Abstract:
An optical information processing apparatus for irradiating a focused light beam to an optical recording medium to record and/or reproduce information. The apparatus includes a device for focus-controlling the light beam, a focus pull-in device for switching the focus control device from an inactive state to an active state, a device for moving an irradiation position of the light beam in a surface of the medium to effect tracking control, and a device for holding the light beam at a center of a movable range by the tracking control device, during the focus pull-in.

Description:
This applicaiton is a continuation of prior application Se.r No. 07/312,655, filed Feb. 21, 1989, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical information processing apparatus which irradiates a focused light beam to an optical recording medium to record and/or reproduce information. 
     2. Related Background Art 
     Various disk, card and tape media which record information by light and reproduce the recorded information have been known. 
     For example, in an optical information processing apparatus which uses an optical disk, the optical disk is scanned by a light beam which is modulated by recording information and focused to a fine spot, and information is recorded as optically detectable record pit tracks (information tracks). In order to exactly record the information without difficulty, such as crossing of the information tracks, it is necessary to control the irradiation position of the light spot on a plane of the optical disk in a direction perpendicular to the scan direction (auto-tracking, hereinafter referred to as AT). Further, in order to irradiate the light spot as a fine spot which is stable in spite of warp or mechanical tolerance of the optical disk, it is necessary to control the irradiation position normal to the plane of the optical disk (auto-focusing, hereinafter referred to as AF). In a reproduction mode, both AT and AF are necessary, too. Various techniques for AT and AF have been known. Usually, a focusing error signal and a tracking error signal are derived from differences between photo-sensing planes of focusing and tracking photo-detectors, and an objective lens is driven by AT and AF actuators. 
     In the above apparatus, when recording or reproduction is to be started, the focusing control means is switched from an inactive state to an active state, that is, so-called focus pull-in is effected. In the focus pull-in, a focus control loop is opened and a triangular wave signal is applied to a focus actuator to move the objective lens up and down, as described in U.S. Pat. No. 4,542,491. When the objective lens reaches a position corresponding to a linear region of a focus error signal, the loop is closed. During the focus pull-in, the tracking control loop is kept open. 
     In the above method, since no drive force along the tracking path is applied to the objective lens in the focus pull-in, the objective lens may deviate from an intended mechanical center (a center of a movable range by the tracking actuator, at which an optical axis of the light beam and an optical axis of the objective lens coincide). Such a deviation may be caused by a residual strain of a spring which movably supports the objective lens or by an offset of an input circuit to the tracking actuator. If such a deviation occurs, the operation range may be unbalanced when the tracking actuator is activated after the focus pull-in. For example, assuming that the optical system of the apparatus assures an optical output in a range of ±250 μm from the optical center and that there occurs a deviation of +100 μm, the assurance of the optical output is in a range of -350 μm to +100 μm. Accordingly, if the light spot is track-jumped in a positive direction by 100 tracks (1.6 μm/track) after the focus pull-in, the light beam is out of the range of assurance of the optical output. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an optical information processing apparatus which solves the above problems and prevents the deviation of the light beam along the tracking path from occurring in the focus pull-in. 
     The above object of the present invention is achieved by 
     an optical information processing apparatus for irradiating a focused light beam to an optical recording medium to record and/or reproduce information, comprising: 
     means for focus-controlling the ilght beam; 
     focus pull-in means for switching said focus control means from an inactive state to an active state: 
     means for moving an irradiation position of the light beam in a surface of the medium to effect tracking control; and 
     means for holding the light beam at a center of a movable range by said tracking control means, during the focus pull-in. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a schematic view of one embodiment of the present invention applied to an optical disk apparatus, 
     FIG. 2 shows a block diagram of a control circuit of the apparatus of FIG. 1, and 
     FIGS. 3A to 3D show signal waveforms in the circuits of FIG. 2. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows an embodiment of the present invention applied to an optical disk apparatus. An optical disk 31 is rotated by a spindle motor 32. An optical head 33 for recording and reproducing information on and from the optical disk 31 is moved radially of the disk by drive means (not shown). 
     A semiconductor laser 34 is provided in the optical head 33. A light beam emitted from the semiconductor laser 34 passes through a collimater lens 35 and a beam splitter 36 and is focused onto the optical disk 31 by an objective lens 1. The light reflected by the optical disk 31 passes through the objective lens 1 and is reflected by a beam splitter 36. The reflected beam is focused by a sensor lens 38 which comprises a rotating symmetrical lens and a cylindrical lens, and is detected by a four-element detector 8, which detects a focusing error signal and a tracking error signal by using an astigmatism method and a push-pull method, and supplies those signals to an actuator, which in turn drives the objective lens along the optical axis and the tracking path which is perpendicular to the optical axis, in accordance with those signals to effect the AF and AT controls. 
