Optical recording method and optical recording medium

Details Optical recording method and optical recording medium Optical recording method and optical recording medium

2002-05-10

2003-12-09

Edun, Muhammad (Department: 2655)

Dynamic information storage or retrieval

Binary pulse train information signal

Binary signal processing for controlling recording light...

C369S059120, C369S059200, C369S116000

Reexamination Certificate

active

06661760

ABSTRACT:

TECHNICAL FIELD
The present invention relates to an optical recording method and an optical recording medium.
BACKGROUND ART
As the amount of information increases in recent years, there are growing demands for a recording medium capable of writing and retrieving a large amount of data at high speed and in high density. There are growing expectations that the optical disks will meet this demand.
There are two types of optical disks: a write-once type that allows the user to record data only once, and a rewritable type that allows the user to record and erase data as many times as they wish. Examples of the rewritable optical disk include a magnetooptical recording medium that utilizes a magneto-optical effect and a phase-change type recording medium that utilizes a change in reflectance accompanying a reversible crystal state change.
The principle of recording an optical disk involves applying a recording power to a recording layer to raise the temperature of that layer to or above a predetermined critical temperature to cause a physical or chemical change for data recording. This principle applies to all of the following media: a write-once medium utilizing pitting or deformation, an magnetooptical medium utilizing a magnetic reversal at the vicinity of the Curie point, and a phase change medium utilizing a phase transition between amorphous and crystal states of the recording layer.
Further, taking advantage of the 1-beam-overwrite ability (erasing and writing at the same time) of the phase change recording medium, rewritable compact disks compatible with CDs and DVDs (CD-ReWritable and CD-RW) and rewritable DVDs have been developed.
Almost all of these optical recording media in recent years employ a mark length recording method, which is suited for increasing the recording density.
The mark length recording is a method that records data by changing both the lengths of marks and the lengths of spaces. Compared with a mark position recording method which changes only the lengths of the spaces, this method is more suited to increasing the recording density and can increase the recording density by as much as 1.5 times. However, to retrieve data accurately makes the detection of the time length of the mark stringent, thus requiring precise control of the shape of mark edges. Further, there is another difficulty that a plurality of kinds of marks with different lengths, from short marks to long marks, need to be formed.
In the following descriptions, the spatial length of a mark is referred to as a mark length and a time length of the mark as a mark time length. When a reference clock period is determined, the mark length and the mark time length have a one-to-one correspondence.
In the mark length recording, when writing an nT mark (a mark having a mark time length of nT where T is a reference clock period of data and n is a natural number), simply radiating a recording power of square wave with the time length of nT or with the length finely adjusted will result in the front and rear ends of each mark differing in temperature distribution, which in turn causes the rear end portion in particular to accumulate heat and widen, forming an mark with an asymmetric geometry. This raises difficulties in precisely controlling the mark length and suppressing variations of the mark edge.
To uniformly shape the marks, from short marks to long marks, various means have been employed, such as division of recording pulses and use of off pulses. For example, the following techniques have been adopted in the phase change media.
That is, a recording pulse is divided to adjust the geometry of an amorphous mark (JP-A62-259229, JP-A63-266632). This approach is also utilized in the write-once medium that is not overwritten. Further, an off pulse is widely employed as a mark shape compensation means (JP-A 63-22439, etc.)
Other proposed methods include one which deliberately dull a trailing edge of the recording pulse to adjust the mark length and the mark time length (JP-A 7-37252); one which shifts a recording pulse radiation time (JP-A 8-287465); one which, in a multipulse recording method, differentiates a value of bias power during the mark writing operation from that during the space writing operation or erasing operation (JP-A 7-37251); and one which controls a cooling time according to a linear velocity (JP-A 9-7176).
The recording method based on the above pulse division approach is also used in the magnetooptical recording medium and the write-once type optical recording medium. In the magnetooptical and write-once type mediums, this approach aims to prevent heat from becoming localized. In the phase change medium, this approach has additional objective of preventing recrystallization.
Common examples of mark length modulation recording include a CD compatible medium using an EFM (Eight-Fourteen Modulation), a DVD compatible medium using an EFM+ modulation, a variation of 8-16 modulation, and a magnetooptical recording medium using a (1, 7)-RLL-NRZI (Ruu-Length Limited Non-Return to Zero Inverted) modulation. The EFM modulation provides 3T to 11T marks; the EFM+ modulation provides 3T to 14T marks; and the (1, 7)-RLL-NRZI modulation provides 2T to 8T marks. Of these, the EFM+ modulation and the (1, 7)-RLL-NRZI modulation are known as modulation methods for high-density mark length modulation recording.
As the recording pulse division scheme for the mark length modulation recording media such as CD, the following method is widely used.
That is, when a mark to be recorded has a time length of nT (T is a reference clock period and n is a natural number equal to or greater than 2), the time (n−&eegr;)T is divided into
&agr;
1
T, &bgr;
1
T, &agr;
2
T, &bgr;
2
T, . . . , &agr;
m
T, &bgr;
m
T
(where &Sgr;&agr;
i
+&Sgr;&bgr;
i
=n−&eegr;; &eegr; 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 &agr;
i
T (1≦i≦m) as the recording pulse section, recording light with a recording power Pw is radiated. In a time duration of &bgr;
i
T (1≦i≦m) as the off pulse section, recording light with a bias power Pb, less than Pw, is radiated.
FIG. 2
is a schematic diagram showing a power pattern of the recording light used in this recording method. To form a mark of a length shown in FIG.
2
(
a
), a pattern shown in FIG.
2
(
b
) is used. When forming a mark that is mark-length-modulated to the length of nT (T is a reference clock period; and n is a mark length, an integer value, that can be taken in the mark length modulation recording), (n−&eegr;)T is divided into m=n−k (k is 1 or 2) recording pulses (in the case of FIG.
2
(
b
), k=1 and &eegr;=0.5), and the individual recording pulse widths are set to &agr;
i
T (1≦i≦m), each followed by the off pulse section of &bgr;
i
T (1≦i≦m). In the &agr;
i
T (1≦i≦m) section during the recording, the recording light with the recording power Pw is radiated and, in the &bgr;
i
T (1≦i≦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, &Sgr;&agr;
i
+&Sgr;&bgr;
i
may be set slightly smaller than n, and the following setting is made: &Sgr;&agr;
i
+&Sgr;&bgr;
i
=n−&eegr; (&eegr; is a real number in 0.0≦&eegr;≦2.0).
That is, in the conventional technique, when the recording light to be radiated to form an nT mark is divided, the recording pulse is divided into m pieces (m=n−k, where k is 1 or 2), m being obtained by uniformly subtracting k from n (as described in JP-A 9-282661), and then a predetermined number is subtracted from the number of divisions m of the recording pulse to control the mark time length accurately (in the following, such a pulse division scheme is called an “n−k division” scheme).
Generally, the reference clock period T decreases as the density or speed increases. For example, T decreases in the following cases.
(1) When the Recordi

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