Abstract:
Recording pulses are each provided in a train of divided pulses, one of which has a greater pulse width than the other divided pulse. Laser light power emitted by a laser diode is detected at predetermined timing corresponding to the greater-width divided pulse, and an electric current for driving the laser diode is controlled in such a manner that the detected laser light power appropriately follows a predetermined laser power value. Thus, in the case where the recording pulses are each provided in a train of divided pulses, this arrangement can accurately detect the laser light power and thereby achieves high-precision control of the laser light power.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a laser light power control method for use in recording on optical disks, such as a CD-R, CD-RW, DVD-R, DVD-RAM and M0, to control the recording laser light power to follow a predetermined reference power value, and a laser diode driving circuit using such a laser light power control method. More particularly, the present invention relates to an improved technique which can control the recording laser light power from a laser diode with high precision by accurately detecting the laser light power in cases where each recording pulse is provided in a train of divided pulses. 
     In recording or reproducing data on an optical disk by use of laser light power, it is necessary to control, with high precision, recording or reproducing laser light power that is predetermined depending on the optical disk used. To this end, the so-called ALPC (Automatic Laser Power Control) technique has been used which constantly detects the laser light power during the recording or reproduction operation and performs control to provide the predetermined recording or reproducing laser light power on the basis of the detected power value. 
     In FIG. 2, there is shown one example of a conventional laser diode driving circuit for optical recording based on such an ALPC technique. Laser diode  10  emits laser light  12  for recording or reproducing data to or from an optical disk. The emitted laser light  12  from the diode  10  is received by a monitor diode  14  provided within an optical pickup, and an output electric current from the monitor diode  14  is converted into a voltage signal via a current-to-voltage converter  16 . Peak value detector circuit  18  detects a peak value of the output voltage from the current-to-voltage converter  16 ; the detected peak value represents laser light power that is actually irradiated onto the optical disk. Offset detector circuit  20  detects a difference or offset between the detected peak value from the peak value detector circuit  18  and a predetermined reference laser light power value and thereby outputs an offset voltage value representative of the offset. Value of an electric current to drive the laser diode  10  is then controlled in accordance with the offset voltage from the offset detector circuit  20  so that the laser light  12  is constantly controlled to provide predetermined recording laser light power. 
     In FIG. 3, there is shown another example of the conventional laser diode driving circuit, which includes a laser diode  10 , monitor diode  14  and current-to-voltage converter  16  similar to those of FIG.  2 . In the example of FIG. 3, an output voltage from the current-to-voltage converter  16  is sent to an analog gate circuit  22 , where it is sampled in response to a sampling pulse that is generated at predetermined timing corresponding to a recording pulse. The sampled voltage value is held by a hold circuit  24 ; the thus-held voltage value represents laser light power that is actually irradiated onto the optical disk. Offset detector circuit  20  detects a difference or offset between the voltage value held in the hold circuit  24  and a target laser light power value and thereby outputs an offset voltage value representative of the offset. Value of an electric current to drive the laser diode  10  is then controlled in accordance with the offset voltage from the offset detector circuit  20  so that the laser light  12  is constantly controlled to provide predetermined recording or reproducing power. 
     Another-type laser diode driving circuit has been known, which is designed to constantly detect a value of a driving current flowing through the laser diode and control the laser-driving current value to follow a predetermined reference value for the recording or reproduction purpose. 
     Among various known techniques for recording data on an optical disk is the so-called “divided pulse recording”, which is characterized by dividing each recording pulse, for forming a single pit on the optical disk, into a train of smaller-width pulses (hereinafter called “divided pulses”). This divided pulse recording technique has the advantage that it can effectively minimize errors in the pit width and length due to excessive heat accumulation. 
     However, in cases where the laser diode driving circuit of FIG. 2 or  3  is employed in the divided pulse recording, each of the divided pulses tends to have too small a width with the result that the current-to-voltage converter  16  is unable to appropriately follow the pulse frequency, which would result in the output waveform of the converter  16  loosing necessary sharpness, i.e., becoming dull. Such a dull output waveform of the converter  16  would prevent accurate detection of the laser light power, and thus the laser light power could not be controlled with high precision. Further, the laser diode driving circuit of FIG. 3 could not achieve high-speed, high-density recording using the divided pulse recording technique, because of a limited switching speed of the analog gate circuit  22 . 
