Abstract:
The present invention realizes stable focus entry control even in the case where a wideband focus error signal is used with which the focus error signal sensitivity and the level in the vicinity of a focus point vary depending on a light path length. A second focus error signal having a wide range is used, causing an objective lens to approach a disk while maintaining a predetermined rate of change with respect to time, and, during approach, a transition to focus servo is performed by detecting the focus position according to the first focus error signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a U.S. continuation application, filed under 35 USC 111(a) and claiming the benefit under 35 USC 120 and 365(c), of PCT application PCT/JP02/08273, filed Aug. 14, 2002. The foregoing application is hereby incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to optical storage devices and servo control methods thereof and, more particularly, to an optical storage device and a servo control method that oscillate a lens with respect to an object. 
     2. Description of the Related Art 
     In optical storage devices such as a disk device, in order to perform accurate recording/reproducing, focus servo control is performed such that a laser beam is focused on a recording film surface of a disk. In the focus servo control, by feeding back a focus error signal, an objective lens is controlled such that the distance between the objective lens and the disk is maintained to be constant. Recently, with the increase in recording density, the diameter of a laser beam spot is reduced. Thereby, the distance between a disk and an objective lens is reduced. In addition, the focus error signal, which is a linear error signal representing an axial relative position of an objective lens with respect to a disk, functions in the range of ±1 μm or less, which is a very narrow range. Conventionally, in order to orient an objective lens within the range in which feed back control is possible, the signal level of the focus error signal, so-called S curve, has been detected while oscillating the objective lens by open-loop control. In addition, focus entry control has been used in which a focus control loop is closed after it is detected that the position of the objective lens falls within a linear range so as to perform focus control by close-loop control. 
       FIG. 1  is a block diagram of a disk device.  FIGS. 2A ,  2 B and  2 C show conceptual diagrams of a separate optical system and positional relationships between a fixed head and a movable head. 
     In  FIG. 1 , a disk device  1  is mainly formed by a control unit  2  and a disk enclosure  3 . 
     The control unit  2  includes a higher interface  11 , a buffer memory  12 , a MPU  13 , an optical disk controller  14 , a read/write LSI  15 , a DSP  16 , a focus error signal (FES) detection circuit  17 , a track error signal (TES) detection circuit  18 , a zero-cross detection circuit  19 , and drivers  20  through  23 . In addition, the enclosure  3  includes a laser diode unit (LDU)  31 , a detector  32  for ID/MO signal detection, a head amp  33 , a spindle motor  34 , a magnetic field application part  35 , a detector  36   a  for focus error detection, a detector  36   b  for track error detection, a lens actuator  37 , and a voice coil motor  38 . 
     The separate optical system of  FIGS. 2A ,  2 B and  2 C includes a fixed head  201 , a movable head  202 , a base  203 , and a guide rail  204 . Additionally, the spindle motor  34  is mounted to the base  203 , and a disk  206  is mounted to the spindle motor  34 . The movable head  202  includes a movable head carriage  210 , a mirror  211 , a lens holder  212 , an objective lens  213 , a focus coil  214  that moves the objective lens in a direction perpendicular to a surface of the disk  206 , and a leaf spring  215 . Light emitted from the laser diode unit  31  shown in  FIG. 1  is guided to the movable head  202  via the fixed head  201 . The light reflected by the mirror  211  in the movable head  202  is directed to the disk  206  and focused on the disk  206  via the objective lens  213 . 
     The higher interface  11  performs interfacing with a higher apparatus. Data transmitted to/received from the higher apparatus are temporarily stored in the buffer memory  12 . The operation of the device  1  is controlled by the MPU  13  and the disk controller  14 . 
     The read/write LSI  15  performs modulation/demodulation of data. When writing data on a disk, the read/write LSI  15  modulates and supplies write data to the laser diode unit  31 , and when reading data from a disk, the read/write LSI  15  controls the laser diode unit  31  such that light for reading is emitted from the laser diode unit  31 . 
