Patent ID: 8264766

Claim:
A wavelength conversion element comprising a monocrystalline nonlinear optical crystal belonging to point group 32 or point group 3, 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 (1) or Formula (2) below, along a Z-axis direction of the crystal between the entrance face and the exit face, [ Math ⁢ ⁢ 79 ] d = m ⁢ ⁢ λ 2 ⁢ ( n + ⁡ ( 2 ⁢ ω ) - n - ⁡ ( ω ) ) ( 1 ) d = m ⁢ ⁢ λ 2 ⁢ ( n - ⁡ ( 2 ⁢ ω ) - n + ⁡ ( ω ) ) ( 2 ) where n + (2ω) is a refractive index of right-handed circularly polarized light in the Z-axis direction of the nonlinear optical crystal for the second harmonic, n + (ω) a refractive index of right-handed circularly polarized light in the Z-axis direction of the nonlinear optical crystal for the fundamental incident wave, n − (2ω) a refractive index of left-handed circularly polarized light in the Z-axis direction of the nonlinear optical crystal for the second harmonic, n − (ω) a refractive index of left-handed circularly polarized light in the Z-axis direction of the nonlinear optical crystal for the fundamental incident wave, and m an order of QPM (quasi phase matching) and natural number.