U.S. patent application number 09/884121 was filed with the patent office on 2001-12-20 for optical recording method and optical recording medium.
This patent application is currently assigned to MITSUBISHI CHEMICAL CORPORATION. Invention is credited to Horie, Michikazu, Nobukuni, Natsuko.
Application Number | 20010053115 09/884121 |
Document ID | / |
Family ID | 26471209 |
Filed Date | 2001-12-20 |
United States Patent
Application |
20010053115 |
Kind Code |
A1 |
Nobukuni, Natsuko ; et
al. |
December 20, 2001 |
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.nT,.alpha..sub.1T,
.beta..sub.1T, .alpha..sub.2T, .beta..sub.2T, . . . ,
.alpha..sub.1T, .beta..sub.1T, . . . , .alpha..sub.mT,
.beta..sub.mT, .eta..sub.2T in that order (m is a pulse division
number; .SIGMA..sub.i(.alpha..sub.1+.-
beta..sub.1)+.eta..sub.1+.eta..sub.2=n; .alpha..sub.1
(1.ltoreq.i.ltoreq.m) is a real number >0; .beta..sub.1
(1.ltoreq.i.ltoreq.m-1) is a real number >0; .beta..sub.m is a
real number .ltoreq.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.1 in a time duration of .alpha..sub.1T
(1.ltoreq.i.ltoreq.m), and radiating recording light with a bias
power Pb.sub.1 in a time duration of .beta..sub.iT
(1.ltoreq.i.ltoreq.m), the bias power being Pb.sub.1<Pw.sub.1
and Pb.sub.1<Pw.sub.1+1; and (ii) changing m, .alpha..sub.1,
.beta..sub.1, .eta..sub.1, .eta..sub.2, Pw.sub.1 and Pb.sub.1
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) |
Correspondence
Address: |
OBLON SPIVAK MCCLELLAND MAIER & NEUSTADT PC
FOURTH FLOOR
1755 JEFFERSON DAVIS HIGHWAY
ARLINGTON
VA
22202
US
|
Assignee: |
MITSUBISHI CHEMICAL
CORPORATION
Tokyo
JP
|
Family ID: |
26471209 |
Appl. No.: |
09/884121 |
Filed: |
June 20, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09884121 |
Jun 20, 2001 |
|
|
|
PCT/JP00/03036 |
May 11, 2000 |
|
|
|
Current U.S.
Class: |
369/59.12 ;
G9B/11.022; G9B/7.014; G9B/7.016; G9B/7.028; G9B/7.099 |
Current CPC
Class: |
G11B 2007/2431 20130101;
G11B 11/1053 20130101; G11B 7/243 20130101; G11B 7/126 20130101;
G11B 7/00456 20130101; G11B 7/2534 20130101; G11B 2007/24308
20130101; G11B 2007/24316 20130101; G11B 7/2585 20130101; G11B
2007/24312 20130101; G11B 7/0062 20130101; G11B 11/10595 20130101;
G11B 2007/24314 20130101; G11B 2007/2432 20130101; G11B 2007/24304
20130101; G11B 2007/24306 20130101; G11B 11/10528 20130101; G11B
11/10506 20130101 |
Class at
Publication: |
369/59.12 |
International
Class: |
G11B 007/125 |
Foreign Application Data
Date |
Code |
Application Number |
May 19, 1999 |
JP |
11-138067 |
Mar 17, 2000 |
JP |
2000-076514 |
Claims
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.1T, .beta..sub.1T, . . . ,
.alpha..sub.mT, .beta..sub.mT, .eta..sub.2T in that order (m is a
pulse division number;
.SIGMA..sub.1(.alpha..sub.1+.beta..sub.1)+.eta..su-
b.1+.eta..sub.2=n; .alpha..sub.1 (1.ltoreq.i.ltoreq.m) is a real
number larger than 0; .beta..sub.1 (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); radiating recording light with a
recording power Pw.sub.1 in a time duration of .alpha..sub.1T
(1.ltoreq.i.ltoreq.m); and radiating recording light with a bias
power Pb.sub.1 in a time duration of .beta..sub.1T
(1.ltoreq.i.ltoreq.m-1), the bias power being Pb.sub.1<Pw.sub.1
and Pb.sub.1<Pw.sub.1+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
2. An optical recording method according to claim 1, wherein
.beta..sub.1 (1.ltoreq.i.ltoreq.m-1) is 0.5 to 2.5.
3. An optical recording method according to claim 1, wherein for
the time length of all the recording marks, an average of
.alpha..sub.1T (1.ltoreq.i.ltoreq.m) is 3 nanoseconds or more and
an average of .beta..sub.1T (1.ltoreq.i.ltoreq.m-1) is 3
nanoseconds or more.
4. An optical recording method according to claim 1, wherein for
the time length of all the recording marks, .alpha..sub.1T.gtoreq.3
nanoseconds (1.ltoreq.i.ltoreq.m) and .beta..sub.1T.gtoreq.3
nanoseconds (1.ltoreq.i.ltoreq.m-1) for each i.
5. An optical recording method according to claim 1, wherein for
the time length of all the recording marks, n/m.gtoreq.1.5 is
met.
6. An optical recording method according to claim 1 , wherein
.alpha..sub.1+.beta..sub.1 (2.ltoreq.i.ltoreq.m-1) or
.beta..sub.1-1+.alpha..sub.1 (2.ltoreq.i.ltoreq.m-1) takes a value
of either 1.5, 2 or 2.5.
7. An optical recording method according to claim 1, wherein for at
least two recording marks with different n's, the same pulse
division number m is used and, at least one of .alpha..sub.1
(1.ltoreq.i.ltoreq.m), .beta..sub.1 (1.ltoreq.i.ltoreq.m),
.eta..sub.1, .eta..sub.2, Pw.sub.1 (1.ltoreq.i.ltoreq.m) and
Pb.sub.1 (1.ltoreq.i.ltoreq.m) is different from any one of said at
least two recording marks.
8. An optical recording method according to claim 7, wherein when
the mark length is expressed as nT=2LT (L is an integer equal to or
larger than 2), the mark is divided into a division number m=L of
sections and .alpha..sub.1 and .beta..sub.1 in recording pulse
sections .alpha..sub.1T and off pulse sections .beta..sub.1T (these
can change according to a value of L) are defined as follows:
.alpha..sub.1+.beta..sub.1=2+.delta..- sub.1
.alpha..sub.1+.beta..sub.1=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 satisfying
-0.5.ltoreq..delta..sub.1.ltore- q.0.5 and
-1.ltoreq..delta..sub.2.ltoreq.1 respectively; and when L=2, only
.alpha..sub.1, .beta..sub.1, .alpha..sub.m and .beta..sub.m exist);
when the mark length is expressed as nT=(2L+1)T, the mark is
divided into a division number m=L of sections and .alpha..sub.1'
and .beta..sub.1' in recording pulse sections .alpha..sub.1'T and
off pulse sections .beta..sub.1'T (these can change according to a
value of L) are defined as follows:
.alpha..sub.1'+.beta..sub.1'=2.5+.delta..sub.1'.alpha..sub.1'-
+.beta..sub.1'=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 satisfying
-0.5.ltoreq..delta..sub.1'.ltoreq.0.5 and
-1.ltoreq..delta..sub.2'.ltoreq.1 respectively; and when L=2, only
.alpha..sub.1', .beta..sub.1', .alpha..sub.m' and .beta..sub.m'
exist); and .alpha..sub.1, .beta..sub.1, .alpha..sub.m,
.beta..sub.m, .alpha..sub.1', .beta..sub.1', .alpha..sub.m' and
.beta..sub.m' satisfy the following equation
.alpha..sub.1+.beta..sub.1+.alpha..sub.m+.beta..su-
b.m+.DELTA.=.alpha..sub.1'+.beta..sub.1'+.alpha..sub.m'+.beta..sub.m'(wher-
e .DELTA.=0.8 to 1.2).