     Furthermore, with the above-mentioned conventional technique which detects a value of a driving current flowing through the laser diode and controls the driving current value to follow a predetermined reference value for recording or reproduction, it was not possible to control the laser light power with high precision due to the fact that a “driving-current vs. output-laser-lightpower” characteristic of the laser diode would greatly vary due to thermal drift and various other physical changes occurring with the passage of time. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a laser light power control method which, in divided pulse recording, can accurately detect laser light power and thereby control the laser light power with high precision, as well as a laser diode driving circuit using such a method. 
     According to an aspect of the present invention, there is provided a method of controlling laser light power to be used for recording information on an optical disk in accordance with a mark-length recording scheme using recording laser light power emitted from a laser diode driven by a recording signal including recording pulses, which comprises the steps of: providing each of the recording pulse in divided pulses, one of the divided pulses having a greater pulse width than the other divided pulses; detecting the recording laser light power at predetermined timing corresponding to the one divided pulse having the greater pulse width; and controlling an electric current for driving the laser diode in such a manner that the recording laser light power detected by the step of detecting follows a predetermined reference power value. 
     Because of the arrangement that one of the divided pulses in the recording pulse has a greater pulse width than the other divided pulse and the recording laser light power is controlled at predetermined timing corresponding to such a greater-width divided pulse, a circuit for detecting laser light power need not have a high-speed response characteristic. Thus, the laser light power control method of the present invention can accurately detect the laser light power and thereby control the laser light power to appropriately follow a predetermined reference value with high precision. 
     As shown in FIG. 4, the greater-width divided pulse can be placed selectively at a central position (FIG.  4 A), leading or fore position (FIG. 4B) or trailing or rear position (FIG. 4C) of the divided pulse train. However, placing the greater-width divided pulse at the central position or leading position of the divided pulse train is more preferable in that it can minimize the possibility of a rear end portion of a pit being excessively expanded rearward under the influence of residual heat. 
     Further, such a greater-width divided pulse may be placed only in a particular divided pulse train for forming a selected pit length, rather than being placed in every divided pulse train irrespective of the pit length. Particularly, if the greater-width divided pulse is placed only in the divided pulse train for a greatest- or near-greatest pit length, the proportion of the greater-width divided pulse to the total length of a resultant pit can be reduced to a significant degree, and thus it is possible to minimize adverse influence of the greater-width divided pulse on the pit formation, such as an erroneous pit length. For example, in the case of recording on an optical disk based on the CD standard, such as a CD-R or CD-RW, where pits are formed to lengths between 3T and 11T, the laser light power can be detected and controlled at regular intervals if the greater-width divided pulse is placed in a pit-forming divided pulse train of each 11T—11T synchronizing signal. 
     Similarly, in the case of recording on an optical disk based on the DVD standard, such as a DVD-R or DVDRAM, where pit lengths are between 3T and 11T for data and 14T for each synchronizing signal, the laser light power can be detected and controlled at regular intervals if the greater-width divided pulse is placed in a pit-forming pulse train of each 14T synchronizing signal. In stead of or in addition to placing the greater-width divided pulse in a divided pulse train for a greatest pit length, the greater-width divided pulse may be placed in a divided pulse train for a near-greatest pit length, such as 10T length in the CD-standard disk or 11T length in the DVD-standard disk. 
     Value of the electric current for driving the laser diode (i.e., laser-driving current value) may be uniform irrespective of the different lengths of the divided pulses as in the illustrated example of FIG.  4 . Alternatively, the laser-driving current value for the greater-width divided pulse may be made smaller than that for the other (smaller-width) divided pulse as shown in FIG. 5; this alternative can effectively reduce the possibility that a pit formed by the greater-width divided pulse is excessively expanded widthwise or a rear end portion of the pit is expanded rearward. Because the laser light power produced by the smaller-width divided pulse (recording laser light power) can not be detected directly in this case, detection is made of the laser light power in between the recording pulses (hereinafter referred to as bottom laser light power) and the laser light power produced by the greater-width divided pulse (intermediate laser light power), and the laser-driving current is controlled in such a manner that the bottom laser light power and intermediate laser light power follow respective predetermined reference values. Recording laser-driving current value to achieve a reference recording laser light power value that is predetermined for the smaller-width divided pulse is then determined on the basis of the thus-controlled laser-driving current values and a “driving-current vs. output-laser-lightpower” characteristic of the laser diode, and the laser-driving current value for the smaller-width divided pulse is ultimately controlled by the thus-determined recording laser-driving current value. This arrangement can control the recording laser light power to follow the predetermined reference value with high precision. 