     The light emitted from the laser diode unit  31  to the disk  206  via the fixed head  201  shown in  FIGS. 2A ,  2 B and  2 C and the movable head  202  is reflected by the disk  206 , returned to the fixed head  201  via the movable head  202  shown in  FIGS. 2A ,  2 B and  2 C, and supplied to the detector  32  for ID/MO signal detection, the detector  36   a  for focus error detection, and the detector  36   b  for track error detection in the fixed head. The detector  32  for ID/MO signal detection detects an ID/MO signal component from the reflected light from the disk  206 , and supplies the detected ID/MO signal to the head amp  33 . The head amp  33  amplifies and supplies the ID/MO signal to the read/write LSI  15 . The read/write LSI  15  demodulates an ID and data from the ID/MO signal. The data demodulated in the read/write LSI  15  are stored in the buffer memory  12 . 
     The detector  36   a  for focus error detection converts incident light into an electronic signal and supplies it to the focus error signal detection circuit  17 . The focus error signal detection circuit  17  generates a focus error signal based on the electronic signal from the detector  36   a  for focus error detection. 
     The focus error signal generated in the focus error signal detection circuit  17  is supplied to the DSP  16 . The DSP  16  generates and supplies to the driver  22  a focus control signal based on the focus error signal generated in the focus error signal detection circuit  17 . Based on the focus control signal from the DSP  16 , the driver  22  supplies a driving current to the actuator  37 . Based on the driving current from the driver  22 , the lens actuator  37  moves the objective lens shown in  FIGS. 2A ,  2 B and  2 C in a focus direction, that is, a direction perpendicular to a surface of the disk  206 . By moving the objective lens of  FIGS. 2A ,  2 B and  2 C in the focus direction, the laser light emitted from the laser diode unit  31  is focused on the disk  206 . 
     In addition, the detector  36   b  for track error detection converts incident light into an electronic signal and supplies it to the track error signal detection circuit  18 . The track error signal detection circuit  18  generates a track error signal based on the signal detected in the detector  36   b  for track error detection. The track error signal detected in the track error signal detection circuit  18  is supplied to the DSP  16  and the track zero-cross signal detection circuit  19 . The track zero-cross signal detection circuit  19  generates and supplies to the DSP  16  a track zero-cross signal based on the track error signal. Based on the track error signal and the track zero-cross signal, the DSP  16  generates and supplies a tracking control signal to the driver  23 . 
     The driver  23  supplies a driving current to the voice coil motor  38  based on the tracking control signal from the DSP  16 . The voice coil motor  38  is driven based on the driving current from the driver  23 , and moves the movable head  202  in a radial direction of the disk  206  to perform a track following operation. 
     Further, the MPU  13  generates and supplies a spindle motor control signal to the driver  20 . Based on the spindle motor control signal from the MPU  13 , the driver  20  rotates the spindle motor  34 . 
     Furthermore, the MPU  13  generates and supplies a magnetic field control signal to the driver  21 . Based on the magnetic field control signal from the MPU  13 , the driver  21  supplies a driving current to the magnetic field application part  35 . The magnetic field application part  35  produces a bias magnetic field corresponding to the driving current from the driver  21 . The bias magnetic field produced by the magnetic field application part  35  is applied to the disk  206  and used for recording and/or reproduction of information. 
     Next, a detailed description is given of the operation of conventional focus entry control. 
     In an objective lens oscillating operation in the focus entry control, an objective lens is oscillated such that the objective lens surely passes a focus position. In a case where focus entry fails in the oscillating operation, there is a possibility that the objective lens may contact a disk. Thus, in order to prevent this, there is a method of providing a stopper between the disk and the objective lens so as to physically limit displacement of the objective lens. According to the method, even in the case where focus entry fails, the objective lens does not contact the disk, and thus it is possible to prevent data from being damaged. 
     However, when the focus distance becomes short, in a state where the laser light beam is focused on the disk, the distance between the objective lens and the disk becomes several dozen μm or less. Considering that vertical run-out of a disk due to rotation is several dozen to several hundred μm, it is impossible to provide a stopper for avoiding collision between the objective lens and the disk. Accordingly, in order to avoid collision between the objective lens and the disk, it is necessary to more positively perform focus entry control. 
