Optical disk apparatus for shaping pits recorded on an optical disk

In an optical disk apparatus for recording information in pits formed on an optical disk by projecting a light beam onto the optical disk and heating an area of the optical disk corresponding to the pulse pattern of the information, the pulse pattern of the information to be recorded is discriminated, and on the basis of the discriminated pulse pattern, the timing to start on and end the emission of the light beam from the light source to the optical disk is modified, or the number of emissions and the duration of each emission is controlled so that the pattern of the pits are corrected to the shapes that ensure accurate regeneration of the information.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an optical disk apparatus for recording 
information on an optical disk by projecting a light beam onto the optical 
disk. 
2. Description of the Related Art 
For high density recording of information on an optical disk, the pit-edge 
recording method is known in which information "1" is recorded at both the 
leading edge and trailing edge of one recording pit, as contrasted with 
the conventional pit-position recording method in which information "1" is 
recorded in one pit position. 
FIG. 1 is a diagram explaining the information recording and regeneration 
signals and the states recording pits. The data word of information to be 
recorded is converted, for example, by "2/7" coding, into recording data 
WD (RZ modulation) as shown in FIG. 1(a) in the case of the pit-position 
recording method, and into recording data WD (NRZI modulation) as shown in 
FIG. 1(b) in the case of the pit-edge recording method, to record the 
information on an optical disk. In the recording of information "1", the 
information "1" is recorded during one pulse period in the case of FIG. 
1(a). On the other hand, in the case of FIG. 1(b), the information "1" is 
recorded at both the leading edge and trailing edge of a recording pulse. 
In other words, in the case of FIG. 1(a), a single "1" can be recorded 
with one pulse, while in the case of FIG. 1(b), two "1"s can be recorded 
with one pulse. 
In the pit edge recording, a laser diode is driven by a laser driving 
signal LP having the same pulse width as that of the pulse of the 
recording data WD shown in FIG. 1(b), and the light beam emitted from thus 
driven laser diode is projected onto the optical disk. The area of the 
optical disk illuminated by the light beam is heated. The area in the 
recording layer which exceeds the Curie temperature by heating is to be 
magnetized in the direction of an externally applied magnetic field, 
thereby forming a recording pit WP whose length corresponds to that of the 
laser drive signal LP, as shown in FIG. 1(d). However, immediately after 
the start of illuminating the light beam, the temperature of the optical 
disk surface near the leading edge of the pit is lower, so that the area 
that exceeds the Curie temperature is restricted; therefore, the pit width 
W of the recording pit is narrower immediately after the start of 
illuminating the light beam. Thereafter, as the heat of preceding light 
beam is accumulated and thus the temperature of the optical disk surface 
rises, the pit width W increases, thus forming the recording pit in a 
teardrop shape. 
On the other hand, when regenerating the recorded information, a light beam 
is projected onto the optical disk, and the light reflected from the 
optical disk is detected to obtain a regeneration signal RD as shown in 
FIG. 1(e). The regeneration signal RD is then compared with a present 
threshold level Vth to obtain a pit detection signal DP as shown in FIG. 
1(f). The pit detection signal DP is used to regenerate the recorded 
information. However, if the recorded pit WP is of a teardrop shape, the 
level of the regeneration signal RD at the leading edge of the recorded 
pit, i.e. the narrower portion is low, and the regeneration level 
gradually increases as the pit width increases as shown in FIG. 1(e). As a 
result, the period that exceeds the threshold level Vth becomes narrower 
than the pulse width of the recording data DW. In other words, the pulse 
width of the pit detection pulse DP shown in FIG. 1(f) is significantly 
shorter as compared with the pulse width of the recording data WD shown in 
FIG. 1(a) or (b) corresponding to information " 1". This prevents accurate 
recording and regeneration of the information. 
An information recording method aiming to overcome the above problem is 
published in Proc. Int. Symp. on Optical Memory, 1987 Japanese Journal of 
Applied Physics, Vol. 26 (1987) Supplement 26-4. FIG. 2 is a timing chart 
of recording and regeneration signals and recording pits according to that 
method. The recording data WD shown in FIG. 2(a) and (b) have the same 
patterns as the recording data WD shown in FIG. 1(a) and (b), 
respectively. As shown in FIG. 2(c), a correction prior to recording is 
made to the laser driving signal LP in such a manner that the pulse level 
thereof is significantly increased for a prescribed period from the 
leading edge of the pulse thereby increasing the power of the laser. As a 
result, the temperature of the optical disk surface quickly rises 
immediately after the start of illuminating the light beam, allowing the 
recording pit WP to be formed in an oval shape, as shown in FIG. 2(d), 
having a wide pit width W from the leading edge. Therefore, when 
regenerating the recorded information, the regeneration signal RD shown in 
FIG. 2(e) is obtained from the light reflected from the optical disk, 
which sharply rises. Consequently, the period during which the 
regeneration signal RD exceeds the threshold level Vth becomes longer than 
in the case of the teardrop-shaped recording pit shown in FIG. 1(d), so 
that the pit detection pulse DP has a wider pulse width as shown in FIG. 
