Patent Publication Number: US-2011058459-A1

Title: Method and device for adjusting tilt of recording medium

Description:
TECHNICAL FIELD 
     The present invention relates to a tilt control method and apparatus of a recording medium, and more particularly to, a tilt control method and apparatus of a recording medium, which can automatically compensate a tilt error occurring between a recording medium and a lens of an optical recording apparatus. 
     BACKGROUND ART 
     As a demand for high quality moving picture processing increases due to consumer&#39;s upgraded taste, a large-scaled optical storage disk is required. In this respect, an optical recording medium of high density, which can record and store video data of high picture quality and audio data of high sound quality for a long time, has been recently developed. 
     Examples of the optical recording medium include blue-ray disk and HD-DVD. The DVD has a recording capacity of 4.7 GB, approximately, while the blue-ray disk has a recording capacity of 25 GB, approximately. 
     recording capacity of 25 GB, approximately. 
     Since such an optical recording medium of high density, an optical recording device of high density based on techniques such as super-RENS, Holography, and near field recording has been developed. 
     The near field recording technique is provided with a near field lens such as a solid immersion lens (SIL) to have a high numerical aperture equivalent to 2, thereby increasing recording density. The near field recording disk has a recording capacity of 140 GB to 160 GB, and should have a numerical aperture higher than that of an exist optical disk to record data at ultra-high density in a disk. Accordingly, a solid immersion lens (SIL) having a shape such as hemisphere is formed on a front surface of an object lens to increase a numerical aperture. A near field optical system which achieves such near field information recording technique enables high density recording and reproduction by overcoming diffraction limitation of a far field using the SIL. 
     Generally, in an optical disk recording and reproducing apparatus, an optical pickup reads data recorded on a surface of an optical disk by irradiating laser to the optical disk and detecting intensity of light reflected on the optical disk. At this time, for exact reading of data, the optical pickup should be arranged to be perpendicular to the surface of the optical disk. 
     However, the optical disk fails to be arranged to be exactly perpendicular to the optical pickup due to a problem occurring in a fabricating process. The case where the optical disk is not perpendicular to the optical pickup will be referred to as a tilt. In order to compensate such a tilt, it is necessary to control the tilt so that the optical pickup is perpendicular to the optical disk. 
     Particularly, in the near field optical recording apparatus, since the distance between the disk and a lens is very narrow, a tilt margin between them is very narrow. If the tilt margin is narrow, it is impossible to obtain stable servo, and scratch may occur due to collision between the disk and the lens. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Accordingly, the present invention is directed to a tilt control method and apparatus of a recording medium, which substantially obviates one or more problems due to limitations and disadvantages of the related art. 
     An object of the present invention is to provide a tilt control method and apparatus of a recording medium, which can efficiently compensate a tilt error occurring between a recording medium and a lens of an optical recording apparatus. 
     To achieve these objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, a tilt control method of a recording medium according to the present invention comprises fixing the recording medium to a recording reproducing apparatus; performing first tilt compensation in a state that the recording medium is stopped; and performing second tilt compensation in a state that the recording medium in which the first tilt compensation has been performed is rotated. 
     The step of performing first tilt compensation includes detecting a gap error signal from the recording medium; and controlling an angle of the recording medium based on the detected gap error signal. 
     The step of controlling an angle of the recording medium includes controlling a tilted angle in a radial direction of the recording medium; and controlling a tilted angle in a tangential direction of the recording medium. 
     The angle of the recording medium is controlled so that the gap error signal reaches a minimum value. 
     The step of performing second tilt compensation includes detecting a gap error signal in a state that the recording medium is rotated; and controlling an angle of the recording medium based on the detected gap error signal. 
     The step of controlling an angle of the recording medium includes controlling a tilted angle in a radial direction of the recording medium; and controlling a tilted angle in a tangential direction of the recording medium. 
     The angle of the recording medium is controlled so that size variation of the gap error signal is minimized. 
