Abstract:
There is a problem that when the relative angle between laser light and an optical disc is changed by temperature, or a chucking state of an optical disc and so forth, the formerly recorded data is overwritten. The above-mentioned problem is solved by performing focus control and tracking control independently with respect to a servo layer and a recording layer in a grooveless disc.

Description:
RELATED APPLICATIONS 
     This application is the U.S. National Phase under 35 U.S.C. §371 of International Application No. PCT/JP2012/001481, filed on Mar. 5, 2012, which in turn claims the benefit of Japanese Application No. 2011-100389, filed on Apr. 28, 2011, the disclosures of which Applications are incorporated by reference herein. 
     TECHNICAL FIELD 
     The present invention relates to an optical disc device for reproducing information from an optical disc, or recording or reproducing information into an optical disc, using laser light. 
     BACKGROUND ART 
     In recent years, in optical discs of the Blu-ray Disc™ standard, an optical disc including three or four recording layers has been developed and standardized in order to increase the recording capacity. Moreover, it is expected that the development of a four-or-more recording-layer optical disc will be performed from now on with an objective of implementing even larger capacity. For example, in Non-Patent Literature 1, the description has been given concerning the following optical disc (i.e. grooveless disc): Namely, in this grooveless disc, there is provided a layer (which, hereinafter, will be referred to as a servo layer) that is equipped with a physical groove structure for performing the tracking servo control. Furthermore, there are provided layers for performing recording/reproduction (“recording layers”) that are equipped with none of the land/groove structure. It is considered that this grooveless disc is easy to fabricate even if a large number of recording layers are to be multilayered. 
     Also, in the abstract of Patent Literature 1, the disclosure has been made as follows: “An additional-writing start position is detected which is continuous to the formerly-recorded area in one of the recording layers of a guide-layer-separated-type optical recording medium. At the time of starting the additional recording, the irradiation spot of a servo-use first laser beam is displaced to the position on a guide track which is directly opposed to a position in the recording layer that is apart from the additional-writing start position onto the unrecorded-area side. The irradiation spot of a recording-or-reproduction-use second laser beam onto the recording layer is displaced in a follow-up manner by this first laser&#39;s irradiation-spot displacement. The additional recording into the recording layer is started from the irradiation-spot position of the second laser beam after this follow-up displacement is over.” 
     CITATION LIST 
     Patent Literature 
     
         
         PATENT LITERATURE 1: JP-A-2010-40093 
       
    
     Non-Patent Literature 
     
         
         NON-PATENT LITERATURE 1: M. Ogasawara et al., “16 Layers Write Once Disc with a Separated Guide Layer”, ISOM2010, Th-L-07 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     As one of the problems when the additional recording is performed into a grooveless disc as described above, there exist the following dangers: Namely, if the relative angle between each laser beam and this optical disc is changed by such a factor as temperature or optical disc&#39;s chucking state, there exists a danger that previously-recorded data will be overwritten. Also, in a rewritable grooveless disc, if the relative angle between each laser beam and this optical disc is changed, there exists a danger that a recording position different from the desired recording position will erroneously be overwritten by new data. 
     In Patent Literature 1, the disclosure has been made regarding the additional recording. In this literature, the additional recording is started with an interval from the finally-recorded position of the formerly-recorded area. This structure makes it possible to suppress the additional recording from being performed in the manner of being overlapped with the formerly-recorded area. This suppression is made possible, even if, as illustrated in  FIG. 6 , a tilt (i.e. an inclination) of the optical disc with respect to the optical axis of each laser beam exists due to such a factor as optical disc&#39;s time-lapse-based warp, or difference in the recording devices. 
     In the solving method of Patent Literature 1, however, the wasted area is formed every time the additional recording is performed. As a result, there exists a problem that this results in a lowering in the disc capacity. 
     Accordingly, an object of the present invention is to provide an optical disc device that allows the recording to be performed at an appropriate and proper position of the grooveless disc. 
     Solution to Problem 
     The above-described problem is solved by the invention disclosed in the appended claims, for example. 
     Advantageous Effects of Invention 
     According to the present invention, it becomes possible to provide the optical disc device that allows the recording to be performed at an appropriate and proper position of the grooveless disc. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a configuration diagram for illustrating an optical disc device of Embodiment 1 to Embodiment 2; 
         FIG. 2  illustrates the structure of an optical disc; 
         FIG. 3  illustrates the relationships among optical spots that are focused onto a recording layer and a servo layer at the recording time; 
         FIG. 4  is a flowchart for the recording-control switching in Embodiment 2; 
         FIG. 5  is a configuration diagram for illustrating the optical disc device of Embodiment 3; 
         FIG. 6  illustrates the relationships among the optical spots that are focused onto a recording layer and the servo layer at the recording time; 
         FIG. 7  is a configuration diagram for illustrating the optical disc device of Embodiment 4; 
         FIG. 8  is a configuration diagram for illustrating the optical disc device of Embodiment 5; and 
         FIG. 9  illustrates the relationships among the optical spots that are focused onto a recording layer and the servo layer at the recording time. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, referring to the drawings, the explanation will be given below concerning embodiments for carrying out the present invention. Also, the configuration that will be explained here indicates just examples of the embodiments. Namely, the present invention is not limited to the embodiments. 
     Embodiment 1 
     Hereinafter, the explanation will be given below concerning Embodiment 1 of the present invention. 
     (Disc Structure) 
       FIG. 2  illustrates the structure (i.e. cross section) of an optical disc that is employed as the target in the present embodiment. A reference numeral  101  on the present drawing denotes the following optical disc: Namely, this optical disc  101  includes a single servo layer where grooves are formed, and a single recording layer or a plurality of recording layers. Here, each recording layer is equipped with none of the grooves, and thus it is a flat layer. As is the case with the grooves seen in the disc such as DVD or BD, the grooves of the servo layer are spiral grooves with the disc&#39;s rotation axis positioned at its center. 
     Also, a reference numeral  1211  on the present drawing denotes an objective lens of an (not-illustrated) optical disc device for focusing laser light onto the optical disc  101 . The present drawing illustrates the following situation: Namely, two different light fluxes pass through the objective lens  1211 . Of these light fluxes, one light flux is focused onto the servo layer of the optical disc  101 ; whereas the other light flux is focused onto one of the plurality of recording layers of the optical disc  101 . The optical disc employed as the target in the present embodiment performs the recording or reproduction by using the two light fluxes in this way, or by using two or more light fluxes. 
     Features of the Present Embodiment 
     Referring to  FIG. 3 , the explanation will be given below concerning the features of the present embodiment.  FIG. 3  illustrates a partial portion of the optical disc, which has the structure illustrated in  FIG. 2 , is cut out and enlarged. 
       FIG. 3  illustrates the following situation: Namely, an optical spot  300  is focused onto one of the recording layers. This optical spot  300  is now recording a mark while travelling in the tangential direction of the optical disc  101 . Simultaneously, an optical spot  301  and an optical spot  302  are respectively focused onto a formerly-recorded area&#39;s recording mark and an area (i.e. unrecorded area) onto which a mark is supposed to be recorded later. At this time, the optical spots  301  and  302  are maintained at a constant distance (i.e. spacing) from the optical spot  300 . Moreover, an optical spot  311  is focused onto a groove (i.e. track) of the servo layer, which is positioned substantially directly below the optical spot  300 . Incidentally, the optical spots  300 ,  301 , and  302  are split from the same light flux originally, and are emitted through a single (not-illustrated) objective lens. Meanwhile, the optical spot  311  is a light flux that is different from the optical spots  300 ,  301 , and  302 , but is emitted through the same objective lens as the objective lens through which these optical spots are emitted. 
     The features of the present embodiment are the following points. Namely, the operations such as CLU control and recording-timing generation at the recording time are performed based on the information acquired from the servo groove. Meanwhile, if formerly-recorded marks exist on the recording layer, the tracking is controlled so that the optical spot  301  will trace and follow the formerly-recorded marks. This tracking control makes it possible to fixedly determine the position of the optical spot  300  in the disc&#39;s radial direction. Subsequently, while fixedly determining the position of the optical spot  300  in this direction, a mark is recorded onto the recording layer. 
     In the tracking control method in accordance with the present embodiment like this, the train (i.e. track) of the formerly-recorded marks is recoded with a constant spacing placed between these recording marks in the disc&#39;s radial direction. This configuration makes it possible to suppress the overwriting of a mark over the train of the formerly-recorded marks. Incidentally, this type of overwriting was a problem in the conventional methods, where the tracking is controlled by always taking advantage of the groove of the servo layer regardless of the recording state of the recording layer. 
     The focus control and the tracking control are performed over the servo layer and each recording layer independently by driving actuators independently. The effects acquired from this configuration are the following points: Namely, it becomes possible to suppress a formerly-recorded area from being overwritten-recorded, even if a tilt (i.e. inclination) of the optical disc with respect to the optical axis of the two laser beams exists due to such a factor as optical disc&#39;s time-lapse-based warp, or difference in the recording devices. Also, it becomes possible to suppress a formerly-recorded area from being overwritten-recorded, even if the tilt (i.e. inclination) of the optical disc with respect to the optical axis of the two laser beams exists due to such a factor as not only the optical disc&#39;s time-lapse-based warp, but also optical disc&#39;s warp of its own, or in-layer deviation between the servo layer and each recording layer. 
     Implementation Method for the Present Embodiment 
       FIG. 1  illustrates an example of the optical disc device for carrying out the above-described tracking control in accordance with the present embodiment. 
     The optical disc device illustrated in  FIG. 1  is constituted from the following configuration components: an optical pickup  102 , a signal processing circuit  103 , a spindle motor  104 , a servo-error-signal generation circuit  105 , a reproduced-signal processing circuit  106 , a spindle driving circuit  107 , an actuator driving circuit  108 , a relay-lens driving circuit  109 , and an aberration-correcting-element driving circuit  110 . 
