Balanced heterodyne laser interferometer with optical axis shift

A laser interferometer comprising:

• a laser light source configured to emit laser light;
• a first light splitter configured to split the laser light into first split light and second split light;
• an optical modulator including a resonator element, the optical modulator being configured to modulate the first split light into reference light having a different frequency from the first split light by using the resonator element;
• a second light splitter configured to reflect a part of interference light, which is between the reference light and object light generated by an object to be measured, to generate third split light and transmit the other part of the interference light to generate fourth split light;
• a first light receiving element configured to receive the third split light and output a first light reception signal;
• a second light receiving element disposed at a position different from the first light receiving element and configured to receive the fourth split light and output a second light reception signal;
• a collimator lens configured to generate collimated light by collimating the laser light;
• and
• an oscillation circuit configured to output a reference signal using the resonator element as a signal source, wherein
• the third split light is incident on the first light receiving element,
• the fourth split light is incident on the second light receiving element,
• an optical axis of the first split light that travels from the first light splitter toward the optical modulator is shifted from an optical axis of the reference light that travels from the optical modulator toward the first light splitter,
• a difference between an optical path length of the third split light from the second light splitter to the first light receiving element and an optical path length of the fourth split light from the second light splitter to the second light receiving element is 20 mm or less,
• an optical axis of the collimated light is defined as a first optical axis,
• return light traveling toward the laser light source is generated by the reference light or the object light, and an optical axis of the return light is defined as a second optical axis,
• a position at which the collimated light is generated in the collimator lens is a reference position, and
• the first optical axis and the second optical axis are shifted from each other so as to satisfy the following equation (A):
• κ
• 2
• +
• 1
• 2
• ⁢
• (
• R
• +
• 2
• ⁢
• L
• R
• ⁢
• λ
• )
• ≦
• Δ
• ⁢
• y
• (
• A
• )
• where Δy is a distance between the first optical axis and the second optical axis at the reference position, wherein Δy is adjusted by inclining the optical modulator,
• κ is an effective diameter of the collimator lens,
• R is a light diameter of the collimated light in the collimator lens,
• L is a distance between the reference position and the optical modulator, and
• Δ is a wavelength of the collimated light.