     The recording medium is a near field optical recording medium, and the recording reproducing apparatus is a near field optical recording reproducing apparatus. 
     In another aspect of the present invention, a tilt control apparatus of a recording medium comprises a tilt driver controlling a tilted angle of the recording medium; and a tilt controller controlling the tilt driver to perform tilt control in each of a state that the recording medium is stopped and a state that the recording medium is rotated. 
     The tilt driver includes a first tilt driver controlling a tilted angle in a tangential direction of the recording medium; and a second tilt driver controlling a tilted angle in a radial direction of the recording medium. 
     The tilt control apparatus further comprises a gap error signal detector detecting a gap error signal from the recording medium, wherein the tilt controller controls the tilt driver based on the gap error signal input from the gap error signal detector. 
     The tilt controller controls the tilt driver to obtain a minimum value of the gap error signal in a state that the recording medium is stopped. 
     The tilt controller controls the tilt driver so that size variation of the gap error signal is minimized. 
     According to the tilt control method and apparatus of the present invention, a tilt error between the recording medium and the lens can be compensated automatically. As a result, more improved reproduction quality can be obtained and reliability of the system can be obtained. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flow chart illustrating a tilt control method of a recording medium according to the embodiment of the present invention; 
         FIG. 2  is a brief diagram illustrating a tilt between a recording medium and a lens of a recording reproducing apparatus; 
         FIG. 3  is a graph illustrating a relation between a tilt angle and a gap error signal in a state that a recording medium is stopped; 
         FIG. 4  is a graph illustrating a relation between variance of a tilt angle and a gap error signal in a state that a recording medium is rotated; 
         FIG. 5   a  and  FIG. 5   b  are graphs illustrating a size of a time variable gap error signal at specific tilt angles of  FIG. 4 ; 
         FIG. 6  is a block diagram illustrating a tilt control apparatus of a recording medium according to the embodiment of the present invention; 
         FIG. 7   a  and  FIG. 7   b  are plane view and sectional view illustrating a recording reproducing apparatus according to the embodiment of the present invention; 
         FIG. 8   a  and  FIG. 8   b  are plane view and sectional view illustrating a first tilt driver and a disk driver; 
         FIG. 9   a  and  FIG. 9   b  are plane view and sectional view illustrating a configuration of a first tilt driver; 
         FIG. 10  is a block diagram illustrating a recording reproducing apparatus according to the embodiment of the present invention; and 
         FIG. 11  is a flow chart illustrating a tilt control method of a recording medium according to the second embodiment of the present invention. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, the preferred embodiments of the present invention will be described with reference to the accompanying drawings so that a person with ordinary skill in the art to which the present invention pertains carries out the present invention. 
     Hereinafter, a tilt control method according to the embodiment of the present invention will be described in detail.  FIG. 1  is a flow chart illustrating a tilt control method of a recording medium according to the embodiment of the present invention. Hereinafter, a near field recording (NFR) medium based on near field optics will be described as a recording medium. 
     Accordingly, a tilt means a gradient occurring between a recording medium and a solid immersion lens (SIL) provided in an optical recording reproducing apparatus using near field. 
     First of all, after a recording medium is loaded in a recording reproducing apparatus (S 10 ), a gap error signal is detected (S 20 ) in a state that the recording medium is stopped, and a tilt is controlled based on the gap error signal (S 30 , S 40 ) in a state that the recording medium is stopped. 
     The gap error signal used to compensate a tilt error is detected through a photodiode of a pickup.  FIG. 2  is a brief diagram illustrating a tilt between a recording medium and an SIL  10  of a recording reproducing apparatus, and  FIG. 3  is a graph illustrating a gap error signal with respect to the tilt. 
     As illustrated in  FIG. 3 , if the SIL  10  is parallel with the recording medium, the gap error signal is minimized. If a tilt error increases as a tilt angle of the recording medium  100  increases, the gap error signal increases. 
