Abstract:
There is provided an apparatus for recording/reproducing data on/from a recording medium including: a pickup unit including an objective lens and a solid immersion lens(SIL) to allow a light to be incident onto a recording medium; a photoelectric element for receiving the light reflected from the recording medium to output a controlling signal; and a controller to control the pickup unit using the controlling signal outputted from the photoelectric element, wherein the controlling signal includes a tilting error signal by skew of the recording medium.

Description:
CLAIM FOR PRIORITY 
     This application is based on and claims priority to Korean Patent Application Nos. 10-2005-0100371 filed on Oct. 24, 2005, 10-2006-0034922 file on Apr. 18, 2006, 10-2006-0060472 file on Jun. 30, 2006 and 10-2006-0063344 filed on Jul. 6, 2006 in the Korean Intellectual Property Office (KIPO), the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus and a method for recording or reproducing data using light. 
     2. Description of the Related Art 
     Recently, there emerges an optical recording medium that can rewrite data in high density, capable of recording and storing high quality video data and high quality audio data for a long time. For example, such medium includes a blue-ray disc. 
     As standardization for the blue-ray disc is under rapid process, related products are developed and brought to the market in preparation for commercialization. The blue-ray disc can store data of about 25 GB. When the blue-ray disc is manufactured in a dual layer, it can store high capacity data of about 50 GB. 
     Meanwhile, method of shortening a wavelength of a laser beam or increasing a numerical aperture (NA) of an objective lens have been used in order to increase a recording capacity of a recording medium. 
       FIG. 1  is a view of an optical system of an optical recording/reproducing apparatus according to a related art. 
     Referring to  FIG. 1 , a beam generated from a laser diode  10  is converted into a parallel beam by a collimator lens  11 , passes through a beam splitter  12 , and is condensed onto a recording medium  14  by an objective lens  13 . 
     Also, a beam reflected by the recording medium  14  passes through the objective lens  13 , and is reflected by the beam splitter  12 , and then condensed by a lens  15  onto a light detector  16 , so that the beam is detected as an electrical signal. 
     However, a method of shortening, at an optical system of  FIG. 1 , a wavelength of light in order to increase a recording capacity has reached almost limitation physically. Also, regarding a method of increasing an NA of an objective lens, it is difficult to make a great improvement using a related art far field recording method. 
     Accordingly, a near field recording method using a solid immersion lens is under active development, which is illustrated in  FIG. 2 . 
     Referring to  FIG. 2 , an SIL  22  is disposed below an objective lens  21  in the near field recording method. The SIL  22  is formed in a hemisphere shape made of a medium having a refractive index n greater than 1. The NA of the objective lens  21  is made to nxNA that is greater than 1 using the SIL  22 , so that a recording capacity is increased even more. 
     Meanwhile, a gap between the SIL  22  and the recording medium  23  is tens of nano meters in the near field recording method. Therefore, in the case where skew or tilting is generated to the recording medium  23 , the gap between the SIL  22  and the recording medium  23  cannot be maintained accurately, which may cause collision between the SIL  22  and the recording medium  23 . An object of the present invention is to provide a recording/reproducing apparatus and a recording/reproducing method, capable of effectively obtaining a tilting error signal of a recording medium such that a gap between an SIL and the recording medium is maintained properly, and performing a servo operation allowing the gap between the SIL and the recording medium to be maintained properly using a tilting error signal. 
     SUMMARY OF THE INVENTION 
     There is provided an apparatus for recording/reproducing data on/from a recording medium including: a pickup unit including an objective lens and a solid immersion lens(SIL) to allow a light to be incident onto a recording medium; a photoelectric element for receiving the light reflected from the recording medium to output a controlling signal; and a controller to control the pickup unit using the controlling signal outputted from the photoelectric element, wherein the controlling signal includes a tilting error signal by skew of the recording medium. 
     In another aspect of the present invention, there is also provided an apparatus for recording/reproducing data on/from a recording medium including : a first optical system for detecting an electrical signal from a first light reflected by a recording medium using the first light; a second optical system for detecting a servo signal from a second light reflected by a solid immersion lens using the second light; and a pickup unit including an objective lens and the solid immersion lens to allow the first light to be incident onto the recording medium and which is driven in response to a controlling signal by a servo signal of the second light. 
     In further another aspect of the present invention, there is also provided a method for recording/reproducing data on/from a recording medium comprising: splitting a light, the light being incident onto the recording medium by a pickup unit, the pickup unit including an objective lens and a solid immersion lens; converting the splitted light to a control signal for controlling servo operation of the pickup unit; wherein the controlling signal includes a tilting error signal by skew of the recording medium. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view of an optical system of an optical recording/reproducing apparatus according to a related art; 
         FIG. 2  is a view explaining an optical system of a near field recording method; 
         FIG. 3  is a side view of an SIL and a recording medium provided to a recording/reproducing apparatus using a near field; 
         FIG. 4  is a view illustrating a relation between an amount of returning light and a gap; 
         FIG. 5  is a view explaining a recording/reproducing apparatus according to an embodiment of the present invention; 
         FIG. 6  is a view illustrating a beam spot observed by a photoelectric element in the case where skew is not generated to a recording medium; 
         FIG. 7  is a view explaining the case where skew is generated to a recording medium; 
         FIG. 8  is a view illustrating a beam spot observed by a photoelectric element in the case where skew is generated to a recording medium; 
         FIG. 9  is a graph illustrating an amount of light incident to a photoelectric element in the case where skew is generated to a recording medium; 
         FIG. 10  is a graph illustrating an amount of light incident to a photoelectric element in the case where skew is not generated to a recording medium; 
         FIGS. 11 and 12  are views illustrating a beam spot detected by a photoelectric element in the case where skew is generated at a recording medium; 
         FIG. 13  is a view explaining a method for detecting an electrical signal from a photoelectric element; 
         FIG. 14  is a view explaining a method for detecting an electrical signal from a first photoelectric element and a second photoelectric element; 
         FIG. 15  is a view explaining a recording/reproducing apparatus according to another embodiment of the present invention; 
         FIG. 16  is a view illustrating beams emitted from a first light source and a second light source are incident onto a recording medium; 
         FIG. 17  is a view comparing a tilt margin by a beam emitted from a first light source with a tilt margin by a beam emitted from a second light source; 
         FIGS. 18 and 19  are views explaining a beam spot at a photoelectric element by skew of a recording medium; 
         FIG. 20  is a view illustrating another example of a photoelectric element in a recording/reproducing apparatus according to an embodiment of the present invention; 
         FIG. 21  is a view explaining a recording/reproducing apparatus according to another embodiment of the present invention; 
         FIG. 22  is a view explaining light is incident onto a recording medium where a cover layer is not formed; 
         FIGS. 23 and 24  are views illustrating a beam spot at a photoelectric element by skew of a recording medium where a cover layer is not formed; 
         FIG. 25  is a view explaining a beam is incident onto a recording medium where a cover layer is formed; and 
         FIGS. 26 and 27  are views explaining a beam spot at a photoelectric element by tilting of a recording medium where a cover layer is formed. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
     A recording/reproducing apparatus according to an embodiment of the present invention includes an apparatus that can record/reproduce data to/from a recording medium, and also includes an apparatus that can perform record/reproduce operations of data. 
