Dynamic information storage or retrieval – Condition indicating – monitoring – or testing – Including radiation storage or retrieval
Reexamination Certificate
1999-06-30
2001-11-13
Young, W. R. (Department: 2651)
Dynamic information storage or retrieval
Condition indicating, monitoring, or testing
Including radiation storage or retrieval
C369S112190, C369S112210
Reexamination Certificate
active
06317400
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to an optical pickup apparatus used for recording and reproducing record information by emitting a light beam from a light source onto an information record medium, and more particularly to an optical pickup apparatus for performing a power control of the light beam by detecting the light amount of the light beam emitted from the light source by means of a front monitor method.
2. Description of the Related Art
In a semiconductor laser as a light source of the optical pickup, since the emission power is changed due to a temperature change and aging, a stabilization of the power level of the light beam emitted onto the information record medium such as an optical disc is performed by conducting a power control by means of the APC (Automatic Power Control). As a representative method of this APC, there is a rear monitor method and a front monitor method.
Since, in the rear monitor method, the light beam emitted from the end surface of the semiconductor laser on the opposite side of the emission surface, there is a problem in a detection accuracy etc., resulting in that the front monitor is in fact employed. In the front monitor method, one portion of the light beam emitted from the semiconductor laser is monitored and is fed-back to a driving circuit of the semiconductor laser, so as to keep constant the power of the light beam.
For example, in the front monitor method, the light beam emitted from a semiconductor laser is inputted to a beam splitter. This beam splitter has a half reflection film, which transmits about 90% of the light beam emitted from the semiconductor laser and reflects the rest of it i.e., about 10% of it. By the action of this half reflection film of the beam splitter, about 90% of the light beam inputted to the beam splitter is transmitted therethrough to an optical disc through some more optical elements such as an objective lens.
On the other hand, about 10% of the light beam, which is reflected by the half reflection film of the beam splitter, is guided to a front monitor detector. The front monitor detector outputs an electric signal corresponding to a light amount of the emitted light beam. This electric signal is supplied to an APC circuit including a laser control circuit for performing a power control of the semiconductor laser. The APC circuit obtains a driving signal optimum for driving the semiconductor laser in accordance with the electric signal from the front monitor detector. In this manner, the output power of the semiconductor laser is controlled by the APC circuit on the basis of the electric signal outputted from the front monitor detector.
However, in the above mentioned optical pickup apparatus employing the front monitor method, since about 10% of the light beam is reflected by the half reflection film of the beam splitter and is guided to the front monitor detector, the power of the light beam emitted onto the optical disc is correspondingly reduced, resulting in that the usage efficiency of the light beam is reduced, which is a problem.
Further, since the property of the half reflection film of the beam splitter such as the reflectivity, the transmissibility or the like has a dispersion of about ±5%, it is necessary to suppose the change in the reflection of the light beam within a wide range of 5% to 15%, so that the design and the adjustment become difficult to perform. Furthermore, the property such as the reflectivity, the transmissibility or the like of the half reflection film of the beam splitter is changed in accordance with the environmental temperature, the humidity and the like, it becomes a problem that the reliability of the power control by means of the APC is certainly degraded.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an optical pickup apparatus, which can keep the usage efficiency of the light beam high and which is hardly influenced by the dispersion of the production or the environmental change, to thereby perform a highly reliable APC.
The above object of the present invention can be achieved by an optical pickup apparatus provided with: a light source for emitting a light beam; an optical system, which comprises an optical element having an incident surface, to which an AR (Anti-Reflection) coat is not applied and to which the light beam emitted from the light source is inputted obliquely by a predetermined incident angle, for guiding one portion of the emitted light beam transmitted through the incident surface onto an information record medium and guiding a reflection light of one portion of the emitted light beam reflected from the information record medium, and also reflecting another portion of the emitted light beam by the incident surface; a main light detector for receiving the reflection light from the information record medium guided by the optical system and outputting a main detection signal corresponding to record information recorded on the information record medium; a monitor detector for receiving another portion of the emitted light beam reflected by the incident surface of the optical element and outputting a monitor detection signal corresponding to a light amount received by the monitor detector; and a controlling device for controlling an output power of the light beam emitted from the light source on the basis of the monitor detection signal.
According to the optical pickup apparatus of the present invention, the light beam emitted from the light source such as a semiconductor laser is inputted to the incident surface of the optical element obliquely by the predetermined incident angle. At this time, since the AR coat is not applied to this incident surface of the optical element, the portion of the light beam is reflected by a predetermined reflection coefficient and is received by the monitor detector. Then, the monitor detection signal is outputted by the monitor detector and is fed back to the light source through the controlling device, so that the output power of the light beam is controlled by the controlling device. Therefore, the reflection coefficient of the incident surface (i.e., the half reflection surface) of the optical element with respect to the monitor detector is determined to be constant mainly in dependence upon the incident angle. Accordingly, the light amount which can be kept in stable is reflected and guided to the monitor detector, so that it is possible to perform the highly reliable APC of the front monitor method which is hardly influenced by the temperature or humidity.
In one aspect of the optical pickup apparatus of the present invention, the optical element has a shaped prism.
According to this aspect, in case of constructing the optical system by the shaped prism, it is enough not to apply the AR coat to the incident surface of the shaped prism. Thus, it is possible to perform the APC of the front monitor method and it is possible to realize a highly reliable APC without the change due to the temperature or humidity, by use of a rather simplified structure while employing an existing optical system.
In another aspect of the optical pickup apparatus of the present invention, the optical element has a polarization beam splitter.
According to this aspect, in case of constructing the optical system by the polarization beam splitter, it is enough not to apply the AR coat to the incident surface of the polarization beam splitter. Thus, it is possible to perform the APC of the front monitor method and it is possible to realize a highly reliable APC without the change due to the temperature or humidity and with little light amount loss, by use of a rather simplified structure while employing an existing optical system.
In another aspect of the optical pickup apparatus of the present invention, the optical element comprises a shaped prism including a polarization beam splitter.
According to this aspect, in case of constructing the optical system by the shaped prism including the polarization beam splitter, it is enough no
Nagahara Shinichi
Yanagawa Naoharu
Pioneer Electronic Corporation
Pitney Hardin Kipp & Szuch LLP
Young W. R.
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