Patent Publication Number: US-2003235128-A1

Title: Information recording and reproducing apparatus, and information recording method

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] This invention relates to a technique for recording information on an optical disc using a laser beam or other means.  
       [0003] 2. Description of Related Art  
       [0004] Onto a recordable or rewritable optical disc such as a DVD-R (DVD-Recordable) or a DVD-RW (DVD-Rerecordable), information is recorded thereon by irradiating a laser beam on its recording surface. At the areas on the recording surface of the optical disc where the laser beam is irradiated, the property of the optical recording medium forming the optical disc is physically changed because of the increased temperature. This produces recording marks on the recording surface.  
       [0005] Namely, the laser beam is modulated by recording pulses having time widths corresponding to information to be recorded, so that the laser pulses having lengths corresponding to information to be recorded are generated and irradiated on the optical disc. Thus, recording marks having lengths corresponding to the information to be recorded can be formed on the optical disc.  
       [0006] One approach recently used is to form a recording mark by a pulse train having a plurality of short pulses, rather than by a single laser pulse. This approach, called “write strategy”, introduces less heat accumulation on the recording surface of the optical disc compared to the approach irradiating a single recording laser pulse. Therefore, uniform temperature distribution can be achieved on the recording surface on which the recording marks are formed. This can prevent undesired teardrop-shaped recording marks from being formed, and enables the formation of the recording marks of preferred shape.  
       [0007] In the case of DVD-R, for example, the recording pulse train consists of a plurality of pulses which magnitudes varying between a certain read power level and write power level. That is, based on recording data, the areas on the recording surface of the optical disc where no recording marks are to be formed (referred to as “space periods” hereafter) are irradiated with the laser beam of the read power. The areas on the recording surface of the optical disc where recording marks are to be formed (referred to as “mark periods” hereafter) are irradiated with the laser beam of the power corresponding to the recording pulse train having magnitudes varying between the read power and the write power. Consequently, the recording marks are formed on the recording surface. In the case of DVD-RW, the recording pulse train has a waveform which magnitudes varying between three power levels, i.e., the read power level, the erase power level for erasing recording mark already formed, and the write power level.  
       [0008] In the case that the magnitudes of the recording pulses vary between the read power level and the write power level, or between the read power level, the erase power level and the write power level, the laser beam is irradiated on the disc surface with the large power for recording when the recording pulse is at the write power level, and the temperature of the disc surface increases. However, when the recording pulse is at the read power level, the laser beam is irradiated on the disc surface with reading power, and the temperature of the disc surface decreases for some degrees. Thus, the recording pulse train according to the write strategy is designed to irradiate the laser beams with the write power and the read power alternately to repeat heating and cooling, thereby preventing the temperature of the disc surface from becoming so high due to the heat accumulation or else.  
       [0009] However, even if the power of the laser beam is switched between the read power level and the write power level (or further between the erase power level in the case of DVD-RW) as described above, the laser beam is still being irradiated on the disc during the period of the read power level. The temperature increase of the disc surface can be suppressed to some extent, compared with the continuous irradiation of the laser beam of write power level because the read power level is much lower than the write power level. However, the heat accumulation and the heat distribution on the disc surface are very important problems in order to maintain the desired recording accuracy, and it is further desired to effectively suppress the heat accumulation and the heat distribution at the time of recording information by using the recording pulse train.  
       SUMMARY OF THE INVENTION  
       [0010] It is an object of the present invention to provide an information recording and reproducing apparatus and an information recording method capable of suppressing the influence of the heat accumulation and the heat distribution to improve the recording accuracy in case of recording information by using the write strategy of so-called multi-pulse type which includes a plurality of pulses and uses a bias level.  
       [0011] According to one aspect of the present invention, there is provided an information recording and reproducing apparatus which irradiates a laser light onto a storage medium to form recording marks corresponding to recording data, including: a light source which emits the laser light; and a control unit which drives the light source based on the recording data to irradiate laser pulses onto the storage medium, wherein, at a time of recording, the control unit controls a level of the laser pulse to vary between a write power level necessary to form the recording mark and a bias level during a mark period of the recording data, and wherein the bias level is lower than a read power level necessary to reproduce the recording data from the storage medium.  
