In this case, however, there was the problem that since a laser beam incident on the photodetector is reduced not only at the time of recording but also at the time of reproduction, the S/N ratio of a reproducing signal becomes small. To solve this problem, it is conceivable to provide a light quantity cut filter on the light incident side of the photodector for interrupting a reflected light having a large intensity, so as to reduce an intensity of the incident light beam by a fixed quantity with respect to the entire band at the time of recording, thus to prevent saturation of the photodetector elements. As a result, the focusing or tracking control servo control becomes unstable because the photodetector element itself becomes saturated. If the recording power is high, since the R-CDs have a large reflectivity, an excessive reflected light is incident on photodetectors (light receiving element).
However, if an attempt is made to record information onto the R-CDs at a high linear velocity, in order to improve the transfer rate in future, the recording power should be higher. At present, since information is recorded at a low linear velocity (1.2 to 1.4 m/sec) in these R-CDs, the recording can be carried out with a recording beam of relatively small power (light intensity). On the other hand, there have been recently developed a recordable disc having a high reflectivity which is called an R-CD (Recordable Compact disc). Accordingly, since such discs are only applicable to the optical pick-up device exclusively used for discs having a small reflectivity, they cannot be reproduced by an apparatus exclusive for reproduction which can only reproduce discs having a large reflectivity. Since a semiconducor laser having a low output power (intensity) is conventionally used, optical discs having a small reflectivity have been used.
At the time of reproduction, a laser beam of a maximum light intensity in a range where information cannot be recorded is irradiated onto the disc, so as to keep S/N ratios of control signals of the servo system and an RF signal of the reproducing system to be sufficiently large.įurther, in the case of recordable discs reproduced by a conventional optical pick-up device, the sensitivity becomes higher when the reflectivity is lower. On the other hand, at the time of reproducing information, a laser beam having a smaller light intensity than that for recording is emitted from the optical pick-up device and irradiated onto the disc to reproduce recorded information, on the basis of a light signal reflected by the disc surface. At the time of recording information, a laser beam having a large light intensity, emitted from the optical pick-up device, is irradiated onto the disc to record information on the disc. To record information on the disc or to reproduce recorded information therefrom, an optical pick-up device irradiates, onto a disc, laser beams having appropriate intensity for recording and reproduction. This invention relates to an optical pick up device used in an apparatus for optically recording information on or reproducing information from a recordable optical disc.
is carried out on the basis of the detected signal. The light beam thus produced is detected by the signal detecting unit, and the servo control such as focusing servo, etc. The light beam switching unit carries out switching between a light having the wavelength for recording information and a light having the wavelength for reproducing information.The light intensity change unit transmits or reflects the light beam having the wavelength for recording information with attenuation to prevent a saturation of the photodiodes and transmits or reflects the light beam having the wavelength for reproducing information without attenuation. An optical pick-up device comprises a light source, a light beam switching unit, a light intensity change unit and a signal detection unit including photodiodes.A light beam emitted from the light source and irradiated on an information recording medium has a wavelength for recording information and a wavelength, different from the former, for the reproducing information.
0 kommentar(er)
