Optical distance-measuring device

Abstract

The invention relates to an optical rangefinding device which consists of two frequency-stabilized multi-mode lasers. Two modes of each laser are superimposed so that each laser generates its own respective amplitude-modulated beam; and each beam is used, alternately, as the measuring light beam. The amplitude-modulation frequencies are selected so that the electronic detection of the two amplitude-modulated light beams, followed by electronic mixing of the individually detected signals, generates an electronic pulse train having a difference frequency which is only a fraction of the two modulation frequencies. The emitted and reflected frequencies of each of these amplitude-modulated beams are phase-compared. Each of these phase comparisons and the difference between them are used to determine distance to the target. Since phase shifts are digitally measured at the relatively low difference frequency by a digital clock rate based upon the relatively high frequency-modulated signal of one of the lasers, phase shift is determined with high accuracy. Further, the device provides measurements having a relatively accuracy which corresponds to the difference in wavelengths of the two amplitude-modulated beams which, in turn, have an accuracy corresponding to the spectral sharpness of the frequency-stabilized lasers.