       [0012] The above information recording and reproducing apparatus irradiates the laser light onto a storage medium such as an optical disc, e.g., DVD-R or DVD-RW, and forms recording marks on the storage medium, thereby recording information. In addition, by using the reflected light from the optical disc, the recorded information is reproduced. Specifically, the information recording and reproducing apparatus drives the light source based on the recording data to irradiate the laser pulses on the storage medium and forms the recording marks. The level of the laser pulse is varied between the write power level necessary to form the recording marks and the bias level during the mark periods. Since the bias level is set to be lower than the read power level necessary to reproduce recording marks, the quantity of the laser light irradiated onto the storage medium during the recording may be reduced, and the adverse effect due to the heat accumulation or the heat distribution can be diminished. As a result, the recording marks of appropriate shape can be formed on the storage medium, and the information recording accuracy can be improved.  
       [0013] In the above information recording and reproducing apparatus, the bias level may be a level at which the light source does not emit the laser light. Thus, the laser light is irradiated on the storage medium only in the period in which the level of the laser pulse is at the write power level necessary to form the recording mark. Therefore, unnecessary laser light is not irradiated on the storage medium, and the heat accumulation may further be diminished.  
       [0014] According to an embodiment of the information recording and reproducing apparatus, the recording data may include, within the mark period, multi-pulse periods in which the level of the laser pulse varies between the write power level and the bias level. In this case, in the case of DVD-R, for example, the level of the laser can be lowered to the bias level independently for each laser pulse, and hence the heat accumulation by irradiating the laser light can be suppressed.  
       [0015] The control unit may set the level of the laser to the read power level at a time of reproduction and set the level of the laser to a level lower than the read power level during a space period of the recording data at the time of recording. By this, the irradiation quantity of the laser light can be reduced not only in the mark periods, but also in the space periods, and hence the heat accumulation can be reduced further.  
       [0016] In another embodiment of the information recording and reproducing apparatus, the recording data may include, within the mark period, a period in which the level of the laser pulse is at an erase power level necessary to erase a recorded mark and a multi-pulse period in which the level of the laser pulse varies between the write power level and the bias level. In this embodiment, in the case of DVD-RW, for example, the level of the laser can be lowered to the bias level independently for each laser pulse, and hence the heat accumulation by irradiating the laser light can be suppressed.  
       [0017] The recording data may include a cooling period of a certain time length within a space period and immediately after the preceding mark period, and the level of the laser pulse during the cooling period may be equal to the bias level. By this, the irradiation quantity of the laser light can be reduced even after the mark period, and the ending edge of the recording mark may be made clear.  
       [0018] In another embodiment of the information recording and reproducing apparatus, the recording data may include, within the mark period, a plurality of smaller periods having different levels including the write power level and the bias power level, and the control unit may control the level of the laser pulse independently for the respective smaller periods. In this case, regardless of the write strategy used for the recording, the mark period is divided into plural periods and the level of the laser is adjusted for each period. Therefore, the irradiation quantity of the laser light can be precisely controlled.  
       [0019] In a preferred embodiment, the light source may be a laser diode, the bias level may be an output level of the laser light emitted by the light source when a bias current is supplied to the light source, and the bias current may be larger than a threshold current of the laser diode. In such a case, the current to be supplied to the laser diode and the respective levels of the laser pulse can be defined within the linear area of the output characteristic of the laser diode, and the level of the laser pulse may be stabilized.  
       [0020] According to another aspect of the present invention, there is provided an information recording method which irradiates a laser light onto a storage medium to form recording marks corresponding to recording data, including the step of driving the light source based on the recording data to irradiate a laser pulse on the storage medium, wherein the driving step controls, at a time of recording, a level of the laser pulse to vary between a write power level necessary to form the recording mark and a bias level during a mark period of the recording data, and wherein the bias level is lower than a read power level necessary to reproduce the recording data.  
       [0021] The above information recording and reproducing method irradiates the laser light onto a storage medium such as an optical disc, e.g., DVD-R or DVD-RW, and forms recording marks on the storage medium, thereby recording information. Specifically, the light source is driven based on the recording data to irradiate the laser pulses on the storage medium and forms the recording marks. The level of the laser pulse is varied between the write power level necessary to form the recording marks and the bias level during the mark periods. Since the bias level is set to be lower than the read power level necessary to reproduce recording marks, the quantity of the laser light irradiated onto the storage medium during the recording may be reduced, and the adverse effect due to the heat accumulation or the heat distribution can be diminished. As a result, the recording marks of appropriate shape can be formed on the storage medium, and the information recording accuracy can be improved.  
