Abstract:
A means for measuring the temperature in the surroundings of a semiconductor laser is provided and, if the temperature is not above a certain level, power correction is performed at high speed by linearly approximating the I-H characteristic according to one item out of reproduction power and multi-pulse recording power, or if the temperature is higher, accurate power correction of the semiconductor laser is accomplished by detecting a plurality of items out of reproduction power and multi-pulse recording power.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to an optical disk device and a luminescent power control method for semiconductor laser, whereby a laser beam emitted from a laser beam source is focused into a minute light spot to irradiate an optical disk, which is an information recording medium to optically record or reproduce information, and more particularly to a luminescent power control technique for semiconductor laser to permit precise control of the luminescent power of the semiconductor laser according to changes in ambient temperature.  
         DESCRIPTION OF THE RELATED ART  
         [0002]    For optical recording/reproduction device including optical disk devices, the density, recording speed and reliability are progressively increasing. In an optical disk device, a spindle motor or the like is used to turn an optical disk, and a laser beam emitted from a semiconductor laser is focused into a minute light spot of about  1  pm in diameter to irradiate the optical disk. Recording of information is achieved by modulating the luminescent power of the semiconductor laser into a pulse shape on according to information signals to vary the intensity of the light spot to irradiate the optical disk, and forming a recording mark by using temperature variations occurring on the recording film of the optical disk. Information is reproduced by keeping the luminescent power of the semiconductor laser at a constant low level, detecting the intensity variations and other factors of the reflected light from the optical disk, and converting them into electric signals. Erasion of recording marks is accomplished by irradiating the optical disk, with the luminescent power of the semiconductor laser kept at a constant level between the power level at the time reproduction and the peak level of the recording pulse. In order to achieve information recording in a high density, it is necessary to form minute marks in the same shape all the time, and this requires highly precise and fast control of the luminescent power of the semiconductor laser.  
           [0003]    One of the known methods for this control is to divide the pulse of the recording power (in a multi-pulse system) to form minute marks or pits on the recording face of the optical disk, and to vary its power level in multiple values. One example of this method is disclosed in JP-A-2000-244054 as a method for setting a multi-pulse recording waveform and recording power. FIG. 6A illustrates the shape of recording marks on an optical disk, wherein reference numeral  101  denotes recording marks, and  102 , a space between recording marks. In FIG. 6B, a solid line  103  represents a multi-pulse recording waveform; a horizontal axis  104 , the time; and a longitudinal axis  105  the luminescent power level of the semiconductor laser. When the space  102  is to be recorded, the semiconductor laser is caused to emit light with erasion power of Bias  1  to erase the background mark. To record a mark  101 , the laser power is set to a plurality of levels including Peak  1 , Peak  2 , Bias  2  and Bias  3 , and the plurality of laser power levels are pulse-modulated to uniformize the heat working on the recording mark, resulting in a stable recording mark  101 . Read in the diagram represents the laser power at the time of reproduction. In the recording waveform shown in FIG. 6, recording requires setting of a total of five levels of laser power.  
           [0004]    The laser power setting method disclosed in JP-A-2000-244054 will be described below. First, the semiconductor laser is caused to emit light, and drive currents to give Peak  1 , Bias  1  and Bias  3  are set. They are counterparts to points P 1 , B 1  and B 3 , respectively, in the graph representing the I-L characteristic in the left part of FIG. 7. In a linear region  113   a  in which the I-L characteristic can sufficiently approximate a straight line, a linear formula represented by a broken line L 1  is obtained from the two points, P 1  and B 1 , to figure out a semiconductor laser driving amperage which will give Peak  2 . Next, a one-dot chain line L 2  is obtained from the two points, B 1  and B 3 , to figure out a semiconductor laser driving amperage which will give Bias  2 , corresponding to a nonlinear region  113   b.  Thus it is made possible to set five-point power levels from three-point measurement results by approximating a power level corresponding to the linear region and another power level corresponding to the nonlinear region from separate linear formulas and, in each region, obtaining a third point by interpolation into two-point measurement results using a linear expression.  
           [0005]    The linear approximation method described above with reference to an example of the prior art whereby a third point is obtained by interpolation into two-point measurement results using a linear expression is suitable for an optical disk device for continuous recording for a long duration, because it does not take a long time for measurement and arithmetic operation and permits real-time recording power correction. However, for an optical disk recorder whose interior is subject to a high temperature rise when used for continuous recording or reproduction for many hours or an optical disk camera often used in a high-temperature ambience, such as outdoors in summer time, the surroundings of the semiconductor laser are highly heated, and even on the high power side of the semiconductor laser the I-L characteristic may deviate from a straight line and become curved. There is a problem that, where the I-L characteristic of the semiconductor laser becomes curved, calculation by linear approximation as in the above-cited example of the prior art is subject to deterioration of the power setting accuracy of the recording pulse waveform, unevenness of recording marks, deterioration in the quality of reproduced signals and resultant difficulty to reproduce information in high density. Nor is there adequate consideration for power correction of the semiconductor laser or speed increase.  