2(f). The pulse width of the pit detection pulse DP is approximately equal 
to the pulse width of the recording data WD shown in FIG. 2(a) or (b), 
thus ensuring accurate regeneration of the information recorded on the 
optical disk. 
However, in order to raise the laser diode power by increasing the level of 
the laser driving signal LP for a prescribed period on the leading edge 
side, as described above, it requires the use of a laser diode having a 
high power, the resulting problem being that the reliability of the 
optical disk apparatus is impaired and life of the apparatus is reduced. 
SUMMARY OF THE INVENTION 
The present invention has been devised to overcome the above enumerated 
problems, and it is a primary object of the invention to provide an 
optical disk apparatus in which the timing to start or end the light beam 
projection on an optical disk or the number of emissions and the duration 
of each emission of the light beam is controlled according to the pulse 
pattern of the information to be recorded, thereby correcting the 
recording pit pattern on the optical disk using the existing light source 
power so as to ensure accurate regeneration of the recorded information. 
The above and further objects and features of the invention will more fully 
be apparent from the following detailed description with accompanying 
drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The preferred embodiments of the invention will now be described with 
reference to the accompanying drawings. 
FIG. 3 is a block diagram of the essential parts of an optical disk 
apparatus according to the invention. There are provided a latch circuit 
10, a pattern discriminating circuit 11, a timing data generating unit 12, 
and a laser driving signal generating circuit 13, which together 
constitute a laser driving circuit 14. 
The latch circuit 10 is supplied with recording data WD of information and 
a latch timing signal TD, the recording data WD being latched by the latch 
circuit 10 in synchronism with the timing signal TD. The data latched by 
the latch circuit 10 is fed to the pattern discriminating circuit 11 as 
well as to the laser driving signal generating circuit 13, and the pattern 
discriminating circuit 11 discriminates the pattern of the recording data 
WD. The output of the pattern discriminating circuit 11 is supplied to the 
timing data generating unit 12. The timing data generating unit 12 
previously stores timing data corresponding to various patterns of 
recording data WD and produces the timing data corresponding to the 
pattern of the recording data WD from the pattern discriminating circuit 
11 to supply the timing data the laser driving signal generating circuit 
13. The laser driving signal generating circuit 13 generates a laser 
driving signal obtained by modifying the leading edge of the pulse of the 
supplied recording data WD in accordance with the timing data given from 
the timing data generating unit 12. The laser driving signal is fed to a 
laser control circuit 15. 
The laser control circuit 15 controls a laser diode 4a to adjust the light 
power of the laser diode 4a to the level suitable for recording or 
regeneration of information. 
A magneto-optical disk 1 is rotated by a spindle motor 2. On one side of 
the magneto-optical disk 1, there is disposed an electromagnet 3 as a 
magnetic head for applying a magnetic field to the magneto-optical disk 1, 
while an optical head 4 is disposed on the other side of the 
magneto-optical disk 1. Light emitted from the laser diode 4a in the 
optical head 4 passes through a collimator lens 4b, a beam splitter 4c and 
a lens 4d, and is projected onto the magneto-optical disk 1. Light 
reflected by the beam splitter 4c passes through a .lambda./4 plate 4e, a 
polarizing beam splitter 4f and a first converging lens 4g, and enters a 
first photo-detector 4h. Light reflected by the polarizing beam splitter 
4f passes through a second converging lens 4i and enters a second 
photo-detector 4j. The outputs of the first and second photo-detectors 4h 
and 4j are respectively supplied to the two input terminals 5a and 5b of a 
preamplifier 5. The output signal from the preamplifier 5 is fed to one 
input terminal 16a of a comparator 16, while a threshold voltage Vth is 
given to the other terminal 16b of the comparator 16. As a result, the 
comparator 16 outputs a pit detection signal. 
The operation of the thus constructed magneto-optical disk apparatus will 
now be described with reference to the timing chart of FIG. 4 for 
recording and regeneration signals of information and recording pits. 
To record information on the magneto-optical disk 1, the electromagnet 3 is 
energized to apply a magnetic field to the magneto-optical disk 1 which is 
rotated by the spindle motor 2, and light emitted from the laser diode 4a 
is projected onto the rotating magneto-optical disk 1 thereby to record 
information thereon. 