       [0022] The nature, utility, and further features of this invention will be more clearly apparent from the following detailed description with respect to preferred embodiment of the invention when read in conjunction with the accompanying drawings briefly described below. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0023]FIGS. 1A and 1B are waveform diagrams each showing a recording waveform at the time of a recording control according to an embodiment of the present invention;  
     [0024]FIGS. 2A and 2B are waveform diagrams each showing a recording waveform of a comparative example;  
     [0025]FIG. 3 is a block diagram showing a schematic configuration of an information recording/reproducing apparatus according to the present invention;  
     [0026]FIG. 4 is a block diagram showing the recording control unit shown in FIG. 3;  
     [0027]FIG. 5 is a graph showing a laser output characteristic of a laser diode; and  
     [0028]FIGS. 7A and 7B are waveform diagrams each showing a recording pulse waveform according to modifications of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0029] The preferred embodiments of the present invention will now be described below with reference to the attached drawings.  
     Recording Pulse Waveform  
     [0030] The present invention is characterized in that the bias level is set to be lower than the read power level within the portion in which the recording mark is formed by the laser beam irradiation (hereinafter referred to as “mark period”) in the recording pulse waveform according to the write strategy using the bias level, such as so-called multi-pulse type write strategy. Thus, the heat accumulation due to the multi-pulse, for example, is effectively suppressed to enable the formation of recording mark having appropriate shape.  
     [0031]FIGS. 1A and 1B show the recording pulse waveforms according to the present invention. FIG. 1A shows the recording pulse waveform for a DVD-R as the recording medium, and FIG. 1B is the recording pulse waveform for a DVD-RW for the recording medium.  
     [0032] As shown in FIG. 1A, the recording data has the mark periods and the space periods (in which no recording mark is formed). FIG. 1A shows the 8T mark period. The recording pulse waveform used in the present invention includes, within the mark period, one top pulse and individual pulses of the number corresponding to the length of the mark period. For example, the mark period of the recording pulse waveform corresponding to 3T mark only includes one top pulse. The mark period corresponding to 4T mark includes one top pulse and one individual pulse, and the mark period corresponding to 5T mark includes one top pulse and two individual pulses. The 8T mark period shown in FIG. 1A includes one top pulse  60  and five individual pulses  61 . It is noted that the portion including the top pulse  60  and the plural individual pulses  61  will be here in after referred to as “multi-pulse portion  62 ”.  
     [0033] The power level of the recording pulse waveform takes one of the laser-off level Po (the laser output power =0) when the laser output is switched off, the bias level Pb, the read power level Pr and the write power level Pw. In the present invention, the level of the multi-pulse portion  62  is varied between the bias level Pb and the write power level Pw during the mark period of the recording pulse waveform, and the bias level Pb is set to be lower than the read power level Pr. Namely, as shown in FIG. 1A, during the mark period, the respective pulses  60  and  61  constituting the multi-pulse portion  62  vary between the bias level Pb lower than the read power level Pr and the write power level Pw.  
     [0034] For the sake of comparison, FIG. 2A shows a general example of multi-pulse type recording waveform. The levels of the respective pulses  60  and  61  in the mark period vary between the read power level Pr and the write power level Pw.  
     [0035] As described above, in the present invention, the level of the recording pulse waveform in the mark period becomes the bias level, which is lower than the read power level Pr, in the portion other than the portion of the write power level Pw, and hence the problem such as the heat accumulation may be reduced.  
     [0036]FIG. 1B shows the example in which the present invention is applied to the recording pulse waveform for DVD-RW. In the case of DVD-RW, the level of the recording pulse waveform is maintained at the erase power level Pe by an APC (Automatic Power Control) in the portion other than the multi-pulse portion  62  within the mark period and the space period expect the portion  66 . On the other hand, in the multi-pulse portion  62  in the mark period, similarly to the above-mentioned DVD-R, the level of the recording pulse waveform varies between the bias level Pb and the write power level Pw.  
     [0037] For the sake of comparison, a general example of a recording pulse waveform of DVD-RW is shown in FIG. 2B. In the example shown in FIG. 2B, the level of the recording pulse waveform varies between the read power level Pr and the write power level Pw in the multi-pulse portion  62  in the mark period. Thus, also in the case of DVD-RW, the adverse effect of the heat accumulation and the heat distribution to the formation of the recording mark may be diminished by varying the level of the recording pulse waveform in the multi-pulse portion  62  between the write power level Pw and the bias power level Pb which is lower than the read power level Pr.  