     According to another aspect of the present invention, there is provided a laser diode driving circuit which employs the above-mentioned laser light power control method. Namely, the laser diode driving circuit comprises: a recording signal output circuit that outputs a recording signal including recording pulses, each of the recording pulses being provided in a train of divided pulses, one of the divided pulses having a greater pulse width than another of the divided pulses; a laser diode that is driven by the recording signal output by the recording signal output circuit, to generate recording laser light power to be used for recording information on an optical disk with a mark-length recording scheme; a recording power detector circuit that detects the recording laser light power emitted by the laser diode, at predetermined timing corresponding to the one divided pulse having the greater pulse width; and a recording laser-driving current control circuit that controls a laser-driving current of the recording pulse, in such a manner that a value of the recording laser light power detected by the recording power detector circuit follows a predetermined reference value. 
     According to still another aspect of the present invention, there is provided a laser diode driving circuit which is characterized by setting a smaller laser-driving current value for the greater-width divided pulse than for the other or smaller-width divided pulse. Namely, this laser diode driving circuit comprises: a recording signal output circuit that outputs a recording signal including a recording pulse, the recording pulse being provided in a train of divided pulses, a smaller laser-driving current value being set for the one divided pulse than for the other divided pulse, the recording signal including a finite value of laser-driving current for generating bottom laser light power, smaller than recordable-level laser light power, in a part or whole of a section in between the recording pulses; a laser diode that is driven by the recording signal output by the recording signal output circuit, to generate recording laser light power to be used for recording information on an optical disk with a mark-length recording scheme; a bottom power detector circuit that detects the recording laser light power emitted by the laser diode, at predetermined timing corresponding to the bottom laser light power; an intermediate power detector circuit that detects the recording laser light power emitted by the laser diode, at predetermined timing corresponding to the one divided pulse having the greater pulse width; a bottom laser-driving current control circuit that controls a laser-driving current for the bottom laser light power, in such a manner that a value of the laser light power detected by the bottom power detector circuit follows a predetermined reference bottom power value; an intermediate laser-driving current control circuit that controls the laser-driving current provided by the one divided pulse having the greater pulse width, in such a manner that a value of the laser light power detected by the intermediate power detector circuit follows a predetermined reference intermediate power value; a recording laser-driving current computing circuit that computes a recording laser-driving current value to achieve a reference recording laser light power value predetermined for the other divided pulse having a smaller pulse width than the one divided pulse, on the basis of values of the laser-driving currents controlled by the bottom laser-driving current control circuit and the intermediate laser-driving current control circuit and a driving-current vs. output-laser-light-power characteristic of the laser diode; and a recording laser-driving current control circuit that controls the laser-driving current provided by the other divided pulse, in such a manner that a value of the laser-driving current follows the recording laser-driving current value computed by the recording laser-driving current computing circuit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For better understanding of the above and other features of the present invention, preferred embodiments of the invention will hereinafter be described in detail with reference to the accompanying drawings, in which: 
     FIG. 1 is a block diagram illustrating a laser diode driving circuit in accordance with a first embodiment of the present invention; 
     FIG. 2 is a circuit diagram showing an example of a conventional laser diode driving circuit; 
     FIG. 3 is a circuit diagram showing another example of the conventional laser diode driving circuit; 
     FIGS. 4A to  4 C are waveform diagrams showing examples of recording pulses employed in the present invention; 
     FIG. 5 is a waveform diagram showing another example of the recording pulses employed in the present invention; 
     FIG. 6 is a waveform diagram explanatory of behavior of the laser diode driving circuit of FIG. 1; 
     FIG. 7 is a diagram showing an exemplary detailed construction of the laser diode driving circuit of FIG. 1; 
     FIG. 8 is a block diagram illustrating a laser diode driving circuit in accordance with a second embodiment of the present invention; 
     FIG. 9 is a waveform diagram explanatory of behavior of the laser diode driving circuit of FIG. 8; 