     It should be noted that terms of skew and tilting are used together in connection with tilting by skew generated at a recording medium. 
       FIG. 3  is a side view of an SIL and a recording medium provided to a recording/reproducing apparatus using a near field, and  FIG. 4  is a view illustrating a relation between an amount of returning light and a gap. 
     A light that has passed through an objective lens is incident to an SIL  52  before the light is incident onto the recording medium  51 . A portion of the light incident to the SIL  52  that is incident at a predetermined critical angle or more is totally reflected at a reflection surface of the SIL  52  and propagates to a photoelectric element (e.g., a photo detector) for observing the totally reflected light. 
     Also, in connection with a size of a light formed on the reflection surface of the SIL  52  and a gap d between the SIL  52  and the recording medium  51 , a region defined as ≦λ/2 by a wavelength λ of the light incident to the SIL  52  is a near field. 
     Also, a far field is defined as a state where the gap d satisfies d ≧λ/2 and a light does not extravasate to a data recoding layer. 
     However, in the case of a far field state, a light incident onto a cross-section of the SIL  52  at an angle of a critical angle or more is totally reflected and becomes returning light. Therefore, referring to  FIG. 4 , an amount of returning light of total reflection in the far field state is represented as a constant value. 
     On the other hand, in the case of a near field state, a portion of light incident onto a reflection surface of the SIL  52  at an angle of a critical angle or more extravasates to the data recording layer of the recording medium  51  at the reflection surface (i.e., a reflection boundary) of the SIL  52  as described above. 
     Therefore, referring to  FIG. 4 , it is revealed that an amount of returning light of total reflection exponentially reduces as it approaches the data recording/reproducing layer (exactly, a surface of the recording medium) of the recording medium  51 . 
     Therefore, when a cross-section position of the SIL  52  is in a near field state, it is possible to control a gap between the cross-section of the SIL  52  and the data recording/reproducing layer of the recording medium  51  to be constant by performing a feedback servo operation on a linear portion that changes depending on a size of the gap d using a gap error signal. 
     A controller controls a pickup unit and a light power of a source, and the pickup unit includes a SIL and objective lens. 
     The controller controls the pickup unit by using a controlling signal, and the controlling signal includes a tilting error signal. 
     For example, referring to  FIG. 4 , when a control is performed so that an amount of returning light of total reflection becomes a control object value P, the gap d is maintained constant. 
     For an apparatus and a method for recording data or reproducing recorded data during a near field state, the present invention provides a more effective servo operation during recording or reproducing of data through a variety of embodiments. 
     Regarding a construction of an apparatus for recording/reproducing data to/from a recording medium, embodiments of the present invention include a case where one light source for creating a light and emitting the same is formed, and a case where two light sources are formed. These constructions should not be construed to limit the scope of the present invention, and a variety of embodiments can be proposed using these constructions. 
       FIG. 5  is a view explaining a recording/reproducing apparatus according to an embodiment of the present invention. 
     Referring to  FIG. 5 , the recording/reproducing apparatus includes a radio frequency (RF) optical block for recording/reproducing data to/from a recording medium  123 , a servo optical block for controlling a gap between the recording medium  123  and an SIL  122 , and a pickup unit  120  including the SIL  122  and an objective lens  121 . 
     Each of the RF optical block and the servo optical block includes a polarization light splitter (PBS) for changing a path of a moving light, and a photoelectric element for converting an incident light into an electrical signal. 
     The recording/reproducing apparatus of  FIG. 5  includes both an RF block and a servo optical block on a path of a moving bean emitted from a single light source  110 . 
     In detail, the recording/reproducing apparatus includes a first light source  110  for emitting a light, a collimator lens  111  for changing the light emitted from the light source  110  into a parallel light, a first PBS  112  and a second PBS  132  for transmitting or reflecting an incident light depending on a polarization component of the incident light, a quarter wave plate (QWP)  116  for changing a wavelength of the incident light, and a reflection mirror  119  for guiding the light to a recording medium. 
     Also, the recording/reproducing apparatus further includes a first photoelectric element (e.g., a photodetector)  113  for converting a light reflected by the recording medium  123  and incident via the first PBS  112  into an electrical signal, and a second photoelectric element  115  for converting a light incident via the second PBS  132  into an electrical signal. 
     Here, the second PBS  132  and the second photoelectric element  115  constitute an RF optical block for obtaining an RF signal from a light reflected by the recording medium  123 . It should be noted that an electrical signal obtained by the second photoelectric element  115  can be used for a purpose of obtaining a tilting error signal of the recording medium as well as detecting an RF signal. 
     Also, the first PBS  112  and the first photoelectric element  113  constitute a servo optical block for detecting a tilting error signal of the recording medium  123  having a difference in a moving path with respect to light incident from the second photoelectric element  115  of the light reflected by the recording medium  123 . 