     In order to minimize a tilt error, the angle of the recording medium  100  should be controlled to minimize the gap error signal. At this time, the tilt between the recording medium  100  and the SIL  10  can be divided into a tilt component in a radial direction and a tilt component in a tangential direction. 
     A tilt control method will be described in detail. First of all, a tilt in a radial direction is controlled (S 30 ). If a level of the gap error signal is measured, the recording medium is titled at a predetermined angle with respect to the SIL. At this time, it is determined whether the gap error signal level is reduced or increased. If the gap error signal level is increased, since it means that the tilt is more increased, the recording medium is tilted at a predetermined angle in an opposite radial direction. 
     If the initial gap error signal level is reduced, or if the gap error signal level is collected to a random value as the gap error signal level is reduced through direction control to an opposite direction, the tilt is stopped. Namely, the gap error signal is used as a feedback signal for tilt compensation. 
     Next, the tilt in a tangential direction is controlled (S 40 ). At this time, the method for compensating the tilt in a radial direction can equally be applied to the tilt in a tangential direction. Namely, the recording medium is tilted at a predetermined angle in a tangential direction with respect to the SIL. At this time, it is determined whether the gap error signal level is reduced and collected to a random value. If the gap error signal level is increased, the recording medium is tilted at a predetermined angle in an opposite tangential direction. In this way, if the gap error signal is reduced and then collected to a random value, the tilt is stopped. 
     It has been described that tilt compensation in the radial direction is first performed and then tilt compensation in the tangential direction is performed. However, the order of tilt compensation may be changed, and tilt compensations in the two directions may be performed simultaneously. 
     At this time, the tilt in the radial direction can be compensated by controlling an angle of a pickup device  20  with respect to the recording medium while the tilt in the tangential direction can be compensated by controlling an angle of a disk driver of the recording reproducing apparatus. Alternatively, the tilt compensation can be performed vice versa. 
     As described above, the angle between the recording medium and the SIL is controlled in due order to search a timing point when the gap error signal is minimized. The reason why that the tilt is controlled in a state that the recording medium is stopped is to prevent the SIL or the recording medium from being damaged and obtain a stable gap error signal by preventing strong collision between the recording medium, which is rotating, and the SIL, wherein the strong collision may occur due to an excessive tilt error. 
     However, even though the angle between the recording medium and the lens is controlled at the time when the gap error signal is minimized in a state that the recording medium is stopped, it means that the tilt error is locally compensated. When the recording medium is rotated, collision between the recording medium and the SIL may occur due to seismic isolation of the recording medium, or rolling of the gap error signal may be caused by angle change. 
     Accordingly, the gap error signal is detected (S 60 ) in a state that the recording medium is rotated (S 50 ). The tilt angle is controlled based on the gap error signal in a state that the recording medium is rotated (S 70 , S 80 ). 
       FIG. 4  is a graph illustrating a variance of a gap error signal depending on a tilt angle in a state that a recording medium is rotated, and  FIG. 5   a  and  FIG. 5   b  are graphs illustrating a time variable gap error signal at specific tilt angles (A and B). 
     As shown, if the tilt error of the recording medium is great, the variance of the gap error signal is great. If the tilt error of the recording medium is small, the variance of the gap error signal is reduced. At this time, if the angle of the recording medium is changed to the time when the variance of the gap error signal is minimized, the angle error between the recording medium and the SIL can be minimized. 
     As described above, the tilt of the recording medium can be divided into the tilt component in the radial direction and the tilt component in the tangential direction. Accordingly, tilt control for each directional component is required as described in detail below. 
