Optical distance measuring instrument

Abstract

An optical instrument (10), such as a stability monitor or a target range finder, uses an unstabilized laser (12) to project a composite optical signal of coherent light having two naturally occurring longitudinal mode components. A beamsplitter (24) divides the signal into a reference beam (26) which is directed toward one photodetector (28) and a transmitted beam (42) which illuminates and is reflected from a distant target (44) onto a second photodetector (52) optically isolated from the first photodetector. Both photodetectors are operated on the square law principle to provide electrical signals modulated at a frequency equal to the separation between the frequencies of the two longitudinal mode components of the optical signal projected by the laser (12). Slight movement of the target may be detected and measured by electrically monitoring the phase difference between the two signals provided by the photodetectors and the range of the target measured with the aid of microprocessor (132) by changing the separation between the longitudinal modes by shifting the length of the resonator cavity in an iterative series of increments.