Abstract:
According to one embodiment, an optical head device provides a signal processing circuit which sets a control amount to move an objective lens so that a distance between the objective lens and a given recording layer of the an optical disc coincides with a focal position, an optical path length correction mechanism which corrects an influence of an aberration component producing an error in the focal distance, a thickness difference detection circuit which finds an amount of correction to be made by the optical path length, and an aberration correction circuit which generates a correction signal to correct the influence of the aberration component producing the error in the focal distance detected by the thickness difference detection circuit, and supplies the correction signal to the optical path length correction mechanism.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2007-050311, filed Feb. 28, 2007, the entire contents of which are incorporated herein by reference. 
       BACKGROUND 
       [0002]    1. Field 
         [0003]    One embodiment of the present invention relates to an information recording/reproducing apparatus and an information recording/reproducing method, which can record information on an information recording medium capable of recording information by using light with two wavelengths. 
         [0004]    2. Description of the Related Art 
         [0005]    In optical discs such as a DVD-R disc and a DVD-RW available on the market, address information is previously recorded in a land pre-pit, and a record mark is formed on a wobbled pre-groove. 
         [0006]    A reproducing signal from a land pre-pit or a pre-groove is used for reproducing address information or as a tracking servo signal. For stable tracking and correct reproduction of address information, the shape of a land pre-pit or a pre-groove is optimized such that a reproducing signal becomes larger. 
         [0007]    A current optical disc drive also adopts a method of optimizing recording conditions, such as a recording power and a recording pulse width, in order to realize more stable information recording. 
         [0008]    For example, Japanese Patent Application Publication (KOKAI) No. 2001-266362 discloses 
         [0009]    a) a large detection signal is obtained from a land pre-pit to ensure the reliability of reproduction from address information recorded in a land pre-pit, and 
         [0010]    b) a large track displacement detection signal is obtained from a pre-groove to ensure high tracking stability at the time of forming a record mark, when a record mark is newly recorded on an information recording medium having a pre-groove and a land pre-pit. 
         [0011]    In the Japanese Patent Application Publication (KOKAI) No. 2004-192679 explains an example of calculating optimum recording power, or optimizing recording conditions when recording information, based on a detected value of an optical phase difference among optical discs. 
         [0012]    In the Japanese Patent Application Publication (KOKAI) No. 6-131688 discloses an example using a reproducing light sources and a recording light source, which are different in the wavelength of output laser beams. 
         [0013]    However, in a read-only DVD-ROM disc, address information and tracks are formed by a record mark formed in an emboss pit, and a land pre-pit and a pre-groove are not formed. Therefore, a read-only optical disc drive is optimized for reproducing information of a record mark, and if a reproducing signal of a land pre-pit or pre-groove specific to a recording optical disc is mixed there, it becomes a noise component. In this case, even if a recording condition is an optimized recording mark, a reproducing characteristic is degraded. 
         [0014]    In the example shown in the Publication No. 2001-266362, as the wavelength of a laser beam for tracking when recording a record mark is the same as the wavelength of a laser beam for reproducing information from a record mark, the following problems occur. 
         [0015]    1. A crosstalk signal from a land pre-pit is mixed into a reproducing signal from a record mark, and the characteristic of a reproducing signal from a record mark is degraded, and the reliability of reproduction from a record mark is largely lowered. 
         [0016]    2. By the influence of a diffracted light from a pre-groove, the characteristic of track displacement detection by a differential phase detect (DPD) method is degraded, and the stability of detection of a track displacement from a record mark is lowered. 
         [0017]    3. As a DC level from a pre-groove is decreased upon reproduction, the amplitude of a reproducing signal from a record mark is lowered, and the reliability of reproduction from a record mark is largely lowered. 
         [0018]    Even by changing the wavelengths of laser beams for recording and reproducing as shown in the Publication No. 2004-192679 or 6-131688, it is difficult to increase the amplitude of a signal upon reproduction as indicated in the Publication No. 2001-266362. 
         [0019]    As described above, there is a problem that the record mark reading characteristic is different between a current DVD-R/DVD-RW disc and a read-only DVD-ROM. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0020]    A general architecture that implements the various feature of the invention will now be described with reference to the drawings. The drawings and the associated descriptions are provided to illustrate embodiments of the invention and not to limit the scope of the invention. 