     First of all, the tilt in the radial direction is controlled (S 70 ). If the variance of the gap error signal is measured in a state that the recording medium is rotated, the recording medium is tilted at a predetermined angle in a radial direction with respect to the SIL. At this time, it is determined whether the variance of the gap error signal is reduced or increased. If the variance of the gap error signal is increased, since it means that the tilt is more increased, the recording medium is tilted at a predetermined angle in an opposite radial direction. If the variance of the initial gap error signal is reduced, or if the variance of the gap error signal is minimized as the variance of the gap error signal is reduced through direction control to an opposite direction, the tilt is stopped. 
     Next, the tilt in the tangential direction is controlled (S 80 ). At this time, the method for compensating the tilt in the radial direction can equally be applied to the tilt in the tangential direction. Namely, the recording medium is tilted at a predetermined angle in a tangential direction with respect to the SIL. At this time, it is determined whether the variance of the gap error signal is reduced. If the variance of the gap error signal is increased, the recording medium is tilted at a predetermined angle in an opposite tangential direction. In this way, if the variance of the gap error signal is minimized, the tilt is stopped. 
     It has been described that tilt compensation in the radial direction is first performed and then tilt compensation in the tangential direction is performed. However, the order of tilt compensation may be changed, and tilt compensations in the two directions may be performed simultaneously. 
     At this time, the tilt in the radial direction can be compensated by controlling an angle of a pickup device  20  with respect to the recording medium while the tilt in the tangential direction can be compensated by controlling an angle of a disk driver of the recording reproducing apparatus. Alternatively, the tilt compensation can be performed vice versa. 
     Next, recording or reproduction is performed by finely controlling the tilt using an actuator. In a state that the recording medium is stopped and rotated as above, a tilt of a component difficult to compensate using the actuator is compensated using a gap servo signal to desirably perform recording and reproduction. 
       FIG. 6  is a block diagram illustrating a tilt control apparatus according to the embodiment of the present invention. As illustrated in  FIG. 6 , the tilt control apparatus according to the embodiment of the present invention includes a pickup device  20 , a first tilt driver  30 , a second tilt driver  40 , a gap error signal detector  80 , and a tilt controller  90 . 
     The pickup device  20  records data in a recording medium  100  by irradiating light to the recording medium  100  or reproduces the data recorded in the recording medium  100 . In this embodiment, the pickup device  20  includes SIL that allows near field. 
     The first tilt driver  30  controls a tilt by controlling an angle of the recording medium  100  under the control of the tilt controller  90 . At this time, the first tilt driver  30  can control the tilt of the recording medium  100 . 
     The second tilt driver  40  controls a tilt by controlling an angle of the pickup device  20  under the control of the tilt controller  90 . At this time, the second tilt driver  40  can control the tilt of the recording medium  100 . 
     The gap error signal detector  80  detects a gap error signal based on an optical signal input from the pickup device  20 , and transfers the detected gap error signal to the tilt controller  90  to perform tilt control. 
     The tilt controller  90  determines a tilt in accordance with the gap error signal input from the gap error signal detector  80  and controls the first tilt driver  30  and the second tilt driver  40  to compensate the tilt based on the determined result. 
     The gap error signal detected from the gap error signal detector  80  is converted to a gap error signal level, which is represented by a specific voltage value, by the tilt controller  90 . The measured gap error signal level is transferred to the first and second tilt drivers  30  and  40  and then used for compensation of the tilt. 
     The tilt controller  90  determines whether the gap error signal level or the variance of the gap error signal is increased or reduced. If the gap error signal level or the variance of the gap error signal is reduced, the tilt controller  90  determines whether the gap error signal level or the variance of the gap error signal reaches a minimum value, and provides the determined result to the first tilt driver  30  and the second tilt driver  40 . 
     The first tilt driver  30  and the second tilt driver  40 , which have received a control signal from the tilt controller  90 , respectively compensate the tilt of the recording medium  100  in the tangential direction and the radial direction. The tilt controller  90  can be configured in software or hardware, or in combination of software and hardware. Only one tilt controller  90  may be provided, or a plurality of tilt controllers  90  can be provided. 