     Also, the signal processing circuit  103 , which is a circuit for performing various types of signal processing of the optical disc device, operates with an electric potential Vref used as its reference. The signal processing circuit  103  is constituted from the following configuration components: a system control circuit  1301 , a recording-layer focus control circuit  1302 , a switch  1303 , an adder  1304 , a recording-layer focus driving-voltage generation circuit  1305 , a servo-layer focus control circuit  1306 , a switch  1307 , an adder  1308 , a servo-layer focus driving-voltage generation circuit  1309 , a servo-layer tracking control circuit  1310 , a switch  1311 , a recording-layer tracking control circuit  1312 , a switch  1314 , and a spindle control circuit  1313 . 
     The optical disc  101  is rotated at a prescribed speed of rotation by the spindle motor  104 . The spindle motor  104  is controlled by the spindle control circuit  1313  that receives an instruction signal from the system control circuit  1301  installed on the signal processing circuit  103 . A signal outputted from the spindle control circuit  1313  is amplified by the spindle driving circuit  107 . Then, the amplified signal is supplied to the spindle motor  104 . 
     In this way, the spindle control circuit  1313  in the present embodiment performs the control over the spindle motor  104  as follows: Namely, based on an output signal from the spindle motor  104 , it is rotated at a prescribed speed of rotation regardless of the radial position of the optical disc  101 . Incidentally, the rotation scheme like this is referred to as the CAV control. 
     The optical pickup  102  includes two optical systems whose wavelengths are different from each other, such as, for example, 405 nm and 650 nm 
     First, the explanation will be given below regarding the 405-nm-wavelength optical system. A laser-power control circuit  1201 , which is controlled by the system control circuit  1301 , outputs a driving current for driving a laser diode  1202 . Here, a-few-hundreds-of-MHz radio-frequency wave&#39;s superposition is applied to this driving current in order to suppress the laser noise. The laser diode  1202  emits 405-nm-wavelength laser light whose waveform corresponds to that of this driving current. The emitted laser light is caused to become parallel laser light by a collimator lens  1203 . Then, a partial component of this parallel laser light is reflected by a beam splitter  1204 , then being focused onto a power monitor  1206  by a focusing lens  1205 . The power monitor  1206  feeds back, to the system control circuit  1301 , the current or voltage corresponding to the intensity of this laser light. This feedback allows the intensity of the laser light, which is to be focused onto the recording layer of the optical disc  101 , to be maintained at a desired value such as, for example, 2 mW. Meanwhile, the laser light which has passed through the beam splitter  1204  is caused to become a plurality of light beams (i.e. O-order light&#39;s main beam and its ±1st-order lights&#39; servo beams) by a three-beam grating  111 . Moreover, these light beams are reflected by a polarization beam splitter  1207 . These reflected light beams, whose convergences/divergences are controlled by an aberration-correcting element  1209  driven by the aberration-correcting-element driving circuit  110 , pass through a dichroic mirror  1208 . Here, the dichroic mirror  1208  is an optical element for reflecting light of a specific wavelength, and permitting light of the other wavelengths to pass therethrough. It is assumed here that the dichroic mirror  1208  reflects the 650-nm-wavelength light, and permits 405-nm-wavelength light to pass therethrough. Furthermore, the laser light beams, which have passed through the dichroic mirror  1208 , are caused to become circularly-polarized light beams by a quarterwave plate  1210 , then being focused onto the recording layer of the optical disc  101  by the objective lens  1211 . Here, the position of the objective lens  1211  is controlled by an actuator  1212 . Subsequently, the laser light beams, which have been reflected by the optical disc  101 , are modulated in their intensities in correspondence with the information recorded into the optical disc  101 , then being caused to become linearly-polarized light beams by the quarterwave plate  1210 . The laser light beams then pass through the polarization beam splitter  1207  via the dichroic minor  1208  and the aberration-correcting element  1209 . In addition, the laser light beams, which have passed through the polarization beam splitter  1207 , are focused onto a detector  1214  by a focusing lens  1213 . The detector  1214  detects the intensities of the laser light beams, then outputting signals corresponding thereto to the servo-error-signal generation circuit  105  and the reproduced-signal processing circuit  106 . 
     The servo-error-signal generation circuit  105  generates the following error signals from the signals outputted from the detector  1214  and a detector  1223 : a recording-layer focus error signal (hereinafter, R_FE signal) used for the focus control over the recording layer, a servo-layer focus error signal (hereinafter, S_FE signal) used for the focus control over the servo layer, a recording-layer tracking error signal (hereinafter, R_TE signal) used for the tracking control over the recording layer, and a servo-layer tracking error signal (hereinafter, S_TE signal) used for the tracking control over the servo layer. It is assumed that each error signal is outputted with the electric potential Vref used as its reference. 
     The focus control and the tracking control in the 405-nm-wavelength optical system are performed on the recording layer (i.e. any one layer of the plurality of recording layers). 
     Based on an instruction signal outputted from the system control circuit  1301 , the recording-layer focus control circuit  1302  performs compensations for the gain and phase of the R_FE signal. Moreover, the control circuit  1302  outputs a driving signal for performing the focus control over the recording layer. The driving signal outputted from the recording-layer focus control circuit  1302  is inputted into the actuator driving circuit  108  via the switch  1303  and the adder  1304 . 
     Based on an R_FON signal outputted from the system control circuit  1301 , the switch  1303  selects and outputs either the output signal from the recording-layer focus control circuit  1302 , or the reference electric potential Vref. If High level is inputted as the R_FON signal, the switch  1303  selects its terminal a. As a result, the output signal from the recording-layer focus control circuit  1302  is outputted to the actuator driving circuit  108  via the adder  1304 . Meanwhile, if Low level is inputted as the R_FON signal, the switch  1303  selects its terminal b, thereby outputting the reference electric potential Vref. 
     As a consequence, the R_FON signal becomes a signal for instructing the ON/OFF of the focus control over the recording layer. Also, the switch  1303  functions as a switch for switching the ON/OFF of the focus control over the recording layer. The focus control over the recording layer is switched ON by the R_FON signal&#39;s being switched from Low level to High level. This operation is referred to as focus pull-in operation. 
     Based on an instruction signal outputted from the system control circuit  1301 , the recording-layer focus driving-voltage generation circuit  1305  outputs a prescribed voltage. The recording-layer focus driving-voltage generation circuit  1305  outputs, for example, the sweep voltage in focus sweep operation or the jump voltage at the time of focus jump. 
     The output signal from the recording-layer focus driving-voltage generation circuit  1305  and the output signal from the switch  1303  are added to each other by the adder  1304 . Then, the resultant is outputted to the actuator driving circuit  108  as an R_FOD. 
     In accordance with the R_FOD, the actuator  1212  is driven in a direction that is vertical to the disc surface of the optical disc  101 . This driving causes the objective lens  1211  to be driven in the direction vertical to the disc surface. 
     Next, the explanation will be given below regarding the 650-nm-wavelength optical system. As is the case with the 405-nm-wavelength optical system, the laser-power control circuit  1201  drives a laser diode  1215 . The laser diode  1215  emits 650-nm-wavelength laser light. The power of a partial component of the laser light is monitored by a power monitor  1219  via a collimator lens  1216 , a beam splitter  1217 , and a focusing lens  1218 . The power monitored is fed back to the system control circuit  1301 . This feedback allows the intensity of the laser light, which is to be focused onto the servo layer of the optical disc  101 , to be maintained at a desired power such as, for example, 3 mW. Meanwhile, the laser light which has passed through the beam splitter  1217  passes through a polarization beam splitter  1220 . Here, the convergence/divergence of the laser light is controlled by a relay lens  1221 . Moreover, the laser light, which has passed through the relay lens  1221 , is reflected by the dichroic mirror  1208 . The reflected laser light is then focused onto the servo layer of the optical disc  101  by the objective lens  1211 . Furthermore, the laser light, which has been reflected by the optical disc  101 , is reflected by the polarization beam splitter  1220 . Finally, the reflected laser light is focused onto the detector  1223  by a focusing lens  1222 . 
     The focus control and the tracking control in the 650-nm-wavelength optical system are performed on the servo layer. 
     Based on an instruction signal outputted from the system control circuit  1301 , the servo-layer focus control circuit  1306  performs compensations for the gain and phase of the S_FE signal. Moreover, the control circuit  1306  outputs a driving signal for performing the focus control over the servo layer. The driving signal is inputted into the relay-lens driving circuit  109  via the switch  1307  and the adder  1308 . This operation allows the execution of the focus control over the servo layer. 
     Based on an S_FON signal outputted from the system control circuit  1301 , the switch  1307  selects and outputs either the output signal from the servo-layer focus control circuit  1306 , or the reference electric potential Vref. If High level is inputted as the S_FON signal, the switch  1307  selects its terminal c. Meanwhile, if Low level is inputted as the S_FON signal, the switch  1307  selects its terminal d, thereby outputting the reference electric potential Vref. 
     As a consequence, the S_FON signal becomes a signal for instructing the ON/OFF of the focus control over the servo layer. Also, the switch  1307  functions as a switch for switching the ON/OFF of the focus control over the servo layer. The focus control over the servo layer is switched ON by the S_FON signal&#39;s being switched from Low level to High level. This operation is referred to as focus pull-in operation. 
     Based on an instruction signal outputted from the system control circuit  1301 , the servo-layer focus driving-voltage generation circuit  1309  outputs a prescribed voltage. The servo-layer focus driving-voltage generation circuit  1309  outputs, for example, the sweep voltage in focus sweep operation. 
     The output signal from the servo-layer focus driving-voltage generation circuit  1309  and the output signal from the switch  1307  are added to each other by the adder  1308 . Then, the resultant is outputted to the relay-lens driving circuit  109  as a S_FOD. 
