Abstract:
The mark-length recording system whereby information is recorded by changing the lengths of a recorded portion and an unrecorded or erased portion has a problem that, when new information is recorded in an already recorded region, the newly recorded information may deteriorate in reliability because the length and width of the newly recorded mark are different from those of the previously recorded mark and a part may exist that is not completely erased at the time of overwriting. The invention widens a setting freedom of the recording power and controls the length and width of the recorded mark by making the effective recording pulse length (the length from a rise of a first pulse to a fall of a last pulse) satisfy a relation: (effective recording pulse length)&lt;(recording code length, i.e., the length of data to be recorded)−2T(twice the reference clock cycle).

Description:
FIELD OF THE INVENTION 
   The present invention relates to an information recording and reproducing apparatus for recording and reproducing information on an information recording medium, and more specifically to an information recording method for recording information by a mark-length recording system with the use of laser light, and an information recording apparatus for such an information recording method. 
   BACKGROUND OF THE INVENTION 
   A technology of recording and reproducing information on/from the information recording medium using laser light has already achieved a practical use of an optical disk apparatus etc. As one method for a rewritable optical disk apparatus, there is a phase change type optical disk that utilizes a reversible phase change between crystalline and amorphous states. In order to obtain these two states, light of a high power (recording power) is irradiated on the recording medium, which is heated over a melting point and then is quenched quickly to convert to the amorphous state; light of an intermediate power (erasing power) lying between the above-mentioned high power and a reproducing power is irradiated on the recording medium, which is heated up to the crystallizing temperature and is cooled gradually to convert to a crystalline state. Therefore, overwriting can be performed with light of a single laser. 
   Conventionally, in the recording method based on the above-mentioned light irradiating method, as described in U.S. Pat. No. 5,490,126, U.S. Pat. No. 5,636,194, and the DVD-RAM JIS Standard (120 mm DVD Rewritable Disk JIS X 6243/P86, Attachment H, Definition of Writing Pulse), at least three power levels consisting of the recording power (high power level) for overwriting, the erasing power (medium power level), and the reproducing power for reducing thermal interference (low power level) are used; and the recording mark is classified into three kinds, namely a first part, an intermediate part, and a last part, and, an information is recorded by a recording pulse whose effective length (length from a rise of the first pulse to a fall of the last pulse) corresponds to the length of data to be recorded (recording code length). Further, this recording method adopts a recording method where, in the first part and in the last part, light of the above-mentioned recording power is irradiated for a constant period of time; in the intermediate part, light of the recording power and light of a power smaller than the recording power (erasing power) are switched alternately and irradiated so that the cycle thus created becomes smaller than the above-mentioned constant period of time; and after the light of the recording power irradiated in the last part, light of a power (reproducing power) lower than the erasing power is irradiated for a constant period of time to decrease the thermal interference. 
   SUMMARY OF THE INVENTION 
   In order to increase storage capacity in the next-generation optical disk, it is preferable that the size of a record mark is smaller than that of the present optical disk (for example, DVD-RAM Version 1.0/2.0). However, if the mark whose recording code length is of the order of 0.15 to 0.20 μm is intended to be recorded by a recording method for recording the mark whose recording code length is of the order of 0.42 to 0.615 μm (DVD-RAM Version 1.0/2.0), a mark of a length of the order of 0.21 to 0.28 μm is formed even using an apparatus that features a laser wavelength=0.405 μm and an NA=0.85. That is, there was a problem that a mark about 1.4-times larger than the targeted recording code length is formed. Further, even when the recording is performed using the above-mentioned recording method with reduced laser output, it is difficult to avoid the problem that a mark whose length is 1.4-times larger than the recording code length of the targeted mark is formed. Moreover, if the recording is performed with a laser output that can make the mark agree with the recording code length, there is a problem that a sufficient mark width cannot be obtained, which causes decrease in the signal amplitude at the time of reproducing and hence reduce in the signal-to-noise ratio, and consequently the reliability of the information is impaired and the like. 
   It is the object of the present invention to provide an information recording method whereby a recording operation of recordable optical disk (e.g., DVD-R) or rewritable optical disks (i.e., DVD-RAM and DVD-RW), in both of which the information is recorded with a recording code length shorter than that of the current optical disk, is controlled appropriately and also an information recording apparatus for such an information recording method. 
   In order to attain the above-mentioned object, the recording method according to one aspect of the present invention is a method for recording data on an optical disk by irradiating laser light on the optical disk and forming a mark whose length is an integral multiple of the reference clock length, characterized in that, as irradiation powers, the above-mentioned laser can irradiate the optical disk at any one of four laser powers that satisfy a relation: 
   ti first power&gt;second power&gt;third power&gt;fourth power, 
   and a sum of the lengths of a pulse of the above-mentioned first power and of a pulse of the above-mentioned fourth power at the time of forming the mark is made to be shorter than the length of the mark minus twice the reference clock length. 
   Further, the recording method according to another aspect of the present invention is a method for recording the data on an optical disk by irradiating laser light on the optical disk and forming a mark whose length is an integral multiple of the reference clock length, characterized in that, as irradiation powers, the above-mentioned laser can irradiate the optical disk at any one of four laser powers that satisfy a relation:
 
first power&gt;second power&gt;third power&gt;fourth power,
 
and the length of a first pulse of the first power at the time of forming the mark is made to be shorter than twice the length of a pulse of the fourth power that follows the first light of the first power.
 
