Patent ID: 6788420
Filing Date: 2004-09-07
Classification: G01B

Abstract:
A heterodyne interferometer system with carrier phase modulaution comprising:a laser light source; a phase modulator positioned to receive a beam from the laser light source and apply a sinusoidal carrier phase modulation; a frequency shifter for shifting the frequency of a target beam and a local beam emanating from the phase modulator, the difference between she target beam frequency and the local beam frequency designated as the heterodyne frequency f, a reference photodetector; a signal photodetector; a beam splitter for deflecting a portion of the target beam and local beam to the reference photodetector and directing another portion of the target beam and local beam to a polarizing beam splitter; a polarizing beam splitter for directing the local beam directly to the signal photodetector, and for directing the target beam to a pair of reflectors separated in distance by L, the reflected target beam being returned by the separated reflectors to the polarizing beam splitter, the reflected target beam being directed to the signal photodetector; a signal mixer for mixing the phase-modulation frequency with the output of the signal photodetector to shift the target signal to the heterodyne frequency f and shift a self-interference signal into sidebands about the modulation frequency; a bandpass filter at the heteodyne frequency f to isolate the target signal and exclude the self-interference signal; and a phase meter to receive signals from the reference photodetector and compare the phase with a phase of the target signal filtered by the bandpass filter.