Method and apparatus for two-wavelength interferometry with optical heterodyne processes and use for position or range finding

Claim 1

Method for measuring changes in a variable interference section of an interferometer using two-wave length interferometry and optical heterodyne processes, the method comprising the steps of splitting a first laser light beam of frequency n 1 into two cross-polarized first partial beams, subjecting one of said cross-polarized first partial beams to a frequency displacement by a frequency f 1 in order to obtain a partial beam of frequency n 1 +f 1 , whilst the frequency n 1 of the other of said first partial beams remains unchanged, so that a partial beam pair n 1 , n 1 +f 1 is formed, jointly applying the first partial beams of the partial beam pair n 1 , n 1 +f 1 across a Michelson interferometer and then across a polarizer to a photodetector, so that the two partial beams interfere with one another upstream of a photodetector and a beat of frequency f 1 is formed, which is detected by the photodetector, the improvement comprising the steps of splitting a second laser light beam of frequency n 2 into two cross-polarized second partial beams, subjecting one of said second partial beams to a frequency displacement by a frequency f 2 , in order to obtain a partial beam of frequency n 2 +f 2 , whilst the frequency n 2 of the other of said second partial beams remains unchanged, so that a partial beam pair n 2 , n 2 +f 2 is formed, combining the first partial beams of the partial beam pair n 1 , n 1 +f 1 and the second partial beams of the partial beam pair n 2 , n 2 +f 2 and jointly applying the first and second partial beams across said Michelson interferometer and finally across the polarizer to said photodetector, so that also two second partial beams of the partial beam pair n 2 , n 2 +f 2 interfere with one another upstream of the photodetector, and wherein in addition to the beat of frequency f 1 , at least one beat of frequency f 2 is formed, which is detected by said photodetector, and providing a second polarizer, a second photodetector and an optical element acting as a non-polarizing half-mirror, and passing four combined partial beams prior to entry thereof into said Michelson interferometer across said optical element acting as a non-polarizing half-mirror, whereby said optical element deflects a fraction of said partial beams across said second polarizer to said second photodetector, so that the four combined partial beams interfere with one another and beats form upstream of said second photodetector, of which at least respective ones of frequencies f 1 and frequency f 2 are detected by said second photodetector, demodulating signals of the first and second photodetectors, respectively accordingly to a square of an amplitude thereof, so as to obtain in each case a demodulated signal of frequency f 1 -f 2 , and measuring a phase difference between two demodulated signals, said phase difference varying in a linear manner with changes of an optical path.