Abstract:
In a laser power control apparatus for controlling a laser power of a combination drive, first laser light Lc applied onto a first optical disk and second laser light Ld applied onto a second optical disk is collected by one photodetector  8.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a laser power control apparatus for controlling laser power in a combination drive which records information or reads recorded information on/from different types of optical disks by applying different laser lights onto the optical disks. More particularly, the present invention relates to a laser power control apparatus for controlling the laser power such that the different laser lights are collected by a common photodetector.  
         [0003]     2. Description of the Background Art  
         [0004]     In recent years, combination drives which are capable of reading recorded information or writing information using plural types of optical disks have been widely used. The combination drive may be generally represented by one that records and reproduces information on CDs and reproduces information on DVDs, as suggested in Japanese Patent Laid-Open Publication No. 2001-236726, for example. In such a combination drive, two lasers of different powers, a CD laser and a DVD laser, need to be controlled individually.  
         [0005]     Referring to  FIG. 17 , a laser power control apparatus in a conventional combination drive will be described. A laser power control apparatus LP includes a CD laser power control system LPcd for controlling the power of a CD laser; a DVD laser power control system LPdvd for controlling the power of a DVD laser; and a controller  100  for controlling the operation of the entire laser power control apparatus LP.  
         [0006]     The CD laser power control system LPcd includes an offset adjuster  1  (referred to as “Offset” in the drawing), an adder  20 , a reproduction amplification unit Up, a recording amplification unit Ur, a laser drive circuit  7  (referred to as “LDD” in the drawing), a laser diode LD 1 , and a front monitor  8  (referred to as “PD” in the drawing). The laser drive circuit  7  allows the laser diode LD 1  to emit CD laser light Lc having a predetermined amount of light, based on a reproduction laser drive control signal LDI 1  supplied from the reproduction amplification unit Up or on a recording laser drive control signal LDI 3  supplied from the recording amplification unit Ur.  
         [0007]     The front monitor  8  is composed of a photodetector. The front monitor  8  collects the CD laser light Lc and generates a laser intensity signal PD 01  having a voltage which is based on the amount of the light collected. The offset adjuster  1  adds a potential which corresponds to a predetermined offset value to the laser intensity signal PD 01  through the adder  20 , thereby generating an offset corrected laser intensity signal S 1 .  
         [0008]     The reproduction amplification unit Up has a variable gain circuit  2   p  (referred to as “VGA” in the drawing), a sample/hold circuit  3   p  (referred to as “S/H” in the drawing), and an operational amplifier  4   p . The variable gain circuit  2   p  functions so that the laser intensity signal S 1  has a sufficient amplitude at reproduction, thereby generating a laser intensity signal S 2   p . The sample/hold circuit  3   p  samples and holds the laser intensity signal S 2   p  at a predetermined timing, thereby generating a laser intensity signal S 3   p . The operational amplifier  4   p  compares between a reference potential Vp supplied from a variable voltage source  5   p  and the laser intensity signal S 3   p  and then generates a reproduction laser drive control signal LDI 1  based on the difference obtained from the comparison.  
         [0009]     The recording amplification unit Ur has a variable gain circuit  2   r , a sample/hold circuit  3   r , an operational amplifier  4   r , and a variable voltage source  5   r , as does the reproduction amplification unit Up. The variable gain circuit  2   r  functions so that the laser intensity signal S 1  has a sufficient amplitude at recording, thereby generating a laser intensity signal S 2   r . The sample/hold circuit  3   r  samples and holds the laser intensity signal S 2   r  at a predetermined timing, thereby generating a laser intensity signal S 3   r . The operational amplifier  4   r  compares between a reference potential Vr supplied from a variable voltage source  5   r  and the laser intensity signal S 3   r  and then generates a recording laser drive control signal LDI 3  based on the difference obtained from the comparison.  
         [0010]     Note that the reproduction amplification unit Up and the recording amplification unit Ur have basically the same configuration and function except that the reproduction laser drive control signal LDI 1  and the recording laser drive control signal LDI 3  generated by the reproduction amplification unit Up and the recording amplification unit Ur, respectively, have different levels. Therefore, the reproduction amplification unit Up and the recording amplification unit Ur are hereinafter collectively referred to as an “amplification unit U” as long as there is no particular problem. In addition, the variable gain circuits  2   p  and  2   r  are hereinafter collectively referred to as a “variable gain circuit  2 ”, the sample/hold circuits  3   p  and  3   r  referred to as a “sample/hold circuit  3 ”, the operational amplifiers  4   p  and  4   r  referred to as an “operational amplifier  4 ”, and the variable voltage sources  5   p  and  5   r  referred to as a “variable voltage source  5 ”. The reproduction laser drive control signal LDI 1  and the recording laser drive control signal LDI 3  are hereinafter collectively referred to as a “CD laser drive control signal LDIc”.  
         [0011]     The DVD laser power control system LPdvd has an operational amplifier  6 , a transistor PNP, a laser diode LD 2 , and a back monitor  19  (referred to as “PD” in the drawing). The transistor PNP allows the laser diode LD 2  to emit DVD laser light Ld having a predetermined amount of light, based on a reproduction laser drive control signal LDI 2  supplied from the operational amplifier  6 . The back monitor  19  is composed of a photodetector, as is the front monitor  8 . The back monitor  19  collects the DVD laser light Ld and generates a laser intensity signal PD 02  having a voltage which is based on the amount of the light collected. The operational amplifier  6  compares the laser intensity signal PD 02  to a reference signal PD 2 ref and then generates a reproduction laser drive control signal LDI 2  based on the difference obtained from the comparison.  
