This paper is a brief rev4.ew of the characteristics of semiconductor lasers under microwave-frequency direct intensity-modulation, and multi-gigahertz gain-switching. The role of biasdependent resonance frequency and -3 dB rolloff frequency in multi-gigabit/sec digital modulation is discussed. The intrinsic transient effects of spectral envelope broadening and dynamic line broadening (frequency-chirping) on the single-longitudinal-mode operation of high-speed directly modulated semiconductor lasers are also discussed.
We show that a modelocked pulse train is as coherent as a single mode source at the same average power, even though the additional number of modes from a mode-locked source are subject to noise perturbations.
A high-speed integrated-optic Ti:LiNbO, modulator has been developed. The modulator exhibits smooth resonance-free frequency r6sponse with a 17-GHz 3-dB bandwidth. Complete intensity modulation can be achieved with 120 mW drive power. Optical damage effects, including optically-induced loss, polarization rotation, modulation transfer characteristic shifts, and mode pattern changes in these devices were also investigated.
The design of acousto-optic Bragg cells for EW application is discussed. The critical issues of concern include operating wavelength, bandwidth, resolution, efficiency, optical scatter, intermodulation products and maximum power handling capability. Experimental results obtained at 0.83 micrometer have demonstrated a bandwidth of 1.8 GHz with a diffraction efficiency of about 6 percent per RF watt.
Phased array technology has been developed using Zno thin film transducer for acousto-optic Bragg cells in the microwave frequency range. This technology has the potential of providing efficient microwave Bragg cells with reasonable fabrication cost. A review of this technology is given in this paper.
Spectrum analysis of microwave signals can be performed using Bragg diffraction of light by magnetostatic waves (MSW's). MSW wave propagation must occur in a thin-film magnetic, ferrite medium, such as YIG. Since it is possible to propagate guided optical waves in the same film, rf spectrum analyzers analogous to those using guided optical wave interaction with surface acoustic waves can be constructed. Intrinsic advantages of the optical-MSW device include high center frequencies of operation, up to 20 GHz and beyond, and tunability of the center frequency. The theory of optical-MSW interactions and laboratory demonstrations of such interactions are summarized. Design considerations for a spectrum analyzer device are discussed, including materials, rf-to-MSW transduction, light sources, thin-film light coupling, and photodetectors.
Optical fibers are an attractive media for transmitting microwave signals due to their low attenuation, light weight, immunity from electromagnetic interference and large bandwidth capabilities. In this paper, transmitter and receiver components for microwave fiber optic links are reviewed. Current limitations to link signal to noise imposed by the performance of these components are analyzed and promising trends in component development are discussed.
Analog microwave fiber optic communication links have the potential to replace coaxial systems for transmission of RF signals at frequencies up to 15 GHz and beyond. Potential applications include antenna remoting, distribution networks, and communications. At TRW, prototype bandpass RF fiber optic links have been constructed with center frequencies up to 5 GHz. The performance of these links will be discussed.
A delay line that can accommodate analog signals with a bandwidth exceeding 1 GHz and a delay time of the order of 1 ms is reported. It uses a single mode fiber as the delay medium in a recirculating mode. Detailed system design and expected performance at 1.3 μm and 1.55 μm operating wavelength are described. Preliminary results of the delay line implementation are also reported. Optical transmitters and receivers with extremely flat bandwidth to 1.3 GHz and low noise level have been achieved.
The very short wavelength of optical frequency carriers compared to that of radio frequency carriers and the ability of optical and acoustical waves to interact offer potential for improved communications satellite hardware even in system configurations where the inputs and outputs are at microwave frequencies. Three such potential applications, an optical switch matrix, an optical intersatellite link and an FDMA demultiplexer and demodulator are described in the context of advanced satellite system concepts.
The increasing need for rapid steering of antenna beams has prompted significant research and development efforts in phased-array technology. Advances in solid-state power amplification now make it possible to construct arrays with individual amplifiers at each radiating element. Consequently, the power loss in the beam-forming network is greatly reduced, since it operates in the low-power regime. Traditionally, beam-forming and steering are performed by individually controlled phase shifters and variable gain amplifiers in the signal path of each array element. Waveguide power splitters distribute the signals to the input ports of the antennas.
The use of optical fibres as delay lines configured to act as a beamformer for a phased array is described. Such a beamformer operates at intermediate frequency or low microwave frequency. The relationship between operating frequency and phase accuracy in the beam-former is discussed. A detailed expression for critical system parameters of the beam-former is derived and verified with a simple experimental model operating at 120 MHz. The construction of the nodes in the beamforming network is identified as a critical feature in design on which effort needs to be concentrated.
A number of recent efforts in the area of dielectric waveguide techniques are reviewed for millimeter-wave circuits. Several forms of dielectric waveguides are discussed along with passive components. An isolator based on the nonreciprocal coupling is described in an image guide form. Some attempts for developing active components are included.
This paper describes a microwave method for determining the photo-induced incremental complex permittivity of semiconductor materials such as Si, Ge, Te, and-- also for the determination of the average collision time. An unbalanced bridge technique is used which allows sensitive measurements even at low optical illumination levels, with relatively simple computational procedures. This method is thus useful for evaluating the potential usefulness of semiconductors for optically controlled microwave devices.
We report dc to rf conversion, using picosecond optoelectronic switches for the first time. A rf pulse train of two and one half cycles has been obtained having a period of 4ns and a voltage switch-out efficiency better than 90%. This device exhibits future potential for high power pulse generation at high frequencies.