     In order to positively perform focus entry control, it is necessary to control the overshoot amount of a focus servo control system after closed-loop control is started by closing the focus servo control system such that the focus error signal falls within a substantially linear range. In order to do so, it is necessary to suppress an error in the relative positions and the relative speeds of the objective lens and the disk immediately before starting closed-loop control by closing the focus servo control system. 
     However, in the conventional method where focus entry is performed by an oscillation operation of an objective lens, it is impossible to find a relative state between a disk and the objective lens until the S curve of the focus error signal is detected. In a case where, for example, the vertical run-out is ±100 μm in a disk rotated at 1500 rpm, the amplitude of vertical acceleration is ±15.7 mm/s. In order to perform focus entry based on detection of the S curve of the focus error signal, it is necessary to maintain the relative speed to a positive value in an approaching direction. Accordingly, when the objective lens is made to approach the disk at ±16 mm/s, the relative speed with respect to the disk varies in the range of 0.3-31.7 mm/s. Hence, in the case where the maximum value of the relative speed is 31.7 mm/s depending on the timing of vertical run-out of the disk, only 7.89 μs is required to pass the range where the focal depth is 0.25 μm. It is very difficult to perform focus servo control by a DSP whose sampling time for the focus error signal is 10 μs. 
     Accordingly, in order to realize stable focus entry control, it is necessary to control the relative speed between the disk and the objective lens. In order to do so, a sensor is required that outputs a position signal whose range of focus error detection is wider than that of the focus error signal. For example, in an embodiment shown in Japanese Laid-Open Patent Application No. 11-120569, having the title of the invention “Device and Method for Recording/Reproducing Optical Disk”, a position detection sensor for an objective lens is provided above a focusing actuator of the objective lens. The relative position with respect to a disk is detected by the position detection sensor for the objective lens, thereby aiming to realize stable focus entry. In this embodiment, however, it is necessary to mount the sensor to the actuator, which adversely affects reduction of the size and weight of the actuator. 
     Additionally, in an embodiment shown in Japanese Laid-Open Patent Application No. 7-287850, having the title of the invention “Optical Pickup Device and Focusing Control Method Thereof”, reflected light from a medium is divided into two beams of light: one is incident on a detection optical system having a low sensitivity to focus error detection and is used for a drawing operation; and the other is incident on a detection optical system having a high sensitivity for focus error detection and is used for focus servo control. According to the method, it is unnecessary to provide a sensor to a movable part. Thus, there is an advantage in that the size of an actuator can be reduced. 
     Further, the embodiment is characterized in that the direction in which a focus exists is detected by a first detection optical system having a low sensitivity from a position relatively distant from the focus, and an objective lens is moved in the correct direction. There is no description about a method for stably transit to focus servo by a second detection optical system having a high sensitivity. Thus, in an apparatus where the distance between the objective lens and a disk is very narrow, there is a risk the lens and the disk may collide with each other. 
     Hence, as for a method for outputting a focus error signal having a wide detection range for focus error by improving a conventional photodetector for servo, there is an embodiment shown in Japanese Patent Application No. 2001-93091, having the title of the invention “Optical Device for Recording and Reproducing Information”. With the use of the method, it is possible to detect the relative displacement between an objective lens and a disk from a position distant from a focus position by several dozen μm. Hence, it is possible to perform controlled movement of the objective lens to a focus position at a desired relative speed. 
     Next, a further detailed description is given of a focus error signal detection method described in Japanese Patent Application No. 2001-93091, with reference to  FIGS. 3 ,  4  and  5 . 
       FIG. 3  is a diagram showing the structure of the fixed optical head  201  shown in  FIGS. 2A ,  2 B and  2 C.  FIG. 4  is a block diagram of a first focus error signal detection circuit.  FIG. 5  is a block diagram of a track error signal and a second focus error signal detection circuit. 
     The fixed optical head of  FIG. 3  is formed by a laser diode  301 , a collimate lens  302 , beam splitters  303 ,  304  and  307 , a Wollaston prism  305 , condenser lenses  306 ,  308  and  310 , a Foucault prism  309 , a divided-by-two detector  32 , a divided-by-six detector  36 , and a divided-by-four detector  36   a . 
