| 摘要 |
PROBLEM TO BE SOLVED: To converge light having different wavelengths on disks having a different thickness with the same objective lens by constituting a wavelength selective diaphragm with a volume hologram, transmitting the light having two wavelengths in the central part area containing the optical axis of this diaphragm and diffracting the light having one wavelength and changing an optical path in the peripheral part area not containing the optical axis. SOLUTION: A volume hologram 1 is constituted of a central part area 1b showing a high linear transmissivity for light having two wavelengths of a wavelength 650 nm and a wavelength 780 nm and a peripheral part area 1a being a diaphragm part having wavelength selectivity showing the high linear transmissivity for the light having the wavelength 650 nm and showing a low linear transmissivity for wavelength 780 nm. Since the peripheral part area and central part area of the wavelength selective diaphragm and constituted of the same material also and the thickness and the mean refractive indexes of the peripheral part area and central part area are nearly equal, optical lengths (refractive index × thickness) become nearly equal and a convergent characteristic on a disk becomes excellent. |
