ABSOLUTE LENGTH MEASURING METHOD AND ABSOLUTE LENGTH MEASURING EQUIPMENT USING FM HETERODYNE

PURPOSE:To improve the accuracy in the absolute distance measurement as remarkably as to not larger than the wavelength of a light source by using a Michelson interferometer etc. and an interferometer which utilizes a first and a second reference lengths. CONSTITUTION:An output light from a semiconductor laser 1 is continuously FM modulated during a time t1 - t2 (period Ta), and an interference phase from a photodiode (PD) 8 is measured, whereby the orders of interference Nm and NNL are specified. When the output of a PD 14 is suddenly decreased, a switch 17 is changed over and the frequency is stabilized to be the frequency of the absorption peak. The stable period continues to a time point t3 (period Tb). One cycle is complete at the time point t3. Interference phases thetam, thetaNL, thetaNS are obtained from the photodiodes PDs 8, 28, 25 during the stable period. Thereafter, a controlling/processing part 34 obtains first the order of interference N and then the absolute index of refraction nabs of the air using the interference phases thetaNL, thetaSL. Then, the absolute distance (m) to an object 6 is obtained from the order of interference N, absolute index of refraction nabs, interference phase thetam and wavelength lambdagas of the laser 1 at the frequency of the absorption peak.