Abstract:
An optical disk drive includes a light source, a light focusing element, a light-focusing-element moving mechanism, a detecting unit, a holding unit, and first and second control units. The light focusing element arranged so as to be opposed to a disk on which a signal is recordable is capable of focusing light emitted from the light source as near-field light on the disk. The detecting unit detects a state where the moving mechanism approaches the light focusing element to the disk such that the light is focused as near-field light on the disk and outputs a detection signal upon detection. The holding unit holds a voltage applied to the mechanism in response to the detection signal and is capable of releasing the held voltage. The first and second control units perform first and second control operations to control the distance between the light focusing element and the disk.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     The present invention contains subject matter related to Japanese Patent Application JP 2005-150743 filed in the Japanese Patent Office on May 24, 2005, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical disk drive for performing at least one of writing and reading of signals using near-field light, an optical disk apparatus mounted with the drive, and a method for driving the apparatus. 
     2. Description of the Related Art 
     To increase the recording density of an optical disk on which data is written or read using laser light, an optical disk apparatus for writing or reading signals using near-field light has recently been proposed. According to a proposed technique for the optical disk apparatus utilizing near-field light, a gap between an optical disk and the end surface of a solid immersion lens (SIL) disposed in a light focusing element, such as an objective lens unit, is controlled to be a distance (near field) where near-field light is generated. The distance is generally half the wavelength of input laser light. For example, in the use of blue-violet laser light with a 400-nm wavelength, the distance is approximately 200 nm. 
     Upon starting control of the gap, an overshoot of 1 μm or less may occur. In a far-field optical system in an optical disk apparatus for writing or reading signals on a compact disk (CD) or a digital versatile disk (DVD), the overshoot is insignificant. In an optical recording and playback apparatus using near-field light, the overshoot is a serious problem. In other words, if the overshoot of 1 μm or less occurs upon starting the control, the SIL collides with the disk, resulting in damages of them. 
     One approach to solving the above-mentioned problem uses a technique for controlling the gap on the basis of the amount of returning laser light reflected from the disk. For instance, in the case of using laser light of a 400-nm wavelength, when the gap length is generally half the wavelength or less, a near-field state is produced. Accordingly, assuming that the gap is more than 200 nm, i.e., in a far-field state, when light emitted from a laser source is incident on the end surface of the SIL at an angle at which total reflection occurs, the light is totally reflected by the end surface of the SIL, so that the amount of returning light is constant. When the gap length is 200 nm or less, i.e., in the near-field state, light incident on the end surface of the SIL at the angle of total reflection partially passes through the end surface thereof, so that the amount of returning light is reduced. When the gap between the SIL and the disk is zero, i.e., when the SIL is in contact with the disk, the entire light incident on the end surface of the SIL at the angle of total reflection passes through the end surface thereof, so that the amount of returning light is zero. In this technique, the amount of returning light is detected by a photodetector and data indicating the detected amount is fed back to an actuator (e.g., a two-axis device for performing focusing servo and tracking servo operations) for the SIL, whereby a gap servo operation is performed. This approach is disclosed in, e.g., U.S. Pat. No. 6,701,913 (Patent Document 1). 
     Another approach utilizes a technique for setting a threshold used to determine, e.g., whether the near-field state is produced, approaching the SIL to the disk until the threshold is detected, adding a servo voltage to an approach voltage after the threshold is detected, and then performing the gap servo operation. This approach is disclosed in, e.g., Japanese Unexamined Patent Application Publication No. 2004-30821 (Patent Document 2). In this case, the approach voltage is a ramped voltage (see FIG. 8 of Patent Document 2). Since the SIL is moved at the initial velocity upon starting to approach to the disk, the SIL fluctuates at the start of the approach (refer to FIG. 12 of Patent Document 2). After that, the SIL is moved in accordance with the ramped voltage that is reduced to a target value (corresponding to a target gap of several tens of nm). 
     SUMMARY OF THE INVENTION 
     In an apparatus disclosed in Patent Document 2, however, there is concern about the fluctuation of the SIL caused by the initial velocity of the SIL upon starting the approach. Therefore, any means has to be devised to increase a margin to avoid collision between the SIL and the disk and more stably approach the SIL to the near field or a target point in the near-field. 
     In consideration of the above-described circumstances, it is desirable to provide an optical disk drive capable of reliably preventing collision between a light focusing element and an optical disk, an optical disk apparatus mounted with the drive, and a method for driving the apparatus. 