         SUMMARY OF THE INVENTION  
         [0006]    An object of the present invention is to solve the problem noted above, and provide a semiconductor laser luminescent power control unit capable of accurately controlling the luminescent power of the semiconductor laser in an optical recording/reproduction device, such as an optical disk device wherein information is recorded or reproduced optically by focusing a laser beam emitted from a laser beam source into a minute light spot and irradiating therewith an optical disk, which is an information recording medium, even when the surroundings of the semiconductor laser are heated to a high temperature.  
           [0007]    In order to solve the problems noted above, according to the invention, there are provided a means to measure the ambient temperature of a semiconductor laser and a means to switch over, when the temperature of the semiconductor laser rises to so high a level that the I-L characteristic on the high power side can no longer be linearly approximated, the method for computing the drive current for a current generating means for driving the semiconductor laser on the basis of the output of a power detecting unit.  
           [0008]    More specifically, according to a first aspect of the invention, there is provided an optical disk device provided with a semiconductor laser, a current generator for supplying a D.C. or pulse-shaped drive current to the semiconductor laser, a power detecting unit for detecting the luminescent power of the semiconductor laser, a peak detector for detecting the peak level of the output signal of the power detector, a bottom detector for detecting the bottom level of the output signal of the power detector, an arithmetic and control unit having programs for computing the drive current of the semiconductor laser and controlling the drive current of the current generator according to the result of computation, and a thermal detector for detecting the temperature of the semiconductor laser, wherein the arithmetic and control unit has a plurality of current computing programs for computing the semiconductor laser drive current by different methods and a program for selecting one current computing program, out of the plurality of current computing programs, according to a temperature signal supplied by the thermal detector.  
           [0009]    According to a second aspect of the invention, the plurality of current computing programs possessed by the arithmetic and control unit in the optical disk device according to the first aspect of the invention include a first current computing program for acquiring the temperature of the semiconductor laser from a temperature signal supplied by the thermal detector and computing the drive current of the semiconductor laser from the output signal of the power detecting unit and the output signal of the peak detector, and a second current computing program for computing the drive current of the semiconductor laser from the output signal of the power detecting unit, the output signal of the peak detector and the output signal of the bottom detector, wherein the program for selecting the current computing program selects the first current computing program if the temperature of the semiconductor laser is below a prescribed level or the second current computing program if the temperature of the semiconductor laser is above the prescribed level.  
           [0010]    According to a third aspect of the invention, the optical disk device according to the second aspect of the invention is further provided with an automatic power control circuit for keeping the luminescent power of the semiconductor laser constant by supplying a D.C. drive current to the semiconductor laser, and the plurality of current computing programs possessed by the arithmetic and control unit include a third current computing program for acquiring the temperature of the semiconductor laser from a temperature signal supplied by the thermal detector and computing the drive current of the semiconductor laser from the output signal of the peak detector, and a fourth current computing program for computing the drive current of the semiconductor laser from the output signal of the peak detector and the output signal of the bottom detector, wherein the program for selecting the current computing program selects the third current computing program if the temperature of the semiconductor laser is below a prescribed level or the fourth current computing program if the temperature of the semiconductor laser is above the prescribed level.  
           [0011]    According to a fourth aspect of the invention, there is provided an optical disk device provided with a semiconductor laser, a current generator for supplying a D.C. or pulse-shaped drive current to the semiconductor laser, a power detecting unit for detecting the luminescent power of the semiconductor laser, a peak detector for detecting the peak level of the output signal of the power detector, a bottom detector for detecting the bottom level of the output signal of the power detector, and an arithmetic and control unit having programs for computing the drive current of the semiconductor laser and controlling the drive current of the current generator according to the result of computation, wherein the arithmetic and control unit has a temperature variation detecting program for detecting any temperature variation in the semiconductor laser, a plurality of current computing programs for computing the semiconductor laser drive current by different methods, and a program for selecting one current computing program, out of the plurality of current computing programs, according to the result obtained by the temperature variation detecting program.  
           [0012]    According to a fifth aspect of the invention, the plurality of current computing program possessed by the arithmetic and control unit in the optical disk device according to the fourth aspect of the invention include a first current computing program for computing the drive current of the semiconductor laser from the output signal of the power detecting unit and the output signal of the bottom detector, and a second current computing program for computing the drive current of the semiconductor laser from the output signal of the power detecting unit, the output signal of the peak detector and the output signal of the bottom detector, wherein the program for selecting the current computing program selects the first current computing program if the result obtained by the temperature variation detecting program is below a prescribed level or the second current computing program if the result obtained by the temperature variation detecting program is above the prescribed level.  
