Laser-difference-frequency discriminator

Abstract

A laser difference frequency discriminator that detects the difference between two laser frequencies and outputs a voltage near baseband that is related to the difference frequency. This discriminator power splits a laser beam containing the two laser frequencies into two paths, one which is sent down a delay line to a photo detector while the other is sent directly to a photodiode; there may be an optional phase shifter in either path. The relative phases of the heterodyne signals from the photo detectors are compared in a phase detector, its output voltage is related to the phase difference, which is, in turn, related to the difference frequency. This discriminator has applications in microwave generation, laser radar, and optical communications.

1. A laser frequency discriminator that detects a difference between two laser frequencies to produce, said first and second paths having respectively a first and second optical length, said first and second optical length being different optical lengths in value, thereby a difference output signal and which comprises:
 a means for splitting an input laser beam with two frequencies into a first and second path; a photo detector which is on the first path and produces a detection output signal by processing signals therefrom;  
 a photodiode which is on the second path and which produces a photodiode output signal by processing signals therefrom;  
 a phase detector which compares phases of the photodiode output signal with the detection output signal to produce thereby the difference output signal.  
 
   
   
     2. A laser frequency discriminator, as defined in  claim 1 , wherein said first and second optical paths respectively comprise a first and second optical fiber which have different lengths.