Patent ID: 6094308
Filing Date: 2000-07-25
Classification: G11B

Abstract:
A diffraction type filter, having a wavelength selectivity disposed in a luminous flux; said filter comprising a first region constituting a center portion of said filter, and a second region positioned outside the center portion of said filter, said first region exhibiting no diffracting action for two light beams respectively having wavelengths of .lambda..sub.1 and .lambda..sub.2 which are different from each other, said second region being formed with a diffraction grating having a diffracting action for only one of said two light beams respectively having wavelengths of .lambda..sub.1 and .lambda..sub.2 ;wherein said diffraction grating has a height h satisfying the following conditional expressions (1) and (2):h=L.lambda..sub.1 /(n.sub.1 -1) (1)h=M.lambda..sub.2 /(n.sub.2 -1)+K.lambda..sub.2 /2(n.sub.2 -1)(2)where.lambda..sub.1 and .lambda..sub.2 are the respective wavelengths of the two incident light beams;n.sub.1 is the refractive index of the diffraction grating with respect to light having the wavelength of .lambda..sub.2 ;n.sub.2 is the refractive index of the diffraction grating with respect to light having the wavelength of .lambda..sub.2 ;L is a positive integer;M is a maximum value among 0 and positive integers satisfying a conditional expression of h>M.lambda..sub.2 /(.sub.2 -1); andK is a value at least 0.65 but not greater than 1.35.