9. An optical recording method according to claim 8, wherein
.alpha..sub.1, .beta..sub.1, .alpha..sub.1' and .beta..sub.1'
satisfy the following equation:
.alpha..sub.1+.beta..sub.1+.DELTA..sub.1
=.alpha..sub.1'+.beta..sub.1'(where .DELTA..sub.1=0.4 to 0.6).
10. An optical recording method according to claim 7, wherein when
the mark length is expressed as nT=2LT (L is an integer equal to or
larger than 2), the mark is divided into a division number m=L of
sections and .alpha..sub.1 and .beta..sub.1 in recording pulse
sections .alpha..sub.1T and off pulse sections .beta..sub.1T (these
can change according to a value of L) are defined as follows:
T.sub.d1+.alpha..sub.1=2+.epsilon..su- b.1
.beta..sub.1-1+.alpha..sub.1=2 (2.ltoreq.i.ltoreq.m) when the mark
length is expressed as nT=(2L+1)T, the mark is divided into a
division number m=L of sections and .alpha..sub.1' and
.beta..sub.1' in recording pulse sections .alpha..sub.1'T and off
pulse sections .beta..sub.1'T (these can change according to a
value of L) 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.1-1'+.alpha..sub.1'=2
(3.ltoreq.i.ltoreq.m-1)
.beta..sub.m-1'+.alpha..sub.m'=2.5+.epsilon..sub.- 3'(where when
L=2, .beta..sub.1'+.alpha..sub.2'=2.5+.epsilon..sub.2' or
.beta..sub.1'+.alpha..sub.2'=3+.epsilon..sub.2'; T.sub.d1 and
T.sub.d1' are almost constant real numbers between -2 and 2, not
dependent on L; and .epsilon..sub.1, .epsilon..sub.1',
.epsilon..sub.2' and .epsilon..sub.3' are real numbers between -1
and 1); and .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=.beta..sub.1'+.alpha..sub.2'+.alpha..sub.m-1'+.alpha..sub.m'(where
.DELTA..sub.2=0.8 to 1.2).
11. An optical recording method according to claim 10, wherein for
L equal to or more than 3, .beta..sub.1'=.beta..sub.1+approximately
0.5, .beta..sub.m-1'=.beta..sub.m-1+approximately 0.5,
.alpha..sub.1=0.8 .alpha..sub.1' to 1.2 .alpha..sub.1',
.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'.
12. An optical recording method according to claim 8, wherein in
recording a mark with the mark length of nT=2T or 3T, the mark is
divided into a division number m=1 of sections.
13. An optical recording method according to claim 8 , wherein when
L is larger than 3, .alpha..sub.1 is held constant at
.alpha..sub.1=.alpha.c and .alpha.i' is held constant at
.alpha..sub.1'=.alpha.c' for 2.ltoreq.i.ltoreq.m-1.
14. An optical recording method according to claim 13, wherein when
L is larger than 3, .alpha.c and .alpha.c' are constant, not
dependent on L.
15. An optical recording method according to claim 13, wherein when
L is larger than 3, .alpha.c=.alpha.c'.
16. An optical recording method according to claim 8, wherein when
L is larger than 3, each of T.sub.d1, T.sub.d1', .alpha..sub.1,
.alpha..sub.1', .beta..sub.1, .beta..sub.1' takes a constant
value.
17. An optical recording method according to claim 8, wherein when
L is larger than 3, each of .alpha..sub.m, .alpha..sub.m',
.beta..sub.m and .beta..sub.m' takes a constant value.
18. An optical recording method according to claim 8 , wherein by
using a first reference clock 1 with a period of T and a second
reference clock 2 with a period of T, which is shifted 0.5T from
the first reference clock, .alpha..sub.1 (1.ltoreq.i.ltoreq.m) is
generated in synchronism with a reference clock 3 with a period of
2T that is produced by dividing the reference clock 1, and
.alpha..sub.1' (2.ltoreq.i.ltoreq.m-1) is generated in synchronism
with a reference clock 4 with a period of 2T that is produced by
dividing the reference clock 2.
19. An optical recording method according to claim 8, wherein for
all L, a delay time T.sub.d1 with respect to a front end of a mark
length to be recorded is provided at rising edges of recording
pulses .alpha..sub.1T and .alpha..sub.1'T; a reference time
T.sub.sync corresponding to a clock mark formed at a predetermined
position on a recording track is generated; a modulation signal
corresponding to each mark length and space is generated by taking
the reference time T.sub.sync a start point; four reference clocks
are generated, the four reference clocks being a reference clock 1a
with a period of 2T which is generated with the delay time T.sub.d1
from the reference time T.sub.sync taken as a start point, a
reference clock 2a with a period of 2T which leads the reference
clock 1a by 0.5T, a reference clock 1b with a period of 2T which
leads the reference clock 1a by 1T, and a reference clock 2b with a
period of 2T which leads the reference clock 1a by 1.5T; when
recording a mark of nT=2LT, gate groups G1a and G1b corresponding
to timings of .alpha..sub.1T, .alpha..sub.1T
(2.ltoreq.i.ltoreq.m-1) and .alpha..sub.mT sections are generated
in synchronism with either the reference clock 1a or 1b; when
recording a mark of nT=(2L+1)T, gate groups G2a and G2b
corresponding to timings of .alpha..sub.1'T, .alpha..sub.1'T
(2.ltoreq.i.ltoreq.m-1) and .alpha..sub.m'T sections are generated
in synchronism with either the reference clock 2a or 2b; when n is
even, a gate G3 of .SIGMA.(.alpha..sub.1+.beta..sub.1)T is
generated with the delay time T.sub.d1 from the front end of the nT
mark taken as a reference; when n is odd, a gate G4 of
.SIGMA.(.alpha..sub.1'+.beta..sub.- 1')T is generated with the
delay time T'.sub.d1 from the front end of the nT mark taken as a
reference; a time that elapses from the reference time T.sub.sync
as a start point to the front end of the nT mark is counted as the
number of reference clocks T; when the elapsed time is an even
number times the reference clock T, the gate signal group G1a or
G2b is selected according to whether n is even or odd; when the
elapsed time is an odd number times the reference clock T, the gate
signal group G1b or G2a is selected according to whether n is even
or odd; When both G3 and G4 are off, recording light with an erase
power Pe is radiated; when either G3 or G4 is on, recording light
with a bias power Pb is radiated; when G3 and G1a are on at the
same time, recording light with a recording power Pw is radiated in
response to a G1a-on section; when G3 and G1b are on at the same
time, recording light with a recording power Pw is radiated in
response to a G1b-on section; when G4 and G2a are on at the same
time, recording light with a recording power Pw is radiated in
response to a G2a-on section; and when G4 and G2b are on at the
same time, recording light with a recording power Pw is radiated in
response to a G2b-on section.
