Laser doppler displacement measuring system and apparatus

Claim 1

Apparatus for measuring the displacement of a moving retro-reflective target from a reference position comprising: a laser for producing a laser beam of a particular frequency ω o ; oscillator means for producing an electrical heterodyning reference signal of a particular frequency Ω; an optical modulator interposed between the laser and the target for passing the laser beam ω o from the laser to the target to be reflected thereby as a reflected laser beam ω o +ω D parallel to and displaced from the laser beam ω o and having a frequency shift ω D proportional to the velocity of the target and connected to said oscillator means for producing a refracted laser beam ω o ±Ω having a frequency shifted by a predetermined amount Ω; beam combining means positioned in the paths of the refracted beam ω 0 ±Ω from the modulator and the reflected beam ω o +ω D from the target for combining the two beams and for producing a combined output beam Ω±ω D in response thereto; a photodetector positioned in the path of the combined output beam Ω+ω D from the beam combining means for producing an electrical beat output signal Ω+ω D in response thereto; means including a phase-demodulator coupled to said detector and to said oscillator means and responsive to the beat output signal Ω+ω D from said detector for comparing the phases of said beat output signal Ω+ω D and said reference signal Ω to produce an output signal Δφ having a phase displacement from said reference signal Ω proportional to the displacement ΔZ of the target from the reference position, ##EQU9## where C is the speed of light, N is the number of half wavelengths, and φ is the phase angle less than 2π; and for applying the heterodyning signal to the optical modulator and to the phase demodulator; and utilization means coupled to said phase demodulator and responsive to the output signal therefrom for providing a measurement of the displacement of the target from the reference position.