Optical systems utilizing three-wave heterodyne detectors

Claim 1

In an optical ranging system for determining the distance to a remote reflecting target, the combination of means for generating at a first site a composite signal having two optical signal components ω 1 and ω 2 ; means for directing said composite signal toward said remote target and for receiving the signal reflected therefrom; a first and second optical signal detector which operate on the heterodyne principle; means for directing said composite signal onto said first detector and said received reflected signal onto said second detector; a first and second mixer; means for coupling the output of said first detector to one input of said first mixer and the output of said second detector to one input of said second mixer; a radio frequency oscillator; means for generating a microwave signal, ω mw ; means for coupling said microwave signal to both said first and second detectors, the two optical signal components of said composite signal and said received reflected signal interacting with said microwave signal to produce first and second difference signals of frequency ω mw - ω 2 + ω 1 ; and means for coupling said radio frequency oscillator to the other input of said first mixer; means for coupling the output of said first mixer to the other input of said second mixer whereby the output of said second mixer is a signal whose frequency corresponds to that of said radio frequency oscillator and whose phase is independent of any slow phase variations in said composite signal or said microwave signal; and means for determining the phase relationship between said difference signals whereby to obtain a parameter which is related to the distance from said site to said remote target.