         [0012]     To the controller  100  a feedback signal Sf which indicates the operational conditions of the components of the laser power control apparatus LP is inputted from the laser power control apparatus LP. Further, to the controller  100  an operation mode signal Sm which indicates a user&#39;s instruction is inputted from the combination drive. The controller  100  generates a control signal Sc for controlling the operation of each component of the laser power control apparatus LP, based on the operation mode signal Sm and the feedback signal Sf.  
         [0013]     As described above, in the conventional combination drive, the CD laser power control system LPcd and the DVD laser power control system LPdvd are independently configured, and thus a space-consuming photodetector is redundantly provided on each of the font and back monitors, which inhibits production of a small size combination drive.  
       SUMMARY OF THE INVENTION  
       [0014]     Therefore, an object of the present invention is to provide a small size laser power control apparatus obtained by providing commonality of redundantly provided front and back monitors. The present invention has the following features to attain the object mentioned above.  
         [0015]     A first aspect of the present invention is directed to a laser power control apparatus for controlling an output of laser light which is exclusively applied onto a first optical disk and a second optical disk in a combination drive to record or read information thereon or therefrom, the apparatus comprising: a first laser light generator for generating first laser light to be applied onto the first optical disk; a first drive current generator for generating a drive current of the first laser light generator; a second laser light generator for generating second laser light to be applied onto the second optical disk; a second drive current generator for generating a drive current of the second laser light generator; a laser light intensity detector for exclusively collecting the first laser light and the second laser light and generating a first laser light intensity signal which indicates an intensity of the collected laser light; a first laser light generating current controller for controlling the first laser light generator based on the first laser light intensity signal; and a second laser light generating current controller for controlling the second laser light generator based on the first laser light intensity signal.  
         [0016]     As described above, in the present invention, in the combination drive capable of reproducing or recording information on plural types of optical disks, only one photodetector is provided which collects laser light to control the power of the laser light, thereby making it possible to produce a compact laser power control apparatus.  
         [0017]     These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  is a block diagram illustrating a configuration of a laser power control apparatus according to a first embodiment of the present invention;  
         [0019]      FIG. 2  is a block diagram illustrating a configuration of a laser power control apparatus according to a second embodiment of the present invention;  
         [0020]      FIG. 3  is a block diagram illustrating a configuration of a laser power control apparatus according to a third embodiment of the present invention;  
         [0021]      FIG. 4  is a block diagram illustrating a configuration of a laser power control apparatus according to a fourth embodiment of the present invention;  
         [0022]      FIG. 5  is a block diagram illustrating a configuration of a laser power control apparatus according to a fifth embodiment of the present invention;  
         [0023]      FIG. 6  is a block diagram illustrating a configuration of a laser power control apparatus according to a sixth embodiment of the present invention;  
         [0024]      FIG. 7  is a block diagram illustrating a configuration of a laser power control apparatus according to a seventh embodiment of the present invention;  
         [0025]      FIG. 8  is a block diagram illustrating a configuration of a laser power control apparatus according to an eighth embodiment of the present invention;  
         [0026]      FIG. 9  is a block diagram illustrating a configuration of a laser power control apparatus according to a ninth embodiment of the present invention;  
         [0027]      FIG. 10  is a block diagram illustrating a configuration of a laser power control apparatus according to a tenth embodiment of the present invention;  
         [0028]      FIG. 11  is a block diagram illustrating a configuration of a laser power control apparatus according to an eleventh embodiment of the present invention;  
         [0029]      FIG. 12  is a block diagram illustrating a configuration of a laser power control apparatus according to a twelfth embodiment of the present invention;  
         [0030]      FIG. 13  is a block diagram illustrating a configuration of a laser power control apparatus according to a thirteenth embodiment of the present invention;  
         [0031]      FIG. 14  is a block diagram illustrating a configuration of a laser power control apparatus according to a fourteenth embodiment of the present invention;  
         [0032]      FIG. 15  is a block diagram illustrating a configuration of a laser power control apparatus according to a fifteenth embodiment of the present invention;  
         [0033]      FIG. 16  is a block diagram illustrating a configuration of a laser power control apparatus according to a sixteenth embodiment of the present invention; and  
         [0034]      FIG. 17  is a block diagram illustrating a configuration of a laser power control apparatus used in a conventional combination drive. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     First Embodiment  
       [0035]     Referring to  FIG. 1 , a laser power control apparatus according to a first embodiment of the present invention will be described. Put simply, a laser power control apparatus LPC 1  has a configuration such that in the conventional laser power control apparatus LP, shown in  FIG. 17 , the back monitor  19  of the DVD laser power control system LPdvd is eliminated, and the CD laser power control system LPcd and the DVD laser power control system LPdvd are connected to each other at a junction  11 . In addition, the controller  100  is replaced with a controller  100   a.    
         [0036]     Specifically, in the laser power control apparatus LPC 1 , a DVD laser power control unit LPdvd_ 1  configured by eliminating the back monitor  19  from the DVD laser power control system LPdvd, and a CD laser power control unit LPcd_ 1  configured by eliminating the front monitor  8  from the CD laser power control system LPcd, share a front monitor  8 . The front monitor  8  collects either CD laser light Lc or DVD laser light Ld and generates a laser intensity signal PD 0 . The laser intensity signal PD 0  is then supplied to the CD laser power control unit LPcd_ 1  and the DVD laser power control unit LPdvd_ 1  through the junction  11 .  