     The light source  110  may be a laser diode for emitting a laser having an excellent rectilinear characteristic. A light detector  127  for observing intensity of a light emitted from the light source  110  can be further provided. Intensity of a light emitted from the light source  110  can be maintained to constant intensity required for recording/reproducing operation by feeding back a signal from the light detector  127 . 
     Also, the pickup unit  120  includes an SIL  122  and an objective lens  121  for condensing an incident light onto the recording medium  123 . 
     In operation, a light emitted from the light source  110  is changed into a parallel light by the collimator lens  1   11 , and incident onto the QWP  116  via the first and second PBSs  112  and  132 . 
     Also, the light is converted from a linear polarized light to a circular polarized light at the QWP  116  and incident onto the reflection mirror  119 . 
     A light reflected by the reflection mirror  119  passes through the objective lens  121  and the SIL  122  and is incident onto the recording medium  123 . At this point, the pickup unit  120  includes a coil and a magnetic circuit to perform a servo operation for a gap error and a tilting error. 
     Here, a speed of a light incident to the SIL  122  is slowed down by a refractive index n of the SIL  122 , and a wavelength of the light is shortened by 1/n. Therefore, a diffraction limit inside the SIL  122  is reduced to less than a general value of 1/n, and an effect that a numerical aperture (NA) of the objective lens  121  is increased by n times is achieved. 
     Meanwhile, a polarized component of a light reflected by the recording medium  123  is changed by 180° and converted into a linear polarized light at the QWP  116 . Here, a polarization direction is changed into a polarization direction perpendicular to an original polarization direction. 
     Also, a reflected light whose polarization direction has changed cannot pass through the second PBS  132  and is reflected to be incident to the second photoelectric element  115 . 
     At this point, a portion of the reflected light has a distorted polarization and passes through the second PBS  132  and is reflected by the first PBS  112  to be incident to the first photoelectric element  113 . 
     The first and second photoelectric elements  113  and  115  convert an incident light into an electrical signal, and can detect a tiling error signal regarding tilting of the recording medium  123  using the created electrical signal. 
     Here, the second photoelectric element  115  receives an RF signal containing data recorded on the recording medium  123  and converts the RF signal into an electrical signal, thereby performing a reproducing operation of data. 
     Also, a signal converted into an electrical signal by the first photoelectric element  113  is used for the purpose of measuring skew of the recording medium  123 . Particularly, an electrical signal converted by the second photoelectric element  115  is also used for detecting a tilting error signal of the recording medium. 
     Next, a method for detecting skew or a tilting error signal of the recording medium  123  will be described below. 
       FIG. 6  is a view illustrating a light spot observed by a photoelectric element in the case where skew is not generated to a recording medium,  FIG. 7  is a view explaining the case where skew is generated to a recording medium, and  FIG. 8  is a view illustrating a light spot observed by a photoelectric element in the case where skew is generated to a recording medium. 
     In the case where tilting is not generated to the recording medium  123  while a predetermined gap is maintained between the SIL  122  and the recording medium  123 , a light spot having four portions of somewhat darkness and constant brightness is observed ( FIG. 6 ). 
     Meanwhile, referring to  FIG. 7 , when tilting (or skew) is generated to the recording medium  123  while a light  141  is incident onto the recording medium  123 , a tilting amount of the recording medium  123  can be detected using an electrical signal of a light spot observed by the first and second photoelectric elements  113  and  115 . Accordingly, a gap servo operation that can prevent damage of the SIL  122  caused by collision between the SIL  122  and the recording medium  123  can be performed. 
     In the case where skew is generated to the recording medium  123  as illustrated in  FIG. 7 , a light spot observed by the photoelectric element has dark portions and bright portions irregularly formed as illustrated in  FIG. 8 . According to the present invention, a tilting error signal of the recording medium can be detected using a light spot observed by the photoelectric element, 
     Also, a difference in a light amount observed by the photoelectric element for converting a light reflected by the recording medium into an electrical signal when skew is generated to the recording medium, will be described below. 
       FIG. 9  is a graph illustrating an amount of light incident to a photoelectric element in the case where skew is generated to a recording medium, and  FIG. 10  is a graph illustrating an amount of light incident to a photoelectric element in the case where skew is not generated to a recording medium. 
     In the graphs illustrated in  FIGS. 9 and 10 , an X-axis represents time as a variable.  FIGS. 9 and 10  mean a gap between the SIL and the recording medium gradually decreases as a time elapses. 
     Referring to  FIG. 9 , a maximum amount of light  151  is incident to the photoelectric element at first. As the SIL gradually approaches closely to the recording medium, an amount of light incident to the photoelectric element gradually decreases. Also, even when the SIL contacts the recording medium, a predetermined amount of light is detected. 
     In this case, skew is generated to the recording medium, and a gap servo operation for correcting tilting of the recording medium can be performed. 
     On the other hand, referring to  FIG. 10 , when skew is not generated to the recording medium, a maximum amount of light  161  is incident to the photoelectric element, but when a gap between the SIL and the recording medium gradually narrows, a light that can detect a skew amount (or tilting amount) of the recording medium from a light reflected by the recording medium is not received ( 162 ). 
     That is, when skew or tilting is generated to the recording medium, it is possible to detect an RF signal using a light reflected from a data recording layer of the recording medium, and simultaneously, to detect a tilting amount of the recording medium using a light reflected from a total reflection surface of the SIL. 
     However, in the case where skew is not generated to the recording medium, only a light reflected by the data recording layer of the recording medium is detected, and a light reflected from the total reflection surface of the SIL does not appear. In this case, a difference in a light amount of a light spot detected by the first photoelectric element  113  illustrated in  FIG. 5  does not exist. 
     Hereinafter, a method for more accurately measuring skew generated to a recording medium using a light spot observed by the photoelectric element will be described. 
       FIGS. 11 and 12  are views illustrating a light spot detected by a photoelectric element in the case where skew is generated at a recording medium. 