           [0021]      FIG. 1  is an exemplary diagram showing an example of an optical disc drive according to an embodiment of the invention; 
           [0022]      FIG. 2A  is an exemplary diagram showing an example of arrangement of photodetecting areas in a photodetector incorporated in an optical head of the optical disc drive shown in  FIG. 1 , according to an embodiment of the invention 
           [0023]      FIG. 2B  is an exemplary diagram showing an example of arrangement of photodetecting areas in a photodetector incorporated in an optical head of the optical disc drive shown in  FIG. 1 , according to an embodiment of the invention; 
           [0024]      FIG. 3  is a graph explaining an example of a relationship between the absorbance (reflection characteristic) of a recording layer of a recording medium and the wavelengths of light from first and second light sources for recording by using the optical disc drive shown in  FIG. 1 , according to an embodiment of the invention; 
           [0025]      FIG. 4  is an exemplary diagram showing an example of an optical head apparatus of the optical disc drive shown in  FIG. 1 , according to an embodiment of the invention; 
           [0026]      FIG. 5  is an exemplary diagram showing an example of another optical head apparatus of the optical disc drive shown in  FIG. 1 , according to an embodiment of the invention; 
           [0027]      FIG. 6  is an exemplary diagram showing an example of another optical head apparatus of the optical disc drive shown in  FIG. 1 , according to an embodiment of the invention; 
           [0028]      FIG. 7  is an exemplary diagram showing an example of another optical head apparatus of the optical disc drive shown in  FIG. 1 , according to an embodiment of the invention; 
           [0029]      FIG. 8  is an exemplary diagram showing an example of another optical head apparatus of the optical disc drive shown in  FIG. 1 , according to an embodiment of the invention; 
           [0030]      FIG. 9  is an exemplary diagram showing an example of another optical head apparatus of the optical disc drive shown in  FIG. 1 , according to an embodiment of the invention; 
           [0031]      FIG. 10  is an exemplary diagram showing an example of another optical head apparatus of the optical disc drive shown in  FIG. 1 , according to an embodiment of the invention; 
           [0032]      FIG. 11  is a graph explaining an example of a relationship between a groove depth and a push-pull signal amplitude when light with a wavelength of 405 nm is condensed by an objective lens with NA=0.65, according to an embodiment of the invention; and 
           [0033]      FIG. 12  is a graph explaining an example of a relationship between a groove depth and a push-pull signal amplitude when light with a wavelength of 650 nm is condensed by an objective lens with NA=0.60, according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0034]    Various embodiments according to the invention will be described hereinafter with reference to the accompanying drawings. In general, according to one embodiment of the invention, an optical head device for recording information on a recording medium which reflects light from a first light source to output light with a first wavelength by a predetermined light power, and diffracts a little light from a second light source to output a light with a second wavelength longer than the light with a first wavelength from the first light source in a non-recording state, comprising: an objective lens which condenses the light from the first and second light sources on a recording layer of a recording medium; an actuator which movably holds the objective lens to permit focusing to match a focal position of the objective lens with a recording layer of a recording medium, and to permit tracking to match the light condensed by the objective lens with a predetermined position in the radial direction of a recording medium; and a photodetector which outputs a signal corresponding to the light power mount of a reflected light from a recording layer of a recording medium captured by the objective lens, wherein recording information on a recording layer of a recording medium by the light emitted from the second light source, simultaneously outputting light from the first and second light sources, controlling a position of the objective lens in the direction of an optical axis by the light emitted from the first or second light source and reflected from recording layer of a recording medium, and controlling a position of the object lens in the radial direction of a recording medium by the output of the photo-detector which detects a component of the light emitted from the first light source and reflected from a recording layer of a recording medium, and wherein reproducing information recorded on a recording medium is only by the light emitted from the second light source, and controlling a position of the objective lens in the direction of an optical axis and in the radial direction of a recording medium by the output of the photo-detector which detects a component of the light emitted from the second light source and reflected from a recording layer of a recording medium. 
         [0035]    Embodiments of this invention will be described in detail with reference to the drawings. 
         [0036]    An optical disc drive (disc drive)  1  shown in  FIG. 1  has a disc motor  31  which supports and rotates a recording medium, or an optical disc D at a predetermined speed, and a pickup (optical) head (PUH)  101  which is located at a predetermined position to a recording surface of the optical disc D, records information on the recording surface of the optical disc D, or reproduces information from the recoding surface of the optical disc D. 
         [0037]    The PUH  101  includes a first semiconductor laser element (LD  1 )  113  capable of outputting a laser beam with a first wavelength, a second semiconductor laser element (LD  2 )  115  capable of outputting a laser beam with a second wavelength longer than the first wavelength, an objective lens  151  which guides laser beams from the first and second laser elements to a recording surface of an optical disc D, and captures a reflected laser beam reflected from the recording surface of the optical disc D, and an actuator (ACT)  111  holding the objective lens  151 , as explained later. 
         [0038]    The PUH  101  has a monitoring photodetector (APC-PD, in  FIG. 4 )  103  which detects laser beams for monitoring from the first and second semiconductor laser elements, converts them into signals corresponding to the intensities of those laser beams, and outputs the signals, and a data photodetector (D-PD, in  FIG. 4 )  105  which detects reflected laser beams from the recording surface of the optical disc D, converts them into signals corresponding to the intensity of those laser beams, and outputs the signals, and other various optical elements to be described later. 