     [0038] It is noted that, while the bias level Pb is set to a certain intermediate level between the laser-off level Po (i.e., the laser output power =0) and the read power level Pr, this is merely an example of determination of the bias level Pb. In the present invention, the level of the recording pulse waveform is varied between the bias level Pb and the write power level Pw in the multi-pulse portion  62  of the mark period, and hence the adverse effect of the heat accumulation can be diminished if the following condition is satisfied:  
     [0039] Bias level Pb &lt;Read power level Pr.  
     [0040] Also, in theory, the bias level Pb may be equal to the laser-off level Po, and the level of the recording pulse waveform in the multi-pulse portion  62  in the mark period may be varied between the laser-off level Po and the write power level Pw. The method of determining the bias level Pb will be described later.  
     [0041] In the example of the recording pulse waveform of DVD-R shown in FIG. 1A, the recording pulse waveform is maintained at the read power level Pr in the space period. However, as shown by the arrow  65 , the level of the recording pulse waveform in the space period may be lowered near the bias level Pb. By this, the quantity of heat given to the optical disc after the mark period is reduced, and it is expected that the shape of the recording mark, particularly the shape of its end portion, becomes appropriate. It is also expected that, if the space period is short, the shape of the following recording mark, particularly the shape of its leading portion, becomes appropriate.  
     [0042] In the case of DVD-RW shown in FIG. 1B, a cooling pulse may be provided immediately after the mark period (i.e., at the head of the space period). In that case, the level of the cooling pulse may also be set to be equal to the bias level Pb, not to the be equal to the read power level Pr (see. the portion  66 ).  
     Configuration of Information Recording-Reproducing Apparatus  
     [0043] Next, a configuration of an information recording and reproducing apparatus to which the invention is applied will be described. FIG. 3 schematically shows an entire configuration of the information recording and reproducing apparatus according to the embodiment of the invention. The information recording and reproducing apparatus  1  records information on an optical disc D and reproduces information from the optical disc D. For example, the optical disc D may be a CD-R (Compact Disc-Recordable) and a DVD-R for recording only once, and a CD-RW (Compact Disc-Rewritable) and a DVD-RW that allow for repeated erasing and recording of information.  
     [0044] The information recording and reproducing apparatus  1  includes an optical pickup  2  for irradiating a recording beam and a reproduction beam to the optical disc D, a spindle motor  3  for controlling rotation of the optical disc D, a recording control unit  10  for controlling recording of information on the optical disc D, a reproduction control unit  20  for controlling reproduction of information recorded on the optical disc D, and a servo control unit  30  for various kinds of servo controls. The servo controls include a spindle servo for controlling rotation of the spindle motor  3 , and a focus servo and tracking servo for controlling a relative position of the optical pickup  2  to the optical disc D.  
     [0045] The recording control unit  10  receives recording data. Then, according to processing to be described later, the recording control unit  10  generates a driving signal SD for driving a laser diode in the optical pickup  2  and supplies driving signal S D  to the optical pickup  2 .  
     [0046] The reproduction control unit  20  receives a read-out signal Srf output from the optical pickup  2  and performs predetermined processing on read-out signal Srf, such as demodulation and decoding, to generate and output reproduction data.  
     [0047] The servo control unit  30  receives the read-out signal Srf from the optical pickup  2 . Based on the signal Srf, the servo control unit  30  supplies a servo signal S 101  such as a tracking error signal and a focus signal to the optical pickup  2  and supplies a spindle servo signal S 102  to the spindle motor  3 . Thus, various kinds of servo processing are performed, such as the tracking servo, the focus servo, and the spindle servo.  
     [0048] Since the invention mainly relates to the recording operations in the recording control unit  10  and various known methods can be applied to the reproduction control and the servo control, these controls will not be described in detail.  
     [0049] In addition, although FIG. 1 illustrates the information recording and reproducing apparatus as an embodiment of the invention, the invention may also be applied to an information recording apparatus dedicated to recording.  
     [0050]FIG. 4 shows an internal configuration of the optical pickup  2  and the recording control unit  10 . As shown in FIG. 4, the optical pickup  2  includes a laser diode (LD)  11  that generates the recording beam for recording information on the optical disc D and the reproduction beam for reproducing information from the optical disc D. The optical pickup  2  also includes a front monitor diode (FMD)  16  that receives the laser beam emitted by the laser diode  11  and outputs a laser power level signal S 10  corresponding to the laser beam.  