           [0013]    According to a sixth aspect of the invention, the optical disk device according to the fifth aspect of the invention is further provided with an automatic power control circuit for keeping the luminescent power of the semiconductor laser constant by supplying a D.C. drive current to the semiconductor laser, wherein the plurality of current computing programs possessed by the arithmetic and control unit include a third current computing program for computing the drive current of the semiconductor laser from the output signal of the bottom detector, and a fourth current computing program for computing the drive current of the semiconductor laser from the output signal of the peak detector and the output signal of the bottom detector, the program for selecting the current computing program selecting the third current computing program if the result obtained by the temperature variation detecting program is below a prescribed level or the fourth current computing program if the result obtained by the temperature variation detecting program is above the prescribed level.  
           [0014]    According to a seventh aspect of the invention, there is provided a semiconductor laser luminescent power control method comprising a step of supplying a D.C. or pulse-shaped drive current to a semiconductor laser, a power detecting step of detecting the luminescent power of the semiconductor laser, a peak detecting step of detecting the peak level of the output signal obtained at the power detecting step, a bottom detecting step of detecting the bottom level of the output signal of the power detecting step, steps of computing the drive current of the semiconductor laser, and a thermal detecting step of detecting the temperature of the semiconductor laser, wherein the steps of computing the drive current of the semiconductor laser have a plurality of current computing steps of computing the semiconductor laser drive current by different methods and a step of selecting one current computing step, out of the plurality of current computing steps, according to a temperature signal detected at the thermal detecting step.  
           [0015]    According to an eighth aspect of the invention, the steps of computing the drive current of the semiconductor laser according to the seventh aspect of the invention include a first current computing step of acquiring the temperature of the semiconductor laser at the thermal detecting step and computing the drive current of the semiconductor laser from the output signals of the power detecting step and the peak detecting step, and a second current computing step of computing the drive current of the semiconductor laser from the output signal of the power detecting step, the output signal of the peak detecting step and the output signal of the bottom detecting step, wherein at the step of selecting the current computing step there is selected the first current computing step if the temperature of the semiconductor laser is below a prescribed level or the second current computing step if the temperature of the semiconductor laser is above the prescribed level.  
           [0016]    According to a ninth aspect of the invention, the method according to the eighth aspect of the invention is further provided with an automatic power control step of keeping the luminescent power of the semiconductor laser constant by supplying a D.C. drive current to the semiconductor laser, wherein the plurality of current computing steps include a third current computing step of acquiring the temperature of the semiconductor laser from a temperature signal supplied at the thermal detecting step and computing the drive current of the semiconductor laser from the output signal of the peak detecting step, and a fourth current computing step of computing the drive current of the semiconductor laser from the output signal of the peak detecting step and the output signal of the bottom detecting step, the current computing step of selecting the current computing step selecting the third current computing step if the temperature of the semiconductor laser is below a prescribed level or the fourth current computing step if the temperature of the semiconductor laser is above the prescribed level.  
           [0017]    According to a 10th aspect of the invention, there is provided a semiconductor laser luminescent power control method comprising a step of supplying a D.C. or pulse-shaped drive current to a semiconductor laser, a power detecting step of detecting the luminescent power of the semiconductor laser, a peak detecting step of detecting the peak level of the output signal of the power detecting step, a bottom detecting step of detecting the bottom level of the output signal of the power detecting step, and computing and controlling steps of computing and controlling the drive current of the semiconductor laser, wherein the computing and controlling steps have a temperature variation detecting step of detecting any temperature variation in the semiconductor laser, a plurality of current computing steps of computing the semiconductor laser drive current by different methods and a step of selecting one current computing step, out of the plurality of current computing steps, according to the result obtained at the temperature variation detecting step.  
           [0018]    According to an 11th aspect of the invention, the plurality of current computing steps according to the 10th aspect of the invention include a first current computing step of computing the drive current of the semiconductor laser from the output signal of the power detecting step and the output signal of the bottom detecting step, and a second current computing step of computing the drive current of the semiconductor laser from the output signal of the power detecting step, the output signal of the peak detecting step and the output signal of the bottom detecting step, wherein the step of selecting the current computing step selects the first current computing step if the result obtained at the temperature variation detecting step is below a prescribed level or the second current computing step if the result obtained at the temperature variation detecting step is above a prescribed level.  
           [0019]    According to a 12th aspect of the invention, the method according to the eighth aspect of the invention is further provided with an automatic power control step of keeping the luminescent power of the semiconductor laser constant by supplying a D.C. drive current to the semiconductor laser, wherein the plurality of current computing steps include a third current computing step of computing the drive current of the semiconductor laser from the output signal of the bottom detecting step, and a fourth current computing step of computing the drive current of the semiconductor laser from the output signal of the peak detecting step and the output signal of the bottom detecting step, at the plurality of current computing steps of selecting the plurality of current computing steps there is selected the third current computing step if the result obtained at the temperature variation detecting step is below a prescribed level or the fourth current computing step if the result obtained at the temperature variation detecting step is above the prescribed level.  