         [0037]     Note that although the laser intensity signal PD 0  is generated from one front monitor  8 , the laser intensity signal PD 0  is essentially the same as the aforementioned laser intensity signals PD 01  and PD 02 . Note also that in  FIG. 1 , due to space limitations, a reproduction amplification unit Up and a recording amplification unit Ur are collectively referred to as an “amplification unit U” and the following description also collectively refers them as the “amplification unit U”, as is the case above.  
         [0038]     Now, the operation of the controller  100   a  will be described. The controller  100   a  detects whether to use a DVD drive or a CD drive based on an operation mode signal Sm inputted from a combination drive (not shown), and generates a control signal Sca for controlling each component of the laser power control apparatus LPC 1 , in accordance with the drive to be used. Specifically, in the case where the CD drive is used, i.e, in the case where a laser diode LD 1  emits light, an operational amplifier  6  of the DVD laser power control unit LPdvd_ 1  is turned off so that a laser diode LD 2  will not emit light. More specifically, the output of the operational amplifier  6  is pulled up to the power supply voltage. Accordingly, only the CD laser power control system LPcd (CD laser power control unit LPcd_ 1 ) functions and thus the laser power of the laser diode LD 1  is appropriately controlled.  
         [0039]     On the other hand, in the case where the DVD drive is used, i.e., in the case where the laser diode LD 2  emits light, an offset adjuster  1 , a variable gain circuit  2 , a sample/hold circuit  3 , and an operational amplifier  4  of the CD laser power control unit LPcd_ 1  are turned off. Accordingly, only the DVD laser power control system LPdvd (DVD laser power control unit LPdvd_ 1 ) functions and thus the laser power of the laser diode LD 2  is appropriately controlled.— 
         [0040]     As described above, in the present embodiment, while a laser intensify signal PD 0  outputted from the front monitor  8  is supplied through the junction  11  to both the CD laser power control system LPcd and the DVD laser power control system LPdvd, the laser power of only the drive to be used is appropriately controlled based on the operation mode (i.e., an operation mode signal Sm) of the combination drive.  
       Second Embodiment  
       [0041]     Referring to  FIG. 2 , a laser power control apparatus according to the present embodiment will be described. A laser power control apparatus LPC 2  is a modified version of the laser power control apparatus LPC 1  shown in  FIG. 1 , in which the laser power control unit LPdvd_ 1  is replaced with a DVD laser power control unit LPdvd_ 2  and the controller  100   a  is replaced with a controller  100   b . The DVD laser power control unit LPdvd_ 2  is different from the DVD laser power control unit LPdvd_ 1  in that a variable gain circuit  9  (referred to as “VGA” in the drawing) is inserted between an operational amplifier  6  and a junction  11 .  
         [0042]     A laser intensity signal PD 02  (PD 0 ) is gain adjusted by the variable gain circuit  9  and then inputted to the operational amplifier  6 . That is, in order to allow a front monitor  8 , which is essentially used to detect CD laser light Lc, to detect DVD laser light Ld having different intensity characteristics, etc., from the CD laser light Lc, it is necessary to change the sensitivity (voltage gain) of the variable gain circuit  9  to the DVD laser light Ld. In terms of this, in the present embodiment, the necessity of changing the power supply gain of the front monitor  8  itself is eliminated by gain adjusting the laser intensity signal PD 0  by the variable gain circuit  9 . In other words, the front monitor  8  which is used for CD laser light Lc can also be used for DVD laser light Ld having a higher energy, with appropriate sensitivity, without the need to adjust the front monitor  8  itself.  
         [0043]     Now the operation of the controller  100   b  will be described. As is the case with the controller  100   a , the controller  100   b  also generates a control signal Scb based on an operation mode signal Sm and controls the laser power control apparatus LPC 2 . In the case where the CD drive is used, i.e., in the case where a laser diode LD 1  emits light, the operational amplifier  6  and variable gain circuit  9  of the DVD laser power control unit LPdvd_ 2  are turned off so that a laser diode LD 2  will not emit light. Accordingly, only the CD laser power control unit LPcd_ 1  functions and thus the laser power of the laser diode LD 1  is appropriately controlled.  
         [0044]     On the other hand, in the case where the DVD drive is used, i.e., in the case where the laser diode LD 2  emits light, the controller  100   b  turns off the offset adjuster  1 , variable gain circuit  2 , sample/hold circuit  3 , and operational amplifier  4  of the CD laser power control unit LPcd_ 1 . Accordingly, only the DVD laser power control unit LPdvd_ 2  functions and thus the laser power of the laser diode LD 2  is appropriately controlled.  
       Third Embodiment  
       [0045]     Referring to  FIG. 3 , a laser power control apparatus according to the present embodiment will be described. A laser power control apparatus LPC 3  is a modified version of the laser power control apparatus LPC 2  shown in  FIG. 2 , in which the laser power control unit LPdvd_ 2  is replaced with a DVD laser power control unit LPdvd_ 3  and the controller  100   b  is replaced with a controller  100   c . The DVD laser power control unit LPdvd_ 3  is different from the DVD laser power control unit LPdvd_ 2  in that an offset adjuster  18  is additionally provided between a variable gain circuit  9  and a junction  11  through an adder  22 .  
         [0046]     By using the offset adjuster  18 , the sensitivity to DVD laser light Ld applied from a laser diode LD 2  can be more accurately adjusted than the laser power control apparatus LPC 2 , without the need to change the voltage gain of a front monitor  8 .  