     FIG. 10 is a diagram showing a “driving-current vs. output-laser-light-power” characteristic of a laser diode employed in the second embodiment; and 
     FIG. 11 is a diagram showing an exemplary detailed construction of the laser diode driving circuit of FIG.  8 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     [First Embodiment] 
     FIG. 1 is a block diagram illustrating a laser diode driving circuit in accordance with a first embodiment of the present invention, and FIG. 6 is a waveform diagram explanatory of behavior of the laser diode driving circuit of FIG.  1 . Sections (b) to (f) of FIG. 6 show voltage or current waveforms of signals denoted at (b) to (f), respectively, of FIG.  1 . In the laser diode driving circuit  11  of FIG. 1, components enclosed by a dotted-line block  13  together constitute a circuit for outputting recording signals (i.e., a recording signal output circuit). Laser diode  10  emits laser light  12  for recording or reproducing data to or from an optical disk. The emitted laser light  12  from the diode  10  is received by a monitor diode  14  provided within an optical pickup. The monitor diode  14  may, for example, be arranged as a back monitor for receiving the laser light irradiated backward from the laser diode  10 . Output current from the monitor diode  14  is converted into a voltage signal via a current-to-voltage converter  16 , and the converted output voltage from the converter  16  is passed to both a bottom-power sample and hold circuit  26  and a recording-power sample and hold circuit  28 . 
     Pulse generator circuit  31  outputs pulse signals as shown in sections (c) to (e) of FIG. 6 in accordance with input recording information shown in section (f) of FIG.  6 . More specifically, section (e) shows divided pulses corresponding to the input recording information; in the illustrated example, only a central one of the divided pulses, for forming a 11T pit of a synchronizing signal having a 11T—11T pattern based on the CD standard, has a relatively great pulse width (which will therefore hereinafter be referred to as a “greater-width divided pulse”), and all of the other divided pulses have a smaller pulse width (which will therefore hereinafter be referred to as “smaller-width divided pulses”). Section (d) shows a pulse for sampling the recording laser light power (i.e., a recording-power sampling pulse), which is generated at timing corresponding to the central region of the greater-width divided pulse. Section (c) shows a pulse for sampling bottom laser light power (i.e., a bottom-power sampling pulse), which is generated at timing corresponding to bottom laser light power generated at a point in between the recording pulses (in the illustrated example, at a point corresponding to the central region of the divided pulse train forming an 11T land of the 11T—11T synchronizing signal). 
     In response to the bottom-power sampling pulse, the bottom-power sample and hold circuit  26  of FIG. 1 samples and holds the output voltage of the current-to-voltage converter  16  as a detected bottom laser light power value. Bottom power setting device  30  outputs a reference voltage corresponding to a reference bottom laser light power value Pr predetermined depending on the optical disk used. Offset detector circuit  32  outputs an offset voltage corresponding to a difference or offset between the voltage sampled and held by the sample and hold circuit  26  and the reference voltage output from the bottom power setting device  30 . Bottom laser-driving current source  34  outputs a bottom laser-driving current Ir, the value of which is controlled in accordance with the offset voltage output from the offset detector circuit  32 , to drive the laser diode  10 . With this control scheme, the bottom power of the laser light  12  is controlled to follow the predetermined reference bottom laser light power value Pr. 
     In response to the recording-power sampling pulse, the recording-power sample and hold circuit  28  samples and holds the output voltage of the current-to-voltage converter  16  as a detected recording laser light power value. Recording power setting device  36  outputs a reference voltage corresponding to a reference recording laser light power value Pw that is predetermined depending on the optical disk used: in this case, the reference recording laser light power value Pw is given as a difference Pw′ from the reference bottom laser light power value Pr. Offset detector circuit  38  outputs an offset voltage corresponding to a difference or offset between the voltage sampled and held by the sample and hold circuit  28  and the reference voltage output by the recording power setting device  36 . Recording laser-driving current source  40  outputs a recording laser-driving current Iw′, the value of which is controlled in accordance with the offset voltage output from the offset detector circuit  38 , to drive the laser diode  10 . Specifically, the recording laser-driving current Iw′ is passed by a switching circuit  42  in response to the divided pulses shown in section (e) of FIG.  6  and added with the bottom laser-driving current Ir, and the resultant added current drives the laser diode  10 . With this control scheme, the recording power of the laser light  12  is controlled to follow the predetermined reference recording laser light power value Pw. 
