Laser interferometer and method for controlling laser interferometer

A method for controlling a laser interferometer including

• a laser light source configured to emit emission light,
• a light splitter configured to split the emission light into first split light, and second split light incident on an object to be measured,
• a light modulator disposed on an optical path on which the first split light advances, including an oscillator oscillating by applying a drive signal and at whose surface a mirror film is formed, and configured to modulate the first split light being incident on the mirror film of the oscillator into a reference light having a different frequency from a frequency of the first split light,
• an optical path length change unit provided between the light splitter and the light modulator, wherein
• the optical path length change unit is configured to change a first optical path length,
• the first optical path length is an optical path length between the light splitter and the light modulator, and
• the first optical path length is the optical path length of an optical path of the reference light,
• a photoreceptor configured to receive an interference light of the reference light and an object light generated by reflecting the emission light at the object to be measured, and to output a light reception signal,
• a controller configured to control operation of the optical path length change unit to change, based on a second optical path length, the first optical path length, wherein
• the second optical path length is an optical path length between the light splitter and the object to be measured, and
• the second optical path length is the optical path length of an optical path of the second split light,
• a demodulation circuit configured to demodulate a sample signal based on a reference signal, wherein
• the sample signal is demodulated from the light reception signal, and
• the sample signal is derived from the object to be measured;
• and
• an oscillating circuit configured to:
• output the drive signal to oscillate the oscillator;
• and
• output the reference signal to the demodulation circuit, wherein the oscillator is a signal source of the oscillating circuit, the method for controlling comprising:
• preparing a sample of which one of a displacement or a velocity is known, as the object to be measured;
• acquiring the light reception signal when the first optical path length between the light splitter and the light modulator is a first length;
• calculating a first measurement value being a displacement or a velocity of the sample based on the light reception signal acquired;
• controlling operation of the optical path length change unit such that the first optical path length between the light splitter and the light modulator is a second length, wherein
• the controlling operation of the optical path length change unit is based on an optical path difference between the first optical path length and the second optical path length;
• acquiring the light reception signal when the first optical path length between the light splitter and the light modulator is the second length;
• calculating a second measurement value being a displacement or a velocity of the sample based on the light reception signal acquired;
• and
• setting the first optical path length between the light splitter and the light modulator to the first length, when the first measurement value is close to the displacement or the velocity of the sample which is known, and setting the first optical path length between the light splitter and the light modulator to the second length, when the second measurement value is close to the displacement or the velocity of the sample which is known.