U.S. patent number RE42,786 [Application Number 10/825,439] was granted by the patent office on 2011-10-04 for optical recording method and optical recording medium.
This patent grant is currently assigned to Mitsubishi Kagaku Media Co., Ltd.. Invention is credited to Michikazu Horie, Natsuko Nobukuni.
United States Patent |
RE42,786 |
Nobukuni , et al. |
October 4, 2011 |
Optical recording method and optical recording medium
Abstract
An optical recording method for recording mark length-modulated
information on a recording medium by using a plurality of recording
mark lengths. The optical recording method comprises the steps of:
when a time length of one recording mark is denoted nT (T is a
reference clock period equal to or less than 25 ns, and n is a
natural number equal to or more than 2), (i) dividing the time
length of the recording mark nT into .eta..sub.1T, .alpha..sub.1T,
.beta..sub.1T, .alpha..sub.2T, .beta..sub.2T, . . . ,
.alpha..sub.iT, .beta..sub.iT, . . . , .alpha..sub.mT,
.beta..sub.mT, .eta..sub.2T in that order (m is a pulse division
number,
.SIGMA..sub.i(.alpha..sub.i+.beta..sub.i)+.eta..sub.1+.eta..sub.2=n;
.alpha..sub.i (1.ltoreq.i.ltoreq.m) is a real number >0;
.beta..sub.i (1.ltoreq.i.ltoreq.m-1) is a real number>0;
.beta..sub.m is a real number.gtoreq.0; and .eta..sub.1 is a real
number of -2.ltoreq..eta..sub.1.ltoreq.2 and .eta..sub.2 is a real
number of -2.ltoreq..eta..sub.2.ltoreq.2); radiating recording
light with a recording power Pw.sub.i in a time duration of
.alpha..sub.iT (1.ltoreq.i.ltoreq.m), and radiating recording light
with a bias power Pb.sub.i in a time duration of .beta..sub.iT
(1.ltoreq.i.ltoreq.m), the bias power being Pb.sub.i<Pw.sub.i
and Pb.sub.i<Pw.sub.i+1; and (ii) changing m, .alpha..sub.i,
.beta..sub.i, .eta..sub.1, .eta..sub.2, Pw.sub.i and Pb.sub.i
according to n of the time length nT of the recording mark; wherein
the pulse division number m is 2 or more for the time duration of
at least one recording mark and meets n/m.gtoreq.1.25 for the time
length of all the recording marks.
Inventors: |
Nobukuni; Natsuko (Kanagawa,
JP), Horie; Michikazu (Kanagawa, JP) |
Assignee: |
Mitsubishi Kagaku Media Co.,
Ltd. (Tokyo, JP)
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Family
ID: |
26471209 |
Appl.
No.: |
10/825,439 |
Filed: |
April 16, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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09884121 |
Jun 20, 2001 |
6411579 |
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PCT/JP00/03036 |
May 11, 2000 |
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Reissue of: |
10141981 |
May 10, 2002 |
6661760 |
Dec 9, 2003 |
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Foreign Application Priority Data
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May 19, 1999 [JP] |
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11-138067 |
Mar 17, 2000 [JP] |
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2000-076514 |
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Current U.S.
Class: |
369/59.11;
369/59.2; 369/116; 369/59.12 |
Current CPC
Class: |
G11B
7/0062 (20130101); G11B 7/243 (20130101); G11B
11/1053 (20130101); G11B 7/00456 (20130101); G11B
11/10528 (20130101); G11B 7/126 (20130101); G11B
2007/24308 (20130101); G11B 11/10595 (20130101); G11B
2007/24304 (20130101); G11B 2007/24312 (20130101); G11B
2007/24306 (20130101); G11B 2007/24316 (20130101); G11B
7/2534 (20130101); G11B 2007/2432 (20130101); G11B
11/10506 (20130101); G11B 2007/2431 (20130101); G11B
2007/24314 (20130101); G11B 7/2585 (20130101) |
Current International
Class: |
G11B
7/00 (20060101) |
Field of
Search: |
;369/47.1,47.28,47.5,53.11,59.12,59.2,116,59.11 |
References Cited
[Referenced By]
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62-259229 |
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63-22439 |
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63-266632 |
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4-325288 |
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JP |
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7-37251 |
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JP |
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7-37252 |
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Feb 1995 |
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JP |
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8-32482 |
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JP |
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8-287465 |
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Nov 1996 |
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JP |
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9-7176 |
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JP |
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9-71049 |
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JP |
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9-282661 |
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Oct 1997 |
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JP |
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10-112063 |
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Apr 1998 |
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JP |
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WO 99/06220 |
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Feb 1999 |
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WO |
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Other References
US. Appl. No. 11/251,941, filed Oct. 18, 2005, Horie et al. cited
by other .
U.S. Appl. No. 11/374,042, filed Mar. 14, 2006, Ohno. cited by
other .
U.S. Appl. No. 10/657,121, filed Sep. 9, 2003, Nobukuni et al.
cited by other .
U.S. Appl. No. 11/217,593, filed Sep. 2, 2005, Nobukuni et al.
cited by other .
M. Horie, et al., Optical Data Storage, Proceedings of SPIE, vol.
4342, XP-002267102, pp. 76-87, "Material Characterization and
Application of Eutectic SbTe Based Phase-Change Optical Recording
Media", Apr. 2001. cited by other .
Patent Abstracts of Japan, JP 10-241160, Sep. 11, 1998. cited by
other .
Patent Abstracts of Japan, JP 61-020236, Jan. 29, 1986. cited by
other .
N. Nobukuni, et al., J. Appl Physics, "Microstructural changes in
GeSbTe film during repititious overwriting in phase-change optical
recording," Dec. 15, 1995, pp. 6980-6988. cited by other.
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Primary Examiner: Edun; Muhammad N
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier
& Neustadt, L.L.P.
Parent Case Text
This is a division of application Ser. No. 09/884,121 filed Jun.
20, 2001, now U.S. Pat. No. 6,411,579, which is a continuation
application of International patent application No.PCT/JP00/03036,
filed May 11, 2000.
Claims
What is claimed is:
1. An optical recording method for recording mark length-modulated
information with a plurality of recording mark lengths by
irradiating a recording medium with a light, the optical recording
method comprising the steps of: when a time length of one recording
mark is denoted nT (T is a reference clock period equal to or less
than 25 ns, and n is a natural number equal to or more than 2),
dividing the time length of the recording mark nT into
.eta..sub.1T, .alpha..sub.1T, .beta..sub.1T, .alpha..sub.2T,
.beta..sub.2T, . . . , .alpha..sub.iT, .beta..sub.iT, . . . ,
.alpha..sub.mT, .beta..sub.mT, .eta..sub.2T in that order (m is a
pulse division number; .[..SIGMA.hdi.].
.Iadd..SIGMA..sub.i.Iaddend.(.alpha..sub.i+.beta..sub.i)+.eta..sub.1+.eta-
..sub.2=n; .alpha..sub.i .[.(1.ltoreq.i.ltoreq.m-1).].
(.Iadd.1.ltoreq.i.ltoreq.m.Iaddend.) is a real number larger than
0; .[..beta.i.]. .Iadd..beta..sub.i.Iaddend.
(1.ltoreq.i.ltoreq.m-1) is a real number larger than 0;
.beta..sub.m is a real number larger than or equal to 0;
.alpha..sub.i+.beta..sub.i (2.ltoreq.i.ltoreq.m-1) or
.[..beta..sub.i-1.]. .Iadd..beta..sub.i-l.Iaddend.+.alpha..sub.i
(2.ltoreq.i.ltoreq.m-1) is kept constant independently of said real
number i; and .eta..sub.1 and .eta..sub.2 are real numbers between
-2 and 2); radiating recording light with a recording power
Pw.sub.i.Iadd., .Iaddend.in a time duration of .alpha..sub.iT
(1.ltoreq.i.ltoreq.m); and radiating recording light with a bias
power Pb.sub.i in a time duration of
.beta..sub.iT(1.ltoreq.i.ltoreq.m-1), the bias power being
Pb.sub.i<Pw.sub.i and Pb.sub.i<Pw.sub.i+1; wherein the pulse
division number m is 2 or more for the time duration of at least
one recording mark and meets n/m.gtoreq.1.25 for the time length of
all the recording marks, further wherein when the same pulse
division number m is used on at least two recording marks with
different n values, a difference mark length is formed by changing
at least one of .beta..sub.1, .beta..sub.m-1, and .[..beta.m.].
.Iadd..beta..sub.m.Iaddend..
2. An optical recording method according to claim 1, wherein
.alpha..sub.i+.beta..sub.i (2.ltoreq.i.ltoreq.m-1) or
.[..beta..sub.i-1.]. .Iadd..beta..sub.i-l.Iaddend.+.alpha..sub.i
(2.ltoreq.i.ltoreq.m-1) is 2 independently of said real number
i.
3. An optical recording method according to claim 1, wherein
.alpha..sub.i is kept constant as a constant value .alpha.c where
said .Iadd.i .Iaddend.is (2.ltoreq.i.ltoreq.m-1).
4. An optical recording method according to claim 1, wherein
.alpha..sub.i(2.ltoreq.i.ltoreq.m-1) is kept constant in the time
length of the recording mark with having a pulse division number m
being at least 3.
5. An optical recording method according to claim 1, wherein when
performing a mark length modulation scheme recording on the same
recording medium by using a plurality of linear velocities v while
keeping v.times.T constant, for m equal to or greater than 2,
(.alpha..sub.i+.beta..sub.i) in 2.ltoreq.i.ltoreq.m-1 is kept
constant independently of the linear velocity, .[.Pw.sub.1.].
.Iadd.PW.sub.i.Iaddend., Pb.sub.i and Pe in each i are kept almost
constant independently of the linear velocity, and .alpha..sub.i
(2.ltoreq.i.ltoreq.m) is decreased as the linear velocity
lowers.
6. An optical recording method according to claim 1, wherein when
performing a mark length modulation scheme recording on the same
recording medium by using a plurality of linear velocities v while
keeping v.times.T constant, for m equal to or greater than 2,
(.[..beta..sub.i-1.]. .Iadd..beta..sub.i-l.Iaddend.+.alpha..sub.i)
in 2.ltoreq.i.ltoreq.m are kept constant independently of the
linear velocity, .[.Pw.sub.1.]. .Iadd.PW.sub.i.Iaddend., Pb.sub.1
and Pe in each i are kept almost constant independently of the
linear velocity, and .alpha..sub.i (2.ltoreq.i.ltoreq.m) are
decreased as the linear velocity lowers.
7. An optical recording .Iadd.method .Iaddend.according to claim 5
or 6, wherein .alpha..sub.iT (2.ltoreq.i.ltoreq.m-1) is kept almost
constant independently of the linear velocity.
8. An optical recording method according to claim 1, the phase
change type optical recording medium having a recording layer made
of M.sub.zGe.sub.y(Sb.sub.xTe.sub.1-x).sub.1-y-z alloy (where
0.ltoreq..sub.z.ltoreq.0.1, 0<.sub.y.ltoreq.0.3, 0.8.ltoreq.x;
and M is at least one of In, Ga, Si, Sn, Pb Pd, Pt, Zn, Au, Ag, Zr,
Hf, V, Nb, Ta, Cr, Co, Mo, Mn, Bi, O, N and S).
9. .[.An.]. .Iadd.A non-transitory .Iaddend.optical information
recording medium having a recording layer, containing excessive Sb
in SbTe eutectic point, in which phase change is made reciprocally
between a crystal state and amorphous state with optical
characteristic being differed from each other by irradiation of an
optical beam, wherein said crystal condition is defined as
polycrystal made of a substantial single crystal phase of a
hexagonal crystal.
10. An optical information recording medium according to claim 9,
wherein said recording layer is made of
M.sub.zGe.sub.y(Sb.sub.xTe.sub.1-x).sub.1-y-z alloy (where
0.ltoreq.z.ltoreq.0.1 0<y.ltoreq.0.3, 0.8.ltoreq.x; and M is at
least any one of In, Ga, Si, Sn, Pb, Pd, Pt, Zn, Au, Ag, Zr, Hf, V,
Nb, Ta, Cr, Co, Mo, Mn, Bi, O, N and S).
11. An optical information recording medium according to claim 9 or
10, wherein said crystal state of said recording layer is defined
as an unrecorded state .[.and an erased state, while said amorphous
state thereof is defined as a recorded state.]. and an erased
state, while said amorphous state thereof is defined as a recorded
state so as to .[.performed.]. .Iadd.perform .Iaddend.recording or
erasing of information.
12. A method of manufacturing an optical information recording
medium having a recording layer, containing excessive Sb in SbTe
eutectic point, in which phase change is made recriprocally between
a crystal state and amorphous state with optical characteristic
being differed from each other by irradiation of an optical beam,
wherein an initialization step is performed with another optical
beam having an elliptical beam shape of which minor axis is 0.1-10
.mu.m after forming at least said recording layer on a substrate,
by scanning said another optical beam to the recording layer in a
direction of said minor axis so as to make the recording layer in
the crystal state, further wherein said scanning of said optical
beam is performed in a speed in a range of 50-80% of a maximum
usable linear velocity for over-writing of the recording layer.
Description
TECHNICAL FIELD
The present invention relates to an optical recording method and an
optical recording medium.
BACKGROUND ART
As the amount of information increases in recent years, there are
growing demands for a recording medium capable of writing and
retrieving a large amount of data at high speed and in high
density. There are growing expectations that the optical disks will
meet this demand.
There are two types of optical disks: a write-once type that allows
the user to record data only once, and a rewritable type that
allows the user to record and erase data as many times as they
wish. Examples of the rewritable optical disk include a
magnetooptical recording medium that utilizes a magneto-optical
effect and a phase-change type recording medium that utilizes a
change in reflectance accompanying a reversible crystal state
change.
The principle of recording an optical disk involves applying a
recording power to a recording layer to raise the temperature of
that layer to or above a predetermined critical temperature to
cause a physical or chemical change for data recording. This
principle applies to all of the following media: a write-once
medium utilizing pitting or deformation, an magnetooptical medium
utilizing a magnetic reversal at the vicinity of the Curie point,
and a phase change medium utilizing a phase transition between
amorphous and crystal states of the recording layer.
Further, taking advantage of the 1-beam-overwrite ability (erasing
and writing at the same time) of the phase change recording medium,
rewritable compact disks compatible with CDs and DVDs
(CD-ReWritable and CD-RW) and rewritable DVDs have been
developed.
Almost all of these optical recording media in recent years employ
a mark length recording method, which is suited for increasing the
recording density.
The mark length recording is a method that records data by changing
both the lengths of marks and the lengths of spaces. Compared with
a mark position recording method which changes only the lengths of
the spaces, this method is more suited to increasing the recording
density and can increase the recording density by as much as 1.5
times. However, to retrieve data accurately makes the detection of
the time length of the mark stringent, thus requiring precise
control of the shape of mark edges. Further, there is another
difficulty that a plurality of kinds of marks with different
lengths, from short marks to long marks, need to be formed.
In the following descriptions, the spatial length of a mark is
referred to as a mark length and a time length of the mark as a
mark time length. When a reference clock period is determined, the
mark length and the mark time length have a one-to-one
correspondence.
In the mark length recording. when writing an nT mark (a mark
having a mark time length of nT where T is a reference clock period
of data and n is a natural number), simply radiating a recording
power of square wave with the time length of nT or with the length
finely adjusted will result in the front and rear ends of each mark
differing in temperature distribution, which in turn causes the
rear end portion in particular to accumulate heat and widen,
forming an mark with an asymmetric geometry. This raises
difficulties in precisely controlling the mark length and
suppressing variations of the mark edge.
To uniformly shape the marks, from short marks to long marks,
various means have been employed, such as division of recording
pulses and use of off pulses. For example, the following techniques
have been adopted in the phase change media.
That is, a recording pulse is divided to adjust the geometry of an
amorphous mark (JP-A62-259229, JP-A63-266632). This approach is
also utilized in the write-once medium that is not overwritten.
Further, an off pulse is widely employed as a mark shape
compensation means (JP-A 63-22439, etc.)
Other proposed methods include one which deliberately dull a
trailing edge of the recording pulse to adjust the mark length and
the mark time length (JP-A 7-37252); one which shifts a recording
pulse radiation time (JP-A 8-287465); one which, in a multipulse
recording method, differentiates a value of bias power during the
mark writing operation from that during the space writing operation
or erasing operation (JP-A 7-37251); and one which controls a
cooling time according to a linear velocity (JP-A 9-7176).
The recording method based on the above pulse division approach is
also used in the magnetooptical recording medium and the write-once
type optical recording medium. In the magnetooptical and write-once
type mediums, this approach aims to prevent heat from becoming
localized. In the phase change medium, this approach has additional
objective of preventing recrystallization.
Common examples of mark length modulation recording include a CD
compatible medium using an EFM (Eight-Fourteen Modulation), a DVD
compatible medium using an EFM+ modulation, a variation of 8-16
modulation, and a magnetooptical recording medium using a (1,
7)-RLL-NRZI (Ruu-Length Limited Non-Return to Zero Inverted)
modulation. The EFM modulation provides 3T to 11T marks; the EFM+
modulation provides 3T to 14T marks; and the (1, 7)-RLL-NRZI
modulation provides 2T to 8T marks. Of these, the EFM+ modulation
and the (1, 7)-RLL-NRZI modulation are known as modulation methods
for high-density mark length modulation recording.
As the recording pulse division scheme for the mark length
modulation recording media such as CD, the following method is
widely used.
That is, when a mark to be recorded has a time length of nT (T is a
reference clock period and n is a natural number equal to or
greater than 2), the time (n-.eta.)T is divided into
.alpha..sub.1T, .beta..sub.1T, .alpha..sub.2T, .beta..sub.2T, . . .
, .alpha..sub.mT, .beta..sub.mT (where
.SIGMA..alpha..sub.i+.SIGMA..beta..sub.i=n-.eta.; .eta. is a real
number from 0 to 2; m is a number satisfying m=m-k; and k is 1 or
2). In a time duration of .alpha..sub.iT (1.ltoreq.i.ltoreq.m) as
the recording pulse section, recording light with a recording power
Pw is radiated. In a time duration of .beta..sub.iT
(1.ltoreq.i.ltoreq.m) as the off pulse section, recording light
with a bias power Pb, less than Pw, is radiated.
FIG. 2 is a schematic diagram showing a power pattern of the
recording light used in this recording method. To form a mark of a
length shown in FIG. 2(a), a pattern shown in FIG. 2(b) is used.
When forming a mark that is mark-length-modulated to the length of
nT (T is a reference clock period; and n is a mark length, an
integer value, that can be taken in the mark length modulation
recording), (n-.eta.)T is divided into m=n-k (k is 1 or 2)
recording pulses (in the case of FIG. 2(b), k=1 and .eta.=0.5), and
the individual recording pulse widths are set to .alpha..sub.iT
(1.ltoreq.i.ltoreq.m), each followed by the off pulse section of
.beta..sub.iT (1.ltoreq.i.ltoreq.m). In the .alpha..sub.iT
(1.ltoreq.i.ltoreq.m) section during the recording, the recording
light with the recording power Pw is radiated and, in the
.beta..sub.iT (1.ltoreq.i.ltoreq.m) section, the bias power Pb
(Pb<Pw) is radiated. At this time, to ensure that an accurate nT
mark can be obtained during the detection of the mark length,
.SIGMA..alpha..sub.i+.SIGMA..beta..sub.i may be set slightly
smaller than n, and the following setting is made:
.SIGMA..alpha..sub.i+.SIGMA..beta..sub.i=n-.eta. (.eta. is a real
number in 0.0.ltoreq..eta..ltoreq.2.0).
That is, in the conventional technique, when the recording light to
be radiated to form an nT mark is divided, the recording pulse is
divided into m pieces (m=n-k, where k is 1 or 2), m being obtained
by uniformly subtracting k from n (as described in JP-A 9-282661),
and then a predetermined number is subtracted from the number of
divisions m of the recording pulse to control the mark time length
accurately (in the following, such a pulse division scheme is
called an "n-k division" scheme).
Generally, the reference clock period T decreases as the density or
speed increases. For example, T decreases in the following
cases.
(1) When the Recording Density is Enhanced to Increase the
Recording Capacity
As the mark length and the mark time length are reduced, the
density increases. In this case, a clock frequency needs to be
increased to reduce the reference clock period T.
(2) When the Recording Linear Velocity is Increased to Increase a
Data Transfer Rate
In the high-speed recording of recordable CDs and DVDs, the clock
frequency is increased to reduce the reference clock period T. In a
CD-based medium such as a rewritable compact disk, for example, the
reference clock period T during a .times.1-speed operation (linear
velocity is 1.2-1.4 m/s) is 231 nanoseconds; but during a
.times.10-speed operation the reference clock period T becomes very
short, 23.1 nanoseconds. In the DVD-based medium, while the
reference clock frequency T during a .times.1-speed operation (3.5
m/s) is 38.2 nanoseconds, it is 19.1 ns during a .times.2-speed
operation.
As can be seen from the (1) and (2), in large-capacity optical
disks and CDs and DVDs with high data transfer rates, the reference
clock period T is very short. As a result, the recording pulse
section .alpha..sub.iT and the off pulse section .beta..sub.iT also
tend to become short. Under these circumstances the following
problems arise.