20. An optical recording method according to claim 10, wherein for
all L, a delay time T.sub.d1 or T.sub.d1' with respect to a front
end of a mark length to be recorded is provided at rising edges of
recording pulses .alpha..sub.1T and .alpha..sub.1'T; a reference
time T.sub.sync corresponding to a clock mark formed at a
predetermined position on a recording track is generated; a
modulation signal corresponding to each mark length and space is
generated by taking the reference time T.sub.sync as a start point;
four reference clocks are generated, the four reference clocks
being a reference clock la with a period of 2T which is generated
from the reference time T.sub.sync taken as a start point, a
reference clock 2a with a period of 2T which leads the reference
clock 1a by 0.5T, a reference clock 1b with a period of 2T which
leads the reference clock 1a by 1T, and a reference clock 2b with a
period of 2T which leads the reference clock 1a by 1.5T; when
recording a mark of nT=2LT, gate groups G1a and G1b corresponding
to timings of .alpha..sub.1T, .alpha..sub.1T
(2.ltoreq.i.ltoreq.m-1) and .alpha..sub.mT sections are generated
in synchronism with either the reference clock 1a or 1b; when
recording a mark of nT=(2L+)T, gate groups G2a and G2b
corresponding to timings of .alpha..sub.1'T, .alpha..sub.1'T
(2.ltoreq.i.ltoreq.m-1) and .alpha..sub.m'T sections are generated
in synchronism with either the reference clock 2a or 2b; when n is
even, a gate G3 of .SIGMA.(.alpha..sub.1+.beta..sub.1)T is
generated with the delay time T.sub.d1 from the front end of the nT
mark taken as a reference; when n is odd, a gate G4 of
.SIGMA.(.alpha..sub.1'+.beta..sub.- 1')T is generated with the
delay time T.sub.d1 from the front end of the nT mark taken as a
reference; a time that elapses from the reference time T.sub.sync
as a start point to the front end of the nT mark is counted as the
number of reference clocks T; when the elapsed time is an even
number times the reference clock T, the gate signal group G1a or
G2b is selected according to whether n is even or odd; when the
elapsed time is an odd number times the reference clock T, the gate
signal group G1b or G2a is selected according to whether n is even
or odd; When both G3 and G4 are off, recording light with an erase
power Pe is radiated; when either G3 or G4 is on, recording light
with a bias power Pb is radiated; when G3 and G1a are on at the
same time, recording light with a recording power Pw is radiated in
response to a G1a-on section; when G3 and G1b are on at the same
time, recording light with a recording power Pw is radiated in
response to a G1b-on section; when G4 and G2a are on at the same
time, recording light with a recording power Pw is radiated in
response to a G2a-on section; and when G4 and G2b are on at the
same time, recording light with a recording power Pw is radiated in
response to a G2b-on section.
21. An optical recording method according to claim 8, 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 L equal to or greater than 2, the
periods of (.alpha..sub.1+.beta..sub.1)T and
(.alpha..sub.1'+.beta..sub.i')T in 2.ltoreq.i.ltoreq.m-1 are kept
constant independently of the linear velocity, Pw.sub.1, Pb.sub.1
and Pe in each i are kept almost constant independently of the
linear velocity, and .alpha..sub.1 and .alpha..sub.1'
(2.ltoreq.i.ltoreq.m) are decreased as the linear velocity
lowers.
22. An optical recording method according to claim 10, 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 L equal to or greater than 2, the
periods of (.beta..sub.1-1+.alpha..sub.1)T and
(.alpha..sub.101'+.alpha..sub.i')T in 2.ltoreq.i.ltoreq.m are kept
constant independently of the linear velocity, Pw.sub.1, Pb.sub.1
and Pe in each i are kept almost constant independently of the
linear velocity, and .alpha..sub.1 and
.alpha..sub.1'(2.ltoreq.i.ltoreq.m) are decreased as the linear
velocity lowers.
23. An optical recording method according to claim 21, wherein
.alpha..sub.1T and .alpha..sub.1'T (2.ltoreq.i.ltoreq.m-1) are kept
almost constant independently of the linear velocity.
24. An optical recording method according to claim 1 , wherein the
erase power Pe of Pb.sub.1.ltoreq.Pe.ltoreq.Pw.sub.1
(1.ltoreq.i.ltoreq.m) is radiated in a time length of the
spaces.
25. An optical recording method according to claim 1 , wherein the
recording medium is a phase change type optical recording medium in
which a crystal state is taken as an unrecorded/erased state and an
amorphous state is taken as a recorded mark.
26. An optical recording method according to claim 1, wherein for
the time length of all the recording marks,
4.gtoreq.n/m.gtoreq.1.5, .SIGMA..sub.i(.alpha..sub.i).ltoreq.0.6 n
and Pb.sub.1/Pe.ltoreq.0.2 are satisfied.
27. An optical recording method according to claim 1, wherein the
linear velocity during recording is 10 m/s or higher and a minimum
mark length is less than 0.8 .mu.m.
28. An optical recording method according to claim 1, wherein a
wavelength of the recording light is less than 500 nm, a numerical
aperture of a lens for focusing the recording light is 0.6 or more,
and the minimum mark length is less than 0.3 .mu.m.
29. An optical recording method according to claim 1, wherein the
mark length modulation scheme is an 8-16 modulation scheme or a (1,
7)-RLL-NRZI modulation scheme.
30. An optical recording method according to claim 1, wherein the
mark length modulation scheme is an EFM modulation scheme in which
the recording is performed by setting the linear velocity during
recording to 10 or more times a CD reference linear velocity of 1.2
m/s to 1.4 m/s and keeping the recording linear density
constant.
31. An optical recording method according to claim 1, wherein the
mark length modulation scheme is an EFM modulation scheme in which
the recording is performed by setting the linear velocity during
recording to two or more times a DVD reference linear velocity of
3.49 m/s and keeping the recording linear density constant.
32. A phase change type optical recording medium recorded by the
optical recording method claimed in 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.z.ltoreq.0.1, 0.ltoreq.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).
33. An optical recording method according to claim 6, wherein
.alpha..sub.1+.beta..sub.i (2.ltoreq.i.ltoreq.m-1) or
.beta..sub.101+.alpha..sub.1 (2.ltoreq.i.ltoreq.m-1) is kept
constant independently of a real number i.
34. An optical recording method according to claim 33, wherein
.alpha..sub.1+.beta..sub.1 (2.ltoreq.i.ltoreq.m-1) or
.beta..sub.1-1+.alpha..sub.1 (2.ltoreq.i.ltoreq.m-1) takes a value
of 2 independently of a real number i, further wherein
.alpha..sub.1=.alpha.c with respect to any one of i in a range of
2.ltoreq.i.ltoreq.m-1, said .alpha.c being a constant value.
35. An optical recording method according to claims 33 or 34,
wherein .alpha..sub.1 (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.
36. An optical recording method according to claim 7, wherein said
at least two recording marks with different n's have each time
length of the recording mark such that said each time length is
adjacent to another.