           [0020]    These and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    [0021]FIG. 1 is a block diagram of an optical disk device, which is a first preferred embodiment of the present invention.  
         [0022]    [0022]FIG. 2A shows the waveform witnessed when the luminescent power of the semiconductor laser is detected in the first preferred embodiment of the invention, and FIG. 2B, the characteristic of power detection signals by a peak detecting circuit and a bottom detecting circuit.  
         [0023]    [0023]FIG. 3 is a characteristic diagram showing the relationship between values set by a recording power register and the luminescent power of the semiconductor laser in the first preferred embodiment of the invention.  
         [0024]    [0024]FIG. 4 is a flow chart for describing correction to be applied to a current generating means by using a thermal sensor in a third preferred embodiment of the invention at the time of recording.  
         [0025]    [0025]FIG. 5 is a flow for describing correction to be applied to the current generating in the third preferred embodiment of the invention.  
         [0026]    [0026]FIG. 6A illustrates recording marks in an optical disk recording/reproduction device according to the prior art, and FIG. 6B, a recording pulse waveform.  
         [0027]    [0027]FIG. 7 shows the relation between the I-L characteristic and the recording pulse waveform in the optical disk recording/reproduction device according to the prior art and the concept of the approximating method for controlling luminescent power.  
         [0028]    [0028]FIG. 8 shows the fitting position of a thermistor  19  in the first preferred embodiment of the invention.  
         [0029]    [0029]FIG. 9 is a circuit diagram illustrating the configuration of the thermistor  19  and a voltage converting circuit  36  in the first preferred embodiment of the invention.  
         [0030]    [0030]FIG. 10 illustrates mounting onto an optical pickup in another preferred embodiment of the invention using a thermal sensor IC.  
         [0031]    [0031]FIG. 11 is a flow chart for describing correction to be applied to a current generating means by using variations in peak power in the fourth preferred embodiment of the invention at the time of recording.  
         [0032]    [0032]FIG. 12 is a partial block diagram of an optical disk device, which is a second preferred embodiment of the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0033]    A first preferred embodiment of the present invention will be described below with reference to FIG. 1 through FIG. 3, FIG. 8 and FIG. 9.  
         [0034]    [0034]FIG. 1 is a block diagram of an optical disk device, which is the first preferred embodiment of the invention. A one-dot chain line  1  represents an optical disk device, and reference numeral  2  denotes an optical disk,  20 , a disk motor, and  21 , a microprocessor unit (MPU). When the optical disk  2  is inserted into and fitted to the disk motor  20 , the disk motor  20  begins turning, actuated by a signal  22  from the MPU  21 , and the optical disk  2  also turns. Reference numeral  23  denotes an optical pickup, consisting of a semiconductor laser  16 , a collimating lens  24 , a beam splitter  25 , a focusing lens  26 , a condensing lens  27 , a photodiode  17  and an optical detector  28 . The photodiode  17  supplies a current signal according to the luminous energy of the received laser beam. Reference numeral  18  denotes an I-V converting circuit, which converts a current signal supplied from the photodiode  17  into a voltage signal. Reference numeral  7  denotes an automatic reproduction power control circuit, which computes and outputs the difference between the voltage signal supplied by the I-V converting circuit  18  and an internal reference voltage. A current generating circuit  15  has an amplifier  11  and an adder  30 . The amplifier  11  converts the voltage signal supplied by automatic reproduction power control circuit  7  into a current, amplifies it and supplies it to the semiconductor laser  16  of the optical pickup  23  via the adder  30 .  
         [0035]    As the optical disk  2  turns, the signal  22  of the MPU  21  causes the reference voltage in the automatic reproduction power control circuit  7  to be set to a prescribed level matching an information reproduction mode. Then, a differential signal supplied from the automatic reproduction power control circuit  7  is turned into a semiconductor laser drive current by the amplifier  11  and the adder  30  and supplied to the semiconductor laser  16 , which then emits a laser beam. The laser beam emitted from the semiconductor laser  16  is collimated by the collimating lens  24  into a parallel beam, part of which is reflected by the beam splitter  25  and carried by the condensing lens  27  to the photodiode  17 . The luminous energy of the laser beam received by the photodiode  17  is entered into the automatic reproduction power control circuit  7  through the I-V converting circuit  18 , and the automatic reproduction power control circuit  7 , comparing with the reference voltage in it, supplies a differential signal. This enables the luminescent power of the semiconductor laser  16  to be controlled to be kept at a prescribed reproduction power level even if the temperature of the semiconductor laser  16  varies.  
