Optical pickup and information reproducing apparatus

An optical pickup for reading information recorded on a plurality of information tracks of a recording medium by using a main light beam, a first subsidiary light beam and a second subsidiary light beam, the main light beam being applied to a reading track which is one of the plurality of information tracks on which the information to be read is recorded, the first subsidiary light beam being applied to a first adjacent track which is different one of the plurality of information tracks adjacent to one side of the reading track, the second subsidiary light beam being applied to a second adjacent track which is further different one of the plurality of information tracks adjacent to another side of the reading track, a spacing between an irradiation position of the main light beam and an irradiation position of the first subsidiary light beam is equal to a spacing between an irradiation position of the main light beam and an irradiation position of the second subsidiary light beam, the optical pickup comprising:

• a light source for emitting an original light beam;
• a diffraction device for diffracting the original light beam to divide the original light beam into the main light beam, the first subsidiary light beam and the second subsidiary light beam, so that the main light beam, the first subsidiary light beam and the second subsidiary light beam are applied to the reading track, the first adjacent track and the second adjacent track, respectively;
• and
• a receiving device for receiving the main light beam, the first subsidiary light beam and the second subsidiary light beam reflected by the recording medium and generating a main signal corresponding to the received main light beam, a first subsidiary signal corresponding to the received first subsidiary light beam and a second subsidiary signal corresponding to the received second subsidiary light beam,
• wherein the diffracting device is made of an optical material having coefficient of linear expansion β, which is determined as follows:
• α−Δα1≦β≦α+Δα2,
• α=Δλ/λ0,
• Δα1={(1+α×Δtmax)×ΔLmax}/{(L0+ΔLmax)×Δtmax},
• Δα2={(1+α×Δtmax)×ΔLmax}/{(L0−ΔLmax)×Δtmax},
• where λ0 is a wavelength of the original light beam at a design temperature of the optical pickup, Δλ is an amount of a change of the wavelength of the original light beam per unit temperature, Δtmax is a predetermined permissible amount of a change of an ambient temperature at which the optical pickup is used, L0 is a spacing between the irradiation position of the first subsidiary light beam and the irradiation position of the main light beam at the design temperature, and ΔLmax is a predetermined permissible amount of a change of the spacing between the irradiation position of the first subsidiary light beam and the irradiation position of the main light beam.

An apparatus for reading information recorded on a plurality of information tracks of a recording medium by using a main light beam, a first subsidiary light beam and a second subsidiary light beam and reproducing the read information, the main light beam being applied to a reading track which is one of the plurality of information tracks on which the information to be read is recorded, the first subsidiary light beam being applied to a first adjacent track which is different one of the plurality of information tracks adjacent to one side of the reading track, the second subsidiary light beam being applied to a second adjacent track which is further different one of the plurality of information tracks adjacent to another side of the reading track, a spacing between an irradiation position of the main light beam and an irradiation position of the first subsidiary light beam is equal to a spacing between an irradiation position of the main light beam and an irradiation position of the second subsidiary light beam, the apparatus comprising:

• (a) an optical pickup for emitting the main light beam, the first subsidiary light beam and the second subsidiary light beam to the recording medium, and generating a main signal corresponding to the main light beam reflected by the recording medium, a first subsidiary signal corresponding to the first subsidiary light beam reflected by the recording medium and a second subsidiary signal corresponding to the second subsidiary reflected by the recording medium;
• (b) a removing device for removing crosstalk caused by the first adjacent track and the second adjacent track from the main signal by using the first subsidiary signal and the second subsidiary signal;
• and
• (c) a reproducing device for reproducing the information on the basis of the main signal supplied from the removing device,
• wherein the optical pickup comprises:
• a light source for emitting an original light beam;
• a diffraction device for diffracting the original light beam to divide the original light beam into the main light beam, the first subsidiary light beam and the second subsidiary light beam, so that the main light beam, the first subsidiary light beam and the second subsidiary light beam are applied to the reading track, the first adjacent track and the second adjacent track, respectively;
• and
• a receiving device for receiving the main light beam, the first subsidiary light beam and the second subsidiary light beam reflected by the recording medium and generating the main signal, the first subsidiary signal and the second subsidiary signal,
• wherein the diffracting device is made of an optical material having coefficient of linear expansion β, which is determined as follows:
• α−Δα1≦β≦α+Δα2,
• α=Δλ/λ0,
• Δα1={(1+α×Δtmax)×ΔLmax}/{(L0+ΔLmax)×Δtmax},
• Δα2={(1+α×Δtmax)×ΔLmax}/{(L0−ΔLmax)×Δtmax},
• where λ0 is a wavelength of the original light beam at a design temperature of the optical pickup, Δλ is an amount of a change of the wavelength of the original light beam per unit temperature, Δtmax is a predetermined permissible amount of a change of an ambient temperature at which the optical pickup is used, L0 is a spacing between the irradiation position of the first subsidiary light beam and the irradiation position of the main light beam at the design temperature, and ΔLmax is a predetermined permissible amount of a change of the spacing between the irradiation position of the first subsidiary light beam and the irradiation position of the main light beam.