     The above-described arrangements permit a sufficient pulse width of the recording-power sampling pulse shown in section (d) of FIG. 6, so that it achieves accurate detection of the recording laser light power even in high-speed, high-density recording and thus can control the recording laser light power with high precision. Further, because a long bottom level period and hence a sufficient pulse width of the bottom-power sampling pulse (section (c) of FIG. 6) are guaranteed, the above-described arrangements also achieve accurate detection of the bottom laser light power even in high-speed, high-density recording and thus can control the bottom laser light power with high precision. 
     In reproduction from the thus-recorded optical disk, the bottom laser-driving current Ir is output from the bottom laser-driving current source  34  as a reproducing laser-driving current while no recording laser-driving current Iw′ is output, so that the laser light  12  of the bottom laser light power value is emitted from the laser diode  10  to reproduce the recorded data from the optical disk. 
     FIG. 7 is a diagram showing an exemplary detailed construction of the laser diode driving circuit  11  of FIG.  1 . In this figure, reference characters (b) to (e) show parts to which are supplied the signals shown in sections (b) to (e) of FIG. 6, respectively. The bottom-power sample and hold circuit  26  turns on an analog gate circuit (FET)  44  in response to the bottom-power sampling pulse ((c) of FIG. 6) and holds a sampled voltage value in a condenser  46 . The offset detector circuit  32  includes a differential amplifier  48  which outputs an offset voltage representing a difference between the voltage held by the condenser  46  and a reference voltage corresponding to a reference bottom laser light power value Pr set by the bottom power setting device  30 , and this offset voltage is then smoothed via a resistance  50  and condenser  52 . The bottom laser-driving current source  34  includes a transistor  55  that is controlled in accordance with the offset voltage output from the offset detector circuit  32 , to supply a bottom laser-driving current Ir to the laser diode  10 . 
     The recording-power sample and hold circuit  28  turns on an analog gate circuit (FET)  54  in response to the recording-power sampling pulse ((d) of FIG. 6) and holds a sampled voltage value in a condenser  56 . The offset detector circuit  38  includes a differential amplifier  58  which outputs a voltage representing a difference between the voltage held by the condenser  56  and a reference voltage corresponding to a reference recording laser light power value Pw′ set by the recording power setting device  36 , and this offset voltage is then smoothed via a resistance  60  and condenser  62 . The recording laser-driving current source  40  includes a transistor  64  that is controlled by the offset voltage output from the offset detector circuit  38 , to supply a recording laser-driving current Iw′ to the laser diode  10 . Specifically, in response to the divided pulses shown in section (e) of FIG. 6, the recording laser-driving current Iw′ is passed by the switching circuit  42 , comprising an analog switch (e.g., switching transistor), for addition with the bottom laser-driving current Ir, and the resultant added current is fed to the laser diode  10 . 
     [Second Embodiment] 
     FIG. 8 is a block diagram illustrating a laser diode driving circuit in accordance with a second embodiment of the present invention, and FIG. 9 is a waveform diagram explanatory of behavior of the laser diode driving circuit of FIG.  8 . Sections (b) to (g) of FIG. 9 show voltage or current waveforms of signals denoted at (b) to (g), respectively, of FIG.  8 . In the laser diode driving circuit  68  of FIG. 8, components enclosed by a dotted-line block  69  together constitute a circuit for outputting recording signals (i.e., a recording signal output circuit). Laser diode  10  emits laser light  12  for recording or reproducing data to or from an optical disk. The emitted laser light  12  from the diode  10  is received by a monitor diode  14  provided within an optical pickup. The monitor diode  14  may, for example, be arranged as a back monitor for receiving the laser light irradiated backward from the laser diode  10 . Output current from the monitor diode  14  is converted into a voltage signal via a current-to-voltage converter  16 , and the converted output voltage from the converter  16  is passed to both a bottom-power sample and hold circuit  26  and an intermediate-power sample and hold circuit  70 . 
