Patent ID: 7548678

Claim:
An optical element, comprising: a base substrate; a waveguide substrate; and a thin film layer provided between the base substrate and the waveguide substrate, having a single-layer structure or a multilayer structure including a film containing Ta 2 O 5 or Nb 2 O 5 as a principal component, wherein the waveguide substrate is made of a crystal that is transparent with respect to light having a wavelength λ and that has a refractive index n 1 with respect to the light having the wavelength λ, the base substrate is made of a material having a refractive index n 2 with respect to the light having the wavelength λ, the thin film layer is made of a material having a refractive index n 3 with respect to the light having the wavelength λ, and has a thickness T satisfying, when n 1 >n 3 and n 2 >n 3 , T≠(k×λ)/(2×n 3 ), and when n 1 >n 3 >n 2 , T≠((2k−1)×λ)/(4×n 3 ) where n 3 ≠n 1 ≠n 2 , and k represents a natural number.