The data word of information to be recorded is converted, for example, by 
"2/7" coding, into recording data (RZ modulation) as shown in FIG. 4(a) in 
the case of pit position recording, and into recording data (NRZI 
modulation) as shown in FIG. 4(b) in the case of pit edge recording. The 
information is recorded using one or other of the recording data. 
In the recording of information "1", the information "1" is recorded during 
one pulse period in the case of FIG. 4(a). On the other hand, in the case 
of FIG. 4(b), the information "1" is recorded at both the leading edge and 
trailing edge of a recording pulse. In other words, in the case of FIG. 
4(a), a single "1" is recorded with one pulse, while in the case of FIG. 
4(b), two "1"s are recorded with one pulse. 
For example, when the recording data WD shown in FIG. 4(b) and the timing 
data TD are given to the latch circuit 10 in the laser driving circuit 14, 
the latch circuit 10 latches the recording data WD in synchronism with the 
timing data TD and supplies the parallel/serial converted recording data 
WD to the pattern discriminating circuit 11 and the laser driving signal 
generating circuit 13. The pattern discriminating circuit 11 discriminates 
the waveform pattern of the recording data WD given from the latch circuit 
10 and supplies the discrimination data to the timing data generating unit 
12. In response, the timing data generating unit 12 outputs the timing 
data corresponding to the discriminated pattern to the laser driving 
signal generating circuit 13. 
Based on the timing data from the timing data generating unit 12, the laser 
driving signal generating circuit 13 generates a pulse LP1 of a laser 
driving signal LP as shown in FIG. 4(c), with the leading edge in advance 
of that of the first pulse P of the recording data WD of FIG. 4(b) 
supplied from the latch circuit 10. The leading edge of the pulse LP1 thus 
precedes the leading edge of the first pulse P of the recording data WD by 
.DELTA.t0. Likewise, the leading edge of the second pulse LP2 of the laser 
driving signal LP precedes the leading edge of the second pulse P of the 
recording data WD by .DELTA.t1. On the other hand, the leading edge of the 
third pulse LP3 is delayed from the leading edge of the third pulse P of 
the recording data WD by .DELTA.t2. In other words, when the pulses Ps of 
the recording data WD are generated repetitively at short intervals, the 
pit pattern formed by the succeeding pulse P becomes larger as it is 
affected by the heat of the preceding pulse. Therefore, the leading edge 
of the succeeding pulse of the laser driving signal LP is delayed from the 
leading edge of the corresponding pulse P to form the pit pattern in a 
proper shape. 
Thus generated laser driving pulse LP is supplied to the laser control 
circuit 15 which controls the laser diode 4a in accordance with the laser 
driving signal LP to obtain the light power for recording information. The 
laser beam emitted from the laser diode 4a passes through the collimator 
lens 4b, the beam splitter 4c, and the lens 4d, and is projected onto the 
magneto-optical disk 1. As a result, the illuminated area of the 
magneto-optical disk 1 is heated by the laser beam up to the Curie 
temperature to be magnetized in the direction of an externally applied 
magnetic field, thus forming the recording pit shown in FIG. 4(d). Since 
the leading edge of each of the pulses LP1, LP2, LP3 of the laser driving 
signal LP is modified in accordance with the pattern of the preceding 
recording data WD, it ensures that each area of the magneto-optical disk 1 
corresponding to the leading edge of the pulse P of the recording data WD 
is heated up to approximately the same temperature. 
When forming a succeeding pit after a short time interval, the effect of 
heat from the preceding pit serves to accelerate the formation of the 
succeeding pit. As a result, the width of the pulse DP of detecting the 
pit will become wider than the pulse width of the recording data WD. 
Therefore, in this embodiment, when the pulses Ps of the recording data WD 
are generated repetitively at short intervals, the leading edge of each 
succeeding pulse LP is delayed from that of the corresponding pulse P. The 
delay time is optimized with reference to the preceding pattern of the 
recording data WD. On the other hand, when there is no effect of heat from 
the preceding pulse LP, a teardrop-shaped pit is formed, as previously 
noted, in which case the width of the pulse DP of detecting the pit will 
become narrower than the pulse width of the recording data WD. Therefore, 
a correction is made before recording to the pulse width which is expected 
to be reduced, so that the laser driving signal LP is generated with its 
leading edge in advance of that of the recording data WD, in order to form 
the recording pit WP. 