37. An optical recording method according to claim 36, where at
least one of (.alpha..sub.1+.beta..sub.1)T and
(.alpha..sub.m+.beta..sub.m)T is different from any one of the at
least two recording marks with different n's.
38. An optical recording method according to claim 36, where at
least one of (.beta..sub.1+.alpha..sub.2)T and
(.beta..sub.m-1+.alpha..sub.m)T is different from any one of the at
least two recording marks with different n's.
39. An optical recording method according to claim 7, wherein
.alpha..sub.1+.beta..sub.i (2.ltoreq.i.ltoreq.m-1) or
.beta..sub.1-1+.alpha..sub.1 (2.ltoreq.i.ltoreq.m-1) takes a value
of 2 independently of a real number i.
Description
[0001] (This is a continuation application of International patent
application No.PCT/JP00/03036)
TECHNICAL FIELD
[0002] The present invention relates to an optical recording method
and an optical recording medium.
BACKGROUND ART
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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.
[0007] Almost all of these optical recording media in recent years
employ a mark length recording method, which is suited for
increasing the recording density.
[0008] 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.
[0009] 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.
[0010] 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.
[0011] 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.
[0012] That is, a recording pulse is divided to adjust the geometry
of an amorphous mark (JP-A 62-259229, JP-A 63-266632). 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.)
[0013] 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).
[0014] 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.
[0015] 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.
[0016] As the recording pulse division scheme for the mark length
modulation recording media such as CD, the following method is
widely used.
[0017] 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
[0018] (where .SIGMA..alpha..sub.1+.SIGMA..beta..sub.1=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.1T
(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.1T (1.ltoreq.i.ltoreq.m) as the off pulse section,
recording light with a bias power Pb, less than Pw.sub.1 is
radiated.
[0019] 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.1T
(1.ltoreq.i.ltoreq.m), each followed by the off pulse section of
.beta..sub.1T (1.ltoreq.i.ltoreq.m). In the .alpha..sub.1T
(1.ltoreq.i.ltoreq.m) section during the recording, the recording
light with the recording power Pw is radiated and, in the
.beta..sub.1T (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.1+.SIGMA..beta..sub.- 1 may be set slightly
smaller than n, and the following setting is made:
.SIGMA..alpha..sub.1+.SIGMA..beta..sub.1=n-.eta. (.eta. is a real
number in 0.0.ltoreq..eta..ltoreq.2.0).
[0020] 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).
[0021] Generally, the reference clock period T decreases as the
density or speed increases. For example, T decreases in the
following cases.
[0022] (1) When the recording density is enhanced to increase the
recording capacity:
[0023] 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.
[0024] (2) When the recording linear velocity is increased to
increase a data transfer rate:
[0025] 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.
[0026] 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.1T and the off pulse section
.beta..sub.1T also tend to become short. Under these circumstances
the following problems arise.
[0027] (Problem a)
[0028] The recording pulse section .alpha..sub.1T 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.
[0029] (Problem b)
[0030] When the off pulse section .beta..sub.1T 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.
[0031] This problem will be explained by taking a phase change
medium as an example.
[0032] 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.
[0033] 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.
[0034] 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.
[0035] 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.
[0036] 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.
[0037] 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=l), 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
[0038] 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.
[0039] 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.
[0040] 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.
[0041] 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.
[0042] 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=0.50 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.
[0043] 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
[0044] 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.
[0045] 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:
[0046] 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),
[0047] 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.1T, .beta..sub.1T, . . . ,
.alpha..sub.mT, .beta..sub.mT, .eta..sub.2T
[0048] in that order (m is a pulse division number;
.SIGMA..sub.1(.alpha..sub.1+.beta..sub.1)+.eta..sub.1+.eta..sub.2=n;
.alpha..sub.1 (1.ltoreq.i.ltoreq.m) is a real number larger than 0;
.beta..sub.1 (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
[0049] radiating recording light with a recording power Pw.sub.1 in
a time duration of .alpha..sub.iT (1.ltoreq.i.ltoreq.m), and
radiating recording light with a bias power Pb.sub.1 in a time
duration of .beta..sub.1T (1.ltoreq.i.ltoreq.m-1), the bias power
being Pb.sub.1<Pw.sub.1 and Pb.sub.1<Pw.sub.1+1;
[0050] 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
[0051] 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-x 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
[0052] FIG. 1 is an explanatory diagram showing an example
recording pulse division scheme and an example method of generating
the recording pulses according to the invention.
[0053] FIG. 2 is an explanatory diagram showing a conventional
recording pulse division scheme.
[0054] FIG. 3 is a schematic diagram showing a shape of a recorded
mark and a change of reflectance in a phase change optical
recording medium.
[0055] FIG. 4 is an example of temperature history when recording
light is radiated against the recording layer of the phase change
optical recording medium.
[0056] FIG. 5 is a schematic diagram of retrieved waveforms
(eye-pattern) of an EFM modulation signal.
[0057] FIG. 6 is an example of division scheme of a recording pulse
for an 11T mark according to an embodiment of the invention.
[0058] FIG. 7 is a graph showing a relation between .alpha..sub.1
and a mark time length in the embodiment 1 of the invention.
[0059] FIG. 8 is a graph showing a relation between .beta..sub.m
and a mark time length in the embodiment 1 of the invention.
[0060] FIG. 9 is an example of division scheme of a recording pulse
for an EFM random pattern in the embodiment 1 of the invention.
[0061] 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.
[0062] FIG. 11 is an example of conventional division scheme of a
recording pulse for a 11T mark/11T space.
[0063] FIG. 12 is an explanatory diagram showing an example of a
pulse division scheme according to the invention.
[0064] FIG. 13 is an explanatory diagram showing a timing for
generating a gate in the pulse division scheme of FIG. 12.
[0065] FIG. 14 is an explanatory diagram showing a pulse division
scheme in (1) of embodiment 3.
[0066] FIG. 15 is a graph showing a dependency of a modulation in
(1) of embodiment 3.
[0067] FIG. 16 is an explanatory diagram showing a pulse division
scheme in (2) of embodiment 3.
[0068] FIG. 17 is a graph showing a dependency of .alpha..sub.1 of
a mark length (-.tangle-solidup.-) and a space length
(-.largecircle.-) in (2) of embodiment 3.
[0069] FIG. 18 is a graph showing a dependency of .beta..sub.1 of a
mark length (-.tangle-solidup.-) and a space length
(-.largecircle.-) in (2) of embodiment 3.
[0070] FIG. 19 is a graph showing a dependency of .beta..sub.m of a
mark length (-.tangle-solidup.-) and a space length
(-.largecircle.-) in (2) of embodiment 3.
[0071] FIG. 20 is an explanatory diagram showing a pulse division
scheme in (3) of embodiment 3.
[0072] FIG. 21 is a graph showing a mark length (-.largecircle.-)
and a space length (-.circle-solid.-), and their jitters in (3) of
embodiment 3.
[0073] FIG. 22 is an explanatory diagram showing a pulse division
scheme in (4) of embodiment 3.
[0074] FIG. 23 is a graph showing a mark length (-.largecircle.-)
and a space length (-.circle-solid.-), and their jitters in (4) of
embodiment 3.
[0075] FIG. 24 is an explanatory diagram showing an example of a
pulse division scheme according to the invention.
