GB2458766A - Recording method using cooling pulse and erase boost pulse - Google Patents

Abstract

In an information recording method which uses short-pulse light using relaxation oscillation of semiconductor laser, the direct overwrite characteristics can be improved by use of light having a cooling pulse of duration Tc driven by a drive current lower than an emission threshold current of the semiconductor laser after a recording pulse of duration Tw at the rear end of a short pulse train which records a mark of a predetermined length, and a boost pulse of duration Tbst driven by a drive current higher than an erase pulse drive current at the head of an erase pulse. The cooling pulse Tc may be greater than 0.8T where T is the clock channel width and Tw may be less than 1.5 ns.

Description

TITLE OF THE INVENTION

METHOD AND APPARATUS FOR RECORDING INFORMATION

BACKGROUND OF THE INVENTION

The present invention relates to an information recording method for recording information on a recording medium having a recording layer by laser light from a light source, and to an information recording apparatus such as an optical disc drive unit using such recording method.

As one optical recording medium in which recording is achieved by applying light onto the recording medium and forming an optically identifiable mark by generated heat, there is a phase-transition medium in which recording is achieved by using a thermal phase transition between crystalline and amorphous states.

A recording film of the phase-transition medium is crystalline in a steady state, but its regions to which light is applied cause a phase transition into an amorphous state by being cooled off after heated and melted.

This principle is utilized to form an amorphous portion as a mark, such that information can be recorded. In such a phase-transition medium, a pulse width modulation (PWM) scheme in which the positions of the leading and terminal ends of the mark are matched to binary information to be recorded is effective in improving recording density.

Generally, in the case where the PWM scheme is applied to an optical disc, what is called multi-pulse recording is performed using, as irradiating light, an optical pulse split into parts instead of a single optical pulse when a long mark is to be formed, in order to reduce thermal storage effects attributed to the application of light.

As a further advanced example, a development group including the inventor of the present application has already proposed a scheme in which a sub-nanosecond-class short optical pulse using relaxation oscillation is used to form a unit mark in order to enable high-quality recording without thermal interference and recrystallization even in high double speed recording at high linear density and at a small track pitch.

In the meantime, it is known that, in overwriting, erase power smaller than recording power can be applied to a space portion to directly overwrite in a previously recorded region.

Japanese Patent Application Publication (KOKAI) No. 2006-209935 has shown the improvement of overwrite characteristics by the addition of an erase head pulse to the head of erase power.

However, the simple addition of the head pulse as in the method described in the above-mentioned publication increases unnecessary input of energy, and there is fear that increasing the number of overwriting may rather accelerate the degradation of the recording film. Therefore, in the method described in the above-mentioned publication, a recording waveform needs to be optimized considering the characteristics of a medium in the high double speed recording.

In addition, it is known that in the phase-transition recording technique, rapid cooling after heating is important for the mark formation. It is known that particularly when heating is rapidly performed using a short recording pulse of a sub-nanosecond-class to adapt to the high double speed recording, the absence of a sufficient cooling period results in insufficient cooling and susceptibility to defective mark formation. That is, setting the cooling period is important in maintaining initial recording characteristics.

On the other hand, laser power is required to be decreased to less than the erase power in the cooling period. Thus, a longer cooling period after the recording pulse leads to a further delay in the start time of the erase power and increases unerased regions, and there is fear that the previous recording mark may remain unerased at the time of overwrite, which degrades direct overwrite characteristics.

An object of the invention is to provide an information recording method capable of improving direct overwrite characteristics, and an information recording apparatus using the recording method.

BRIEF SUMMARY OF THE INVENTION

According to an aspect of the invention, there is provided a method of recording information by short-pulse light generated by relaxation oscillation of semiconductor laser characterized by comprising: preparing a cooling pulse driven by a drive current lower than an emission threshold current of the semiconductor laser after a recording pulse at the rearmost end of a short pulse train which records a mark of a predetermined length; and preparing light having a boost pulse driven by a drive current higher than an erase pulse drive current at the head of an erase pulse.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is an exemplary diagram showing an example of an information reproducing apparatus (an optical disc apparatus) according to an embodiment of the invention; FIG. 2 is an exemplary diagram showing an example of a laser drive circuit used in the optical disc apparatus shown in FIG. 1, according to an embodiment of the invention; FIG. 3 is a flowchart showing an example of calibration processing performed by the laser drive circuit shown in FIG. 2 for the strength of laser light which is generated by relaxation oscillation and which is emitted in an extremely short period, according to an embodiment of the invention; FIGS. 4A to 4D are exemplary diagrams each showing an example of the relationship between light emission with relaxation oscillations of the laser element and a laser drive current in the PUH of the optical disc apparatus shown in FIG. 2, according to an embodiment of the invention; FIG. 5 is an exemplary diagram showing an example of processing for recording with the relaxation oscillations of the laser element, according to an embodiment of the invention; FIGS. 6A to 6C are exemplary diagrams each showing a characteristic example of a recording mark which may be generated by recording processing shown in FIG. 5 and an example of a recording mark according to the present proposal; FIG. 7 is an exemplary diagram showing examples of a recording pulse, a cooling pulse, an erase pulse and an erase boost pulse (off-pulse) of the present proposal, according to an embodiment of the invention; FIG. 8 is a graph explaining the relationship between the cooling pulse width of the present proposal and a bit error rate during direct overwrite (DOW), according to an embodiment of the invention; and FIG. 9 is a graph explaining how the bit error rate is improved by the use of the cooling pulse and erase boost pulse of the present proposal, according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention will be explained in detail hereinafter with reference to the accompanying drawings. The various modules of the systems described herein can be implemented as software applications, hardware and/or software modules, or components on one or more computers, such as servers. While the various modules are illustrated separately, they may share some or all of the same underlying logic or code.

FIG. 1 is a block diagram showing one example of the configuration of an information recording/reproducing apparatus (optical disc drive) to which the present invention is applicable.

The information recording/reproducing apparatus (optical disc drive) records information in a recording surface of an information recordIng medium (optical disc) 100 or reproduces information recorded on the recording surface.

A concentric or spiral groove is cut in the recording surface of the optical disc 100. A concave portion of the groove is called a land while a convex portion of the groove is called a groove, and one circuit of the groove or land is called a track.

Laser light whose strength has been modulated is applied along the track (the groove alone or the groove and land) to form a recording mark, such that user data is recorded. The reproduction of the data is achieved by applying laser light of read power weaker than that in recording along the track and thereby detecting a change in the strength of light reflected by the recording mark on the track.

The erasure of recorded data can be achieved by applying laser light of erase power stronger than that of the read power along the track and thereby crystallizing a recording layer.

The optical disc 100 is rotated at a predetermined velocity by a spindle motor 63.

A rotational angle signal is output from a rotary encoder 63A provided in the spindle motor 63. One rotation of the spindle motor 63 produces, for example, five pulses of the rotational angle signal. By this rotational angle signal, a spindle motor control circuit module 64 determines the rotational angle and rotation number of the spindle motor 63.

The recording of information on the optical disc and the reproduction of information from the optical disc 100 are achieved by an optical pickup (hereinafter referred to as a pickup head or PUH) 65.

The PUH 65 is coupled to a feed motor 67 via a gear and a screw shaft. The feed motor 67 is controlled by a feed motor control circuit module 68.

The feed motor 67 is rotated by a feed motor drive current from the feed motor control circuit module 68, such that the optical head 65 moves in the radial direction of the optical disc 100.

The PtJH 65 is provided with an objective lens 70 which is supported by an unshowri wire or leaf spring to be movable over a predetermined distance in a direction perpendicular to the recording surface of the optical disc 100 or in the radial direction of the optical disc 100. The objective lens 70 can be moved in a focusing direction (the direction perpendicular to the recording surface, i.e., the optical axis direction of the objective lens 70) by the driving of a drive coil 72, and can also be moved in a tracking direction (the radial direction of the optical disc 100, i.e., a direction perpendicular to the optical axis of the objective lens 70) by the driving of a drive coil 71.

In recording information (the formation of a mark), a laser modulation control circuit module 75 supplies a write signal to a laser diode (laser light emitting element) 79 on the basis of recording data supplied from a host device 94 via an interface circuit module 93.

The laser light generated by the laser diode 79 enters a half mirror 96. The half mirror 96 branches the laser light generated by the laser diode 79 by a fixed ratio.

A monitor light detector (FM-PD) 95 configured by a photodiode receives part of the laser light from the half mirror 96. The monitor light detector (FM-PD) 95 detects part of the laser light proportionate to irradiation power, and supplies a light reception signal to the laser modulation control circuit module 75.

The laser modulation control circuit module 75 controls the laser diode 79 on the basis of the strength of reflection laser light received by the monitor light detector 95 so that reproduction laser power, recording laser power and erase laser power that have been set by a main arithmetic processing block module 90 including a central processing unit (CPU) may be suitably obtained.

The laser diode 79 generates laser light in accordance with a drive current supplied from the laser modulation control circuit module 75. The laser light emitted from the laser diode 79 is applied onto the optical disc 100 via a collimator lens 80, a half prism 81 and the objective lens 70. Reflection light from the optical disc 100 is guided to a photodetector 84 via the objective lens 70, the half prism 81, a collecting lens 82 and a cylindrical lens 83.