     According to an embodiment of the present invention, there is provided an optical disk drive including the following elements. A light source emits light. A light focusing element arranged so as to be opposed to a disk on which a signal is recordable is capable of focusing the light emitted from the light source as near-field light on the disk. A moving mechanism is configured to move the light focusing element closer to or farther away from the disk on the basis of a change in voltage. A detecting unit is configured to detect a state where the moving mechanism approaches the light focusing element to the disk such that the distance between the light focusing element and the disk is equal to a first distance where the light is focused as near-field light on the disk through the light focusing element and to output a detection signal upon detecting the state. A holding unit is configured to hold a voltage applied to the moving mechanism in response to the detection signal output from the detecting unit. The holding unit is capable of releasing the held voltage. A first control unit is configured to perform a first control operation of applying a voltage, whose maximum value is equal to the held voltage, to the moving mechanism so that the distance between the light focusing element and the disk is equal to or smaller than the first distance while the held voltage is released. A second control unit is configured to perform a second control operation of controlling the distance between the light focusing element and the disk to a second distance smaller than the first distance on the basis of the detection signal while the distance therebetween is equal to or smaller than the first distance. 
     According to this embodiment, the optical disk drive may further include a switch for switching from the first control operation by the first control unit to the second control operation by the second control unit in response to a detection signal output from the detecting unit. 
     According to this embodiment, preferably, the second control unit performs the second control operation while the maximum voltage is being applied to the moving mechanism through the first control unit. 
     According to this embodiment, the detecting unit may include a measuring unit for measuring the amount of returning light that is emitted from the light source and is then reflected by the light focusing element, and the first or second control unit may perform the control operation on the basis of the amount of returning light measured by the measuring unit. 
     According to this embodiment, the light focusing element may include a solid immersion lens. 
     According to this embodiment, preferably, the light emitted from the light source is blue or blue-violet laser light. 
     According to another embodiment of the present invention, there is provided an optical disk apparatus including the following elements. A light source emits light. A light focusing element arranged so as to be opposed to a disk on which a signal is recordable is capable of focusing the light emitted from the light source as near-field light on the disk. A moving mechanism is configured to move the light focusing element closer to or farther away from the disk on the basis of a change in voltage. A detecting unit is configured to detect a state where the moving mechanism approaches the light focusing element to the disk such that the distance between the light focusing element and the disk is equal to a first distance where the light is focused as near-field light on the disk through the light focusing element and outputting a detection signal upon detecting the state. A holding unit is configured to hold a voltage applied to the moving mechanism in response to the detection signal output from the detecting unit. The holding unit is capable of releasing the held voltage. A first control unit is configured to perform a first control operation of applying a voltage, whose maximum value is equal to the held voltage, to the moving mechanism so that the distance between the light focusing element and the disk is equal to or smaller than the first distance while the held voltage is released. A second control unit is configured to perform a second control operation of controlling the distance between the light focusing element and the disk to a second distance smaller than the first distance on the basis of the detection signal while the distance therebetween is equal to or smaller than the first distance. A writing/reading mechanism is capable of performing at least one of writing and reading of the signal to/from the disk while the distance therebetween is being controlled to the second distance through the second control unit. 
     According to this embodiment, the optical disk apparatus may further include a switch for switching from the first control operation by the first control unit to the second control operation by the second control unit in response to a detection signal output from the detecting unit. 
     According to this embodiment, the second control unit may perform the second control operation while the maximum voltage is being applied to the moving mechanism through the first control unit. 
     According to this embodiment, the detecting unit may include a measuring unit for measuring the amount of returning light that is emitted from the light source and is then reflected by the light focusing element, and the first or second control unit may perform the control operation on the basis of the amount of returning light measured by the measuring unit. 
     According to this embodiment, the light focusing element may include a solid immersion lens. 
     According to this embodiment, preferably, the light emitted from the light source is blue or blue-violet laser light. 