Problem a
The recording pulse section .alpha..sub.iT may be too short for the
rising/falling edge speed of radiated light, particularly a laser,
to follow. A rise time is a time taken by the projected power of
radiated light such as laser to reach a set value, and a fall time
is a time taken by the projected power of the radiated light such
as laser to fall from the set value to a complete off level. At
present the rise and fall times take at least 2-3 nanoseconds
respectively. Hence, when the pulse width is less than 15 ns. for
example, the time it takes for the light to actually project a
required power is a few nanoseconds. Further, when the pulse width
is less than five nanoseconds, the projected power begins to fall
before it reaches the set value, so that the temperature of the
recording layer does not rise sufficiently, failing to produce a
predetermined mark size. These issues of response speed limits of a
signal source and a laser beam cannot be dealt with by making
improvements on the wavelength of a light source, on the method of
radiating light onto substrate/film surface, or on other recording
methods.
Problem b
When the off pulse section .beta..sub.iT is narrow, the recording
medium cannot take a sufficient time to cool down and the off pulse
function (cooling speed control function) does not work although
the off pulse section is provided, leaving heat to be accumulated
in the rear end part of the mark, making it impossible to form the
correct shape of the mark. This problem becomes more serious as the
length of the mark increases.
This problem will be explained by taking a phase change medium as
an example.
The currently available phase change medium typically takes crystal
portions as an unrecorded state or erased state and amorphous
portions as a recorded state. To form an amorphous mark involves
radiating a laser onto a tiny area of the recording layer to melt
that tiny portion and quickly cooling it to form an amorphous mark.
When, for example, a long mark (a mark more than about 5T in length
based on the EFM modulation recording for CD format) is formed
using a rectangular waveform of recording power with no off pulse
section at all, as shown in FIG. 3(a), then an amorphous mark with
a narrow rear end is formed as shown in FIG. 3(b) and a distorted
retrieve waveform is observed as shown in FIG. 3(c). This is
because, in the rear part of the long mark in particular, heat is
accumulated by heat diffusion from the front part enlarging the
melted area in the rear part but the cooling speed deteriorates
significantly allowing the melted area to recrystallize as it
solidifies. This tendency becomes conspicuous as the linear
velocity for recording decreases because the cooling speed of the
recording layer becomes slower as the linear velocity
decreases.
Conversely, if the cooling speed is so high as to render
recrystallization almost negligible, when a long mark is recorded,
an amorphous mark with a thicker rear end is formed as shown in
FIG. 3(d), producing a distorted retrieve waveform as shown in FIG.
3(e). This is explained as follows. In the rear end of the long
mark in particular, heat is accumulated by heat diffusion from the
front part enlarging the melted area in the rear part and the shape
of the melted area is transformed into the shape of an amorphous
mark relatively precisely because the cooling speed is kept
relatively high over the entire area.
When a plurality of off pulse sections are not distributed and
properly used over the entire mark length, recrystallization
becomes conspicuous somewhere in the mark, as shown in FIGS. 3(b)
and 3(d) though in different degrees, preventing a good formation
of an amorphous long mark and causing distortions in the retrieve
waveform.
Inserting the off pulse sections makes sharp the temperature change
over time of the recording layer ranging from the front end to the
rear end of the long mark, preventing degradation of the mark due
to recrystallization during recording.
However, as the reference clock frequency T becomes shorter because
of increased density and speed as described above, the rapid
cooling becomes difficult to achieve even with the off pulse
sections provided in a conventional manner, resulting in the front
half of the mark being recrystallized.
For example, when a mark with a time length of 4T is to be recorded
on a CD-RW, a phase change type rewritable compact disk, by the
conventional n-k division scheme (k=1), the following pulses are
radiated during the process of forming the amorphous mark:
.alpha..sub.1T, .beta..sub.1T, .alpha..sub.2T, .beta..sub.2T,
.alpha..sub.3T, .beta..sub.3T
Here, the starting end of the mark is melted by the application of
the recording pulse .alpha..sub.1T and then heat produced by the
application of the subsequent recording pulses .alpha..sub.2T,
.alpha..sub.3T conducts toward the front part of the mark. FIG. 4
is a schematic temperature history of the mark starting end, with
FIG. 4(a) representing a case in which the linear velocity is low
and FIG. 4(b) a case in which the linear velocity is high. In
either case, three temperature rising processes due to
.alpha..sub.1T, .alpha..sub.2T, .alpha..sub.3T and three cooling
processes due to .beta..sub.1T, .beta..sub.2T, .beta..sub.3T are
observed.
In the case of low linear velocity, as shown in FIG. 4(a), there
are sufficient cooling times at .beta..sub.1T, .beta..sub.2T,
during each of which the temperature of the cooling layer can fall
below the crystallization temperature. In the case of high linear
velocity, however, because the reference clock period T decreases
in inverse proportion to the linear velocity, the recording layer
melted by the .alpha..sub.1T is heated by the next .alpha..sub.2T
and further by .alpha..sub.3T without cooling below the
crystallization temperature range, as shown in FIG. 4(b). The time
during which the recording layer stays in the crystallization
temperature range is much longer for T.sub.4+T.sub.5+T.sub.6 of the
high linear velocity than for T.sub.1+T.sub.2+T.sub.3 of the low
linear velocity, so it is understood that the recrystallization is
more likely to take place at the fast linear velocity. In an alloy
with a composition close to a SbTe eutectic composition and used as
a phase change recording layer, a crystal is likely to grow at the
amorphous/crystal boundary and therefore recrystallization easily
occurs outer area of the mark. Here, the low speed refers to less
than about .times.10-speed (T=less than 23.1 nanoseconds) and the
high speed refers to about .times.10-speed or more.
As described above, in the phase change medium, as the reference
clock period T becomes short due to an increased density and speed,
recrystallization is likely to occur with the conventional pulse
division scheme, giving rise to a serious problem that a required
degree of modulation fails to be generated at the central part of
the long mark.
In the phase change medium in which an amorphous mark is recorded
over a crystal area, although it is generally easy at high linear
velocity to secure an enough cooling speed to form an amorphous
solid, the crystallization time is difficult to secure. Hence, the
phase change medium often employs a recording layer of a
composition which tends to be easily crystallized, i.e., a
recording layer of an easily recrystallizable composition.
Therefore, it is important to increase the off pulse section to
enhance the cooling effect, but during the high linear velocity the
off pulse section becomes short to the contrary.
The similar problem is also encountered when the wavelength of a
laser source is reduced or a numerical aperture is increased to
reduce a beam diameter for enhancing the density of the phase
change medium. For example, when a laser with a wavelength of 780
nm and a numerical aperture of NA=50 is changed to a laser with a
wavelength of 400 nm and a numerical aperture of 0.65, the beam
diameter is throttled to almost one-half. At this time, the energy
distribution in the beam becomes steep so that the heated portion
is easily cooled, allowing an amorphous mark to be formed easily.
This however makes the recording layer more difficult to
crystallize. In this case, too, it is necessary to increase the
cooling effect.
The present invention has been accomplished to solve the
aforementioned problems. It is an object of the invention to
provide an optical recording method and an optical recording medium
suited for the method, which can perform recording in a
satisfactory manner even during a mark length recording using a
short clock period suited for high density recording and high speed
recording.
DISCLOSURE OF THE INVENTION
The inventors of this invention have found that the above objective
can be realized by reducing the number of divisions m in the pulse
division scheme from the conventional division number.
Viewed from one aspect the present invention provides an optical
recording method for recording mark length-modulated information
with a plurality of recording mark lengths by radiating light
against a recording medium, the optical recording method comprising
the steps of: when a time length of one recording mark is denoted
nT (T is a reference clock period equal to or less than 25 ns, and
n is a natural number equal to or more than 2), dividing the time
length of the recording mark nT into .eta..sub.1T, .alpha..sub.1T,
.beta..sub.1T, .alpha..sub.2T, .beta..sub.2T, . . . ,
.alpha..sub.iT, .beta..sub.iT, . . . , .alpha..sub.mT,
.beta..sub.mT, .eta..sub.2T in that order (m is a pulse division
number;
.SIGMA..sub.i(.alpha..sub.i+.beta..sub.i)+.eta..sub.1+.eta..sub.2=n;
.alpha..sub.i (1.ltoreq.i.ltoreq.m) is a real number larger than 0;
.beta..sub.i (1.ltoreq.i.ltoreq.m-1) is a real number larger than
0; .beta..sub.m is a real number larger than or equal to 0; and
.eta..sub.1 and .eta..sub.2 are real numbers between -2 and 2); and
radiating recording light with a recording power Pw.sub.i in a time
duration of .alpha..sub.1T (1.ltoreq.i.ltoreq.m), and radiating
recording light with a bias power Pb.sub.i in a time duration of
.beta..sub.iT (1.gtoreq.i .gtoreq.m-1), the bias power being
Pb.sub.i<Pw.sub.i and Pb.sub.i<Pw.sub.i+1; wherein the pulse
division number m is 2 or more for the time duration of at least
one recording mark and meets n/m.gtoreq.1.25 for the time length of
all the recording marks.
Viewed from another aspect, the present invention provides a phase
change type optical recording medium recorded by the optical
recording method, the phase change type optical recording medium
having a recording layer made of
M.sub.zGe.sub.y(Sb.sub.xTe.sub.1-x).sub.1-y-z alloy (where
0.ltoreq.z.ltoreq.0.1, 0<y.ltoreq.0.3, 0.8.ltoreq.x; and M is at
least one of In, Ga, Si, Sn, Pb, Pd, Pt, Zn, Au, Ag, Zr, Hf, V, Nb,
Ta, Cr, Co, Mo, Mn, Bi, O, N and S).
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1(a)-(c) are explanatory diagrams showing an example
recording pulse division scheme and an example method of generating
the recording pulses according to the invention.
FIGS. 2(a)-(b) are explanatory diagrams showing a conventional
recording pulse division scheme.
FIGS. 3(a)-(e) are schematic diagrams showing a shape of a recorded
mark and a change of reflectance in a phase change optical
recording medium.
FIGS. 4(a)-(b) are examples of temperature history when recording
light is radiated against the recording layer of the phase change
optical recording medium.
FIG. 5 is a schematic diagram of retrieved waveforms (eye-pattern)
of an EFM modulation signal.
FIGS. 6(a)-(c) are examples of division scheme of a recording pulse
for an 11T mark according to an embodiment of the invention.
FIG. 7 is a graph showing a relation between .alpha..sub.1 and a
mark time length in the embodiment 1 of the invention.
FIG. 8 is a graph showing a relation between .beta..sub.m and a
mark time length in the embodiment 1 of the invention.
FIG. 9 is an example of division scheme of a recording pulse for an
EFM random pattern in the embodiment 1 of the invention.
FIG. 10 is a graph showing a relation of measured values of mark
time length/space time length with respect to theoretical values in
the embodiment 1 of the invention.
FIGS. 11(a)-(b) are examples of conventional division scheme of a
recording pulse for a 11T mark/11T space.
FIGS. 12(a)-(c) are explanatory diagrams showing an example of a
pulse division scheme according to the invention.
FIGS. 13(a)-(d) are explanatory diagrams showing a timing for
generating a gate in the pulse division scheme of FIG. 12.
FIGS. 14(a)-(b) are explanatory diagrams showing a pulse division
scheme in (1) of embodiment 3.
FIGS. 15(a)-(b) are graphs showing a dependency of a modulation in
(1) of embodiment 3.
FIGS. 16(a)-(c) are explanatory diagrams showing a pulse division
scheme in (2) of embodiment 3.
FIGS. 17(a)-(b) are graphs showing a dependency of .alpha..sub.1 of
a mark length (-.tangle-solidup.-) and a space length
(-.largecircle.-) in (2) of embodiment 3.
FIGS. 18(a)-(b) are graphs showing a dependency of .beta..sub.1 of
a mark length (-.tangle-solidup.-) and a space length
(-.largecircle.-) in (2) of embodiment 3.
FIGS. 19(a)-(b) are graphs showing a dependency of .beta..sub.m of
a mark length (-.tangle-solidup.-) and a space length
(-.largecircle.-) in (2) of embodiment 3.
FIG. 20 is an explanatory diagram showing a pulse division scheme
in (3) of embodiment 3.
FIGS. 21(a)-(b) are graphs showing a mark length (-.diamond.-) and
a space length (-.circle-solid.-), and their jitters in (3) of
embodiment 3.
FIG. 22 is an explanatory diagram showing a pulse division scheme
in (4) of embodiment 3.
FIGS. 23(a)-(b) are graphs showing a mark length (-.diamond.-) and
a space length (-.circle-solid.-), and their jitters in (4) of
embodiment 3.
FIGS. 24(a)-(c) are explanatory diagrams showing an example of a
pulse division scheme according to the invention.
FIGS. 25(a)-(c) are explanatory diagrams showing an example of a
pulse division scheme according to embodiment 4 and a dependency on
Tw/T of a modulation obtained.
FIG. 26 is an explanatory diagram showing an example of a pulse
division scheme according to embodiment 4 of the invention.
FIGS. 27(a)-(c) are diagrams showing a dependency on power of
modulation and jitter and a dependency of jitter on the number of
overwrites.
FIG. 28 is an explanatory diagram showing another example of a
pulse division scheme according to embodiment 4.
PREFERRED EMBODIMENTS OF THE INVENTION
Now, the present invention will be described in detail by referring
to the accompanying drawings.
The optical recording method of this invention reduces the number
of divisions in the pulse division scheme, i.e., elongates each
pulse of recording light to make the time during which to heat a
light-irradiated portion of the optical recording medium
sufficiently long with respect to the response speed of the laser
pulse and also sets the time during which to cool the
light-irradiated portion sufficiently long. This enables
satisfactory mark length recording even with a clock period as low
as 25 nm or less.
In more concrete terms, suppose the time length of a recording mark
is nT (T is a reference clock period equal to or less than 25 ns;
and n is a natural number equal to or more than 2). The time length
nT of the recording mark is divided in the following order:
.eta..sub.1T, .alpha..sub.1T, .beta..sub.1T, .alpha..sub.2T,
.beta..sub.2T, . . . , .alpha..sub.iT, .beta..sub.iT, . . . ,
.alpha..sub.mT, .beta..sub.mT, .eta..sub.2T (m is a number of pulse
divisions;
.SIGMA..sub.i(.alpha..sub.i+.beta..sub.i)+.eta..sub.1+.eta..sub.2=n;
.alpha..sub.i(1.ltoreq.i.ltoreq.m) is a real number larger than 0,
.beta..sub.i(1.ltoreq.i.ltoreq.m-1) is a real number larger than 0,
and .beta..sub.m is a real number equal to or larger than 0; and
.eta..sub.1 and .eta..sub.2 are real numbers equal to or larger
than -2, preferably 0, and equal to or smaller than 2, preferably
1). In the time length of .alpha..sub.iT (1.ltoreq.i.ltoreq.m),
recording light with a recording power Pw.sub.i is radiated; and in
the time length of .beta..sub.iT (1.ltoreq.i.ltoreq.m), recording
light with a bias power Pb.sub.i, which has the relation of
Pb.sub.i<Pw.sub.i and Pb.sub.i<Pw.sub.i+1, is radiated. As
for the time length of at least one recording mark, the above pulse
division number m is set to 2 or more; and as for the time length
of all recording marks, n/m.gtoreq.1.25.
That is, while the conventional n-k division scheme sets the pulse
division number m equal to n-k (k is 1 or 2), this invention
defines the pulse division number m from a different
perspective.
In this invention, as to the time length of at least one recording
mark the above pulse division number m is set to 2 or more. It
should be noted, however, that there is no need to perform the
pulse division for all nT marks (marks with a time length of nT; T
is a reference clock period; and n is a natural number equal to or
larger than 2). In short marks such as 2T, 3T and 4T, the problem
of heat accumulation is relatively small but the response speed of
the pulse being unable to follow the pulse division poses a more
serious problem. It is therefore preferred that only one pulse of
recording light with a recording power of Pw be radiated or that
one pulse of recording light with the recording power of Pw and one
pulse of recording light with a bias power of Pb be radiated.
In this invention, as to the time lengths of all recording marks,
it is assumed that n/m.gtoreq.1.25.
Suppose that .eta..sub.1 and .eta..sub.2 are both 0. Then because
.SIGMA..sub.i(.alpha..sub.i+.beta..sub.i)/m=n/m, the value of n/m
corresponds to an average length of (.alpha..sub.i+.beta..sub.i)
and the value of (n/m)T corresponds to an average period of the
divided pulse.
In the conventional n-k division scheme, m=n-k and k is fixed to 1
or 2, so that n/m=n/(n-1) or n/m=n/(n-2). This value decreases as n
increases. Thus, if we let the longest mark time length be
n.sub.maxT, then n/m becomes minimum for n.sub.max. That is,
because the average period of the divided pulses is longest for the
shortest mark and shortest for, the longest mark, .alpha..sub.iT
and .beta..sub.iT are shortest for the longest mark.
For example, in the EFM modulation, n=3-11 and k=2, so
(n.sub.max/m)=11/(11-2)=about 1.22
Similarly, in the EFM+ modulation, n=3-14 and k=2, so
(n.sub.max/m)=14/(14-2)=about 1.16
In the (1, 7)-RLL-NRZI modulation, n=2-8 and k=1, so
(n.sub.max/m)=8/(8-1)=about 1.14
As can be seen from the above, in the conventional scheme the
values of n/m are approximately 1.22, 1.16 and 1.14. When the
reference clock period T becomes shorter than about 25 nanoseconds.
the average period of the divided pulses in the longest mark is
generally less than 25 nanoseconds and the average value of the
recording pulse section .alpha..sub.iT or the average value of the
off pulse section .beta..sub.iT is less than 12.5 nanoseconds. This
means that for at least one i, either .alpha..sub.iT or
.beta..sub.iT is less than 12.5 nanoseconds. Further, when the
clock period T goes below approximately 20 seconds, either
.alpha..sub.iT or .beta..sub.iT becomes further smaller.
In the above explanation, if a particular .alpha..sub.i or
.beta..sub.i becomes longer than the average, this means that other
.alpha..sub.i or .beta..sub.i becomes shorter and the fact still
remains that either .alpha..sub.iT or .beta..sub.iT becomes
smaller.
To describe more accurately, in the n-k division scheme
.SIGMA.(.alpha..sub.i+.beta..sub.i) is not necessarily equal to n
and may be equal to n-.eta. (.eta.=0 to 2). In this case, the
average value of .alpha..sub.i and .beta..sub.i becomes further
smaller, making the problem more serious.
In the optical recording method of this invention, m is set to
satisfy the condition of n/m.gtoreq.1.25 as to the time length of
all recording marks ranging from short to long marks. As a result,
the lengths of .alpha..sub.iT and .beta..sub.iT are made
sufficiently long. For example, the recording pulse section
.alpha..sub.iT and the off pulse section .beta..sub.iT can
generally be set slightly longer than 0.5T to sufficiently heat the
recording layer and at the same time limit the heat being supplied
from the subsequent pulses and thereby produce a sufficient cooling
effect.
When a mark is long in particular, the shape of a mark is easily
deformed by the accumulated heat. Hence, for marks 7T or longer in
time length, n/m should preferably be set to 1.5 or more. It is of
course preferred that, also for short marks 6T or shorter, n/m be
set to 1.5 or more, more preferably to 1.8 or more.
It is noted, however, that because too large a value of n/m
increases the heat accumulation, normally n/m is preferably set to
4 or less, more preferably 3 or less.
The optical recording method of this invention produces a greater
effect as the reference clock period T decreases, and it is
preferred that the reference clock period be set to 20 nm or less
or more preferably 15 ns or less. A very short clock period is
difficult to achieve in practice and it is normally preferred that
the clock period have 0.1 ns or more, or preferably 1 ns or more,
or more preferably 3 ns or more. As the clock period T decreases,
it is desired that the minimum value of n/m be increased.
The recording mark in this invention is recognized as a physical
mark formed continuously in a recording medium and optically
distinguishable from other portions. That is, the invention does
not join, through processing by a reproducing system, 2T, 3T and 4T
marks of the conventional n-k division scheme that meet the
condition of n/m.gtoreq.1.25 and recognize them as a single long
mark. In this invention, however, the recording mark may be formed
of a plurality of physical marks that are below the optical
resolution power of the retrieveing light. If we let the numerical
aperture of an objective for focusing the retrieveing light be NA
and the wavelength of the retrieveing light be .lamda., when the
physical marks are spaced from each other by 0.2 (.lamda./NA) or
more, these physical marks can be optically distinguishable as
separate marks. Hence, when forming a recording mark using a
plurality of physical marks, they should preferably be spaced
within 0.2 (.lamda./NA) of each other.
In this invention, the parameters associated with the divided
pulses such as .alpha..sub.i, .beta..sub.i, .eta..sub.1, and
.eta..sub.2, Pw and Pb can be changed as required according to the
mark length and i.