         [0039]    The optical disc drive  1  has a CPU  21 , a RAM  23 , a ROM  25  and an interface  27 , which are connected to a bus  11 . The bus  11  is connected to a signal processing circuit  61  to give a predetermined characteristic to the output from the D-PD  105  of the PUH  101 , a servo circuit  63  to control a position of an ACT  111  by using the output from the signal processing circuit  61 , and a data reproducing circuit  65  to reproduce data (information) recorded in the optical disc D from the output of the signal processing circuit. The bus  11  is also connected to a PLL circuit  67 , a laser drive circuit (laser diode driver, LDD)  51 , and a disc motor control circuit  41 . The LDD  51  includes a laser control circuit  53  and a modulation circuit  55 , controls the strengths and waveforms of laser beams output from the first and second laser elements mounted in the PUH  101 , and controls the output and stop of a laser beam. The LDD  51  can drive the first and second semiconductor laser elements at the same time. 
         [0040]    In the optical disc drive  1  shown in  FIG. 1 , the PUH  101  is moved in the radial direction (the tracking direction) of the optical disc D by a not-shown pickup feeding mechanism. 
         [0041]    The modulation circuit  55  modulates recording data supplied from a host set (an external apparatus) connected through the interface  27 , at the time of recording information, and supplies the modulated data to the laser control circuit  53 . 
         [0042]    The laser control circuit  53  supplies a writing signal to at least one of the first and second laser elements in the PUH  101 , at the time of recording information (at the time of forming a mark), based on the modulated data supplied from the modulation circuit  55 . At the time of reproducing information, a laser beam fixed to a reproducing power is supplied to at least one of the first and second laser elements of the PUH  101 . 
         [0043]    The laser beam according to a signal supplied from the laser control circuit  53  output from the PUH  101  is focused on the optical disc D. A monitoring signal corresponding to the intensity of a laser beam is generated by a monitoring PD  103  of the PUH  101 , and output to the laser control circuit  55 . Then, a writing signal is adjusted. 
         [0044]    An output signal based on a reflected light from the optical disc D is generated by the data PD  105  of the PUH  101 , and supplied to the servo circuit  63  and data reproducing circuit  65  through the signal processing circuit  61 . The signal processing circuit  61  generates a focus error signal and a tracking signal, and outputs them to the servo circuit  63 . 
         [0045]    The servo circuit  63  generates a focusing control signal and a tracking signal for controlling the position of the ACT  111 , and outputs them to not-shown focus coil and tracking coil of the ACT  111 . As a result, a laser beam condensed on the recording surface of the optical disc D by an objective lens of the ACT  111  is controlled to be just-focused on the recording layer of the recording surface of the optical disc D, and then controlled to follow a track. 
         [0046]    The output from the signal processing circuit  61  supplied from the data reproducing circuit  65  is reproduced as recorded data (on the optical disk D), based on a reproducing clock signal from the phase-locked loop (PLL) circuit  67 . 
         [0047]    The reproduced data reproduced by the data reproducing circuit  65  is output to a host set (an external apparatus) or a storage device (an HDD or a work memory) through the interface circuit  27 . 
         [0048]    It is needless to say that the disc motor control circuit  41 , modulation circuit  55  (LDD  51 ), laser control circuit  53  (LDD  51 ), servo circuit  63 , data reproducing circuit  65 , and PLL circuit  67  are controlled by the central processing unit (CPU)  21 . The CPU  21  controls all operations of the optical disc drive  1 , according to operation commands supplied from the host set through the interface circuit  27 . The CPU  21  uses the random access memory (RAM)  23  as a work area, and is operated according to a program stored in the read-only memory (ROM)  25 . This is not substantially different from a known disc drive apparatus, and explanation will be omitted. 
         [0049]    Next, an explanation will be given on a photodetector incorporated in an optical head (PUH) of the optical disc drive shown in  FIG. 1  by using  FIG. 2A . A photodetector (PD) is available in two types, a monitoring PD for monitoring the intensities of laser beams with first and second wavelengths (photodiode integrated circuit [PDIC]), and a data PD (PDIC) as explained later. In  FIG. 2A , a data PD (D-PDIC  105 ) will be explained. In  FIG. 2B , a monitoring PD (APC-PDIC  103 ) will be explained. 
         [0050]    The data PDIC  105  has a main photodetecting area at a position in the optical head (PUH  101 ,  FIG. 1 ) including a designed optical axis (also called an optical axis of a system) passing through the center of an objective lens described later. In many cases, a pair of (two) sub photodetecting areas is provided on both sides of the main photodetecting area. Each photodetecting area is divided into four parts by division lines intersecting at right angles. Namely, each photodetecting area has 4 channels. 
         [0051]    Each photodetecting area receives light with a wavelength  1  (400 to 410 nm), outputs a push-pull (PP) signal usable for a focus error signal and a tracking error signal, and outputs a data reproducing signal, or radio frequency (RF) signal from an area to receive light with a wavelength  2  (650 to 680 nm). 
