Abstract:
An optical disk apparatus carries out focusing operation at a position shifted by a very small distance from the surface of a disk or from the recording surface of the disk in such a condition that the disk is stopped or is rotating at a speed sufficiently lower than a normal operational speed for the first focusing operation, after that, the disk is rotated at the normal rotational speed and a focus deviation amount is stored, and then focusing operation on an information recording surface is performed while applying the stored focus deviation component to a focus moving means.

Description:
CLAIM OF PRIORITY 
       [0001]    The present application claims priority from Japanese applications JP-2006-330111 filed on Dec. 7, 2006 the content of which is hereby incorporated by reference into this application. 
       BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to an optical disk apparatus and a focusing method. 
         [0003]    One of background arts in the present technical field is disclosed in, for example, JP-A-07-65382. The Publication states “A value for the focus deviation of an optical disk 5 is found from an output of a focusing detector 10 and then stored in a memory circuit 25. the moving speed of a objective lens 4 is changed according to the focus deviation amount of the focus deviation memory circuit 25 upon refocusing operation to delay a relative speed of a light beam and the optical disk 5 and to obtain stable focusing operation”. 
       SUMMARY OF THE INVENTION 
       [0004]    Focusing operation in the prior art optical disk apparatus will be briefly explained. 
         [0005]    The controlled focusing operation is carried out by rotating a recording medium (which will be referred to as the optical disk, hereinafter) at a predetermined rotational speed under control of a motor, converging, projecting a light beam emitted from a light source such as a semiconductor laser, and moving the objective lens. 
         [0006]    When the focusing position of the light beam is moved to cause the light beam to pass over a recording surface of the disk, a focus error signal of an S curve shape is obtained. A range of detection of the S-shaped focus error signal is as narrow as several um. For this reason, in order to obtain stable focusing operation by decreasing an overshoot in the focusing operation, it is desirable to decrease the moving speed of the objective lens. 
         [0007]    To this end, there is disclosed in JP-A-07-65382 a method of storing focus deviation amount as a movement in the vertical direction of the optical disk in the recording surface of the optical disk and applying the movement to a focus moving means upon refocusing operation in such a manner that a relative speed of the light beam to the optical disk is not influenced by the focus deviation. 
         [0008]    As a result, the relative speed of the light beam to the optical disk upon the focusing operation can be reduced, thus enabling stable focusing control. 
         [0009]    With the aforementioned arrangement, however, since the relative speed of the light beam to the optical disk is influenced by the focus deviation upon the first focusing operation before the focus deviation is stored, stable focusing control cannot be attained. 
         [0010]    It is an object of the present invention to provide an optical disk apparatus which can attain stable focusing operation even upon the first focusing operation and also a focusing method. 
         [0011]    The above object can be attained, as an example, by performing focusing operation in such a condition that a disk is stopped or the disk is rotating at a low speed for the first focusing operation. 
         [0012]    In accordance with an aspect of the present invention, there can be provided an optical disk apparatus which performs stable focusing operation even upon the first focusing operation and also a focusing method. 
         [0013]    Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a block diagram of a focusing device in a first embodiment: 
           [0015]      FIG. 2  shows a waveform of a focus error signal in the first embodiment; 
           [0016]      FIG. 3  shows waveforms of signals appearing in the first embodiment for explaining focusing operation; 
           [0017]      FIG. 4  shows a flow chart of the focusing operation in the first embodiment; 
           [0018]      FIG. 5  shows a block diagram of a focusing device in a second embodiment: 
           [0019]      FIG. 6  shows a waveform of a focus error signal in the second embodiment; 
           [0020]      FIG. 7  shows waveforms of signals appearing in the second embodiment for explaining focusing operation; and 
           [0021]      FIG. 8  shows a flow chart of the focusing operation in the second embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    The present invention will be explained as to embodiments of the invention with reference to the accompanying drawings. 
         [0023]    In the following embodiment, it is assumed that focusing operation is performed in front of a disk surface or recording surface in such a condition that a disk is not rotated yet. However, the disk may be rotated in such conditions that the disk is rotated at a speed sufficiently lower than a normal rotational speed and is not influenced by focus deviation. 
       Embodiment 1 
       [0024]      FIG. 1  shows an arrangement of an optical disk apparatus in accordance with a first embodiment. 