     According to another embodiment of the present invention, there is provided a method for driving an optical disk drive including a light source that emits light, a light focusing element which is arranged so as to be opposed to a disk on which a signal is recordable and is capable of focusing the light emitted from the light source as near-field light on the disk, and a moving mechanism for moving the light focusing element closer to or farther away from the disk on the basis of a change in voltage. The method includes the steps of: detecting a state where the moving mechanism approaches the light focusing element to the disk such that the distance between the light focusing element and the disk is equal to a first distance where the light is focused as near-field light on the disk through the light focusing element; outputting a detection signal when the state is detected; holding a voltage applied to the moving mechanism in response to the detection signal; releasing the held voltage; performing a first control operation of applying a voltage, whose maximum value is equal to the held voltage, to the moving mechanism so that the distance between the light focusing element and the disk is equal to or smaller than the first distance while the held voltage is released; and performing a second control operation of controlling the distance between the light focusing element and the disk to a second distance smaller than the first distance on the basis of the detection signal while the distance therebetween is equal to or smaller than the first distance. 
     As described above, according to the present invention, collision between a light focusing element and an optical disk can be prevented reliably. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing the structure of an optical disk drive according to an embodiment of the present invention; 
         FIG. 2  is a side plan view showing a light focusing element and an optical disk; 
         FIG. 3  is a block diagram of the configuration of a gap servo module; 
         FIG. 4  is a block diagram of the configuration of a data processor; 
         FIG. 5  is a graph showing the relationship between the amount of totally reflected returning light and a gap between the end surface of an SIL and the surface of an optical disk; 
         FIG. 6  is a block diagram of the configuration of an approach voltage generating unit; 
         FIG. 7  is a diagram showing low-pass filtering of a stepped voltage; 
         FIG. 8  is a flowchart showing an example of the operation of the gap servo module; 
         FIG. 9  is a timing diagram showing a voltage applied to a three-axis actuator and the amount of totally reflected returning light in the operation of  FIG. 8 ; and 
         FIG. 10  is a diagram of a change in gap-servo target value generated by a gap-servo target value setting unit. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of the present invention will now be described below with reference to the drawings. 
       FIG. 1  is a diagram of the structure of an optical disk drive according to one embodiment of the present invention. An optical disk drive  1  includes an optical head  28 , a servo controller  40 , and a spindle motor  48 . The optical head  28  includes: a laser diode (LD)  31 , serving as a light source; collimator lenses  32  and  46 ; an anamorphic prism pair  33  for shaping laser light; a beam splitter (BS)  34 ; a quarter wave plate (QWP)  43 ; an achromatic lens  44 ; a diverging lens  45  that diverges a laser beam; a Wollaston prism  35 ; converging lenses  36  and  38 ; a light focusing element  5 ; photodetectors (PDs)  37  and  39 ; an automatic power controller (APC)  41 ; and an LD driver  42 . 
     The Wollaston prism  35  consists of two prisms. The Wollaston prism  35  permits incident light to pass through as two beams which are mutually perpendicularly polarized. The PD  37  outputs an RF read signal to read a signal written on an optical disk, and a tracking error signal and a gap error signal which are necessary for servo controls to the servo controller  40 . 
     The servo controller  40  includes a gap servo module  51 , which will be describe later, a tracking servo module  52 , a tilt servo module  53 , and a spindle servo module  54 . The tracking servo module  52  controls the tracking operation of the light focusing element  5  in accordance with a tracking error signal. The tilt servo module  53  controls the tilt angle of the light focusing element  5 . The spindle servo module  54  controls the rotation of the spindle motor  48 . 
     The APC  41  outputs a predetermined signal to the LD driver  42  on the basis of a signal output from the PD  39  so that the power of laser light output from the LD  31  is constant. 
     The operation of the optical disk drive  1  will now be described. For example, an optical disk  47 , serving as a recording medium, is loaded into the optical disk drive  1 . Laser light emitted from the LD  31  is collimated through the collimator lens  32  and is then shaped through the anamorphic prism pair  33 . The laser light incident on the BS  34  is split into a beam that is incident on the QWP  43  and a beam that is incident on the converging lens  38 . The power of the beam incident on the converging lens  38  is controlled to be constant by the APC  41  as mentioned above. As for the beam incident on the QWP  43 , the incident beam, which is linearly polarized, is circularly polarized by the QWP  43 . The resultant beam is subjected to correction for chromatic aberration by the achromatic lens  44 . The resultant beam passes through the diverging lens  45  and the collimator lens  46  and is then incident on the light focusing element  5 . 