         [0052]    For example, the data PDIC  105  shown in  FIG. 2A  can receive a laser beam with the wavelength  2  in a central (main) photodetecting area, and can obtain an RF signal by signal processing by the data reproducing circuit  65 . The data PDIC  105  can also receive a laser beam with the wavelength  1  reflected from the recording layer of the recording surface of the optical disc D in two sub photodetecting areas, and can obtain a tracking signal. 
         [0053]    It is possible to obtain a focus error signal by using the outputs of four channels (CH) of the central (main) photodetecting area. Further, it is also possible to obtain a differential phase detection (DPD) signal by using the outputs of four channels of the central area. Therefore, it is possible to perform tracking even for an optical disc having a record mark (a string of pits) by DPD by using a laser beam with the wavelength  2 . 
         [0054]    The monitoring PDIC  103  is provided at a predetermined position in the optical head (PUH)  101 , and has a photodetecting area  103 - 1  to receive light with the wavelength  1  (400 to 410 nm), and a photodetecting area  103 - 2  to receive light with the wavelength  2  (650 to 680 nm), as shown in  FIG. 4  to  FIG. 10 . Each photodetecting area outputs a signal linearly proportional to the optical output emitted from each laser element. 
         [0055]    A filter or a polarizer  123  may be provided between the PDIC  103  and a beam splitter (a dichroic prism described later with reference to  FIG. 4  to  FIG. 10 ). The filter or a polarizer  123  capable of transmitting a laser beam with a wavelength corresponding to the photodetecting areas  103 - 1  and  103 - 2  has been shown in  FIG. 2B . In this case, a laser beam with one of the wavelengths is interrupted or reduced in intensity by the corresponding photodetecting area  123 - 1  or  123 - 2 , and only a laser beam with one of the wavelengths is applied to each photodetecting area. Therefore, a noise is reduced, or a detection error is prevented. The same effect can be obtained by evaporating a thin film to transmit only a laser beam with the wavelength  1  and a thin film to transmit only a laser beam with the wavelength  2  (thin films capable of transmitting laser beams with corresponding wavelengths) on a cover glass of the PDIC (APC-PD)  103 , instead of using the filter  123 . 
         [0056]      FIG. 3  shows an example of the characteristics of a recording medium usable when changing wavelengths of laser beams for recording and reproducing, that is, a recording layer of a recording surface of an optical disc D, in the optical disc drive shown in  FIG. 1 . First and second wavelengths are set to 400 to 410 and 650 to 680 nm, respectively, because of the following reason (has been shown). 
         [0057]    In current DVD-R and DVD-RW discs, 650±5 nm is assumed to be a wavelength of light for reproducing. Therefore, a laser beam with a wavelength of 650 nm can be obtained by using the PUH  101  of the optical disc drive shown in  FIG. 1 . Further, as a recording layer of an optical disc having the characteristic (absorbance) shown in  FIG. 3 , it is required to be reproduced by a laser beam with the wavelength of 650±5 nm, to ensure reproduction compatibility between current DVD-R and DVD-RW discs. 
         [0058]    Actually, an optical disc having a wide usable wavelength range is advantageous in cost performance, and reproducible by a 650±5 nm laser beam. Therefore, one of wavelengths of a laser beam to be output from a laser element mounted in the PUH  101  is decided to 650±5 nm. 
         [0059]    A wavelength λw used for a laser beam used for tracking may be any value shorter than 650 nm. In contrast, as a laser beam (a semiconductor laser source) with a wavelength of 405 nm has been used in the HD DVD and Blu-ray disc (BD) standards, one of wavelengths of a laser beam to be output from a laser element mounted in the PUH  101  is desirably 405±5 nm. 
         [0060]    As recording is performed by using light with a wavelength of 650 nm, the characteristic (absorbance) of the recording layer of the optical disc shown in  FIG. 3  is preferably higher in a wavelength of 650 nm (second wavelength) than in a wavelength of 405 nm (first wavelength) either before recording denotes with curve A or after recording denotes with curve B, and the absorbance for the second wavelength is preferably lower after recording compared with before recording. 
         [0061]    Namely, the characteristic of the recording layer of the optical disc shown in  FIG. 3  shows a peak of absorbance in a wavelength longer than the second wavelength (red). When recording or reproducing information by using a laser beam with the second wavelength, a laser beam with the first wavelength is substantially not absorbed, and reflectivity for a laser beam with the first wavelength is substantially constant, regardless of whether recording is made by using a laser beam with the second wavelength. 
         [0062]    Therefore, by using a recording layer having the absorbance (reflectivity) shown in  FIG. 3 , stable tracking is possible without changing an offset of a tracking signal. It should be noted that when reflectivity (absorbance) is different in recording and non-recording (existence of recording) for a laser beam with the first wavelength, a difference in brightness (reflection) in a recording area and a non-recording area (existence of recording) becomes an offset of a push-pull signal, and stable tracking is difficult. 