     The laser light emitted from the laser diode  301  is emitted from the fixed head  201  of  FIGS. 2A ,  2 B and  2 C via the collimate lens  302  and the beam splitter  303 , guided to the movable head  202 , and directed onto the optical disk  206 . The returning light reflected by the optical disk  206  is reflected by the beam splitter  303  and guided to the beam splitter  304 . The beam splitter  304  splits the incident light into two light beams and guides the respective light beams to the Wollaston prism  305  and the beam splitter  307 . The light incident on the Wollaston prism  305  is focused on the divided-by-two detector  32  via the condenser lens  306 , and the ID/MO signal is detected. 
     On the other hand, the light guided to the beam splitter  307  is divided into two light beams by the beam splitter  307  and the beam splitter  307  guides the respective light beams to the condenser lens  308  and the Foucault prism  309 . The light guided to the condenser lens  308  is focused on the divided-by-six detector  36   b . The light guided to the Foucault prism  309  is focused on the divided-by-four detector  36   a  via the condenser lens  310 . 
       FIG. 4  is a block diagram of the first focus error signal detection circuit  17 , which is formed mainly by a current-to-voltage (I-V) conversion circuit  401 , a first focus error signal (FE 1 ) operation circuit  402 , a focus sum (FS) operation circuit  403 , and an automatic gain control (AGC) circuit  404 . The divided-by-four detector  36   a  is formed by four detectors F, G, H and I. 
     According to the Foucault method, each output current of each of the detectors F, G, H and I of the divided-by-four detector  36   a , which output current is produced from the returning light focused on the divided-by-four detector  36   a , is converted into a voltage signal by the current-to-voltage (I-V) conversion circuit  401 . Then, the first focus error signal (FE 1 ) operation circuit  402  subtracts the sum signal of the voltage signals with respect to the detectors G and H from the sum signal of the voltage signals with respect to the detectors F and I, and outputs the resulting difference. On the other hand, the focus sum (FS) operation circuit  403  outputs the sum signal of the voltage signals with respect to the detectors F, G, H and I. Then, the automatic gain control (AGC) circuit  404  divides the output of the first focus error signal (FE 1 ) operation circuit  402  by the output of the focus sum (FS) operation circuit  403  to detect a focus error signal, which is used for normal focus servo. 
       FIG. 5  is a block diagram of the track error signal and the second focus error signal detection circuit, which is formed by a current-to-voltage (I-V) conversion circuit  501 , a track error (TE) operation circuit  502 , a second focus error signal (FE 2 ) operation circuit  503 , a track sum (TS) operation circuit  504 , and automatic gain control (AGC) circuits  505  and  506 . The divided-by-six detector  36   b  is divided into three in the direction orthogonal to a divided-by-two detector for tracking error signal detection according to a conventional push-pull method, and is formed by detectors A 1 , A 2 , B 1 , B 2 , C 1  and C 2 . 
     Each output current of each of the detectors A 1 , A 2 , B 1 , B 2 , C 1  and C 2  of the divided-by-six detector  36   b , which output current is produced from the returning light focused on the divided-by-six detector  36   b , is converted into a voltage signal by the current-to-voltage (I-V) conversion circuit  501 . The current-to-voltage (I-V) conversion circuit  501  outputs: a voltage signal A (=A 1 +A 2 ) obtained by converting the output currents of the detectors A 1  and A 2  into voltages and adding them together; a voltage signal B (=B 1 +B 2 ) obtained by converting the output currents of the detectors B 1  and B 2  into voltages and adding them together; a voltage signal C (=C 1 +C 2 ) obtained by converting the output currents of the detectors C 1  and C 2  into voltages and adding them together; a voltage signal D (=A 1 +B 1 +C 1 ) obtained by converting the output currents of the detectors A 1 , B 1  and C 1  into voltages and adding them together; and a voltage signal E (=A 2  +B 2  +C 2 ) obtained by converting the output currents of the detectors A 2 , B 2  and C 2  into voltages and adding them together. 