     [0051] The optical pickup  2  further includes known components, which will not be shown or described in detail. These components include a photo-detector for receiving a reflection beam of the reproduction beam reflected from the optical disc D and generating the read-out signal Srf, and an optical system for guiding the recording and reproduction beams and the reflection beam to appropriate directions.  
     [0052] The recording control unit  10  includes a laser diode (LD) driver  12 , an APC (Automatic Power Control) circuit  13 , a sample-and-hold (S/H) circuit  14 , a controller  15 , and a buffer  17 .  
     [0053] The LD driver  12  supplies the laser diode (LD)  11  with a current corresponding to the recording data and causes information to be recorded on the optical disc D. As shown in FIG. 4, the LD driver  12  includes a voltage-to-current (V/I) converter  121 , an interface (I/F)  122 , D/A converters  123  and  124 , drivers  125  and  126 , and switches SW 1  and SW 2 .  
     [0054] The sample-and-hold circuit  14  samples and holds the level of the laser power level signal S 10  at the timing prescribed by a sample-and-hold signal S 5 .  
     [0055] The APC circuit  13  controls the power of the laser diode  11  based on a signal S 11  output from the sample-and-hold circuit  14 . Specifically, in the case of DVD-R, the APC circuit  13  works the LD driver  12  so that the read power level of the laser beam emitted by the laser diode  11  is maintained constant. In the case of DVD-RW, the sample-and-hold circuit  14  controls the LD driver  12  so that the erase power level of the laser beam is maintained constant.  
     [0056] The controller  15  mainly performs recording operation and APC control. As shown in FIG. 4, the controller  15  includes a switching controller  151 , a recording level controller  154 , and an APC controller  155 .  
     [0057] The switching controller  151  includes a write pulse generator  152  and an inverter circuit  153 , and generates switching signals S 1  and S 2  for the switches SW 1  and SW 2  in the LD driver  12  based on the recording data input to the controller  15 .  
     [0058] The recording level controller  154  generates a recording level signal S 3  for determining a power level such as the read power level, the write power level, or the erase power level in cooperation with the APC circuit  13 . The recording level controller  154  then supplies the recording level signal S 3  to the I/F  122  in the LD driver  12 .  
     [0059] The APC controller  155  generates an APC target value S 4  which is a target value for servo control performed by the APC loop and supplies it to the APC circuit  13 . The APC controller  155  also supplies the sample-and-hold circuit  14  with a sample-and-hold signal S 5  that indicates the sampling and holding timings of the sample-and-hold circuit  14 .  
     [0060] Although the above configuration uses the sample-and-hold circuit  14  to form the APC loop, a bottom hold circuit may be used instead of the sample-and-hold circuit  14 . In that case, the APC servo may be performed by using a bottom value of the laser power level signal S 10  output from the front monitor diode  16 .  
     Recording/Reproduction Operations  
     [0061] Next, recording control performed by the recording control unit  10  shown in FIG. 4 using the optical pickup  2  will be described. The recording control unit  10  performs the recording/reproduction control and the APC control.  
     [0062] (I) Recording/Reproduction Control  
     [0063] First, the recording/reproduction control will be described. It is noted that the description is first directed to the case of DVD-R. In the recording operation, the recording level controller  154  in the controller  15  supplies the LD driver  12  with the recording level data S 3  for generating currents I 2  and I 3 . The currents I 2  and I 3  are used to create the read power level Pr and the write power level Pw.  
     [0064] Out of the recording level signal S 3 , data for the current I 2  is supplied to the D/A converter  123  through the I/F  122  in the LD driver  12 . The D/A converter  123  generates a corresponding analog signal, and drives the driver  125  by the analog signal to generate the current I 2  and supply the current I 2  to the switch SW 1 . Out of the recording level signal S 3 , data for the current I 3  is supplied to the D/A converter  124  through the I/F  122  in the LD driver  12 . The D/A converter  124  generates a corresponding analog signal, and drives the driver  126  with the analog signal to generate the current I 3  and supply the current I 3  to the switch SW 2 .  