[0076] FIG. 25 is an explanatory diagram showing an example of a
pulse division scheme according to embodiment 4 and a dependency on
Tw/T of a modulation obtained.
[0077] FIG. 26 is an explanatory diagram showing an example of a
pulse division scheme according to embodiment 4 of the
invention.
[0078] FIG. 27 is a diagram showing a dependency on power of
modulation and jitter and a dependency of jitter on the number of
overwrites.
[0079] FIG. 28 is an explanatory diagram showing another example of
a pulse division scheme according to embodiment 4.
PREFERRED EMBODIMENTS OF THE INVENTION
[0080] Now, the present invention will be described in detail by
referring to the accompanying drawings.
[0081] 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.
[0082] 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.1T, .beta..sub.1T, . . . ,
.alpha..sub.mT, .beta..sub.mT, .eta..sub.2T
[0083] (m is a number of pulse divisions;
.SIGMA..sub.1(.alpha..sub.1+.bet-
a..sub.1)+.eta..sub.1+.eta..sub.2=n; .alpha..sub.1
(1.ltoreq.i.ltoreq.m) is a real number larger than 0, .beta..sub.1
(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.1T (1.ltoreq.i.ltoreq.m),
recording light with a recording power Pw.sub.1 is radiated; and in
the time length of .beta..sub.1T (1.ltoreq.i.ltoreq.m), recording
light with a bias power Pb.sub.1, which has the relation of
Pb.sub.1<Pw.sub.1 and Pb.sub.1<Pw.sub.1+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.
[0084] 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.
[0085] 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.
[0086] In this invention, as to the time lengths of all recording
marks, it is assumed that n/m.gtoreq.1.25.
[0087] Suppose that .eta..sub.1 and .eta..sub.2 are both 0. Then
because .SIGMA..sub.1(.alpha..sub.1+.beta..sub.1)/m=n/m, the value
of n/m corresponds to an average length of
(.alpha..sub.1+.beta..sub.1) and the value of (n/m)T corresponds to
an average period of the divided pulse.
[0088] 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.1T
and .beta..sub.1T are shortest for the longest mark.
[0089] For example, in the EFM modulation, n=3-11 and k=2, so
(n.sub.max/m)=11/(11-2)=about 1.22
[0090] Similarly, in the EFM+ modulation, n=3-14 and k=2, so
[0091] (n.sub.max/m)=14/(14-2)=about 1.16
[0092] In the (1, 7)-RLL-NRZI modulation, n=2-8 and k=1, so
[0093] (n.sub.max/m)=8/(8-1)=about 1.14
[0094] 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.1T or the average value of the
off pulse section .beta..sub.1T is less than 12.5 nanoseconds. This
means that for at least one i, either .alpha..sub.1T or
.beta..sub.1T is less than 12.5 nanoseconds. Further, when the
clock period T goes below approximately 20 seconds, either
.alpha..sub.1T or .beta..sub.1T becomes further smaller.
[0095] In the above explanation, if a particular .alpha..sub.1 or
.beta..sub.1 becomes longer than the average, this means that other
.alpha..sub.1 or .beta..sub.1 becomes shorter and the fact still
remains that either .alpha..sub.1T or .beta..sub.1T becomes
smaller.
[0096] To describe more accurately, in the n-k division scheme
.SIGMA.(.alpha..sub.1+.beta..sub.1) 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.1 and .beta..sub.1 becomes further
smaller, making the problem more serious.
[0097] 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.1T and .beta..sub.1T are made
sufficiently long. For example, the recording pulse section
.alpha..sub.1T and the off pulse section .beta..sub.1T 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.
[0098] 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.
[0099] 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.
[0100] 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.
[0101] 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
.lambda., when the physical marks are spaced from each other by 0.2
(.lambda./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 (.lambda./NA) of each other.
[0102] In this invention, the parameters associated with the
divided pulses such as .alpha..sub.1, .beta..sub.1, .eta..sub.1,
.eta..sub.2, Pw and Pb can be changed as required according to the
mark length and i.
[0103] Further, in this invention it is preferred that the average
value of the recording pulse section .alpha..sub.1T
(1.ltoreq.i.ltoreq.m) and the average value of the off pulse
section .beta..sub.1T (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.1T (1.ltoreq.i.ltoreq.m) and .beta..sub.1T
(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.1T (1.ltoreq.i.ltoreq.m) and
.beta..sub.1T (1.ltoreq.i.ltoreq.m).
[0104] In this invention, although it is possible to set
.beta..sub.m 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.
[0105] When the recording pulse section .alpha..sub.1T
(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.1 although
there is a problem of the rising/falling edge of the recording
light.
[0106] On the other hand, when the off pulse section .beta..sub.1T
(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.
[0107] To obtain a still greater cooling effect, it is desired that
.SIGMA..sub.1(.alpha..sub.1) associated with the time length of all
recording marks be set to 0.6n or less, particularly 0.5 n or less.
More preferably, .SIGMA..sub.1(.alpha..sub.1) is set to 0.4 n or
less. That is, the sum of the recording pulse sections
.SIGMA..sub.1(.alpha..sub.1T) is set shorter than
.SIGMA..sub.1(.beta..sub.1T) 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.1T.ltoreq..beta..sub.1T, i.e., in the recording
pulse train following at least a second pulse, .beta..sub.1T is
made longer.
[0108] In the recording method of this invention, the values of
.alpha..sub.1 (1.ltoreq.i.ltoreq.m) and .beta..sub.1
(1.ltoreq.i.ltoreq.m-1) are set appropriately according to the
values of the recording pulse section .alpha..sub.1T
(1.ltoreq.i.ltoreq.m) and the off pulse section .beta..sub.1T
(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.1 (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.1 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.1T that has a great effect on the shape of the
front end of the mark.
[0109] 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.1T
(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.1+.beta..sub.1)T
(2.ltoreq.i.ltoreq.m-1) or (.alpha..sub.1+.beta..sub.1-1)T
(2.ltoreq.i.ltoreq.m-1) is preferably set to l.5T, 2T or 2.5T.
[0110] In this invention, the recording light power Pb.sub.1
radiated during the off pulse section .beta..sub.1T
(1.ltoreq.i.ltoreq.m-1) is set smaller than the powers Pw.sub.1 and
Pw.sub.1+1 of the recording light radiated during the recording
pulse sections .alpha..sub.1T and .alpha..sub.1+1T. To obtain a
large cooling effect, it is preferred that Pb.sub.1<Pw.sub.1 be
set for the time lengths of all recording marks. More preferably
Pb.sub.1/Pw.ltoreq.0.5 and still more preferably
Pb.sub.1/Pw.sub.1.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.
[0111] For the time length of one particular recording mark, two or
more different values of Pb.sub.1 and/or Pw.sub.1 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.1 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.1 in the intermediate recording pulse sections
.alpha..sub.1T (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.1
in the off pulse sections .beta..sub.1T (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.1 and/or Pb.sub.1
for the same i.
[0112] 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.1.ltoreq.Pe<Pw.sub.1. 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.1 and equal to or lower than
the erase power Pe be radiated. Setting the light power equal to
the bias power Pb.sub.1 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.
[0113] 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.
[0114] 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.
[0115] 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.
[0116] 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.
[0117] 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.
[0118] 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.