Further, in this invention it is preferred that the average value
of the recording pulse section .alpha..sub.iT (1.ltoreq.i.ltoreq.m)
and the average value of the off pulse section .beta..sub.iT
(1.ltoreq.i.ltoreq.m-1) both be set to 3 nanoseconds or more,
preferably 5 nanoseconds or more, or more preferably 10 nanoseconds
or more in terms of securing the response capability of the
radiated light. More preferably, individual .alpha..sub.iT
(1.ltoreq.i.ltoreq.m) and .beta..sub.iT (1.ltoreq.i.ltoreq.m-1) are
set to 3 nanoseconds or more, or 5 nanoseconds or more, or more
specifically 10 nanoseconds or more. The rise time and fall time of
the power of the laser beam normally used during the process of
recording should preferably be set 50% or less of the minimum
.alpha..sub.iT (1.ltoreq.i.ltoreq.m) and .beta..sub.iT
(1.ltoreq.i.ltoreq.m).
In this invention, although it is possible to set .beta..sub.m to 0
not to radiate light during the last off pulse section of
.beta..sub.mT, if the heat accumulation problem at the end of the
mark is grave, .beta..sub.mT should preferably be provided. In that
case, it is preferred that .beta..sub.mT be set normally to 3
nanoseconds or more, or specifically to 5 nanoseconds or more, or
more preferably to 10 nanoseconds or more.
When the recording pulse section .alpha..sub.iT
(1.ltoreq.i.ltoreq.m) is three nanoseconds or more, especially 5
nanoseconds or more, the radiation energy required for recording
can be secured by increasing the recording power Pw.sub.i although
there is a problem of the rising/falling edge of the recording
light.
On the other hand, when the off pulse section .beta..sub.iT
(1.ltoreq.i.ltoreq.m-1), too, is 3 nanoseconds or more, especially
5 nanoseconds or more, the cooling effect can be secured by
reducing the bias power Pb down to nearly the retrieveing light
power Pr or to 0 as long as this is not detrimental to a tracking
servo or others.
To obtain a still greater cooling effect, it is desired that
.SIGMA..sub.i(.alpha..sub.i) associated with the time length of all
recording marks be set to 0.6 n or less, particularly 0.5 n or
less. More preferably, .SIGMA..sub.i(.alpha..sub.i) is set to 0.4 n
or less. That is, the sum of the recording pulse sections
.SIGMA..sub.i(.alpha..sub.iT) is set shorter than
.SIGMA..sub.i(.beta..sub.iT) so that the off pulse section in each
mark is longer. It is particularly preferred that, for all i of i=2
to m-1, .alpha..sub.iT.ltoreq..beta..sub.iT, i.e., in the recording
pulse train following at least a second pulse, .beta..sub.iT is
made longer.
In the recording method of this invention, the values of
.alpha..sub.i (1.ltoreq.i.ltoreq.m) and .beta..sub.i
(1.ltoreq.i.ltoreq.m-1) are set appropriately according to the
values of the recording pulse section .alpha..sub.iT
(1.ltoreq.i.ltoreq.m) and the off pulse section .beta..sub.iT
(1.ltoreq.i.ltoreq.m-1) and are normally set to 0.01 or more,
preferably 0.05 or more, and normally 5 or less, preferably 3 or
less. Too small a value of .beta..sub.i (1.ltoreq.i.ltoreq.m-1) may
result in an insufficient cooling effect and hence it is preferably
set to 0.5 or more, specifically 1 or more. On the other hand, too
large a value of .beta..sub.i may cause an excessive cooling and
result in the recording mark being optically separated. Hence it is
preferably set to 2.5 or less, specifically 2 or less. The effect
of this setting is particularly large in the first off pulse
section .beta..sub.iT that has a great effect on the shape of the
front end of the mark.
What has been described above can also be said of the last off
pulse section .beta..sub.mT that has a great effect on the shape of
the rear end of the mark. Hence, .beta..sub.m is normally set to
0.1 or more, preferably 0.5 or more, more preferably 1 or more, and
2.5 or less, preferably 2 or less. The switching period of
intermediate pulse sections (group) .alpha..sub.iT
(2.ltoreq.i.ltoreq.m-1) between the start pulse section
.alpha..sub.1T and the last pulse section .alpha..sub.mT should
preferably be set constant in terms of simplifying the circuit. In
more concrete terms, (.alpha..sub.i+.beta..sub.i)T
(2.ltoreq.i.ltoreq.m-1) or (.alpha..sub.i+.beta..sub.i-1)T
(2.ltoreq.i.ltoreq.m-1) is preferably set to 1.5T, 2T or 2.5T.
In this invention, the recording light power Pb.sub.i radiated
during the off pulse section .beta..sub.iT (1.ltoreq.i.ltoreq.m-1)
is set smaller than the powers Pw.sub.i and Pw.sub.i+1 of the
recording light radiated during the recording pulse sections
.alpha..sub.iT and .alpha..sub.i+1T. To obtain a large cooling
effect, it is preferred that Pb.sub.i<Pw.sub.i be set for the
time lengths of all recording marks. More preferably
Pb.sub.i/Pw.ltoreq.0.5 and still more preferably
Pb.sub.i/Pw.sub.i.ltoreq.0.2. The bias power Pb can be set equal to
the power Pr of the light radiated during retrieving. This
simplifies the setting of the divided pulse circuit required for
the pulse division.
For the time length of one particular recording mark, two or more
different values of Pb.sub.i and/or Pw.sub.i may be used according
to i. Particularly, setting the recording powers Pw.sub.1 and
Pw.sub.m used in the start recording pulse section .alpha..sub.1T
and the last recording pulse section .alpha..sub.mT to values
different from the recording power Pw.sub.i used in the
intermediate recording pulse sections .alpha..sub.iT
(2.ltoreq.i.ltoreq.m-1) can control the shape of the front and rear
ends of the mark accurately. It is preferred that the recording
powers Pw.sub.i in the intermediate recording pulse sections
.alpha..sub.iT (2.ltoreq.i.ltoreq.m-1) be set equal as practically
as possible as this simplifies the setting of the divided pulse
circuit. Similarly, it is preferred that the bias powers Pb.sub.i
in the off pulse sections .beta..sub.iT (1.ltoreq.i.ltoreq.m-1) be
all set to the same value as practically as possible unless there
is any justifiable reason. At least two recording marks with
different n's may have different values of Pw.sub.i and/or Pb.sub.i
for the same i.
In this invention, although there are no limiting specifications as
to what power of light shall be radiated onto the spaces where no
recording marks are formed, the light to be radiated should
preferably have a power Pe, which is
Pb.sub.i.ltoreq.Pe<Pw.sub.i. In the rewritable recording medium,
the power Pe is an erase power used to erase the recorded marks. In
this case, it is preferred that during a
(n-(.eta..sub.1+.eta..sub.2))T section, light with a power equal to
or higher than the bias power Pb.sub.i and equal to or lower than
the erase power Pe be radiated. Setting the light power equal to
the bias power Pb.sub.i or the erase power Pe facilitates the
setting of the divided pulse circuit. When light with the bias
power Pb is radiated during an .eta..sub.1T section, the light with
the bias power Pb is radiated prior to the start recording pulse
section .alpha..sub.1T, thus minimizing the influences of heat from
the preceding recording mark.
The recording power Pw and bias power Pb or erase power Pe have
different physical functions depending on the type of the optical
recording medium used.
In the case of the magnetooptical medium, for example, Pw or Pe is
a power necessary to raise the temperature of the recording layer
at least above the vicinity of the Curie temperature to make the
occurrence of the magnetization inversion easy. In the so-called
optical modulation overwritable magnetooptical medium, Pw is
greater than Pe and is a power to raise the temperatures of a
plurality of magnetic layers with different Curie points above one
of the Curie point temperatures.
In the case of the phase change medium, when performing the
recording through crystallization, Pw is a power to raise the
recording layer to a temperature higher than the crystallization
temperature. Or when performing the recording through
transformation into amorphous state, Pw is a power to raise the
recording layer at least to a temperature higher than its melting
point. When performing overwriting through amorphisation recording
and crystallization erasing, Pe is a power to raise the recording
layer at least above the crystallization temperature.
In the write-once medium that performs recording through pitting or
deformation of a metallic or organic recording layer, Pw is a power
necessary to raise the recording layer to a temperature that
induces softening, melting, evaporation, decomposition or chemical
reaction.
Although the values of the recording power Pw and bias power Pb
differ from one kind of recording medium used to another, in the
rewritable phase change medium for example the recording power Pw
is normally about 1-100 mW and the bias power Pb about 0.01-10
mW.
Whichever medium is used, the recording power Pw is a laser beam
power necessary to raise the recording layer to a temperature that
induces some optical changes in the recording layer, or to hold
that temperature. The bias power Pb on the other hand is a power at
least lower than the recording power Pw. Normally, the bias power
Pb is lower than the recording power Pw and the erase power Pe and
does not induce any physical changes in the recording layer.
The heat accumulation problem described above is common to a wide
range of optical disks that perform the mark length modulation
recording, such as phase change type, magnetooptical type and
write-once type optical recording media.
In the overwritable phase change medium among others, because the
mark recording and mark erasing are performed at the same time by
precisely controlling two temperature parameters, the heating speed
and cooling speed of the recording layer, the function of cooling
the recording layer by the off pulses bears more importance than in
other write-once medium and magnetooptical medium. Hence, this
invention is particularly effective for the phase change type
recording medium.
In the recording method using the pulse division of this invention,
the same pulse division number m may be used on at least two
recording marks which have different n's of time lengths nT of the
pulse recording marks. Normally, the same m values are used for the
nT marks having adjoining time lengths, such as 3T mark and 4T
mark. With m values set equal, at least one of .alpha..sub.i
(1.ltoreq.i.ltoreq.m), .beta..sub.i (1.ltoreq.i.ltoreq.m),
.eta..sub.1, .eta..sub.2, Pw.sub.i (1.ltoreq.i.ltoreq.m) and
Pb.sub.i (1.ltoreq.i.ltoreq.m) is made to differ from others. This
makes it possible to differentiate the time lengths of the marks
from one another that have the same division numbers.
The division numbers m may be arranged irrelevant to the magnitudes
of the n values but it is preferred that the division numbers m be
set to monotonously increase as the mark becomes longer, i.e., the
value of m increases (including the case of staying the same).
Examples of pulse division scheme according to this invention are
shown below.
Example 1 of Division Scheme
For example, in the EFM modulation that forms 3T to 11T marks, m=1
for n=3 and m is increased for n.gtoreq.4 (4, 5, 6, 7, 8, 9, 10,
11). That is, the division number m is increased to m=1, 2, 2, 3,
4, 5, 6, 7, 8 as the n value increases to n=3, 4, 5, 6, 7, 8, 9,
10, 11.
The value of n/m is minimum at 1.38 when n=11 and maximum at 3 when
n=3.
Example 2 of Division Scheme
In the same EFM modulation, the division number m is increased to
m=1, 2, 2, 3, 4, 5, 6, 6, 6 as the n value increases to n=3, 4, 5,
6, 7, 8, 9, 10, 11.
The value of n/m is minimum at 1.5 when n=9 and maximum at 3 when
n=3.
Example 3 of Division Scheme
In the same EFM modulation, the division number m is increased to
m=1, 2, 2, 3, 3, 4, 5, 5, 5 as the n value increases to n=3, 4, 5,
6, 7, 8, 9, 10, 11.
The value of n/m is minimum at 1.8 when n=9 and maximum at 3 when
n=3.
When the same pulse division number m is used on at least two
recording marks with different n values, a pulse period
.tau..sub.i+.alpha..sub.i+.beta..sub.i and a duty ratio
(.alpha..sub.i/(.alpha..sub.i+.beta..sub.i) may be changed.
Examples of this procedure are shown below.
Example 4 of Division Scheme
The simplest division scheme is to make an equal division such that
the pulse period .tau..sub.i=nT/m when m.gtoreq.2.
However, simply dividing nT into equal parts may result in
.tau..sub.i assuming a value totally irrelevant to the timing and
length of the reference clock period T.
Example 5 of Division Scheme
The pulse period .tau..sub.i is preferably synchronized to the
reference clock period T or to the reference clock period T divided
by an integer (preferably 1/2T, 1/4T, 1/5T, 1/10T) as this allows
the rising/falling edge of the pulse to be controlled with one base
clock taken as a reference. At this time,
.SIGMA..sub.i(.tau..sub.i)=.SIGMA..sub.i(.alpha..sub.i+.beta..sub.i)
does not necessarily agree with n and an excess time is produced,
so that the pulse length must be corrected. It is preferred that
the sum of the pulse irradiation times be set smaller than n
because setting the sum greater than n makes the mark length too
long.
Hence, sections .eta..sub.1T, .eta..sub.2T are provided such that
.SIGMA..sub.i(.alpha..sub.i+.beta..sub.i)+(.eta..sub.1+.eta..sub.2)=n
(.eta..sub.1 and .eta..sub.2 are each real numbers such that
0.ltoreq..eta..sub.1 and 0.ltoreq..eta..sub.2), and these sections
are changed in each of two recording marks that have the same
division numbers m but different lengths. During the sections
.eta..sub.1T, .eta..sub.2T light with the bias power Pb may be
radiated. At this time, it is preferred that
0.ltoreq.(.eta..sub.1+.eta..sub.2).ltoreq.1.
The above .eta..sub.1 and .eta..sub.2 can also be used to correct
the effect of heat transferred from other preceding and/or
subsequent marks. In this case, the time lengths of .eta..sub.1T
and .eta..sub.2T are made variable according to the mark lengths
and/or space lengths of the preceding and/or subsequent marks.
It is possible to use only the first .eta..sub.1T or last
.eta..sub.2T of the divided pulses and set the other to 0, or to
use both of them in the range of
0.ltoreq.(.eta..sub.1+.eta..sub.2).ltoreq.1. It is also possible to
radiate light having other than the bias power Pb during the
sections .eta..sub.1T, .eta..sub.2T to align the mark lengths or to
more precisely control the influence of heat transferred from the
preceding and/or subsequent marks.
Example 6 of Division Scheme
The divided pulse period .tau..sub.i and the duty ratio
(.alpha..sub.i/(.alpha..sub.i+.beta..sub.i)) are made variable
according to i. With this method, jitters (fluctuations) in the
front and rear ends of the mark, which are important in the mark
length recording, can be improved.
More specifically, the first recording pulse period .tau..sub.1
and/or the last recording pulse period .tau..sub.m are made to
differ from a recording pulse period .tau..sub.i
(2.ltoreq.i.ltoreq.m-1) of intermediate pulses.
At this time it is possible to slightly adjust .tau..sub.1,
.alpha..sub.1, .beta..sub.1, .tau..sub.m, .alpha..sub.m and
.beta..sub.m of the first and/or last pulse according to the
preceding and/or subsequent mark length or space length.
It is preferred that the first recording pulse section
.alpha..sub.1T be set larger than any of the subsequent recording
pulse sections .alpha..sub.2T, . . . , .alpha..sub.mT. It is also
preferred that the recording power Pw.sub.1 be set higher than the
recording power Pw.sub.i in the succeeding recording pulse sections
.alpha..sub.2T, . . . , .alpha..sub.mT. These methods are effective
in improving an asymmetry value of the retrieve signal described
later.
The heat accumulation effect is small in short marks such as those
with time lengths of 3T and 4T so that the mark tends to be formed
slightly shorter than required. In such a case, the mark time
length may be strictly controlled by elongating the recording pulse
section .alpha..sub.1T to some extent or setting the recording
power Pw.sub.1 in the recording pulse section .alpha..sub.1T
slightly higher than required.
The method of changing the first pulse or last pulse is
particularly effective when overwriting an amorphous mark in the
crystal area of the phase change medium.
Changing the first recording pulse section .alpha..sub.1T can
control the width of an area of the recording layer in the phase
change medium that first melts.
The last off pulse section .beta..sub.mT is important in preventing
the recording layer of the phase change medium from getting
recrystallized and is also an important pulse that determines the
area in which the recording layer is made amorphous.
When an amorphous mark is formed, an area in the rear end part of
the mark that has melted crystallizes again, making the actually
formed amorphous mark smaller than the melted area. Elongating the
off pulse section, i.e., extending the cooling time length, can
prevent recrystallization and elongates the amorphous portion.
Hence, by changing the length of the last off pulse section
.beta..sub.mT it is possible to change the time during which the
rear end portion of the mark is kept in the crystallization time
and thereby change the mark length in significant degrees.
Conversely, by changing the intermediate parameters .tau..sub.i,
.alpha..sub.i, .beta..sub.i (2.ltoreq.i.ltoreq.m-1) without
changing .tau..sub.1, .alpha..sub.1, .beta..sub.1, .tau..sub.m,
.alpha..sub.m and .beta..sub.m, the degree of modulation can be
controlled without affecting the mark edges.
Now, the method of generating divided recording pulses that
realizes the above-described division scheme will be explained
below.
The above pulse division can basically be realized by making the
division scheme for each mark time length nT programmable and
incorporating it into a ROM chip. However, adding a very wide range
of flexibility to the same pulse generating circuit will render the
circuit complex. So, the following two pulse generating methods may
preferably be used. They can provide pulses capable of dealing with
almost all media with ease.
Divided Recording Pulse Generating Method 1
For the mark length modulated data 100, which is EFM-modulated as
shown in FIG. 1(a), a division scheme 101 shown in FIG. 1(b) is
applied. That is, the division is made as follows: m=1, 1, 2, 3, 3,
4, 5, 5, 5 for n=3, 4, 5, 6, 7, 8, 9, 10, 11. At this time, the
circuits Gate1, Gate2, Gate3, Gate4 that generate clocks at timings
shown in FIG. 1(c) are combined to realize the division scheme of
FIG. 1(b).
In FIG. 1(c), the Gate1 denoted 102 generates the first recording
pulse .alpha..sub.1T with a delay time of T.sub.d1. The Gate2
denoted 103 generates a group of second and subsequent intermediate
recording pulses .alpha..sub.iT with a delay time of T.sub.d2. The
Gate3 denoted 104 generates pulses with a bias power Pb and pulses
with power Pe. That is, when recording pulses are not generated by
the Gate1, Gate2 and Gate4, off pulses .beta..sub.iT with a bias
power Pb aretrieved when the level is low and pulses with a power
Pe aretrieved when the level is high. The Gate3 and T.sub.d1
determine (n-(.eta..sub.1+.eta..sub.2))T. The Gate4 denoted 105
generates a last recording pulse .alpha..sub.mT with a delay time
of T.sub.d3 after the intermediate recording pulse group
.alpha..sub.iT has been generated. In the sections in which the
Gate3 is at low level, when the recording pulses are at high level,
they have priority over the off pulses.
.beta..sub.1T can be controlled independently by the delay time
T.sub.d2 and .alpha..sub.1T, and .beta..sub.mT can be controlled
independently by Gate3 and .alpha..sub.mT.
In the section where the .alpha..sub.1T pulse is generated by the
Gate1, a recording power Pw.sub.1 is used; in the sections where
the intermediate pulse group .alpha..sub.iT is generated by the
Gate2, a recording power Pw.sub.2 is used; and in the section where
the .alpha..sub.mT pulse is generated by the Gate4, a recording
power Pw.sub.3 is used. This arrangement allows the recording power
to be controlled independently in each of the first pulse section,
the intermediate pulse section group and the last pulse
section.
To independently control the recording pulse width and the
recording power in the first and last sections, the period of the
intermediate pulses is defined by
.gamma..sub.i=.alpha..sub.i+.beta..sub.i-1 (2.ltoreq.i.ltoreq.m-1)
with T.sub.d2 as a start point, and .gamma..sub.i is set almost
constant at .gamma..sub.i=1 to 3. In this case, .beta..sub.i is
automatically determined. In FIG. 1, .gamma..sub.i=1.5. It is
noted, however, that T.sub.d2 is defined so as to make a correction
of (T.sub.d2-(T.sub.d1+.alpha..sub.1T)) for .beta..sub.1 and thus
.beta..sub.1 can be handled as an independent parameter.
In either case, it is assumed that the Gate timing is synchronized
with the reference clock period T or with a base clock, which is
the reference clock period divided by an integer, and that
.alpha..sub.i and .beta..sub.i are defined by the duty ratio with
respect to the base clock.
If n is smaller than a predetermined value n.sub.c, then m=1 and
the intermediate pulse group are not generated by the Gate2. If n
is equal to or larger than n.sub.c, a predetermined number of
pulses aretrieved according to the above (division scheme example
3). In FIG. 1, n.sub.c is set to 5 and when n is equal to or
smaller than 4, then m=1; and when n is 5 or more, the intermediate
pulses are generated. Here, it is assumed that the intermediate
pulses are generated, according to n, in numbers equal to the
division number stored in the ROM memory.
The last pulse .alpha..sub.mT generated by the Gate4 is generated
only when n.gtoreq.n.sub.c+1. This is indicated by, a 9T mark in
FIG. 1.
When n=n.sub.c, the pulse is divided into two pulses, the first
pulse and one intermediate pulse. In FIG. 1 this is represented by
a 5T mark.
When a plurality of marks with different time lengths are each
divided into the same number of divisions, if a 3T mark and a 4T
mark in FIG. 1 for example are both recorded by a pair of recording
pulse and an off pulse, at least .alpha..sub.1, .beta..sub.1,
.eta..sub.1 and .eta..sub.2 and, if further required, Pw.sub.1 and
Pw.sub.3 need to be differentiated between the 3T mark and the 4T
mark.