When regenerating thus recorded information, the laser control circuit 15 
controls the laser diode 4a to emit a light beam with power suitable for 
regeneration of information. The light emitted from the laser diode 4a 
passes through the collimator lens 4b, the beam splitter 4c and the lens 
4d, and is projected onto the rotating magneto-optical disk 1. The light 
reflected from the magneto-optical disk 1 passes through the lens 4d and 
is reflected by the beam splitter 4c. The reflected light then passes 
through the .lambda./4 plate 4e, reflected by the polarizing beam splitter 
4f, and enters the first photo-detector 4h through the converging lens 4g. 
On the other hand, the light reflected by the polarizing beam splitter 4f 
passes through the converging lens 4i and enters the second photo-detector 
4j. Each of the photo-detectors 4h and 4j outputs an analog signal 
corresponding to the incident illumination by photoelectric transfer, and 
the analog signal is amplified by the preamplifier 5 to obtain a 
regeneration signal RD shown in FIG. 4(e). The regeneration signal RD is 
compared with the threshold voltage Vth by the comparator 16. When the 
regeneration signal RD exceeds the threshold voltage, the comparator 16 
outputs a pit detection signal DP, shown in FIG. 4(f), whose pulse width 
is approximately equal to that of the pulse D of the recording data WD, 
thus enabling accurate regeneration of the recorded information. 
Therefore, information can be accurately recorded on the magneto-optical 
disk without temporarily increasing the power of the laser diode 4a during 
the recording of information. 
In the above embodiment, the leading edge of each of the pulses LP1, LP2, 
LP3 of the laser driving signal LP is modified relating to the pattern of 
the preceding recording data WD, but alternatively, when the trailing edge 
of each of the pulses LP1, LP2, LP3 of the laser driving signal LP may be 
modified relating to the pattern of the succeeding recording data WD, as 
shown in FIG. 5, each area of the magneto-optical disk 1 corresponding to 
the leading edge of the pulse P of the recording data WD can be heated up 
to approximately the same temperature. 
The same recording process can also be applied to the RZ modulation 
recording data WD shown in FIG. 4(a). 
FIG. 6 is a timing chart for recording and regeneration signals of 
information and recording pits according to another embodiment of the 
optical disk of the invention. As shown in FIG. 6(c), in this embodiment, 
the pulse of the laser driving signal LP during the recording period of 
information "1" is subdivided into a group of pulses of different widths, 
the first pulse in the pulse group having a different width from the other 
pulses in the group corresponding to the pattern of the recording data WD. 
The second and subsequent pulses 2P, 3P in each pulse group are provided 
with gradually diminishing widths or with the same widths. The pulse 
number data and timing data of various pulse groups for the laser driving 
signal LP are stored in the timing data generating unit 12 shown in FIG. 
3, and the pulse number data and timing data corresponding to the pattern 
of the recording data WD are supplied to the driving signal generating 
circuit 13 to obtain the laser driving signal LP as shown in FIG. 6(c). 
When the laser diode 4a is driven using the above laser driving signal LP, 
the temperature of the surface of the magneto-optical disk 1 illuminated 
by a light beam quickly rises upon the illumination, but the subsequent 
heating up of the disk surface is suppressed. This serves to reduce the 
effect of heat from the preceding recording pit WP when the recording pits 
WP are formed in close proximity to each other. As a result, each 
recording pit WP is formed in an oval shape as shown in FIG. 6(d), the 
width being maintained approximately the same across the length in the 
longitudinal direction thereof. 
Also, when regenerating the recorded information, a pit detection signal DP 
is obtained which has approximately the same width as that of the pulse of 
the recording data WP, as shown in FIG. 6(f), thus permitting accurate 
regeneration of the recorded information. 
In the above embodiment, recording and regeneration of information to/from 
a magneto-optical disk has been described, but it will be appreciated that 
the same effects as stated herein can also be obtained when the invention 
is applied to an optical disk apparatus in which information is recorded 
to and regenerated from an optical disk without applying a magnetic field. 
Also, in the above embodiment, the laser diode is driven by a laser driving 
signal corresponding to the pattern of information, but it will be 
appreciated that the same effects as stated herein can be obtained when a 
shutter capable of blocking the laser diode light to be projected onto the 
optical disk is provided and the shutter is driven by a control signal 
similar to the laser driving signal. 
Further, light to be projected onto an optical or magneto-optical disk is 
not restricted to a laser light. 
As this invention may be embodied in several forms without departing from 
the spirit of essential characteristics thereof, the present embodiment is 
therefore illustrative and not restrictive, since the scope of the 
invention is defined by the appended claims rather than by the description 
preceding them, and all changes that fall within metes and bounds of the 
claims, or equivalence of such metes and bounds thereof are therefore 
intended to be embraced by the claims.