JPS643584A - Laser doppler type speed measuring instrument - Google Patents

Abstract

PURPOSE:To improve measurement sensitivity by providing interactive projecting means which project 1st laser light >=2 times to an object to be measure so as to subject said light to >=2 times of Doppler shifts. CONSTITUTION:Of the laser light (Lz+Lx) outputted fro a laser apparatus 10, the laser light Lx advancing rectilinearly in a polarizing beam splitter 16 is circularly polarized by a quarter-wave plate 22 and is reflected in a Z direction by a plane mirror 24 so as to be imaged by a condenser lens 26 to one point on the object 28 to be measured. The reflected light Lx* reflected by the surface of the object 28 to be measured is again entered to the condenser lens 26 by passing the reflection optical pat symmetrical with the incident optical pat around the optical axis P of the lens 26. This light is reflected by the mirror 30 to return backward in the previously passed optical path, is reflected by the object 28, is further passed through the lens 26, is reflected by the mirror 24 toward the X direction, is converted to the linearly polarized light of the Z direction by the plate 22 and is entered to the splitter 16. Since the laser light is reflected twice by the object 28, the laser light is subjected to 2 times of the Doppler shifts and the frequency deviation is detected by a photodetecting element 32.