1. Field of the Invention
The present invention relates in general to an optical disc driver, and more particularly to a tilt detector for detecting a tilt of an optical axis of a beam of light, which is emitted by an optical pickup on a recording surface of an optical disc for the reading of information written on the recording surface, relative to the surface.
2. Description of the Prior Art
It is commonly required that an optical pickup should be improved in precision to meet requirements for a densification of an optical disc. In particular, it is currently required that an optical axis of a beam of light, which is emitted by the optical pickup on a recording surface of the optical disc for the reading of information written on the recording surface, that is, an optical axis of an object lens installed in the optical pickup should be perfectly perpendicular to the recording surface.
On the other hand, with the densification of the optical disc, the number of apertures (NA) of the object lens installed in the optical pickup increases and the beam of light emitted from the optical pickup has a shorter wavelength. As a result, a coma-aberration increases significantly due to a tilt of the optical axis of the beam of light emitted from the optical pickup relative to the recording surface of the optical disc, which leads to a keen need for detection of a tilt amount (for example, a tilted angle).
Particularly, provided that a liquid crystal device is used to correct the tilt on the basis of the detected tilt amount, an aberration will have to be generated to offset the tilt. In this regard, it is required to accurately detect and measure a tilt amount (for example, a tilted angle).
FIG. 9 shows the construction of a conventional tilt detector 102 for detecting a tilt amount (for example, a tilted angle). The tilt detector 102 is installed in an optical pickup, which generates a beam of light for the reading of information written on a recording surface of an optical disc 101. Separately from the beam of light generated by the optical pickup, a light emitting diode (LED) 103 emits a divergent beam of light on the recording surface of the optical disc 101. The divergent beam of light emitted from the LED 103 has an intensity of Gaussian distribution. A beam of light reflected from the recording surface of the optical disc 101 is received by a photodiode (PD) 106. The PD 106 is adapted to detect the centroid of the intensity of the received beam of light.
If the recording surface of the optical disc 101 is tilted as indicated by a broken line a in FIG. 9, the centroid of the intensity of a beam of light received on a receiving surface of the PD 106 moves accordingly. This centroid movement is detected by the PD 106, and a tilt amount is in turn detected on the basis of the detected centroid movement.
However, the above-mentioned conventional tilt detector has the following problem. That is, the centroid of the intensity of the beam of light received on the receiving surface of the PD 106 may move even with, for example, a variation in distance from the tilt detector 102 to the recording surface of the optical disc 101 due to a surface vibration of the disc 101, etc. In this case, the detected tilt amount value varies, thereby making it impossible to detect the tilt amount at a high degree of precision.
The above problem will hereinafter be described in detail referring again to FIG. 9. If the recording surface of the optical disc 1 is tilted as indicated by the broken line a, the centroid of the intensity of the beam of light on the receiving surface of the PD 106 moves beyond the LED 103. On the other hand, even when the recording surface of the optical disc 101 becomes more distant from a transmitting surface of the LED 103 and the receiving surface of the PD 106 as indicated by a broken line b, the centroid of the intensity of the beam of light on the receiving surface of the PD 106 moves beyond the LED 103.
Therefore, the present invention has been made in view of the above problem, and it is an object of the present invention to provide a tilt detector which is capable of detecting a tilt amount at a high degree of precision.
In accordance with the present invention, the above and other objects can be accomplished by the provision of a tilt detector for detecting a tilt of a recording surface of an optical disc, comprising light emitting means for emitting a divergent beam of light to be incident on the recording surface of the optical disc; a collimating lens for collimating the divergent beam of light emitted from the light emitting means and directing the collimated beam of light onto the recording surface of the optical disc; a condensing lens for condensing the collimated beam of light from the collimating lens, directed onto the recording surface of the optical disc and then reflected from the recording surface; and light receiving means for receiving the collimated beam of light condensed by the condensing lens and detecting the centroid of the intensity of the received beam of light, the light receiving means including a receiving surface partitioned into a plurality of areas for receiving the collimated beam of light condensed by the condensing lens.
Preferably, the collimating lens and condensing lens may constitute a single transmitting/receiving lens.
More preferably, the light emitting means and light receiving means may be arranged symmetrically on the basis of an optical axis of the transmitting/receiving lens.
As an alternative, the light emitting means and light receiving means may be arranged on the optical axis of the transmitting/receiving lens.
More preferably, the transmitting/receiving lens may have two centers of curvature.