     FIG. 2 shows a block diagram of an AT/AF control circuit of the apparatus of FIG. 1. 
     The objective lens 1 is fixed to a bodytube 2 on which a light emitting diode (LED) 3 is mounted. A two-element detector 4 is arranged on a line of light irradiated from the LED 3. When the objective lens 1 moves in the tracking direction, the light irradiated from the LED 3 is moved on the two-element detector 4 by the same distance. Outputs of the two-element detector 4 are supplied to a differential amplifier 5. 
     The four-element detector 8 detects the light reflected by the optical disk 31. Outputs of the respective photo-sensing elements of the four-element detector 8 are supplied to a tracking error detecting circuit 9 and a focusing error detecting circuit 11, which detect a tracking error signal and a focusing error signal, respectively. Those error detecting circuits may be constructed as shown in U.S. Pat. No. 4,410,969. The outputs of the tracking error detecting circuit 9 and the focusing error detecting circuit 11 are supplied to a tracking equalizer 10 and a focusing equalizer 12 through a switch (SW) 17 and a switch (SW) 18, respectively. The output of the tracking equalizer 10 and the output of the differential amplifier 5 through the SW 16 are supplied to a subtractor 21, whose output is amplified by a tracking amplifier 6, whose output is supplied to a tracking actuator 15. On the other hand, the output of the focusing equalizer 12 and an output of a triangular wave generator 13 through the SW 19 are supplied to an adder 22, whose output is amplified by a focusing amplifier 14, whose output is applied to a focusing actuator 7. 
     On the other hand, the outputs of the respective photo-sensing elements of the four-element detector 8 are added by a summation signal detecting circuit 23, whose output is supplied to a microprocessor unit (MPU) 24. The MPU 24 generates a focus pull-in timing signal based on the focusing error signal to control the switching of SW 16-SW 19. 
     The operation of the optical disk apparatus is now explained. 
     In the focus pull-in, the MPU 24 first opens the SW 17 and the SW 18 to deactivate the focusing servo and the tracking servo. Under this condition, the SW 16 is closed. Thus, the tracking amplifier 6, the tracking actuator 15, the LED 3, the two-element detector 4 and the differential amplifier 5 constitute a servo loop so that the objective lens 1 is positioned to and held at the mechanical center (optical adjustment point) which is the center of movable range in the tracking direction. Under this condition, the SW 19 is closed so that the output of the triangular wave generator 13 is applied to the focusing actuator 7 through the focusing amplifier 14 and the objective lens 1 is moved up and down to effect focus searching. At an in-focus position, the MPU 24 opens the SW 19 and closes the SW 18 to terminate the focus pull-in. At this moment, the objective lens is held at the mechanical center and no deviation occurs. Then, the MPU 24 opens the SW 16 and closes the SW 17, and activates the tracking servo at the mechanical center. 
     The focus pull-in timing signal is generated by the MPU 24 in the following manner. When the objective lens is moved parallel to the optical axis by a pattern signal generated by the triangular wave generator 13, the summation signal detecting circuit 23 supplies a signal shown in FIG. 3A to the MPU 24. On the other hand, the focusing error detecting circuit 11 supplies a focusing error signal shown in FIG. 3C to the MPU 24. The MPU 24 slices the signal of FIG. 3A at a level L to generate a window pulse shown in FIG. 3B. During a high level period of the signal of FIG. 3B, a zero-crossing point of the signal of FIG. 3C is detected to generate a timing signal shown in FIG. 3D. The timing signal is supplied to the SW 18 and the SW 19 from the MPU 24 so that the SW 19 is opened and the SW 18 is closed. 
     In accordance with the optical information processing apparatus of the present invention, the focus pull-in is effected while the optical system for irradiating the light beam to the optical recording medium is kept at the predetermined position. Thus, the light beam may be track-jumped to a maximum point in the range of optical assurance even immediately after the focus pull-in. 
     Various modifications of the present invention may be made in addition to the embodiment described above. For example, the optical recording medium is not limited to a disk, but it may be a tape, card or any other shape. The present invention covers all of those modifications without departing from the scope of the following claims.