Patent ID: 7471396

Claim:
An optical interferometer that measures change in thickness of a workpiece, comprising: a first optical interferometer disposed on a front surface side of the workpiece; and a second optical interferometer disposed on a rear surface side of the workpiece, wherein the first optical interferometer and the second optical interferometer each include: a light emitting section; a polarization separator disposed between the light emitting section and the workpiece, the polarization separator having a reference surface, the polarization separator arranged such that a polarization direction of reference light that is reflected light reflected by the reference surface is orthogonal to a polarization direction of object light that is reflected light reflected by the workpiece; and an interference fringe sensor that acquires an interference fringe generated by interference between the reference light and the object light, the polarization separator of the first optical interferometer and the polarization separator of the second optical interferometer are disposed such that directions of polarization axes thereof are not in parallel to each other, and when the workpiece is not set, the reference surface of the polarization separator of the first optical interferometer reflects light from the second optical interferometer to generate object light; and the reference surface of the polarization separator of the second optical interferometer reflects light from the first optical interferometer to generate object light.