         [0063]    The depth of land/groove specific to an optical disc can be obtained from the result of simulation showing the relationship between the groove depth and push-pull signal amplitude when a laser beam with a wavelength of 405 nm is condensed by an objective lens with a numerical aperture (NA) of 0.65, as shown in  FIG. 11 , and the result of simulation showing the relationship between the groove depth and push-pull signal amplitude when a laser beam with a wavelength of 650 nm is condensed by an objective lens of NA=0.6, as shown in  FIG. 12 . 
         [0064]      FIG. 11  shows the calculation of changes in the output when the ratio of the width of a land to the width of a groove at the center of a trapezoidal slope is changed, assuming the depth of a pre-groove to be a parameter. In  FIG. 11 , the curve a shows an example with a groove depth of 10 nm, the curve b shows an example with a groove depth of 20 nm, the curve c shows an example with a groove depth of 30 nm, the curve d shows an example with a groove depth of 40 nm, the curve e shows an example with a groove depth of 50 nm, the curve f shows an example with a groove depth of 60 nm, and the curve q shows an example with a groove depth of 70 nm, respectively.  FIG. 12  shows the result of simulation when a recording light with a wavelength of 650 nm is condensed by an objective lens with the NA of 0.6 under the same conditions. In  FIG. 12 , the curves A to G indicate the groove depths, and correspond to those written in lowercase shown in  FIG. 11 . 
         [0065]    Namely, the push-pull signal amplitude is very large in  FIG. 11 , compared with  FIG. 12 . 
         [0066]    For example, when the depth of a groove of a recording layer of an optical disc D is 20 nm, the width of a push-pull signal is large for a laser beam with the wavelength  1  (405 nm), but small for a laser beam with the wavelength  2  (650 nm). By giving such a groove to the recording layer of the optical disc D, a push-pull signal is substantially not output when reproducing the optical disc D with a laser beam with the wavelength  2 , after recording a signal on an optical disc by tracking by a laser beam with the wavelength  1 . As a result, the influence of a track cross (crosstalk) in a focus signal is reduced, and stable focusing is possible. Even when reproducing a recorded optical disc by a second laser beam by using optional optical head and optical disc drive specific to a current DVD standard optical disc, as a record mark (a string of pits) is recorded, tracking in the DPD method is possible, and no problem arises from the fact that a push-pull signal is not output. 
         [0067]      FIG. 4  shows a preferable embodiment of an optical head (PUH) incorporated in the optical disc drive shown in  FIG. 1 . 
         [0068]    An optical head (PUH)  101  shown in  FIG. 4  has a blue laser element (LD  1 )  113  (to output a laser beam with a first wavelength), a red laser element (LD  2 )  115  (to output a laser beam with a second wavelength), a dichroic prism  121 , a polarization beam splitter  131 , a collimator lens (CL)  141 , an objective lens (OL)  151 , a monitoring photodiode integrated circuit (PDIC)  103  (photodetector) for auto power control (APC), a data PDIC  105  (photodetector), and an actuator (ACT)  11 . 
         [0069]    The wavelength of a laser beam output from the first laser element (LD  1 )  113  is 405±5 nm (400 to 410 nm), and blue, as already explained. 
         [0070]    The wavelength of a laser beam output from the second laser element (LD  2 )  115  is 650 to 680 nm (or 650±15 nm), and red, as already explained. 
         [0071]    The dichroic prism  121  transmits most (over 90%) of a laser beam with a first wavelength, and reflects most (over 90%) of a laser beam with a second wavelength. 
         [0072]    The polarization beam splitter  131  transmits a p-polarized light beam, and reflects an s-polarized light beam. 
         [0073]    The APC PDIC  103  consists of two or more photodetecting areas (usually, one main area and two sub-areas on both sides) partitioned by division lines intersecting at right angles, as shown in  FIG. 2B , and can independently monitor (receive) laser beams with the wavelengths  1  and  2 . For example, a focus error signal and a tracking error signal (a push-pull signal) are obtained from the area (main area) to receive light with the wavelength  1 . 
         [0074]    As shown in  FIG. 2B , the APC PDIC  103  has two photodetecting areas to receive light with the wavelength  1  (400 to 410 nm) and light with the wavelength  2  (650 to 680 nm), respectively. The light with the wavelength  1  and light with the wavelength  2  guided to the APC PDIC  103  is optimized in the photodetecting areas and wavelengths by the polarizer or filter  123  provided between the dichroic prism  121  and APC PDIC  103 . 
         [0075]    The data PDIC  105  consists of two or more photodetecting areas (usually, one main area and two sub-areas on both sides) partitioned by division lines intersection at right angles, as shown in  FIG. 2A . A focus error signal and a data output (a RF signal) are obtained from the area (main area) to receive light with the wavelength  2 . If information has been recorded on a disc, a DPD signal is also obtained. 
         [0076]    In the optical head (PUH)  101  shown in  FIG. 4 , the blue laser element (LD  1 )  113  outputs laser beam having p-polarization and the first wavelength, and at the same time, the red laser element (LD  2 )  115  outputs laser beam having p-polarization and the second wavelength. 