     First, since skew is not generated to the recording medium  123  in only a predetermined direction, skew generated to the recording medium can be quantified in two cases as illustrated in  FIGS. 11 and 12 . 
     Each of the photoelectric elements according to an embodiment of the present invention, i.e., the first and second photoelectric elements  113  and  115  can include a photodetector divided into four parts, but is not limited thereto. Embodiments of the present invention can be applied to any photodetector allowing a difference in a light amount caused by skew of a recording medium. 
     Skew generated to the recording medium can be divided into a radial direction R and a tangential direction T of the recording medium. At this point, the radial direction R and the tangential direction T are defined using a line connecting a center of the recording medium with a position onto which a light is illuminated for a reference. 
     That is, a direction of a line extending from a virtual reference line connecting the center of the recording medium with the position onto which the light is illuminated is defined as the radial direction R of the recording medium. A direction perpendicular to the virtual reference line is defined as the tangential direction T. At this point, a light amount detected by each of the photoelectric elements  113  and  115  has a difference depending on skew of the recording medium. 
       FIG. 11  illustrates the case where skew is generated in the radial direction R of the recording medium, and  FIG. 12  illustrates the case where skew is generated in the tangential direction T of the recording medium. 
     As described above, since intensity of reflected light shows difference depending on a gap between the SIL and the recording medium, a difference in a light amount received to the photoelectric element is substantially generated when skew is generated to the recording medium. 
     Referring to  FIGS. 11 and 12 , a bright portion of a light signal represents a gap between the SIL and the recording medium is relatively large, and a dark portion represents a gap between the SIL and the recording medium is relatively small. 
     Therefore, a skew direction generated to the recording medium can be known using a light spot observed by the photoelectric element, and a method for detecting an error signal regarding generated skew will be described below. 
       FIG. 13  is a view explaining a method for detecting an electrical signal from a photoelectric element, and  FIG. 14  is a view explaining a method for detecting an electrical signal from a first photoelectric element and a second photoelectric element. 
     First, referring to  FIG. 13 , the photoelectric element can include photodetector divided into four parts. Here, the photoelectric element can be the first photoelectric element illustrated in  FIG. 5 . 
     Also, each of detecting devices A, B, C, and D outputs a signal corresponding to light amounts received thereto. At this point, the output signal is shown as each of A, B, C, and D for easy understanding. 
     At this point, it is possible to create tilting error signals in the radial direction R and the tangential direction T using A, B, C, and D output from the respective detecting devices. The tilting error signal TE 1  in the radial direction R can be created as a difference signal of a signal (A+B/C+D) detected by the detecting devices divided in the radial direction R. 
     Also, a tilting error signal TE 2  in the tangential direction T can be created as a difference signal of a signal (A+D/B+C) detected by the detecting devices divided in the tangential direction R. 
     Here, TE 1 =k 1 [(A+B)−(C+D)], and TE 2 =k 2 [(A+D)−(B+C)]. 
     That is, since the difference signal represents a difference in a light amount depending on a direction of skew generated to the recording medium, it is used as a tilting error signal. At this point, a signal formed by a reflection light can be divided in the tangential direction and the radial direction of the recording medium to control tilting. Through this process, a servo operation of controlling the SIL can be performed to cancel skew generated to the recording medium. 
     Besides this method, a method for more accurately detecting a tiling error signal is shown in  FIG. 14 . A light passing through the SIL cannot be accurately illuminated onto the recording medium by not only skew generated to the recording medium, eccentricity of the recording medium, or other reasons. 
     That is, there is a track error which can be generated when a center of the recording medium itself is formed to have eccentricity when a light moves during rotation of the recording medium. In this case, a difference signal detected by the method with reference to  FIG. 13  can contain the track error signal therein. 
     Therefore, an error needs to be compensated for by removing an influence caused by the track error from a difference signal detected in  FIG. 13 . 
     For this purpose, it is necessary to judge whether a track error by a movement of a track exists and to create a tilting error signal capable of compensating for a track error signal. That is, since a tilting signal operation equation for the radial direction operated at each of the photoelectric elements  113  and  115  is the same as an operation equation for detecting a track error signal, a difference of a signal detected by the two photoelectric elements  113  and  115  is obtained using a method for canceling a track error component to remove an influence of a track error. 
     At this point, an error of a tilting error signal by a track error can be compensated for tilting error signals of the radial direction and the tangential direction. Since an operation can be performed using the same method, the present invention will be described in detail with reference to  FIG. 14  using an embodiment as a tilting error signal of the radial direction. 
     A light illuminated onto the recording medium  123  via the SIL formed on the pickup unit  120  is reflected by the recording medium  123  and reflected by the second and first PBSs  132  and  112 , and received by the second photoelectric element  115  and the first photoelectric element  113 . 
     A light received by the second photoelectric element  113  creates a tilting error signal using signals A, B, C, and D output from respective detecting devices constituting the second photoelectric element. That is, as described above, a tilting error signal is a difference of signals that are separated in the radial direction and summed (i.e., [(A+B)−(C+D)]) when a radial direction of the recording medium is used for a reference. 
     Also, a portion of the light reflected by the recording medium  123  is received by the first photoelectric element  113  with a predetermined path difference with respect to a light incident to the second photoelectric element  115 . 
     The light received by the first photoelectric element  113  creates a tilting error signal using signals a, b, c, and d output from respective detecting devices constituting the first photoelectric element. 
     That is, as described above, a tilting error signal is a difference of signals that are separated in the radial direction and summed (i.e., [(a+b)−(c+d)]) when a radial direction of the recording medium is used for a reference. 
     Referring to  FIG. 14 , a difference in a value obtained by multiplying a tilting error signal by a proportional constant is obtained to form an error-compensated tilting error signal in order to remove an influence caused by a movement of the track movement from the tilting error signal. 
     That is, a tilting error in the radial direction of the recording medium and a tilting error in the tangential direction can be calculated using the following equations.
 
k3[(A+B)−(C+D)]−k4[(a+b)−(c+d)]  Equation 1
 
k5[(A+D)−(B+C)]−k6[(a+d)−(b+c)]  Equation 2
 
     Equation 1 is a tilting error signal in the radial direction, and Equation 2 is a tilting error signal in the tangential direction. 
