Patent EP-1736974-A2 (EP)
Inventor: MATSUMOTO HIROSHI
IPC: G11B7/125
Filed: 2006-06-13
Abstract: An optical-path compensating device comprises: a first wavelength plate inputting linearly polarized light that includes three different wavelengths λ 1  , λ 2  and λ 3  of which polarized directions are the same and optical paths are in parallel; a first birefringent plate inputting the linearly polarized light including the three different wavelengths that were emitted from the first wavelength plate; a second wavelength plate inputting the linear polarized light including the three different wavelengths that transmitted through the first birefringent plate; a second birefringent plate inputting the linear polarized light including the three different wavelengths that were emitted from the second wavelength plate; a third wavelength plate inputting the linear polarized light including the three different wavelengths that transmitted through the second birefringent plate. The first wavelength plate gives a phase difference 2πm 1  to the linearly polarized light including the wavelength λ 1 , a phase difference π(2n 1 -1) to the linearly polarized light including the wavelength λ 2  and a phase difference 2πq 1  to the linearly polarized light including the wavelength λ 3  respectively ( m 1 , n 1  and q 1  are integers). The first birefringent plate is arranged so as to make the linear polarized light including the wavelength λ 2  be an ordinary ray and the linear polarized light including the wavelengths λ 1  and λ 3  be an extraordinary ray, when these linear polarized light are emitted from the first wavelength plate along the optical axis of the first birefringent plate. The following equation is satisfied:        t   1     =     d   1     ⋅       n   ⁢     0   2     ⋅   tan   θ   +   n   ⁢     e   2       /       n   ⁢     0   2     −   n   ⁢     e   2       ⋅   tan   θ       ,         
where a distance for compensating an optical path is d 1  a refractive index of the birefringent plate to an ordinary ray is n0,a refractive index of the birefringent plate to an extraordinary ray is ne, an angle between an optical axis and a normal line to a main plane of the birefringent plate is θ and the thickness of the birefringent plate is ti. 
The second wavelength plate gives a phase difference π(2m 2 -1) to the linearly polarized light including the wavelength λ 1 , a phase difference 2πn 2  to the linearly polarized light including the wavelength λ 2  and a phase difference 2πq 2  to the linearly polarized light including the wavelength λ 3  respectively ( m 2 , n 2  and q 2  are integers). The second birefringent plate is arranged so as to make the linear polarized light including the wavelengths λ 1  and λ 2  be an extraordinary ray and the linear polarized light including the wavelength λ 3  be an ordinary ray, where these linear polarized light are emitted from the first wavelength plate along the optical axis of the second birefringent plate. The following equation is satisfied:       t   2     =     d   2     ⋅       n   ⁢     0   2     ⋅   tan   θ   +   n   ⁢     e   2       /       n   ⁢     0   2     −   n   ⁢     e   2       ⋅   tan   θ       ,         
where a distance for compensating an optical path is d 2  a refractive index of the birefringent plate to an ordinary ray is n0,a refractive index of the birefringent plate to an extraordinary ray is ne, an angle between an optical axis and a normal line to a main plane of the birefringent plate is θ and the thickness of the birefringent plate is t 2 . 
The third wavelength plate gives a phase difference π(2m 3 -1) to the linearly polarized light including the wavelength λ 1 , a phase difference π(2n 3 -1) to the linearly polarized light including the wavelength λ 2  and a phase difference 2πq 3  to the linearly polarized light including the wavelength λ 3  respectively ( m 3 , n 3  and q 3  are integers).