The photodetector 84 includes, for example, four photodetector cells, and detection signals of these photodetector cells are output to an RF amplifier module 85. The RF amplifier module 85 processes the -10 -signals from the photodetector cells, and generates a focus error signal FE indicating a deviation from a focal position, a tracking error signal TE indicating a difference between the beam spot center of the laser S light and the center of the track, and a reproduction signal which is a total sum signal of the signals of the photodetector cells.

The focus error signal FE is supplied to a focus control circuit module 87. The focus control circuit module 87 generates a focus drive (control) signal in accordance with the focus error signal FE. The focus drive signal is supplied to the drive coil 71 in the focusing direction. Thus, focus servo is performed whereby the laser light is always just focused on the recording film of the optical disc 100.

The tracking error signal TE is supplied to a tracking control circuit module 88. The tracking control circuit module 88 generates a tracking drive signal in accordance with the tracking error signal TE.

The tracking drive (control) signal output from the tracking control circuit module 88 is supplied to the drive coil 72 in the tracking direction. Thus, tracking servo is performed whereby the laser light always traces on the track formed on the optical disc 100.

Such focus servo and tracking servo are performed, such that the change of the reflection light from, for -11 -example, a recording mark formed on the track of the optical disc 100 in accordance with recording information is reflected in a total sum signal RF of the output signals from the photodetector cells of the photodetector 84. This signal is supplied to a data reproduction circuit module 78. The data reproduction circuit module 78 reproduces the recording data on the basis of a reproduction clock signal from a PLL circuit module 76.

When the objective lens 70 is controlled by the tracking control circuit module 88, the position of the feed motor 67, that is, the position of the optical head (PUH) 65 is also controlled by the feed motor control circuit module 68 so that the objective lens 70 is located in the vicinity of a predetermined position within the optical head 65.

The spindle motor control circuit module 64, the feed motor control circuit module 68, a laser control circuit 73, the phase locked loop (PLL) circuit module 76, the data reproduction circuit module 78, the focus control circuit module 87, the tracking control circuit module 88, an error correction circuit module 62, etc. are controlled by the main arithmetic processing block module (CPU) 90 via a bus 89. The Cpu (main arithmetic processing block module) 90 controls the overall operation of the recording/reproducing apparatus in accordance with operation commands provided from the -12 -host device 94 through the interface circuit module 93.

Moreover, the CPU 90 uses a random access memory module (RAN) 91 as a working area, and performs a predetermined operation in accordance with a control program including a program by the present invention recorded in a read-only memory module (ROM) 92 by appropriately referring to a parameter for each individual apparatus recorded in a nonvolatile random access memory module 99. In addition, it goes without saying that the error correction circuit module 62 corrects errors in the reproduction signal.

Furthermore, a wavelength ? of the laser light output by the laser diode (semiconductor laser element) 79 is, for example, 450 rim or less, and is preferably 405�5 nm.

Still further, the numerical aperture (NA) of the objective lens 70 is, for example, 0.6 to 0.65, and a minimum mark length L is preferably A./(4xNA)�=L�=/(2.5xNA) where X is the wavelength of the laser light output from the laser diode 79 between a pitch (space) TP of a track (guide groove) formed in the optical disc 100 and the minimum mark length L (2T) described later.

Further yet, the track pitch TP is preferably I (2xNA)�=TE'�=)./ (1. 3xNA) where X is the wavelength of the laser light output from the laser diode 79 between the pitch (space) TP of -13 -the track (guide groove) formed in the optical disc 100 and the minimum mark length L (2T) described later.

FIG. 2 shows an example of the configuration of a laser drive circuit (laser modulation control circuit module) in the information recording/reproducing apparatus shown in FIG. 1.

A laser drive circuit (laser modulation control circuit module) includes a write strategy (recording waveform (laser light) optimizing) circuit module 7503 for generating a recording waveform from a recording clock and recording data, digital-to-analog converter (DAC) modules 7504 to 7507 for setting indicated values of drive currents to be supplied to the laser diode 79 to enable the emission of lasers of peak power (write power), erase power, read power and bias power, current sources 7508 to 7511 for the peak power (write power), erase power, read power and bias power, and an interface module 7501 for interpreting a control signal from the signal bus 89 and controlling all the drive currents supplied from the laser modulation control circuit module 75 to the laser diode 79.

Although not described in detail, the write strategy circuit module 7503 includes a phase locked loop (PLL) circuit and a modulation circuit. The PLL circuit receives the recording clock to generate a timing signal necessary for the modulation circuit.

The modulation circuit interprets the recording data to -14 -generate a recording waveform in accordance with the control signal including strategy information such as a pulse width and edge timing set by an internal bus 7502, and decomposes the recording waveform into timing signals indicating the timing of turning on/off the respective current sources 7508 to 7511.