         [0047]     Next, the operation of the controller  100   c  will be described. The controller  100   c  generates a control signal Scc based on an operation mode signal Sm and controls the laser power control apparatus LPC 3 . In the case where the CD drive is used, i.e., in the case where a laser diode LD 1  emits light, the operational amplifier  6 , variable gain circuit  9 , and offset adjuster  18  of the DVD laser power control unit LPdvd_ 3  are turned off so that the laser diode LD 2  will not emit light. Accordingly, only the CD laser power control unit LPcd_ 1  functions and thus the laser power of the laser diode LD 1  is appropriately controlled.  
         [0048]     On the other hand, in the case where the DVD drive is used, i.e., in the case where the laser diode LD 2  emits light, the controller  100   c  turns off the offset adjuster  1 , variable gain circuit  2 , sample/hold circuit  3 , and operational amplifier  4  of the CD laser power control unit LPcd_ 1 . Accordingly, only the DVD laser power control unit LPdvd_ 3  functions and thus the laser power of the laser diode LD 2  is appropriately controlled.  
       Fourth Embodiment  
       [0049]     Referring to  FIG. 4 , a laser power control apparatus according to the present embodiment will be described. A laser power control apparatus LPC 4  is a modified version of the laser power control apparatus LPC 2  shown in  FIG. 2 , in which the laser power control unit LPdvd_ 2  is replaced with a DVD laser power control unit LPdvd_ 4  and the controller  100   b  is replaced with a controller  100   d . In the DVD laser power control unit LPdvd_ 4 , instead of the reference potential PD 2 ref, a variable voltage source  25  is connected to an operational amplifier  6 . By using the variable voltage source  25 , a given voltage within a predetermined range can be set as a reference potential, and therefore the sensitivity to DVD laser light Ld can be more accurately adjusted than the laser power control apparatus LPC 2 .  
         [0050]     Next, the operation of the controller  100   d  will be described. The controller  100   d  generates a control signal Scd based on an operation mode signal Sm and controls the laser power control apparatus LPC 4 . Specifically, in the case where the CD drive is used, the operational amplifier  6  and variable gain circuit  9  of the DVD laser power control unit LPdvd_ 4  are turned off so that a laser diode LD 2  will not emit light. Accordingly, only the CD laser power control unit LPcd_ 1  functions and thus the laser power of a laser diode LD 1  is appropriately controlled.  
         [0051]     On the other hand, in the case where the DVD drive is used, the controller  100   d  turns off the offset adjuster  1 , variable gain circuit  2 , sample/hold circuit  3 , and operational amplifier  4  of the CD laser power control unit LPcd_ 1 . Accordingly, only the DVD laser power control unit LPdvd_ 4  functions and thus the laser power of the laser diode LD 2  is appropriately controlled.  
       Fifth Embodiment  
       [0052]     Referring to  FIG. 5 , a laser power control apparatus according to the present embodiment will be described. A laser power control apparatus LPC 5  is a modified version of the laser power control apparatus LPC 1  shown in  FIG. 1 , in which the laser power control unit LPdvd_ 1  is replaced with a DVD laser power control unit LPdvd_ 5  and the controller  100   a  is replaced with a controller  100   e . In the DVD laser power control unit LPdvd_ 5 , an operational amplifier  6  is connected between the adder  20  and amplification unit U of the CD laser power control unit LPcd_ 1  through a junction  12  instead of the junction  11 .  
         [0053]     A laser intensity signal PD 0  (PD 01 ) which is an output from a front monitor  8  is offset adjusted by an offset adjuster  1  and then inputted to the operational amplifier  6 . That is, the laser power control apparatus LPC 5  has an effect of accommodating the difference in the light emission levels of two different types of lasers, as does the aforementioned sample/hold circuit  3  of the laser power control apparatus LPC 1 . In the present embodiment, by sharing the offset adjuster  1  with the CD laser power control unit LPcd_ 1  and the DVD laser power control unit LPdvd_ 5 , the laser power control apparatus LPC 5  can be reduced in circuit size compared to the laser power control apparatus LPC 3 .  
         [0054]     Next, the operation of the controller  100   e  will be described. The controller  100   e  generates a control signal Sce based on an operation mode signal Sm and controls the laser power control apparatus LPC 5 . Specifically, in the case where the CD drive is used, the operational amplifier  6  of the DVD laser power control unit LPdvd_ 5  is turned off so that a laser diode LD 2  will not emit light. Accordingly, only the CD laser power control unit LPcd_ 1  functions and thus the laser power of the laser diode LD 1  is appropriately controlled.  
         [0055]     On the other hand, in the case where the DVD drive is used, the controller  100   e  turns off the variable gain circuit  2 , sample/hold circuit  3 , and operational amplifier  4  of the CD laser power control unit LPcd_ 1 . Note that the offset value of the offset adjuster  1  is changed to a value appropriate for the laser diode LD 2 . That is, in the present embodiment, the controller  100   e  changes the offset value of the offset adjuster  1  between at least two types, i.e., CD and DVD applications.  
       Sixth Embodiment  
       [0056]     Referring to  FIG. 6 , a laser power control apparatus according to the present embodiment will be described. A laser power control apparatus LPC 6  is a modified version of the laser power control apparatus LPC 5  shown in  FIG. 5 , in which the laser power control unit LPdvd_ 5  is replaced with a DVD laser power control unit LPdvd_ 6  and the controller  100   e  is replaced with a controller  100   f . In the DVD laser power control unit LPdvd_ 6 , a variable gain circuit  9  is additionally provided between an operational amplifier  6  and a junction  12 .  
