Method and apparatus for measuring heterodyne optical interference utilizing adjustable polarizing plate

An apparatus for measuring an optical heterodyne interference, comprising:

• a polarized light generator configured to generate a reference light and a measurement light having different frequencies, the reference light being one of a P-polarization light and a S-polarization light and the measurement light, being the other of the P-polarization light and the S-polarization light;
• a polarization light beam splitter arranged to transmit the reference light in a direction of a reference mirror and to reflect the measurement light in a direction of a target to be measured;
• a first λ/4 wavelength plate positioned between the polarization light beam splitter and the target;
• a second λ/4 wavelength plate positioned between the polarization light beam splitter and the mirror;
• a light receiving element arranged to detect an interference light of the measurement light and the reference light and to output an electric signal indicative of the interference light;
• a polarizing plate, positioned between the polarization light beam splitter and the light receiving element configured to transmit the measurement light reflected from the target to be measured and passed through the first λ/4 wavelength plate and the polarization light beam splitter and the reference light reflected from the reference mirror and passed through the second λ/4 wavelength plate and the polarization light beam splitter;
• a polarizing plate rotation mechanism configured to rotatably support the polarizing slate and to fix an axis of the polarizing plate as rotated into an angular position;
• a light shutoff mechanism , positioned between the polarized light generator and the polarization light beam splitter, which is configured to selectively shut off the reference light;
• and
• wherein the axis of the polarizing plate is set in an angular position by the polarizing plate rotation mechanism so that the electric signal outputted by the light receiving element becomes a minimum when the reference light is shut off by the light shutoff mechanism.