In this paper, transmitter and receiver components for microwave fiber optic links are reviewed. Present link signal to noise limitations imposed by the performance of these components are analyzed, and promising trends in component development are discussed.

A record room temperature small-signal modulation bandwidth of 17 GHz is reported for vapor phase regrown 1.3 μm InGaAsP buried heterostructure (BH) lasers operated at a pulse bias optical power of only 12 mW/facet. Under cw bias conditions a band-width of 12 GHz is achieved. The optical modulation amplitude remains flat in sharp contrast to other types of BH lasers which exhibit strong signal roll-off at frequencies well below the resonance frequency. The modulation bandwidth is attained by increasing the p-doping level in the active region and by the choice of short cavity length. The device is grown on a conductive substrate indicating that it is unnecessary to use a semi-insulating substrate to obtain flat optical response in these vapor phase regrown BH lasers.

Q-switched diode lasers have been operated in a continuous-inversion mode of Q-switching at pulse repetition rates of 14 GHz with a measurement-system-limited 40-ps FWHM pulsewidth. In continuous-inversion Q-switching, the gain in the amplifier section of the device is continuously high while the laser is modulated by an intracavity electroabsorption modulator. The low-capacitance of the modulator section of a Q-switched laser makes it attractive for high-speed modulation, especially when integrated with other electronic devices.

A new type of high-performance, planar PIN photodetector operating in the 1.0-1.6 μm wavelength region has been fabricated on semi-insulating InP and InGaAs entirely by formation of alloyed contacts. Their planar geometry and simple fabrication process, that does not require a separate step to form the junction, together with their high speed under reverse bias (FWHM . 50 ps) and sensitivity (n ≈ 40% at λ = 1.24μm, without anti-reflection coating) make these devices attractive for integration with FETs and for photodetector arrays.

It is demonstrated that an ultra-high speed window buried heterostructure GaAlAs laser fabricated on semi-insulating substrate can be used as narrow band signal transmitters in the Ku-band frequency range (12-20GHz). The modulation efficiency can be increased over a limited bandwidth by a weak optical feedback. A stronger optical feedback enables one to actively mode-lock the laser diode at a very high repetition rate up to 17.5GHz, producing pulses of = 12ps long.

A Visible-Millimeter Wave mixing system for testing high frequency devices has been set up with all components operating satisfactorily and locked to stabilized cavities. Using this system, mixing has been obtained, with frequency separations ranging to 100 GHz, in a number of GaAs and GaAs/AlGaAs devices. These devices include commercial FETs, state of the art industrial FETs as well as modulation doped HEMT structures and Heterojunction bi-polar transistors.

An optical receiver front end was built which detects optical heterodyne signals up to 80 GHz. The unit combines optical detection in a GaAs photodiode with RF mixing of that same detected signal in the same photodiode. At low frequencies the diode is biased with DC, working as a conventional detector. Above 2 GHz the photodiode in the unit can be excited simultaneously with both light and a local oscillator voltage. The unit then produces intermedite frequency mixer products. When the signal mixes with the fundamental of the local oscillator (LO), these products are only 5 dB weaker than the original heterodyne signal. The LO voltage was 1.4 VRMS and its frequency 2-6 GHz. Above 6 GHz higher harmonics of the LO voltage mix with the RF photosignal. They mix unusually well in this receiver, and deliver significant power to the IF amplifier. Near 80 GHz, the receiver's IF power of -88 dBm was generated when the 14th harmonic of the LO mixed with the signal.

The complex dielectric constants of two ferroelectric crystals SBU and BSKNN have been determlned for millimeter waves betweeen 55 and 110 GHz as a function of temperature. These measurements used Fabry-Perot fringes produced by crystal surface reflections. Absorption was found to decrease markedly upon cooling for incident waves polarized along the crystal polar axis. Since exploitation of the large millimeter wave electooptic coefficients in these crystals is limited at room temperature by absorption losses, these results indicate that cooled crystals can he used for efficient low-loss electrooptic devices.

There is a growing interest in optically controlled microwave devices and systems. This paper is concerned with the design, fabrication and application of optically stimulated microwave PIN diodes. A microwave PIN device was modified to facilitate optical injection of carriers into the intrinsic region of the diode. The optical port transforms the PIN into a three terminal device. The device was characterized at low frequencies (110 MHz) and its utilization as a phase-shifter and as a switch were explored at X-band. Based on these results recommendations for improved performance were developed.

