Patent ID: 8305864
Filing Date: 2012-11-06
Classification: G11B

Abstract:
1. A compound objective lens comprising: an optical element having a diffraction structure; and a refractive lens, wherein the diffraction structure includes a first region, a second region, and a first boundary band provided between the first region and the second region, a diffraction structure having a sawtooth or stepwise cross section is formed in the first region and the second region, a height of the sawtooth or stepwise cross section formed in the first region provides a light beam, which has a predetermined wavelength, with a difference in optical path length of N times the predetermined wavelength, as compared with a case of transmission in air; a height of the sawtooth or stepwise cross section formed in the second region provides the light beam, which has the predetermined wavelength, with a difference in optical path length of J times the predetermined wavelength, as compared with a case of transmission in air, and at least one of a difference in height between both ends of the first boundary band and a width of the first boundary band provides the light beam, which has the predetermined wavelength, with a difference in optical path length of (N+J)/2 times (N and J are mutually different natural numbers) the predetermined wavelength, as compared with a case of transmission in air.