Divided Recording Pulse Generating Method 2
The following description concerns a divided recording pulse
generating method based on a clock signal with a period of 2T which
is obtained by dividing the reference clock period T. This method
has more limitations than the divided recording pulse generating
method 1 but has an advantage of allowing for the design of logic
circuits based on more regular rules.
The pulse generating method 2 is characterized in that the
procedure depends on whether the value that n of an nT mark can
take is odd or even.
That is, for the recording of a mark in which n is even, i.e., the
mark length is nT=2LT (L is an integer equal to 2 or more), the
mark is divided into the number of sections m=L and the
.alpha..sub.1 and .beta..sub.1 in the recording pulse sections
.alpha..sub.1T and the off pulse sections .beta..sub.1T are defined
as follows. .alpha..sub.1+.beta..sub.1=2+.delta..sub.1
.alpha..sub.i+.beta..sub.i=2(2.ltoreq.i.ltoreq.m-1)
.alpha..sub.m+.beta..sub.m=2+.delta..sub.2 (where .delta..sub.1 and
.delta..sub.2 are real numbers that satisfy
-0.5.ltoreq..delta..sub.1.ltoreq.0.5 and
-1.ltoreq..delta..sub.2.ltoreq.1; and when L=2, it is assumed that
only .alpha..sub.1, .beta..sub.1, .alpha..sub.m and .beta..sub.m
exist).
For the recording of a mark in which n is odd, i.e., the mark
length is nT=(2L+1)T, on the other hand, the mark is divided into
the number of sections m=L and the .alpha..sub.i' and .beta..sub.i'
in the recording pulse sections .alpha..sub.i'T and the off pulse
sections .beta..sub.i'T are defined as follows.
.alpha..sub.1'+.beta..sub.1'=2.5+.delta..sub.1'
.alpha..sub.i'+.beta..sub.i'=2(2.ltoreq.i.ltoreq.m-1)
.alpha..sub.m'+.beta..sub.m'=2.5+.delta..sub.2' (where
.delta..sub.1' and .delta..sub.2' are real numbers that satisfy
-0.5.ltoreq..delta..sub.1'.ltoreq.0.5 and
-1.ltoreq..delta..sub.2'.ltoreq.1; and when L=2, it is assumed that
only .alpha..sub.1', .beta..sub.1', .alpha..sub.m' and
.beta..sub.m' exist).
Further, in the pulse generating method 2, the following equation
is satisfied.
.alpha..sub.1+.beta..sub.1+.alpha..sub.m+.beta..sub.m+.DELTA.=.alpha..sub-
.1'+.beta..sub.1'+.alpha..sub.m'+.beta..sub.m' (where .DELTA.=0.8
to 1.2).
In the above pulse generating method 2, .alpha..sub.1,
.beta..sub.1, .alpha..sub.1', .beta..sub.1', .delta..sub.1,
.delta..sub.2, .delta..sub.1', and .delta..sub.2' may change
according to the value of L. In the pulse generating method 2, in
the process of forming recording marks with n=2L and n=(2L+1), they
are both divided into the same division number L of recording
pulses. That is, when n is 2, 3, 4, 5, 6, 7, 8, 9, . . . , in that
order, then the division number m is set to 1, 1, 2, 2, 3, 3, 4, 4,
. . . in that order. More specifically, in the EFM modulation
signal, for n=3, 4, 5, 6, 7, 8, 9, 10, 11, the division number m is
sequentially set to m=1, 2, 2, 3, 3, 4, 4, 5, 5 in that order. In
the EFM+ signal, n=14 is added. In that case, the division number m
is set to 7. In the (1, 7)-RLL-NRZI modulation there is a case of
n=2, in which case the division number m is set to 1.
In the pulse generation method 2, two recording marks with the same
division numbers m=L and different lengths have only the first
pulse period (.alpha..sub.1+.beta..sub.1)T and the last pulse
period (.alpha..sub.m+.beta..sub.m)T differ from each other. That
is, for (.alpha..sub.1+.beta..sub.1+.alpha..sub.m+.beta..sub.m),
(.alpha..sub.1'+.beta..sub.1'+.alpha..sub.m'+.beta..sub.m') is
increased by .DELTA.(.DELTA.=0.8 to 1.2). The .DELTA. is normally 1
but can be changed in a range of about 0.8 to 1.2, considering the
influence of heat interference from the preceding and subsequent
recording marks.
.delta..sub.1 and .delta..sub.2, and .delta..sub.1' and
.delta..sub.2' are adjusted to ensure that each mark length will be
precisely nT and to reduce jitters at the ends of the mark. They
are normally -0.5.ltoreq..delta..sub.1.ltoreq.0.5,
-0.5.ltoreq..delta..sub.1'.ltoreq.0.5,
-1.ltoreq..delta..sub.2.ltoreq.1 and
-1.ltoreq..delta..sub.2'.ltoreq.1. The correction amounts at the
front end and rear end are preferably set equal, i.e.,
|.delta..sub.2/.delta..sub.1| and |.delta..sub.2'/.delta..sub.1'|
are each preferably in the range of 0.8 to 1.2.
The two recording marks with the same division numbers are
preferably formed in such a manner that their mark length
difference 1T is about 0.5T at the front end side and about 0.5T at
the rear end side. That is,
.alpha..sub.1+.beta..sub.1+.DELTA..sub.1=.alpha..sub.1'+.beta..sub.1'
(where .DELTA..sub.1=0.4 to 0.6) In this case, the rear end side is
normally
.alpha..sub.m+.beta..sub.m+.DELTA..sub.2=.alpha..sub.m'+.beta..s-
ub.m' (where .DELTA..sub.2=0.4 to 0.6 and
.DELTA..sub.1+.DELTA..sub.2=.DELTA.)
Setting .delta..sub.1=about 0 and .delta..sub.1'=about 0 is
particularly preferred as this allows the use of a circuit that can
generate divided pulses in synchronism with the front end of the
mark. The position of the front end of the mark is determined
almost by the rising edge of the recording power laser beam at
.alpha..sub.1T and its jitter is determined by the duty ratio of
.alpha..sub.1 and .beta..sub.1 and by the duty ratio of
.alpha..sub.1' and .beta..sub.1'. Hence, in this method, setting
.delta..sub.1=0 and .delta..sub.1'=0.5 can control the mark front
end position and the jitter satisfactorily.
The mark rear end position depends on .delta..sub.2 (and
.delta..sub.2'), i.e., the value of the divided pulse period
(.alpha..sub.m+.beta..sub.m)T (and (.alpha..sub.m'+.beta..sub.m')T)
at the rear end of the mark and also on the value of the duty ratio
of .alpha..sub.m and .beta..sub.m (and the duty ratio of
.alpha..sub.m' and .beta..sub.m'). Further, the mark rear end
position also depends on the position of the falling edge of the
recording pulse .alpha..sub.mT (and .beta..sub.m'T) at the rear end
and on the cooling process of the recording layer before and after
that falling edge position. In the phase change medium where
amorphous marks are formed, in particular, the mark rear end
position depends on the value of the off pulse section
.beta..sub.mT (and .beta..sub.m'T) at the rear end that has a great
effect on the cooling speed of the recording layer. Hence, the
divided pulse period (.alpha..sub.m+.beta..sub.m)T at the rear end
does not need to be 0.5T or 1T, and fine adjustment can be made
with a resolution power of about 0.1T, preferably 0.05T, or more
preferably 0.025T.
In the pulse generating method 2, the duty ratio between
.alpha..sub.i and .beta..sub.i,
.alpha..sub.i/(.alpha..sub.i+.beta..sub.i), can be optimized for
each mark length, but for simplification of the pulse generating
circuit, it is preferred that the duty ratios in the intermediate
pulses situated between the first pulse and the last pulse be set
to a fixed value. That is, when L.gtoreq.3 in which case
intermediate pulses can exist, it is preferred that, for all i
ranging from 2 to (m-1) in two recording marks with the same
division numbers m=L, .alpha..sub.i and .alpha..sub.i' be set to
.alpha..sub.i=.alpha.c (fixed value) and .alpha..sub.i'=.alpha.c'
(fixed value). Further, when L is 3 or more, .alpha.c and .alpha.c'
are preferably set to a fixed value, particularly
.alpha.c=.alpha.c', not dependent on the value of L because this
further simplifies the circuit.
For the simplified pulse generating circuit in the pulse generating
method 2, it is preferred that in the recording mark with n being
even, .alpha..sub.1 and .beta..sub.1 assume fixed values for all L
equal to 3 or more. For all L equal to 2 or more, it is preferred
that .alpha..sub.1+.beta..sub.1 be set to 2 as this causes the
period (.alpha..sub.i+.beta..sub.i)T to become 2T for all i ranging
from 1 to (m-1).
Similarly, for the simplified pulse generating circuit in the pulse
generating method 2, it is preferred that in the recording mark
with n being odd, .alpha..sub.1' and .beta..sub.1' assume fixed
values for all L equal to 3 or more. For all L equal to 2 or more,
it is preferred that .alpha..sub.1+.beta..sub.1 be set to 2.5 as
this makes it easy to synchronize with the subsequent divided pulse
period 2T.
Further, for the simplification of the pulse generating circuit in
the pulse generating method 2, .alpha..sub.m, .beta..sub.m,
.alpha..sub.m' and .beta..sub.m' each preferably assume the same
values for all L equal to 3 or more, specifically 2 or more. Here,
if
.DELTA..sub.2=(.alpha..sub.m'+.beta..sub.m')-(.alpha..sub.m+.beta..sub.m)-
=0.5, the circuit can be further simplified.
When n is 2 or 3, the division number m is 1. In that case, the
.alpha..sub.1-.beta..sub.1 duty ratio and .delta..sub.1 (or the
.alpha..sub.1'-.beta..sub.1' duty ratio and .delta..sub.1') can be
adjusted to achieve a desired mark length and jitter. Here, it is
desired that .delta..sub.1'-.delta..sub.1=1.
In the pulse generating method 2, as described above, it is
particularly desired that .delta..sub.1=.delta..sub.1'=0. In that
case, the pulse generating circuit should preferably be controlled
to ensure that .alpha..sub.i (1.ltoreq.i.ltoreq.m) is generated in
synchronism with a frequency-divided first reference clock 3 with a
period 2T which is produced by frequency-dividing a first reference
clock 1 with a period T; that .alpha..sub.i' (2.ltoreq.i.ltoreq.m)
is generated in synchronism with a frequency-divided second
reference clock 4 with a period 2T which is obtained by
frequency-dividing a second reference clock 2 that has the same
period T as that of the first reference clock 1 and is shifted 0.5T
from the first reference clock 1; and that .alpha..sub.1' rises
2.5T before .alpha..sub.2' rises. The use of a plurality of
reference clocks can simplify the pulse generating circuit.
There is a case in which the rising edges of .alpha..sub.1 and
.alpha..sub.1' need to be delayed or advanced with respect to the
rising or falling edge of a square wave modulated according to the
mark length to be recorded. In such a case it is preferred that the
same delay times T.sub.d1 be added in order to make the lengths of
spaces constant. T.sub.d1 is a real number between -2 and 2. When
the value of T.sub.d1 is negative, it indicates a advance time.
FIG. 12 shows an example relation between recording pulses when the
pulse division scheme in the recording method of this invention is
implemented by using a plurality of reference clocks described
above. In FIG. 12, the delay times T.sub.d1 of .alpha..sub.1T and
.alpha..sub.1'T with respect to the front end of the nT mark are 0;
the recording power in the recording pulse section .alpha..sub.iT
(1.ltoreq.i.ltoreq.m) is Pw which is constant; the bias power in
the off pulse section .beta..sub.iT (1.ltoreq.i.ltoreq.m) is Pb
which is constant; and the power of light radiated in the spaces
and in other than .alpha..sub.iT (1.ltoreq.i.ltoreq.m) and
.beta..sub.iT (1.ltoreq.i.ltoreq.m) is an erase power Pe which is
constant. Here Pb.ltoreq.Pe.ltoreq.Pw.
In FIG. 12, reference number 200 denotes a reference clock with a
period T.
FIG. 12(a) shows a pulse waveform corresponding to a recording mark
with a length of nT, with reference number 201 representing the
length of a 2LT recording mark and 202 representing the length of a
(2L+1)T recording mark. FIG. 12(a) illustrates a case of L=5.
FIG. 12(b) shows a divided recording pulse waveform when n=2L (=10)
and FIG. 12(c) shows a divided recording pulse waveform when n=2L+1
(=11).
In FIG. 12(b), a frequency-divided first reference clock 205 with a
period 2T is obtained by frequency-dividing a first reference clock
203 which has a zero phase delay from the reference clock 200 with
a period T. Because .alpha..sub.1+.beta..sub.1=2, the rising edge
of each recording pulse section .alpha..sub.iT
(1.ltoreq.i.ltoreq.m) is synchronized with the frequency-divided
first reference clock 205 with a period 2T. In synchronism with the
frequency-divided first reference clock 205, the duty ratio of
.alpha..sub.i-.beta..sub.i is adjusted to produce a recording pulse
waveform 207.
In FIG. 12(c), a frequency-divided second reference clock 206 with
a period 2T is obtained by frequency-dividing a second reference
clock 204 with a period T which has a phase shift of 0.5T from the
reference clock 200. The leading edge of each recording pulse
section .alpha..sub.iT (2.ltoreq.i.ltoreq.m) is synchronized with
the frequency-divided second reference clock 206 with a period 2T.
Because .alpha..sub.1+.beta..sub.1=2.5, only .alpha..sub.1T rises
0.5T before the clock. In synchronism with the frequency-divided
second reference clock 206, the duty ratio of
.alpha..sub.i-.beta..sub.i is adjusted to produce a recording pulse
waveform 208.
In FIG. 12, the mark lengths 2LT and (2L+1)T are depicted so that
their rear ends are aligned at T2 and T4. Hence, there are only two
possible relations (b) and (c) between the reference clocks 205 and
206, both with the period of 2T. In reality, however, when the
2T-period reference clocks are used, the front end positions of
these mark lengths can be 1T out of phase with each other. Further,
considering the cases of n being even and n being odd, there are
four possible relations as shown in FIGS. 13(a), (b), (c) and (d).
It is therefore desirable to adopt the following gate generating
method to deal with this situation.
FIG. 13 is a timing chart explaining the above gate generating
method. The gate generating method of FIG. 13 involves the
following steps: (1) it generates a reference time T.sub.sync
corresponding to the clock mark formed at a predetermined position
on the recording track; (2) it generates four reference clocks, a
2T-period reference clock 1a lagging the reference time T.sub.sync
as a start point by the delay time T.sub.d1, a 2T-period reference
clock 2a leading the reference clock 1a by 0.5T, a 2T-period
reference clock 1b leading the reference clock 1a by 1T, and a
2T-period reference clock 2b leading the reference clock 1a by
1.5T; (3) when recording a mark of nT=2LT, it generates gate groups
G1a, G1b in synchronism with either the reference clock 1a or 1b at
timings corresponding to the .alpha..sub.1T, .alpha..sub.iT
(2.ltoreq.i.ltoreq.m-1) and .alpha..sub.mT sections; and (4) when
recording a mark of nT=(2L+1), it generates gate groups G2a, G2b in
synchronism with either the reference clock 2a or 2b at timings
corresponding to .alpha..sub.1'T, .alpha..sub.i'T'
(2.ltoreq.i.ltoreq.m-1) and .alpha..sub.m'T.
In FIG. 13 the reference number 300 represents the reference clock
with a period T (reference clock for data). To record data at a
particular address on the recording medium, the recording system
normally makes a phase comparison between the synchronization
signal T.sub.sync generated at clock marks arranged on the medium
for each minimum unit of address (e.g., synchronization signal such
as VFO formed as a pit train on the medium and arranged for each
sector, and a synchronization pattern arranged for each ATIP frame
(every 1/75 second) formed as a groove meander on the medium) and
the T-period reference clock generated in advance in order to
establish synchronization.
FIG. 13 shows an example case in which the front end of the mark
appears an even number of periods T after the T.sub.sync. An
even-numbered length mark 301 with n being even is shown in FIG.
13(a) and an odd-numbered length mark 304 with n being odd is shown
in FIG. 13(d). As an example in which the front end of the mark
appears an odd number of periods T after the T.sub.sync, an
even-numbered length mark 302 with n being even (FIG. 13(b)) and an
odd-numbered length mark 303 with n being odd (FIG. 13(c)) are
shown.
In each of FIGS. 13(a) to 13(d), when reference clocks are
generated by shifting them 0.5T from one another in a manner
similar to that of FIG. 12, four different clock trains are
produced, as indicated by 305, 306, 307 and 308. That is, with the
reference clock 305 taken as a reference, the clock 307 is shifted
by 0.5T, the clock 306 is shifted by 1T, and the clock 308 is
shifted by 1.5T. These clocks are all formed by frequency-dividing
the T-period reference clock having its origin at T.sub.sync and
then shifting their phases by 0.5T from one another.
In the case of FIG. 13(a), in synchronism with each of periods P1a,
P2a, P3a, P4a and P5a the gate group G1a corresponding to the
recording pulse sections .alpha..sub.1T, .alpha..sub.2T,
.alpha..sub.3T, .alpha..sub.4T, .alpha..sub.5T.
In the case of FIG. 13(b), in synchronism with each of periods P1b,
P2b, P3b, P4b and P5b the gate group G1b corresponding to the
recording pulses .alpha..sub.1T, .alpha..sub.2T, .alpha..sub.3T,
.alpha..sub.4T, .alpha..sub.5T.
In the case of FIG. 13(c), in synchronism with each of periods R1a,
R2a, R3a, R4a and Rya the gate group G2a corresponding to the
recording pulses .alpha..sub.1'T, .alpha..sub.2'T, .alpha..sub.3'T,
.alpha..sub.4'T, .alpha..sub.5'T.
In the case of FIG. 13(d), in synchronism with each of periods R1b,
R2b, R3b, R4b and R5b the gate group G2b corresponding to the
recording pulses .alpha..sub.1'T, .alpha..sub.2'T, .alpha..sub.3'T,
.alpha..sub.4'T, .alpha..sub.5'T.
Here, the recording pulse generating gate groups G1a, G1b, G2a and
G2b are identical to the Gate1, Gate2 and Gate4 combined in FIG. 1.
That is, in FIG. 1, the Gate1 for generating the first pulse
.alpha..sub.1T, the Gate2 for generating the intermediate pulse
group .alpha..sub.iT (2.ltoreq.i.ltoreq.m-1), and the Gate4 for
generating the last pulse .alpha..sub.mT are produced separately
and then combined to generate the gate groups G1a and G1b. In FIG.
1, the first pulse .alpha..sub.1'T, the intermediate pulse group
.alpha..sub.i'T (2.ltoreq.i.ltoreq.m-1), and the last pulse
.alpha..sub.m'T are produced separately and then combined to
generate the gate groups G2a and G2b.
Generating the first pulse independently as with the Gate1 of FIG.
1 can deal with the situation where (.alpha..sub.1'+.beta..sub.1')
is 2.5 when n is odd, by generating the gate for .alpha..sub.1'T in
synchronism with the front end of nT and generating the 2T-period
intermediate pulse group .alpha..sub.i'T with a delay of 2.5T. This
is equivalent to setting the T.sub.d2 for Gate2 in FIGS. 1 to 2.5T
(when there is a delay T.sub.d1, another delay T.sub.d1 is
made).
The gate groups G1a, G1b, G2a and G2b are selected as follows.
First, with T.sub.sync taken a reference, the starting point of the
T-period reference clock 300 is determined, and it is checked
whether the mark length nT rises an even number of clock periods T
or an odd number of clock periods T after the starting point. More
specifically, a 1-bit adder is used which is reset at T.sub.sync
and adds 1 every period. If the result is 0, it is decided that the
elapsed time is determined to be an even number of periods; and if
the result is 1, the elapsed time is determined to be an odd number
of periods. That is, if the elapsed time from the reference time
T.sub.sync to the front end of the nT mark is an even number times
the period T, then the gate signal group G1a or G2b is selected
depending on whether n is even or odd. If the elapsed time from the
reference time T.sub.sync to the front end of the nT mark is an odd
number times the period T, then the gate signal group G1b or G2a is
selected depending on whether n is even or odd. It is therefore
possible to generate all the recording pulses in a series of nT
marks which are generated, with T.sub.0 as a starting point, by
using combinations of the four 2T-period reference clocks shifted
0.5T from one another.