The timing signals are connected to a selector module 7512, and the current sources 7508 to 7511 set to output different output values by the current setting DAC modules 7504 to 7507 for which current command values are set by the internal bus 7502 are turned on/off, such that the strength of the laser drive current supplied to the laser element 79 increases or decreases. Thus, the strength of the irradiation power during recording is modulated.

Furthermore, in the embodiment of the present invention, the pulse width and the current values are supplied as the control signal from the CPU 90 to the laser modulation control circuit module 75 through the signal bus 89.

Moreover, the pulse width and the current values are sent to the write strategy circuit module 7503 and the current setting DAC modules 7504 to 7507 via the interface module 7501 and the internal bus 7502.

In addition, although the PEAK DAC 7504 and the PEAK current source 7508 are shared by a peak pulse and an erase boost pulse in the example in FIG. 2, it goes -15 -without saying that a current source and a current setting DAC (current source) for erase boost may be separately provided -Next, calibration processing which is one embodiment of the present invention is described with FIG. 3, and the calibration processing is performed for recording processing using sub-nanosecond-class pulse laser light, that is, laser light which is generated by relaxation oscillation and which is emitted in an extremely short period.

In order to perform the recording processing using the pulse laser light, that is, the laser light which is generated by relaxation oscillation and which is emitted in an extremely short (sub-nanosecond class) period, it is necessary to determine a write strategy by the write strategy circuit module 7503 described with FIG. 2.

To this end, for example, a predetermined control signal is supplied to the laser modulation control circuit module 75 connected to the Cpu 90 by the signal bus 89 to perform calibration in the following manner.

In accordance with a flowchart shown in FIG. 3, initial setting conditions prepared in the optical disc or a servo driver 30 are first read (BLOCK 11).

Then, one peak current and one drive pulse width are temporarily determined from among, for example, five kinds of peak current values and drive pulse widths on -16 -the basis of the read initial setting values (BLOCK 12) Then, using, as a recording signal, a signal including a long mark which allows a sufficient modulation degree to be obtained for the spatial frequency transfer characteristics of the optical pickup head used for recording/reproduction, calibration of the peak current is performed so that a sufficient modulation degree can be obtained (BLOCK 13). Here, the long mark which allows a sufficient modulation degree to be obtained is, by way of example, liT, and can be any one of 6T to 13T. In addition, T indicates one clock cycle, and the shortest mark is 2T.

Subsequently, read laser light is applied to a region on the optical disc 100 in which test write has been performed by the laser light emitted at the above-mentioned value and containing the relaxation oscillation, and the reflected light is detected (BLOCK 14). On the basis of the detection result, a write strategy (optimization of the recording waveform (laser light) is determined.

For example, a peak current to be set is gradually increased, and whether the amplitude of the detection signal of the reflected light is maximized is determined (BLOCK 15). A peak current value at the maximum amplitude is determined to be the peak current value of the write strategy, and stored in a storage -17 -area, which is not described in detail, of the write strategy circuit module 7503 (BLOCK 16).

Then, the pulse width is calibrated using a recording signal in which the minimum length mark and a sufficiently long mark are mixed, for example, using a signal in which a mark and a space of any one of 6T to 13T, for example, liT are mixed with a mark and a space of 2T which is the shortest mark. That is, for the temporarily determined pulse width, test write is performed on the optical disc 100 by laser light which has relaxation oscillation and which is emitted at the value of this pulse width (BLOCK 17).

Subsequently, the read laser light is applied to the region on the optical disc 100 in which the test write has been performed by the laser light containing relaxation oscillation in BLOCK 17, and the reflected light is detected (BLOCK 18).

Then, asymmetry (an index for evaluating whether the center levels of the signals of the respective Ts are equal) of the reproduction signal is calculated, and the pulse width is adjusted so that the asymmetry may be about zero (BLOCK 19). Further, the pulse width at this point is determined to be a strategy, and stored in the storage area, which is not described in detail, of the write strategy circuit module 7503 (BLOCK 20).

For the above-mentioned asymmetry, a value -18 - (asymmetry value) should be used which is represented by Asymmetry value = (Al+A2)/(Al-A2) where Al (normally a positive value) is a signal obtained by detecting a positive (upper) amplitude level with reference to 0 V using a peak level detection circuit while a reproduction RF signal is put in an AC-coupled state by being passed through a high-pass filter, and A2 (normally a negative value) is a signal obtained by detecting a negative (lower) amplitude level with reference to 0 V using a bottom level detection circuit.

Furthermore, equivalent effects can be obtained here by a pulse width determining method which includes: searching for a pulse width that enables a relationship where a signal level up to the amplitude center of the detection signal when successive patterns of liT from the reproduction signal level of an unrecorded region are used as recording patterns may be about double the signal level up to the amplitude center of the detection signal when successive patterns of the minimum mark length (2T, two clocks when iT is a clock) from the reproduction signal level of the unrecorded region are used as recording patterns; determining this pulse width to be a strategy; and storing the pulse width in a storage area of, for example, a write strategy section 41.

-19 -Thus, the strategy used for the relaxation oscillation recording processing is characterized in that it is defined by the minimum pulse width alone, riot by pulse edge timing for each recording mark length and by edge timing compensation values adapted to the preceding and succeeding mark lengths/space lengths and to respective recording mark lengths as in conventional recording processing.

The write strategy thus found and stored in the write strategy section 41 is read during data recording processing under the control of a control section 31.

In the recording processing using the relaxation oscillation of laser, a peak current value and an erase current value, for example, are determined on the basis of a write strategy that has been read, and recording on an optical disc is performed (BLOCK 21).

In addition, the relaxation oscillation is generally useful for generating a sharp recording pulse of duration less than 1 ns, and is characterized in that the rise/fall time of the pulse of the laser drive current supplied to the laser diode 79 as a recording pulse is below 100 Ps.

Furthermore, regarding a condition for generating relaxation oscillation, a bias current bi and a peak current pe are given as drive currents, as shown in FIGS. 4A and 4C. In this case, the bias current Ibi set at a level slightly higher than a threshold current

-

Ith at which the laser diode 79 starts laser oscillation is first generated, and the laser diode 79 is preliminarily driven, as shown in FIG. 4A. Then, until the level is dropped down to the bias current bi at time B, the peak current pe for obtaining desired peak power is applied at time A. In this manner, the peak current Tpe is applied between time A and time B, such that the laser output (a change in the strength of laser emission light with time) as shown in FIG. 4B is obtained.

That is, until time A at which the intensity of the laser drive current is the bias current bi' the strength of the emitted light has significantly low power which does not enable the laser light output from the laser diode 79 to record data on the optical disc 100, but the peak current pe is applied so that the strength of the laser light increases to recording power. It is appreciated that the strength of the emitted light is again at low power at and after time B. If the strength of the emitted light is observed in more detail, it is seen in FIG. 4B that when the strength is increased to the recording power at time A, the strength instantaneously increases and then decreases before it stabilizes at steady recording power (an arrow c portion in FIG. 4B). This is attributed to the relaxation oscillation of the laser -21 -diode 79, and this relaxation oscillation is controlled to the minimum in normal recording pulse generation.

The relaxation oscillation is a transitional oscillation phenomenon which occurs when the drive current rapidly increases from a certain level to a fixed level far exceeding the threshold current in the semiconductor laser as described above.

In addition, the relaxation oscillation decreases every time the oscillation is repeated, and eventually settles down.

In the optical recording apparatus of the present invention, this relaxation oscillation is actively utilized for recording.

That is, although the generation of the relaxation oscillation should originally be inhibited, the present invention intends to stably obtain a sharp recording pulse having a short length by use of, as the characteristics of the relaxation oscillation, a short pulse length arid an energy amount (an integral value of laser power as an optical output) which may be able to change the recording film of the optical disc 100 to a recording level.

As shown in FIG. 4C, when a drive current with a predetermined characteristic is supplied to the laser diode 79 from the laser modulation control circuit module 75, oscillation is involved as seen in FIG. 4D, but a laser output with a high peak level is obtained -22 -for a short period.

More specifically, the bias current bi. set at a level lower than the threshold current th is supplied to the laser diode 79. Further, with predetermined timing, that is, at time A, the drive current is rapidly raised to the peak current level pe higher than the threshold current th at a rise time earlier than in the normal recording pulse generation. Then, the drive current is returned to the bias current bi at time D after the lapse of a nanosecond-level instant shorter than in the normal recording pulse generation.

In this case, a laser output (a change in the strength of laser emission light with time) is obtained, as shown in FIG. 4D.

That is, in FIG. 4D, the laser diode 79 has not started the laser oscillation until time A at which it is driven by the bias current bi lower than the threshold current 1th' and this is a negligible level at which it only emits light as a light emitting diode.

Then, a current is rapidly applied at time A, such that the relaxation oscillation is caused and the strength of the emitted light rapidly increases.

Subsequently, the amplitude of the relaxation oscillation gradually converges onto a steady level, but a predetermined time, that is, time C is set, and then the drive current is set to the bi lower than the threshold current th' such that laser light having a -23 -certain energy amount is obtained. In addition, as is apparent from FIGS. 4C and 4D, the time D is determined to be timing whereby a second period pulse of the relaxation oscillation is generated.