     The track error signal (TE) operation circuit  502  calculates a (D−E) signal and outputs a track error signal. The second focus error signal (FE 2 ) operation circuit  503  calculates a (A+B−C) signal, and a focus error signal according to the spot size detection method (hereinafter referred to as the SSD method) is output. A focus error signal according to the SSD method can obtain a second focus error signal having a wider detection area compared to a focus error signal according to the conventional Foucault method, and is suitable for the case where focus control is performed from a position distant from a focus position. Then, the track sum (TS) operation circuit  504  outputs a (D+E) signal. 
     Then, the automatic gain control (AGC) circuit  505  performs automatic gain control by dividing the track error signal output (D−E) by the sum signal (D+E). In addition, the automatic gain control (AGC) circuit  506  performs automatic gain control by dividing the second focus error signal (A+B−C) by the sum signal (D+E). 
       FIG. 6A  shows an example of the first focus error signal according to the Foucault method, and  FIG. 6B  shows an example of the second focus error signal according to the SSD method. While the use range of the first focus error signal of  FIG. 6A  is ±0.25 μm, the second focus error signal of  FIG. 6B  can be used in the range of −5 to +20 μm. 
       FIG. 7  shows an operating waveform of focus entry control using the second focus error signal. In  FIG. 7 , the vertical axis represents the distance between a focus position and an objective lens, and the horizontal axis represents time. 
     In this embodiment, the objective lens is made to approach a disk by open-loop control in a zone  710 , up to the position that is distant from the focus position by 10 μm. Then, when the position of the objective lens is within 10 μm from the focus position, position feedback control according to the second focus error signal is performed. A broken line  701  indicates a control target position signal, and a continuous line  702  indicates the operation of the position feedback control according to the second focus error signal. By approximating the target position signal  701  to zero over time, the position of the objective lens is made close to the focus position by the position feedback control according to the second focus error signal. When the target position signal  701  reaches zero, the input of the position feedback control is switched from the second focus error signal to the first focus error signal. A continuous line  703  indicates the operation of the position feedback control according to the first focus error signal. In the aforementioned manner, it is possible to make the relative position and relative speed of the objective lens with respect to the focus position at the time of switching the first focus error signal to be zero, and thus it is possible to realize stable focus entry control. 
     However, in the second focus error signal detection according to the SSD method, when the movable head  202  shown in  FIGS. 2A ,  2 B and  2 C moves and the total light path length varies, error signal sensitivity and signal level vary. Thus, there is a problem in that the distance from the focus and the signal sensitivity depend on the total light path length. 
       FIG. 8  shows the characteristics of the second focus error signal at the time when the movable head  202  shown in  FIG. 2B  is moved by +20 μm ( FIG. 2A ) and −20 μm ( FIG. 2C ) and the light path length from the fixed head is changed by 40 mm. In  FIG. 8 , a line  801  indicates the characteristics of the second focus error signal in the case where the movable head  202  is at the position of  FIG. 2A  (+20 μm), a line  802  indicates the characteristics of the second focus error signal in the case where the movable head  202  is at the position of  FIG. 2B  (0 μm), and a line  803  indicates the characteristics of the second focus error signal in the case where the movable head  202  is at the position of  FIG. 2C  (−20 μm). 
     Under such characteristics, in the case where the movable head  202  is at the position of  FIG. 2B , by switching to the first focus error signal at the time when the level of the second focus error signal becomes zero, it is possible to make the relative position and the relative speed of the objective lens with respect to the focus position at the time of switching the first focus error signal to be zero. Thus, it is possible to realize stable focus entry. However, in the case where, for example, the movable head is at the position of +20 mm shown in  FIG. 2A , when the zero level of the second focus error signal is detected, actually, the objective lens exists at the position that is distant from the focus position by −2.5 μm. On this occasion, since the position of the objective lens is outside the linear range of the first focus error signal, it is impossible to stably switch to the first focus error signal. 