     Also, tilting error signals detected by the first and second photoelectric elements  113  and  115  include both error data regarding tilting and error data regarding track movement. 
     The first and second photoelectric elements  113  and  115  receive the same light reflected by the recording medium  123  but the received lights have path differences, respectively. Therefore, a change in the light caused by skew of the recording medium  123  is the same, but a movement path difference of the light or a light amount differs, so that a degree of a change can has a difference. 
     In this case, an error by a track movement can be removed and an error-compensated tilting error signal can be created by multiplying predetermined proportional constants k 3 , k 4 , k 5 , and k 6 . 
     As described above, the light reflected by the recording medium  123  are received by the two photoelectric elements  113  and  115 , which can create tilting error signals of the radial direction R and the tangential direction T. At this point, the tilting error signal uses a characteristic that the light linearly changes depending on a change in a gap between the SIL and the recording medium  123 , which has been described in detail in the above. 
     Also, tilting error signals described in Equations 1 and 2 create error-compensated tilting error signals by calculating a difference in a value obtained by multiplying a proportional constant to cancel an error caused by a track movement in order to remove an error caused by the track movement. 
     That is, Equations 1 and 2 become tilting error signals of the recording medium in which an error caused by the track movement has been canceled with respect to the radial direction and the tangential direction of the recording medium. 
     A slop of the SIL by the pickup unit  120  is controlled during recording or reproduction operation of data using the above-created error-compensated tilting error signal, and horizontality is maintained within a limit range, so that stable data processing can be performed. 
     Here, the second photoelectric element  115  creates a recording/reproducing signal (RF signal) or a track error signal, and the first photoelectric element  113  creates a gap error signal. 
     The above embodiment of the present invention has described a servo operation for the case where one light source for emitting a light is provided. Hereinafter, a servo operation for the case where two light sources for emitting lights having different wavelengths, respectively, are provided as another embodiment of the present invention. 
       FIG. 15  is a view explaining a recording/reproducing apparatus according to another embodiment of the present invention. 
     Referring to  FIG. 15 , a recording/reproducing apparatus includes an RF optical block for recording/reproducing data to/from a recording medium  223 , a gap servo optical block for controlling a gap between the recording medium  223  and an SIL  222  and a pickup unit  220  having an objective lens  221  and the SIL  222  for allowing light to be incident onto the recording medium  223  and onto which light reflected by the recording medium  223  is incident. 
     A servo signal used hereinafter includes a tilting amount of the recording medium  223 ( 323 ) and a movement amount of the objective lens  221 ( 321 ) or the pickup unit  220 . The servo signal corresponding to the tilting amount of the recording medium  223 ( 323 ) is a first servo signal, and the servo signal corresponding to the movement amount of the objective lens  221 ( 321 ) or the pickup unit  220  is a second servo signal. 
     The RF optical block includes a first light source  210  for emitting a blue light in a wavelength band of 405 nm, a first collimator lens  211  for changing the light emitted from the first light source  210  into parallel light, a first PBS  212  for transmitting or reflecting an incident light depending on a polarization component of the incident light, a first light expander  215  for controlling a diverging angle or a converging angle of the light, a first QWP  216  for changing a wavelength of the incident light, and a light splitter  217 . 
     Also, the RF optical block also includes a first photoelectric element (e.g., a photodetector  113 ) for detecting an RF signal reflected by the recording medium  223  and incident via the first PBS  212 . 
     Also, the gap servo optical block includes a second light source  230  for emitting a red light in a wavelength band of 650 nm, a second collimator lens  231  for changing the light emitted from the second light source  230  into parallel light, a second PBS  232  for transmitting or reflecting an incident light depending on a polarization component of the incident light, a second light expander  235  for controlling a diverging angle or a converging angle of the light, and a second QWP  236  for changing a wavelength of the incident light. 
     Also, the gap servo optical block also includes a first photoelectric element for detecting a gap error signal totally reflected by the SIL  222  and incident via the second PBS  232 . 
     Also, the pickup unit  220  includes the SIL  222  and the objective lens  221  for condensing the incident light onto the recording medium  223 . 
     In operation, a first light emitted from the first light source  210  is changed into parallel light at the first collimator lens  211 , and passes through the first PBS  212 . A diverging angle or a converging angle of the first light is controlled by the first light expander  215 . 
     Also, the first light is converted from a linear polarized light to a circular polarized light at the first QWP  216 , and is incident to the reflection mirror  218 . 
     The first light reflected by the reflection mirror  218  passes through the objective lens  221  and the SIL  222 , and is incident onto the recording medium  223 . At this point, the pickup unit  221  includes a coil and a magnetic circuit to perform a servo operation for a gap error and a tracking error. 
     Meanwhile, a polarization component of an RF signal reflected by the recording medium  223  is changed by 180° and is converted into linear polarized light at the first QWP  216 . At this point, the light has a polarization direction perpendicular to an original polarization direction. 
     Therefore, the RF signal is reflected by the first PBS  212  and incident to the first photoelectric element  213 . 
     The first photoelectric element  213  converts the RF signal into an electrical signal to reproduce data stored in the recording medium  223 . 
     Meanwhile, an optical recording/reproducing apparatus according to an embodiment of the present invention includes a gap servo optical block for controlling a gap between an SIL  222  and a recording medium  223 . 
     A second light emitted from the second laser diode  210  (second light source) is changed into parallel light at the second collimator lens  231 , and passes through the second PBS  232 . A diverging angle or a converging angle of the second light is controlled by the second light expander  235 . 
     Also, a polarization of the second light is converted at the second QWP  236 , and is incident to the reflection mirror  218 . 
     The second light reflected by the reflection mirror  218  passes through the objective lens  221  and the SIL  222 , and is incident onto the recording medium  223 . 