[0119] 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.
[0120] 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.
[0121] 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.1
(1.ltoreq.i.ltoreq.m), .beta..sub.1 (1.ltoreq.i.ltoreq.m),
.eta..sub.1, .eta..sub.2, Pw.sub.1 (1.ltoreq.i.ltoreq.m) and
Pb.sub.1 (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.
[0122] 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).
[0123] Examples of pulse division scheme according to this
invention are shown below.
EXAMPLE 1 OF DIVISION SCHEME
[0124] For example, in the EFM modulation that forms 3T to 11T
marks, m=1 for n=3 and m is increased for n.ltoreq.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
[0125] as the n value increases to
n=3, 4, 5, 6, 7, 8, 9, 10, 11.
[0126] The value of n/m is minimum at 1.38 when n=11 and maximum at
3when n=3.
EXAMPLE 2 OF DIVISION SCHEME
[0127] In the same EFM modulation, the division number m is
increased to
m=1, 2, 2, 3, 4, 5, 6, 6, 6
[0128] as the n value increases to
n=3, 4, 5, 6, 7, 8, 9, 10, 11.
[0129] 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
[0130] In the same EFM modulation, the division number m is
increased to
m=1, 2, 2, 3, 3, 4, 5, 5, 5
[0131] as the n value increases to
n=3, 4, 5, 6, 7, 8, 9, 10, 11.
[0132] The value of n/m is minimum at 1.8 when n=9 and maximum at 3
when n=3.
[0133] When the same pulse division number m is used on at least
two recording marks with different n values, a pulse period
.tau..sub.1=.alpha..sub.1+.beta..sub.1 and a duty ratio
(.alpha..sub.1/(.alpha..sub.2+.beta..sub.1)) may be changed.
Examples of this procedure are shown below.
EXAMPLE 4 OF DIVISION SCHEME
[0134] The simplest division scheme is to make an equal division
such that the pulse period .tau..sub.1=nT/m when m.ltoreq.2.
[0135] However, simply dividing nT into equal parts may result in
.tau..sub.1 assuming a value totally irrelevant to the timing and
length of the reference clock period T.
EXAMPLE 5 OF DIVISION SCHEME
[0136] The pulse period .tau..sub.1 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.1(.tau..sub.1)=.SIGMA..su-
b.i(.alpha..sub.1+.beta..sub.1) 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.
[0137] Hence, sections .eta..sub.1T, .eta..sub.2T are provided such
that
.SIGMA..sub.1(.alpha..sub.1+.beta..sub.1)+(.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.
[0138] 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.
[0139] 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
[0140] The divided pulse period .tau..sub.1 and the duty ratio
(.alpha..sub.1/(.alpha..sub.1+.beta..sub.1)) 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.
[0141] 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.1
(2.ltoreq.i.ltoreq.m-1) of intermediate pulses.
[0142] At this time it is possible to slightly adjust
.alpha..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.
[0143] 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.1 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.
[0144] 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.
[0145] 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.
[0146] 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.
[0147] 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.
[0148] 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.
[0149] Conversely, by changing the intermediate parameters
.tau..sub.1, .alpha..sub.1, .beta..sub.1 (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.
[0150] Now, the method of generating divided recording pulses that
realizes the above-described division scheme will be explained
below.
[0151] 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.
[0152] (Divided Recording Pulse Generating Method 1)
[0153] 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.
[0154] At this time, the circuits Gate1, Gate 2, Gate3, Gate4 that
generate clocks at timings shown in FIG. 1(c) are combined to
realize the division scheme of FIG. 1(b).
[0155] 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.1T 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.1T
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.1T 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.
[0156] .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.
[0157] 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.1T 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.
[0158] 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.1=.alpha..sub.1+.beta..sub.- 1-1
(2.ltoreq.i.ltoreq.m-1) with Td.sub.2 as a start point, and
.gamma., is set almost constant at .gamma..sub.1=1 to 3. In this
case, .beta..sub.1 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.
[0159] 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.1 and .beta..sub.1 are defined by the duty
ratio with respect to the base clock.
[0160] 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.
[0161] The last pulse .alpha..sub.mT generated by the Gate4 is
generated only when n.ltoreq.n.sub.c+1. This is indicated by a 9T
mark in FIG. 1.
[0162] 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.
[0163] 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.
[0164] (Divided Recording Pulse Generating Method 2)
[0165] 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.
[0166] 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.
[0167] 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.1+.beta..sub.1=2 (2.ltoreq.i.ltoreq.m-1)
.alpha..sub.m+.beta..sub.m=2+.delta..sub.2
[0168] (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).
[0169] 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.1' and
.beta..sub.1' in the recording pulse sections .alpha..sub.1'T and
the off pulse sections .beta..sub.1'T are defined as follows.
.alpha..sub.1'+.beta..sub.1'=2.5+.delta..sub.1'
.alpha..sub.1'+.beta..sub.1'=2 (1.ltoreq.i.ltoreq.m-1)
.alpha..sub.m'+.beta..sub.m'=2.5+.delta..sub.2'
[0170] (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).
[0171] 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'
[0172] (where .DELTA.=0.8 to 1.2).
[0173] 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.
[0174] 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 A 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.
[0175] .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.,
.vertline..delta..sub.2/.delta..sub.1.vertlin- e. and
.vertline..delta..sub.2'/.delta..sub.1'.vertline. are each
preferably in the range of 0.8 to 1.2.
[0176] 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'
[0177] (where .DELTA..sub.1=0.4 to 0.6)
[0178] In this case, the rear end side is normally
.alpha..sub.m+.beta..sub.m+.DELTA..sub.2=.alpha..sub.m'+.beta..sub.m'
[0179] (where .DELTA..sub.2=0.4 to 0.6 and
.DELTA..sub.1+.DELTA..sub.2=.DE- LTA.)
[0180] 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.
[0181] 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 .alpha..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.
[0182] In the pulse generating method 2, the duty ratio between
.alpha..sub.1 and .beta..sub.i,
.alpha..sub.2/(.alpha..sub.1+.beta..sub.1- ), 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.1 and .alpha..sub.i' be set to
.alpha..sub.1=.alpha.c (fixed value) and .alpha..sub.1'=.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.
[0183] 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.1+.beta..sub.i)T to become 2T for
all i ranging from 1 to (m-1).
[0184] 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.
[0185] 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.
[0186] 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.
[0187] 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.1 (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.1' (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.
[0188] 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.
[0189] 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.1T (1.ltoreq.i.ltoreq.m) is Pw which is
constant; the bias power in the off pulse section .beta..sub.1T
(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.1T
(1.ltoreq.i.ltoreq.m) and .beta..sub.1T (1.ltoreq.i.ltoreq.m) is an
erase power Pe which is constant. Here Pb.ltoreq.Pe.ltoreq.Pw.
[0190] In FIG. 12, reference number 200 denotes a reference clock
with a period T.
[0191] 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.
[0192] 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).
[0193] 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.1T
(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.1-.beta..sub.1 is adjusted to produce a recording pulse
waveform 207.
[0194] 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.1T (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.1-.beta..sub.1 is adjusted to produce a recording pulse
waveform 208.
[0195] 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.
[0196] 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 la 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.1T
(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.1'T
(2.ltoreq.i.ltoreq.m-1) and .alpha..sub.m'T.