     In accordance with the S_FOD, the relay lens  1221  is driven so that the position of the 650-nm-wavelength optical spot is controlled in a direction that is vertical to the disc surface of the optical disc  101 . For example, in the case of  FIG. 1 , in order to drive the 650-nm-wavelength optical spot in the direction vertical to the disc surface of the optical disc  101 , it is advisable to drive the relay lens  1221  in a direction that is horizontal to the disc surface. The present invention, however, is not limited to this configuration. Namely, the following configuration of the optical pickup  102  is also allowable: Namely, in order to control the position of the 650-nm-wavelength optical spot in the direction vertical to the disc surface of the optical disc  101 , the relay lens  1221  is driven into the direction vertical to the disc surface. 
     The relay-lens driving circuit  109  drives the relay lens  1221  installed inside the optical pickup  102 . This driving allows the focus control and the tracking control to be performed over the servo layer. 
     The relay-lens driving circuit  109  and the servo-layer focus control circuit  1306  operate as described earlier. This operation allows the focus control over the servo layer to be performed in accordance with the following manner: Namely, the 650-nm-wavelength laser spot, with which the optical disc  101  is irradiated, is always focused on the surface of the servo layer of the optical disc  101 . 
     Here, High level and Low level of the R_FON signal and the S_FON signal are not necessarily required to be in the states described earlier. For example, it is also allowable to control the switch so that the switch selects the terminal a when the R_FON signal is at Low level. 
     Next, the explanation will be given below concerning the tracking control over the servo layer in the present embodiment. 
     Based on an instruction signal outputted from the system control circuit  1301 , the servo-layer tracking control circuit  1310  performs compensations for the gain and phase of the servo-layer tracking error signal (hereinafter, S_TE signal). Moreover, the control circuit  1310  outputs a driving signal for performing the tracking control over the servo layer. The driving signal outputted from the servo-layer tracking control circuit  1310  is inputted into the relay-lens driving circuit  109  via the switch  1311 . 
     Based on a S_TON signal outputted from the system control circuit  1301 , the switch  1311  selects and outputs either the output signal from the servo-layer tracking control circuit  1310 , or the reference electric potential Vref. Moreover, the switch  1311  outputs the resultant to the relay-lens driving circuit  109  as a tracking driving signal, S_TRD. If High level is inputted as the S_TON signal, the switch  1311  selects its terminal e. As a result, the output signal from the servo-layer tracking control circuit  1310  is outputted to the relay-lens driving circuit  109 . Meanwhile, if Low level is inputted as the S_TON signal, the switch  1311  selects its terminal f, thereby outputting the reference electric potential Vref. 
     As a consequence, the S_TON signal becomes a signal for instructing the ON/OFF of the tracking control. Also, the switch  1311  functions as a switch for switching the ON/OFF of the servo-layer tracking control. The servo-layer tracking control is switched ON by the S_TON signal&#39;s being switched from Low level to High level. This operation is referred to as servo-layer track pull-in operation. 
     Next, the explanation will be given below concerning the tracking control over the recording layer in the present embodiment. 
     Based on an instruction signal outputted from the system control circuit  1301 , the recording-layer tracking control circuit  1312  performs compensations for the gain and phase of the recording-layer tracking error signal (hereinafter, R_TE signal) and outputs a driving signal for performing the tracking control. The driving signal outputted from the recording-layer tracking control circuit  1312  is inputted into the actuator driving circuit  108  via the switch  1314 . 
     Based on a R_TON signal outputted from the system control circuit  1301 , the switch  1314  selects either the output signal from the recording-layer tracking control circuit  1312 , or the reference electric potential Vref and outputs to the actuator driving circuit  108  as a tracking driving signal (hereinafter, R_TRD). If High level is inputted as the R_TON signal, the switch  1314  selects its terminal g. As a result, the output signal from the recording-layer tracking control circuit  1312  is outputted to the actuator driving circuit  108 . Meanwhile, if Low level is inputted as the R_TON signal, the switch  1314  selects its terminal h, thereby outputting the reference electric potential Vref. 
     As a consequence, the R_TON signal becomes a signal for instructing the ON/OFF of the tracking control over the recording layer. Also, the switch  1314  functions as a switch for switching the ON/OFF of the tracking control over the recording layer. The servo-layer tracking control is switched ON by the R_TON signal&#39;s being switched from Low level to High level. This operation is referred to as recording-layer track pull-in operation. Here, High level and Low level of the R_TON signal and the S_TON signal are not necessarily required to be in the states described earlier. For example, it is also allowable to control the switch so that the switch selects the terminal g when the R_TON signal is at Low level. 
     In accordance with the tracking driving signal (hereinafter, R_TRD), the actuator driving circuit  108  drives the actuator  1212  in a direction that is parallel to the disc surface. This driving allows the objective lens  1211  to be driven in the disc&#39;s radial direction. In this way, the actuator driving circuit  108  in the present embodiment is so constituted as to include both the in-focus-direction driving circuit and the in-tracking-direction driving circuit. 
     In accordance with the servo-layer tracking driving signal (hereinafter, S_TRD), in order to drive the position of the 650-nm-wavelength optical spot in the direction parallel to the disc surface, the relay-lens driving circuit  109  drives the relay lens  1221  in the direction vertical to the disc surface of the optical disc  101 . The present invention, however, is not limited to this configuration. Namely, the following configuration of the optical pickup  102  is also allowable: Namely, in order to control the position of the 650-nm-wavelength optical spot in the direction parallel to the disc surface of the optical disc  101 , the relay lens  1221  is driven in the direction parallel to the disc surface. In this way, the relay-lens driving circuit  109  in the present embodiment is so constituted as to include both the in-focus-direction driving circuit and the in-tracking-direction driving circuit. 
     The servo-error-signal generation circuit  105 , the servo-layer tracking control circuit  1310 , and the relay-lens driving circuit  109  operate as described earlier. At the time of recording information, this operation allows the tracking control to be performed in such a manner that the 650-nm-wavelength laser spot follows the servo groove formed in the servo layer. Also, the servo-error-signal generation circuit  105 , the recording-layer tracking control circuit  1312 , and the actuator driving circuit  108  operate as described earlier. At the time of recording information, this operation allows the R_TE to be generated by the servo-error-signal generation circuit  105  from the recording marks formed on the recording layer. Subsequently, this operation allows the tracking control to be performed in such a manner that the 405-nm-wavelength laser spot follows these recording marks. 
     Furthermore, at the time of reproducing information, the tracking control is performed in such a manner that the 405-nm-wavelength laser spot follows the recording marks formed on the recording layer. 
     Also, the reproduced-signal processing circuit  106  applies an equalizing processing to the electrical signals detected by the detectors  1214  and  1223 , then outputting as a reproduced signal. The reproduced signal is then inputted into the system control circuit  1301 . Inside the system control circuit  1301 , such processings as amplification, equalization, and decoding are performed, thereby creating information read from the optical disc  101  (such as recording timing, recorded data, and present address information). 
     At the time of recording information, the reproduced-signal processing circuit  106  creates the recording timing from the wobble of the servo groove formed in the servo layer of the optical disc  101 . As this wobble, the wobble specified in the DVD, CD, and BD standards may be physically formed in the servo layer, for example. This wobble frequency, however, is not limited to the specification of the DVD, CD, and BD standards. 
     Based on an instruction signal outputted from the system control circuit  1301 , the aberration-correcting-element driving circuit  110  generates a driving voltage for driving the aberration-correcting element  1209 , thereby driving the aberration-correcting element  1209 . 
     (Recording-Time Tracking Control for Implementing Features and Effects of the Present Embodiment) 
       FIG. 3  illustrates the relationships among the optical spots that are focused onto the recording layer and the servo layer at the recording time in the present embodiment. 
     At the time of the recording operation, the S_TON signal, the R_TON signal, the S_FON signal, and the R_FON signal illustrated in  FIG. 1  are switched at High level. Moreover, the terminal of the switch  1303 , the terminal of the switch  1307 , the terminal of the switch  1311 , and the terminal of the switch  1314  are switched to a, c, e, and g, respectively. Also, it is assumed that the focus control and the tracking control are performed over the recording layer and the servo layer. 
     The 405-nm-wavelength laser light emitted from the laser diode  1202  is caused to become the three beams by the grating  111  illustrated in  FIG. 1 . Moreover, the three optical spots resulting therefrom are focused onto a recording layer (i.e. any one layer of the plurality of recording layers) of the optical disc  101  by the objective lens  1211 . In  FIG. 3 , the tracking control is performed at the recording marks by the optical spot  301 . The recording and the focus control are performed by the optical spot  300 . The confirmation of the unrecorded is performed by the optical spot  302 . For example, as a method for confirming the unrecorded using the optical spot  302 , it is conceivable to take a difference value between the total amount of reflection light of the optical spot  302  and that of the optical spot  301 . Here, it is assumed that the optical disc  101  is the following type of optical disc; namely, the amount of reflection light is higher on an unrecorded area where no recording mark is formed, whereas the amount of reflection light is lower on a recorded area where a recording mark is formed. At this time, the total amount of reflection light of the optical spot  302  travelling on the unrecorded area is subtracted from the total amount of reflection light of the optical spot  301  following the recording marks. Obviously, the calculation result of this subtraction turns out to be negative. In this way, the sign obtained after the difference value is taken makes it possible to confirm whether or not the tracking follow operation at the recording marks is being successful. Also, in the case of the optical disc  101  of the type where, conversely, the amount of reflection light becomes higher if a recording mark is formed, the sigh becomes inverted to the sigh of the above-described former case. 
     Also, the 650-nm-wavelength laser light emitted from the laser diode  1215  in  FIG. 1  is focused onto the servo layer of the optical disc  101  by the relay lens  1221  and the objective lens  1211 . Moreover, the optical spot  311  resulting therefrom is used for reproducing, from the servo layer in  FIG. 3 , the information needed for performing the CLV control. Here, the CLV control means a control of controlling the spindle control circuit  1313  not by using the recording timing and the output signal from the spindle motor  104 , but by using the information read from the optical disc  101 . Also, the use of the reproduced-signal processing circuit  106  allows the reproduced signal and the address to be read from the optical spot  301  that is following the recording marks, and the optical spot  311 . Accordingly, it is also allowable to use this address in order to confirm whether or not the recording at the recording time is being performed at a proper position and on a proper recording layer. 
