Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reexamination Certificate
1998-10-13
2001-09-04
Edun, Muhammad (Department: 2651)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
C369S053100, C369S059170
Reexamination Certificate
active
06285647
ABSTRACT:
The invention relates to a method of recording marks representing data in an information layer of an optical record carrier by irradiating the information layer by a pulsed radiation beam, each mark being written by a sequence of pulses. The invention also relates to an optical recording device for carrying out the recording method. The method is suitable for direct-overwrite on a record carrier, i.e. by writing information to be recorded in the information layer of the carrier and at the same time erasing information previously written in the information layer. The method can be used in direct-overwriting on an information layer made of a phase-change material.
A recording method according to the preamble is known from the publication “Improved High-Density Phase-Change Recording” by B. A. J. Jacobs et al in the Japanese Journal of Applied Physics, Vol. 36(1997) pp. 491-494. A mark is written by a sequence of write pulses and the previously written marks between the marks being written are erased by applying an erase power in between the sequences. The known sequence has a first power level or bias level between the pulses and a return to the bias level after the last pulse of the sequence before rising to an erase power level. It is a disadvantage of the known method that it does not allow a sufficient reduction of the jitter in the read signal obtained from reading marks written by using the known method. The jitter is the standard deviation of the time differences between level transitions in the digitized read signal and the corresponding transitions in a clock signal, the time differences being normalized by the duration of one period of the clock.
It is an object of the invention to provide a recording method for writing marks having a reduced jitter.
This object is achieved when the method of the preamble is characterised in that the radiation beam has a first power level between the pulses, a second power level after the last pulse of the sequence, followed by a third power level, and in that the first power level is lower than the second power level and the second power level is lower than the third power level.
It has been found that the disadvantage of the known method resides in the fact that the method does not provide an independent control of the jitter caused by the leading edge of a mark and the jitter caused by the trailing edge of a mark. A change of the first or bias level in the known method affects both the leading-edge jitter and the trailing-edge jitter. The method according to the invention introduces a new degree of freedom by making the low power level after the last pulse of a sequence independent of the low power level between the pulses. The first level between the pulses now influences mainly the leading-edge jitter, whereas the second level after the last pulse of the sequence influences mainly the trailing-edge jitter. When the first power level is smaller than the second power level and the second power level smaller than the third power level, the jitter of the marks is smaller than the jitter of marks written by the known method.
The tolerance on the setting of the bias power level in the known method is relatively tight. The reason for this turns out to be that the bias power level and the cooling power level have only partly overlapping tolerance ranges. The known method uses an equal value for the bias power level and the cooling power level. The value selected is a compromise between an optimum bias power level and an optimum cooling power level. Since the compromise value is not optimum for each of the power levels, a small change in the level causes a relatively large change in the jitter, resulting in a small tolerance range. When, according to the invention, both power levels are adjusted separately, each level has a wide tolerance range, simplifying the adjustment and maintenance of a level and making the recording device more tolerant for variations in the properties of record carriers.
The second power level, i.e. the power level immediately after the last pulse of a sequence, is preferably lower than 0.75 times the third power level. The second power level is preferably higher than 0.20 times the third power level. Within this range the cooling of the information layer after the last pulse of a sequence is sufficiently rapid for a proper formation of the mark, and the heating is sufficient for the erasure of the previously written marks immediately after the just written mark.
The first power level, i.e. the level between the pulses in a sequence, lies preferably in a range from 0 to 0.30 times the third power level. It has the advantage that it allows larger duty cycles of the pulses in a sequence, thereby allowing a lower peak output of the radiation source. The difference between the first power level and the second power level is preferably larger than 0.1 times the third power level. In some embodiments the secondpower level is preferably larger than 1.5×the first power level.
When the writing speed used in the method is changed, the duration of the period during which the second power level is maintained, i.e. the so-called cooling period, is preferably made dependent on the writing speed. The cooling period folling the last pulse of the sequence and preceding the start of the erasure at the third power level provides cooling of the information layer at the end of the sequence. If the cooling period is too short, the erasure starts too soon and will erase too much of the just written mark. If, on the other hand, the cooling period is too long, the erasure starts too late and previously written marks immediately following the just written mark will not be erased. There is an optimum duration of the cooling period when writing at a certain speed. When changing the writing speed, it turns out that the duration of the cooling period must be changed in dependence on the writing speed in order to obtain a proper transition from the write operation to the erasure operation.
In general, marks of different lengths are written on a record carrier by pulse sequences of different lengths. If a pulse sequence contains only one pulse, the power level during the cooling period following the single pulse is preferably equal to the second power level.
In a preferred embodiment of the method according to the invention the duration of the cooling period is linearly related to the inverse of the writing speed. Preferably, the length on the information layer corresponding to the cooling period has a constant value, independent of the writing speed and the type of record carrier.
The length on the information layer corresponding to the cooling period depends on the overlap of the areas heated by the last radiation pulse of the sequence and the start of the subsequent erasure. The size of a heated area is proportional to the size of the diffraction-limited spot formed by the focused radiation beam on the information layer. The length of the cooling period is therefore preferably proportional to &lgr;/NA and lies in a range from 0.09 to 0.27 times &lgr;/NA, where &lgr; is the wavelength of the radiation and NA is the numerical aperture of the radiation beam incident on the information layer. This means that the cooling period has a duration preferably between 0.09 and 0.27 times &lgr;/(NA v), where v is the writing speed. In terms of channel-bit periods, the duration of the cooling period lies preferably within the range from 2.85 10
6
to 8.54 10
6
times &lgr;/(NA v), where the two constants have the dimension of second
−1
. When the duration of the cooling period is chosen in the indicated range, the corresponding length provides such an overlap of the heated areas that the rear edge of the written mark is defined properly. As a result, the jitter on reading the marks is reduced.
The pulses in a sequence for writing a mark have preferably a substantially equal pulse width and a mark is written by a substantially constant number of pulses per unit of length of the mark independent of the writing speed. The control unit of a recordi
Borg Hermanus J.
Duchateau Johan P. W. B.
Van Woudenberg Roel
Belk Michael E.
Edun Muhammad
U.S. Philis Corporation
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