         [0057]     A laser intensity signal PD 0  (PD 01 ) outputted from the front monitor  8  is offset adjusted by an adder  20  and an offset adjuster  1 , then further gain adjusted by the variable gain circuit  9 , and then inputted to the operational amplifier  6 . This configuration provides the same advantageous effect as that obtained by the laser power control apparatus LPC 3  shown in  FIG. 3 . In addition, by sharing the offset adjuster  1  and the adder  20  with the CD laser power control unit LPcd_ 1  and the DVD laser power control unit LPdvd_ 6 , the laser power control apparatus LPC 6  can be reduced in circuit size compared to the laser power control apparatus LPC 3 .  
         [0058]     Next, the operation of the controller  100   f  will be described. The controller  100   f  generates a control signal Scf based on an operation mode signal Sm and controls the laser power control apparatus LPC 6 . Specifically, in the case where the CD drive is used, the operational amplifier  6  and variable gain circuit  9  of the DVD laser power control unit LPdvd_ 6  are turned off so that a laser diode LD 2  will not emit light. Accordingly, only the CD laser power control unit LPcd_ 1  functions and thus the laser power of a laser diode LD 1  is appropriately controlled.  
         [0059]     On the other hand, in the case where the DVD drive is used, the controller  100   f  turns off the variable gain circuit  2 , sample/hold circuit  3 , and operational amplifier  4  of the CD laser power control unit LPcd_ 1 . Note that the offset value of the offset adjuster  1  is changed to a value appropriate for the laser diode LD 2 . The variable gain circuits  2  and  9  have fixed gain values.  
       Seventh Embodiment  
       [0060]     Referring to  FIG. 7 , a laser power control apparatus according to the present embodiment will be described. A laser power control apparatus LPC 7  is a modified version of the laser power control apparatus LPC 6  shown in  FIG. 6 , in which the laser power control unit LPdvd_ 6  is replaced with a DVD laser power control unit LPdvd_ 7  and the controller  100   f  is replaced with a controller  100   g . In the DVD laser power control unit LPdvd_ 7 , the reference potential PD 2 ref of the operational amplifier  6  is replaced with a variable voltage source  25 . By this, the reference potential can be set to a given value within a predetermined range, and therefore the amplification characteristics provided by the operational amplifier  6  can be more minutely adjusted than the case of the laser power control apparatus LPC 6 .  
         [0061]     Next, the operation of the controller  100   g  will be described. The controller  100   g  generates a control signal Scg based on an operation mode signal Sm and controls the laser power control apparatus LPC 7 . Specifically, in the case where the CD drive is used, the operational amplifier  6  and variable gain circuit  9  of the DVD laser power control unit LPdvd_ 7  are turned off so that a laser diode LD 2  will not emit light. Accordingly, only the CD laser power control unit LPcd_ 1  functions and thus the laser power of a laser diode LD 1  is appropriately controlled.  
         [0062]     On the other hand, in the case where the DVD drive is used, the controller  100   g  turns off the variable gain circuit  2 , sample/hold circuit  3 , and operational amplifier  4  of the CD laser power control unit LPcd_ 1 . Note that the offset value of an offset adjuster  1  is changed to a value appropriate for the laser diode LD 2 . The variable gain circuits  2  and  9  have fixed gain values. The voltage values of the variable voltage sources  5  and  25  are fixed to predetermined values.  
       Eighth Embodiment  
       [0063]     Referring to  FIG. 8 , a laser power control apparatus according to the present embodiment will be described. A laser power control apparatus LPC 8  is a modified version of the laser power control apparatus LPC 1  shown in  FIG. 1 , in which the laser power control unit LPdvd_ 1  is replaced with a DVD laser power control unit LPdvd_ 8  and the controller  100   a  is replaced with a controller  100   h . In the DVD laser power control unit LPdvd_ 8 , a laser intensity signal S 2 , which is an output from a variable gain circuit  2  of a CD laser power control unit LPcd_ 1 , is inputted to an operational amplifier  6  of the DVD laser power control unit LPdvd_ 8  through a junction  13 . That is, this configuration is such that in the laser power control apparatus LPC 3  shown in  FIG. 3  the variable gain circuit  9 , offset adjuster  18 , and adder  22  of the DVD laser power control unit LPdvd_ 3  are eliminated and the functions of the eliminated components are performed by the offset adjuster  1 , variable gain circuit  2 , and adder  20  of the CD laser power control unit LPcd_ 1 . With this configuration, the laser power control apparatus LPC 8  can be reduced in circuit size compared to the laser power control apparatus LPC 3 .  
         [0064]     Next, the operation of the controller  100   h  will be described. The controller  100   h  generates a control signal Sch based on an operation mode signal Sm and controls the laser power control apparatus LPC 8 . Specifically, in the case where the CD drive is used, the operational amplifier  6  of the DVD laser power control unit LPdvd_ 8  is turned off so that a laser diode LD 2  will not emit light. Accordingly, only the CD laser power control unit LPcd_ 1  functions and thus the laser power of a laser diode LD 1  is appropriately controlled.  
         [0065]     On the other hand, in the case where the DVD drive is used, the controller  100   h  turns off the variable gain circuit  2 , sample/hold circuit  3 , and operational amplifier  4  of the CD laser power control unit LPcd_ 1 . Note that the offset value of the offset adjuster  1  and the gain value of the variable gain circuit  2  are changed to values appropriate for the laser diode LD 2 .  