     At this time, the first tilt driver  30  and the second tilt driver  40  control the tilt by tilting the pickup device  20  or the recording medium  100 . 
     The respective elements of the aforementioned tilt control apparatus can be configured in software or hardware to perform the aforementioned functions, or can be configured in combination of software and hardware. 
     Hereinafter, a detailed example of the recording reproducing apparatus for performing tilt control will be described with reference to the accompanying drawing. The apparatus described hereinafter is only exemplary to describe the present invention, and the present invention is not limited to the following description. 
       FIG. 7   a  is a plane view illustrating an example of a recording reproducing apparatus according to the embodiment of the present invention, and  FIG. 7   b  is a sectional view of  FIG. 7   a . As shown, the recording reproducing apparatus includes a pickup device  20 , a first tilt driver  30 , a second tilt driver  40 , a disk driver  50 , and first and second rotation support assemblies  60  and  70 . 
     The pickup device  20  includes a pickup driving unit  26 , a pickup unit  22  fed by the pickup driving unit  26 , a guide rail  24  guiding feeding of the pickup unit  22 , and a pickup device base  28  for mounting the units. 
     The first tilt driver  30  compensates a tilt of the recording medium by controlling an angle of the disk driver  50 . The second tilt driver  40  compensates the tilt of the recording medium by controlling an angle of the pickup device  20 . 
     The disk driver  50  includes a driving motor  54 , and a driving unit base  52  provided with the driving motor  54 . 
     The first rotation support assembly  60  and the second rotation support assembly  70  are respectively connected with the second tilt driver  40  and the first tilt driver  30 . 
     The first rotation support assembly  60  and the second rotation support assembly  70  respectively include elastic support units  62  and  72 , hinge supports  64  and  74 , and hinges  66  and  76 . Also, the elastic support units  62  and  72  respectively include clamp shafts  62   a  and  72   a  and elastic members  62   b  and  72   b.    
     Hereinafter, angle control of the disk driver  50  through the operation of the first tilt driver  30  will be described in detail with reference to  FIG. 8   a  and  FIG. 8   b.    
     The first tilt driver  30  is provided to tilt the driving unit base  52  provided with the driving motor  54 . 
     The hinge  76  of a dome shape and the hinge support  74  are provided at the lower part of the driving unit base  52 . 
     The driving unit base  52  can be tilted to correspond to the recording medium by the hinge  76  and the hinge support  74 . 
     In a state that the driving unit base  52  is rotated (tilt driving) at a certain angle by the hinge  76  and the hinge support  74 , the elastic support unit  72  that applies a rotational force to the driving base unit  52  is provided in the driving unit base  52 . The elastic support unit  72  includes the clamp shaft  72   a  and the elastic member  72   b.    
     The clamp shaft  72   a  constituting the elastic support unit  72  as above may be provided to pass through the driving unit base  52 , or may be provided to adjoin the driving unit base  52 . The elastic member  72   b  is arranged between the clamp shaft  72   a  and the driving unit base  52  to apply elasticity to the driving unit base  52 . Namely, the driving unit base  52  can be configured to be pushed downwardly. 
     The elastic support unit  72  can be configured based on the hinge  76  and the hinge support  74  to apply a potential rotational force to one side of the driving base unit  52 . At this time, the potential rotational force applied to the driving unit base  52  by the elastic support unit  72  will be applied to a direction where the first tilt driver  30  which will be described later is located. This is because that the rotational force should be applied to the driving unit base  52  by the first tilt driver  30  in an opposite direction of the direction of the rotational force applied to the driving unit base  52  by the elastic support unit  72  based on the hinge  76  and the hinge support  74 . 
     The first tilt driver  30  and the second tilt driver  40  are comprised of a cam structure so that the driving base unit  52  and the pickup device  20  can be tilted, respectively. 