       FIGS. 9A ,  9 B and  9 C show focus entry waveforms on this occasion.  FIG. 9A  shows the distance between the focus position and the objective lens,  FIG. 9B  shows the second focus error signal, and  FIG. 9C  shows the first focus error signal.  901 ,  902  and  903  of  FIGS. 9A ,  9 B and  9 C correspond to the positions of the movable head  202  of  FIGS. 2A ,  2 B and  2 C respectively. In the case of  FIG. 2B  where the movable head position is ±0, even if focus servo control according to the second focus error signal is switched to focus servo control according to the first focus error signal in the vicinity of the time 78.5 ms, the transition is smooth. However, in the case where, for example,  FIG. 2A  where the position of the movable head is +20 mm, as indicated by the lines  901  in  FIGS. 9A ,  9 B and  9 C, though the second focus error signal is zero in the vicinity of the time 78.5 ms, the actual shift of the position of the objective lens from the focus position is −2 μm. Thus, at this time point, since it is outside the linear signal detection range of the first focus error signal, large overshooting occurs at the time of transition to servo control according to the first focus error signal. The same applies to the case of  FIG. 2C  where the position of the movable head is −20 mm as indicated by the line  903 . 
     In view of the above, it is an object of the present invention to provide an optical storage device and a servo control device therefor that can perform stable focus entry by smoothly performing a transition to focus servo control according to the first focus error signal even in the case where the characteristics of the second focus error signal vary due to a variation in the light path length. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an optical storage device and a servo control device therefor in which the above-mentioned problems in the prior art are eliminated. 
     It is another object of the present invention to realize stable focus entry control even in the case where a focus error signal is used with which the focus error signal sensitivity and the level in the vicinity of a focus point vary depending on the light path length. 
     In order to achieve the above-mentioned objects, the present invention uses a second focus error signal having a wide detection range, causing an objective lens to approach a disk while maintaining a predetermined rate of change with respect to time, and, during approach, a transition to focus servo is performed by detecting the focus position by using the first focus error signal. 
     According to the present invention, by using the second focus error signal having the wide detection range, the objective lens is caused to approach the disk while maintaining the predetermined rate of change with respect to time. Hence, it is possible to positively make the focus point to be reached while maintaining arbitrary relationships of relative distance/relative speed between the objective lens and the disk. Accordingly, when switching to focus servo according to a first accurate focus error signal, it is possible to control an initial condition to fall within a fixed range. Thus, it is possible to realize stable focus entry. Hence, even in an apparatus having a short working distance, it is possible to significantly reduce the risk of collision of the objective lens with the disk at the time of focus entry. 
     In addition, according to the present invention, the rate of change with respect to time of a moving objecting lens is controlled. Thus, it is possible to maintain focus entry speed within a fixed range. Accordingly, even in the case where the characteristics of the second focus error signal vary depending on the light path length, it is possible to realize stable focus entry. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a block diagram showing a disk device; 
         FIG. 2A  is a diagram showing the concept of a separate optical system and the positional relationship between a fixed head and a movable head; 
         FIG. 2B  is a diagram showing the concept of the separate optical system and the positional relationship between the fixed head and the movable head; 
         FIG. 2C  is a diagram showing the concept of the separate optical system and the positional relationship between the fixed head and the movable head; 
         FIG. 3  is a diagram showing the structure of the fixed optical head; 
         FIG. 4  is a block diagram of a first focus error signal detection circuit; 
         FIG. 5  is a block diagram of a track error signal and a second focus error signal detection circuit; 
         FIG. 6A  is a graph showing an example of a first focus error signal; 
         FIG. 6B  is a graph showing an example of a second focus error signal; 
         FIG. 7  is a graph showing a waveform of focus entry control using the second focus error signal; 
         FIG. 8  is a graph showing an example of the second focus error signal; 
         FIG. 9A  is a graph showing the time waveform representing an operation according to a conventional method; 
         FIG. 9B  is a graph showing the time waveform representing an operation according to the conventional method; 
         FIG. 9C  is a graph showing the time waveform representing an operation according to the conventional method; 
         FIG. 10  is a block diagram of one embodiment of a focus entry control part according to the present invention; 
         FIG. 11A  is a graph showing the time waveform representing an operation of focus entry control of the present invention; 
         FIG. 11B  is a graph showing the time waveform representing an operation of the focus entry control of the present invention; and 
         FIG. 11C  is a graph showing the time waveform representing an operation of the focus entry control of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A description is given below of an embodiment for performing the present invention with reference to the drawings. 