         [0077]    The light with the first wavelength (indicated by a solid line) passes through the dichroic prism  121 , and the light with the second wavelength (indicated by a broken line) reflects on the dichroic prism  121 . The first laser element (LD  1 )  113  and second laser element (LD 2 )  115  are arranged, so that the output lights with the first and second wavelengths are positioned on the same axial line. 
         [0078]    The light with the first wavelength passing through the dichroic prism  121  and the light with the second wavelength reflected from the dichroic prism  121  are transmitted through the polarization beam splitter  131 , converted to parallel light by the collimator lens (CL)  141 , transmitted through a not-shown λ4 plate, given a predetermined convergence by the objective lens (OL)  151 , and condensed on the recording layer of the recording surface of the optical disc D. 
         [0079]    The reflected laser beams (the laser beams with the first and second wavelengths overlapped) reflected from the recording surface of the optical disc D are captured by the objective lens  151 , converted to parallel light, transmitted through a not-shown λ/4 plate and turned  90  degree compared with the laser beam having the polarizing direction toward the optical disc D, and changed from a p-polarized beam to a s-polarized beam. 
         [0080]    The reflected laser beam transmitted through the %/ 4  plate is given convergence by the collimator lens  141 , and applied to the polarization beam splitter  131 . At this time, as the polarizing direction is changed to a s-polarization, then the reflected laser beam is reflected toward the data PDIC  105 . 
         [0081]    Thereafter, the reflected laser beam is photoelectrically converted by the PDIC  105 , and supplied to the signal processing circuit  61  (refer to  FIG. 1 ). The output of the PDIC  105  is converted to the predetermined characteristic or output format demanded by the servo circuit  63  ( FIG. 1 ) and data reproduction circuit  65  (refer to  FIG. 1 ), by the signal processing circuit  61  when it is output from the optical head (PUH)  101 . As shown in  FIG. 2A , the PDIC  105  has three photodetecting areas, a central (main) area and sub areas (two on both sides), and they correspond to a 0th-order light or a light beam on the axis guided to the main (central) photodetecting area, and a ±1 st -order light beam guided to the sub areas (two on both sides), when a not-shown diffraction element or a hologram plate having a predetermined diffraction pattern is inserted between the dichroic prism  121  and collimator lens  141 , for example. Therefore, a focus error signal and data output (RF signal) are obtained from the output of the PDIC  105 . If information has been recorded on a disc, a DPD signal is also obtained. 
         [0082]    A laser beam with the first wavelength reflected from the dichroic prism  121  by a predetermined ratio and a laser beam with the second wavelength passing through the dichroic prism  121  by a predetermined ratio are guided to the APC PDIC  103 , photoelectrically converted by the PDIC  103 , and supplied to the laser control circuit  53  (refer to  FIG. 1 ) of the LDD  51 . Then, the first and second laser beams are set to predetermined intensities. 
         [0083]      FIG. 5  shows another preferable embodiment of an optical head (PUH) incorporated in the optical disc drive shown in  FIG. 1  (the optical head and signal processing shown in  FIG. 4  are different). The components substantially the same as those shown in  FIG. 4  are given the same reference numbers, and detailed explanation on these components will be omitted. 
         [0084]    An optical head (PUH)  201  shown in  FIG. 5 , records data by using a laser beam with the wavelength  2  from the LD  2  (red), by tracking by using a laser beam with the wavelength  1  from the LD  1  (blue)  113 , after focusing by using a laser beam with the wavelength  2  from the LD  2  (red). 
         [0085]    The data PDIC  105  obtains a focus error signal and an RF signal from a laser beam with the second wavelength output from the main photodetecting area diagrammatically shown in  FIG. 2A , and obtains a tracking error signal from a laser beam with the first wavelength output from the sub photodetecting areas (two). 
         [0086]    Namely, as a laser beam with the wavelength  2  is used for focusing, a spot of a laser beam with the wavelength  2  emitted from the objective lens (OL)  151  can be condensed without defocusing. By obtaining a tracking error signal from a laser beam with the wavelength  1 , even if a recording layer of an optical disc is displaced from a focal position of the objective lens  151 , a tracking error signal can be obtained without being so influenced in contradistinction to an RF signal. 
         [0087]    The output of the APC PDIC  103  is used for setting the intensity of a laser beam output from each laser element, as in the example of  FIG. 4 . 
         [0088]      FIG. 6  shows another preferable embodiment of an optical head (PUH) incorporated in the optical disc drive shown in  FIG. 1 . The components substantially the same as those shown in  FIG. 4  are given the same reference numbers, and detailed explanation on these components will be omitted. 