[0197] 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 {fraction (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.
[0198] 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.
[0199] 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.
[0200] 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.
[0201] In the case of FIG. 13(c), in synchronism with each of
periods R1a, R2a, R3a, R4a and R5a 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.
[0202] 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.
[0203] 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 Gate 1 for generating the first
pulse .alpha..sub.1T, the Gate2 for generating the intermediate
pulse group .alpha..sub.1T (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.1'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.
[0204] Generating the first pulse independently as with the Gate 1
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.1'T with a delay of 2.5T. This is equivalent to setting
the T.sub.d2 for Gate2 in FIG. 1 to 2.5T (when there is a delay
T.sub.d1, another delay T.sub.d1 is made).
[0205] 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.
[0206] 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.1+.beta..sub.1) is generated with a delay
time of T.sub.d1 when n is even; and a gate G4 of
.SIGMA.(.alpha..sub.1'+.beta..sub.1') 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.
[0207] 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.
[0208] 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.
[0209] (Divided Recording Pulse Generating Method 3)
[0210] 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.
[0211] 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.
[0212] 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.
T.sub.d1+.alpha..sub.1=2+.epsilon..sub.1
.beta..sub.1-1+.alpha..sub.1=2 (2.ltoreq.i.ltoreq.m)
[0213] 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.1' and
.beta..sub.1' in the recording pulse sections .alpha..sub.1'T and
the off pulse sections .beta..sub.1'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.1-1'+.alpha..sub.1'=2 (3.ltoreq.i.ltoreq.m-1)
.beta..sub.m-1'+.alpha..sub.m'=2.5+.epsilon..sub.3'
[0214] (When L=2, it is assumed that
.beta..sub.1'+.alpha..sub.2'=2.5+.eps- ilon..sub.2' or
.beta..sub.1'+.alpha..sub.2'=3+.epsilon..sub.2')
[0215] 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=.bet-
a..sub.1'+.alpha..sub.2'+.beta..sub.m-1'+.alpha..sub.m'
[0216] (where .DELTA..sub.2=0.8 to 1.2).
[0217] The values of .alpha..sub.1, .beta..sub.1, .alpha..sub.1',
.beta..sub.1', T.sub.d1, T.sub.d1', .epsilon..sub.1,
.epsilon..sub.1', .epsilon..sub.2' and .epsilon..sub.3' can vary
according to L.
[0218] 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.
[0219] .alpha..sub.1, .beta..sub.1, .alpha..sub.1' and
.beta..sub.1' are real numbers normally between 0 and 2, preferably
between 0.5 and 1.5.
[0220] .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.1-1+.alpha..sub.1)T that form the period 2T.
[0221] 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.
[0222] 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.
[0223] 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.1-.beta..sub.1 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.
[0224] 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.
[0225] 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,
.alpha..sub.m-1'=.beta..sub.m-1+- 0.5 .alpha..sub.m=.alpha..sub.m',
and .beta..sub.m=.beta..sub.m'.
[0226] 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.T and .alpha..sub.m'T
and the rear end of the mark length nT be synchronized, with a
predetermined time difference therebetween.
[0227] 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.
[0228] As to the jitter at the rear end, on the other hand, if
.beta..sub.m-1, .beta..sub.m-', .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.
[0229] 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.
[0230] For further simplification of the pulse generating circuit,
when L is 3 or more, .alpha..sub.1 and .alpha..sub.1' 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'
[0231] 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.
[0232] More preferably, when L is 3 or more, the values of
.alpha..sub.1 and .alpha..sub.1' 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.1=.alpha..sub.1'=.alpha.c
(2.ltoreq.i.ltoreq.m-1) as described above.
[0233] It is more preferred that .alpha..sub.m and .alpha..sub.m'
be set to the same values of .alpha..sub.1 and .alpha..sub.1' for
2.ltoreq.i.ltoreq.m-1.
[0234] 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.
[0235] 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.
[0236] 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.1T and .beta..sub.1T
(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.1+.beta..sub.1-1 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.1'+.beta..sub.1-1' are generated.
[0237] In the above example, also when .epsilon..sub.1,
.epsilon..sub.1', .epsilon..sub.2' and .alpha..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.
[0238] 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.
[0239] 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.sub.1 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.1 in other sections .alpha..sub.1T (i=2 to m-1).
[0240] 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.
[0241] In FIG. 24, reference number 220 represents a T-period
reference clock.
[0242] 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.
[0243] 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.
[0244] 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.1T (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.1-.beta..sub.1
is adjusted to produce a recording pulse waveform 227.
[0245] 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.1'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.1-1-.alpha..sub.1 is adjusted to produce a recording
pulse waveform 228.
[0246] In this way, by using the T-period first reference clock 1
(223) and the T-period second reference clock 2 (224) 0.5 T out of
phase with the T-period first reference clock, .alpha..sub.1
(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.1'
(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.
[0247] 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).
[0248] 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.
[0249] 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.
[0250] 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.1T
(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.1'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.
[0251] 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 .alpha..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.
[0252] That is, when n is even, a gate G3 of
.SIGMA.(.alpha..sub.1+.beta.)- 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.1'+.beta..sub.1- ')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.
[0253] In summary, all the gates for generating the recording pulse
sections .alpha..sub.1T 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.1T (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.1'T (2.ltoreq.i.ltoreq.m-1) and .alpha..sub.m'T are
generated in synchronism with either the reference clock 2a or
2b.
[0254] 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.
[0255] 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.1+.beta..sub.1)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.1-.beta..sub.1 and duty ratio of
.alpha..sub.1'-.beta..sub.1', 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.
[0256] 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.
[0257] 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.
[0258] 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.1+.beta..sub.1)T and
(.alpha..sub.1'+.beta..sub.1')T for 2.ltoreq.i.ltoreq.m-1 constant
irrespective of the linear velocity, also keeps Pw.sub.1, Pb.sub.1
and Pe for each i almost constant irrespective of the linear
velocity, and reduces .alpha..sub.1 and .alpha..sub.1'
(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.
[0259] 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.1-1+.alpha..sub.1)T and
(.beta..sub.1-1'+.alpha..sub.1')T for 2.ltoreq.i.ltoreq.m constant
irrespective of the linear velocity, also keeps Pw.sub.1, Pb.sub.1
and Pe for each i almost constant irrespective of the linear
velocity, and monotonously reduces .alpha..sub.1 and .alpha..sub.1'
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.
[0260] In the above two examples, the expression "Pw.sub.1,
Pb.sub.1 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.1, Pb.sub.1 and Pe are virtually constant, not dependent of
the linear velocity at all.
[0261] In the above two examples, the method of reducing
.alpha..sub.1 and increasing .beta..sub.1 in
(.alpha..sub.1+.beta..sub.1)T and reducing .alpha..sub.1 and
increasing .beta..sub.i-1 in (.alpha..sub.1+.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.1. In that case, for all linear velocities v used and
for all L, it is preferred that .beta..sub.1 and .beta..sub.1' be
set to 0.5<.beta..sub.1.ltoreq.- 2.5 and
0.5<.beta..sub.1'.ltoreq.2.5, more preferably
1.ltoreq..beta..sub.1.ltoreq.2 and 1.ltoreq..beta..sub.1'.ltoreq.2,
to secure the cooling time to change the medium into the amorphous
state.