     The rotation of the optical disc  101  causes the optical spot  300 , the optical spot  301 , the optical spot  302 , and the optical spot  311  to move in the recording direction, which is illustrated in  FIG. 3 , with respect to the optical disc  101 . Subsequently, while performing the tracking control by the optical spot  301 , a recording mark is formed by the optical spot  300 . 
     The track pitch, i.e. the distance in the radial direction between two of the optical spot  300 , the optical spot  301 , and the optical spot  302 , is so adjusted as to become equal to 0.32 um. This adjustment is made by an optical element such as the grating  111 . The distance in the circumferential direction, however, is permissible as long as it is a distance that can be resolved by the detector  1214 . Here, the design of the radial-direction track pitch of the optical spot  300 , the optical spot  301 , and the optical spot  302  changes, depending on factors such as the grating  111  and the recording density. Accordingly, 0.32 um is specified merely as one example. Also, in the present embodiment, the laser light has been caused to become the three beams by the grating  111 . The following plurality of light beams, however, are also conceivable, as long as the scheme is a scheme where the recording marks on the recording layer are followed: two beams of the optical spot  300  and the optical spot  301 , or a plurality of beams (such as, for example, five beams of O-order light&#39;s main beam, its ±1st-order lights&#39; servo beams, and its ±2nd-order lights&#39; servo beams). This situation will be basically the same in the following embodiments as well. 
     The intensity ratio among the three optical spots, i.e. the optical spot  300 , the optical spot  301 , and the optical spot  302 , at the recording time is set at 1:10:1, for example. This setting is employed in order to implement the following situation. Namely, a recording mark is formed by the optical spot  300 , and the already-recorded recording marks are not overwritten by the optical spot  301 , and the unrecorded area is not recorded by the optical spot  302 . The intensity ratio among these optical spots, however, is not limited thereto. Namely, whatever intensity ratio is allowable, as long as the situation is implementable, i.e. a recording mark is formed by the optical spot  300 , and the already-recorded recording marks are not overwritten by the optical spot  301 , and the unrecorded area is not recorded by the optical spot  302 . This situation will be basically the same in the following embodiments as well. 
     In the present embodiment, when the recording is performed based on the CAV control, the speed of rotation of the optical disc  101  is constant independently of the radius. Accordingly, the control over the spindle motor  104  becomes easier as compared with the case of the CLU control, but the linear velocity changes depending on the radius. As a result, constraints are added to the mechanical structure of the optical disc  101  and the recording film of the recording layer. Meanwhile, the execution of the CLV control requires the execution of a control of making the linear velocity constant independently of the radius. The execution of the CLV control, however, allows the acquisition of an effect of being capable of reducing the constraints added to the mechanical structure of the optical disc  101  and the recording film of the recording layer. In view of this situation, in the present embodiment, the configuration is employed which is controllable by both the CAV control and the CLV control. This situation will be basically the same in the following embodiments as well. 
     Also, in the present embodiment, the example is employed where the focus control and the tracking control over the servo layer are performed using the single light beam. It is also allowable, however, that the three light beams (i.e. O-order light&#39;s main beam, and its ±1st-order lights&#39; servo beams) are used by deploying the grating between the beam splitter  1217  and the polarization beam splitter  1220 . The signals generated by the servo-error-signal generation circuit  105  in the present embodiment are implemented as follows. Namely, the schemes such as differential push-pull method (DPP method) and push-pull method are used in the case of the tracking signals whereas the schemes such as knife-edge method and differential astigmatic aberration method are used in the case of the focus signals. The schemes described earlier, however, are not limited, and thus different schemes are also usable. This situation will be basically the same in the following embodiments as well. 
     In Embodiment 1 of the present invention, the above-described configuration allows the focus control and the tracking control to be performed over the recording layer and the servo layer independently of each other. As a result, the train of the formerly-recorded marks is recoded with a constant spacing placed between these recording marks in the disc&#39;s radial direction. This configuration makes it possible to suppress the overwriting of a recording mark over the train of the formerly-recorded marks. Moreover, the operations such as the CLV control and recording-timing generation at the recording time can be performed based on the information acquired from the servo groove. 
     Incidentally, the optical disc device performs communications with a (not-illustrated) host such as PC (Personal Computer) via an interface such as SATA (Serial Advanced Technology Attachment). This situation will be basically the same in the following embodiments as well. 
     Embodiment 2 
     Hereinafter, the explanation will be given below concerning Embodiment 2 of the present invention. 
     (Disc Structure) 
       FIG. 2  illustrates the disc structure of the optical disc  101 . Since the structure illustrated in  FIG. 2  is the same as the one illustrated in Embodiment 1, the explanation thereof will be omitted here. 
     Features of the Present Embodiment 
     In Embodiment 2, if, on the recording layer, there exists none of the recording marks to be used for the tracking control by the optical spot  301 , or none of a groove equivalent to the recording marks, the information that is needed for the servo-error-signal generation circuit  105  to generate the R_TE signal is not made available from the optical spot  301  on the recording layer. In view of this situation, the tracking control is performed at the servo groove of the servo layer by the optical spot  311 . Subsequently, the formation of a recording mark is performed onto the recording layer by the optical spot  300  along the servo groove of the servo layer. On account of this formation of the recording mark, the information that allows the R_TE signal to be generated by the servo-error-signal generation circuit  105  becomes available from the optical spot  301  on the recording layer. Once the R_TE signal becomes available, the recording operation is implemented as follows. Namely, the tracking control is performed at the recording marks on the recording layer by the optical spot  301 . Subsequently, the recording is performed by the optical spot  300 . 
     The effects acquired by the present embodiment are the following points. Namely, the present embodiment makes it possible to record information into the optical disc  101  regardless of the presence or absence of the recording marks to be used for the tracking control over the recording layer. Moreover, the present embodiment makes it possible to acquire basically the same effects as those in Embodiment 1. 
     First of all, in the optical disc device, the various types of processings are performed, such as disc recognition, focus pull-in, tracking pull-in, aberration adjustment, and management information&#39;s reproduction. Having received a recording or reproduction instruction from the host, these processings are performed in order to implement the state where the information recording or reproduction is executable. Hereinafter, these processings will be referred to as set-up processings. In these set-up processings, the management information is read which is recorded on the recording layers or the servo layer of the optical disc  101  in  FIG. 2 . Here, the following conditions are assumed. Namely, the recording layer L 0  is in an entirely-recorded state. Moreover, the recording layer L 1  is in the unrecorded state, where there exists none of the recording marks to be used for the tracking control by the optical spot  301  in  FIG. 3 , or none of the groove equivalent to the recording marks. 
     Also, in the present embodiment, the following conditions are assumed. Namely, the set-up processings are finished on the L 0 , and the focus jump from the L 0  to the L 1  is completed in order to record information onto the L 1 . Furthermore, on the recording layers, only the focus control is switched ON, and the tracking control is swished OFF. On the servo layer, both the focus control and the tracking control are swished ON. 
     Implementation Method for the Present Embodiment 
       FIG. 1  illustrates an example of the optical disc device for carrying out the above-described tracking control in accordance with the present embodiment. Since the structure illustrated in  FIG. 1  had been explained in Embodiment 1, the explanation thereof will be omitted here. 
     (Recording-Time Tracking Control for Implementing Features and Effects of the Present Embodiment) 
       FIG. 4  illustrates a flowchart for the preparation processing of the tracking control, which is to be performed before the recording in accordance with the present embodiment. 
     For example, when the system control circuit  1301  in  FIG. 1  receives the recording instruction of recording information onto the L 1  from the host to the optical disc device, the pre-recording preparation processing is started (Step S 101 ). This processing is a processing for performing a switching as to whether to perform the tracking control at the recording marks on the recording layer by the optical spot  301  in  FIG. 3 , or to record information onto the recording layer in accordance with the tracking control at the servo groove of the servo layer by the optical spot  311 . 
     First, the explanation will be given below regarding the case of performing a processing (Step S 104 ), where the tracking control is performed based on the track groove of the servo layer. 
     When the pre-recording preparation processing is started (Step S 101 ), first, based on the above-described management information included in the optical disc  101 , it is judged whether or not the recording layer, onto which information is to be recorded, is in the unrecorded state (Step S 102 ). As a result of reproducing the management information, if, on the L 1 , there exists none of the recording marks to be used for the tracking control by the optical spot  301  in  FIG. 3  (Yes at Step S 102 ), it is judged whether or not the recording-marks-based control is executable (Step S 103 ). For example, this judgment is a processing that assumes a case where the management information is recorded in a collective manner when the optical disc  101  is taken out of the optical disc device. In this case, the management information is not updated until the optical disc  101  is taken out of the device. Accordingly, if the recording operation is continued to be performed without taking out the optical disc  101 , the execution of the management-information-based confirmation at Step S 102  always results in the judgment that the recording layer is in the unrecorded state (Yes at Step S 102 ). In order not to create the condition under which the recording-marks-based tracking control is always prevented from being executed, it is judged whether or not the recording-marks-based control is executable (Step S 103 ). Here, if the R_TE signal can be generated by the servo-error-signal generation circuit  105  in  FIG. 1  using the information from the optical spot  5302  (No at Step S 103 ), the R_TON is switched at High level, and the terminal of the switch  1314  is switched from h to g. This switching operation allows the starting of the tracking control where the optical spot  301  follows the recording marks (Step S 105 ). For example, the confirmation can be made as follows as to whether or not it is possible to generate the R_TE signal. Namely, it is impossible to generate the R_TE signal from the optical spot  301  at the recording-starting time like the present embodiment. If, however, the recording is being performed by the optical spot  300 , signals from the recorded marks are made available to the optical spot  301  as well. This situation makes it possible to judge that the tracking control where the optical spot  301  follows the recording marks becomes executable. 
