I wouldn't agree with that. The OP was talking about using a heterodyne technique, and of course you can do that optically- indeed, it is regularly done in optical signal processing. I think the point here is that usually when you talk about down converting, the assumption is that you are down converting a _carrier_ that is somehow modulated with much lower frequency information. In this case, however, there is no modulation- the carrier itself is the signal of interest. While you can technically mix that with another optical signal and produce a new signal in a very low frequency range (low enough to be transmitted electrically on wire), there is no point in doing so since you have destroyed the essential original information in the process.
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Chris L Peterson Cloudbait Observatory