       Ninth Embodiment  
       [0066]     Referring to  FIG. 9 , a laser power control apparatus according to the present embodiment will be described. A laser power control apparatus LPC 9  is a modified version of the laser power control apparatus LPC 8  shown in  FIG. 8 , in which the laser power control unit LPdvd_ 8  is replaced with a DVD laser power control unit LPdvd_ 9  and the controller  100   h  is replaced with a controller  100   i . In the DVD laser power control unit LPdvd_ 9 , the reference potential PD 2 ref of the operational amplifier  6  is replaced with a variable voltage source  25 . By this, the reference potential can be set to an appropriate value within a predetermined range, and therefore the amplification provided by the operational amplifier  6  can be more minutely adjusted than the case of the laser power control apparatus LPC 8 .  
         [0067]     Next, the operation of the controller  100   i  will be described. The controller  100   i  generates a control signal Sci based on an operation mode signal Sm and controls the laser power control apparatus LPC 9 . Specifically, in the case where the CD drive is used, the operational amplifier  6  of the DVD laser power control unit LPdvd_ 9  is turned off so that a laser diode LD 2  will not emit light. Accordingly, only the CD laser power control unit LPcd_ 1  functions and thus the laser power of a laser diode LD 1  is appropriately controlled.  
         [0068]     On the other hand, in the case where the DVD drive is used, the controller  100   i  turns off the sample/hold circuit  3  and operational amplifier  4  of the CD laser power control unit LPcd_ 1 . Then, the offset value of an offset adjuster  1  and the gain value of a variable gain circuit  2  are changed to values appropriate for the laser diodes LD 1  and LD 2 . The values of the variable voltage sources  5  and  25  are fixed to predetermined values.  
       Tenth Embodiment  
       [0069]     Referring to  FIG. 10 , a laser power control apparatus according to the present embodiment will be described. A laser power control apparatus LPC 10  is a modified version of the laser power control apparatus LPC 8  shown in  FIG. 8 , in which the laser power control unit LPdvd_ 8  is replaced with a DVD laser power control unit LPdvd_ 10  and the controller  100   h  is replaced with a controller  100   j . In the DVD laser power control unit LPdvd_ 10 , a laser intensity signal S 3 , which is an output from a sample/hold circuit  3  of a CD laser power control unit LPcd_ 1 , is inputted to an operational amplifier  6  of the DVD laser power control unit LPdvd_ 10  through a junction  14 . By this, it is possible to input to the operational amplifier  6  a laser intensity signal S 3  from which high-frequency noise has been eliminated by a filter provided in the sample/hold circuit  3 .  
         [0070]     Next, the operation of the controller  100   j  will be described. The controller  100   j  generates a control signal Scj based on an operation mode signal Sm and controls the laser power control apparatus LPC 10 . Specifically, in the case where the CD drive is used, the operational amplifier  6  of the DVD laser power control unit LPdvd_ 10  is turned off so that a laser diode LD 2  will not emit light. Accordingly, only the CD laser power control unit LPcd_ 1  functions and thus the laser power of a laser diode LD 1  is appropriately controlled.  
         [0071]     On the other hand, in the case where the DVD drive is used, the controller  100   j  turns off an operational amplifier  4  of the CD laser power control unit LPcd_ 1 , while allowing the sample/hold circuit  3  to operate at all times. Note that the offset value of an offset adjuster  1  and the gain value of a variable gain circuit  2  are changed to values appropriate for the laser diodes LD 1  and LD 2 .  
       Eleventh Embodiment  
       [0072]     Referring to  FIG. 11 , a laser power control apparatus according to the present embodiment will be described. A laser power control apparatus LPC 11  is a modified version of the laser power control apparatus LPC 10  shown in  FIG. 10 , in which the laser power control unit LPdvd_ 10  is replaced with a DVD laser power control unit LPdvd_ 11  and the controller  100   j  is replaced with a controller  100   k . In the DVD laser power control unit LPdvd_ 11 , the reference potential PD 2 ref of the operational amplifier  6  is replaced with a variable voltage source  25 . By this, the reference potential can be set to an appropriate value within a predetermined range, and therefore the amplification characteristics provided by the operational amplifier  6  can be more minutely adjusted than the case of the laser power control apparatus LPC 10 .  
         [0073]     Next, the operation of the controller  100   k  will be described. The controller  100   k  generates a control signal Sck based on an operation mode signal Sm and controls the laser power control apparatus LPC 11 . Specifically, in the case where the CD drive is used, the operational amplifier  6  of the DVD laser power control unit LPdvd_ 11  is turned off so that a laser diode LD 2  will not emit light. Accordingly, only the CD laser power control unit LPcd_ 1  functions and thus the laser power of a laser diode LD 1  is appropriately controlled.  
         [0074]     On the other hand, in the case where the DVD drive is used, the controller  100   k  turns off an operational amplifier  4  of the CD laser power control unit LPcd_ 1 , while allowing a sample/hold circuit  3  to operate at all times. Note that the offset value of an offset adjuster  1  and the gain value of a variable gain circuit  2  are changed to values appropriate for the laser diodes LD 1  and LD 2 . The values of variable voltage sources  5  and  25  are fixed to predetermined values.  
       Twelfth Embodiment  
       [0075]     Referring to  FIG. 12 , a laser power control apparatus according to the present embodiment will be described. A laser power control apparatus LPC 12  is a modified version of the laser power control apparatus LPC 11  shown in  FIG. 11 , in which the laser power control unit LPdvd_ 11  is replaced with a DVD laser power control unit LPdvd_ 12  and the controller  100   k  is replaced with a controller  100   l . In the DVD laser power control unit LPdvd_ 12 , the operational amplifier  6  and the variable voltage source  25  are eliminated from the DVD laser power control unit LPdvd_ 11 . Further, a CD laser drive control signal LDIc, which is outputted from an amplification unit U of a CD laser power control unit LPcd_ 1 , is inputted to a transistor PNP.  