       FIG. 10  is a block diagram of a focus entry control part of an optical disk device. Although a description is given of the case where the focus entry control part is hardware, the focus entry control part may be realized as software that is executed in the DSP  16  of  FIG. 1 . The focus entry control part shown in  FIG. 10  is formed mainly by comparing parts  1001 ,  1002 ,  1003  and  1004 , an approach control output generation part  1005 , a target trajectory generation part  1006 , a subtracting part  1007 , a first phase compensation part  1008 , a second phase compensation part  1009 , and a focus actuator driving part  1010 . In addition, a focus entry control start signal  1020 , a second focus error signal  1021 , and a threshold value  1022  are input to the comparing part  1001 . An output signal  1027  of the comparing part  1001 , the second focus error signal  1021 , and a threshold value  1023  are input to the comparing part  1002 . An output signal  1028  of the comparing part  1002 , a first focus error signal  1024 , and a threshold value  1025  are input to the comparing part  1003 . An output signal  1029  of the comparing part  1003 , the first focus error signal  1024 , and a threshold value  1026  are input to the comparing part  1004 . 
     Upon input of the focus entry control start signal  1020 , the comparing part  1001  and the approach control output generation part  1005  start operations. A signal that makes an objective lens to gradually approach a disk is output from the approach control output generation part  1005 , sent to the focus actuator driving part  1010 , and causes the objective lens to move in a direction in which the objective lens approaches the disk. 
     When the level of the second focus error signal  1021  becomes equal to or more than the threshold value  1022 , the comparing part  1001  sends the output signal  1027  to the comparing part  1002 , thereby starting the operation of the comparing part  1002 . On this occasion, the operation of the comparing part  1001  stops. 
     When the level of the second focus error signal  1021  becomes equal to or less than the threshold value  1023 , the comparing part  1002  operates so to start the operations of the comparing part  1003 , the phase compensation part  1008 , and the target trajectory generation part  1006 , and stops the approach control output generation part  1005 . On this occasion, the operation of the comparing part  1002  stops. 
     The target trajectory generation part  1006  generates a signal whose level is gradually decreased from the threshold value  1023  at a constant rate with respect to time. In addition, if the polarity of the subtracting part  1007  is reversed, a signal whose level is gradually increased at a constant rate with respect to time may be generated. Alternatively, a target trajectory signal may be generated that is continuously changed with respect to time and the rate of change is of a single polarity. 
     The subtracting part  1007  calculates the difference between the output signal of the target trajectory generation part  1006  and the second focus error signal  1021 . Then, the difference signal between the output signal of the target trajectory generation part  1006  and the second focus error signal  1021 , which is calculated by the subtracting part  1007 , is input to the phase compensation part  1008 . The first phase compensation part  1008  outputs the difference signal after performing phase compensation thereon so that a focus actuator control system is stabilized. The signal subjected to the phase compensation is sent to the focus actuator driving part  1010 , and the position of the objective lens is controlled in the direction of approaching the disk. 
     When the level of the first focus error signal  1024  becomes equal to or more than the threshold value  1025 , the comparing part  1003  starts the operation of the comparing part  1004 . On this occasion, the operation of the comparing part  1003  stops. 
     When the level of the first focus error signal  1024  becomes equal to or less than the threshold value  1026 , the comparing part  1004  activates the second phase compensation part  1009 , and stops the first phase compensation part  1008  and the target trajectory generation part  1006 . On this occasion, the operation of the comparing part  1004  stops. 
     The first focus error signal  1024  is input to the second phase compensation part  1009 . The second phase compensation part  1009  outputs a signal whose phase is compensated such that the focus actuator control system is stabilized. The phase-compensated signal is sent to the focus actuator driving part  1010 , and the position of the objective lens is controlled to maintain a focus position. 
     In the above-mentioned structure, it is detected by the comparing parts  1001  and  1002  that the level of the second focus error signal is equal to or less than the predetermined minimum level after detecting that the level of the second focus error signal is equal to or more than the predetermined maximum level. However, similar effects are obtained in an adverse structure in which the polarities of input terminals of the comparing parts  1001  and  1002  are reversed, and it is detected that the level of the second focus error signal is equal to or more than the predetermined maximum level after detecting that the level of the second focus error signal is equal to or less than the predetermined minimum level. 