     Also, another confirmation can be made in such a manner that the tracking control that is following the servo layer is switched OFF. As described earlier, if the tracking control is switched OFF, the track-crossing signal from the servo groove is made available from the S_TE signal in the case of the servo layer. Also, the track-crossing signal available from the recording marks on the recording layer is made available as the R_TE signal. If, at this time, the R_TE signal is proximate to the reference electric potential Vref, the recording layer is in the unrecorded state. Consequently, it can also be judged from the track-crossing signal that it is impossible to generate the R_TE signal. 
     If it is judged that the recording-marks-based control is not executable (Yes at Step S 103 ), the control is performed based on the track groove of the servo layer (Step S 104 ). Concretely, the laser diode  1215  emits the 650-nm-wavelength laser light. This emitted laser light is focused onto the servo layer of the optical disc  101  by the relay lens  1221  and the objective lens  1211 . Moreover, based on the servo signal generated by the servo-error-signal generation circuit  105  from this focused optical spot, the relay lens  1221  is caused to follow the servo groove of the servo layer by the relay-lens driving circuit  109 . 
     At this time, the signal that is driven in the disc&#39;s radial direction by the objective lens  1211  is not inputted into the actuator driving circuit  108 . Accordingly, the signal operates freely. Consequently, it is conceivable that the signal exerts an influence onto the recording quality. In a case like this, for example, a voltage for fixing the relative position between the relay lens  1221  and the objective lens  1211  may also be applied to the actuator driving circuit  108  from the recording-layer tracking control circuit  1312  in correspondence with the S_TE signal. As a result, it turns out that the relative position between the relay lens  1221  and the objective lens  1211  does not change while the recording is underway. 
     Also, the explanation will be given below regarding the case of performing a processing (Step S 105 ), where the control is performed based on the recording marks on the recording layer. 
     If the recording layer is in the recorded state (No at Step S 102 ) as a result of the management-information-based confirmation, like Embodiment 1, the tracking control is performed by the optical spot  301 &#39;s following the recording marks on the recording layer. The recording is started after this pre-recording preparation processing is completed. 
     Also, as is the case with the above-described No at Step S 103 , if it becomes possible to generate the S_TE signal by the servo-error-signal generation circuit  105  using the optical spot  301  while the recording is underway, the tracking control may also be switched so that the recording onto the L 1  of the optical disc  101  is performed while the optical spot  301  is following the recording marks formed on the L 1 . Otherwise, the recording may also be started again by stopping the recording once temporarily, and performing the track pull-in of the optical spot  301  onto the recording marks. Furthermore, when the additional recording is performed in the state where the optical disc  101  is not taken out, the recording like Embodiment 1 may also be performed. 
     In the present embodiment, the following conditions have been assumed. Namely, the set-up processings are finished on the L 0 , and the focus jump from the L 0  to the L 1  is completed in order to record information onto the L 1 . Furthermore, on the recording layers, only the focus control is switched ON, and the tracking control is swished OFF. If, however, there exists none of the recording marks, the signal available from the optical spot  301  is proximate to the potential Vref. Accordingly, the S_TE becomes equal to zero, and the tracking error signal becomes equal to zero. As a result, even if the tracking control over the recording layer is swished ON, no influence is exerted. In view of this situation, the following operation is also executable. Namely, the tracking control over the recording layer is maintained at ON. Then, once a recording mark is formed, the optical spot is caused to follow this recording mark. 
     The tracking control over the servo layer in the present embodiment is the same as the one in Embodiment 1. Consequently, the explanation thereof will be omitted here. 
     In the embodiment, the three-beam scheme and the one-beam scheme have been applied to the recording layer and the servo layer, respectively. The following plurality of light beams, however, are also allowable. The three-beam scheme is applied to the recording layer and the servo layer, respectively. Otherwise, two beams, or a plurality of beams (such as, for example, five beams of O-order light&#39;s main beam, its ±1st-order lights&#39; servo beams, and its ±2nd-order lights&#39; servo beams) are applied thereto, respectively. 
     In Embodiment 2 of the present invention, the above-described configuration makes it possible to acquire basically the same effects as those in Embodiment 1. Simultaneously, it becomes possible to record information into the optical disc  101  regardless of the presence or absence of the recording marks to be used for the tracking control over the recording layer. 
     Embodiment 3 
     Hereinafter, the explanation will be given below concerning Embodiment 3 of the present invention. 
     (Disc Structure) 
       FIG. 2  illustrates the disc structure of the optical disc  101 . The structure of the optical disc  101  in the present embodiment has been explained already. Consequently, the explanation thereof will be omitted here. 
     Features of the Present Embodiment 
     In the embodiments explained so far, the optical pickup has been so configured as to include the single objective lens. In contrast to this configuration, a feature of the present embodiment is the employment of an optical pickup including the following configuration. Namely, two laser optical paths are so structured as to become independent of each other by deploying two objective lenses in the radial direction. 
     Referring to  FIG. 6 , the explanation will be given below regarding features of the present embodiment other than the above-described feature.  FIG. 6  illustrates a partial portion of the optical disc, which has the structure illustrated in  FIG. 2 , is cut out and enlarged. 
       FIG. 6  illustrates the following situation. Namely, the optical spot  300  is focused onto one of the recording layers. This optical spot  300  is recording a recording mark at present, while travelling in the tangential direction of the optical disc  101 . Simultaneously, the optical spot  301  and the optical spot  302  are maintained at a constant distance (i.e. spacing) from the optical spot  300 . These optical spots  301  and  302  are respectively focused onto a formerly-recorded mark and an area (i.e. unrecorded area) onto which a recording mark is supposed to be recorded later. Moreover, in the present embodiment, the two objective lenses are deployed in the radial direction. This condition gives rise to the occurrence of a shift between the optical spot  300  and the optical spot  311  that is focused onto a groove (i.e. track) of the servo layer. This shift corresponds to the deployment of these two spots in the radial direction. Namely, it turns out that these two spots are focused at the positions that are a few tracks away from each other when converted to the tracks on the servo layer (on the drawing, three tracks away). Incidentally, the optical spots  300 ,  301 , and  302  are split from the same light flux originally, and are emitted through a single (not-illustrated) objective lens. Meanwhile, the optical spot  311  is a light flux that is different from the optical spots  300 ,  301 , and  302 , and is emitted through the different objective lens. 
     Incidentally, the two objective lenses exist. This condition gives rise to the occurrence of a shift in the addresses, when information is reproduced which is acquired from the optical spots on the recording layer and the servo layer. This shift corresponds to a spacing between the objective lenses. The correction for this shift can be successfully made using the following method. The relative address shift is corrected by calculating the mutual address difference from the addresses of the optical spots on the recording layer and the servo layer. Consequently, the following point is also a feature of the present embodiment. The mutual address difference is corrected which is acquired from the addresses of the optical spots on the recording layer and the servo layer. Subsequently, the operations such as CLU control and recording-timing generation at the recording time are performed based on the address information acquired from the servo groove. 
     The present embodiment makes it possible to acquire basically the same effects as those in Embodiments 1 and 2. Simultaneously, it becomes possible to provide users with the inexpensive optical disc device. This is because specification of the objective lenses can be relaxed. Also, it becomes easier to implement the compatibility with an optical disc (for example, DVD or BD) that performs the recording/reproduction using a single type of laser light-source. 
     Implementation Method for the Present Embodiment 
       FIG. 5  illustrates an example of the optical disc device for carrying out the above-described recording-time tracking control in accordance with the present embodiment. The configuration in  FIG. 5  differs from the one in  FIG. 1  in the portion of an optical pickup  113 . In the present embodiment, the structure other than the optical pickup  113  is the same as the one in  FIG. 1 . Consequently, the overlapped explanation thereof will be omitted here. 
     In the optical disc device, the objective lens  1211  focuses the laser light that has entered the recording layer (i.e. any one layer of the plurality of recording layers), thereby causing the laser spot to be generated. Also, an objective lens  1226  focuses the laser light that has entered the servo layer, thereby causing a laser spot to be generated. 
     The optical disc device in the present embodiment is constituted from the following configuration components: the optical pickup  113 , the signal processing circuit  103 , the spindle motor  104 , the servo-error-signal generation circuit  105 , the reproduced-signal processing circuit  106 , the spindle driving circuit  107 , the actuator driving circuit  108 , an actuator driving circuit  109 , and the aberration-correcting-element driving circuit  110 . 
     The explanation of the control over the signal processing circuit  103  and the spindle motor  104  is the same as the one in  FIG. 1 . Consequently, the explanation thereof will be omitted here. 
     The optical pickup  113  includes the two optical systems whose wavelengths are different from each other, such as, for example, the 405-nm-wavelength and 650-nm-wavelength. 
     First, the explanation will be given below regarding the 405-nm-wavelength optical system. The laser-power control circuit  1201 , which is controlled by the system control circuit  1301 , outputs the driving current for driving the laser diode  1202 . Here, a-few-hundreds-of-MHz radio-frequency wave&#39;s superposition is applied to this driving current in order to suppress the laser noise. The laser diode  1202  emits the 405-nm-wavelength laser light whose waveform corresponds to that of this driving current. The emitted laser light is caused to become the parallel laser light by the collimator lens  1203 . Then, a partial component of this parallel laser light is reflected by the beam splitter  1204 , then being focused onto the power monitor  1206  by the focusing lens  1205 . The power monitor  1206  feeds back, to the system control circuit  1301 , the current or voltage corresponding to the intensity of this laser light. This feedback allows the intensity of the laser light, which is to be focused onto the recording layer of the optical disc  101 , to be maintained at the desired value such as, for example, 2 mW. Meanwhile, the laser light which has passed through the beam splitter  1204  is caused to become the plurality of light beams (i.e. O-order light&#39;s main beam and its ±1st-order lights&#39; servo beams) by the three-beam grating  111 . Moreover, these light beams are reflected by the polarization beam splitter  1207 . The convergences/divergences are controlled by the aberration-correcting element  1209  driven by the aberration-correcting-element driving circuit  110 . Furthermore, the laser light beams, whose convergences/divergences have been controlled, are caused to become the circularly-polarized light beams by the quarterwave plate  1210 , then being focused onto the recording layer of the optical disc  101  by the objective lens  1211 . Here, the position of the objective lens  1211  is controlled by the actuator  1212 . Subsequently, the laser light beams, which have been reflected by the optical disc  101 , are modulated in their intensities in correspondence with the information recorded into the optical disc  101 , then being caused to become the linearly-polarized light beams by the quarterwave plate  1210 . The laser light beams then pass through the polarization beam splitter  1207  via the aberration-correcting element  1209 . In addition, the laser light beams, which have passed through the polarization beam splitter  1207 , are focused onto the detector  1214  by the focusing lens  1213 . The detector  1214  detects the intensities of the laser light beams, then outputting signals corresponding thereto to the servo-error-signal generation circuit  105  and the reproduced-signal processing circuit  106 . 