[0262] In the above two examples, it is further preferred that, for
all linear velocities, .alpha..sub.1T and .alpha..sub.1'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.1 and .alpha..sub.1' 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.
[0263] 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.
[0264] Examples in which the present invention proves particularly
effective are described below.
[0265] 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.
[0266] 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.
[0267] 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.
[0268] 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.
[0269] 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.
[0270] Next, the quality of the mark length modulation signal will
be described by referring to the drawings.
[0271] 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.
[0272] 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. 1 m 11 = I 11 I top .times.
100 ( % ) ( 1 )
[0273] 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%.
[0274] Further it is preferred that the asymmetry value Asym
defined by the equation below be set as close to 0 as possible. 2
Asym = ( I slice I 11 - 1 2 ) ( % ) ( 2 )
[0275] 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.
[0276] Next, a preferred optical recording medium for use in the
above-described optical recording method will be explained.
[0277] 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.
[0278] 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.
[0279] 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).
[0280] The alloy with the above composition, as explained above, is
a binary alloy containing excessive Sb at the
Sb.sub.70Te.sub.3-eutectic point and which contains Ge for 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.
[0281] 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.
[0282] As the element M in the above composition, In and Ga may be
used. In 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.bG-
e.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, Bi, N, O and S; and A.sup.2 is In and/or Ga).
[0283] 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.
[0284] 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).
[0285] 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.
[0286] 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.
[0287] 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.
[0288] 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 recrystallization 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.
[0289] 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.
[0290] 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.
[0291] 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.
[0292] 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.
[0293] 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.
[0294] 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.
[0295] 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.
[0296] 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.
[0297] 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.
[0298] 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.
[0299] 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.
[0300] In the case of the recording medium of this invention, the
difference between R1 and R2 is preferably set as small as
possible.
[0301] 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. 3 2 R1 - R2 R1 + R2 .times. 100 (
% )
[0302] For example, in the phase change medium with R1 of around
17%, R2 needs to be in the range of 16-18%.
[0303] 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.
[0304] 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.
[0305] 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.
[0306] 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.
[0307] 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.
[0308] 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.
[0309] 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..multidot.m and
100 n.OMEGA..multidot.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.
[0310] Possible materials for the reflection layer include
aluminum, silver and alloys of these materials as main
components.
[0311] 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.
[0312] 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.
[0313] 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.
[0314] 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).
[0315] 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.
[0316] The reflection layer may be formed in multiple layers to
increase the heat dissipating effect and the reliability of the
medium.
[0317] 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.
[0318] Forming the reflection layer in multiple layers is effective
also for obtaining a desired sheet resistivity at a desired
thickness of layer.
[0319] 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.
[0320] Embodiment 1
[0321] 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.
[0322] 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.
[0323] 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. Apattern in which these marks with
different mark time lengths are randomly generated is an EFM
modulation random pattern.
[0324] 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.
[0325] 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.
[0326] 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.
[0327] 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.
[0328] 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.
[0329] 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/s 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.
[0330] 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.
[0331] 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.
[0332] 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.
[0333] 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.
[0334] Embodiment 2
[0335] 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.
[0336] 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.
[0337] 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.
[0338] 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.
[0339] 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.
[0340] 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.
[0341] An EFM modulation random pattern was recorded and retrieved
in a manner similar to the embodiment 1. The result was
satisfactory.
EXAMPLE FOR COMPARISION 1
[0342] 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.
[0343] 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.
[0344] 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.
[0345] Embodiment 3
[0346] 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).s- ub.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..multidot.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.
[0347] 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.
[0348] 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.
[0349] 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.
[0350] (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.1 was set constant at 20 mW, the bias
power Pb.sub.1 was also set constant at 0.8 mW and the erase power
Pe for spaces was set to 10 mW.
[0351] As shown in FIG. 14(a), in the divided recording pulses
having constant .alpha..sub.1=1, .beta..sub.1 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.
[0352] 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.
[0353] 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.1 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.
[0354] (2) Next, the divided recording pulses of FIG. 16 with the
intermediate pulse group fixed to .alpha..sub.1=1 and
.beta..sub.1=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.
[0355] 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.
[0356] 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.
[0357] 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.
[0358] 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.
[0359] (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.
[0360] 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.1 where the recording power Pw.sub.1 is
to be radiated and the off pulse section .beta..sub.1 where the
bias power Pb.sub.1 is to be radiated are set as follows:
.alpha..sub.1+.beta..sub.1=2
.alpha..sub.1+.beta..sub.1=2 (2.ltoreq.i.ltoreq.m-1)
.alpha..sub.m+.beta..sub.m=1.6
[0361] 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.1'+.beta..sub.1'=2 (2.ltoreq.i.ltoreq.m-1)
.alpha..sub.m'+.beta..sub.m'=2.1
[0362] 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.
[0363] 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.1=0.8 and
.beta..sub.1=1.2 (2.ltoreq.i.ltoreq.m-1) irrespective of the n
value.
[0364] 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.
[0365] The recording power Pw.sub.1 and the bias power Pb.sub.1
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.
[0366] 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.
[0367] (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.1T
(1.ltoreq.i.ltoreq.m) was held almost constant. That is, the
intermediate recording pulse group was held constant at
.alpha..sub.1=0.5 and .beta..sub.1=1.5 (2.ltoreq.i.ltoreq.m-1).
[0368] 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..sub.m=1.4. When n was odd, pulses were
set to .alpha..sub.1'=0.6, .beta..sub.1'=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 {fraction (16/10)} (inversely
proportional to the linear velocity) while holding the recording
pulse length obtained in FIG. 20 constant. The recording power
Pw.sub.1 and the bias power Pb.sub.1 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.
[0369] 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.
[0370] 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.
[0371] (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.
[0372] 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.
[0373] 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.
[0374] 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.
[0375] Embodiment 4
[0376] 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).s- ub.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..multidot.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.
[0377] 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.
[0378] 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).
[0379] 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.
[0380] 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.1=.alpha..sub.1'=1 and
.beta..sub.1=.beta..sub.1'=1.
[0381] 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.
[0382] 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.
[0383] 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.1 and
.beta..sub.1 in the recording pulse section .alpha..sub.1T and the
off pulse section .beta..sub.1T are set as follows:
T.sub.d1+.alpha..sub.1=2 (T.sub.d1=0.95)
.beta..sub.101+.alpha..sub.1=2 (2.ltoreq.i.ltoreq.m-1)
[0384] 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.1 and .beta..sub.1 in the recording pulse section
.alpha..sub.1T and the off pulse section .beta..sub.1T 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.1-1'+.alpha..sub.1'=2 (3.ltoreq.i.ltoreq.m-1)
.beta..sub.m-1'+.alpha..sub.m'=2.45
[0385] 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.
[0386] In the case of L.gtoreq.3, the intermediate recording pulse
group was set to constant values: .alpha..sub.1'=.alpha..sub.1=1
and .beta..sub.1'=.beta..sub.1=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.
[0387] 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.
[0388] The bias power Pb.sub.1 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.1 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 FIG. 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.
[0389] 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).
[0390] 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.1 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.
[0391] 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.
[0392] Industrial Applicability
[0393] 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.
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