         [0036]    The laser beam of the semiconductor laser  16  controlled to be kept at the prescribed reproduction power level by the automatic reproduction power control circuit  7  is collimated by the collimating lens  24  into a parallel beam, which passes the beam splitter  25  and is focused by the focusing lens  26  onto the optical disk  2 . The laser beam reflected by the optical disk  2  is again collimated by the focusing lens  26  into a parallel beam, which is then reflected by the beam splitter  25  to be received by the optical detector  28 . From the optical detector  28  are supplied a defocusing signal, an off-track detection signal and a signal reproducing information recorded on the optical disk  2  as signals  31 . The defocusing signal and the off-track detection signal are supplied to a two-dimensional lens actuator of a known configuration (not shown), and control of the position of the focusing lens  26  in two directions makes it possible to supply stable reproduced signals. The reproduced signals are transmitted as signals  32  via the MPU  21  to an externally connected computer and the like. The procedure so far described makes possible achievement of the reproduction of information recorded on the optical disk  2 .  
         [0037]    The luminous energy signal of the laser beam received by the photodiode  17  of the optical pickup  23  is converted into a voltage signal by the I-V converting circuit  18 , and the converted signal is entered into both a peak detecting circuit  5  and a bottom detecting circuit  6 . If the luminescent power of the semiconductor laser  16  varies in a pulse shape when recording information, the peak detecting circuit  5  detects the peak level of the luminous energy signal supplied by the photodiode  17 , and the bottom detecting circuit  6  detects the bottom level of the luminous energy signal supplied by the photodiode  17 . Reference numeral  4  denotes a switch-over circuit, which selects with a switch-over signal  33  one out of the output signal of the peak detecting circuit  5 , the output signal of the I-V converting circuit  18  and the output signal of the bottom detecting circuit  6 , and supplies a selected signal  34 . An arithmetic and control circuit  3  receives user data  35  to be recorded, sent via the MPU  21  from a personal computer or some other superior unit, and issues the data and an instruction to the current generating circuit  15  to have the data recorded on the optical disk  2 .  
         [0038]    The current generating circuit  15  sets values in recording power registers  10  and a slope control digital-to-analog converter (DAC)  12  via an internal interface  8 . Referring to FIG. 1, the recording power registers  10  can set four kinds of power levels including Pa-reg, Pb-reg, Pc-reg and Pd-reg. One of these recording power registers  10  is selected by a switching circuit  13  controlled by a write strategy circuit  9 , and a multi-pulse for recording data from the arithmetic and control circuit  3  is generated. A value Dp of a power control DAC  14  that determines the amperage of the drive current to be supplied to the semiconductor laser  16  is given by Dp=Rw/Da, where Da is the value of the slope control DAC  12  and Rw, the value of the recording power register  10  selected by the switching circuit  13 . The drive current amperage of the semiconductor laser  16  is the sum of the amperage from the power control DAC  14  and the amperage from the automatic reproduction power control circuit  7 , both mentioned above. The reason why the slope control DAC  12  is used is to ensure that the value set in the recording power register  10  match the luminescent power of the semiconductor laser  16  unaffected by any fluctuation of the I-L characteristic of each DAC or the semiconductor laser  16 , but this embodiment is applicable even without using such a slope control DAC  12 . The output current of the power control DAC  14  is added to the output current supplied from the automatic reproduction power control circuit  7  to the semiconductor laser  16  via the amplifier  11 .  
         [0039]    Reference numeral  19  denotes a thermistor, which detects the temperature of the semiconductor laser  16 , and  36 , a voltage converting circuit, which converts the output signal of the thermistor  19  into a voltage signal  37 . The voltage signal  37  is entered into the arithmetic and control circuit  3 , and used for the selection of the method for computation of the semiconductor laser drive current by the arithmetic and control circuit  3 .  
         [0040]    [0040]FIG. 8 shows the fitting position of the thermistor  19 . The thermistor  19 , fitted near the semiconductor laser  16  of the optical pickup  23 , detects the temperature of the semiconductor laser  1 . FIG. 9 is a circuit diagram illustrating the configuration of the thermistor  19  and a voltage converting circuit  36 . A resistor R 0  of the voltage converting circuit  36  and the thermistor  19  are connected in series, and applies a constant voltage Vo. As the internal resistance Rs of the thermistor  19  varies with a temperature change, the potential at a point Q varies. The potential at point Q is amplified by an operational amplifier  55  and another amplifier using a resistor R 1  and a resistor R 2  and turned into the voltage signal  37 .  