   Moreover, the recording method according to still another aspect of the present invention is a method for recording the data on an optical disk by irradiating laser on the optical disk and forming a mark whose length is an integral multiple of the reference clock length, characterized in that, as irradiation powers, the above-mentioned laser can irradiate the optical disk at any one of four laser powers that satisfy a relation:
 
first power&gt;second power&gt;third power&gt;fourth power,
 
and the length of a first pulse of the fourth power at the time of forming the mark is made to be longer than the length of a pulse of the fourth power that follows the first pulse of the fourth power.
 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of an information recording apparatus in one embodiment according to the present invention. 
       FIG. 2  is a waveform chart for explaining the recording method in the one embodiment according to the present invention. 
       FIG. 3  is a recording waveform chart in the one embodiment according to the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows the block diagram of the one embodiment of the information recording apparatus that implements the recording method according to the present invention. In the block diagram, the numeral  1  is a laser, the numeral  2  is an APC (Auto Power Control) circuit, the numeral  3  is a high-frequency superimposing circuit, the numeral  4  is a current source for reproducing, the numeral  5  is a first recording current source, the numeral  6  is a second recording current source, the numeral  7  is a third recording current source, the numeral  8  is a fourth recording current source, the numeral  9  is a first switch, the numeral  10  is a second switch, the numeral  11  is a third switch, the numeral  12  is a fourth switch, the numeral  14  is a counter, the numeral  15  is a memory, the numeral  13  is a recording pulse generating circuit composed of the counter  14  and the memory  15 , the numeral  16  is a reference clock, the numeral  17  is a sequence of recording codes, the numeral  18  is a first recording pulse, the numeral  19  is a second recording pulse, the numeral  20  is a third recording pulse, and the numeral  21  is a fourth record pulse. 
   At the time of reproducing, the laser  1  is oscillated at a reproducing power level Pr by the APC circuit  2 . The high-frequency superimposing circuit  3  is provided to reduce laser noise arising from the laser  1 , but the high-frequency superimposition may be stopped at the time of recording/erasing from the viewpoint of a laser life. At the time of recording, the switches  9 - 12  for controlling the currents from the recording current sources  5 - 8  are controlled by four kinds of recording pulses  18 - 21 , which enables the laser  1  to emit the laser power necessary for recording the information. The memory  15  stores combinations of recording pulses (hereinafter referred to as the recording pulse trains) used to form nine kinds of marks of 3T (channel bits) to 11T, which are necessary in the mark-length recording system, and outputs a recording pulse train in response to an input from the counter  14 . The reference clock  16  and the sequence of recording codes  17  that is information to be recorded are inputted into the counter  14 , where the sequence of recording codes  17  is decomposed into marks (recorded portions) and spaces (erased portions) to be recorded in synchronization with the reference clock  16 , the recording code length is counted, and consequently the memory  15  outputs a recording pulse train that corresponds to the recording code length. 
   An outline of the combination of the control signal and the recording waveform length at the time of recording 3T and 6T marks on the optical disk, where the information is recorded with a recording code length shorter than that of the current optical disk, will be described referring to  FIG. 2 .  FIG. 2(A)  and  FIG. 2(B)  show the reference clock  16  and the sequence of recording codes  17 , respectively, which are inputted into the counter  14 . The counter  14  decomposes the sequence of recording codes  17  into the recorded (mark) portions (H level) and erased (space) portions (L level) at positions of the rise or fall of the reference clock  16 , and counts the recording code length; and then, the memory  15  outputs a recording pulse train corresponding to the recording code length.  FIG. 2(C)  shows the recording pulse outputted from the memory  15 , and  FIG. 2(D)  shows a laser power emitted from the laser  1  to the optical disk. 
   Here, the first recording pulse  18  has a power level of Pw (recording power level), the second record pulse  19  has a power level of Pe (erasing power level), the third recording pulse  20  has a power level of Pb, and the fourth recording pulse  21  has a power level of Pc. At the time of recording 3T and 6T marks, the high-frequency superimposing circuit  3  is stopped, a power level Pr′ that is maintained by the APC circuit  2  is used as a base power, on which the power levels (Pw, Pe, Pc, Pb) are superimposed. Here Pr′ is a power level when the high-frequency superimposition circuit is stopped, and this power level is lower than the reproducing power level Pr. Further, for the space portions, the power level of Pe is superimposed on the power level of Pr′ and is irradiated on the optical disk. 
   In the intermediate part of a portion where the 3T mark is to be formed (when the second recording pulse is off), the light at the power level of Pw heats up the recording medium, and the light at the power level of Pc suppresses the thermal interference and also controls shapes of front and rear ends of the mark that is being formed on the recording medium. Further, in the intermediate part of a portion where the 6T mark is to be formed, the light at the power levels of Pw, Pb, and Pc controls the shapes of front and rear ends of the mark that is being formed on the recording medium. That is, at the time of recording 3T and 6T marks, the laser light is irradiated on the recording medium at the laser powers shown in  FIG. 2(D) . 
