Patent ID: 8582112

Claim:
An interferometer comprising: a light splitting device configured to split light from a light source apparatus into reference light and measurement light, a reference mirror configured to reflect the reference light which enters the reference mirror from a first direction into reflected light; a measurement mirror configured to reflect the measurement light which enters the measurement mirror from a second direction into reflected light, and to be displaced along the second direction; an optical system configured so that the reflected light from the reference mirror and the reflected light from the measurement mirror enter the optical system via the light splitting device; a reflective device configured to receive the reflected lights from the optical system and to reflect the reflected light from the reference mirror back onto the reference mirror via the optical system and the light splitting device, and to reflect the reflected light from the measurement mirror back onto the measurement mirror via the optical system and the light splitting device, the lights, after having been reflected twice on the reference mirror and the measurement mirror, respectively, being multiplexed to an multiplexed light by the light splitting device; and a light receiving device configured to receive the multiplexed light, wherein in a first plane that includes first light that exits the light splitting device via reflection of the light from the light source apparatus and second light that originates from the first light and re-enters the light splitting device to be multiplexed to the multiplexed light, and in a second plane that includes the light that enters the light splitting device from the light source apparatus and the multiplexed light, the light from the light source apparatus enters, via the light splitting device, the reference mirror and the measurement mirror at an oblique angle (α), respectively, and each of the lights to be multiplexed to the multiplexed light exits corresponding one of the reference mirror and the measurement mirror at the oblique angle (α); the optical system has an optical power, other than zero, to cause the reflected lights from the reference mirror and the measurement mirror, passed therethrough, to be parallel with the second plane, the reflective device is configured to reflect the lights from the optical system in a direction parallel with the second plane; and at least one of the reference mirror and the measurement mirror is tilted, in a normal plane normal to the first plane and the second plane, so that a normal direction of the at least one of the reference mirror and the measurement mirror differs from corresponding one of the first and second directions by a predetermined angle (θ).