     The laser beam incident on the light focusing element  5  is focused as near-field light, which will be described below, on the optical disk  47  to write a signal to the optical disk  47 . Alternatively, the laser beam focused as near-field light on the optical disk  47  is incident on the optical disk  47  to read a signal written on the optical disk  47 . Light reflected or diffracted by the optical disk  47  is received by the light focusing element  5 . The received light, serving as returning light, passes through the light focusing element  5 , the collimator lens  46 , the diverging lens  45 , the achromatic lens  44 , and the QWP  43 , and is then incident on the BS  34 . The incident light is totally reflected by the BS  34  and is then incident on the PD  37  through the Wollaston prism  35  and the converging lens  36 . An RF read signal and servo control signals are generated by the PD  37 . The servo control signals are supplied to the servo controller  40 , so that the servo controls are performed. 
       FIG. 2  is a side plan view showing the light focusing element  5  and the optical disk  47 . The light focusing element  5  is opposed to the optical disk  47 . The light focusing element  5  includes an SIL  2 , an aspheric lens  3 , and a lens holder  4 . The lens holder  4  receives the SIL  2  and the aspheric lens  3 . The structure of the light focusing element  5  is not limited to the above. The light focusing element  5  may be configured to guide laser light  24  as near-field light to the optical disk  47 . The SIL  2  is disposed such that the end surface  2   a  thereof is opposed to the recording surface  47   a  of the disk  47 . The lens holder  4  is arranged in a three-axis actuator  6 , which constitutes at least part of a mechanism for moving the light focusing element  5  closer to or farther away from the disk  47 . The three-axis actuator  6  includes, e.g., coils in three orthogonal directions, a yoke, etc. which are not shown. When a predetermined servo voltage is applied to the three-axis actuator  6 , current flows through each coil, whereby a tracking servo, focusing servo, or tilt servo operation is controlled. The focusing servo operation includes a gap servo operation. In the case where the present invention is applied to the optical disk drive  1  according to the present embodiment, the tracking servo module  52  and the tilt servo module  53  are not necessarily included. 
       FIG. 3  is a block diagram showing the outline of the gap servo module  51 . A control target of the gap servo module  51  is the three-axis actuator  6 . The amount to be detected (amount to be controlled) is the amount of totally reflected returning light  24 , which is detected by the PD  37  as described above. The detected amount of totally reflected returning light  24  is normalized to, e.g., 1 V by a gain normalizer  18 . The resultant signal is converted into digital data through an analog-to-digital (AD) converter  19 . The digital data, indicating the amount of totally reflected returning light, is supplied to a data processor  10 . A voltage to approach the SIL  2  of the light focusing element  5  to the optical disk  47  is output from the data processor  10 . The output voltage is converted into an analog signal by a digital-to-analog (DA) converter  11  and the analog signal is output as an approach voltage  14 . On the other hand, a gap error signal  27  is supplied to a filter  13 . The signal is converted into an analog signal through a DA converter  12  and the resultant signal is output as a servo voltage  15 . The servo voltage  15  is added to the approach voltage  14 . The resultant voltage is supplied to a driver  16 . The driver  16  drives the three-axis actuator  6  so that a gap error becomes zero. 
       FIG. 4  is a block diagram of the detailed configuration of the data processor  10 . 
     The data processor  10  receives the data indicating the amount of totally reflected returning light  24  and a signal  9  output from a gap servo switch. For example, the gap servo switch signal  9  is supplied to the data processor  10  when the optical disk  47  is loaded into the optical disk drive  1 . Input timing is not limited to the above. 
     A near-field detection level setting unit  21  sets a near-field detection level (threshold voltage to start the gap servo operation)  8  and inputs the set level  8  to a system controller  20 . The system controller  20  compares the amount of totally reflected returning light  24  with the near-field detection level  8 . On the basis of the result, the system controller  20  outputs predetermined control signals to an approach voltage generating unit  23  and a switch  26 , as will be described below. 
     The near-field detection level  8  may be set as shown in, e.g.,  FIG. 5 . In other words, the near-field detection level  8  is set to a value that lies within a near-field zone and is higher than a gap-servo target value  7 . Referring to  FIG. 5 , for instance, when the amount of totally reflected returning light  24  in a far-field zone is normalized to 1 (V), the near-field detection level  8  is set to 0.8 (V) in a linear region. The gap-servo target value  7  is set by a gap-servo target value setting unit  22  (see  FIG. 4 ). As shown in  FIG. 5 , the gap-servo target value  7  is set to a value that lies within the linear region and is lower than 0.8 (V), e.g., 0.5 (V). 