Recent work has demonstrated that light can be used to both phase-lock and switch IMPATT diodes. However, because the light that was used has energy E higher than band gap Ea, it cannot effectively reach the deeply-buried active region of the diode. The result hds been low optical coupling efficiencies (< 2%). We find that Burstein shift and internal photoemission can be used to enhance the efficiency by tuning E around Eg. These two mechanisms also can be used to define a better phase relationship with the locked microwaves with adjusting the distribution of photoinduced current injected into the active region. Our photoresponse study of Ti/W-GaAs Schottky IMPATT diodes has shown that efficiencies can be increased to 50% and 13% at E = 1.42 eV (using Burstein shift) and 1.38 eV (using internal photoemission), respectively. In the former case, the current is generated over the entire active region. In the latter case, the current is injected from a narrow region, i.e., the junction area.
A dynamic bridge method has been developed to measure the time varying phase shift and attenuation of millimeter-waves in semiconductor waveguides when perturbed by optically induced plasmas. The resolution of this technique is 2 ns in real time. Comparison of this method with a theoretical model is also discussed.
An approximate theoretical analysis of the optical injection locking phenomenon inMESFET oscillators is presented. The locking ranges estimated in the frame of this analysis are in quite fair agreement with those found experimentally.
This paper discusses the classification of EW receivers according to their applications and structures. The characteristics of each type of receiver will be discussed briefly. The development stage and problems in each type of receiver will be discussed according to the receiver structure.
Small, compact, and rugged IF spectrometers are needed in high resolution heterodyne receivers designed for a new generation of space-borne telescopes planned for the next decade. Acousto-optic Spectrometers (AOS) promise to provide the necessary bandwidth and resolution in a package which is compact, power efficient, and ruggedized for space applications. Sensitivity, linearity, and stability are the primary goals of an astronomical receiver and these features must be demonstrated in order for an AOS to be accepted in place of more conventional RF or digital technology.
An acousto-optic architecture for simultaneously obtaining time integration correlation and high-speed power spectrum analysis has been constructed using commercially available Te02 modulators and photodiode detector-arrays. The correlator section of the processor uses coherent interferometry to attain maximum bandwidth and dynamic range while achieving a time-bandwidth product of 106. Two correlator outputs are achieved in this system configuration. One is optically filtered and magnified 2 : 1 to decrease the spatial frequency to a level where a 25-MHz bandwidth may be sampled by a 62-mm array with elements on 25-μm centers. The other output is magnified by a factor of 10 such that the center 4 μS of information is available for estimation of time-difference-of-arrival to within 10 ns. The Bragg cell spectrum-analyzer section, which also has two outputs, resolves a 25-MHz instantaneous bandwidth to 25 kHz and can determine discrete-frequency reception time to within 15 μs. A microprocessor combines spectrum analysis information with that obtained from the correlator.
This paper describes the configuration and operation of a channelized receiver that uses an acousto-optic bragg cell as a filter bank followed by an encoder that develops pulse descriptor words for each pulse signal received.
We have investigated and identified the important sources affecting the AOSP (acousto-optic signal processor) dynamic range. With spatial filtering techniques to clean up laser source noise and selection of good optical elements, the background scattering noise was reduced down to -62.5 dB. Using the heterodyne detection scheme, we have demonstrated ~70 dB dynamic range and picowatt minimum detectable power.
This paper reports the demonstration of a coherent frequency channelization and down conversion system for RF spectrum analysis. In this system, RF local oscillator signals modulate an optical beam which is then demultiplexed by a wedged etalon. Each of the resulting spatially separated channels are mixed with an incoming RF signal by heterodyne detection which down converts and channelizes the signals for further AO spectrum analysis. The performance of a concept demonstration system is discussed.
Thin-film Luneburg lenses are an alternative to geodesic and grating type lenses for optical microelectronic devices requiring the collimation or focusing of a guided optical wave on a substrate. They have the advantage of circular symmetry and ease of fabrication by semiconductor-type sputtering processes. However, close tolerance levels must be maintained for control of refractive index, loss and scatter, profile and position. These issues have been addressed theoretically, with Luneburg-type lens structures, and experimentally, with films of niobium pentoxide (Nb2O5) on Ti-indiffused lithium niobate (Ti:LiNb03) planar waveguides. Films with loss levels of 0.5 dB/cm and low in-plane scatter were obtained by using reactive rf sputtering from a niobium target. Refractive indices near 2.29 at X = 0.633 Am can be repeatedly obtained with partial pressure-controlled and gas-analyzed sputter environment. Tolerance levels on refractive index have been theoretically determined and are compared to experimental values. Time-temperature experiments have determined aging and environmental ruggedness. The data indicate that high index thin-film lenses can be reproducibly fabricated that meet inte-grated, optical spectrum analyzer requirements.
Future electromagnetic environments will have much higher signal densities than current system processors are capable of detecting, sorting, and classifying. New system processors utilizing an acousto-optical Bragg cell followed by advanced detector arrays and microelec-tronics to minimize the signal rate at the digital processor have the potential to meet future system processing needs. Detector considerations for the new system processors are discussed.
A GaAs photodetector array integrated with GaAs CCD designed with a parallel input scheme has been fabricated, and the multiplexing of optically generated signals fed through the parallel input system has been demonstrated. High speed operation of a GaAs CCD device with parallel input capability has also been demonstrated at 1 GHz clocking rate with a CTE value > 0.997.
A parallel channel detector array was developed to function in an acousto-optical signal processor. Each channel is independent of all the other channels with a photodetector pitch of 254 micrometers. The design provided for a 50 db dynamic range for a -54 dbm to -4 dbm 0.85 micrometer optical power input with a logarithmic output transfer characteristic. A monolithic N-MOS process with 3 micrometer design rules was utilized to fabricate the detector array.