     The first tilt driver  30  and the second tilt driver  40  include a cam structure and a driving motor that can rotate the cam structure. The first tilt driver  30  and the second tilt driver  40  will be described in detail. Hereinafter, the first tilt driver  30  will exemplarily be described, and the description of the second tilt driver  40  similar to the first tilt driver  30  will be omitted. 
       FIG. 9   a  and  FIG. 9   b  are plane view and sectional view illustrating the first tilt driver  30 . As illustrated in  FIG. 9   a  and  FIG. 9   b , the first tilt driver  30  includes a driving gear  310  generating a rotational force, a tilting driving unit base  320  receiving the rotational force from the driving gear  310 , and a cam unit  330  fixed to the tilting driving unit base  320  and arranged to adjoin the driving unit base  52 . 
     The first tilt driver  30  arranged to adjoin the driving unit base  52  will be described in more detail. A gear  324  is formed at a side of the tilting driving unit base  320  constituting the first tilt driver  30 , a rotation shaft  322  is provided below the tilting driving unit base  320 , and the cam unit  330  is provided on the tilting driving unit base  320 . 
     According to the aforementioned configuration, the tilting driving unit base  320  provided with the gear  325  at the side is arranged at one side of the driving unit base  52  by the rotation shaft  322  provided below the tilting driving unit base  320 . 
     The cam unit  330  fixed onto the tilting driving unit base  320  is configured to apply the rotational force to the driving unit base  52  in an opposite direction of the direction of the rotational force applied by the elastic support unit  72  based on the hinge  76  and the hinge support  74  in contact with one end of the driving unit base  52 . 
     The driving gear  310  includes a driving motor  312  generating a rotational force, and a gear unit  314  for transferring the rotational force generated from the driving motor  312  to the tilting driving unit base  320 . 
     The gear unit  314  is configured to apply the rotational force in a state that it is engaged with the gear  324  formed at the side of the tilting driving unit base  320 . The gear unit  314  may be comprised of a plurality of gear assemblies, or may be comprised of a pinion gear generally fixed to the driving motor  312 . 
     In accordance with the aforementioned configuration, the rotational force generated by the driving motor  312  is transferred to the cam unit  330  through the gear unit  314  and the tilting driving unit base  320 . The cam unit  330  is rotated by the rotational force in a state that its upper surface adjoins the end of the driving unit base  52 . 
     When a peak point, the lowest point, or a center point of the cam unit  330  is located at the end of the driving unit base  52  by the aforementioned rotation, the rotation state of the driving unit  52  is determined as a clockwise, counterclockwise, or parallel state based on the hinge  76  and the hinge support  74 . 
       FIG. 10  is a block diagram illustrating a recording reproducing apparatus according to the embodiment of the present invention. The recording reproducing apparatus is configured to detect light irradiated from a pickup  1100  and reflected on a recording medium  1200 , and control tilt or trace of a track to correspond to the detected light, thereby irradiating the light to the exact position. Hereinafter, an optical system included in the pickup  1100  will be described in detail. 
     The pickup  1100  includes a light source  110 . The light source  110  could be a laser having good straightness, such as a laser diode. The light irradiated from the light source  110  to the recording medium could be parallel light. Accordingly, the recording reproducing apparatus according to the embodiment of the present invention includes a collimate lens  120  makes a path of the light emitted from the light source  110  parallel. 
     Split synthesizers  130  and  140  split the path of light entered from one direction or synthesize paths of light entered from different directions. A data recording apparatus according to the embodiment of the present invention includes a first split synthesizer  130  and a second split synthesizer  140 . 
     The first split synthesizer  130  partially transmits and reflects incident light. For example, the first split synthesizer  130  could be a non-polarized beam splitter. The second split synthesizer  140  could be a polarized beam splitter that transmits polarized light of a specific direction in accordance with a polarized direction. If linear polarized light is used, the second split synthesizer  140  can be configured to transmit polarized components of a vertical direction and reflect polarized components of a horizontal direction. By contrast, the second split synthesizer  140  may be configured to transmit the polarized components of the horizontal direction and reflect the polarized components of the vertical direction. 