         [0041]    The method for collection of laser power in the first embodiment of the invention will now be described in detail with reference to FIG. 2 and FIG. 3. In FIG. 2A, showing the variation of the luminescent power of the semiconductor laser  16  over time, the horizontal axis  41  represents time, the longitudinal axis  42 , the luminescent power, and the solid line  43 , the luminescent power of the semiconductor laser  16 . In this embodiment, two of the recording power registers  10  are used to form the multi-pulse represented by the solid line  43  in FIG. 2A. Here it is supposed that Pa-reg is responsible for the luminescent power at P 1 , and Pb-reg, for that at P 2 . The luminescent power of the semiconductor laser  16  is higher in the order of P 2  and P 1  at the time of reproduction, wherein P 2  and P 1  respectively correspond to erasion power and recording power for a changeable-phase optical disk. In FIG. 2B, showing the variations of the powers detected by the peak detecting circuit  5  and the bottom detecting circuit  6  over time, the horizontal axis  41  represents time as in FIG. 2A, the longitudinal axis  44 , detection power, the solid line  45 , the output signal of the peak detecting circuit  5 , and the solid line  46 , the output signal of the bottom detecting circuit  6 . The peak detecting circuit  5 , consisting of a circuit for detecting the peak level of the voltage signal supplied by the I-V converting circuit  18 , can detect the P 1  power of the multi-pulse  43  shown in FIG. 2A. On the other hand, the bottom detecting circuit  6 , consisting of a circuit for detecting the bottom level of the voltage signal supplied by the I-V converting circuit  18 , can similarly detect the P 2  power.  
         [0042]    [0042]FIG. 3 is a graph showing the relationship between values set by the recording power register  10  and by the luminescent power of the semiconductor laser  16 . The horizontal axis  51  represents the value set by the recording power register  10 , the longitudinal axis  52 , the luminescent power of the semiconductor laser  16 , and the straight line F(N), the luminescent power of the semiconductor laser  16  in the vicinity of the room temperature. Point α is the point of operation by the automatic reproduction power control circuit  7  alone, where the semiconductor laser  16  is emitting light at reproduction power Pα with all the recording power registers  10  at a zero value.  
         [0043]    Point α indicates that the functioning of the automatic reproduction power control circuit  7  keeps reproduction power Pα constant all the time even when the temperature of the semiconductor laser  16  varies. A W 1  is set in Pa-reg of the recording power registers  10 , a value W 2  in Pb-reg of the same, and a value Da in the slope control DAC  12 . The value of the power control DAC  14  is Dp 1 =W 1 /Da when Pa-reg is selected, or Dp 2 =W 2 /Da when Pb-reg is selected, by the switching circuit  13 . The power control DAC  14  supplies a current matching Dp 1  or Dp  2 , the adder  30  adds a current matching reproduction power Pα, and the semiconductor laser  16  emits luminescent power at P 1  or P 2 . As stated above, the peak detecting circuit  5  and the bottom detecting circuit  6  detect luminescent powers P 1  and P 2 , respectively.  
         [0044]    The first embodiment of the invention is characteristic in that the method for recording power correction is altered in response to the output signal  37  of a temperature detecting means by the thermistor  19  and the voltage converting circuit  36  in FIG. 1. This correcting method will be described below. First, when the temperature of the semiconductor laser  16  is not higher than a certain level, the I-L characteristic can sufficiently approximated by a linear function, and therefore the line F(N) is computed by the arithmetic and control circuit  3  from the output of the peak detecting circuit  5  and the two points of reproduction power Pα kept by the automatic reproduction power control circuit  7 , and the value Da of the slope control DAC  12  is corrected to Da′. In this case, since reproduction power Pα is fixed in the arithmetic and control circuit  3 , the only variable used in the computation is the output from the peak detecting circuit  5 . Thus, the use of the automatic reproduction power control circuit provides the advantage that only one datum suffices where two data would otherwise be needed, and only two are sufficient even at a high temperature where three would be usually required. To add, where no automatic reproduction power control circuit is used, obviously the present invention can still be applied though a greater number of data would have to be measured.  
         [0045]    On the other hand, if the temperature rises and the I-L characteristic becomes curved, the correction of recording power is altered to a two-step method. Curve F(N′) in FIG. 3 shows the relationship of the recording power registers  10  versus luminescent power before the correction of recording power when the semiconductor laser  16  is high. The first step of correction is to correct the value of the slope control DAC  12  according to detection power P 1 ′ of the peak detecting circuit  5  and target power P 1 . Thus, the value of the slope control DAC  12  is corrected to Da′ so at to adjust the detection power at the peak detecting circuit  5  to P 1 . The result of this first step of correction is curve F(T). As is evident from the graph, the luminescent power matching W 1  set by the recording power register Pa-reg is P 1  and there is no problem about it, but the luminescent power matching W 2  set by the recording power register Pb-reg is P 2 T, far deviating from P 2 , which is the desired power. Then, as the second step of correction, the value of the recording power register Pb-reg is corrected according to detection power P 2 T of the bottom detecting circuit  6  and target power P 2 . Thus the value of Pb-reg is corrected from W 2  to W 2 T so as to adjust the detection power at the bottom detecting circuit  6  to P 2 . The quantity of correction can be, for instance, W 2 T=W 2 ×P 2 /P 2 T. Thus, the presence of the temperature detecting means consisting of the thermistor  19  and the voltage converting circuit  36  makes possible recording power correction matching the temperature of the semiconductor laser, which is impossible according to the prior art. Where the temperature of the semiconductor laser is not higher than a certain level, high speed operation is made possible by causing only peak detection to function, and even where the temperature of the semiconductor laser is high enough to give the I-L characteristic a curved shape, recording power can be controlled accurately.  
