A frequency method for detecting weak reflected or scattered coherent optical signals

A frequency method for the detection of weak laser emission backscatter is proposed which is based on the dependence of the beat frequency of two axial modes on the amplitude and phase of the reflected signal entering the laser cavity. The sensitivity of the method is evaluated theoretically and experimentally, and the advantages of the method are examined in comparison with a backscatter detection method based on the emission intensity modulation. For a He-Ne laser with a power output of 1 mW, the minimal detectable backscatter coefficient is shown to be 6 x 10 to the -8th sq rt Hz.