Development of laser interferometer with picometer resolution using frequency–tunable laser

Abstract

A phase modulation homodyne interferometer (PMHI), in which an incident light phase is modulated with Pockels–Cell and the output signal is demodulated with a lock–in amplifier, can determine an optical path difference of wavelength times integer with an accuracy of picometer using the null–method. In the case of dark fringe, the output of the lock–in amplifier of the PMHI must be zero. We have constructed a PMHI with a tunable laser diode (TLD) to calibrate nanometer sensor. In the interferometer, a displacement of the movable mirror, which can be linked with the nanometer sensor, can be determined by tracking the dark fringe and measuring a change of the TLD frequency.