Patent ID: 8477272

Claim:
A quarter wave plate that is a crystal plate made of an inorganic material having birefringence and optical rotatory power and has an optical axis, comprising: an incident surface positioned on one surface of the crystal plate; and an emitting surface positioned on another surface, opposed to the incident surface, of the crystal plate, wherein linearly-polarized light incident from the incident surface is converted into circularly-polarized light so as to be emitted from the emitting surface, circularly-polarized light incident from the incident surface is converted into linearly-polarized light so as to be emitted from the emitting surface, an angle φ formed by a normal line on the incident surface and the optical axis is set in a range of 0°<φ<90°, an optical axis azimuth angle θ formed by an optical axis projection line, the optical axis projection line being formed by projecting the optical axis on the incident surface, and a polarization plane of the linearly-polarized light is set in one of a range of 0°<θ<90° under θ≠45° and a range of 90°<θ<180° under θ≠135°, and the crystal plate is made of quartz crystal, wherein when the quartz crystal is right-handed quartz crystal, the optical axis azimuth angle θ is set as 0°<θ<45° and phase difference Γ due to the birefringence of the quartz crystal is set as Γ>90, and when the quartz crystal is left-handed quartz crystal, the optical axis azimuth angle θ is set as 45°<θ<90° and the phase difference Γ due to the birefringence of the quartz crystal is set as Γ<90°.