     [0065] The switching controller  151  in the controller  15  generates switching signals S 1  and S 2  based on the recording data. Specifically, the write pulse generator  152  generates a write pulse signal that consists of a plurality of pulse trains based on the recording data shown in FIG. 1A, and supplies the write pulse signal as the switching signal S 2  to the LD driver  12 . The inverter  153  generates inverted data of the recording data, and supplies it to the LD driver  12  as the switching signal S 1 .  
     [0066] In the LD driver  12 , the current I 1  is being supplied from the V/I converter  121  to the laser diode  11 . As shown in FIG. 1A, the current I 1  defines the bias level of the recording pulse signal.  
     [0067] Referring to FIG. 1A, during the mark period, the switch SW 1  is controlled by the switching signal S 1  obtained by inverting the recording data. Therefore, the switch SW 1  is turned off, and the current I 2  is not supplied to the laser diode  11 . Also, in the mark period, the switch SW 2  is controlled by the switching signal S 2  which is identical to the write pulse signal. Therefore, the switch S 2  is turned on and the current I 3  is supplied to the laser diode  11  intermittently. As a result, as shown in FIG. 1A, during the mark period, the recording pulse waveform is obtained, which level intermittently varies between the bias level (corresponding to the current I 1 ) and the write power level (corresponding to the current I 1 +I 3 ).  
     [0068] On the other hand, during the space period, the write pulse generator  152  generates no write pulse. Therefore, the switch SW 2  is kept turned off and the current I 3  is not supplied to the laser diode  11 . The switching signal S 1  obtained by inverting the recording data switches the switch SW 1  so that the switch SW  1  is kept turned on during the space period, thereby allowing the current I 2  to be supplied to the laser diode  11 . Thus, as shown in FIG. 1A, the write pulse signal is maintained at the read power level Pr (corresponding to the current I 1 +I 2 ) during the space period.  
     [0069] At the time of reproduction, the current I 1 +I 2  is always supplied to the laser diode  11 , similarly to the space period, and the recording pulse signal is maintained at the read power level Pr. This allows the recording data to be reproduced.  
     [0070] When the read power level Pr in the space period at the time of recording is set to be lower than the read power level Pr at the time of reproduction, it is necessary that the read power level Pr at the time of recording is different from the read power level Pr at the time of reproduction. Namely, during the space period at the time of recording, the read power level Pr may be lowered within such a range that the APC servo described later and the forcus/tracking servo control is ensured. However, at the time of reproduction, it is necessary that the read power level Pr is maintained at a normal level so that the reproduction of the recorded data is enabled and the reproduction quality is ensured. Therefore, the APC target value supplied to the APC circuit  13  is different at the time of reproduction and in the space period at the time of recording.  
     [0071] Next, the description will be given to the case of DVD-RW. Also in the case of DVD-RW, the basic operation is the same as the case of the DVD-R. Namely, the recording pulse waveform is maintained at the read power level Pr at the time of reproduction. In the space period at the time of recording, the current I 1 +I 2  is supplied to the laser diode  11 , but the value of the current I 2  is increased to set the recording pulse waveform at the erase power level Pe. In the mark period at the time of recording, the current I 3  is selectively added to the bias current I 1  to generate the recording pulse waveform shown in FIG. 1B.  
     [0072] (II) APC Control  
     [0073] Next, the APC control will be described. The APC control is performed at the time of reproduction and during the space period expect the portion  66  at the time of recording, but is not performed during the mark period at the time of recording. The APC control is performed by the APC loop formed by the laser diode  11 , the front monitor diode  16 , the buffer  17 , the sample-and-hold circuit  14 , the APC circuit  13 , and the V/I converter  121 .  
     [0074] First, the description will be given to the case of DVD-R. In the case of DVD-R, the APC control adjusts the level of the bias current I 1  supplied from the LD driver  12  to the laser diode such that the level of the laser beam emitted by the laser diode  11  is always maintained at the read power level Pr. Namely, out of the space period of the recording data (which is 8-16 modulated and includes the mark period and the space period of the length of 3T to 11T and 14T), in the long space period (e.g., space period of 5T to 11T and 14T) the bias current I 1  from the LD driver  12  is adjusted such that the read power level Pr is constant.  
     [0075] Details of the operations are as follows. The controller  15  generates the recording pulse waveform corresponding to the recording data as described above, and drives the LD driver  12  according to the recording pulse waveform to causes the laser diode  11  to emit the laser beam.  
