LASER LENGTH MEASURING INSTRUMENT AND CORRECTING METHOD FOR AIR FLUCTUATION INSTRUMENT IN THE INSTRUMENT

PURPOSE:To obtain a highly accurate output by splitting a beam into two beam, using one beam for measuring displacement, using the other beam for correction, monitoring the refractive index of air, receiving the measured results with both beams, and offsetting the changes in refractive indexes. CONSTITUTION:The linearly polarized laser light from a laser oscillator 1 is split through a beam splitter 2 into a correcting beam 23 and a measuring beam 24. Then, the beam 23 is inputted into the polarization beam splitter of a correcting interferometer 4 and split into two beams of correcting reference light and correcting signal light. Since a correcting reference light path 7 is all in glass, the path does not receive the effect of the fluctuation of air, and the length of the light path is stable. On the contrary, when the refractive index of air on a correcting signal light path is changed by fluctuation, the phase of reflected light is changed and the intensity of interferance light is changed. The changing amount of the phase due to the fluctuation of the air based on the change in the intensity is obtained as an output A from a light-path- difference measuring device 9. The device 9 becomes a monitor for the change in refractive index of air. When the measured results with both beams are operated in an operating device 11, the highly accurate output is obtained.