     The system controller  20  compares the near-field detection level  8  with the amount of totally reflected returning light  24 , i.e., a voltage corresponding thereto. As the result of comparison by the system controller  20 , when the amount of totally reflected returning light  24  is higher than the near-field detection level  8 , i.e., when the end surface  2   a  of the SIL  2  is located in the far field, a signal  29  output from the system controller  20  to the switch  26  becomes a low level. On the other hand, when the amount of totally reflected returning light  24  is equal to or lower than the near-field detection level  8 , i.e., when the end surface  2   a  of the SIL  2  is located in the near field, the output signal  29  becomes to a high level. At the time when the output signal  29  of the system controller  20  goes to the high level, the switch  26  is turned on, whereby the gap servo operation starts. A deviation between the amount of totally reflected returning light  24  and the gap-servo target value  7  is obtained and is supplied as a deviation signal  25  to the switch  26 . 
     When the switch  26  is turned on, i.e., when the gap servo operation starts, the switch  26  outputs the supplied deviation signal  25  as a servo voltage  27 . In the above-described gap servo operation, the gap between the end surface  2   a  of the SIL  2  and the recording surface  47   a  is controlled so that the voltage applied to the three-axis actuator  6  is equal to the gap-servo target value  7 . 
       FIG. 6  is a block diagram showing the configuration of the approach voltage generating unit  23 . The approach voltage generating unit  23  includes a ramped-voltage generating unit  55 , a sample-hold circuit  57 , a stepped-voltage generating unit  56 , a low-pass filter  58 , and a switch  59 . 
     When receiving a control signal  65  based on the above-described gap servo switch signal  9  from the system controller  20 , the ramped-voltage generating unit  55  generates a ramped voltage, which linearly increases, and outputs the voltage to the sample-hold circuit  57 . 
     The system controller  20  outputs a hold signal  67  to the sample-hold circuit  57  when the amount of totally reflected returning light  24  is lower than the near-field detection level  8 . When receiving the hold signal  67 , the sample-hold circuit  57  holds the voltage generated by the ramped-voltage generating unit  55 . In addition, the sample-hold circuit  57  outputs a signal  62  corresponding to the input or held ramped voltage  62  to the switch  59  and the stepped-voltage generating unit  56 . 
     The system controller  20  outputs a control signal  66  to the stepped-voltage generating unit  56  when the amount of totally reflected returning light  24  is lower than the near-field detection level  8 . When receiving the control signal  66 , the stepped-voltage generating unit  56  generates a stepped voltage and outputs a signal corresponding to the voltage  64  to the low-pass filter  58 . 
     As shown in  FIG. 7 , the low-pass filter  58  integrates the voltage signal  64  to obtain a voltage signal  63  and outputs the signal  63  to the switch  59 . 
     In accordance with a control signal  69  from the system controller  20 , the switch  59  selects either the voltage signal  62  of the sample-hold circuit  57  or the output signal  63  of the low-pass filter  58 . Then, the switch  59  outputs the selected signal as the approach voltage  14 . 
     The operation of the gap servo module  51  with the above-described configuration will now be described.  FIG. 8  is a flowchart of an example of the operation thereof.  FIG. 9  is a timing diagram showing a voltage applied to the three-axis actuator  6  and the amount of totally reflected returning light. 
     At time t 0 , the gap servo switch is turned on and the gap servo switch signal  9  is supplied to the system controller  20  (step  801 ). The system controller  20  outputs the control signal  65  to the ramped-voltage generating unit  55 . In response to the control signal  65 , the ramped-voltage generating unit  55  generates a ramped voltage  71  (refer to  FIG. 9 ) (step  802 ). The ramped voltage  71  is supplied as the voltage signal  62  to the switch  59 . The system controller  20  permits the switch  59  to output the voltage signal  62  as the approach voltage  14 . The approach voltage  14  is supplied to the driver  16 . The driver  16  drives the three-axis actuator  6  on the basis of the approach voltage  14 . Consequently, the light focusing element  5  is moved closer to the optical disk  47  and the amount of totally reflected returning light  24  starts to drop, so that the end surface  2   a  of the SIL  2  enters the near field. 
     As shown in  FIG. 5 , at time t 1  when a voltage corresponding to the amount of totally reflected returning light  24  is equal to or lower than 0.8 V which corresponds to the near-field detection level  8  (YES in step  803 ), the system controller  20  outputs the hold signal  67  to the sample-hold circuit  57 . In response to the hold signal  67 , the sample-hold circuit  57  holds the input ramped voltage (step  804 ). Referring to  FIG. 9 , let V (V) denote the held voltage. Simultaneously with outputting of the hold signal  67 , the system controller  20  switches the switch  59  to temporarily release the ramped approach voltage  14  applied up to that time (step  805 ). 
