Patent ID: 8107351

Claim:
A half-waveplate comprising: a first wave plate; and a second wave plate in which the first wave plate and the second wave plate are arranged so that optical axes thereof intersect each other, wherein when phase differences of the first wave plate and the second wave plate with respect to a wavelength λ are represented by Γ 1 and Γ 2 , in-plane bearing angles formed by a polarization plane of a linearly-polarized beam incident on the half-wave plate and optical axes of the first and second wave plates are represented by θ 1 and θ 2 , an angle formed by a polarization direction of the linearly-polarized beam incident on the half-wave plate and a polarization direction of the linearly-polarized beam emitted from the half-wave plate is represented by φ, and an optical axis adjustment amount is represented by a, the following expressions are satisfied: Γ 1 =180 °+n× 360°; Γ 2 =180 °+n× 360° (where n in Γ 1 and Γ 2 is a non-negative integer); θ 1 =φ/4 +a ; and θ 2 =3φ/4 −a, wherein the optical axis adjustment amount a is set so that 0°<a<a max , and the maximum value a max of the optical axis adjustment amount is determined by the following expressions: a max =A 0 +A 1 ·φ+A 2 ·φ 2 ; A 0 =−0.06032; A 1 =0.0717; and A 2 =−0.00035, or, a max =A 0 +A 1 ·φ+A 2 ·φ 2 ; A 0 =−0.15869; A 1 =0.13912; and A 2 =−0.00066.