     Pulse generator circuit  31  outputs pulse signals as shown in sections (c) to (f) of FIG. 9 in accordance with input recording information shown in section (g) of FIG.  9 . More specifically, section (f) shows divided pulses, corresponding to the recording information, from which a greater-width divided pulse is excluded and hence all of which are a smaller-width pulse. Section (e) shows the greater-width divided pulse extracted from among the divided pulses, which is generated at timing corresponding to the central region of the divided pulses forming an 11T pit of a synchronizing signal having a 11T—11T pattern based on the CD standard. Section (d) shows a pulse for sampling intermediate laser light power (i.e., an intermediate-power sampling pulse), which is generated at timing corresponding to the central location of the greater-width divided pulse. Section (c) shows a pulse for sampling bottom laser light power (i.e., a bottom-power sampling pulse), which is generated at timing corresponding to bottom laser light power between the recording pulses (in the illustrated example, at a point corresponding to the central region of the divided pulses forming a 11T land of the 11T—11T synchronizing signal). 
     In response to the bottom-power sampling pulse, the bottom-power sample and hold circuit  26  of FIG. 1 samples and holds the output voltage of the current-to-voltage converter  16  as a detected bottom laser light power value. Bottom power setting device  30  outputs a reference voltage corresponding to a reference bottom laser light power value Pr that is predetermined depending on the optical disk used. Offset detector circuit  32  outputs an offset voltage corresponding to a difference or offset between the voltage sampled and held by the sample and hold circuit  26  and the reference voltage output from the bottom power setting device  30 . Bottom laser-driving current source  34  outputs a bottom laser-driving current Ir, the value of which is controlled in accordance with the offset voltage output from the offset detector circuit  32 , to drive the laser diode  10 . With this control scheme, the bottom power of the laser light  12  is controlled to follow the predetermined reference bottom laser light power value Pr. 
     In response to the intermediate-power sampling pulse, the intermediate-power sample and hold circuit  70  samples and holds the output voltage of the current-to-voltage converter  16  as a detected intermediate laser light power value. Intermediate power setting device  72  outputs a reference voltage corresponding to a reference intermediate laser light power value Pm that is predetermined depending on the optical disk used; in this case, the reference intermediate laser light power value Pm is given as a difference Pm′ from the reference bottom laser light power value Pr. Offset detector circuit  74  outputs an offset voltage corresponding to a difference or offset between the voltage sampled and held by the sample and hold circuit  70  and the reference voltage output by the setting device  72 . Intermediate laser-driving current source  76  outputs an intermediate laser-driving current Im′, the value of which is controlled in accordance with the offset voltage output from the offset detector circuit  74 , to drive the laser diode  10 . Specifically, the intermediate laser-driving current Im′ is passed by a switching circuit  78  in response to the greater-width divided pulse shown in section (e) of FIG.  9  and added with the bottom laser-driving current Ir, and the resultant added current drives the laser diode  10 . With this control scheme, the recording power of the laser light  12  is controlled to follow the predetermined reference intermediate laser light power value Pm. 
     Intermediate laser-driving current detector circuit  80  of FIG. 8 detects a value of the intermediate laser-driving current Im′ having been controlled in the above-described manner. On the basis of the detected value of the intermediate laser-driving current Im′ and a “driving-current vs. output-laser-light-power” characteristic of the laser diode  10 , a microcomputer  82  computes a value of a laser-driving current to achieve a reference recording laser light power value Pw that is predetermined for the smaller-width divided pulses; in this instance, the laser-driving current value Iw′ is given as a difference from the reference bottom laser light power value Pr. As shown in FIG. 10, the “driving-current vs. output-laser-light-power” characteristic of the laser diode  10  presents some linearity and varies with a different inclination depending on an ambient temperature. Because the reference bottom laser light power value Pr, reference intermediate laser light power value Pm and reference recording laser light power value Pw are predetermined depending on the optical disk used and the intermediate laser-driving current Im′ has been detected by the intermediate laser-driving current detector circuit  80 , the laser-driving current value Iw′ for controlling the recording laser light power to follow the reference recording laser light power value Pw may be computed from the following equation using the abovementioned values: 
     
       
           Iw′=Im′ ( Pw′/Pm′ ),  
       
     
     where Pw′=Pw−Pr, and Pm′=Pm−Pr. 