   The details of the control signal and the recording waveform length at the time of the recording of the optical disk on which the information is recorded with a recording code length shorter than that of the current optical disk will be described referring to  FIG. 3 . The 8/16 modulation is one whereby information is recorded using marks and spaces of 3T to 11T. Although the laser powers corresponding to the recording codes of 3T to 5T are made to vary intricately as shown in the figure, the laser powers for the recording codes of 6T to 11T are such that a pulse train (T MP , T MC ) synchronized with the reference clock  16  is added to the recording code 5T by the difference number of cycles between the recording code 5T and a recording code in question. Here, the laser is driven alternately at two power levels of Pw and Pb. The reason why similar waveforms are used for 6T and thereafter is that the size (the length) of the mark that is formed depends on outflow of the heat in the recording medium, the size of the irradiated light spot, and the length of the mark to be formed. Hereafter, the effective recording pulse length for each recording code etc. will be described concretely referring to  FIG. 3(C) . 
   First, in order to form the recording code 3T, the light is irradiated at the recording power Pw for a first pulse length T FP =0.563T. The effective recording pulse length consists of the first pulse length T FP , and needs to be shortened from the recording code length by the amount: (recording code length 3T)−T FP =2.437T as a control in the time-axis direction. 
   For the recording code 4T, the effective recording pulse length becomes 1.688T that is a sum of the lengths of three kinds of pulses: a first pulse length T FP =0.625T, a first cooling pulse length T FC =0.5T, and a last pulse length T LP =0.563T, and it needs to be shortened from the recording code length by the amount: (recording code length 4T)−(effective recording pulse length)=2.312T as a control in the time-axis direction. Further, the ratio of the first pulse length to the first cooling pulse length (T FP /T FC ) is 1.25. 
   The effective recording pulse length for the recording code 5T is 2.315T which is a similar recording waveform as the recording code 4T. Also in this case, as with the recording code 4T, the effective recording pulse length needs to be shortened from the recording code length by 2.685T as a control in the time-axis direction. Further, a ratio of the first pulse length to the first cooling pulse length (T FP /T FC ) is approximately 1.25. 
   For the recording code 6T, the effective recording pulse length becomes 3.315T that is a sum of the lengths of five kinds of pulses: a first pulse length T FP =0.94T, a first cooling pulse length T FC =0.75T, an intermediate pulse length T MP =0.375T, an intermediate cooling pulse length T MC =0.625T, and a last pulse length T LP =0.625T, and it needs to be shortened from the recording code length by the amount: (recording code length 6T)−effective recording pulse length)=2.685T as a control in the time-axis direction. Further, the ratio of the first pulse length to the first cooling pulse length (T FP /T FC ) is approximately 1.25 as with the case of the recording code 5T. Moreover, the ratio of the first cooling pulse length to the intermediate cooling pulse length (T FC /T MC ) is 1.2. 
   For the recording codes 7T to 11T, the recording waveforms are ones that a combination of the intermediate pulse length T MP =0.375T and the intermediate cooling pulse length T MC =0.625T is added to the intermediate part of the recording waveform for the recording code 6T, one by one, respectively. 
   By performing such recording waveform control as explained in the forgoing, a heat storage effect can be kept constant and the width of the mark can be controlled constant regardless of the length of a recording code. This control is extremely effective for the mark-length recording system where a mark smaller than a light spot size that is defined by λ/NA is used as a shortest mark. Further, also in the hindmost part of the mark, both a shape of the rear end of the mark formed in the recording medium and the thermal interference can be controlled by the fourth recording pulse  21 , so that a mark whose recording code length is of the order of 0.15 to 0.20 μm can be recorded adequately. 
   Note that, although this embodiment adopts a configuration in which recording pulse trains corresponding to the nine kinds of recording code lengths of 3T to 11T are stored, the embodiment may adopt a configuration in which a recording pulse train corresponding to a recording code length of 2T is stored additionally according to the recording medium and the recording apparatus. Moreover, in this embodiment, the differences between the recording code lengths and the effective recording pulse lengths are set to 2.315T to 2.685T, but even if this difference is set to 2T or so, a similar effect can be obtained. Furthermore, the ratio of the first pulse length to the first cooling pulse length (T FP /T FC ) is set to approximately 1.25, but even if this ratio is set to 2 or so, a similar effect can be obtained. 
   According to the present invention, in the mark-length recording system where a mark smaller than a light spot size that is defined by λ/NA is used as a shortest mark, there can be offered the effect that the length and width of the record mark are controlled in a highly accurate manner regardless of variation in lengths of the record marks and spaces, which enables the information to be recorded densely, and the effect that the reliability of the information is improved.