     The servo-error-signal generation circuit  105  generates the following error signals from the signals outputted from the detector  1214  and the detector  1223 : the R_FE signal used for the focus control over the recording layer, the S_FE signal used for the focus control over the servo layer, the R_TE signal used for the tracking control over the recording layer, and the S_TE signal used for the tracking control over the servo layer. It is assumed that each error signal is outputted with the electric potential Vref used as its reference. 
     The focus control and the tracking control in the 405-nm-wavelength optical system are performed on the recording layer (i.e. any one layer of the plurality of recording layers). 
     Next, the explanation will be given below regarding the 650-nm-wavelength optical system. As is the case with the 405-nm-wavelength optical system, the laser-power control circuit  1201  drives the laser diode  1215 . The laser diode  1215  emits the 650-nm-wavelength laser light. The power of a partial component of the laser light is monitored by the power monitor  1219  via the collimator lens  1216 , the beam splitter  1217 , and the focusing lens  1218 . The power monitored is fed back to the system control circuit  1301 . This feedback allows the intensity of the laser light, which is to be focused onto the servo layer of the optical disc  101 , to be maintained at the desired power such as, for example, 3 mW. Meanwhile, the laser light which has passed through the beam splitter  1217  is reflected by the polarization beam splitter  1220 . Moreover, the laser light passes through a quarterwave plate  1225 , then being focused onto the servo layer of the optical disc  101  by the objective lens  1226 . Furthermore, the laser light, which has been reflected by the optical disc  101 , passes through the polarization beam splitter  1220 . Finally, the laser light is focused onto the detector  1223  by the focusing lens  1222 . 
     The focus control and the tracking control in the 650-nm-wavelength optical system are performed on the servo layer. 
     Based on an instruction signal outputted from the system control circuit  1301 , the servo-layer focus control circuit  1306  performs compensations for the gain and phase of the S_FE signal. Moreover, the control circuit  1306  outputs a driving signal for performing the focus control over the servo layer. The driving signal is inputted into the actuator driving circuit  109  via the switch  1307  and the adder  1308 . This operation allows the execution of the focus control over the servo layer. 
     The actuator driving circuit  109  drives an actuator  1224  installed inside the optical pickup  113 . This driving allows the focus control to be performed over the servo layer. The actuator driving circuit  109  and the servo-layer focus control circuit  1306  operate as described earlier. This operation allows the focus control over the servo layer to be performed in accordance with the following manner. Namely, the 650-nm-wavelength laser spot, with which the optical disc  101  is irradiated, is always focused on the surface of the servo layer of the optical disc  101 . 
     Here, High level and Low level of the R_FON signal and the S_FON signal are not necessarily required to be in the states described earlier. For example, it is also allowable to control the switch so that the switch selects the terminal a when the R_FON signal is at Low level. 
     Also, in accordance with the S_TRD, the actuator driving circuit  109  drives the actuator  1224  in the direction that is parallel to the disc surface. This driving allows the objective lens  1226  to be driven in the disc&#39;s radial direction. In this way, the actuator driving circuit  109  in the present embodiment is so constituted as to include both the in-focus-direction driving circuit and the in-tracking-direction driving circuit. 
     The servo-error-signal generation circuit  105 , the servo-layer tracking control circuit  1310 , and the actuator driving circuit  109  operate as described earlier. At the time of recording information, this operation allows the tracking control to be performed in such a manner that the 650-nm-wavelength laser spot follows the servo groove formed in the servo layer. Also, the servo-error-signal generation circuit  105 , the recording-layer tracking control circuit  1312 , and the actuator driving circuit  108  operate as described earlier. At the time of recording information, this operation allows the R_TE to be generated from the recording marks formed on the recording layer. Subsequently, this operation allows the tracking control to be performed in such a manner that the 405-nm-wavelength laser spot follows these recording marks. 
     (Recording-Time Tracking Control for Implementing Features and Effects of the Present Embodiment) 
       FIG. 6  illustrates the relationships among the optical spots that are focused onto the recording layer and the servo layer at the recording time in the present embodiment. 
     At the time of the recording operation, the S_TON signal, the R_TON signal, the S_FON signal, and the R_FON signal illustrated in  FIG. 5  are switched at High level. Moreover, the terminal of the switch  1303 , the terminal of the switch  1307 , the terminal of the switch  1311 , and the terminal of the switch  1314  are selected to a, c, e, and g, respectively. Also, it is assumed that the focus control and the tracking control are performed over the recording layer and the servo layer. 
     The 405-nm-wavelength laser light emitted from the laser diode  1202  is caused to become the three beams by the grating  111  illustrated in  FIG. 5 . Moreover, the three optical spots resulting therefrom are focused onto a recording layer (i.e. any one layer of the plurality of recording layers) of the optical disc  101  by the objective lens  1211 . In  FIG. 6 , the tracking control is performed at the recording marks by the optical spot  301 . The recording and the focus control are performed by the optical spot  300 . The confirmation of the unrecorded is performed by the optical spot  302 . For example, as a method for confirming the unrecorded using the optical spot  302 , it is conceivable to take a difference value between the total amount of reflection light of the optical spot  302  and that of the optical spot  301 . Here, it is assumed that the optical disc  101  is the following type of optical disc; namely, the amount of reflection light is higher on an unrecorded area where no recording mark is formed, whereas the amount of reflection light is lower on a recorded area where a recording mark is formed. At this time, the total amount of reflection light of the optical spot  302  travelling on the unrecorded area is subtracted from the total amount of reflection light of the optical spot  301  following the recording marks. Obviously, the calculation result of this subtraction turns out to be negative. In this way, the sign obtained after the difference value is taken makes it possible to confirm whether or not the tracking follow operation at the recording marks is being successful. Also, in the case of the optical disc  101  of the type where, conversely, the amount of reflection light becomes higher if a recording mark is formed, the sigh becomes inverted to the sigh of the above-described former case. 
     Also, the 650-nm-wavelength laser light emitted from the laser diode  1215  in  FIG. 5  is focused onto the servo layer of the optical disc  101  by the objective lens  1226 . Moreover, the optical spot  311  resulting therefrom is used for reproducing, from the servo layer in  FIG. 6 , the information needed for performing the CLV control. Here, the CLV control means a control of controlling the spindle control circuit  1313  not by using the recording timing and the output signal from the spindle motor  104 , but by using the information read from the optical disc  101 . Also, the use of the reproduced-signal processing circuit  106  allows the reproduced signal and the address to be read from the optical spot  301  that is following the recording marks, and the optical spot  311 . Accordingly, it is also allowable to use this address in order to confirm whether or not the recording at the recording time is being performed at a proper position and on a proper recording layer. 
     The rotation of the optical disc  101  causes the optical spot  300 , the optical spot  301 , the optical spot  302 , and the optical spot  311  to move in the recording direction, which is illustrated in  FIG. 6 , with respect to the optical disc  101 . Subsequently, while performing the tracking control by the optical spot  301 , a recording mark is formed by the optical spot  300 . 
     The track pitch, i.e. the distance in the radial direction between two of the optical spot  300 , the optical spot  301 , and the optical spot  302 , is so adjusted as to become equal to 0.32 um. This adjustment is made by an optical element such as the grating  111 . The distance in the circumferential direction, however, is permissible as long as it is a distance that can be resolved by the detector  1214 . Here, the design of the radial-direction track pitch of the optical spot  300 , the optical spot  301 , and the optical spot  302  changes, depending on factors such as the grating  111  and the recording density. Accordingly, 0.32 um is specified merely as one example. 
     Also, in the present embodiment, the two objective lenses are deployed in the radial direction. It is also allowable, however, that the two objective lenses are deployed in the circumferential direction (which, hereinafter, will be referred to as a “tangential direction”). 
     In Embodiment 3 of the present invention, unlike Embodiment 1 and Embodiment 2, the above-described configuration allows the two objective lenses to be deployed inside the optical pickup. As a result, the positions of the optical spots, which are focused onto the servo layer and the recording layer, are made different from each other by a few tracks. The correction for this difference, however, is made in such a manner that the address obtained from the servo layer and the address obtained from the recording layer are corrected. This correction, like Embodiment 1 and Embodiment 2, allows the focus control and the tracking control to be performed over the recording layer and the servo layer independently of each other. As a result, the train of the formerly-recorded marks is recoded with a constant spacing placed between these recording marks in the disc&#39;s radial direction. This configuration makes it possible to suppress the overwriting of a recording mark over the train of the formerly-recorded marks. Moreover, the operations such as the CLU control and recording-timing generation at the recording time can be performed based on the information acquired from the servo groove. Furthermore, in the present embodiment, the specification of the objective lenses can be relaxed. This configuration makes it possible to provide users with the inexpensive optical disc device. Also, it becomes easier to implement the compatibility with an optical disc (for example, DVD or BD) that performs the recording/reproduction using a single type of laser light-source. 
     Embodiment 4 
     Hereinafter, the explanation will be given below concerning Embodiment 4 of the present invention. 
     (Disc Structure) 
       FIG. 2  illustrates the disc structure of the optical disc  101 . The structure of the optical disc  101  in the present embodiment has been explained already. Consequently, the explanation thereof will be omitted here. 