     At this point, a portion of the second light is totally reflected by the SIL  222 . When a gap between the SIL  222  and the recording medium  223  is small, an amount of a light that is totally reflected is small. On the other hand, when the gap between the SIL  222  and the recording medium  223  is large, the amount of the light that is totally reflected is large. 
     This is due to relationship among the SIL  222 , the recording medium  223 , a refractive index of air contained between the SIL  222  and the recording medium  223 , and a wavelength of the light. When the gap between the SIL  222  and the recording medium  223  is 100 nm or less, a gap between the SIL  222  and the recording medium  223 , and an amount of a light that is totally reflected by the SIL  222  has a correlation, which is the same as that described with reference to  FIGS. 5 to 14 . 
     A second light reflected by the SIL  222  is reflected by the second PBS  232  and incident to the second photoelectric element  233 . The second photoelectric element  233  detects a gap servo signal from the second light reflected by the SIL  222 . 
     As described above, the first light emitted from the first light source  210  is for detecting an RF signal, and the second light emitted from the second light source  230  is for detecting a gap servo signal. 
     According to the present invention, a tilting amount by skew of the recording medium  223  can be known using a gap servo signal representing a gap between the SIL  222  and the recording medium  222 . Also, the SIL  222  and the recording medium  223  are controlled such that they does not collide with each other through a gap servo operation. 
       FIG. 16  is a view illustrating lights emitted from a first light source and a second light source are incident onto a recording medium. 
     Referring to  FIG. 16 , the first light emitted from the first light source  210  and the second light emitted from the second light source  230  have different light sizes, respectively, at the SIL  222 . 
     The first light  240  can be a light in a wavelength band of blue light having a short wavelength, and the second light  241  can be a light in a wavelength band of red light having a relatively long wavelength. 
     Since the second light  241  is incident in a relatively large size onto the SIL  222 , a gap error signal by skew of the recording medium  223  can be more accurately detected from the second light  241 . 
       FIG. 17  is a view comparing a tilt margin by a light emitted from a first light source with a tilt margin by a light emitted from a second light source. 
     The first light  240  emitted from the first light source  210  has a small light size at the SIL  222 , and a tilt margin of the recording medium  223  that can be detected using the small light size is small. 
     That is, in the case where a gap servo is performed using the first light  240 , a tilting state of the recording medium  223  can be detected when the recording medium  223  is located at a first position  223   b . Therefore, when the recording medium  223  is located on the first position  223   b , the pickup unit  220  performs a control operation by a gap servo in order to prevent collision between the SIL  222  and the recording medium  223 . 
     On the other hand, the second light emitted from the second light source  230  has a large light size at the SIL  222 , and a tilt margin of the recording medium  223  that can be detected using the large light size is greater than the tilt margin of the recording medium  223  that can be detected using the first light  240 . 
     That is, in the case where the gap servo is performed using the second light  241 , a tilting state of the recording medium  223  can be detected when the recording medium  223  is located at a second position  223   a.    
     Therefore, in the case where the gap servo is performed using the second light  241 , a movement of the recording medium  223  can be more sensitively detected, and a control operation by the gap servo can be more swiftly performed. 
     Also, since the second light  241  has a relatively large light size at the SIL  222 , a signal more sensitive to skew of the recording medium  223  can be obtained. 
       FIGS. 18 and 19  are views explaining a light spot at a photoelectric element by skew of a recording medium. 
     Referring to  FIGS. 18 and 19 , when skew is generated to the recording medium  223  while the second light  241  is incident onto the recording medium  223 , the second photoelectric element  233  can detect skew of the recording medium  223  using a value of a signal of (A+B)−(C+D). Accordingly, a gap servo operation for preventing damage of the SIL  122  caused by collision between the SIL  222  and the recording medium  223  can be performed. 
     That is, an optical recording/reproducing apparatus according to an embodiment of the present invention includes the first light source  210  for recording or reproducing data, and the second light source  230  for the gap servo operation. Skew of the recording medium  223  can be sensitively detected by making a wavelength of the light emitted from the second light source  230  longer than that of the first light source  210 . 
       FIG. 20  is a view illustrating another example of a photoelectric element in a recording/reproducing apparatus according to an embodiment of the present invention. 
     Unlike the second photoelectric element  233  illustrated in  FIG. 19 , the second photoelectric element  233  illustrated in  FIG. 20  is divided into sixteen cells, and has an advantage of more sensitively detecting skew of the recording medium  223  compared to the photoelectric element divided into four cells ( FIG. 19 ). 
     Though not shown, the present invention can more sensitively detect skew of the recording medium  223  by providing a photoelectric detector including four cells or more. 
       FIG. 21  is a view explaining a recording/reproducing apparatus according to another embodiment of the present invention. 
     Referring to  FIG. 21 , the recording/reproducing apparatus includes a first optical system for recording/reproducing data to/from a recording medium  323 , a second optical system for detecting a gap error signal between the recording medium  323  and an SIL  322 , and a pickup unit  320  onto which light reflected by the recording medium  323  is incident and including the SIL  322  and an objective lens  321  allowing light to be incident onto the recording medium  323 . 
     In detail, one of the first and second optical systems performs an operation of detecting a servo signal for detecting a tiling amount of the recording medium  323 , and the other performs an operation of detecting an error signal generated by movement of the objective lens  321  during the tilting servo operation. 
     That is, it is possible to measure a tilting amount of the recording medium  323  and a movement amount of the objective lens  321  using a photoelectric element (e.g., a photo detector) provided to the first and second optical systems. 
     The first optical system includes a first light source  310  for emitting a blue light in a wavelength band of  405  nm, a first collimator lens  311  for changing the light emitted from the first light source  310  into parallel light, a first PBS  312  and a third PBS  314  for transmitting or reflecting incident light depending on a polarized component of the incident light, a third PBS  314 , a first expander  316  for controlling a diverging angle or a converging angle of a light to control a light size, a first QWP  317  for changing a wavelength of an incident light, and a light splitter  318 . 
     Also, the first optical system also includes a third photoelectric element  315  for detecting an RF signal reflected by the recording medium  323  and incident via the third PBS  314 , and a firth photoelectric element  313  for detecting a gap error signal reflected by the recording medium  323  and incident via the first PBS  312 . 
     For reference, the present invention is described using a photodetector as a photoelectric element. 
     The second optical system includes a second light source  330  for emitting a red light in a wavelength band of  650  nm, a second collimator lens  331  for changing the light emitted from the second light source  330  into parallel light, a second PBS  332  for transmitting or reflecting incident light depending on a polarized component of the incident light, a second expander  313  for controlling a diverging angle or a converging angle of a light to control a light size, and a second QWP  337  for changing a wavelength of an incident light. 
     Also, the second optical system also includes a second photoelectric element  333  for detecting a gap error signal reflected by the recording medium  323  and incident via the second PBS  332 . 
     The pickup unit  320  includes an SIL  322  and an objective lens  321  for condensing an incident light onto the recording medium  323 . 
     In operation, a first light emitted from the first light source  310  is changed into parallel light at the first collimator lens  311 , and passes through the first PBS  312  and the third PBS  314 . A diverging angle or a converging angle of the first light is controlled by the first expander  316 , so that a light size is changed. 
     Also, the first light is converted from a linear polarized light to a circular polarized light at the first QWP  317 , and is incident to the reflection mirror  319 . 
     The first light reflected by the reflection mirror  319  passes through the objective lens  321  and the SIL  322 , and is incident onto the recording medium  323 . At this point, the pickup unit  321  includes a coil and a magnetic circuit to perform a servo operation for a gap error and a tracking error. 
     Meanwhile, the light reflected by the recording medium  323  is changed into a linear polarized light at the first QWP  317 . Here, a polarization direction is changed into a polarization direction perpendicular to an original polarization direction. 
     In the case where the first light is used for a purpose of reproducing data recorded on the recording medium  323 , the first light is not only reflected by a data recording/reproducing layer of the recording medium  323  but also a portion of the first light is totally reflected by the SIL  322  before incident onto the recording medium  323 . Here, a surface by which the first light is totally reflected is a reflection surface of the SIL  322 , and the reflection surface is a flat surface of the SIL  322  facing the recording medium  323 . 
     In a near field optical recording/reproducing apparatus, coupling is generated by a correlation between a gap between the SIL  322  and the recording medium  323 , and refractive indexes of the SIL  322 , the recording medium  323 , and air, so that a portion of the light incident onto the SIL  322  is totally reflected, and the rest is incident onto the recording medium  323 . 
     Therefore, a portion of the light reflected by the recording medium  323  that has a polarization component of being reflected by the third PBS  314  is incident to the third photoelectric element  315 . A signal incident to the third photoelectric element  315  becomes an RF signal. The third photoelectric element  315  converts the RF signal into an electrical signal to reproduce data stored in the recording medium  323 . 
     Also, a portion of the light reflected by the recording medium  323  that has passed through the third PBS  314  (more specifically, the light totally reflected by the SIL) is reflected by the first PBS  312 , and incident to the first photoelectric element  313 . The signal incident to the first photoelectric element  313  serves as a signal for detecting a tilting amount of the recording medium  323 , or serves as a signal for detecting a movement amount of the objective lens  321  or the pickup unit  320 . 
     Also, the second photoelectric element, which will be described later, detects a tilting amount of the recording medium  323 , or detects a movement amount of the objective lens  321  or the pickup unit  320 . 
     Meanwhile, the second light emitted from the second light source  330  is changed into parallel light at the second collimator lens  331 , passes through the second PBS  332 , and is converted at the second QWP  337 , and incident onto the reflection mirror  319 . 
     The second light reflected by the reflection mirror  319  passes through the objective lens  321  and the SIL  322 , and is incident onto the recording medium  323 . 
     Also in this case, a portion of the second light is totally reflected by the SIL  322 . In the case where a gap between the SIL  322  and the recording medium  323  is small, an amount of totally reflected light is small. On the other hand, in the case where the gap between the SIL  322  and the recording medium  323  is large, the amount of totally reflected light is large. 
     This is due to relationship among the SIL  322 , the recording medium  323 , a refractive index of air contained between the SIL  322  and the recording medium  323 , and a wavelength of the light. When the gap between the SIL  322  and the recording medium  323  is 100 nm or less, a gap between the SIL  322  and the recording medium  323 , and an amount of a light that is totally reflected by the SIL  322  has a correlation, which is the same as described above. 
     A second light reflected by the SIL  322  is reflected by the second PBS  332  and incident to the second photoelectric element  333 . The second photoelectric element  333  detects a gap servo signal from the second light reflected by the SIL  322 . 
     Also, a signal detected by the second photoelectric element  333  serves as a signal for detecting a tilting amount of the recording medium  323 , or serves as a signal for detecting a movement amount of the objective lens  321  or the pickup unit  320 . 
     The present invention uses both the first light and the second light, and particularly, uses only portions of the first and second lights that are totally reflected by the SIL  322  in order to detect a tilting error signal. 
     Regarding use of the first and second lights for detecting a tilting error signal, detail description will be made for the case where the recording medium  323  includes a cover layer (protective layer) and the case where the recording medium  323  does not include the cover layer with reference to  FIGS. 21 to 27 . 
     It is important to maintain a gap between the SIL and a surface of the recording medium in a near field recording medium. In the case where a cover layer is formed on the recording medium, a gap between a surface of the cover layer and the SIL needs to be maintained. On the other hand, in the case where the cover layer is not formed on the recording medium, a gap between a data recording/reproducing layer and the SIL needs to be maintained. 
       FIG. 22  is a view explaining light is incident onto a recording medium where a cover layer is not formed, and  FIGS. 23 and 24  are views illustrating a light spot at a photoelectric element by skew of a recording medium where a cover layer is not formed. 
     Referring to  FIG. 22  illustrating the case where a cover layer is not formed on a data recording/reproducing layer of the recording medium  323 . At this point, a first light  340  is used for detecting an RF signal and detecting a DC offset by a movement of the objective lens  321 , and the second light  341  is used for measuring a tilting amount of the recording medium. 