Insufferably high conduction losses for metal-based guiding structures at millimeter/sub-millimeter wavelength range as well as the phenomenal success of dielectric-based optical waveguides and integrated optical circuits prompted us to take a new look at dielectric waveguides as viable guiding structures for mm/sub-mm waves. Our investigation shows that dielectric waveguides may yet be proven to be the best mm/sub-mm waveguiding struc-tures. There are two possible ways of realizing a low loss millimeter/sub-millimeter (mm/sub-mm) waveguide structure: Through the use of low loss material and/or through the use of specially configured structure. This paper will address the latter. New low loss configurations are presented. Detailed propagation characteristics of the dominant mode on these new structures are discussed. The finite element approach perfected for optical waveguide calculations was used to analyze these complex structures.

This paper illustrates an application of integrated optoelectronics to satellite communications. Although Gallium Arsenide technology is employed, this application is a step beyond Monolithic Microwave Integrated Circuits. The type of transponder described here could be operational in the mid-1990's.

Operation of a bulk acoustic wave (BAW) spectrum analyzer is similar to a coherent optical Fourier transformer. In our experimental device an array of phased acoustic transducers serves as a coherent radiation source as well as spatial modulator. This device is modeled by its optical equivalence, applying many of the well developed diffraction formulation used in optics. The theoretical development is supplimented by experimental results. The data indicates effective signal channelization where the diffraction side lobes are suppressed considerably.

Several techniques have been developed for improving the performance of acousto-optic Bragg cells. These techniques include, birefringent phase matching, planar acoustic beam steering, the combination of the two, and optical multiple pass. The principles of these techniques and the enhanced performance, including increasing bandwidth and efficiency will be described.

Microwave applications of optical signal processing requires wideband acousto-optic bragg cells with high efficiency as input transducer. Several novel techniques have been developed to improve the efficiency bandwidth product of such devices. With these newer techniques, useful devices with more than two GHz of bandwidth and up to 10 GHz center frequencies can be anticipated in the near future.

Magnetostatic waves have been used to diffract guided optical waves in ferririagnetic yttrium iron garnet (YIG). Doppler shift of the optical frequency has been observed and a rudimentary optical system for signal correlation in a time integrating architecture has been deployed. Employing substitutional elements in the garnet material can enhance diffraction efficiency and throughput.

It has been suggested that the optical scattering effect in a heterodyne acousto-optical system can be ignored because its beat frequency with the reference beam is outside of the IF passband of the heterodyne detection. Our analysis concludes that this is not true. A finite point spread function convolving with the reference beam of a heterodyne spectrum analyzer causes mixing of photons with different frequency shifts and thus generates IF noise. A signal beam with finite scattering level also contributes to optical cross-talk by mixing with scattered reference beam. This problem is made clear below by analytical derivation and experimental data.

This paper discusses the dynamic range improvement of acousto-optic receivers. Two basic architectures are discussed: the direct detection approach and the interferometric (heterodyne) detection approach. The interferometric spectra analyzer was implemented using a cascade-cell configuration. The system demonstrated a two-tone spurious-free dynamic range of 50 dB above tangential sensitivity.

Passive surveillance systems requiring wideband signal processing may benefit from a recently developed compact acousto-optic time-integrating correlator. The correlator has a bandwidth of 100 MHz about a center frequency (IF) of 300 MHz to provide time-difference-of-arrival information with a resolution of less than 10 nsec useful in locating wideband radar jammers when signals are received at two widely separated antennas. On an airborne platform, the correlator may perform passive ranging. This correlator features the use of counterpropagating surface acoustic waves on a single lithium niobate delay line interacting with light from a laser diode. Diffracted beams are interferometrically combined on an integrating photodiode array. Integration time of 1 msec provides a time-bandwidth product of 105. A two dimensional correlator allows processing of both time-difference-of-arrival and differential Doppler.

Recent advancements in the development of the Real Time Acousto-optic SAR Processor are presented. In particular, the technique for introducing the azimuth reference function into the processor via an acousto-optic Bragg cell is discussed. This approach permits the reference function to be stored in electronic memory, thus giving the processor the flexibility needed to adapt rapidly to changes in the radar/target geometry. The architecture is described and results are presented which show the applicability of the technique to both spot-light and strip-map SAR.