     Features of the Present Embodiment 
       FIG. 7  illustrates the configuration of an optical disc device according to the present embodiment. In the embodiments explained so far, the optical disc device has been so configured as to include the single optical pickup. A feature of the present embodiment, however, is the point of the optical disc device including two optical pickups. Referring to  FIG. 6 , the explanation will be given below regarding features of the present embodiment other than the above-described feature.  FIG. 6  illustrates a partial portion of the optical disc, which has the structure illustrated in  FIG. 2 , is cut out and enlarged. 
       FIG. 6  illustrates the following situation. Namely, the optical spot  300  is focused onto one of the recording layers. This optical spot  300  is recording a recording mark at present, while travelling in the tangential direction of the optical disc  101 . Simultaneously, the optical spot  301  and the optical spot  302  are maintained at a constant distance (i.e. spacing) from the optical spot  300 . These optical spots  301  and  302  are respectively focused onto a formerly-recorded mark and an area (i.e. unrecorded area) onto which a recording mark is supposed to be recorded later. Moreover, in the present embodiment, the two objective lenses are deployed in the radial direction. This condition gives rise to the occurrence of a shift between the optical spot  300  and the optical spot  311  that is focused onto a groove (i.e. track) of the servo layer. This shift corresponds to the deployment of these two spots in the radial direction. Namely, it turns out that these two spots are focused at the positions that are a few tracks away from each other when converted to the tracks on the servo layer (on the drawing, three tracks away). Incidentally, the optical spots  300 ,  301 , and  302  are split from the same light flux originally, and are emitted through a single (not-illustrated) objective lens. Meanwhile, the optical spot  311  is a light flux that is different from the optical spots  300 ,  301 , and  302 , and is emitted through the different objective lens. 
     Incidentally, the two optical pickups exist. This condition gives rise to the occurrence of a shift in the addresses, when information is reproduced which is acquired from the optical spots on the recording layer and the servo layer. This shift corresponds to a spacing between the objective lenses. The correction for this shift can be successfully made using the following method. The relative address shift is corrected by calculating the mutual address difference from the addresses of the optical spots on the recording layer and the servo layer. Consequently, the following point is also a feature of the present embodiment. The mutual address difference is corrected which is acquired from the addresses of the optical spots on the recording layer and the servo layer. Subsequently, the operations such as CLU control and recording-timing generation at the recording time are performed based on the address information acquired from the servo groove. 
     Implementation Method for the Present Embodiment 
     In the present embodiment, the structure other than optical pickups  112  and  114  is the same as the one in  FIG. 1 . Consequently, the overlapped explanation thereof will be omitted here. 
     In the optical disc device, the objective lens  1211  focuses the laser light that has entered the recording layer (i.e. any one layer of the plurality of recording layers), thereby causing the laser spot to be generated. Also, an objective lens  1226  focuses the laser light that has entered the servo layer, thereby causing a laser spot to be generated. 
     The optical disc device in the present embodiment is constituted from the following configuration components: the optical pickup  114 , the optical pickup  112 , the signal processing circuit  103 , the spindle motor  104 , the servo-error-signal generation circuit  105 , the reproduced-signal processing circuit  106 , the spindle driving circuit  107 , the actuator driving circuit  108 , the actuator driving circuit  109 , and the aberration-correcting-element driving circuit  110 . 
     The explanation of the control over the signal processing circuit  103  and the spindle motor  104  is the same as the one in  FIG. 1 . Consequently, the explanation thereof will be omitted here. 
     The optical pickup  112  includes, for example, the 650-nm-wavelength optical system whereas the optical pickup  114  includes, for example, the 405-nm-wavelength optical system. 
     First, the explanation will be given below concerning the 405-nm-wavelength optical system of the optical pickup  114 . The laser-power control circuit  1201 , which is controlled by the system control circuit  1301 , outputs the driving current for driving the laser diode  1202 . Here, a-few-hundreds-of-MHz radio-frequency wave&#39;s superposition is applied to this driving current in order to suppress the laser noise. The laser diode  1202  emits the 405-nm-wavelength laser light whose waveform corresponds to that of this driving current. The emitted laser light is caused to become the parallel laser light by the collimator lens  1203 . Then, a partial component of this parallel laser light is reflected by the beam splitter  1204 , then being focused onto the power monitor  1206  by the focusing lens  1205 . The power monitor  1206  feeds back, to the system control circuit  1301 , the current or voltage corresponding to the intensity of this laser light. This feedback allows the intensity of the laser light, which is to be focused onto the recording layer of the optical disc  101 , to be maintained at the desired value such as, for example, 2 mW. Meanwhile, the laser light which has passed through the beam splitter  1204  is caused to become the plurality of light beams (i.e. O-order light&#39;s main beam and its ±1st-order lights&#39; servo beams) by the three-beam grating  111 . Moreover, these light beams are reflected by the polarization beam splitter  1207 . The convergences/divergences are controlled by the aberration-correcting element  1209  driven by the aberration-correcting-element driving circuit  110 . Furthermore, the laser light beams, whose convergences/divergences have been controlled, are caused to become the circularly-polarized light beams by the quarterwave plate  1210 , then being focused onto the recording layer of the optical disc  101  by the objective lens  1211 . Here, the position of the objective lens  1211  is controlled by the actuator  1212 . Subsequently, the laser light beams, which have been reflected by the optical disc  101 , are modulated in their intensities in correspondence with the information recorded into the optical disc  101 , then being caused to become the linearly-polarized light beams by the quarterwave plate  1210 . The laser light beams then pass through the polarization beam splitter  1207  via the aberration-correcting element  1209 . In addition, the laser light beams, which have passed through the polarization beam splitter  1207 , are focused onto the detector  1214  by the focusing lens  1213 . The detector  1214  detects the intensities of the laser light beams, then outputting the signals corresponding thereto to the servo-error-signal generation circuit  105  and the reproduced-signal processing circuit  106 . 
     Next, the explanation will be given below concerning the 650-nm-wavelength optical system of the optical pickup  112 . As is the case with the 405-nm-wavelength optical system, the laser-power control circuit  1201  drives the laser diode  1215 . The laser diode  1215  driven emits the 650-nm-wavelength laser light. The power of a partial component of the laser light is monitored by the power monitor  1219  via the collimator lens  1216 , the beam splitter  1217 , and the focusing lens  1218 . The power monitored is fed back to the system control circuit  1301 . This feedback allows the intensity of the laser light, which is to be focused onto the servo layer of the optical disc  101 , to be maintained at the desired power such as, for example, 3 mW. Meanwhile, the laser light which has passed through the beam splitter  1217  is reflected by the polarization beam splitter  1220 . Moreover, the laser light passes through the quarterwave plate  1225 , then being focused onto the servo layer of the optical disc  101  by the objective lens  1226 . Furthermore, the laser light, which has been reflected by the optical disc  101 , passes through the polarization beam splitter  1220 . Finally, the laser light is focused onto the detector  1223  by the focusing lens  1222 . 
     The actuator driving circuit  109  drives the actuator  1224  installed inside the optical pickup  112 . This driving allows the focus control to be performed over the servo layer. 
     Also, in accordance with the S_TRD, the actuator driving circuit  109  drives the actuator  1224  in the direction that is parallel to the disc surface. This driving allows the objective lens  1226  to be driven in the disc&#39;s radial direction. In this way, the actuator driving circuit  109  in the present embodiment is so constituted as to include both the in-focus-direction driving circuit and the in-tracking-direction driving circuit. 
     The servo-error-signal generation circuit  105 , the servo-layer tracking control circuit  1310 , and the actuator driving circuit  109  operate as described earlier. At the time of recording information, this operation allows the tracking control to be performed in such a manner that the 650-nm-wavelength laser spot follows the servo groove formed in the servo layer. Also, the servo-error-signal generation circuit  105 , the recording-layer tracking control circuit  1312 , and the actuator driving circuit  108  operate as described earlier. At the time of recording information, this operation allows the R_TE to be generated from the recording marks formed on the recording layer. Subsequently, this operation allows the tracking control to be performed in such a manner that the 405-nm-wavelength laser spot follows these recording marks. 
     (Recording-Time Tracking Control for Implementing Features and Effects of the Present Embodiment) 
       FIG. 6  illustrates the relationships among the optical spots that are focused onto the recording layer and the servo layer at the recording time in the present embodiment. 
     At the time of the recording operation, the S_TON signal, the R_TON signal, the S_FON signal, and the R_FON signal illustrated in  FIG. 7  are switched at High level. Moreover, the terminal of the switch  1303 , the terminal of the switch  1307 , the terminal of the switch  1311 , and the terminal of the switch  1314  are selected to a, c, e, and g, respectively. Also, it is assumed that the focus control and the tracking control are performed over the recording layer and the servo layer. 
     The 405-nm-wavelength laser light emitted from the laser diode  1202  is caused to become the three beams by the grating  111  illustrated in  FIG. 7 . Moreover, the three optical spots resulting therefrom are focused onto a recording layer (i.e. any one layer of the plurality of recording layers) of the optical disc  101  by the objective lens  1211 . In  FIG. 6 , the tracking control is performed at the recording marks by the optical spot  301 . The recording and the focus control are performed by the optical spot  300 . The confirmation of the unrecorded is performed by the optical spot  302 . For example, as a method for confirming the unrecorded using the optical spot  302 , it is conceivable to take a difference value between the total amount of reflection light of the optical spot  302  and that of the optical spot  301 . Here, it is assumed that the optical disc  101  is the following type of optical disc; namely the amount of reflection light is higher on an unrecorded area where no recording mark is formed, whereas the amount of reflection light is lower on a recorded area where a recording mark is formed. At this time, the total amount of reflection light of the optical spot  302  travelling on the unrecorded area is subtracted from the total amount of reflection light of the optical spot  301  following the recording marks. Obviously, the calculation result of this subtraction turns out to be negative. In this way, the sign obtained after the difference value is taken makes it possible to confirm whether or not the tracking follow operation at the recording marks is being successful. Also, in the case of the optical disc  101  of the type where, conversely, the amount of reflection light becomes higher if a recording mark is formed, the sigh becomes inverted to the sigh of the above-described former case. 
