Patent ID: 7986606
Filing Date: 2011-07-26
Classification: G02B,G11B

Abstract:
1. An optical head device comprising a light source for emitting a linearly polarized laser beam, an objective lens for converging the laser beam to guide it to an optical recording medium, and a photodetector for receiving light reflected by the optical recording medium, wherein the laser beam includes three laser beams having wavelengths of λ 1 =405 nm, λ 2 =660 nm and λ 8 =790 nm, a broadband phase plate for controlling phase state of the laser beam is disposed between the light source and the objective lens, the broadband phase plate is constituted by two phase plates laminated so that their optical axes are crossed, and when the three laser beams have wavelengths λ 1 , λ 2 and λ 3 (λ 1 <λ 2 <λ 3 ) respectively, at least one of the phase plates has such retardation values that proportions R(λ 1 )/R(λ 3 ) and R(λ 2 )/R(λ 3 ) between retardation values R(λ) at the wavelengths, satisfy the following formulas: