Patent ID: 8384910

Claim:
An interferometer comprising: a light splitting device configured to split light from a light source into a reference light and a measurement light; a reference mirror configured to reflect the reference light which has been split by the light splitting device and enters the reference mirror from a first direction; a measurement mirror configured to reflect the measurement light which has been split by the light splitting device and enters the measurement mirror from a second direction; an optical system configured so that reflected lights from the reference mirror and the measurement mirror enter the optical system via the light splitting device; a reflective device configured to reflect lights from the optical system to irradiate the reference mirror and the measurement mirror with the reflected lights through the optical system, respectively; and a light receiving device configured to receive light which have been formed by interference between two lights reflected twice at the reference mirror and the measurement mirror, respectively, wherein the reference mirror and the measurement mirror are tilted so that normal directions of the reference mirror and the measurement mirror differ from the first and the second directions, respectively, and the measurement mirror is configured to be displaced along the second direction so that the reference mirror is in a positional relation with the measurement mirror, the optical system having an optical power, other than zero, that causes, in the positional relation, the reflected lights from the reference mirror and the measurement mirror, passed therethrough, to be parallel with each other, and the reflective device being configured to reflect the lights, in the positional relation, backward on respective paths, on which the lights have respectively traveled from the optical system.