     Also, the 650-nm-wavelength laser light emitted from the laser diode  1215  in  FIG. 7  is focused onto the servo layer of the optical disc  101  by the objective lens  1226 . Moreover, the optical spot  311  resulting therefrom is used for reproducing, from the servo layer in  FIG. 6 , the information needed for performing the CLV control. Here, the CLV control means a control of controlling the spindle control circuit  1313  not by using the recording timing and the output signal from the spindle motor  104 , but by using the information read from the optical disc  101 . Also, the use of the reproduced-signal processing circuit  106  allows the reproduced signal and the address to be read from the optical spot  301  that is following the recording marks, and the optical spot  311 . Accordingly, it is also allowable to use this address in order to confirm whether or not the recording at the recording time is being performed at a proper position and on a proper recording layer. 
     The rotation of the optical disc  101  causes the optical spot  300 , the optical spot  301 , the optical spot  302 , and the optical spot  311  to move into the recording direction, which is illustrated in  FIG. 6 , with respect to the optical disc  101 . Subsequently, while performing the tracking control by the optical spot  301 , a recording mark is formed by the optical spot  300 . 
     The track pitch, i.e. the distance in the radial direction between two of the optical spot  300 , the optical spot  301 , and the optical spot  302 , is so adjusted as to become equal to 0.32 um. This adjustment is made by an optical element such as the grating  111 . The distance in the circumferential direction, however, is permissible as long as it is a distance that can be resolved by the detector  1214 . Here, the design of the radial-direction track pitch of the optical spot  300 , the optical spot  301 , and the optical spot  302  changes, depending on factors such as the grating  111  and the recording density. Accordingly, 0.32 um is specified merely as one example. 
     In Embodiment 4 of the present invention, unlike Embodiment 3, the above-described configuration allows the two optical pickups to be deployed. This configuration makes it possible to solve the problem that the positions of the optical spots, which are focused onto the servo layer and the recording layer, are made different from each other by a few tracks. Also, like Embodiment 1 and Embodiment 2, the focus control and the tracking control are performed over the recording layer and the servo layer independently of each other. As a result, the train of the formerly-recorded marks is recoded with a constant spacing placed between these recording marks in the disc&#39;s radial direction. This configuration makes it possible to suppress the overwriting of a recording mark over the train of the formerly-recorded marks. Moreover, the operations such as the CLU control and recording-timing generation at the recording time can be performed based on the information acquired from the servo groove. Furthermore, in the present embodiment, the specification of the objective lenses can be relaxed. This configuration makes it possible to provide users with the inexpensive optical disc device. Also, it becomes easier to implement the compatibility with an optical disc (for example, DVD or BD) that performs the recording/reproduction using a single type of laser light-source. 
     Embodiment 5 
     Hereinafter, the explanation will be given below concerning Embodiment 5 of the present invention. 
     (Disc Structure) 
       FIG. 2  illustrates the disc structure of the optical disc  101 . The structure of the optical disc  101  in the present embodiment has been explained already. Consequently, the explanation thereof will be omitted here. 
     Features of the Present Embodiment 
       FIG. 8  illustrates the configuration of an optical disc device according to the present embodiment. A feature of the present embodiment is the following point. Namely, the optical disc device includes two optical pickups. Moreover, these two optical pickups are deployed at positions that are symmetrical to each other with respect to the rotation axis of the spindle motor. Referring to  FIG. 9 , the explanation will be given below regarding features of the present embodiment other than the above-described feature.  FIG. 9  illustrates a partial portion of the optical disc, which has the structure illustrated in  FIG. 2 , is cut out and enlarged. 
       FIG. 9  illustrates the following situation. Namely, the optical spot  300  is focused onto one of the recording layers. This optical spot  300  is recording a recording mark at present, while travelling in the tangential direction of the optical disc  101 . Simultaneously, the optical spot  301  and the optical spot  302  are maintained at a constant distance (i.e. spacing) from the optical spot  300 . These optical spots  301  and  302  are respectively focused onto a formerly-recorded mark and an area (i.e. unrecorded area) onto which a recording mark is supposed to be recorded later. Moreover, the two objective lenses are deployed at the positions that are symmetrical to each other with respect to the rotation axis of the spindle motor  104 . This condition makes it possible to reduce a track shift between the optical spot  300  and the optical spot  311  that is focused onto a groove (i.e. track) of the servo layer. This track shift corresponds to the radial positions of the optical pickup  112  and the optical pickup  114 . Namely, this reduction in the track shift allows the two optical spots to be focused at the same radial position when converted to the tracks on the servo layer. Incidentally, the optical spots  300 ,  301 , and  302  are split from the same light flux originally, and are emitted through a single (not-illustrated) objective lens. Meanwhile, the optical spot  311  is a light flux that is different from the optical spots  300 ,  301 , and  302 , and is emitted through the different objective lens. 
     Implementation Method for the Present Embodiment 
     In the present embodiment, the configuration other than the positions of the optical pickups  112  and  114  is the same as the one in  FIG. 7 . Consequently, the overlapped explanation thereof will be omitted here. 
     Next, the explanation will be given below regarding the control due to the optical pickup  112  and the optical pickup  114  to be deployed at the positions that are symmetrical to each other with respect to the rotation axis of the spindle motor  104 . In the present embodiment, the two optical pickups exist. This condition allows the addresses of the optical spots to be read when the information is reproduced which is acquired from the optical spots on the recording layer and the servo layer. The shift, however, exists between the addresses read. The correction for this shift can be successfully made as follows, for example. Namely, the relative address-shift amount is determined by calculating the mutual address difference from the addresses of the optical spots on the recording layer and the servo layer. Subsequently, the two optical pickups are controlled so that this relative address-shift amount becomes constant. Consequently, the following point is also a feature of the present embodiment: The mutual address difference is corrected which is acquired from the addresses of the optical spots on the recording layer and the servo layer. Subsequently, the operations such as CLU control and recording-timing generation at the recording time are performed based on the address information acquired from the servo groove. 
     In the present embodiment, the laser-power control circuit  1201  is so configured as to be deployed outside the optical pickup  112  and the optical pickup  114 . It is also allowable, however, that the circuit  1201  is integrated into these optical pickups as a laser-power control circuit for the optical pickup  112  and a laser-power control circuit for the optical pickup  114 . 
     In the present embodiment, the optical pickup  112  and the optical pickup  114  are deployed at the two positions, which are symmetrical to each other with respect to the rotation axis of the spindle motor  104 . 
     In Embodiment 5 of the present invention, the above-described configuration makes it possible to acquire basically the same effects as those in Embodiment 4. Simultaneously, the amount of address shift like the one in Embodiment 4 due to the positions of the optical pickups on the recording layer and the servo layer can be rendered as the amount of circumferential-direction address shift at the same radius. This condition makes it possible to reduce the address-shift amount. 
     Incidentally, the above-described respective embodiments have been explained assuming the employment of a write-once disc. It is needless to say, however, that the present invention is not limited thereto, but can also be applied to a rewritable disc. In this case, the present invention makes it possible to suppress a recording position different from the desired recording position from being erroneously overwritten by new data. 
     Incidentally, the present invention is not limited to the above-described embodiments, but includes a variety of modified embodiments. For example, the above-described embodiments have been explained in detail in order to explain the present invention in an easy-to-understand manner. Namely, the above-described embodiments are not necessarily limited to the ones that include all of the configurations explained. Also, a partial portion of the configuration of a certain embodiment can be replaced by the configuration of another embodiment. Also, the configuration of another embodiment can be added to the configuration of a certain embodiment. Also, the control lines and information lines specified are the ones that can be considered as being necessary from the explanation&#39;s point-of-view. Namely, all of the control lines and information lines are not necessarily specified when seen from the product&#39;s point-of-view. It may also be considered that, actually, almost all of the configurations are mutually connected to each other. 
     REFERENCE SIGNS LIST 
     
         
           101  . . . optical disc 
           102  . . . optical pickup 
           103  . . . signal processing circuit 
           104  . . . spindle motor 
           105  . . . servo-error-signal generation circuit 
           106  . . . reproduced-signal processing circuit 
           107  . . . spindle driving circuit 
           108  . . . actuator driving circuit 
           109  . . . relay-lens driving circuit 
           110  . . . aberration-correcting-element driving circuit 
           111  . . . grating 
           112  . . . optical pickup 
           113  . . . optical pickup 
           114  . . . optical pickup 
           1201  . . . laser-power control circuit 
           1202  . . . laser diode 
           1203  . . . collimator lens 
           1204  . . . beam splitter 
           1205  . . . focusing lens 
           1206  . . . power monitor 
           1207  . . . polarization beam splitter 
           1208  . . . dichroic mirror 
           1209  . . . aberration-correcting element 
           1210  . . . quarterwave plate 
           1211  . . . objective lens 
           1212  . . . actuator 
           1213  . . . focusing lens 
           1214  . . . detector 
           1215  . . . laser diode 
           1216  . . . collimator lens 
           1217  . . . beam splitter 
           1218  . . . focusing lens 
           1219  . . . power monitor 
           1220  . . . polarization beam splitter 
           1221  . . . relay lens 
           1222  . . . focusing lens 
           1223  . . . detector 
           1224  . . . actuator 
           1225  . . . quarterwave plate 
           1226  . . . objective lens 
           1301  . . . system control circuit 
           1302  . . . recording-layer focus control circuit 
           1303  . . . switch 
           1304  . . . adder 
           1305  . . . recording-layer focus driving-voltage generation circuit 
           1306  . . . servo-layer focus control circuit 
           1307  . . . switch 
           1308  . . . adder 
           1309  . . . servo-layer focus driving-voltage generation circuit 
           1310  . . . tracking control circuit 
           1311  . . . switch 
           1312  . . . recording-layer tracking control circuit 
           1313  . . . spindle control circuit 
           1314  . . . switch