To determine the length of the off pulse section and the erase
power Pe light irradiating section requires focusing attention on
the off pulse section .beta..sub.mT. That is, it is desired to
provide the rear end of the mark with not the period 2T but with a
margin of about .+-.1T. In this case, the timing of the last off
pulse .beta..sub.m or .beta..sub.m' needs to be defined
exceptionally. To this end, it is preferable to generate a gate
signal corresponding to the Gate3 of FIG. 1. For example, with the
front end of the nT mark taken as a reference, the gate signals are
generated depending on whether n is even or odd, that is, a gate G3
of .SIGMA.(.alpha..sub.i+.beta..sub.i) is generated with a delay
time of T.sub.d1 when n is even; and a gate G4 of
.SIGMA.(.alpha..sub.i'+.beta..sub.i') is generated with a delay
time of T.sub.d1 when n is odd, to radiate light with different
powers according to the following conditions. (1) in a duration
where both G3 and G4 are off, light with a power Pe is radiated;
(2) in a duration where either G3 or G4 is on, light with a power
Pb is radiated; (3) in a duration where both G3 and G1a are on at
the same time, light with a power Pw is radiated for a G1a-on
section; (4) in a duration where both G3 and G1b are on at the same
time, light with a power Pw is radiated for a G1b -on section; (5)
in a duration where both G4 and G2a are on at the same time, light
with a power Pw is radiated for a G2a-on section; and (6) in a
duration where both G4 and G2b are on at the same time, light with
a power Pw is radiated for a G2b-on section.
The gate priority relationship described above is determined by
matching the gate on/off to logical 0 and 1 levels and performing
an OR operation on each gate controlling logical signal.
FIG. 12 and FIG. 13 represent a case where, for simplicity, the
rising edge of the first recording pulse .alpha..sub.1T,
.alpha..sub.1'T is at the front end of the nT mark, i.e.,
concurrent with the front end of the nT mark being recorded. If the
mark has a delay, it is preferred in terms of keeping the space
length at a desired value that the rising edges of .alpha..sub.1T
and .alpha..sub.1'T be provided with the same delay T.sub.d1.
Divided Recording Pulse Generating Method 3
The following description concerns another example of the divided
recording pulse generating method based on a 2T-period clock signal
which is obtained by dividing the reference clock period T This
method allows for the design of logic circuits based on more
regular rules than those employed in the divided recording pulse
generating method 1.
In more concrete terms, as in the pulse generating method 2, the
procedure depends on whether the value the n of an nT mark can take
is odd or even. In the divided recording pulse generating method 2,
the correction of the mark length difference 1T between an
even-numbered length mark and an odd-numbered length mark, both
having the same number of divisions, is distributed and allocated
to the first and last recording pulse periods. In the pulse
generating method 3, however, the correction of the mark length
difference 1T is done by adjusting the off pulse length
.beta..sub.iT (2.ltoreq.i.ltoreq.m-1) in the intermediate divided
recording pulse group.
That is, for the recording of a mark in which n is even, i.e., the
mark length is nT=2LT (L is an integer equal to 2 or more), the
mark is divided into the number of sections m=L and the
.alpha..sub.i and .beta..sub.i in the recording pulse sections
.alpha..sub.iT and the off pulse sections .beta..sub.iT are defined
as follows. T.sub.d1+.alpha..sub.1=2+.epsilon..sub.1
.beta..sub.i-1+.alpha..sub.i=2(2.ltoreq.i.ltoreq.m)
For the recording of a mark in which n is odd, i.e., the mark
length is nT=(2L+1)T, on the other hand, the mark is divided into
the number of sections m=L and the .alpha..sub.i' and .beta..sub.i'
in the recording pulse sections .alpha..sub.i'T and the off pulse
sections .beta..sub.i'T are defined as follows.
T.sub.d1'+.alpha..sub.1'=2+.epsilon..sub.1'
.beta..sub.1'+.alpha..sub.2'=2.5+.epsilon..sub.2'
.beta..sub.i-1'+.alpha..sub.1'=2(3.ltoreq.i.ltoreq.m-1)
.beta..sub.m-1'+.alpha..sub.m'=2.5+.epsilon..sub.3' (When L=2, it
is assumed that .beta..sub.1'+.alpha..sub.2'=2.5+.epsilon..sub.2'
or .beta..sub.1'+.alpha..sub.2'=3+.epsilon..sub.2')
Then, .beta..sub.1, .alpha..sub.2, .beta..sub.m-1, .alpha..sub.m,
.beta..sub.1', .alpha..sub.2', .beta..sub.m-1 and .alpha..sub.m'
satisfy the following equation.
.beta..sub.1+.alpha..sub.2+.beta..sub.m-1+.alpha..sub.m+.DELTA..sub.2=.be-
ta..sub.1'+.alpha..sub.2'+.beta..sub.m-1'+.alpha..sub.m' (where
.DELTA..sub.2=0.8 to 1.2).
The values of .alpha..sub.i, .beta..sub.i, .alpha..sub.i',
.beta..sub.i', T.sub.d1, T.sub.d1', .epsilon..sub.1,
.epsilon..sub.1', .epsilon..sub.2' and .epsilon..sub.3' can vary
according to L.
T.sub.d1 and T.sub.d1' are delay or advance times from the starting
end of the nT mark in the mark length-modulated original signal
until the first recording pulse .alpha..sub.1T rises. They are real
numbers normally between -2 and 2. The positive values of T.sub.d1
and T.sub.d1' signify delays. T.sub.d1 and T.sub.d1' are preferably
set almost constant regardless of the value of L.
.alpha..sub.i, .beta..sub.i, .alpha..sub.i' and .beta..sub.i' are
real numbers normally between 0 and 2, preferably between 0.5 and
1.5.
.epsilon..sub.1, .epsilon..sub.1', .epsilon..sub.2' and
.epsilon..sub.3' are real numbers normally between -1 and 1,
preferably between -0.5 and 0.5. These are used, as required, as
correction values for realizing precise mark lengths or space
lengths in the divided pulse periods
(.beta..sub.i-1+.alpha..sub.i)T that form the period 2T.
In the pulse generating method 3, two marks corresponding to n=2LT
and n=(2L+1)T, where L's are equal, are divided into the same
division number L of recording pulses in the process of recording.
That is, for n=2, 3, 4, 5, 6, 7, 8, 9, . . . , the number of
recording pulses for the corresponding n is set to 1, 1, 2, 2, 3,
3, 4, 4, 5, 5 . . . For example, in the EFM modulation signal, for
n=3, 4, 5, 6, 7, 8, 9, 10, 11, the division number m is
sequentially set to m=1, 2, 2, 3, 3, 4, 4, 5, 5 in that order. In
the EFM+ signal, n=14 is added. In that case, the division number m
is set to 7. In the (1, 7)-RLL-NRZI modulation, the division number
m is set to 1 also in the case of n=2.
To record two kinds of mark lengths of n=2L and 2L+1 with the same
division numbers, the period (.beta..sub.1+.alpha..sub.2)T and the
period (.beta..sub.m-1+.alpha..sub.m)T are each increased or
decreased by 0.5T to adjust their lengths. What is important in the
mark length recording is the mark end position and the jitter that
are determined by the waveform of the front and rear ends of the
mark. The intermediate portion of the mark does not have a great
effect on the jitter at the ends of the mark as long as the correct
amplitude of the intermediate portion is obtained. The above
adjusting method takes advantage of the fact that as long as the
mark does not appear optically divided, if the recording pulse
period in the intermediate portion of the mark is extended or
reduced by 0.5T, the mark length only increases or decreases by the
corresponding amount and does not greatly affect the jitter at the
ends of the mark.
In the pulse generating method 3, 2T is taken as the base recording
pulse period for any mark length. The duty ratio of
.alpha..sub.i-.beta..sub.i can be optimized for each mark length or
for each i, but it is preferred that the following restrictions be
provided for the simplification of the recording pulse generating
circuit.
As to the front end of the mark, .alpha..sub.1, .beta..sub.1,
.alpha..sub.1' and .beta..sub.1' are each preferably made constant
and independent of L for the L value of 3 or more. More preferably,
.alpha..sub.1'=0.8.alpha..sub.1 to 1.2.alpha..sub.1 and
.beta..sub.1'=.beta..sub.1+about 0.5. Still more preferably,
.beta..sub.1'=.beta..sub.1+0.5, .alpha..sub.1=.alpha..sub.1' and
.beta..sub.1=.beta..sub.1'. The position of the front end of the
mark is almost determined by the leading edge of the first
recording pulse. That is, if the position of the leading edge of
.alpha..sub.1T=.alpha..sub.1'T is set to lag the starting end of
the mark length nT by a constant delay time T.sub.d1, the actual
front end position of the mark is determined almost uniquely. As to
the jitter at the front end of the mark, on the other hand, if
.beta..sub.1T has more than a certain length (in practice, 0.5T, )
assuming that .alpha..sub.1T is nearly equal to .alpha..sub.1'T,
the jitter can be kept within a satisfactory level irrespective of
n value by setting only .beta..sub.1' to approximately
.beta..sub.1'=.beta..sub.1+0.5.
As to the rear end of the mark, .alpha..sub.m, .beta..sub.m,
.alpha..sub.m' and .beta..sub.m' are each preferably made constant
and independent of L for the L value of 3 or more. More preferably,
.beta..sub.m-1'=.beta..sub.m-1+about 0.5,
.alpha..sub.m'=0.8.alpha..sub.m to 1.2.alpha..sub.m, and
.beta..sub.m'=0.8.beta..sub.m to 1.2.beta..sub.m. Still more
preferably, .beta..sub.m-1'=.beta..sub.m-1+0.5,
.alpha..sub.m=.alpha..sub.m', and .beta..sub.m=.beta..sub.m'.
When L=2, it is preferred that .beta..sub.1'=.beta..sub.1+0.5 to
1.5, .alpha..sub.m'=.alpha..sub.m+0 to 1, and
.beta..sub.m'=0.8.beta..sub.m to 1.2.beta..sub.m. However, in
either case, it is desired that the falling edges of the
.alpha..sub.mT and .alpha..sub.m'T and the rear end of the mark
length nT be synchronized, with a predetermined time difference
therebetween.
The rear end position of the mark depends not only on the trailing
edge position of the last recording pulse .alpha..sub.mT (or
.alpha..sub.m'T) but also on the cooling process of the recording
layer temperature before and after the mark rear end position. In
the phase change medium that forms amorphous marks in particular,
the mark rear end position depends on the cooling speed of the
recording layer temperature controlled by the last off pulse
section .beta..sub.mT (or .beta..sub.m'T) Hence, if .alpha..sub.mT
and .alpha..sub.m'T are shifted a predetermined time from the rear
end of the nT mark and .beta..sub.m'=.beta..sub.m, then the mark
rear end position is determined almost uniquely.
As to the jitter at the rear end, on the other hand, if
.beta..sub.m-1, .beta..sub.m-1', .alpha..sub.m and .alpha..sub.m'
are longer than a predetermined length, the jitter produced is
small and governed mostly by .beta..sub.m'=.beta..sub.m. Optimizing
the .beta..sub.m'=.beta..sub.m can produce nearly the best
jitter.
In the pulse generating method 3, too, in the process of high
density recording in particular, the values of T.sub.d1,
.alpha..sub.1, .alpha..sub.1', .beta..sub.1, .beta..sub.1',
.alpha..sub.m, .alpha..sub.m', .beta..sub.m and .beta..sub.m' can
be finely adjusted in the range of about .+-.20% to correct the
heat interference according to marks or spaces immediately before
or after the mark being recorded. In the above explanation, the
expression "about 0.5" or "about 1" means that the fine adjustment
of that degree is allowed.
For further simplification of the pulse generating circuit, when L
is 3 or more, .alpha..sub.i and .alpha..sub.i' are made constant
and independent of i for the i value of 2.ltoreq.i.ltoreq.m-1. That
is, .alpha..sub.2=.alpha..sub.3= . . . =.alpha..sub.m-1
.alpha..sub.2'=.alpha..sub.3'= . . . =.alpha..sub.m-1' Here, the
expression "L is 3 or more" is the condition to establish that the
division number is 3 or more and there is one or more intermediate
divided recording pulses excluding first and last divided
pulses.
More preferably, when L is 3 or more, the values of .alpha..sub.i
and .alpha..sub.i' for 2.ltoreq.i.ltoreq.m-1 are fixed to constant
values of .alpha.c and .alpha.c' respectively, which are
independent of L. Still more preferably, .alpha.c=.alpha.c'. In the
mark length recording, the formation of the intermediate portion of
the mark has little effect on the mark end position and the jitter
as long as the appropriate signal amplitudes are produced. In most
cases therefore it is possible to make a uniform setting of
.alpha..sub.i=.alpha..sub.i'=.alpha.c (2.ltoreq.i.ltoreq.m-1) as
described above.
It is more preferred that .alpha..sub.m and .alpha..sub.m' be set
to the same values of .alpha..sub.i and .alpha..sub.i' for
2.ltoreq.i.ltoreq.m-1.
When L=1, i.e., the mark length nT is 2T or 3T, it is preferred
that m=1. In that case, the period (.alpha..sub.1+.beta..sub.1)T
and the duty ratio of .alpha..sub.1-.beta..sub.1 (or period
(.alpha..sub.1'+.beta..sub.1')T and .alpha..sub.1'-.beta..sub.1'
duty ratio) are adjusted to realize a desired mark length and
jitter. If .beta..sub.1 or .beta..sub.1' is constant for
n.gtoreq.4, it is preferred that .beta..sub.m or .beta..sub.m' also
use the same values of .beta..sub.1 or .beta..sub.1' for
n.gtoreq.4.
These divided recording pulses, when 0.ltoreq.T.sub.d1.ltoreq.2 and
0.ltoreq.T.sub.d1'.ltoreq.2, can be formed as follows.
First, (1) it is assumed that an original mark length modulation
signal is generated in synchronism with the first reference clock
with a period T. With the starting end of the nT mark of the mark
length modulation signal taken as a reference, the first recording
pulse .alpha..sub.1T (or .alpha..sub.1'T) is generated with a delay
time of T.sub.d1 (or T.sub.d1'). Next, (2) the last recording pulse
.alpha..sub.mT (.alpha..sub.m'T) is generated so that its falling
edge aligns, after a time difference of .epsilon..sub.3 (or
.epsilon..sub.3'), with the rear end of the nT mark. Then, (3) as
to .alpha..sub.iT and .beta..sub.iT (2.ltoreq.i.ltoreq.m-1)--the
intermediate divided recording pulses that are produced when L is 3
or more--.alpha..sub.2T falls 4T after the starting end of the nT
mark and thereafter .alpha..sub.i+.beta..sub.i-1 are generated with
a period of 2T. (4) When n is odd (n=2L+1), .alpha..sub.2'T falls
4.5T after the starting end of the nT mark and thereafter
.alpha..sub.i'+.beta..sub.i-1' are generated.
In the above example, also when .epsilon..sub.1, .epsilon..sub.1',
.epsilon..sub.2' and .epsilon..sub.3' are not 0, the falling edge
of at least .alpha..sub.2T or .alpha..sub.2'T in the intermediate
divided recording pulse group is produced precisely the delay time
of 4T or 4.5T after the starting end of the nT mark. Therefore, at
least the intermediate divided recording pulse group can be
generated in synchronism with the 2T-period reference clock, which
was generated by frequency-dividing the T-period reference data
clock in advance.
FIG. 24 shows the relation between the recording pulses when the
recording pulse dividing method of this invention is implemented by
combining a plurality of 2T-period reference clocks.
In FIG. 24, for simplicity, the recording power Pw of light
radiated during the recording pulse sections, the bias power Pb of
light radiated during the off pulse sections, and the erase power
Pe of light radiated during other than these sections are each
shown to be constant for any i. Although these powers are shown to
have the relationship of Pb<Pe<Pw, these powers may be set to
different values depending on the values of n and i. Particularly,
the recording power Pw.sub.1 in .alpha..sub.1T and .alpha..sub.1'T
and the recording power Pw.sub.m in .alpha..sub.mT and
.alpha..sub.m'T may be set different from the recording power
Pw.sub.i other sections .alpha..sub.iT (i=2 to m-1).
Further, in FIG. 24, for simplification, it is assumed that
.epsilon..sub.1=.epsilon..sub.1'=.epsilon..sub.2'=.epsilon..sub.3'=0,
and the first recording pulses .alpha..sub.1T and .alpha..sub.1'T
are shown to fall 2T, after the front end of the nT mark being
recorded and the falling edges of .alpha..sub.mT and
.alpha..sub.m'T are shown to coincide with the rear end of the nT
mark.
In FIG. 24, reference number 220 represents a T-period reference
clock.
FIG. 24(a) shows square waves associated with the nT mark of the
original mark length modulation signal, with 221 representing a
mark 2LT in length and 222 representing a mark (2L+1)T in length.
Here, although two kinds of marks corresponding to L=5 are shown,
it is possible to handle other cases of the L value by adding or
subtracting the period of 2T for the intermediate i of
2.ltoreq.i.ltoreq.m-1 each time L increments or decrements by
1.
FIG. 24(b) represents a waveform of divided recording pulses when
n=2L=10 and FIG. 24(c) represents a waveform of divided recording
pulses when n=2L+1=11.
In FIG. 24(b), the 2T-period reference clock 225 is obtained by
frequency-dividing a T-period reference clock 223 which has no
phase delay with respect to the T-period reference clock 220. When
T.sub.d1+.alpha..sub.1=2, the falling edge of each recording pulse
.alpha..sub.iT (1.ltoreq.i.ltoreq.m) is synchronized with the
2T-period reference clock 225. In synchronism with the 2T-period
reference clock 225, the duty ratio of .alpha..sub.i-.beta..sub.i
is adjusted to produce a recording pulse waveform 227.
In FIG. 24(c), a 2T-period reference clock 226 is obtained by
frequency-dividing a T-period reference clock 224 which is 0.5T out
of phase with the T-period reference clock 220. The falling edge of
each recording pulse .alpha..sub.i'T (2.ltoreq.i.ltoreq.m) is
synchronized with the 2T-period reference clock 226. In synchronism
with the reference clock 226, the duty ratio of
.beta..sub.i-1-.alpha..sub.i is adjusted to produce a recording
pulse waveform 228.
In this way, by using the T-period first reference clock 1 (223)
and the T-period second reference clock 2 (224) 0.5T out of phase
with the T-period first reference clock, .alpha..sub.i
(1.ltoreq.i.ltoreq.m) is generated in synchronism with the
2T-period reference clock 3 (225) which is obtained by
frequency-dividing the reference clock 1 and .alpha..sub.i'
(2.ltoreq.i.ltoreq.m-1) is generated in synchronism with the
2T-period reference clock 4 (226) which is obtained by
frequency-dividing the reference clock 2, thereby producing the
divided recording pulses corresponding to 2L and 2L+1 easily.
In FIG. 24, the mark lengths 2LT and (2L+1)T are depicted to have
their rear ends align with each other at T2 and T4. So, there are
only two possible relations (b) and (c) between the 2T-period
reference clocks 225 and 226. In reality, however, when the
2T-period reference clocks are used, the front end positions of
these mark lengths can be 1T out of phase with each other although
they are in phase with the 2T period. Hence, the divided recording
pulse generating method 3 needs also to consider, as in the divided
recording pulse generating method 2, the fact that there are four
possible relations considering the cases of n being even and n
being odd as shown in FIGS. 13(a), (b), (c) and (d).
Then, by using the 2T-period clock train 4 of FIG. 13, in the case
of (1a), a gate group G1a corresponding to the recording pulse
sections .alpha..sub.1T, .alpha..sub.2T, .alpha..sub.3T,
.alpha..sub.4T, .alpha..sub.5T is generated in synchronism with
each of the periods P1a, P2a, P3a, P4a, P5a; in the case of (1b), a
gate group G1b corresponding to the recording pulses
.alpha..sub.1T, .alpha..sub.2T, .alpha..sub.3T, .alpha..sub.4T,
.alpha..sub.5T is generated in synchronism with each of the periods
P1b, P2b, P3b, P4b, P5b; in the case of (2a), a gate group G2a
corresponding to the recording pulses .alpha..sub.1'T,
.alpha..sub.2'T, .alpha..sub.3'T, .alpha..sub.4'T, .alpha..sub.5'T
is generated in synchronism with each of the periods R1a, R2a, R3a,
R4a, R5a; and in the case of (2b), a gate group G2b corresponding
to the recording pulses .alpha..sub.1'T, .alpha..sub.2'T,
.alpha..sub.3'T, .alpha..sub.4'T, .alpha..sub.5'T is generated in
synchronism with each of the periods Q1b, Q2b, Q3b, Q4b, Q5b.
These recording pulse generating gate groups G1a, G1b, G2a, G2b are
identical to the combinations of Gate 1, 2, and 4 in FIG. 1, as in
the case of the divided recording pulse generating method 2.
That is, in generating G1a and G1b, as shown in FIG. 1, the Gate1
for generating the first pulse .alpha..sub.1T, the Gate2 for
generating the intermediate pulse group .alpha..sub.iT
(2.ltoreq.i.ltoreq.m-1), and the Gate4 for generating the last
pulse .alpha..sub.mT are separately generated and then combined. Or
in generating G2a and G2b, as shown in FIG. 1, the first pulse
.alpha..sub.1'T, the intermediate pulse group .alpha..sub.i'T
(2.ltoreq.i.ltoreq.m-1), and the last pulse .alpha..sub.m'T are
separately produced and then combined. When .epsilon..sub.1,
.epsilon..sub.1', .epsilon..sub.2' and .epsilon..sub.3' are not 0,
the first recording pulses .alpha..sub.1T, .alpha..sub.1'T may be
given a predetermined time difference of period P1a, Q1a, P1b or
Q1b, and the last recording pulses .alpha..sub.mT, .alpha..sub.m'T
are given a predetermined time difference of either period P5a,
P5b, Q5a or Q5b.