         [0025]    In  FIG. 1 , a light beam emitted from a laser  2  is passed through a collimating lens  3  to form collimated light, and then passed through a beam splitter  4 , a ¼ wavelength plate  5 , and an objective lens  6  to be focused on a disk  1 . Light reflected by the disk  1  is again passed through the objective lens  6  to form collimated light, passed through the ¼ wavelength plate  5 , the beam splitter  4 , and a cylindrical lens  7 , and then directed into a light detector  8  where the incident light is converted to an electric signal. 
         [0026]    An output of the light detector  8  is applied to a signal processor  9  which in turn generates a focus error signal FE indicative of a offset in the focus of the light beam focused on the disk  1 . The focus error signal FE is applied to a focus controller  10  to be subjected to phase compensation for stable operation of a focus control system. 
         [0027]    The focus error signal FE is also applied to an FZC (Focus Zero Cross) generator  11 , which in turn compares the signal FE with a predetermined threshold Vth and outputs an FZC signal. 
         [0028]    A controller  12  determines timing for focus control to be turned ON on the basis of the FZC signal, and outputs an FON signal to the focus controller  10 . The focus controller  10  controls an output to an adder  13  according to the FON signal to turn ON/OFF the focus control. 
         [0029]    The adder  13  adds an output of the focus controller  10  and an output of a focus deviation memory  14 , and outputs an added result to an adder  15 . A sweeper  16  outputs a sweep signal SWP to the adder  15  to drive an actuator  18  in a disk surface direction under control of a sweep control signal SCNT received from the controller  12 . The output of the focus controller  10  is applied to the driver  17  via the adders  13  and  15  to drive the actuator  18 . The objective lens  6  is arranged so as to be operated together with the actuator  18  as a single unit. That is, the actuator  18  is driven in a direction vertical to the disk surface for the focus control. 
         [0030]      FIG. 2  shows a waveform of the focus error signal FE when the objective lens  6  is moved close to the disk  1  from a position sufficiently away from the disk. At a time point that the objective lens  6  first passes through a position b where the light beam is focused on the disk surface, a focus error signal called an S-shaped signal is output from the signal processor. At a time point that the objective lens  6  is further moved closer to the disk  1  and passes through at such a position a where the beam is focused on the information recording/reproducing surface, the S-shaped focus error signal is output from the signal processor. 
         [0031]    Explanation will next be made as to focusing operation by referring to  FIGS. 3 and 4 . 
         [0032]    In the focusing operation, it is assumed that the sweep control signal SCNT output from the controller  12  to the sweeper  16  has first a high level of “H” (time t 1 , step S 100 ). As a result, the sweeper  16  outputs such a sweep signal SWP as to move the objective lens  6  to a position sufficiently away from the disk surface and then to move the lens close to the disk surface, to the adder  15 . The driver  17  drives the actuator  18  toward the disk surface according to the sweep signal SWP. When the objective lens  6  reaches a position in the vicinity of the focused position or point b on the disk surface, the S-shaped focus error signal is output from the signal processor. An FZC generator  11  compares the focus error signal with the predetermined threshold Vth and outputs the FZC signal to the controller  12 . The controller  12  determines at a rising edge of the FZC signal that the objective lens  6  reached the focused position or point b on the disk surface (time t 2 , step S 101 ), and sets the sweep control signal SCNT to have a low level of “L” and a focus ON signal FON to have a high level of “H” (time t 2 , step S 102 ). This causes the sweep signal SWP to be stopped, the focus control to be started, and the focusing operation on the disk surface to be carried out. When a spindle ON signal SPON issued to a motor controller  19  is set to have a high level of “H”, the motor controller  19  outputs a control signal to a driver  20  to rotate a motor  21  at a predetermined rotational speed and to turn the disk  1  (time t 3 , step S 103 ). The motor controller  19  outputs a SLOCK signal indicative of the rotational state of the disk  1 , to the controller  12 . At a time point that the rotational speed of the disk  1  reaches a predetermined level, the SLOCK signal is set to have a high level of “H” (time t 4 ). When confirming that the level of the SLOCK signal was changed to “H” (step S 104 ), the controller  12  set the level of a MON signal to be output to the focus deviation memory  14  at “H”, thus starting recording a focus deviation component (step S 105 ). The focus deviation memory  14  stores therein the focus deviation component from the output of the adder  13 , and again adds the stored focus deviation component to the adder  13 , thus forming a so-called repetitive control system. At a time point that the recording of the focus deviation component was started and then recording operation was carried out during a predetermined number N of turns, the controller  12  sets the sweep control signal SCNT to be output to the sweeper  16  therefrom to have a high level of “H” and also sets the focus ON signal FON to have a low level of “L” (time t 5 , step S 106 ). As a