     After that, the system controller  20  outputs the control signal  66  to the stepped-voltage generating unit  56 . As described above, the signal  66  is used to generate a stepped voltage through the stepped-voltage generating unit  56 . In response to the control signal  66 , the stepped-voltage generating unit  56  generates a stepped voltage (refer to  FIG. 7 ) at time t 2  (step  806 ). The stepped voltage is increased to the voltage V (V) at maximum. Data indicating the held voltage V (V) may be stored in, e.g., a memory (not shown) included in the system controller  20 , alternatively, another memory. The stepped voltage is supplied to the switch  59  through the low-pass filter  58 . The supplied voltage is applied as the approach voltage  14  to the three-axis actuator  6 . Since the approach voltage  14  of up to the maximum voltage V (V) is applied after time  2 , when the end surface  2   a  of the SIL  2  reaches a position, corresponding to the near-field detection level  8 , close to the optical disk  47 , the velocity of the SIL  2  becomes approximately zero (shown by point A at time t 3 ). When the voltage corresponding to the amount of totally reflected returning light  24  is equal to or lower than the voltage corresponding to the near-field detection level  8  (YES in step  807 ), the system controller  20  turns on the switch  26  to output a servo voltage  27  on the basis of the gap-servo target value  7  set by the target value setting unit  22 , thus starting the gap servo operation (step  808 ). 
       FIG. 10  shows a change in gap-servo target value generated by the gap-servo target value setting unit  22 . While the approach voltage  14  having the constant value V (V) is being applied to the three-axis actuator  6 , the system controller  20  starts the gap servo operation on the basis of the gap-servo target value so as to follow the target value. 
     As described above, according to the present embodiment, at time t 3  when the gap servo operation is started, the initial velocity of the SIL  2  is zero. The gap servo operation is started under the condition that the initial velocity of the SIL  2  is zero. Therefore, the SIL  2  can be moved closer to the disk  47  such that the applied voltage smoothly follows the gap-servo target value  7  as shown by a waveform  73  indicating the amount of totally reflected returning light in  FIG. 9 . 
     According to the present embodiment, while the approach voltage  14  is applied to the three-axis actuator  6  and the end surface  2   a  of the SIL  2  is located in the near field, a voltage obtained by adding the servo voltage  27  to the approach voltage  14  is applied to the three-axis actuator  6 . Thus, the light focusing element  5  can be smoothly moved closer to the disk  47 . 
     In a conceivable method, before the approach voltage  14  is applied, the initial position of the light focusing element  5  (the SIL  2 ) is previously set so that the initial velocity of the SIL  2  becomes approximately zero at the time when the near-field detection level is detected. In this method, however, the initial position has to be determined by trial and error. According to the present embodiment, since automatic control can be performed, it is unnecessary to design the arrangement in consideration of the type of disk drive and the type of disk. 
     The present invention is not limited to the above-described embodiment but many modifications and variations are possible. 
     Examples of the arrangement and functions of the optical system of the optical head  28  and the sensors include, but are not limited to, those shown in  FIG. 1 . 
     In  FIG. 8 , a ramped voltage is applied in step  802  and a stepped voltage is applied in step  806 . Voltage waveforms are not limited to those. Similar voltage waveforms may be used. For example, in step  802 , a stepped voltage may be filtered by a low-pass filter and the resultant voltage may be applied. In step  806 , a ramped voltage may be filtered through a low-pass filter with a relatively small time constant and the resultant voltage may be applied. 
     In  FIG. 9 , the maximum voltage V is released at time t 1  and the application of the approach voltage is restarted at time t 2 . For example, if data indicating the maximum voltage V is temporarily stored, the following operation may be omitted: After the optical disk  47  is loaded into the optical disk drive, the ramped voltage  71  is applied and is then released every writing or reading operation. In other words, if the data indicative of the maximum voltage V is stored once, the second or later writing or reading operation may be started at time t 2 . When another disk is loaded, data indicating the maximum voltage V may be updated. It is a matter of course that the above-described operation may be started at time t 0  and data indicating the maximum voltage V may be recorded every writing or reading operation with respect to one disk. 
     It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.