         [0046]    Although this embodiment of the invention has been described, for the sake of simplicity of description, with reference to a case in which the multi-pulse is formed of two levels of power, obviously the invention can as well be applied where it is formed of three or more levels of power.  
         [0047]    Further, though the temperature detecting means referred to in the foregoing description consists of the thermistor  19  and the voltage converting circuit  36 , a thermal sensor IC, in which a thermal detection element and an electronic circuit for supplying its detection signal are integrated, can as well be used. For instance, a thermal sensor IC named LM 20  is commercially available from National Semiconductor Corporation.  
         [0048]    [0048]FIG. 10 illustrates mounting onto an optical pickup using a thermal sensor IC. Reference numeral  23  denotes an optical pickup,  63 , a two-dimensional lens actuator,  26 , a focusing lens and  64 , a shaft for shifting the optical pickup  23  in the radial direction of the optical disk. A thermal sensor IC  61  and the semiconductor laser  16 , after being soldered onto an optical head board  62 , which is a flexible electronic circuit board, is closely attached to the housing of the optical pickup  23 . As the heated generating from the semiconductor laser  16  is transmitted to the thermal sensor IC  61  through the housing of the optical pickup  23 , the thermal sensor IC  61  can accurately detect the temperature of the semiconductor laser  16 .  
         [0049]    To add, instead of arranging the thermistor  19  and the thermal sensor IC  61  near the semiconductor laser  16  as the temperature detecting means, it is also possible to infer the temperature from any variation in detection power P 1  at the peak detecting circuit  5 .  
         [0050]    Next will be described a second preferred embodiment of the present invention, which is an optical disk device characterized in that the temperature is inferred from any variation in the output signal of the peak detecting circuit. FIG. 12 is a partial block diagram of the optical disk device, which is the second embodiment. In this optical disk device, which is the second preferred embodiment of the invention, a sampling holding circuit  65  and an operational amplifier  66  are used as the temperature detecting means in place of the thermistor  19  and the thermal sensor IC  61  shown in FIG. 1. Other constituent parts are the same as their respective counterparts in the optical disk device  1 , which is the first embodiment of the invention described with reference to FIG. 1, and assigned the same reference numerals as in FIG. 1. The sampling holding circuit  65 , in response to a the timing signal  67  supplied by the arithmetic and control circuit  3 , takes in a voltage signal supplied by the peak detecting circuit  5 , and holds its value until the next timing signal is issued. A differential amplifier composed of the operational amplifier  66 , a resistor R 5  and a resistor R 6  computes difference between a voltage signal supplied by the peak detecting circuit  5  and the voltage signal held by the sampling holding circuit  65 , and supplies a signal  37 .  
         [0051]    For instance, immediately after the start of recording of information onto the optical disk  2 , the arithmetic and control circuit  3  issues the timing signal  67 , and the sampling holding circuit  65  takes in a voltage signal supplied by the peak detecting circuit  5  and holds its value. Immediately after the start of recording, the voltage signal supplied by the peak detecting circuit  5  and the voltage signal held by the sampling holding circuit  65  are equal, and the signal  37  is at the zero level. As the recording of information is continued, the temperature of the semiconductor laser  16  rises and the laser power supplied by the semiconductor laser  16  drops. Then, as the level of the voltage signal supplied by the peak detecting circuit  5  drops, the level of the signal  37  supplied by the operational amplifier  66  varies. Accordingly, the temperature change in the semiconductor laser  16  can be detected from the variation in the signal  37 .  
         [0052]    A third preferred embodiment of the present invention will now be described with reference to FIG. 4 and FIG. 5. The third embodiment has the same configuration as the optical disk device  1 , which is the first embodiment of the invention, described with reference to FIG. 1, except that the arithmetic and control circuit  3  has a microcomputer and programs to be executed by the microcomputer embodies ingenuity. Therefore, the description the configuration and the constituent parts of the optical disk device will be dispensed, with and the constituent parts will be referred to by the same numerals as their respective counterparts in FIG. 1.  
         [0053]    [0053]FIG. 4 and FIG. 5 are flowcharts of a program to be executed by the microcomputer possessed by the arithmetic and control circuit  3  of the optical disk device effectively utilizing, the temperature detecting means. FIG. 4 and FIG. 5 are flow charts of one program, the flows in the two charts being connected by a terminal A numbered  700 . First, the power supply to the optical disk device  1  is turned on ( 701 ) to perform an initial learning operation ( 702 ). The initial learning operation includes gain offset learning by a servo means and test writing in search of the optimal recording power. Next, a measurement mode  2  is set to detect the bottom power of the multi-pulse recording waveform ( 703 ) to take in initially learned values P 2  ( 704 ). Similarly, a measurement mode  1  is set to detect the peak power of the multi-pulse recording waveform ( 705 ) to take in initially learned values P 1 . Then the optical disk device is placed in a check operation state ( 707 ). If a reproduction request comes from a superior unit or elsewhere, reproduction is performed, followed by a return to the check operation state ( 707 ). If an end request comes, the power supply is turned OFF ( 708 ).  