         [0089]    In an optical head (PUH)  301  shown in  FIG. 6 , independently provided first and second data PDIC (D-PD  1 )  305 - 1  and (D-PD  2 )  305 - 2  receive, respectively, a laser beam with the wavelength  1  from the LD  1  (blue), and a reflected laser beam of a laser beam with the wavelength  2  emitted from the LD  2  (red) and reflected from the recording surface of an optical disc D. As two data PDIC  305 - 1  and  305 - 2  are used, the dichroic prism  121  is arranged close to the collimator lens  141 , and a first polarization beam splitter  331  is positioned between the LD  1  (blue)  113  and dichroic prism  121 , and a second polarization beam splitter  333  is positioned between the LD  2  (red)  115  and dichroic prism  121 . 
         [0090]    By using the PUH  301  shown in  FIG. 6 , when recording on an optical disc capable of recording at a highly increased speed, even if the power of a laser beam with the wavelength  2  reaches 10 times of the power of a laser beam with the wavelength  1 , a scattered light of a laser beam with the wavelength  2  does not enter a photodetecting area to receive light with the wavelength  1  ( 305 - 1 ) when receiving a laser beam with the wavelength  1 , and S/N (ratio of signal/noise) is improved. 
         [0091]      FIG. 7  shows a still another preferable embodiment of an optical head (PUH) incorporated in the optical disc drive shown in  FIG. 1 . The components substantially the same as those shown in  FIG. 4  are given the same reference numbers, and detailed explanation on these components will be omitted. 
         [0092]    In an optical head (PUH)  401  shown in  FIG. 7 , the PUH  301  shown in  FIG. 6  is configured as an independent detection system from the viewpoint of APC, and a laser beam with the wavelength  1  from the LD  1  (blue)  113  and a laser beam with the wavelength  2  from the LD  2  (red)  115  are received by independently provided a first APC PDIC (APC-PD  1 )  463 - 1  and a second APC PDIC (APC-PD  2 )  463 - 2  to monitor the laser beams, respectively. As two APC PDIC  463 - 1  and  463 - 2  are used, the dichroic prism  121  is arranged close to the collimator lens  141 , and a first polarization beam splitter  331  is positioned between the LD  1  (blue)  113  and dichroic prism  121 , and a second polarization beam splitter  333  is positioned between the LD  2  (red)  115  and dichroic prism  121 . 
         [0093]    By using the PUH  401  shown in  FIG. 7 , when recording data on an optical disc capable of recording at a highly increased speed, even if the power of a laser beam with the wavelength  2  reaches  10  times of the power of a laser beam with the wavelength  1 , reception of a laser beam with the wavelength  1  is hardly affected by a scattered light of a laser beam with the wavelength  2 , and an influence upon detection sensitivity caused by a different dynamic range can be reduced, compared with the case of using a single APC PDIC. 
         [0094]      FIG. 8  shows another preferable embodiment of an optical head (PUH) shown in  FIGS. 4 ,  6  and  7 . The components substantially the same as those shown in  FIGS. 4 ,  6  and  7  are given the same reference numbers, and detailed explanation on these components will be omitted. 
         [0095]    In an optical head (PUH)  501  shown in  FIG. 8 , as the APC PDIC  103  is commonly used for the laser beams with the first and second wavelengths, the dichroic prism  121  is arranged close to the collimator lens  141 , the first polarization beam splitter  331  is positioned between the LD  1  (blue)  113  and dichroic prism  121 , the second polarization beam splitter  333  is positioned between the LD  2  (red)  115  and dichroic prism  121 , a laser beam with the first wavelength branched by the first polarization beam splitter  331  is directly guided to the APC PDIC  103 , and a laser beam with the second wavelength branched by the second polarization beam splitter  333  is guided to the APC PDIC  103  through a neutral density (ND) filter  561  and second dichroic prism  571 . 
         [0096]    By using the optical head (PUH) shown in  FIG. 8 , the light amount (power) of a laser beam with the wavelength  2  guided to the APC PDIC  103  can be set to the same as the light amount (power) of a laser beam with the wavelength  1 , and the load to the APC PDIC  103  is reduced (a special PDIC considering a dynamic range becomes unnecessary). 
         [0097]      FIG. 9  and  FIG. 10  show a still another preferable embodiment of an optical head (PUH) incorporated in the optical disc drive shown in  FIG. 1 . The components substantially the same as those shown in the most similar PUH  201  shown in  FIG. 5  or the PUH  101  shown in  FIG. 4  are given the same reference numbers, and detailed explanation on these components will be omitted. 
         [0098]    A PUH  601  shown in  FIG. 9  corrects a focus displacement of a laser beam with the wavelength  2  by using a movable (relay) lens (RL)  681  between the dichroic prism  121  and the second laser element (LD  2 )  115  to output a laser beam with the wavelength  2 . 
         [0099]    Namely, a focus of a laser beam with the wavelength  2  is corrected by adjusting the position of the movable (relay) lens  681  to obtain a best RF signal with the wavelength  2 , thereby realizing stable focus control compared with a system ( FIG. 5 ) not provided with the movable (relay) lens  681 . At the same time, the time required by the focus control is reduced. 
