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Visible Spectrum Levels



This graph shows the signal and noise spectrum levels in an optical heterodyne receiving system. (18.3 kbytes)


This graph shows the calculated signal and noise spectral levels for a 1 GW cw beacon.  A symmetrical 10 m diffraction limited heterodyne system would yield a CNR = 94 dB re 1 Hz over a range of 10 light years.  This corresponds to a carrier-to-noise ratio (CNR) = 19 dB in a 30 MHz bandwidth.  Thus, this link would be be capable of transmitting real time NTSC signals as per terrestrial FM satellite transmissions, with over a 10 dB CNR margin.  This graph can also be used to predict the detectability of monochromatic beacons that the OSETI researcher might like to search for.

The optical bandwidth must be substantially reduced to ensure detectability of even a 1 GW continuous wave beacon.  In the Fraunhofer line, the carrier to background noise ratio is about 43 dB re 100 GHz optical bandwidth for an incoherent optical receiver.  The downside to monochromatic beacon detection is that the "magic frequency" must be guessed, and the optical haystack is very large.


Copyright (c), 1991, The COSETI Observatory

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