         [0054]    If a recording request comes, data are taken in with the measurement mode  1  to detect the peak power kept as it is while recording is carried out ( 709 ). The data taken in are compared with the previous values in the measurement mode  1  (if it is the first time, compared with the initial learned values P 1 ) ( 710 ), and if the variance is not more than 5%, a return to the check operation state follows ( 707 ). Or if the variance is greater than 5%, the value of the slope control DAC is corrected ( 711 ). Then the temperature is measured with the temperature detecting means ( 712 ) and if it is no higher than 60° C. for instance, a return to the check operation state follows. Or if it is over 60° C., it is judged that the I-L characteristic may have deviated from a straight line and become curved, the detector is switched over to the mode  2  for detecting the bottom power ( 713 ) to take in data in the measurement mode  2  ( 714 ). The data taken in are compared with the previous values in the measurement mode  2  (if it is the first time, compared with the initial learned values P 2 ) ( 715 ), and if the variance is not more than 5%, the detector is switched over to the measurement mode  1  ( 711 ), followed by a return to the check operation state ( 707 ). Or if the variance is greater than 5%, the value of the recording power register is corrected ( 716 ), and the detector is switched over to the measurement mode  1  ( 711 ), followed by a return to the check operation state ( 707 ). If the temperature subsequently drops to 60° C. or less, correction in the mode  1  alone is performed.  
         [0055]    The presence of this temperature detecting means makes it possible, at a temperature where the I-L characteristic can be approximated by a linear function with no problem, recording power can be corrected at high speed by linear approximation with the detector fixed to the measurement mode  1  or, at a temperature where linear approximation of the I-L characteristic would expand the error and invite faulty recording, the recording power correction method according to the invention can be applied to accomplish accurate          recording power correction while switching over between the measurement mode  1  and the measurement mode  2 . Incidentally, the variance rate of acquired data and the temperature at which a switch-over to the mode  2  is to take place (5% and 60° C., respectively) can obviously be selected as desired. It is also conceivable to enhance the S/N ratio of data acquisition by taking in and averaging data a number of times and performing correction according to the averaged data.  
         [0056]    To add, instead of arranging the thermistor  19  and the thermal sensor IC  61  near the semiconductor laser  16  as the temperature detecting means, it is also possible to infer the temperature from the value of the slope control DAC  12 .  
         [0057]    [0057]FIG. 11 illustrates a fourth preferred embodiment of the invention, which is an optical disk device characterized in that the temperature is inferred from value of the slope control DAC  12 . This fourth embodiment of the invention has the same configuration as the optical disk device  1 , which is the first embodiment of the invention, described with reference to FIG. 1. However, neither the thermistor  19  and the voltage converting circuit  36  nor the thermal sensor IC  61  is needed. Further, as the fourth embodiment has common parts to the third embodiment where it was described with reference to FIG. 4, their illustration and description are dispensed with.  
         [0058]    [0058]FIG. 11, like FIG. 5, is a flow chart of a program to be executed by the microcomputer possessed by the arithmetic and control circuit  3 , and shows a continuation from the terminal A numbered  700  in FIG. 4. In this embodiment, instead of using the thermistor  19  and the thermal sensor IC  61  as the temperature detecting means, if the results of measurement of peak power at step  709  are compared with the previous values in the measurement mode  1  (if it is the first time, compared with the initial learned values P 1 ) ( 710 ) and the variance is found greater than 5%, it is judged that the I-L characteristic may have deviated from a straight line and become curved, and the detector is switched over to the mode  2  in which the bottom power is detected by step  713  to take in data in the measurement mode  2  by step  714 .  
         [0059]    As a semiconductor laser differs in the performance of radiation of heat generated by the laser chip with the material and structure of the package for mounting the laser chip, the I-L characteristic of the semiconductor laser  16  may deviate from a straight line and become curve even if its temperature has not reached 60° C. Therefore, the method of this embodiment to infer the variation in the I-L characteristic according to the variation in peak power measured at step  709 , instead of using the thermistor  19  and the thermal sensor IC  61  as the temperature detecting means, can prove effective.  
         [0060]    According to the present invention, there can be realized semiconductor laser luminescent power control unit capable of accurately controlling the luminescent power of the semiconductor laser in an optical recording/reproduction device, such as an optical disk device wherein information is recorded or reproduced optically by focusing a laser beam emitted from a laser beam source into a minute light spot and irradiating therewith an optical disk, which is an information recording medium, even when the surroundings of the semiconductor laser are heated to a high temperature.  
         [0061]    The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended rather than by the foregoing description and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.