Parallel Optical Signal Processing

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The schematic 9006-028 shows a method of parallel signal processing to speed up the sky search for Extraterrestrial Intelligence (ETI).  A 2 x 2 array of small photodiodes lies in the focal plane of the telescope.  A single local oscillator field illuminates the entire two dimensional photodetector array with sufficient power so that each photodetector will produce a shot noise limited signal.

The image of the star field is formed on the mosaic of "N" photodetectors and the starlight from each star or small group of stars mixes (beats) with the local oscillator on a different photodiode.  The size of each photodetector array element and the resolution size within the focal plane image must be carefully matched, in order that the signal-to-noise ratio is not degraded by local oscillator power outside a resolution element.

The schematic 9008-035 shows more details of the photodetector array.  Each photodetector had its own front-end amplifier, the output of which feeds a switch or sampling gate.  While the illustration shows discrete front-end amplifiers, these amplifiers may be integrated into the photodetector array.   If cost was no object, we could simultaneously "look" at the outputs of all the array elements.  However, a sequential sampling approach is far more cost effective and should not impact the time taken to "find" a signal in a star-field.

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One high-cost telescope and signal processing system is thus shared with "N" photodetectors by sequentially accessing the output of each photodetector front-end system.  This time-multiplexing Search or Alarm Mode approach considerably reduces the cost of the electronics.  Once a signal is detected in any part of the array, the system would switch into the Detection Mode where it would determine which of the many stars in the star-field is the actual source of the signal.  The time-multiplexing will then stop and the signal processing electronics will lock onto the element that is producing the signal.  It would then switch into the Communications Mode for demodulation and recording of any intelligence that might be on the optical carrier.

The Columbus Optical SETI Observatory
Copyright (c), 1990