| 摘要 |
PURPOSE:To vary interval of interference fringes without replacing a diffraction grating by providing a grating inputting divergent luminous flux from a light source to generate diffracted light, an optical means for converting the diffracted light to parallel luminous fluxes and a photodetecting means. CONSTITUTION:A diode chip is formed on a laser diode 1 as a point light source O. Plus and minus first-order light generated from a diffraction grating 10 is converted via a lens 21 into parallel luminous fluxes, and a material 7 to be measured which moves at a speed V is disposed at an interference fringe forming part of a region where two parallel luminous fluxes are not completely separated. Divergent lights from the material 7 to be measured are condensed to a light receiving part 9a of a photodetector 9 via a condensing lens 8 provided to be decentered from the lens 21. A beat frequency at the photodetector 9 is obtained by a predetermined formula, and a speed V of the material 7 to be measured is calculated. An arithmetic unit PU detects the beat frequency, and calculates the speed V of the material 7 to be measured by using the formula. |