         [0100]    As in a PUH  701  shown in  FIG. 10 , when obtaining a tracking error signal from a laser beam with the wavelength  1  by focusing by using a laser beam with the wavelength  2 , it is also possible to correct a focus displacement of a laser beam with the wavelength  1  by providing a movable (relay) lens  791  between the dichroic prism  121  and the LD  1  (blue) to output a laser beam with the wavelength  1 . 
         [0101]    In the system shown in  FIG. 10 , the position of the movable (relay) lens  791  is set a position which is a position with a tracking signal output from a PDIC for the wavelength  1  becomes maximum and a focus of a laser beam with the wavelength  1  is just on focused. 
         [0102]    For example, by simultaneously lighting the laser element (LD  1 )  113  for the wavelength  1  and the laser element (LD  2 )  115  for the second wavelength, and by obtaining focus error signals of both laser elements at the same time, it is possible to detect a focus displacement of a laser beam with each wavelength (a displacement from a focus position of an objective lens), to perform focusing and tracking by using a laser beam with the wavelength  1 , and to perform recording by using a laser beam with the wavelength  2 . 
         [0103]    For example, when recording, a predetermined offset may be added to a focus error signal, so that a size of the beam spot becomes minimum with respect to light with the wavelength  2 . And for example, In this case, by adjusting an optical path length of a laser beam with at least one of the wavelengths by a corresponding movable (relay) lens, as in the system shown in  FIG. 9  or  FIG. 10 . 
         [0104]    As explained above, it is possible to record information at a highly increased speed (several or several ten times higher) on a write-once optical disc (a recording medium), which outputs a push-pull signal when receiving a laser beam with the wavelength  1  (400 to 410 nm) of the present invention and outputs almost no push-pull signal when receiving a laser beam with the wavelength  2  (650 to 680 nm), by using a laser beam with the second wavelength  2  for recording. 
         [0105]    Namely, according to the invention, when recording data on a write-once optical disc medium, which outputs a push-pull signal when receiving a laser beam with the wavelength  1  (400 to 410 nm) of the present invention and outputs almost no push-pull signal when receiving a laser beam with the wavelength  2  (650 to 680 nm), as it is impossible to follow (track) a face wobble before recording with the wavelength  2 , a record mark to permit reading a signal by light with the wavelength  2  is formed on the medium by tracking with the wavelength  1 , or by recording by light with the wavelength  1  or  2 . As tracking by light with the wavelength  2  is possible by using the DPD method after recording, reproduction is possible only by light with the wavelength  2 . 
         [0106]    Further, according to the invention, as a maximum output of semiconductor laser having the wavelength  2  is larger than the output of a semiconductor laser having the wavelength  1 , recording at a highly increased speed is possible by using a semiconductor laser having the wavelength  2 . Further, as the wavelength  2  is longer than the wavelength  1 , it is necessary to increase NA in order to realize the same spot size. But, when a spreading angle and optical magnification of a laser beam from LD are equal in the wavelengths  1  and  2 , a beam using efficiency is increased when the NA is high. Therefore, from this point of view, recording by using the wavelength  2  is advantageous in the power. 
         [0107]    Further, when the working distances of the wavelengths  1  and  2  are different when recording, there arise a problem that when focusing by using the wavelength  1 , and the light with the wavelength  2  is defocused and a spot of the wavelength  2  cannot be condensed. According to the invention, in the example shown in  FIG. 4 , this problem is solved by setting the working distance of an objective lens substantially the same for light with the wavelengths  1  and light with the wavelength  2 . In the example shown in  FIG. 5 , this problem is solved by focusing by using light with the wavelength  2 . 
         [0108]    Namely, by using the invention, it is possible to record data on a write-once optical disc medium, which outputs a push-pull signal when receiving a laser beam with the wavelength  1  (400 to 410 nm) of the present invention and outputs almost no push-pull signal when receiving a laser beam with the wavelength  2  (650 to 680 nm), by the same wavelength as a reproducing wavelength, by tracking by using a laser beam with the wavelength  1 , and by recording by using a laser beam with the wavelength  2 . Namely, when recording on a medium, it is easily possible to optimize recording conditions to obtain the best quality of a reproducing signal. In addition, when reproducing data, it is possible to obtain a reproducing signal with good quality with less influences of land pre-pit and pre-groove. 
         [0109]    Further, in a semiconductor laser used for an optical disc drive, a laser beam with the wavelength  2  is lower in cost and larger in output. Therefore, it is possible to manufacture at low cost an optical disc drive capable of recording on a write-once optical disc which outputs a push-pull signal when receiving a laser beam with the wavelength  1  (400 to 410 nm) of the present invention and outputs almost no push-pull signal when receiving a laser beam with the wavelength  2  (650 to 680 nm), at a highly increased speed. 
         [0110]    While certain embodiments of the inventions have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions. What is claimed is: