Patent ID: 7283302

Claim:
A wavelength filter comprising: a solid material that is optically transparent and includes a pair of planar surfaces substantially parallel to each other; and a supporting member that supports the solid material on a planar surface of the solid material, other than the pair of planar surfaces, the supporting member having rigidity higher than that of the solid material, wherein the solid material is a birefringent material having an optical axis that makes a predetermined angle with respect to a normal to the pair of planar surfaces, the wavelength filter selects light having a wavelength that is determined by optical length between the pair of planar surfaces, by resonating the light between the pair of planar surfaces, and the predetermined angle is set so that the temperature coefficient of the optical length has a predetermined value with the birefringent material fixed on the supporting member, and absolute value of a sum of (i) a product of (a) difference between linear expansion coefficients of the birefringent material and the supporting member and (b) refractive index of the birefringent material, (ii) thermoptical coefficient of the birefringent material, and (iii) change of refractive index due to a thermal strain between the supporting member and the birefringent material, is minimized.