System and method for optical heterodyne detection of an optical signal including optical pre-selection that is adjusted to accurately track a local oscillator signal

A system for optical heterodyne detection comprising:

• a first optical path for carrying an input signal;
• a second optical path for carrying a swept local oscillator signal;
• optical combining means for combining said input signal and said swept local oscillator signal into a combined optical signal;
• a third optical path for carrying said combined optical signal;
• a photodetector optically arranged to receive said combined optical signal from said third optical path, said photodetector generating an electrical signal in response to said combined optical signal;
• an optical pre-selector optically arranged to filter an optical signal within one of said first, second, and third optical paths, said optical pre-selector having a passband that tracks the frequency of said swept local oscillator signal, said optical pre-selector outputting a filtered portion of said optical signal;
• means for adjusting said optical pre-selector passband in response to a measure of the frequency of said swept local oscillator signal and in response to a measure of a portion of said swept local oscillator signal after said portion of said swept local oscillator signal has optically interacted with said optical pre-selector, said optical pre-selector passband being adjusted to track the frequency of said swept local oscillator signal;
• and
• a phase modulator for modulating at least some portion of said swept local oscillator signal, said phase modulator being responsive to said means for adjusting and being located along an optical path that is before said optical pre-selector.