     The microcomputer  80  determines the laser-driving current value Iw′ by use of the equation above. Recording laser-driving current control circuit  84  controls a recording laser-driving current source  86  in such a manner that the current source  86  outputs the laser-driving current value Iw′. The laser-driving current value Iw′ is passed by a switching circuit  88 , in response to the smaller-width divided pulse, for addition with the bottom laser-driving current Ir, and the resultant added current is fed to the laser diode  10 . With this control scheme, the recording power of the laser light  12  is controlled to follow the predetermined reference recording laser light power value Pw. 
     In reproduction, the bottom laser-driving current Ir is output from the bottom laser-driving current source  34  as a reproducing laser-driving current while no intermediate and recording laser-driving currents Im′ and Iw′ are output, so that the laser light  12  of the bottom laser light power value is emitted from the laser diode  10  to reproduce the recorded data from the optical disk. 
     FIG. 11 is a diagram showing an exemplary detailed construction of the laser diode driving circuit  68  of FIG.  8 . In this figure, reference characters (b) to (f) show parts to which are supplied the signals shown in sections (b) to (e) of FIG. 6, respectively. The bottom-power sample and hold circuit  26  turns on an analog gate circuit (FET)  44  in response to the bottom-power sampling pulse ((c) of FIG. 6) and holds a sampled voltage value in a condenser  46 . The offset detector circuit  32  includes a differential amplifier  48  which outputs an offset voltage representing a difference between the voltage held by the condenser  46  and a reference voltage corresponding to a reference bottom laser light power value Pr set by the bottom power setting device  30 , and this offset voltage is then smoothed via a resistance  50  and condenser  52 . The bottom laser-driving current source  34  includes a transistor  55  that is controlled in accordance with the offset voltage output from the offset detector circuit  32 , to supply a bottom laser-driving current Ir to the laser diode  10 . 
     The intermediate-power sample and hold circuit  70  turns on an analog gate circuit (FET)  90  in response to the intermediate-power sampling pulse ((d) of FIG. 9) and holds a sampled voltage value in a condenser  92 . The offset detector circuit  74  includes a differential amplifier  94  which outputs a voltage representing a difference between the voltage held by the condenser  92  and a reference voltage corresponding to a reference intermediate laser light power value Pm′ set by the intermediate power setting device  72 , and this offset voltage is then smoothed via a resistance  96  and condenser  98 . The intermediate laser-driving current source  76  includes a transistor  100  that is controlled in accordance with the offset voltage output from the offset detector circuit  74 , to supply an intermediate laser-driving current Im′ to the laser diode  10 . Specifically, in response to the greater-width divided pulse shown in section (e) of FIG. 9, the intermediate laser-driving current Im′ is passed by the switching circuit  78 , comprising an analog switch (e.g., switching transistor), for addition with the bottom laser-driving current Ir, and the resultant added current drives the laser diode  10 . 
     Further, an A/D converter  80  in FIG. 11, which constitutes the intermediate laser-driving current detector circuit  80 , converts, into digital representation, emitter potential of the transistor  100  that varies in accordance with the intermediate laser-driving current Im′. The microcomputer  82  computes a recording laser-driving current value Iw′ on the basis of the A/D-converted emitter potential output from the converter  80 . The recording laser-driving current value Iw′ thus computed by the microcomputer  82  is converted into an analog signal via a D/A converter  102 , which is then sent, via an amplifier  104  of the recording laser-driving current control circuit  84 , to a transistor  106  of the recording laser-driving current source  86 . Thus, the transistor  106  is controlled in accordance with the D/A-converted value so that the laser-driving current value Iw′ is output from the current source  86 . Specifically, this laser-driving current value Iw′ is passed by the switching circuit  88 , in response to the smaller-width divided pulse, for addition with the bottom laser-driving current Ir, and the resultant added current drives the laser diode  10 .