On the other hand, to determine the off pulse sections and the Pe
power irradiation sections, one must consider the fact that the
last off pulse section .beta..sub.mT of the mark is irregular. That
is, the period of the rear end of the mark is not necessarily 2T
and must be given a margin of about 2T.+-.1T. This can be dealt
with by defining the last off pulse .beta..sub.m or .beta..sub.m'
exceptionally. For that purpose, the gate signal corresponding to
the Gate3 of FIG. 1 is generated.
That is, when n is even, a gate G3 of
.SIGMA.(.alpha..sub.i+.beta..sub.i)T is generated with a delay time
T.sub.d1 from the front end of the nt mark; and when n is odd, a
gate G4 of .SIGMA.(.alpha..sub.i'+.beta..sub.i')T is generated with
a delay time T.sub.d1' from the front end of the nT mark. Then,
when either G3 or G4 is off, the light with the erase power Pe is
radiated; when either G3 or G4 is on, the light with the bias power
Pb is radiated; when both G3 and G1a are on simultaneously, the
light with the recording power Pw is radiated in response to the
G1a-on section; when both G3 and G1b are on simultaneously, the
light with the recording power Pw is radiated in response to the
G1b-on section; when both G4 and G2a are on simultaneously the
light with the recording power Pw is radiated in response to the
G2a-on section; and when both G4 and G2b are on simultaneously, the
light with the recording power Pw is radiated in response to the
G2b-on section. The gate priority relationship described above is
determined by matching the gate on/off to logical 0 and 1 levels
and performing an OR operation on each gate controlling logical
signal.
In summary, all the gates for generating the recording pulse
sections .alpha..sub.iT can be produced by the following procedure.
(1) A reference time T.sub.sync corresponding to the clock mark
formed at a predetermined position on the recording track is
generated; (2) four reference clocks are generated: a 2T-period
reference clock 1a produced at the reference time T.sub.sync as a
starting point, a 2T-period reference clock 2a produced 0.5T in
advance of the reference clock 1a, 2T-period reference clock 1b
produced 1T, in advance of the reference clock 1a, and a 2T-period
reference clock 2b produced 1.5T in advance of the reference clock
1a; (3) in recording a mark of nT=2LT, the gate groups G1a and G1b
which have timings corresponding to the .alpha..sub.1T,
.alpha..sub.iT (2.ltoreq.i.ltoreq.m-1) and .alpha..sub.mT sections
are generated in synchronism with either the reference clock 1a or
1b; (4) in recording a mark of nT=(2L+1)T, the gate groups G2a and
G2b which have timings corresponding to the .alpha..sub.1'T,
.alpha..sub.i'T (2.ltoreq.i.ltoreq.m-1) and .alpha..sub.m'T are
generated in synchronism with either the reference clock 2a or
2b.
The gate groups G1a, G1b, G2a, G2b can be selected as follows.
First, it is checked whether the mark length nT rises an even
number of clock periods T or an odd number of clock periods T after
the reference time T.sub.sync as a start point. More specifically,
a 1-bit adder is used which is reset at T.sub.sync and adds 1 every
period. If the result is 0, it is decided that the elapsed time is
determined to be an even number of periods; and if the result is 1,
the elapsed time is determined to be an odd number of periods. That
is, if the elapsed time from the reference time T.sub.sync to the
front end of the nT mark is an even number times the period T, then
the gate signal group G1a or G2b is selected depending on whether n
is even or odd. If the elapsed time from the reference time
T.sub.sync to the front end of the nT mark is an odd number times
the period T, then the gate signal group G1b or G2a is selected
depending on whether n is even or odd. It is therefore possible to
generate all the recording pulses in a series of nT marks which are
generated, with T.sub.0 as a starting point, by using combinations
of the four 2T-period reference clocks shifted 0.5T from one
another.
With the divided recording pulse generating methods 1, 2 and 3
described above, by holding constant the switching period of at
least intermediate pulse group (.alpha..sub.i+.beta..sub.i)T or
(.alpha..sub.i+.beta..sub.i-1)T (2.ltoreq.i.ltoreq.m-1) at 1T,
1.5T, 2T or 2.5T, and by changing the duty ratio of
.alpha..sub.i-.beta..sub.i and duty ratio of
.alpha..sub.i'-.beta..sub.i', it is possible to find an optimum
divided recording pulse strategy easily even when mediums with
different characteristics are used or when the same medium is used
at different linear velocities.
The optical recording method of this invention is particularly
effective for a phase change medium in which information is
overwritten by forming an amorphous mark on a crystal-state medium,
the crystal state being taken as an unrecorded or erased state.
The optical recording method of this invention is also effective in
cases where the recording is made on the same medium at different
linear velocities. Generally, a constant density recording is
commonly practiced, which does not depend on the linear velocity
but keeps a product of vT at a plurality of linear velocities
constant, where v is a linear velocity and T is a clock period.
When for example the recording based on the mark length modulation
scheme is to be performed on the same recording medium at a
plurality of linear velocities v in such a way that v.times.T is
constant, the pulse generation method 2, for L equal to or more
than 2, keeps the periods of (.alpha..sub.i+.beta..sub.i)T and
(.alpha..sub.i'+.beta..sub.i')T for 2.ltoreq.i.ltoreq.m-1 constant
irrespective of the linear velocity, also keeps Pw.sub.i, Pb.sub.i
and Pe for each i almost constant irrespective of the linear
velocity, and reduces .alpha..sub.i and .alpha..sub.i'
(1.ltoreq.i.ltoreq.m) as the linear velocity becomes slower (JP-A
9-7176). As a result, a satisfactory overwrite is made possible in
a wide range of linear velocity.
When the recording based on the mark length modulation scheme is to
be performed on the same recording medium at a plurality of linear
velocities v with v.times.T kept constant, the pulse generation
method 3, for L equal to or more than 2, keeps the periods of
(.beta..sub.i-1+.alpha..sub.i)T and
(.beta..sub.i-1'+.alpha..sub.i')T for 2.ltoreq.i.ltoreq.m constant
irrespective of the linear velocity, also keeps Pw.sub.i, Pb.sub.i
and Pe for each i almost constant irrespective of the linear
velocity, and monotonously reduces .alpha..sub.i and .alpha..sub.i'
as the linear velocity becomes slower (JP-A 9-7176). In this case,
too, a satisfactory overwrite is made possible in a wide range of
linear velocity.
In the above two examples, the expression "Pw.sub.i, Pb.sub.i and
Pe are almost constant irrespective of the linear velocity" means
that the minimum value is within about 20% of the maximum value,
more preferably within 10%. Still more preferably, Pw.sub.i,
Pb.sub.i and Pe are virtually constant, not dependent of the linear
velocity at all.
In the above two examples, the method of reducing .alpha..sub.i and
increasing .beta..sub.i in (.alpha..sub.i+.beta..sub.i)T and
reducing .alpha..sub.i and increasing .beta..sub.i-1 in
(.alpha..sub.i+.beta..sub.i)T as the linear velocity decreases is
particularly effective in the phase change medium. This is because
in the phase change medium, the cooling speed of the recording
layer becomes slower as the linear velocity decreases and it is
necessary to accelerate the cooling effect by increasing the ratio
of the off pulse section .beta..sub.i. In that case, for all linear
velocities v used and for all L, it is preferred that .beta..sub.i
and .beta..sub.i' be set to 0.5<.beta..sub.i.ltoreq.2.5 and
0.5.ltoreq..beta..sub.i'.ltoreq.2.5, more preferably
1.ltoreq..beta..sub.i.ltoreq.2 and 1.ltoreq..beta..sub.i'.ltoreq.2,
to secure the cooling time to change the medium into the amorphous
state.
In the above two examples, it is further preferred that, for all
linear velocities, .alpha..sub.iT and .alpha..sub.i'T
(2.ltoreq.i.ltoreq.m-1) be held constant, i.e., the intermediate
recording pulses have almost constant absolute lengths of time. The
expression "almost constant" means that they have a variation range
of about .+-.0.1T at each linear velocity. In that case, the
reference clock T becomes large as the linear velocity decreases,
so .alpha..sub.i and .alpha..sub.i' in the intermediate pulse group
necessarily decrease monotonously. Although the first recording
pulse sections .alpha..sub.1T, .alpha..sub.1'T can be made
constant, they should preferably be finely adjusted at each linear
velocity. The .beta..sub.m and .beta..sub.m' are preferably
fine-adjusted at each linear velocity. In that case, it is
preferred that .beta..sub.m and .beta..sub.m' be set constant or
made to increase as the linear velocity decreases.
In the above two pulse generating methods 1, 2 and 3, when the
reference clock period T is smaller than the .times.1-speed of the
recordable DVD (linear velocity 3.5 m/s; and reference clock period
T is 38.2 nanoseconds), n-(.eta..sub.1+.eta..sub.2) and the first
and last pulses should preferably be controlled according to the
preceding and/or subsequent mark lengths or space lengths.
Examples in which the present invention proves particularly
effective are described below.
A first case is where the linear velocity during the recording is
set as high as 10 m/s or more and the shortest mark length as small
as 0.8 .mu.m or less in order to perform high density recording.
Because the shortest mark length is expressed as nT.times.V where V
is the linear velocity, the reduced shortest mark length results in
the reference clock period T being shortened.
It is also effective to set the wavelength of the recording light
to as short as 500 nm or less, the numerical aperture of the lens
for focusing the recording light to as high as 0.6 or more, the
beam diameter of the recording light to a small value, and the
shortest mark length to as small as 0.3 .mu.m or less to perform
high density recording.
Further, it is also effective to use high density recording
modulation scheme, such as a 8-16 modulation scheme and a (1,
7)-RLL-NRZI modulation scheme, as the mark length modulation
scheme.
Another case is where the mark length modulation scheme is an EFM
modulation scheme and the linear velocity during recording is set
to a very high speed of 10 times the CD reference linear velocity
of 1.2 m/s to 1.4 m/s while keeping the recording line density
constant during the recording.
Still another case is where the mark length modulation scheme is an
EFM+ modulation scheme, the high density recording scheme, and the
linear velocity during recording is set to as high as two or more
times the DVD reference linear velocity of 3.49 m/s while keeping
the recording line density constant during the recording.
Next, the quality of the mark length modulation signal will be
described by referring to the drawings.
FIG. 5 is a schematic diagram showing retrieved waveforms
(eye-pattern) of the EFM modulation signal used in the CD family
including Cd-RW. In the EFM modulation, the recording mark and
space lengths can take a time length of between 3T and 11T and the
eye-pattern virtually randomly includes retrieved waveforms of all
amorphous marks from 3T to 11T. The EFM+ modulation further
includes a mark length of 14T and a space length of 14T.
The upper end I.sub.top of the eye-pattern converted into the
reflectance is an upper end value R.sub.top, and the amplitude of
eye-pattern (in practice, amplitude of 11T mark) I.sub.11
standardized by the I.sub.top is a modulation m.sub.11 of the
recording signal expressed as follows.
.times..times. ##EQU00001## m.sub.11 is preferably set between 40%
and 80% and it is particularly important to set m.sub.11 to 40% or
more. It is preferred that the signal amplitude be set large, but
too large a signal amplitude will result in the gain of the
amplifier of the signal reproducing system becoming excessively
saturated. So, the upper limit of m.sub.11 is set at around 80%.
Too small a signal amplitude on the other hand will reduce the
signal-noise ratio (SN ratio) and thus the lower limit is set at
around 40%.
Further it is preferred that the asymmetry value Asym defined by
the equation below be set as close to 0 as possible.
.times. ##EQU00002##
Further, it is desired that the jitter of each mark and space of
the retrieved signal be almost 10% or less of the reference clock
period T and that the mark length and space length have nearly
nT.times.V (T is a reference clock period of data, n is an integer
from 3 to 11, and v is a linear velocity during reproduction). With
this arrangement, a signal reproduction using a commercially
available CD-ROM drive can be performed at a low error rate. In a
recordable DVD medium using the EFM+ modulation scheme, equations
(1) and (2) are defined by replacing I.sub.11 with an amplitude
I.sub.14 of a 14T mark. The jitter is measured as a so-called
edge-to-clock jitter, which is obtained by passing an analog
retrieved signal through an equalizer to digitize it. In that case,
the value of jitter is preferably 13% or less of the clock period,
particularly 9% or less.
Next, a preferred optical recording medium for use in the
above-described optical recording method will be explained.
Optical recording mediums recorded according to this invention
include a pigment-based organic recording medium, a magnetooptical
recording medium, a phase change recording medium and various other
types of recording mediums. They also include a write-once and
rewritable mediums. Of these mediums, the one that can produce a
particularly significant effect is the phase change recording
medium, particularly a rewritable phase change recording medium in
which an amorphous mark is overwritten on a crystal-state medium,
the crystal state being taken as an unrecorded state.
A particularly preferred material of the recording layer is of a
type in which crystallization initiates at an interface between a
crystal area and a melted area.
Among the preferred phase change mediums are those having a
recording layer containing still more excessive Sb in the SbTe
eutectic composition. A particularly preferred composition is the
one which contains excessive Sb and also Ge in the base
Sb.sub.70Te.sub.30 eutectic composition. The Sb/Te ratio is
particularly preferably set to 4 or more. The content of Ge is
preferably 10 atomic % or less. An example of such a recording
layer is a M.sub.zGe.sub.y(Sb.sub.xTe.sub.1-x).sub.1-y-z alloy
(where 0.ltoreq.z.ltoreq.0.1, 0<y.ltoreq.0.3, 0.8.ltoreq.x; and
M is at least one of In, Ga, Si, Sn, Pb, Pd, Pt, Zn, Au, Ag, Zr,
Hf, V, Nb, Ta, Cr, Co, Mo, Mn, Bi, O, N and S).
The alloy with the above composition, as explained above, is a
binary alloy containing excessive Sb at the Sb.sub.70Te.sub.30
eutectic point and which contains Ge for improving the
time-dependent stability and jitter, and also contains at least one
of the series of elements represented by M for further reduction of
jitter and improvement of linear velocity dependency and optical
characteristics. Alternatively, a composition with the Te amount
close to zero can be regarded as an alloy that has Te or M element
added in the composition near the Ge.sub.15Sb.sub.85 eutectic
point.
In the above composition, Ge acts to enhance the time-dependent
stability of the amorphous mark without degrading the high speed
crystallization function offered by excess Sb. It is considered to
have a capability to raise the crystallization temperature and
enhance the activation energy for crystallization. That is, the
above-mentioned alloy recording layer consisting mainly of GeSbTe
in the base SbTe eutectic composition can increase the Sb/Te ratio
while suppressing the formation of crystal nucleus by the presence
of Ge and thereby increase the speed of crystal growth. Generally,
the forming of crystal nucleus initiates at a lower temperature
than that of the crystal growth and this is not desirable to the
storage stability of the mark at around the room temperature when
amorphous marks are formed. In the alloy recording layer with the
above GeSbTe as a main component, because the crystal growth at
near the melting point is selectively promoted, this alloy is
capable of quick erasure and has an excellent stability of the
amorphous mark at room temperature. In this sense, the alloy
recording layer described above is particularly suited for high
linear velocity recording.
As the element M in the above composition, In and Ga may be used.
In particular is effective in reducing jitter and enlarging the
associated linear velocity margin. A more preferred composition of
the recording layer of the phase change medium is
A.sup.1.sub.aA.sup.2.sub.bGe.sub.c(Sb.sub.dTe.sub.1-d).sub.1-a-b-c
alloy (where 0.ltoreq.a.ltoreq.0.1, 0<b.ltoreq.0.1,
0.02<c.ltoreq.0.3, 0.8.ltoreq.d; A.sup.1 is at least one of Zn,
Pd, Pt, V, Nb, Ta, Cr, Co, Si, Sn, Pb.sub.iBi, N, O and S; and
A.sup.2 is In and/or Ga).
These compositions are preferable because, compared with the
composition near the conventional GeTe--Sb.sub.2Te.sub.3
pseudo-binary alloy, the reflectance of individual fine crystal
grains has a smaller dependency on the direction of crystal plane,
providing these compositions with the ability to reduce noise.
Further, the SbTe-based composition with the above Sb/Te ratio
higher than 80/20 is excellent in that it is capable of quick
erasure at high linear velocities equal to or more than 12 times
the CD linear velocity (about 14 m/s) or 4 times the DVD linear
velocity (about 14 m/s).
This composition, on the other hand, poses a particularly large
problem when the reference clock period is as small as 25 ns or
less. The reason is described as follows.
The erasure of the amorphous mark in the above composition is
virtually governed only by the crystal growth from the boundary
with the crystal area surrounding the amorphous mark, and the
formation of a crystal nucleus inside the amorphous mark and the
process of crystal growth from the crystal nucleus hardly
contribute to the recrystallization process. As the linear velocity
increases (e.g., to more than 10 m/s), the time that the erase
power Pe is irradiated becomes short, extremely reducing the time
that the layer is kept at a high temperature around the melting
point necessary for the crystal growth. In the above composition,
although the crystal growth from the area surrounding the amorphous
mark can be promoted by increasing the Sb content, the increased
content of Sb also increases the crystal growth speed during the
re-solidifying of the melted area. That is, increasing the Sb
content to ensure the quick erasure of the amorphous mark during
the high linear velocity recording makes the formation of good
amorphous marks difficult. In other words, when the speed of
recrystallization from around the amorphous mark is increased above
a certain level, the recrystallization from around the melted area
during the re-solidifying of the melted area formed to record the
amorphous mark is also accelerated.
In the composition described above, there is a problem that an
attempt to perform erasure at high speed to effect a high linear
velocity recording makes the formation of an amorphous mark
difficult. In addition, at a high linear velocity the clock period
is shortened, reducing the off pulse section and degrading the
cooling effect, which in turn renders that problem even more
conspicuous.
The composition problem described above is considered relatively
not so large with the commonly used conventional
GeTe--Sb.sub.2Te.sub.3 pseudo-binary alloy-based composition. In
the GeTe--Sb.sub.2Te.sub.3 pseudo-binary alloy-based composition,
the erasure of the amorphous mark is effected mostly by the
formation of crystal nuclei within the amorphous mark and not very
much by the crystal growth. Further, the formation of crystal
nuclei is more active than the crystal growth at low temperatures.
Hence, in the GeTe--Sb.sub.2Te.sub.3 pseudo-binary alloy-based
composition, the re-crystallization can be achieved by generating a
large number of crystal nuclei even when the crystal growth is
relatively slow. Further, during the process of re-solidification
at temperatures below the melting point, the crystal nuclei are not
generated and the speed of crystal growth is relatively small, so
that the recording layer is easily transformed into the amorphous
state at a relatively small critical cooling speed.
The recording layer having a composition containing excess Sb in
the SbTe eutectic composition, particularly a composition further
including Ge, should preferably have a crystal state consisting of
a virtually single phase, not accompanied by phase separation. The
crystal state can be obtained by performing an initialization
operation, which involves heating and crystallizing the recording
layer of amorphous state produced at an initial phase of the film
deposition process using sputtering. The expression "virtually
single phase" means that the recording layer may be formed of a
single crystal phase or a plurality of crystal phases and that when
it is formed of a plurality of crystal phases, it preferably has no
lattice mismatch. When it is formed of a single crystal phase, the
recording layer may be multiple crystal layers of the same crystal
phase but with different orientations.
The recording layer of such a virtually single phase can improve
characteristics, such as reduced noise, an improved storage
stability and a greater ease with which crystallization can be
effected at high speed. This may be explained as follows. When
various crystal phases, including a crystal phase of a hexagonal
structure, a cubic crystal such as Sb but with a largely differing
lattice constant, a face-centered cubic crystal such as found in
AgSbTe2, and other crystal phases belonging to other space groups,
exist in a mixed state, a grain boundary with a large lattice
mismatch is formed. This is considered to cause disturbances to the
peripheral geometry of the mark and also produce optical noise. In
the recording layer of a single phase, however, such a grain
boundary is not formed.
The type of crystal phase formed in the recording layer depends
largely on the initialization method performed on the recording
layer. That is, to produce a preferred crystal phase in this
invention, the recording layer initializing method should
preferably incorporate the following provisions.
The recording layer is normally formed by a physical vacuum
deposition such as sputtering. The as-deposited state immediately
after the film is formed normally is an amorphous state and thus
should be crystallized to assume an unrecorded/erased state. This
operation is called an initialization. The initialization operation
includes, for example, an oven annealing in a solid phase in a
temperature range from the crystallization temperature (normally
150-300.degree. C.) up to the melting point, an annealing using
light energy irradiation by a laser beam and light of a flash lamp,
and an initialization by melting. To obtain a recording layer of a
preferred crystal state, the melting initialization is preferred.
In the case of annealing in the solid phase there is a time margin
for establishing a thermal equilibrium and thus other crystal
phases are likely to be formed.
In the melting crystallization, it is possible to melt the
recording layer and then directly recrystallize it during the
re-solidification process. Or, it is possible to change the
recording layer to the amorphous state during the re-solidification
process and then recrystallize it in solid phase at near the
melting point. In that case, when the crystallization speed is too
slow, it may bring about a time margin for the thermal equilibrium
to be established thereby forming other crystal phases. Therefore
it is preferred that the cooling speed be increased to some
extent.