     A lens unit  150  is located near the recording medium  1200  to irradiate light to a given region of the recording medium  1200 . 
     A light convert surface  160  and an expander  190  are provided between the lens unit  150  and the second beam splitter  140 . The light convert surface  160  converts a polarized direction of light entered and reflected from the recording medium  1200 . If the light convert surface  160  is a quarter wave plate, the light convert surface  160  polarizes the light entered the recording medium  1200  counterclockwise, and polarizes the light reflected on the recording medium  1200  clockwise. As a result, the reflected light transmitting the light convert surface  160  is polarized in a direction different from that of the incident light. Phase difference of 90° occurs between the reflected light and the incident light. 
     Accordingly, the reflected light of which polarized direction is converted as above is reflected without transmitting the second beam splitter  140  through which the incident light is transmitted, and enters a first detector  170 . At this time, a part of the reflected light is polarized and then transmits the second beam splitter. Then, the part of the reflected light is reflected on the first beam splitter  130  and then enters a second detector  180 . This is because that the part of the reflected light is polarized as the lens unit  150  has a numerical aperture greater than 1. 
     The first detector  170  and the second detector  180  receive the reflected light and generate an electric signal corresponding to the reflected light. In the embodiment of the present invention, the first detector  170  and the second detector  180  respectively generate a tilt error signal and a gap error signal. 
     The expander  190  controls a sectional size of the incident light. Particularly, in the second embodiment of the present invention which will be described later, the expander  190  makes a sectional area of the light entering the lens unit  150  great so as to more efficiently perform tilt control. 
       FIG. 11  is a flow chart illustrating a tilt control method of a recording medium according to the second embodiment of the present invention. In a tilt control procedure of the recording medium, a contact condition is set with respect to a part where the light is actually located in the SIL. Accordingly, if a sectional area of light entering a bottom surface of the SIL or emitted from the bottom surface of the SIL has a large size, it is advantageous to acquire a signal for tilt control. In this respect, in this embodiment, the sectional are of the light entering the lens unit is maximized using the expander  190  (S 100 ). At this time, it is possible to increase the sectional area of the light by controlling a relative position of the expander  190 . 
     Next, in the same manner as the first embodiment of the present invention, in a state that the recording medium is stopped, the gap error signal is measured (S 120 ), and a tilt is controlled in a radial direction and a tangential direction (S 130 ). 
     Subsequently, it is determined whether the sectional area of the light has been completely maximized to correspond to the size of the SIL where the light enters (S 140 ). At this time, variation of a contact level according to expansion of the sectional area of the light is stored in a separate system memory, and gap servo is only operated to finally verify the set contact condition. 
     An initial tilt condition can be set uniformly and exactly for the entire bottom surface of the SIL through the initial condition procedure of the tilt between the disk and the bottom surface of the SIL in the expanded sectional area state of the light with respect to the bottom surface of the SIL. 
     At this time, if it is determined that the sectional area of the light has not been maximized, the sectional area of the light is again controlled using the expander  190  (S 150 ). 
     If it is determined that the sectional area of the light has been maximized, for example, if the size of the bottom surface of the SIL is equivalent to that of the sectional area of the light, tilt compensation of the recording medium is performed (S 160 ) as described in the first embodiment in a state that the recording medium is rotated. 
     According to this embodiment, the contact and tilt condition can be set for a broader bottom surface of the SIL. Accordingly, it is possible to actually improve stability of the gap servo. 
     It will be apparent to those skilled in the art that the present invention can be embodied in other specific forms without departing from the spirit and essential characteristics of the invention. Thus, the above embodiments are to be considered in all respects as illustrative and not restrictive. The scope of the invention should be determined by reasonable interpretation of the appended claims and all change which comes within the equivalent scope of the invention are included in the scope of the invention.