     Further, in the above-mentioned structure, it is detected by the comparing parts  1003  and  1004  that the level of the first focus error signal is equal to or less than the predetermined minimum level after detecting that the level of the first focus error signal is equal to or more than the predetermined maximum level. However, similar effects are obtained in an adverse structure in which the polarities of input terminals of the comparing parts  1003  and  1004  are reversed, and it is detected that the level of the first focus error signal is equal to or more than the predetermined maximum level after detecting that the level of the first focus error signal is equal to or less than the predetermined minimum level. 
     Next,  FIGS. 11A ,  11 B and  11 C show operation signal waveforms in the case of using this embodiment. 
       FIGS. 11A ,  11 B and  11 C show focus entry waveforms according to the present invention. FIG.  11 A shows the distance between the focus position and the objective lens,  FIG. 11B  shows the second focus error signal, and  FIG. 11C  shows the first focus error signal.  1101 ,  1102  and  1103  in  FIGS. 11A ,  11 B and  11 C indicate operation waveforms of the present invention corresponding to the positions of the movable head  202  of  FIGS. 2A ,  2 B and  2 C, and also corresponding to the cases of  901 ,  902  and  903  in  FIGS. 9A ,  9 B and  9 C, which are conventional examples. 
     In  FIGS. 11A ,  11 B and  11 C, up to about the time 76.2 ms, the objective lens is made to approach the disk by the signal from the approach control output generation part  1005 . At this point, the level of the second focus error signal is larger than the threshold value  1022 , and the comparing part  1002  is operating. In the vicinity of the time 76.2, the level of the second focus error signal  1021  becomes equal to or less than the threshold value  1023  (in this case, a level equivalent to 6 μm), and target trajectory control using the second focus error signal  1021  is being performed. Here, the comparing part  1003  monitors the level of the first focus error signal  1023 , and upon detection that the level of the first focus error signal  1024  is equal to or more than the threshold value  1025  (in this case, a level equivalent to 0.4 μm), switching is made to monitoring by the comparing part  1004 . 
     When it is detected by the comparing part  1004  that the level of the first focus error signal  1024  becomes equal to or less than the threshold value  1026  (in this case, a level equivalent to 0.1 μm), focus servo control using the first focus error signal  1024  is being performed. 
     When trajectory control is performed by using the second focus error signal  1021  at a constant rate of change, though actual approaching speed differs depending on the position of the movable head  202  shown in  FIGS. 2A ,  2 B and  2 C, it is possible to monitor the level of the first focus error signal  1024  and, at the zero-cross point thereof, perform a transition from focus servo control using the second focus error signal  1021  to focus servo control using the first focus error signal  1024 . Hence, it can be seen that a smooth transient response is obtained. 
     The focus entry control part may also be realized by hardware, software executed by, for example, a microcomputer or a digital signal processing circuit (DSP), or a combination thereof. 
     In this embodiment, the description is given by taking the disk device as an optical device. However, the disk device is not a limitation and application to an optical storage device using another optical storage medium such as an optical card is also possible. Further, the present invention is not limited to optical storage devices using disks or optical cards, and may be applied to optical devices such as a microscope and a light emitting device. 
     As described above, according to the present invention, it is possible to positively cause the objective lens to approach in the vicinity of the focal point by performing target trajectory control with the use of the second focus servo control. Thereafter, the first focus servo signal is monitored, and focus servo control using the second focus servo signal is switched to focus servo control using the first focus servo signal. Accordingly, there is no error in the relative positions between the focus position and the position of the objective lens. Thus, it is possible to smoothly perform a transition to focus servo control using the first focus error signal within a predetermined error range for relative speed. Thereby, it is possible to stably and positively perform focus entry control. 
     Consequently, even in the case where the characteristics of the second focus error signal vary in accordance with a variation in the light path length of the separate optical system, it is possible to stably and positively perform focus entry control. 
     In addition, according to the present invention, since it is possible to stably and positively perform focus entry control, it is also possible to avoid collision of the objective lens with a disk. 
     The present invention is not limited to the specifically disclosed embodiments, and variations and modifications may be made without departing from the scope of the present invention.