For example, the time during which to hold the recording layer
above the melting point is preferably set normally to 2 .mu.s or
less, more preferably 1 .mu.s or less. For the melting
initialization a laser beam is preferably used. It is particularly
desirable for the initialization to use a laser beam which is
elliptical with its minor axis oriented almost parallel in the
direction of scan (this initialization method may hereinafter be
referred to as a "bulk erase"). In that case, the length of major
axis is normally 10-1,000 .mu.m and the minor axis normally 0.1-10
.mu.m. The lengths of major axis and minor axis of the beam are
defined as a half width of the light energy intensity distribution
measured within the beam. The scan speed is normally about 3-10
m/s. When the scanning is performed at speeds higher than the
maximum usable linear velocity at which at least the phase change
medium of this invention can be overwrite-recorded, the area that
was melted during the initialization scan may be transformed into
the amorphous state. Further, scanning at speeds 30% or more lower
than the maximum usable linear velocity generally causes a phase
separation, making it difficult to produce a single phase. A scan
speed 50-80% of the maximum usable linear velocity is particularly
preferred. The maximum usable linear velocity itself is determined
as the upper limit of a linear velocity that can assure a complete
erasure when the medium is irradiated with the Pe power at that
linear velocity.
A laser beam source may use a semiconductor laser, a gas laser and
others. The power of the leaser beam is normally between
approximately 100 mW and 2 W.
During the initialization by the bulk erase, when a disklike
recording medium is used, for example, it is possible to match the
direction of the minor axis of the elliptical beam almost to the
circumferential direction, scan the disk in the minor axis
direction by rotating the disk, and move the beam in the major axis
(radial) direction for every revolution, thus initializing the
whole surface. The distance moved by the beam in the radial,
direction for each revolution is preferably made shorter than the
beam major axis to overlap the scans so that the same radius is
irradiated with the laser beam a plurality of times. This
arrangement allows for a reliable initialization and avoids an
Uneven initialized state that would be caused by the energy
distribution (normally 10-20%) in the radial direction of the beam.
When the distance traveled is too small, other unwanted crystal
phases are likely to be formed. Hence, the distance of travel in
the radial direction is normally set to 1/2 or more of the beam
major axis.
The melting initialization may also be accomplished by using two
laser beams, melting the recording layer with a preceding beam, and
recrystallizing the recording layer with the second beam. If the
distance between the two beams is long, the area melted by the
preceding beam solidifies first before being recrystallized by the
second beam.
Whether the melting/recrystallization has been performed or not can
be determined by checking whether a reflectance R1 of the erased
state, after the recording layer has been actually overwritten with
an amorphous mark by the recording light about 1 .mu.m across, is
virtually equal to a reflectance R2 of the unrecorded state after
initialization. When a signal pattern for recording amorphous marks
intermittently is used, the measurement of R1 is carried out after
a plurality of overwrites, normally approximately 5 to 100
overwrites, have been performed. This eliminates the influences of
the reflectance of the spaces that could remain in the unrecorded
state after one recording operation alone.
The above erased state may be obtained, rather than by modulating
the focused recording laser beam according to the actual recorded
pulse generation method, but by irradiating the recording power
DC-wise to melt the recording layer and then resolidifying it.
In the case of the recording medium of this invention, the
difference between R1 and R2 is preferably set as small as
possible.
In more concrete terms, it is preferred that a value involving R1
and R2 which is defined as follows be set 10(%) or less,
particularly 5(%) or less.
.times..times..times..times..times..times..times. ##EQU00003##
For example, in the phase change medium with R1 of around 17%, R2
needs to be in the range of 16-18%.
To realize such an initialized state, it is desired that almost the
same thermal history as the actual recording condition be given by
the initialization.
The single crystal phase obtained by such an initialization method
generally tends to be a hexagonal crystal when the Sb/Te ratio is
larger than approximately 4.5 and a face-centered cubic crystal
when the Sb/Te ratio is less than 4.5. But this does not depend
only on the Sb/Te ratio. In the recording at speeds 16 times the CD
linear velocity and four times the DVD linear velocity, it is
preferred that the recording layer be made of a single phase of
hexagonal polycrystal.
The phase change medium of this invention normally has formed on
the substrate a lower protective layer, a phase change recording
layer, an upper protective layer and a reflection layer. It is
particularly preferred to form a so-called rapid cooling structure
in which the recording layer is 10-30 nm thick, the upper
protective layer is 15-50 nm thick and the reflection layer is
30-300 nm thick. When the recording method of this invention is to
be applied to the above optical recording medium, n/m associated
with the time lengths of all recording marks should preferably be
set to 1.5 or more. Further, n/m is more preferably 1.8 or more.
The upper limit of n/m normally is approximately 4, preferably
approximately 3, but can change depending on other conditions such
as the recording power Pw and the bias power Pb. Basically, n/m
needs only to fall in a range that gives a sufficient time length
for cooling.
When the optical recording method is to be applied to a write-once
type medium, a setting should be made such that Pe=Pb=Pr (Pr is a
retrieving light power). It is also possible to set Pe>Pr to
provide a residual heat effect.
The recording method of this invention does not depend on the layer
structure of the recording medium or the light radiating method,
and can be applied not only to a medium which has a layer structure
of substrate/protective layer/recording layer/protective
layer/reflection layer and in which a retrieve/write laser beam is
radiated through the substrate but also to a so-called film-side
incident type medium which has a layer structure of
substrate/reflection layer/protective layer/recording
layer/protective layer and in which the retrieve/write laser beam
is radiated from the side opposite the substrate. Further, the
recording method of this invention can also be applied to a medium
that combines these mediums to form multiple recording layers.
The reflection layer has a function of promoting heat dissipation
and enhancing the cooling speed. Hence, in the recording medium of
this invention, the selection of the reflection layer is important.
Specifically, it is preferred in this invention that a reflection
layer used have a high heat dissipating effect.
The thermal conductivity of the reflection layer is considered to
be nearly inversely proportional to its volume resistivity and the
heat dissipating effect of the reflection layer is proportional to
the film thickness. So, the heat dissipating effect of the
reflection layer is considered generally to be inversely
proportional to the sheet resistivity. In this invention,
therefore, a reflection layer with a sheet resistivity of 0.5
.OMEGA./.quadrature. or less, particularly 0.4 .OMEGA./.quadrature.
or less, is preferably used. The volume resistivity is preferably
in the range of between approximately 20 n.OMEGA.m and 100
n.OMEGA.m. A material with too small a volume resistivity is
practically not usable. A material with too large a volume
resistivity tends not only to have a poor heat dissipating effect
but to degrade the recording sensitivity.
Possible materials for the reflection layer include aluminum,
silver and alloys of these materials as main components.
Examples of aluminum alloy that can be used for the reflection
layer are aluminum alloys having added to Al at least one of Ta,
Ti, Co, Cr, Si, Sc, Hf, Pd, Pt, Mg, Zr, Mo and Mn. The contents of
the additive elements are normally between 0.2 atomic % and 1
atomic %. When these contents are too small, hillock resistance
tends to be insufficient; and when they are too large, the heat
dissipating effect tends to deteriorate.
Examples of silver alloy that can be used for the reflection layer
are silver alloys having added to Ag at least one of Ti, V, Ta, Nb,
W, Co, Cr, Si, Ge, Sn, Sc, Hf, Pd, Rh, Au, Pt, Mg, Zr, Mo and Mn.
The additive elements are preferably at least one of Ti, Mg, Pd and
Cu metal elements in terms of enhancing the time-dependent
stability. The contents of the additive elements are normally
between 0.2 atomic % and 3 atomic %. When these contents are too
small, the corrosion resistance tends to deteriorate; and when they
are too large, the heat dissipating effect tends to
deteriorate.
The volume resistivity increases in proportion to the contents of
the added elements in the Al alloy and to the contents of the added
elements in the Ag alloy.
The reflection layer is normally formed by sputtering and vacuum
deposition methods. Because the total amount of impurities in the
reflection layer, including water and oxygen trapped therein during
the film making, should preferably be 2 atomic % or less, it is
desired that the vacuum level in the process chamber used for
forming the layer be set to 1.times.10.sup.-3 Pa or less. To reduce
the amount of impurities trapped, the deposition rate is preferably
set to 1 nm/sec or higher, particularly 10 nm/sec or higher. The
amount of impurities trapped also depends on the method of
manufacture of an alloy target used in the sputtering and on the
sputter gas (rare gas such as Ar, Ne and Xe).
To enhance the heat dissipating effect of the reflection layer, the
material of the reflection layer preferably consists of only
aluminum and silver, as practically as possible.
The reflection layer may be formed in multiple layers to increase
the heat dissipating effect and the reliability of the medium.
For example, when the reflection layer is made mainly of silver
which has a large heat dissipating effect and a protective layer
containing sulfur is provided between the reflection layer and the
recording layer, the influences of silver and sulfur may pose
problems with the repetitive overwrite characteristic and with a
corrosion resistance under an accelerated test environment at high
temperature and humidity. To avoid these problems an interface
layer formed of an aluminum-based alloy can be provided between
these two layers so that a 2-layer reflection layer consisting of
an aluminum layer and a silver layer can be obtained. In that case,
the thickness of the interface layer is normally between
approximately 5 nm and 100 nm, preferably between 5 nm and 50 nm.
When the interface layer is too thin, the protective effects tends
to be insufficient; and when it is too thick, the heat dissipating
effect tends to deteriorate.
Forming the reflection layer in multiple layers is effective also
for obtaining a desired sheet resistivity at a desired thickness of
layer.
Now, the present invention will be explained in detail by taking
example embodiments. It should be noted that the invention is not
limited to these embodiments but can be applied to whatever
applications are within the spirit of the invention.
Embodiment 1
Over a polycarbonate substrate 1.2 mm thick formed with a tracking
groove (track pitch of 1.6 .mu.m, groove width of about 0.53 .mu.m,
and groove depth of about 37 nm), a (ZnS).sub.80(SiO.sub.2).sub.20
protective layer was deposited to a thickness of 70 nm, a
Ge.sub.5Sb.sub.77Te.sub.18 recording layer to 17 nm, a
(ZnS).sub.85(SiO.sub.2).sub.15 protective layer to 40 nm, and an
Al.sub.99.5Ta.sub.0.5 alloy to 220 nm by sputtering in the vacuum
chamber. An ultraviolet curing protective coat was applied over
this substrate to a thickness of 4 .mu.m and cured to manufacture a
phase change type rewritable optical disk.
This rewritable disk was subjected to the initial crystallization
process using a bulk eraser with a laser waveform of 810 nm and a
beam diameter of about 108 .mu.m.times.1.5 .mu.m at a power of 420
mW. Further in an evaluation apparatus having a laser wavelength of
780 nm and a pickup numerical aperture NA of 0.55, the grooves and
the lands were crystallized once with a DC light of 9.5 mW by
activating a servo to reduce noise of the crystal level.
Then, in the evaluation apparatus with a laser wavelength of 780 nm
and a pickup numerical aperture NA of 0.55, the grooves were
overwritten with an EFM modulation random pattern under the
conditions: linear velocity of 12 m/s (.times.10-speed of CD), base
clock frequency of 43.1 MHz, and reference clock period T of 23.1
nanoseconds. The EFM modulation scheme uses marks having time
lengths ranging from 3T to 11T. A pattern in which these marks with
different mark time lengths are randomly generated is an EFM
modulation random pattern.
These patterns were overwrite-recorded by using the above-described
pulse division scheme 3 (the division number is set to m=1, 2, 2,
3, 3, 4, 5, 5, 5 for n=3, 4, 5, 6, 7, 8, 9, 10, 11) with the
recording power Pw set to 18 mW, the erase power Pe to 9 mW and the
bias power Pb=retrieving power Pr to 0.8 mW. This pulse division
scheme was able to be realized by slightly changing the pulse
generating circuit of FIG. 1.
Retrieving was done at a speed of 2.4 m/s (.times.2-speed of CD)
and the retrieve signal was passed through a 2-kHz high frequency
pass filter and then DC-sliced and retrieve by taking the center of
the signal amplitude as a threshold value.
Before performing the overwrite, the pulse division scheme was
optimized in each of the mark time lengths ranging from 3T to 11T.
Specifically, the first recording pulse section .alpha..sub.1T and
the last off pulse section .beta..sub.mT were optimized.
An example case is shown in which an 11T mark (1.27 microseconds at
.times.2-speed) was divided into five parts and the recording pulse
widths and off pulse widths were determined.
Using the pulse division scheme shown in FIG. 6(a), the pulse
widths were recorded by changing only .alpha..sub.1. The
.alpha..sub.1-dependency of the retrieve mark time length at the
linear velocity of 2.4 m/s is shown in FIG. 7. For
.alpha..sub.1=1.0, the mark time length was 1.28 microseconds,
which was most preferable. The theoretical value is 1.27
microseconds.
Similarly, using the pulse division scheme shown in FIG. 6(b),
measurements were made of the .beta..sub.m (m=5) dependency. FIG. 8
shows the .beta..sub.m-dependency of the retrieve mark time length
at the linear velocity of 2.4 m is equivalent to two times the CD
linear velocity. For .beta..sub.m=.beta..sub.5=1.0, the mark time
length was 1.35 microseconds.
These experiments were conducted on the marks having respective
mark time lengths in order to optimize, in particular, the first
recording pulse .alpha..sub.1 and the last off pulse .beta..sub.5.
The pulse division scheme shown in FIG. 9 was determined. For the
long marks with 8T to 11T lengths, .alpha..sub.1=1.0 and
.beta..sub.m=1.0 were set.
After the optimization, the pulse division scheme of FIG. 9 was
used to overwrite the amorphous marks in the crystal area. The
measurements of the mark time lengths of the retrieve signals for
individual input signals of nT marks are shown in FIG. 10. The mark
length change was linear and the mark length deviation of the
retrieve marks was in a range that allows the 3T-11T marks to be
correctly distinguished and detected. The jitter value here was
low, well below the CD standard's jitter upper limit of 17.5
nanoseconds for the .times.2-speed reproduction, and the modulation
was 0.6 or higher. This indicates that the recording signal thus
obtained was satisfactory. In the figure, the mark length refers to
a mark time length and the space length refers to a space time
length.
Next, by using the pulse division scheme of FIG. 9, the EFM random
signal was overwritten. The random signal was generated using
AWG520 manufactured by Sony Techtronix. At this time the pulse
division was optimized for each mark length. As a result, even when
the random signals were generated, desired mark lengths and
satisfactory mark length jitter and space length jitter below 17.5
ns were obtained during the reproduction at .times.2-speed.
When the random pattern was recorded, it was verified by a
transmission electron microscope that the nT marks were not divided
into a plurality of amorphous portions but formed into a continuous
amorphous mark.
Embodiment 2
Over a polycarbonate substrate 1.2 mm thick formed with a tracking
groove (track pitch of 1.6 .mu.m, groove width of about 0.53 .mu.m,
and groove depth of about 37 nm), a (ZnS).sub.80(SiO.sub.2).sub.20
protective layer was deposited to a thickness of 70 nm, a
Ge.sub.7Sb.sub.79Te.sub.14 recording layer to 17 nm, a
(ZnS).sub.85(SiO.sub.2).sub.15 protective layer to 40 nm, and an
Al.sub.99.5Ta.sub.0.5 alloy to 220 nm by sputtering in the vacuum
chamber. An ultraviolet curing protective coat was applied over
this substrate to a thickness of 4 .mu.m and cured to manufacture
an optical disk.
This rewritable disk was subjected to the initial crystallization
process using a bulk eraser with a laser waveform of 810 nm and a
beam diameter of about 108 .mu.m.times.1.5 .mu.m at a power of 420
mW. Further in an evaluation apparatus having a laser waveform of
780 nm and a pickup numerical aperture NA of 0.55, the grooves and
the lands were crystallized once with a DC light of 9.5 mW by
activating a servo to reduce noise of the crystal level.
Then, in the evaluation apparatus with a laser waveform of 780 nm
and a pickup numerical aperture NA of 0.55, the grooves were
recorded with amorphous marks 11T in time length by using the pulse
division scheme shown in FIG. 6(c) under the conditions: linear
velocity of 19.2 m/s (.times.16-speed of CD), base clock frequency
of 69.1 MHz, and reference clock period T of 14.5 nanoseconds.
The overwrite-recording was performed using the recording power Pw
of 18 mW, the erase power Pe of 9 mW and the bias power
Pb=retrieving power Pr of 0.8 mW.
The retrieving was performed at 2.4 m/s (.times.2-speed of CD) and
the retrieved signal was passed through a 2-kHz high frequency pass
filter and then DC-sliced and retrieve by taking the center of the
signal amplitude as a threshold value.
The mark jitter was 13.1 nanoseconds and the space jitter 13.2
nanoseconds, well below the CD standard's jitter upper limit of
17.5 nanoseconds.
An EFM modulation random pattern was recorded and retrieved in a
manner similar to the embodiment 1. The result was
satisfactory.
Examples for Comparison 1
In the evaluation apparatus with a laser waveform of 780 nm and a
pickup numerical aperture NA of 0.55, the disk manufactured in the
embodiment 2 was recorded with amorphous marks 11T in time lengths
and spaces 11T in time length alternately by using the n-k division
scheme (m=n-k, n=1, the minimum of n/m is 1.1) of FIG. 11 currently
employed in the CD-RW, under the conditions: linear velocity of
19.2 m/s (.times.16-speed of CD), base clock frequency of 69.1 MHz,
and reference clock period T of 14.5 nanoseconds.
The overwrite-recording was performed using the recording power Pw
of 18 mW, the erase power Pe of 9 mW and the bias power
Pb=retrieving power Pr of 0.8 mW.
When the signal was retrieved at the linear velocity of 2.4 m/s,
the reflectance corresponding to a central portion of the mark of
the retrieved signal did not fall sufficiently. Examination of the
mark found that the central portion of the mark was significantly
recrystallized. The jitter exceeded the 17.5-nanosecond limit
substantially and was too high to be measured. To prevent
recrystallization, the recording pulse widths were narrowed while
still in the n-1 division scheme but the modulation of the
recording laser beam could not follow the narrowed pulses,
resulting in an increased recording power Pw and showing no
improvements in the cooling effect.
Embodiment 3
Over a polycarbonate substrate 1.2 mm thick formed with a tracking
groove, which has a track pitch of 1.6 .mu.m, a groove width of
about 0.53 .mu.m and a groove depth of about 37 nm, a
(ZnS).sub.80(SiO.sub.2).sub.20 protective layer was deposited to a
thickness of 70 nm, a Ge.sub.7Sb.sub.78Te.sub.15 recording layer to
17 nm, a (ZnS).sub.80(SiO.sub.2).sub.20 protective layer to 45 nm,
and an Al.sub.99.5Ta.sub.0.5 alloy reflection layer to 220 nm
(volume resistivity of about 100 n.OMEGA.m and sheet resistivity of
0.45 .OMEGA./.quadrature.) by sputtering in the vacuum chamber. An
ultraviolet curing resin protective coat was applied over this
substrate to a thickness of 4 .mu.m. A guide groove for tracking
was given groove meanders 30 nm in amplitude (peak-to-peak) which
were formed by frequency-modulating a 22.05-kHz carrier wave by
.+-.1 kHz. That is, address information was provided in the form of
so-called ATIP along the spiral groove.
As in the embodiment 1 and 2, the disk was arranged so that a major
axis of a focused laser beam was oriented in the direction of the
disk radius, the laser beam having a wavelength of about 810 nm and
an elliptical shape about 108 .mu.m in major axis by about 1.5
.mu.m in minor axis. The disk was scanned at a linear velocity of
3-6 m/s and irradiated with a power of 400-600 mW for
initialization. Further, in the evaluation apparatus with a laser
wavelength of 780 nm and a pickup numerical aperture NA of 0.55, a
servo was activated to crystallize the grooves and the lands once
with 9.5 mW of DC light to reduce the noise of the crystallization
level.
For the retrieve/write evaluation, a Pulsetech DDU1000 (wavelength
of 780 nm, NA=0.55) was used to write into and retrieve from the
grooves. The retrieving was performed at .times.2-speed
irrespective of the linear velocity used for recording. The jitter
tolerance value for the CD format in this case is 17.5 nanoseconds.
As a signal source for generating gate signals, an arbitrary
waveform signal source AWG520 of Sony Techtronix made was used.
First, the recording was made at a linear velocity 16 times the CD
linear velocity (19.2 m/s) and the reference clock period T was
14.5 nanoseconds.
(1) First, the optimum condition for the intermediate pulse group
was studied by using the divided recording pulses of FIG. 14. The
recording power Pw.sub.i was set constant at 20 mW, the bias power
Pb.sub.i was also set constant at 0.8 mW and the erase power Pe for
spaces was set to 10 mW.
As shown in FIG. 14(a), in the divided recording pulses having
constant .alpha..sub.i=1, .beta..sub.i was set to .beta.c (constant
value) and then changed to examine the dependency of the amorphous
mark formation on the off pulse section length.