result, the focus control is turned OFF and simultaneously the sweep signal SWP is output from the sweeper  16  to move the objective lens  6  closer to the recording surface of the disk  1 . The sweep signal and the output of the focus deviation memory  14  are added together at the adders  13  and  15 , and the added signal is sent to the driver  17  to generate such a signal as to drive the actuator  18 . As a result, the objective lens  6  is driven toward the recording surface according to the sweep signal SWP issued from the sweeper  16  while substantially following up the focus deviation amount. When the objective lens  6  reaches a position in the vicinity of the focused position or point a on the recording surface, the S-shaped focus error signal is output from the signal processor. The FZC generator  11  compares the focus error signal with the predetermined threshold Vth and outputs the FZC signal to the controller  12 . The controller  12  determines at a rising edge of the FZC signal that the objective lens  6  reached the focused point a on the recording surface (time t 6 , step S 108 ), and sets the sweep control signal SCNT to have a low level of “L” and the focus ON signal FON to have a high level of “H” (step S 109 ). As a result, the sweep signal SWP is stopped and simultaneously the focus control is started to carry out the focusing operation on the recording surface. 
         [0033]    When the first focusing operation is carried out in such a condition that the disk is not rotated yet or is rotated at a speed much lower than the predetermined rotational speed as in the embodiment 1, the relative speed between the light beam and the optical disk can be made small and stable focusing operation can be attained advantageously. During recording an amount of focus deviation, the light beam is focused on the disk surface not on the recording surface. Thus information on the recording surface is not destroyed by this operation. 
       Embodiment 2 
       [0034]    Next, the second embodiment of the present invention will be explained by referring to  FIGS. 5 to 8 . 
         [0035]    In  FIG. 5 , constituent elements having the same functions as those in  FIG. 1  are denoted by the same reference numerals or symbols. Reference numeral  22  denotes a diffraction grating which divides a light beam issued from the laser  2  into first-order and zero-order light beams. The zero-order light beam is used mainly to detect record/reproduce signals, and the first-order light beam is used together with the zero-order light beam to detect a focus error. A method of detecting a focus error using the first-order light beam and the zero-order light beam is known as a differential astigmatism method. A jumper  23  outputs a jump signal JMP to cause the actuator  18  to be accelerated or decelerated toward the disk surface according to a jump trigger signal JTRIG received from the controller  12   
         [0036]      FIG. 6  shows a waveform of the focus error signal FE when the objective lens  6  is moved close to the disk  1  from a position much spaced from the disk  1  in the differential astigmatism method. At a time point that the objective lens  6  first passes through a point b′ before the light beam is focused on the disk surface, a focus error signal of an S curve shape is output. 
         [0037]    At a time point that the objective lens  6  further passes through a point b at which the light beam is focused on the disk surface the objective lens  6  is moved toward the disk  1 , the S-shaped focus error signal is output. At a time point that the objective lens  6  passed through the point a′ before the light beam is focused on the recording surface as the objective lens  6  is moved toward the disk  1 , the S-shaped focus error signal is output. At a time point that the objective lens  6  passes through the point a at which the light beam is focused on the recording surface as the objective lens  6  is moved toward the disk  1 , the S-shaped focus error signal is output. Accordingly, in the differential astigmatism method, the S-shaped focus error signal is detected four times at positions at which the light beam is focused on the disk surface and on the recording surface and at positions before the former positions as the objective lens  6  is moved closer to the disk  1  from a position sufficiently away from the disk  1 . 
         [0038]    Explanation will next be made as to the focusing operation with reference to  FIGS. 7 and 8 . In the focusing operation, it is assumed that the sweep control signal SCNT output from the controller  12  to the sweeper  16  is first set to have a high level of “H” (time t 1 , step S 100 ). This causes the sweeper  16  to output to the adder  15  the sweep signal SWP to move the objective lens  6  at a position sufficiently away from the disk surface and then to move it closer to the disk surface. The driver  17  drives the actuator  18  toward the disk surface according to the sweep signal SWP. When the objective lens  6  reaches a position in the vicinity of the focused point b′ where the light beam is focused on the disk surface, a first S-shaped focus error signal is output. The FZC generator  11  compares the focus error signal with the predetermined threshold Vth and output the FZC signal to the controller  12 . The controller  12  determines at a rising edge of the FZC signal that the objective lens  6  reached a point b′(time t 2 , step S 101 ). As the objective lens  6  is further moved closer to the disk  1 , a second S-shaped focus error signal is output at a point in the vicinity of a point b at which the light beam is focused. 