     [0076] The front monitor diode  16  is located in proximity to the laser diode  11  in the optical pickup  2 . The front monitor diode  16  receives a part of the laser beam emitted by the laser diode  11  to generate the laser power level signal S 10  indicating the level of the laser beam, and supplies the signal S 10  to the sample-and-hold circuit  14  through the buffer  17 .  
     [0077] The sample-and-hold circuit  14  samples the laser power level signal S 10  supplied by the front monitor diode  16  and holds its level for a certain period at the timings given by the sample-and-hold signal S 5  supplied by the APC controller  155  in the controller  15 . The sample-and-hold signal S 5  output from the controller  15  is a pulse signal indicating the period in which the signal for the APC control is to be generated. More specifically, the sample-and-hold signal S 5  is a pulse signal indicating a certain period (a period during which the APC is performed, also referred to as an “APC period” hereafter) in a relatively long space period (e.g., 5T to 11T) in the recording data. Thus, the sample-and-hold circuit  14  samples the level of the laser power level signal S 10  in the APC period in the space period of the recording data, and holds and supplies the sampled level to the APC circuit  13  during the period other than the APC periods.  
     [0078] The APC circuit  13  is supplied with the APC target value S 4  by the APC controller  155  in the controller  15 . The APC target value S 4  indicates the level of the laser beam at which the laser beam is to be maintained by the APC. In the present case, the APC target value S 4  corresponds to the read power level Pr. The APC circuit  13  supplies the control signal S 12  to the V/I converter  121  in the LD driver  12  to maintain the level of the laser power level signal S 10  in the APC periods at a certain level indicated by the APC target value S 4 . The V/I converter  121  converts the voltage indicated by the input control signal S 12  to a current and outputs the bias current I 1 .  
     [0079] During the space period at the time of recording as well as at the time of reproduction, the current I 2  is supplied to the laser diode  11  via the switch SW 1 . Therefore, the laser diode  11  is driven by the current (I 1 +I 2 ) corresponding to the read power level Pr during the space period, and outputs the laser beam at the read power level Pr. If the output level of the laser beam emitted by the laser diode  11  varies due to the temperature variation or other factor, the APC loop functions to vary the bias current I 1  so as to absorb the variation of the laser output level. As a result, during the space period, the recording pulse waveform is constantly maintained at the read power level Pr.  
     [0080] As mentioned above, in the case of DVD-R shown in FIG. 3A, the read power level Pr during the space period can be lowered by reducing the current I 2 . In that case, the APC target value S 4  supplied from the controller  15  to the APC circuit  13  is reduced.  
     [0081] Next, the description will be given to the case of DVD-RW. In the case of DVD-RW, at the time of reproduction, the output level of the laser beam is maintained at the read power level Pr. Therefore, the APC target value supplied to the APC circuit  13  is set to the value corresponding to the read power level Pr, and the APC loop operates such that the level that the laser diode outputs based on the current I 1 +I 2  is equal to the read power level Pr. On the other hand, during the space period at the time of recording, the output level of the laser beam is maintained at the erase power level Pe. In this case, the value of the current I 2  may be set to be larger than that in the case of DVD-R, as mentioned above. The APC target value corresponding to the erase power level Pe is supplied to the APC circuit  13 , and the APC loop operates such that the laser output level is maintained at the erase power level Pe.  
     Determination of Bias Current I 1   
     [0082] Next, the method of determining the bias current I 1  will be studied. FIG. 5 shows the characteristic of the laser diode. In FIG. 5, the horizontal axis represents the drive current I of the laser diode, and the vertical axis represents the laser output power P. The laser diode hardly emit light when the drive current is smaller than the threshold current Ith, and its output power is almost zero. When the drive current I exceeds the threshold current Ith, the output power P increases generally in proportion to the increase of the drive current. The above-mentioned write power level Pw, the erase power level Pe and the read power level Pr are all set within the range in which the output characteristic shown in FIG. 5 is linear, i.e., the range in which the drive current I is sufficiently larger than the threshold current Ith.  
