar processors which use the large number of degrees-of-freedom available in 3D photorefractive volume holograms to time integrate the adaptive weights to perform beam-steering and jammer-cancellation signal-processing tasks for very large phased-array antennas. We have experimentally demonstrated independently the two primary subsystems of the beam-steering and jammer-nulling phased-array radar processor, the beam-forming subsystem and the jammer-nulling subsystem, as well as simultaneous main beam formation and jammer suppression in the combined processor. The beam-steering subsystem calculates the angle of arrival of a desired signal of interest and steers the antenna pattern in the direction of this desired signal by forming a dynamic holographic grating proportional to the correlation between the incoming signal of interest from the antenna array and the temporal waveform of the desired signal. This grating is formed by repetitively applying the temporal waveform of the desired signal to a single acousto-optic Bragg cell and allowing the diffracted component from the Bragg cell to interfere with an optical mapping of the received phased-array antenna signal at a photorefractive crystal. The diffracted component from this grating is the antenna output modified by an array function pointed towards the desired signal of interest. This beam-steering task is performed with the only a priori information being that of the knowledge of a temporal waveform that correlates well with the desired signal and that the delay of the desired signal remains within the time aperture of the Bragg cell. The jammer-nulling subsystem computes the angles-of- arrival of multiple interfering narrowband radar jammers and adaptively steers nulls in the antenna pattern in order to extinguish the jammers by implementing a modified LMS algorithm in the optical domain. This task is performed in a second photorefractive crystal where holographic gratings are formed which are proportional to the correlation between the unprocessed antenna output and a delayed version of the formed main beam. The diffracted components from these gratings are subtracted from the formed main-beam signal producing a processor output with reduced jammer content.

We experimentally demonstrate the phase shift of a microwave signal using the electrically controlled birefringence of a liquid crystal. A phase shift of 20 degrees at 10.5 GHz for an interaction length of 4 cm and an applied voltage of 16 V is obtained.

We describe recent results from our efforts in characterizing a photorefractive device to be used as a time integrator in the Acousto-optic Null Steering Processor. This processor is being developed for the ARPA Transition of Optical Processing to Systems Program under Rome Laboratory contract F30602-91-C-0044. We present frequency response data for the BSO crystals which were measured using a novel acousto-optic apparatus. We also present measured dynamic range data for the candidate BSO crystals.

An acousto-optic correlator has been developed for application to radar report-track association. This correlator employs a 2D, ferro-electric liquid crystal spatial light modulator for storage of track data while report data is input to the system's Bragg cell. Performance matches that of digital processing with the added capability of operation at much higher processing rates.

Novel space integrating acousto-optic correlators and convolvers are introduced that are based on the in-line AO interferometer. A new notch filter architecture is introduced with a potential of >36 dB rf null depths.