         [0076]     That is, all the amplification functions of the drive current of a laser diode LD 2  are performed by the amplification unit U of the CD laser power control unit LPcd_ 1 , whereby the circuit size of a variable gain circuit  2  of the laser power control apparatus LPC 12  can be reduced. Note that the DVD laser power control unit LPdvd_ 12 , which only includes the transistor PNP, can control the power of DVD laser light Ld of the laser diode LD 2 , based on a CD laser drive control signal LDIc.  
         [0077]     Next, the operation of the controller  100   l  will be described. The controller  100   l  generates a control signal Scl based on an operation mode signal Sm and controls the laser power control apparatus LPC 12 . Specifically, in the case where the CD drive is used, the transistor PNP is turned off and stopped so that the laser diode LD 2  will not emit light. Accordingly, only the CD laser power control unit LPcd_ 1  functions and thus the laser power of a laser diode LD 1  is appropriately controlled.  
         [0078]     On the other hand, in the case where the DVD drive is used, the controller  100   l  turns off a laser drive circuit  7  of the CD laser power control unit LPcd_ 1 , while allowing a sample/hold circuit  3  to operate at all times. Note that the offset value of an offset adjuster  1  and the gain value of the variable gain circuit  2  are changed to values appropriate for the laser diodes LD 1  and LD 2 . The values of a variable voltage source  5  and the variable gain circuit  2  are fixed to predetermined values.  
       Thirteenth Embodiment  
       [0079]     Referring to  FIG. 13 , a laser power control apparatus according to the present embodiment will be described. A laser power control apparatus LPC 13  is a modified version of the laser power control apparatus LPC 12  shown in  FIG. 12 , in which the CD laser power control unit LPcd_ 1  is replaced with a CD laser power control unit LPcd_ 13  and the controller  100   l  is replaced with a controller  100   m . In the CD laser power control unit LPcd_ 13 , a polarity reverser  16  (referred to as “POL sel” in the drawing) is additionally provided between an adder  20  and a front monitor  8 . In other words, it can be said that the polarity reverser  16  is inserted between the CD laser power control unit LPcd_ 13  and the front monitor  8 .  
         [0080]     The polarity reverser  16  has the function of reversing the polarity of a signal to be inputted, in response to an instruction from the controller  100   m , and outputting the signal. Specifically, a laser intensity signal PD 0  (PD 01 ) outputted from the front monitor  8  undergoes a polarity reversal, if necessary, and then is inputted to the CD laser power control unit LPcd_ 13 . More specifically, in the case where the CD laser power control unit LPcd_ 13  and a DVD laser power control unit LPdvd_ 12  have different control polarities, the polarity of the laser intensity signal PD 0  is reversed depending on the diode to be driven (i.e., either a laser diode LD 1  or a laser diode LD 2 ).  
         [0081]     Next, the operation of the controller  100   m  will be described. The controller  100   m  generates a control signal Scm based on an operation mode signal Sm and controls the laser power control apparatus LPC 13 . Specifically, in the case where the CD drive is used, a transistor PNP is turned off and stopped so that the laser diode LD 2  will not emit light. Accordingly, only the CD laser power control unit LPcd_ 13  functions and thus the laser power of the laser diode LD 1  is appropriately controlled.  
         [0082]     On the other hand, in the case where the DVD drive is used, the controller  100   m  turns off a laser drive circuit  7  of the CD laser power control unit LPcd_ 13 , while allowing a sample/hold circuit  3  to operate at all times. Note that the offset value of an offset adjuster  1  and the gain value of a variable gain circuit  2  are changed to values appropriate for the laser diodes LD 1  and LD 2 . The values of a variable voltage source  5  and the variable gain circuit  2  are fixed to predetermined values. In the case where the CD laser power control unit LPcd_ 13  and the DVD laser power control unit LPdvd_ 12  have different control polarities, the polarity of a laser intensity signal PD 0  (PD 01 ) is reversed by the polarity reverser  16 , depending on the diode to be driven (i.e., either the laser diode LD 1  or the laser diode LD 2 ).  
       Fourteenth Embodiment  
       [0083]     Referring to  FIG. 14 , a laser power control apparatus according to the present embodiment will be described. A laser power control apparatus LPC 14  is a modified version of the laser power control apparatus LPC 12  shown in  FIG. 12 , in which the CD laser power control unit LPcd_ 1  is replaced with a CD laser power control unit LPcd_ 14  and the controller  100   l  is replaced with a controller  100   n . In the CD laser power control unit LPcd_ 14 , the operational amplifier  4  is replaced with a polarity-reversible operational amplifier  4 ′. With this configuration, in the case where the CD laser power control unit LPcd_ 14  and a DVD laser power control unit LPdvd_ 12  have different control polarities, as with the laser power control apparatus LPC 13 , the polarity of the polarity-reversible operational amplifier  4 ′ is reversed depending on the diode to be driven (i.e., either a laser diode LD 1  or a laser diode LD 2 ).  
