Patent ID: 8422120

Claim:
A wavelength conversion element comprising a monocrystalline nonlinear optical crystal having an optical rotatory power, wherein the nonlinear optical crystal has: a plurality of first regions having a polarity direction along a predetermined direction; a plurality of second regions having a polarity direction opposite to the predetermined direction; an entrance face into which a fundamental incident wave having a wavelength λ and a frequency ω is incident in a direction substantially perpendicular to the predetermined direction; and an exit face from which a second harmonic with a frequency 2ω generated in the crystal emerges, and wherein the plurality of first and second regions are formed as alternately arranged in a period substantially equal to d expressed by Formula (3), Formula (4), Formula (5), or Formula (6) below, between the entrance face and the exit face, [ Math ⁢ ⁢ 81 ] d = m ⁢ ⁢ λ 2 ⁢ ( n 1 ⁡ ( 2 ⁢ ⁢ ω ) - n 1 ⁡ ( ω ) ) ( 3 ) d = m ⁢ ⁢ λ 2 ⁢ ( n 1 ⁡ ( 2 ⁢ ⁢ ω ) - n 2 ⁡ ( ω ) ) ( 4 ) d = m ⁢ ⁢ λ 2 ⁢ ( n 2 ⁡ ( 2 ⁢ ⁢ ω ) - n 1 ⁡ ( ω ) ) ( 5 ) d = m ⁢ ⁢ λ 2 ⁢ ( n 2 ⁡ ( 2 ⁢ ω ) - n 2 ⁡ ( ω ) ) ( 6 ) where n 1 and n 2 are refractive indices of two orthogonal eigen-polarizations in a ray direction of the nonlinear optical crystal where ellipticity γ of the eigen-polarizations of the crystal is less than 0.1, n 1 (2ω) a refractive index of the nonlinear optical crystal for the second harmonic, n 1 (ω) a refractive index of the nonlinear optical crystal for the fundamental incident wave, n 2 (2ω) a refractive index of the nonlinear optical crystal for the second harmonic, n 2 (ω) a refractive index of the nonlinear optical crystal for the fundamental incident wave, and m an order of QPM (quasi phase matching) and natural number.