When the off pulse section was shorter than about 1T, the signal
amplitude at the front end of the mark was low due to the
recrystallization at the mark front end as shown in FIG. 3(d). At
the rear end, too, the amplitude was somewhat low. The maximum
amplitude in the entire mark length divided by the erase level
signal intensity (.times.100%) was defined as a modulation, and the
dependency of the modulation on the off pulse section is shown in
FIG. 15(a). It is seen that when the off pulse section was short,
the modulation deteriorated due to the influence of the waveform
distortion (bad formation of the amorphous mark). When the off
pulse section exceeded 1T, the modulation became saturated,
producing a waveform close to a rectangular wave without
distortion.
Next, using the divided recording pulses as shown in FIG. 14(b)
with the off pulse section set to 1.5T, the dependency of the
modulation on the recording pulse section was examined. In FIG.
14(b), .alpha..sub.i was set to .alpha.c (a constant value) and
changed uniformly. FIG. 15(b) shows the .alpha.c-dependency of
modulation. It is seen that a nearly saturated modulation was
obtained for .alpha.c=1 to 1.5.
(2) Next, the divided recording pulses of FIG. 16 with the
intermediate pulse group fixed to .alpha..sub.i=1 and
.beta..sub.i=1.5 were used and the control of the mark length and
the characteristic of the mark end was examined by controlling the
first period and the last period. In FIG. 16, one 0.5T recording
pulse section was added at the rear end of the mark to make the
mark length close to 11T accurately. This made both of the mark
length and the space length assume 11T and the condition for
obtaining a satisfactory jitter was searched. The original waveform
was a repetitive pattern of the 11T mark and the 11T space, with
the first recording pulse rising in synchronism with the front end
of the 11T mark. Here, because the .times.2-speed retrieving was
performed, the upper limit of the jitter allowable value was 17.5
nanoseconds (ns) and the 11T was equivalent to about 1.27
microseconds (.mu.s). FIGS. 17, 18 and 19 show these values with
dotted lines.
Using the divided recording pulses as shown in FIG. 16(a), the
dependency on the front recording pulse .alpha..sub.1 length was
checked. FIGS. 17(a) and 17(b) represent the
.alpha..sub.1-dependency of the mark length and space length and
the .alpha..sub.1-dependency of the mark jitter and the space
jitter, respectively. It is seen from FIG. 17(b) that .alpha..sub.1
is preferably set to 0.8-1.8 to keep the jitter below 17.5
nanoseconds.
In FIG. 17(b) the desired 11T was not obtained for the mark length
and space length. So, .alpha..sub.1 was set to .alpha..sub.1=1, and
the divided recording pulses as shown in FIG. 16(b) were used to
examine the dependency on the first off pulse .beta..sub.1T length.
FIGS. 18(a) and 18(b) represent the .beta..sub.1-dependency of the
mark length and the space length and the .beta..sub.1-dependency of
the mark jitter and the space jitter, respectively. It is seen that
almost the desired mark length and space length were obtained for
.beta..sub.1=1.3 and that satisfactory jitters were obtained for a
.beta..sub.1 range of between 1 and 1.7. Here, .beta..sub.1=1.5 was
chosen.
Further, using the divided recording pulses as shown in FIG. 16(c)
and setting .alpha..sub.1=1 and .beta..sub.1=1.5, the dependency on
the last off pulse .beta..sub.m length was studied. FIGS. 19(a) and
19(b) show the .beta..sub.m-dependency of the mark length and the
space length and the .beta..sub.m-dependency of the mark jitter and
the space jitter, respectively. The figures show that the desired
mark length and space length were obtained for .beta..sub.m=around
0.7 and that satisfactory jitters were obtained in a wide range of
.beta..sub.m=0 to 1.8.
These show that setting .alpha..sub.1=1, .beta..sub.1=1.5 and
.beta..sub.m=0.8 results in the desired 11T mark length and minimum
jitters.
(3) With the results of the above (1) and (2) taken into account, a
pulse dividing method based on the (divided recording pulse
generating method 2) described above and using a base period of 2T
was performed, in a range of .alpha..sub.1=1.+-.0.5 and
.beta..sub.1=1.+-.0.5, on the EFM modulation signal which consists
of 3T to 11T mark lengths. The specific pulse dividing method for
each mark length is shown in FIG. 20.
That is, for the mark recording in which n is even, i.e., the mark
length is nT=2LT, where L is an integer equal to or more than 2,
the mark is divided into m=L sections and the recording pulse
section .alpha..sub.i where the recording power Pw.sub.iis to be
radiated and the off pulse section .beta..sub.i where the bias
power Pb.sub.i is to be radiated are set as follows:
.alpha..sub.1+.beta..sub.1=2
.alpha..sub.i+.beta..sub.i=2(2.ltoreq.i.ltoreq.m-1)
.alpha..sub.m+.beta..sub.m=1.6
For the mark recording in which n is odd, i.e., the mark length is
nT=(2L+1)T, the mark is divided into m=L sections and each pulse
sections is set as follows: .alpha..sub.1'+.beta..sub.1'=2.5
.alpha..sub.i'+.beta..sub.i'=2(2.ltoreq.i.ltoreq.m-1)
.alpha..sub.m'+.beta..sub.m'=2.1 Although the division number is
the same m=L for the 2LT mark and the (2L+1)T mark, the first
period and the last period are differentiated between these marks
by giving them a 0.5T difference.
In FIG. 20 the delay of .alpha..sub.1T from the front end of the nT
mark is set to T.sub.d1=0. For n.gtoreq.4, the intermediate pulse
group is held constant at .alpha..sub.i=0.8 and .beta..sub.i=1.2
(2.ltoreq.i.ltoreq.m-1) irrespective of the n value.
Further, when n is even, the following settings are made:
.alpha..sub.1=0.8, .beta..sub.1=1.2, .alpha..sub.m=0.7 and
.beta..sub.m=0.9. When n is odd, the following settings are made:
.alpha..sub.1'=1.0, .beta..sub.1'=1.5, .alpha..sub.m'=1.0 and
.beta..sub.m'=1.1. Only the 3T case was exceptional. A 3T mark
length was obtained for .alpha..sub.1=1.2 and .beta..sub.1=1.5. In
FIG. 20, the recording pulse section and the off pulse section are
represented by the top and bottom portions of the rectangular wave.
Individual lengths of sections are indicated by numbers, and the
depicted lengths of the top and bottom portions in the figure are
not scaled to the exact lengths of the sections.
The recording power Pw.sub.i and the bias power Pb.sub.i were set
constant irrespective of the i value, i.e., Pw=20 mW and Pb=0.8 mW.
the erase power Pe was set to 10 mW.
After 9 overwrites were performed (the initial recording was deemed
a 0-th recording), measurements were made of the mark length and
space length and also jitters for each nT mark and nT space. The
measurements of mark lengths and space lengths are shown in FIG.
21(a) and the measurements of jitters of the marks and spaces are
shown in FIG. 21(b). The mark lengths and space lengths were almost
precisely nT and the jitters were below 17.5 nanoseconds although
the jitters degraded 2-3 nanoseconds from the initial recording due
to overwriting. Instead of performing overwrite, the erase power Pe
was radiated DC-wise for erase operation. This resulted in a jitter
improvement of about 2 nanoseconds. (4) An overwrite was performed
on the same medium at .times.10-speed of CD by changing the clock
period so that the product of the linear velocity v and the clock
period T was constant. That is, the reference clock period T in
this case was 23.1 nanoseconds. For n.gtoreq.4, .alpha..sub.iT
(1.ltoreq.i.ltoreq.m) was held almost constant. That is, the
intermediate recording pulse group was held constant at
.alpha..sub.i=0.5 and .beta..sub.i=1.5 (2.ltoreq.i.ltoreq.m-1).
The divided pulses are as shown in FIG. 22. When n was even, pulses
were set to .alpha..sub.1=0.6, .beta..sub.1=1.4, .alpha..sub.m=0.5
and .beta.m=1.4. When n was odd, pulses were set to
.alpha..sub.i'=0.6, .beta..sub.i'=1.9, .alpha..sub.m'=0.6 and
.beta..sub.m'=1.8. Only the 3T case was exceptional. A 3T mark
length was obtained for .alpha..sub.1=0.8 and .beta..sub.1=2.4.
This divided recording pulses correspond, except for n=3, roughly
to multiplying the clock period by 16/10 (inversely proportional to
the linear velocity) while holding the recording pulse length
obtained in FIG. 20 constant. The recording power Pw.sub.i and the
bias power Pb.sub.i were held constant at Pw=20 mW and Pb=0.8 mW
irrespective of the i value as in the case with the
.times.16-speed. The erase power Pe was also set to 10 mW as in the
case with the .times.16-speed.
After 9 overwrites were performed (the initial recording was deemed
a 0-th recording), measurements were made of the mark length and
space length and also jitters for each nT mark and nT space. The
measurements of mark lengths and space lengths are shown in FIG.
23(a) and the measurements of jitters of the marks and spaces are
shown in FIG. 23 (b). The mark lengths and space lengths were
almost precisely nT and the jitters were below 17.5 nanoseconds
although the jitters degraded 2-3 nanoseconds from the initial
recording due to overwriting.
Instead of performing overwrite, the erase power Pe was radiated
DC-wise for erase operation. This resulted in a jitter improvement
of about 2 nanoseconds.
(5) An overwrite was performed on the same medium by using a
repetitive pattern (11T pattern) consisting of 11T mark with
divided recording pulses and 11T spaces, and a repetitive pattern
(3T pattern) consisting of 3T mark with divided recording pulses
and 3T spaces. After overwriting the 3T pattern nine times, the 11T
pattern was overwritten at the 10th time and a rate of reduction in
the carrier level of the 3T signal (in unit of dB) was measured as
an erase ratio (overwrite erase ratio). Although the 3T pattern was
slightly deviated among different linear velocities, both the 3T
and 11T patterns were basically changed according to the division
method of FIG. 20 so that .alpha..sub.iT (1.ltoreq.i.ltoreq.m)
remained almost constant.
The erase ratio was evaluated by changing the linear velocity while
keeping the product of the linear velocity and the reference clock
period constant. The overwrite erase ratio of 20 dB or more was
obtained for the 10, 12, 16 and 18 times the CD linear
velocity.
When a random pattern was recorded, it was verified with a
transmission electron microscope that the nT marks were not divided
into a plurality of amorphous portions but formed into a continuous
amorphous mark.
The recording layer similar to that used above was peeled off after
being initialized and its crystallinity was observed with a
transmission electron microscope. The observation found that the
recording layer was a polycrystal formed of a single phase of
hexagonal crystal. The crystal phase was found to have no phase
separation and is assumed to have a single phase polycrystalline
structure with the orientations rotated. An examination using an
X-ray diffraction found that it had a hexagonal structure.
Embodiment 4
Over a polycarbonate substrate 0.6 mm thick formed with a tracking
groove, which has a track pitch of 0.74 .mu.m, a groove width of
about 0.27 .mu.m and a groove depth of about 30 nm, a
(ZnS).sub.80(SiO.sub.2).sub.20 protective layer was deposited to a
thickness of 68 nm, a Ge.sub.5Sb.sub.77Te.sub.18 recording layer to
14 nm, a (ZnS).sub.80(SiO.sub.2).sub.20 protective layer to 25 nm,
and an Al.sub.99.5Ta.sub.0.5 alloy reflection layer to 200 nm
(volume resistivity of about 100 n.OMEGA.m and sheet resistivity of
0.5 .OMEGA./.quadrature.) by sputtering in the vacuum chamber. An
ultraviolet curing resin layer was applied over this substrate to a
thickness of 4 .mu.m by a spin coat. This is bonded with another
substrate 0.6 mm thick having the same structure of layers to form
a phase change disk.
As in the embodiment 3, the disk thus obtained was arranged so that
a major axis of a focused laser beam was oriented in the direction
of the disk radius, the laser beam having a wavelength of about 810
nm and an elliptical shape about 108 .mu.m in major axis by about
1.5 .mu.m in minor axis. The disk was scanned at a linear velocity
of 3-6 m/s and irradiated with a power of 400-600 mW for
initialization. Further, in the evaluation apparatus with a laser
wavelength of 660 nm and a pickup numerical aperture NA of 0.65,
tracking and focus servos were activated to scan about 6 mW of DC
light over the grooves once at 4 m/s to reduce the noise of the
crystallization level.
For the retrieve/write evaluation, a Pulsetec DDU1000 (wavelength
of about 660 nm, NA=0.55) was used to write into and retrieve from
the grooves. As a signal source for generating gate signals, an
arbitrary waveform signal source AWG610 manufactured by Sony
Techtronix was used. In this case, the length of a 3T mark was 0.4
.mu.m and the clock period at each linear velocity was so set that
the recording density would be the same as that of DVD (26.16 MHz
at 3.5 m/s).
First, the linear velocity during the recording was set to 16.8 m/s
(clock frequency of 125.93 MHz and clock period of 7.9 nsec)
equivalent to the .times.4.8-speed of DVD; a 14T section was
divided by using simple waveforms as shown in FIG. 25; and the
intermediate divided recording pulses were examined. The space was
set to 14T. The recording power was set to a constant value of
Pw=15 mW, the erase power to Pe=5 mW, and the bias power to Pb=0.5
mW. The recording power application section was denoted Tw and the
bias power application section Tb. Two cases were studied: in the
first case Tw+Tb=1T was set and Pw and Pb were applied for 14T
periods (FIG. 25(a)); and in the second case Tw+Tb=2T was set and
Pw and Pb were applied for 7T periods (FIG. 25(b)). In each of
these two cases, the dependency of the modulation of the recording
mark portion of the retrieved signal on a ratio of Tw to T (Tw/T)
was evaluated. When the Tw/T for 2T periods was 1.0, the signal
obtained was a square wave almost free of distortion and the
modulation was maximum. When the Tw/T ratio was less than 0.5, the
waveform was distorted. This is considered due to the insufficient
recording power application section and therefore an insufficient
temperature rise. Conversely, when the Tw/T is more than 1.0, as Tw
increases, the modulation decreases. This is considered due to the
insufficient cooling time, which prevents the transformation to the
amorphous state by recrystallization. When Tw/T exceeds 1.5, the
modulation falls below 5%, resulting in a distorted waveform (not
shown). For 1T period, the modulation was low over the entire range
and only the distorted waveforms were produced. This is because in
the 1T period there might not be a range where the recording power
application time and the cooling time were both sufficient.
It can be seen from the foregoing discussion that, in the divided
recording pulse generating method 2 or 3, the intermediate divided
recording pulse group for at least 2.ltoreq.i.ltoreq.m-1 is
preferably set to .alpha..sub.i=.alpha..sub.i'=1 and
.beta..sub.i=.beta..sub.i'=1.
Next, it was verified as follows that the disk discussed above was
capable of high speed erasure at high linear velocities of 14 m/s
and 17.5 m/s (equivalent to 4 and 5 times the DVD linear velocity
of 3.5 m/s). That is, the overwrite was performed by using a
repetitive pattern (8T pattern) consisting of 8T mark with divided
recording pulses and 8T spaces, and a repetitive pattern (3T
pattern) consisting of 3T mark with divided recording pulses and 3T
spaces. After overwriting the 3T pattern 9 times, the 8T, pattern
was overwritten at the 10th time and a rate of reduction in the
carrier level of the 3T signal was determined as an overwrite erase
ratio. The overwrite erase ratio was determined by keeping the
product of the linear velocity and the reference clock period
constant so that the same recording density as the DVD was
obtained. The overwrite erase ratio of 25 dB or more was obtained
for 14 m/s and 17.5 m/s.
Further, a pulse dividing method based on the divided recording
pulse generating method 3 described above and using a base period
of 2T, was performed on a EFM+ modulation signal consisting of
3T-11T and 14T, marks. This EFM+ modulation signal was recorded at
14 m/s and 16.8 m/s (3 and 4.8 times the DVD linear velocity of 3.5
m/s). For the .times.4-speed, the clock frequency was 104.9 MHz and
the clock period was 9.5 nsec. For the .times.4.8-speed, the clock
frequency was 125.9 MHz and the clock period was 7.9 nsec. The
specific pulse dividing method is as shown in FIG. 26.
For the mark recording in which n is even, i.e., the mark length is
nT=2LT (L is an integer equal to or more than 2), the mark is
divided into m=L sections and .alpha..sub.i and .beta..sub.i in the
recording pulse section .alpha..sub.iT and the off pulse section
.beta..sub.iT are set as follows:
T.sub.d1+.alpha..sub.1=2(T.sub.d1=0.95)
.beta..sub.i-1+.alpha..sub.i=2(2.ltoreq.i.ltoreq.m-1)
For the mark recording in which n is odd, i.e., the mark length is
nT=(2L+1)T, the mark is divided into m=L sections and .alpha..sub.i
and .beta..sub.i in the recording pulse section .alpha..sub.iT and
the off pulse section .beta..sub.iT are set as follows:
T.sub.d1'+.alpha..sub.1'=2.05(T.sub.d1'=1)
.beta..sub.1'+.alpha..sub.2'=2.45
.beta..sub.i-1'+.alpha..sub.i'=2(3.ltoreq.i.ltoreq.m-1)
.beta..sub.m-1'+.alpha..sub.m'=2.45 In this case, for L=2,
.beta..sub.1'+.alpha..sub.2'=2.9 and .alpha..sub.m=1 and
.alpha..sub.m'=.alpha..sub.m+0.2=1.2.
In the case of L.gtoreq.3, the intermediate recording pulse group
was set to constant values: .alpha..sub.i'=.alpha..sub.i=1 and
.beta..sub.i'=.beta..sub.i=1 (2.ltoreq.i.ltoreq.m-1), and
.alpha..sub.m=.alpha..sub.m'=1. For L.gtoreq.2, they were set to
constant values, not dependent on the n value:
.alpha..sub.1=.alpha..sub.1'=1.05 and
.beta..sub.m=.beta..sub.m'=0.4.
Further, in the case of 3T, a 3T mark length was obtained with
T.sub.d1=1.15, .alpha..sub.1=1.2 and .beta..sub.1=0.8. In FIG. 26,
the recording pulse section and the off pulse section are
represented by the top and bottom portions of the rectangular wave.
Specific lengths of sections are indicated by numbers, and the
depicted lengths of the top and bottom portions in the figure do
not correspond to the lengths of the sections.
The bias power Pb.sub.i was set to a fixed value Pb=0.5 mW, not
dependent on the i value, and the erase power Pe was set to 4.5 mW.
The recording power Pw.sub.i was also set to a fixed value
irrespective of the i value. After overwriting 9 times, the
edge-to-clock jitter and the dependency of the modulation on the
recording power were measured. Retrieving was performed using the
reproducing light power of Pr=0.8 mW and the linear velocity of 3.5
m/s. At either recording linear velocity and with the recording
power of 15.0 mW, the edge-to-clock jitter was less than 10% and
the modulation achieved 60% or higher, as shown in FIGS. 27(a) and
27(b). R.sub.top was about 18%. Measurement of the overwrite
dependency at the recording power of 15.0 mW found that, as shown
in FIG. 27(c), the edge-to-clock jitter was 11% or less even after
10,000 overwrite operations. At this time R.sub.top and the
modulation exhibited almost no change with the overwrite.
Further, a pulse dividing method of FIG. 28 based on the divided
recording pulse generating method 3 described above was performed
on the similar disk by recording an EFM+ modulation signal at a
linear velocity of 7 m/s, equivalent to two times the DVD linear
velocity, and a clock frequency of 52.5 MHz (clock period of 19.1
nsec).
As in the case with 4 and 4.8 times the DVD speed, the bias power
was set constant at Pb=0.5 mW and the erase power Pe at 4.5 mW. The
recording power Pw.sub.i was also set constant, not dependent on
the i value. After nine overwrite operations, the edge-to-clock
jitter and the recording power dependency of the modulation were
measured. As shown in FIGS. 27(a) and 27(b), at the recording power
of 13.0 mW, the edge-to-clock jitter was less than 8% and the
modulation achieved 57% or higher. R.sub.top was about 18%. At the
recording power of 13.0 mW, the overwrite dependency was measured
and it was found that, as shown in FIG. 27(c), the edge-to-clock
jitter was below 11% even after 10,000 overwrite operations. At
this time R.sub.top and the modulation exhibited almost no change
with the overwrite.
From the above discussion, it is understood that the use of the
pulse dividing method based on the divided recording pulse
generation method 3 enables recording in a linear velocity range of
2 to 4.8 times the DVD linear velocity. Hence, with this method the
recording with a constant angular velocity can be performed in a
radial range, for example, from about 24 mm to about 58 mm, which
constitutes a data area of DVD.
INDUSTRIAL APPLICABILITY
According to this invention, even when the reference clock period
is short, a satisfactory mark length modulation recording can be
performed, allowing a higher density and a faster recording of the
optical recording media. This in turn leads to an increase in the
recordable capacity of the optical disk and enables the recording
speed and transfer rate of the optical disk to be enhanced, greatly
expanding the range of its applications for recording large amounts
of data such as music and video and for external storage devices of
computers. For instance, it is possible to realize a rewritable CD
that overwrites EFM modulation marks at speeds more than 12 times
the CD linear velocity and a rewritable DVD that overwrites EFM+
modulation marks at speeds more than 4 times the DVD linear
velocity.
* * * * *