Optical element and method for producing the same

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 Ta2O5 or Nb2O5 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 n1 with respect to the light having the wavelength λ,
• the base substrate is made of a material having a refractive index n2 with respect to the light having the wavelength λ,
• the thin film layer is made of a material having a refractive index n3 with respect to the light having the wavelength λ, and has a thickness T satisfying,
• when n1>n3 and n2>n3, T≠(k×λ)/(2×n3), and
• when n1>n3>n2, T≠((2k−1)×λ)/(4×n3)
• where n3≠n1≠n2, and k represents a natural number.