Thus, the pulse generated by the relaxation oscillation is characterized in that the strength of the emitted light increases in a much shorter time than in the normal recoding pulse and that the strength of the emitted light decreases at a certain period which is determined by the structure of the semiconductor laser. Therefore, the use of the pulse generated by the relaxation oscillation for the recording pulse makes it possible to obtain a short pulse having short rise and fall times and having a high peak strength which can not be obtained by the normal recoding pulse.

Next, the outline of the recording processing using the sub-nanosecond-class pulse laser light, that is, the laser light generated by the relaxation oscillation is described with FIG. 5. In addition, FIG. 5 is a timing chart showing one form of each signal in the case where the 2T mark and 3T mark are formed in the optical disc apparatus. Moreover, an [A) of FIG. 5 shows iT, that is, a clock CLK.

When the 2T mark is formed, one pulse is supplied to the laser diode 79 from the laser modulation control circuit module 75, as shown in a [B] of FIG. 5. In addition, in a [C] of FIG. 5, there are shown a pulse -24 -width T, a peak current PEAK' a bias current BIAS' and an erase current 1EPJSE* Moreover, when the 3T mark is formed, two pulses are supplied to the laser diode 79 from the laser modulation control circuit module 75.

The laser light output from the laser diode 79 exhibits extremely sharp pulsed power as shown in a [DI of FIG. 5, and shows several oscillation waveforms, that is, the relaxation oscillations described with FIGS. 4A to 4D.

It should be noted here that the laser light output using the relaxation oscillation scheme which is one embodiment of the present invention enables recording processing with extremely small energy that is about one fifth of that in a known method in terms of power consumption. That is, conventional driving methods require an energy of about 300 pJ for a peak power of 10 mW, a current value of 80 mA and a multipath width total of 30 ns to record a 4T mark, by way of example. However, when the relaxation oscillation is used, the energy is 20 pJ for the pulse laser light involving the relaxation oscillation with a peak power of 40 niW, a Current value of 150 mA and a signal time width of 1.5 ns. Thus, when three recording pulses (laser light) are used to record the 4T mark, required energy is about 60 pJ, so that the 4T mark can be recorded with extremely small energy that -25 -is about one fifth of that in the conventional driving methods.

In the meantime, in the example shown in FIG. 5, it has been ascertained that a region recrystallized due to the melting of peripheral parts (FIG. 6A) and an unheated region due to the turning off of laser (FIG. 6B) are produced in a period up to the output of the erase pulse following the write pulse, that is, a region up to the output of the erase pulse in the rear of the write pulse, as schematically shown in FIG. 6A or 6B. This might lead to the generation of unerased regions that degrade the reproduction signal when direct overwrite recording is repeated.

Thus, in order to prevent the emergence of the recrystallized region/unheated region shown in FIGS. 6A and 6B, it is preferable to add light emitted by a boost current bst determined by providing each current value with a relationship in Equation 1 below, that is, to add a boost pulse so that the end pulse of the last pulse is set as a cooling period, as indicated by a binarized recording data (NRZI), a drive current for driving the laser diode 79, and a light output waveform in FIG. 7: = c < th < e < 1bst �= 1p (1) where I is the peak current of a recording pulse, e is an erase current, b is a bias current, c is a current in a cooling period, bst is a boost current, -26 -and th is an oscillation threshold current of the laser diode 79.

More specifically, when, for example, a 4T recording mark is formed, the sizes of pulse widths satisfy the relationship: Tb5t �= Tw < 1.5 [as] 0.8T < Tç where T is the width of the recording pulse, Tc is the width of the cooling pulse, Tbst is the width of the boost pulse, and T is a channel clock interval, such that it is possible to prevent, as shown in FIG. 6C, the generation of the recrystallized region shown in FIG. 6A and the unheated region serving as the unerased region shown in FIG. 6B.

That is, in recording (PEAK/WRITE = write pulse output), a channel clock WCLK and binarized recording data WDATA are detected in the write strategy determination module 7503, and a timing signal for controlling the selector module 7512 is generated with timing (pulse width/pulse number) previously set through the internal bus 7502.

In the case where the 4T recording mark shown in [A] of FIG. 7 is written, given only that writing is being performed, a switch (READ_SW in the selector module 7512) connected to the READ current source 7510 is turned off, and with timing (Tbst) of BIAS in [B] of FIG. 7, switches (PEAK_SW in the selector module -27 - 7512/ER.ASESW in the selector module 7512) connected to the PEAK current source 7508 and the ERASE current source 7509 are turned off, and a switch (BIAS_SW in the selector module 7512) connected to the BIAS current source 7511 is turned on.

