Evaluation Of High-Gain Optical Processors For LPI Surveillance And Communications Systems

AbstractLow probability of intercept (LPI) surveillance and communication systems can be ofvital benefit to a number of military applications. The basic concept in all LPI systemsis to spread the transmitted power over as wide a frequency band as possible to makedetection as difficult as possible. The critical technology issue involved in implementingthis concept is to achieve complex real time matched filter signal processing in thereceiver with as large a processing gain as possible. This paper briefly discusses anexperimental program, which combined a surface wave tapped delay line and optical coherentintegrator to demonstrate processing gains on the order of 70 dB; and the evolving tech-nology in optical signal processing, which needs to be developed to achieve feasible sys-tems application.IntroductionUtilization of the electromagnetic spectrum has long been considered for enhancing thecovert and jam resistant properties of radar and communication systems. Spread spectrumtechniques incorporating pseudorandom coding, frequency or time hopping, or combinations ofthese have long been proposed for incorporation into many military electronics systems toenhance the low probability of intercept (LPI) capability of these systems. The LPI capa-bilities offered by spread spectrum techniques can spread narrowband interference signalsin the receiver while at the same time lowering the power spectral density of signalsemitted by the transmitter. Communication systems benefit by the message screening fromeavesdroppers, selective addressing, and multiple access capabilities possible in spreadspectrum systems. The pulse compression feature in a radar makes possible the simultaneousattainment of range and Doppler accuracy and resolution required for tracking and multipletarget applications as well as providing LPI capability.In all cases, the product of the pulse (or code) length T and the bandwidth W is ameasure of this capability. As TW products become larger, the accuracy and complexity ofthe hardware increases and only recently has technology been able to support advancedspread spectrum systems.Optical Processing TechniquesProcessing large TW product signals can be done with several techniques including:digital electronic circuits, surface acoustic wave (SAW) delay lines, charge -coupleddevices, and coherent optical data processors (CODP).The optimum technique for any particular application will depend on the TW product ofthe signal, the bandwidth of the signal and, in addition, the size, power and dollars thatcan be dedicated to the implementation of the technique in actual hardware. A rough com-parison of the approximate range of bandwidth and time bandwidth product for severalprocessing techniques is shown in Figure 1 *.A research program begun at DARPA in 1975 had as its objective the feasibility demon-stration of the processing gain (i.e., element resolution) depicted in Figure l * *. Withthis goal, the following tasks were undertaken: design and fabrication of a signal simu-lator, design and integration of an optical signal processor, and test and evaluation ofthe simulator and processor in order to ascertain the practicability of high gain large TWproduct optical processors.