         [0039]    The controller  12  determines at a rising edge of the FZC signal that the objective lens  6  reached the point b (time t 3 , step S 101 ′). When the objective lens  6  is moved closer to the disk  1 , a third S-shaped focus error signal is output at a point in the vicinity of the point a′ before a point at which the light beam is focused on the recording surface. The controller  12  determines at a rising edge of the FZC signal that the objective lens  6  reached the point a′ (time t 4 , step S 101 ″). The controller sets the sweep control signal SCNT to have a low level of “L” and the focus ON signal FON to have a high level of “H” (time t 4 , step S 102 ). As a result, the sweep signal SWP is stopped, and simultaneously the focus control is started to perform the focusing operation at the point a′ before the recording surface. Next, when the spindle ON signal SPON output to the motor controller  19  is set to have a high level of “H”, a control signal is output from the motor controller  19  to the driver  20  to rotate the motor  21  at a predetermined rotational speed and to turn the disk  1  (time t 5 , step S 103 ). The SLOCK signal indicative of a rotational state of the disk  1  is output from the motor controller  19  to the controller  12 , so that the SLOCK signal has a high level of “H” when the rotational speed of the disk  1  reached a predetermined value (time t 6 ). The controller  12  confirms that the SLOCK signal had a high level of “H” (step S 104 ), sets the level of the MON signal to be output to the focus deviation memory  14  at a high level “H” to start recording the focus deviation component (step S 105 ). The focus deviation memory  14  stores therein a focus deviation component from the output of the adder  13 , and the stored focus deviation component is again sent to the adder  13 , thus forming a so-called repetitive control system. At a time point that the recording operation was carried out by a predetermined number N of turns after the recording of the focus deviation component was started (step S 106 ), the controller  12  sets the level of the jump trigger signal JTRIG to be output from the controller  12  to the jumper  23  at a high level “H” and also sets the level of the focus ON signal FON at a low level “L” (time t 7 , step S 107 ′). As a result, the focus control is turned OFF and simultaneously, the jumper  23  outputs a jump signal JMP to cause the objective lens  6  to be moved toward the recording surface of the disk  1 . The jump signal and the output of the focus deviation memory  14  are added together in the adders  13 ,  15 , and  25  to form a signal. The formed signal is sent to the driver  17  to drive the actuator  18 . As a result, the objective lens  6  is driven toward the recording surface according to the jump signal JMP issued from the jumper  23  while substantially following up the focus deviation amount. When the objective lens  6  reaches a point in the vicinity of the position a where the light beam is focused on the recording surface, a S-shaped focus error signal is output. The FZC generator  11  compares the focus error signal with the predetermined threshold Vth and outputs the FZC signal to the controller  12 . The controller  12  determines at a rising edge of the FZC signal that the objective lens  6  reached the beam focused point a on the recording surface (time t 8 , step S 108 ), and sets the jump trigger signal JTRIG to have a low level “L” and the focus ON signal FON to have a high level “H” (step S 109 ′). As a result, the jump signal JMP is stopped and simultaneously the focus control is started, thus attaining the focusing operation on the recording surface. 
         [0040]    When the first focusing operation is carried out in such a condition that the disk is not rotated yet or is rotated at a speed sufficiently lower than a predetermined rotational speed as in the embodiment 2, the relative speed between the light beam and the optical disk can be made small and the stable focusing operation can be attained advantageously. When an a focus deviation level is recorded, the light beam is not focused on the recording surface by focusing the light beam at a position immediately before the recording surface. Thus information on the recording surface can advantageously be avoided from being destroyed. Since the focusing operation is made at a position away from the recording surface, the focus can advantageously be less influenced by a flaw or dust on the disk surface than when the focusing operation is made on the disk surface. 
         [0041]    It should be further understood by those skilled in the art that although the foregoing description has been on embodiments of the invention, the invention is not limited thereto and various change and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.