| 摘要 |
An optical pickup for reading information recorded on information tracks of a recording medium by using a main light beam and a subsidiary light beam includes a diffraction device for diffracting an original light beam to generate the main light beam and the subsidiary light beam. The main light beam is applied to a reading track which is one of the information tracks on which the information to be read is recorded. The subsidiary light beam is applied to the information track adjacent to the reading track. The diffracting device is made of an optical material having coefficient of linear expansion beta, which is determined as follows:where lambd0 is a wavelength of the original light beam at a design temperature, DELTAlambd is an amount of a change of the wavelength of the original light beam per unit temperature, DELTAtmax is a predetermined permissible amount of a change of an ambient temperature, L0 is a spacing between an irradiation position of the subsidiary light beam and an irradiation position of the main light beam at the design temperature, and DELTALmax is a predetermined permissible amount of a change of the spacing.
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