Basic Optical Heterodyne Receiver

9006-032

The schematic 9006-026 shows a optical heterodyne receiver which detects signals in two orthogonal planes.  A narrow band optical pre-filter may be employed to reduce the Planck background from starlight, though in many situations this filter may not be required.

The local oscillator laser has its Relative Intensity Noise (RIN) reduced by an external "noise eater" (this reduces SNR impairment due to excess intensity noise), and is split into two equal intensity beams by the polarizing beamsplitter.   The single mode local oscillator laser is tunable and controlled by the computer.

The outputs of the two photodetector front-ends are downconverted by a series of electrical local oscillators, and the resulting signals are spectral analyzed by two banks of filters or their equivalents.  If a strong wide-band signal is detected, conventional intensity or frequency demodulators may be connected to the output of the mixers to extract the intelligence on the carrier signal.

To save on electronic complexity, it is possible to time multiplex the downconvertion and/or filter bank electronics, so that each orthogonal plane of polarization is monitored sequentially.  However, better performance will be obtained with the system shown, which will produce a higher signal-to-noise ratio should the alien transmitter employ polarization modulation.

The Columbus Optical SETI Observatory
Copyright (c), 1990