Furthermore, as shown in the [C] of FIG. 7, with the timing of PEAK (LO output), the PEAK_SW (in the selector module 7512) is turned on, and the ERASE SW (in the selector module 7512) is turned off. At this point, the BIAS_SW (in the selector module 7512) is turned on, such that a current in which the PEAK current and the BIAS current are added together can be obtained. In addition, when the PEAK current source alone takes charge of the peak power, the BIAS_SW may be turned of f.

Subsequently, after the predetermined pulse width time Tw has passed, the PEAK_SW is again turned off so that the BIAS current alone is supplied to the laser diode 79, thereby obtaining an optical pulse of an extremely short period containing the relaxation oscillation.

Consequently, using one optical pulse in this extremely short period as a unit pulse, a recording mark (see the [C] of FIG. 5 to the [E) of FIG. 5) is recorded on the optical disc 100 at every unit pulse.

The pulse width T is set within one to five periods of the relaxation oscillation, such that a -28 -recording mark can be stably formed without thermal interference and regardless of the moving velocity of a medium. In addition, although the recording mark formed by the unit pulse is used as the minimum length mark in the present embodiment, it is also possible to use marks formed by a plurality of unit pulses as the minimum length marks.

Subsequently, a bias pulse of about iT period is again output in a BIAS current section, and a PEAK current of the pulse width (period) T is again output.

Thus, when the 4T mark is recorded, three unit pulses are applied at iT intervals.

That is, when the minimum length mark based on a recording modulation scheme is 2T, the shortest mark is represented by M (M: an integral number of 1 or higher) optical pulses in an extremely short period. When the recording mark is NT (N: integral number, T: channel clock width), the recording mark is formed by successive shortest marks at (N-i) one-channel clock intervals.

Thus, when the length of the shortest mark is 3T, the shortest mark is represented by one short optical pulse mentioned above. When the recording mark is NT (N: integral number, T: channel clock width), the shortest marks can be represented by successive shortest marks at (N-2) one-channel clock intervals.

On the other hand, after the output of the last -29 -pulse of a series of recording pulses, a move is made again to the BIAS current section as a cooling period.

Here, after the time Tc has passed, a move is further made to the PEAK current period (PEAK pulse output) as the erase boost pulse.

In addition, this PEAK pulse is a pulse for erasure, and has a different role from the previous PEAK pulse for mark formation. However, when the circuit shown in FIG. 2 is used, the use of the same current source and digital-to-analog converter (DAC) as the PEAK current is assumed to reduce the size of the circuit, so that a pulse having the same current value as the PEAK pulse is output.

Then, after the time width time Tbst has passed, a move is made to an erase current part (erase pulse output).

By setting such a recording waveform (peak current supply) and cooling period, satisfactory recording characteristics and overwrite characteristics can be obtained in a rewritable phase-transition recording medium even under conditions including a high linear velocity, high linear density and a small track pitch.

More specifically, in the short pulse recording using the relaxation oscillation of the semiconductor laser, the cooling pulse having a power level lower than the erase power is provided after the recording pulse at the rear end of a short pulse train which -30 -records a mark of a predetermined length, and the boost pulse having a power level higher than the erase power is added at the head of the erase power, such that direct overwrite characteristics can be improved.

In addition, it is preferable that Tc > 0.8T where T is the drive current pulse width of the cooling pulse, and T is the channel clock width.

Moreover, bst �= I, and Tbst �= where bst is the drive current of the boost pulse, Tbst is the boost pulse width, I is the drive current of the recording pulse, and Tw is a pulse width.

Still further, the drive current pulse width T of the recording pulse is T < 1.5 [ns].

FIG. 8 shows the relationship between the above-mentioned cooling pulse width (Tc) and bit error rate (bER) when repetitive recording (direct overwrite) is continued for an arbitrary number of times. That is, FIG. 8 showp that a lower bER provides a lower error rate in the repetitive recording and holds down the degree of the generation of the recrystallized region and unheated region as shown in FIGS. 6A and 6B.

In addition, FIG. 8 shows the bER in the case where the number of repetitions (direct overwrites [DOW]) is five, wherein it is recognized that erasing -31 -failures are sufficiently reduced regardless of the number of repetitions within a cooling pulse width of 0.8T to l.13T.

FIG. 9 shows the difference of bER between the existing overwrite recording and the overwrite recording of the present proposal described with FIGS. 4A to 4D and 5, that is, how the bER is improved by the overwrite of the present proposal regardless of the number of repetitive recordings.

While it is natural for the bER to rapidly degrade due to the repetition of the direct overwrite, the bER remains at about 1.0 x 10-6 when the overwrite of the present proposal is employed.

