Photodetectors For Acousto-Optic Signal Processing

summary of some of the most important performance issues and promising solid stateapproaches is presented.A -O detectorsDetectors for A -O signal procesing applications generally have significantly higherdynamic range and frame rate requirements then detectors utilized for visable imagingapplications. In addition, different A -O signal processing applications require uniquearray performance specifications and functional characteristics. RF spectrum analysisapplications require high dynamic range and high detector frame rates. However, detectorsfor correlation signal processing applications typically do not have as large dynamicrange requirements and frame rates. For example, time integrating processors have longframe time requirements compared to spectrum analyzers. Detectors for the programmable RFexcision application require detector bandwidths greater then the highest instantaneous RFsignal frequency. A review of detector array types that have been developed for each ofthese applications is presented.The two basic functional types of detector arrays that have been utilized for real timeRF spectrum analysis are the parallel addressed (instantaneous) detector array and theserial addressed (integrating) detector array. Figure 1 shows schematic diagrams of eachdetector type. The instantaneous detector array utilizes parallel videodetector /amplifier channels with no detector reset operation during the detector sensingoperation. The integrating array integrates the optical signal power at the focal planefor a frame period and then transfers the detector output signals to one or several serialoutput multiplexers just before the detector is reset. The instantaneous detector arrayhas the major characteristic of simultaneously determining time of arrival (TOA) andfrequency of pulse RF signals. However, frequency resolution is limited by the number ofpackage pins and power dissipation limitations of the microelectronic photodetector array.The integrating detector array removes the pin limitation and therefore achieves improvedfrequency resolution (large number of detector pixels) by multiplexing the detectors toone or several outputs. However, the time required to perform the multiplexing operationlimits the TOA resolution. In addition, because of the integrating nature of the detectoroperation, the integrating detector array approach offers improved detectivity compared tothe instantaneous detector approach for long pulse signals.A detector array architecture that has been implemented for A -0 programable filter(excison) applications [4] is shown in Figure 2. The A -0 programmable filter detectorarray utilizes coherent optical processing. Both the Bragg diffracted and zero orderbeams illuminate the detector interfering at its surface. Because the diffracted opticalbeam is doppler frequency shifted, a difference frequency component at RF will be presentat each detector pixel. By controlling the state of switches connecting the photodiodesto the summing node, programmable excision of narrowband frequency components from awideband RF frequency spectrum is achieved. The implementation shown in Figure 2 utilizes