     That is, in the case where the cover layer is not formed on the recording medium  323 , a focus of the first light  340  is formed on the data recording/reproducing layer, and a light size of the first light  340  at the SIL  322  is relatively large than that of the second light  341 . 
     A focus of light generated from light source  310 ,  330  is adjusted by movement of the objective lens  321 . The objective lens  321  can be moved to right/left and up/down by pickup unit  320 . 
     Since a tiling amount (or a skew amount) of the recording medium  323  can be more accurately detected when a size of a light formed on the SIL  322  is large, the second light  341  is used for detecting a tilting amount of the recording medium  323 . 
     Also, since a tilting amount of the recording medium  323  is difficult to detect when a size of a light formed on the SIL  322  is small, the first light  340  is used for detecting a movement amount (or a DC offset) of the objective lens  321  or the pickup unit  320  using the first photoelectric element  313  as well as detecting an RF signal. 
     In this case, photo-electric converted signals observed via the first and second photoelectric elements  313  and  333  are illustrated in  FIGS. 23 and 24 . 
     The first photoelectric element  313  can be a photo detector divided into two parts, and the second photoelectric element  333  can be a photo detector divided into four parts. That is, the first light  340  is used for detecting a DC offset by a movement of the objective lens  321  as well as recording and reproducing data using an RF signal. 
     Since a size of the first light  340  is smaller than that of the second light  341 , it is difficult to detect a tiling amount of the recording medium  323  using the first light  340  as described above. 
     A light spot of the totally reflected by the SIL  322  that is photo-electric converted by the first photoelectric element  313  is illustrated in  FIG. 23 , and a DC offset by a movement of the objective lens  321  is k×(E−F). 
     Also, a light spot of the totally reflected by the SIL  322  that is photo-electric converted by the second photoelectric element  333  is illustrated in  FIG. 24 , and a tilting amount of the recoding medium  323  that is detected by the second photoelectric element  333  is [(A+D)−(B+C)]. That is, a difference between a left side and a right side of signals photoelectric-converted by the second photoelectric element  333  divided into four parts is a tiling amount. 
     Therefore, a tilting error signal TE of the recording medium that is measured according to an embodiment of the present invention is [(A+D)−(B+C)]−k×(E−F), and the objective lens  321  can perform a servo operation minimizing the tilting error signal TE. 
     Also, a detecting element for detecting a tilting error signal of the recording medium using the first and second photoelectric elements  313  and  333  can be further provided. In other words, the detecting element(or controller) detects a controlling signal including the tilting error signal. 
     Meanwhile, the case where a cover layer is formed on the recording medium  323  will be described with reference to  FIGS. 25 to 27 . 
     Generally, since a thickness of the cover layer is thicker than a distance between the SIL and a surface of the recording medium, a use of a light for detecting a tilting error signal of the recording medium is different in a recording/reproducing apparatus for a recording medium where a cover layer is formed. 
     That is, an operation of recording or reproducing data using the first light, which is a blue light, is the same for the cases where the cover layer is formed on the recording medium, but a distance between the data recording/reproducing layer and the SIL is relatively larger in the case where the cover layer is formed on the recording medium rather than the case where the cover layer is not formed on the recording medium, so that a size of a light formed on the SIL is different. 
     To explain this,  FIGS. 25 to 27  are attached,  FIG. 25  is a view explaining a light is incident onto a recording medium where a cover layer is formed, and  FIGS. 26 and 27  are views explaining a light spot at a photoelectric element by tilting of a recording medium where a cover layer is formed. Referring to  FIG. 25 , the case of a recording/reproducing apparatus where the cover layer is formed on a data recording/reproducing layer of the recording medium  323  is illustrated. At this point, the first light  340  is used for detecting an RF signal and a tilting amount (or skew) of the recording medium. Also, the second light  341  is used for detecting a DC offset by a movement of the objective lens  321  during a servo control operation. 
     That is, in the case where the cover layer is formed on the recording medium  323 , a light size of the first light  340  is greater than that of the second light  341  formed on the SIL  322 , so that the second light  341  is used for detecting a DC offset. 
     In this case, photo-electric converted signals observed via the first and second photoelectric elements  313  and  333  are illustrated in  FIGS. 26 and 27 . 
     The first photoelectric element  313  can be a photo detector divided into four parts, and the second photoelectric element  333  can be a photo detector divided into two parts. 
     A light spot of the totally reflected by the SIL  322  that is photo-electric converted by the first photoelectric element  313  is illustrated in  FIG. 26 , and a tilting amount of the recoding medium  323  that is detected by the first photoelectric element  313  is [(A+D)−(B+C)]. That is, a difference between a left side and a right side of signals photoelectric-converted by the first photoelectric element  313  divided into four parts is a tiling amount of the recording medium. 
     Also, a light spot of the totally reflected by the SIL  322  that is photo-electric converted by the second photoelectric element  333  is illustrated in  FIG. 27 , and a DC offset caused by a movement of the objective lens  321  and detected by the second photoelectric element  333  is k×(E−F). 
     Therefore, a tilting error signal TE of the recording medium that is measured according to an embodiment of the present invention is [(A+D)−(B+C)]−k×(E−F), and the objective lens  321  can perform a servo operation minimizing the tilting error signal TE using the detecting element. 
     As described above, an optical recording/reproducing apparatus according to the present invention can be embodied for the case of recording/reproducing data to/from a recording medium where a cover layer is formed and for the case of recording/reproducing data to/from a recording medium where a cover layer is not formed. Also, a tilting error signal where a DC offset component caused by a movement of an objective lens is removed can be detected during a servo control operation, so that more accurate servo operation can be performed. 
     According to various embodiments proposed by the present invention, a tilting amount due to skew of a recording medium can be more accurately detected. Furthermore, an efficient recording or reproducing operation of data can be performed 
     Although the preferred embodiments of the present invention have been disclosed for illustrative purpose, those skilled in the art will appreciate that various modifications, additions and substitutions can be made without departing from the scope and spirit of the invention as defined in the accompanying claims.