As described above, using one embodiment of the present invention, a sufficient cooling period is provided, and short-period light emission with power greater than the erase power is added at the head of the succeeding erase power to extend the head of a erase region, such that the amount of input heat can be held down, and degradation of the reproduction signal due to unerased recorded marks can be reduced even when overwrite is repeated.

That is, the direct overwrite characteristics are improved, and a recording mark which makes it possible to obtain a stable reproduction signal is formed even when overwrite is repeated.

Additional advantages and modifications will -32 -readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.

Claims (20)

1. -33 -WHAT IS CLAIMED IS: 1. A method of recording information by short-pulse light generated by relaxation oscillation of semiconductor laser comprising: preparing a cooling pulse driven by a drive current lower than an emission threshold current of the semiconductor laser after a recording pulse at the rearmost end of a short pulse train which records a mark of a predetermined length; and preparing light having a boost pulse driven by a drive current higher than an erase pulse drive current at the head of an erase pulse.
2. 2. The recording method according to claim 1, wherein Tc > 0.8T where Tc is the drive current pulse width of the cooling pulse, and T is a channel clock width.
3. 3. The recording method according to claim 1, wherein bst �= Iv,, and Tbst �= T where bst is the drive current of the boost pulse, Tbst is the boost pulse width, I is a drive current of the recording pulse, and T is a pulse width.
4. 4. The recording method according to claim 3, wherein the drive current pulse width Tw of the recording pulse is -34 -T < 1.5 ns.
5. 5. The recording method according to claim 1, wherein a minimum mark length L is /(4xNA) �= L �= /(2.5xNA) where) is the wavelength of the laser light, and NA is the numerical aperture of an objective lens.
6. 6. The recording method according to claim 1, wherein the track pitch TP is X/(2xNA) �= TP �= ?/(1.3xNA) where is the wavelength of the laser light.
7. 7. The recording method according to claim 5, wherein the wavelength X of the laser light is 450 run or less.
8. 8. The recording method according to claim 6, wherein the wavelength of the laser light is 450 nm or less.
9. 9. An information recording/reproducing apparatus comprising: a light source which outputs light of a predetermined wavelength; a drive circuit which provides the light source with a drive current to output first pulse light of a power level lower than erase power which erases a mark already recorded on a recording medium, a second pulse light of a power level higher than the erase power, and third pulse light of a power level higher than the second pulse light; and -35 -a control circuit which indicates to the drive circuit timing of outputting the first to third pulses.
10. 10. The information recording apparatus according to claim 9, wherein the control circuit causes the drive circuit to output the first pulse after a rearmost pulse of the third pulse and to output the second pulse at the head of the erase pulse.
11. 11. The information recording apparatus according to claim 10, wherein the control circuit causes the drive circuit to successively output the second pulse and the first pulse between the rearmost pulse of the third pulse and the erase pulse.
12. 12. The information recording apparatus according to claim 10, wherein the control circuit causes the drive circuit to output the first pulse within the range of Tc > 0.8T where T is the drive current pulse width of the first pulse, and T is a channel clock width.
13. 13. The information recording apparatus according to claim 10, wherein the control circuit causes the drive circuit to output the second pulse so that bst �= Is,, and Tbst �= T where bst is the drive current of the second pulse, Tbst is the width of the second pulse, I, is a drive current of the third pulse, and Ti., is the width of the -36 -third pulse.
14. 14. The information recording apparatus according to claim 10, wherein the control circuit causes the drive circuit to output the third pulse so that the pulse width Tw of the drive current of the third pulse is T<1.5ns.
15. 15. The information recording apparatus according to claim 10, wherein a minimum mark length L is X / (4xNA) �= L X / (2.5xNA) where A is the wavelength of laser light, and NA is the numerical aperture of an objective lens.
16. 16. The information recording apparatus according to claim 10, wherein the track pitch TP is A/(2xNA)�=TP�= X/(1.3xNA) where A is the wavelength of laser light.
17. 17. The information recording apparatus according to claim 15, wherein the wavelength A. of laser light is 450 nm or less.
18. 18. The information recording apparatus according to claim 16, wherein the wavelength A. of laser light is 450 nm or less, and the control circuit causes the drive circuit to output the third pulse so that the pulse width T of the drive current of the third pulse is T< 1.5 as.
19. 19. A method of recording and erasing information on a recording medium whose characteristics can be altered by localised heating using a semiconductor laser, driven by a short recording pulse which utilises a relaxation oscillation effect, in which the recording pulse includes a cooling-down period comprising a pulse having a drive current lower than the emission threshold of the semiconductor laser at the trailing end -37 -of the recording pulse, and in which an erase pulse includes a boost pulse which is higher than the erase pulse drive currents, at the leading end of the erase pulse.
20. 20. A method and apparatus for recording information, substantially as hereinbefore described with reference to the accompanying drawings.