     [0083] As described above, in the present invention, by varying the level of the multi-pulse portion between the bias level Pb and the write power level Pw during the mark period at the time of recording, and by setting the bias level Pb to be lower than the read power level Pr, the adverse effect of the heat accumulation and the heat distribution is suppressed. Therefore, it is necessary that the bias level Pb is set to be lower than the read power level Pr. In view of diminishing the effect of the heat accumulation as much as possible, it is desired that the bias level Pb is set to be as lower as possible. However, if the bias current is lower than the threshold current Ith, the operating point enters the non-linear range of the output characteristic of the laser diode, thereby possibly making the bias level Pb with respect to the bias current I 1  unstable. As understood from FIGS. 1A, 1B and  5 , the write power level Pw, the erase power level Pe and the read power level Pr are all based on the bias level Pb and are all obtained by adding the currents I 2  and I 3  to the bias current I 1 . Therefore, if the bias level Pb becomes unstable, it is likely that those levels also become unstable. In this view, it is desirable that the bias current I 1  is set within the range in which the output characteristic of the laser diode is linear, i.e., the range near the threshold current Ith or larger than the threshold current Ith. It is more desirable that the bias current I 1  is as near to the threshold current Ith as possible.  
     [0084] After the bias current is thus determined, the currents I 2  and I 3  may be determined such that the respective outputs of the write power level Pw, the erase power level Pe and the read power level Pr are obtained as shown in FIG. 5.  
     [0085] It is noted that, when the read power level Pr is lowered during the space period at the time of recording in the case of DVD-R (the read power level at the time of reproduction is not lowered), it is necessary that the APC servo control can work at the read power level at the time of recording thus lowered.  
     Modification  
     [0086] Next, one modification of the present invention will be described. In the above-described embodiment, out of the recording pulse waveform at the time of recording, during the mark period, the multi-pulse is varied between the bias level Pb and the write power level Pw, and the bias level Pb is set to be lower than the read power level Pr. However, it is not always necessary that the bias level Pb is lower than the read power level Pr throughout the mark period. Namely, the mark period may be divided into smaller periods and the recording pulse waveform may be adjusted such that the bias level of each smaller period is different from each other.  
     [0087]FIG. 6A shows an example in which the recording pulse waveform near one mark period is divided into a plurality of smaller periods. In FIG. 6A, the period A represents the period prior to the recording operation, the period B is a part of the preceding space period, and the period C is a period within the mark period and immediately before the multi-pulse portion. The period D is within the multi-pulse portion, and the period E is immediately after the multi-pulse portion. The periods C and E are not limited to the length of the mark to be formed, and may partly or wholly cover the preceding space period or following space period.  
     [0088] Thus, the recording pulse waveform may be divided into a plurality of smaller periods, and the bias level may be lowered independently for each smaller period, or for certain combination of the smaller periods. Further, the bias level may be set to different levels for each smaller period, independently of each other. Still further, the period D may be further be divided into a plurality of periods, for which the bias level may be controlled independently of each other. For example, the period D may be divided into the further smaller periods corresponding to the respective pulses, and the bias level is lowered only for the periods following the top pulse of the multi pulse portion and is not lowered for other periods.  
     [0089] Next, another modification of the present invention will be described. In the above-described embodiment, the present invention is applied to the multi-pulse type recording pulse waveform and the bias level is adjusted. However, the present invention is applicable to the recording pulse wave form according to the write strategy other than the multi-pulse type. FIG. 6B shows an example in which the present invention is applied to the recording pulse waveform having a single pulse within the mark period. In this example, the bias level periods  70  and  72  exist before and after the single pulse  71  in the mark period, and the bias level of those periods  70  and  72  may be set to be lower than the read power level Pr. Further, the bias level in the period  70  may be different from the bias level in the period  72 .  
     [0090] Next, still another modification of the present invention will be described. In the above-described embodiment, the bias level is adjusted for the plurality of mark periods in the same manner. On the contrary, for the recording pulse waveform including mark periods and space periods of various lengths, the bias level may be adjusted with taking the mark length or space length into account. For example, the bias level in the mark period following the short space period such as 3T to 5T may be much lowered in comparison with the bias level in the mark period following the long space period such as equal to or longer than 6T. Also, the bias level may be adjusted only for the mark period before and after the short space period.  
     [0091] As described above, according to the present invention, since the pulse in the mark period is varied between the bias level, lower than the read power level, and the write power level, the adverse effect of the heat accumulation and the heat distribution may be diminished at the time of recording. By this, recording marks of appropriate shape can be formed, and the recording accuracy can be improved.  
     [0092] The invention may be embodied on other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning an range of equivalency of the claims are therefore intended to embraced therein.  
     [0093] The entire disclosure of Japanese Patent Application No. 2002-130490 filed on May 2, 2002 including the specification, claims, drawings and summary is incorporated herein by reference in its entirety.