         [0084]     Next, the operation of the controller  100   n  will be described. The controller  100   n  generates a control signal Scn based on an operation mode signal Sm and controls the laser power control apparatus LPC 14 . Specifically, in the case where the CD drive is used, a transistor PNP is turned off and stopped so that the laser diode LD 2  will not emit light. Accordingly, only the CD laser power control unit LPcd_ 14  functions and thus the laser power of the laser diode LD 1  is appropriately controlled.  
         [0085]     On the other hand, in the case where the DVD drive is used, the controller  100   n  turns off a laser drive circuit  7  of the CD laser power control unit LPcd_ 14 , while allowing a sample/hold circuit  3  to operate at all times. Note that the offset value of an offset adjuster  1  and the gain value of a variable gain circuit  2  are changed to values appropriate for the laser diodes LD 1  and LD 2 . The values of a variable voltage source  5  and the variable gain circuit  2  are fixed to predetermined values. In the case where the CD laser power control unit LPcd_ 14  and the DVD laser power control unit LPdvd_ 12  have different control polarities, the polarity of the polarity-reversible operational amplifier  4 ′ is reversed depending on the diode to be driven (i.e., either the laser diode LD 1  or the laser diode LD 2 ).  
       Fifteenth Embodiment  
       [0086]     Referring to  FIG. 15 , a laser power control apparatus according to the present embodiment will be described. The configuration of a laser power control apparatus LPC 15  is the same as that of the laser power control apparatus LPC 14  shown in  FIG. 14 , except that the controller  100   n  is replaced with a controller  100   o.    
         [0087]     In the present embodiment, a polarity-reversible operational amplifier  4 ′ includes therein a switch SW 1  and a switch SW 2 . By controlling the operation timing of the two types of switches SW 1  and SW 2  by the controller  100   o , the output polarity can be reversed.  
       Sixteenth Embodiment  
       [0088]     Referring to  FIG. 16 , a laser power control apparatus according to the present embodiment will be described. As shown in  FIG. 16 , a laser power control apparatus LPC 16  includes a laser diode LD 1 , laser drive circuit  7 , a front monitor  8 , a back monitor  19 , an operational amplifier  41 , a variable voltage source  51 , a sample/hold circuit  31 , a sample/hold circuit  32 , a variable gain circuit  2 , an offset adjuster  1 , an adder  20 , an operational amplifier  42 , a variable voltage source  52 , a switch  60 , a DVD laser power control unit LPdvd_ 10 , and a controller  100   q . The operational amplifiers  41  and  42  are each composed of a negative comparator, and an operational amplifier  6  is composed of a positive comparator.  
         [0089]     The switch  60  includes a switch  60   a  and a switch  60   b . The switch  60   a  selects one of the front monitor  8 , the sample/hold circuit  32 , and the sample/hold circuit  31  based on the intensity of an output signal from the back monitor  19 . The switch  60   b  selects either the switch  60   a  or the back monitor  19  based on the intensity of an output signal from the back monitor  19 . Specifically, the switch  60  selects one of the outputs from the sample/hold circuit  31 , the sample/hold circuit  32 , and the front monitor  8  based on an output from the back monitor  19  and outputs the selected output to the DVD laser power control unit LPdvd_ 10 . Note that the order of connection of the offset adjuster, the variable gain circuit, and the sample/hold circuit is not limited to that described above; the order of connection may be changed appropriately.  
         [0090]     Next, the operation of the controller  100   q  will be described. The controller  100   q  generates a control signal Scq based on an operation mode signal Sm and controls the laser power control apparatus LPC 16 . Specifically, in the case where the CD drive is used, the operational amplifier  6  is turned off so that a laser diode LD 2  will not emit light.  
         [0091]     On the other hand, in the case where the DVD drive is used, the controller  100   q  controls the components in the following manner.  
         [0092]     In the case where the switch  60   a  selects the sample/hold circuit  31  and the switch  60   b  selects the switch  60   a , the controller  100   q  turns off the variable gain circuit  2 , the sample/hold circuit  32 , the operational amplifier  41 , and the operational amplifier  42 . Then, when the laser diode LD 1  or LD 2  emits light, the controller  100   q  changes the offset value of the offset adjuster  1 . The sample/hold circuit  31  is allowed to operate at all times.  
         [0093]     In the case where the switch  60   a  selects the sample/hold circuit  32  and the switch  60   b  selects the switch  60   a , the controller  100   q  turns off the sample/hold circuit  31 , the operational amplifier  41 , and the operational amplifier  42 . Then, when the laser diode LD 1  or LD 2  emits light, the controller  100   q  changes the offset value of the offset adjuster  1  and the gain of the variable gain circuit  2 . The sample/hold circuit  32  is allowed to operate at all times.  
         [0094]     In the case where the switch  60   a  selects the back monitor  19  and the switch  60   b  selects the switch  60   a , the controller  100   q  turns off the offset adjuster  1 , the variable gain circuit  2 , the sample/hold circuit  31 , the sample/hold circuit  32 , the operational amplifier  41 , and the operational amplifier  42 .  
         [0095]     In the case where the switch  60   b  selects the back monitor  19 , the controller  100   q  turns off the offset adjuster  1 , the variable gain circuit  2 , the sample/hold circuit  31 , the sample/hold circuit  32 , and the operational amplifier  41 , and the operational amplifier  42 .  
         [0096]     This configuration allows the controller to select from a plurality of characteristics depending on the situation, whereby flexibility is increased. Further, since the back monitor can be used, it is also possible to control a laser power in a conventional system. Therefore, this configuration can be applied to combination drives where a reduction in size is sought.  
         [0097]     While the invention has been described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is understood that numerous other modifications and variations can be devised without departing from the scope of the invention.