Stock material or miscellaneous articles – Circular sheet or circular blank – Recording medium or carrier
Reexamination Certificate
2002-03-28
2004-03-02
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Circular sheet or circular blank
Recording medium or carrier
C428S064200, C428S064800, C428S066600, C369S275400, C430S270140
Reexamination Certificate
active
06699557
ABSTRACT:
DETAILED DESCRIPTION OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical recording medium and an optical recording method for irradiating the optical recording medium with a laser beam by switching in multi-stages at least one of the duration of time and power of the laser beam in response to the data to be recorded in order to form recording marks of multiple types on the recording layer and thereby record information in multi-levels.
2. Description of the Prior Art
A number of studies have been made on a method for recording multiple pieces of data on each of signals having the same length by switching the depth of a read signal (the degree of modulation of a reflected signal) in multi-stages in contrast to a method for recording data on a conventional optical recording medium by changing the length of a read signal (the length of the modulated portion of a reflected signal) in multi-stages.
This optical recording method, in comparison with a case of recording binary data merely by the presence or absence of pits, makes it possible to record multiple pieces of data in the depth direction. This allows more signals to be assigned to a given length and thus the linear record density to be improved. As a method for switching the depth of a read signal in multi-stages, the power of a laser beam is in general switched in multi-stages to thereby form some recording marks of different types. As its recording medium, suggested currently are those that use holograph or have a multi-layered recording layer.
Incidentally, it is called the multi-level recording to record multiple types of record data different in degree of modulation of reflected signal from each other.
On the other hand, these optical methods for multi-level recording had a problem of providing signals that were degraded in quality at the time of reading as the power of a laser beam for use in recording increased, that is, as a reflected signal to be formed was increased in depth. The reason for this has not been made clear until now, however, the inventor predicts that this is conceivably caused by an increase in area of a recording mark (recording mark area) resulting from an increase in the laser power. For example, suppose that a conventional method was used to shorten recording marks to provide a recording medium with an amount of information recorded at a high density, in which the power of a laser beam was switched in multi-stages to perform multi-level recording. In this case, the quality of signals was significantly degraded, resulting in making no use of the merit of the multi-level recording. That is, to employ the multi-level recording, the recording marks had to be spaced widely apart from each other so that data could be positively detected to some extent even when the quality of signals were degraded. For this reason, the conventional method had a problem that recording marks were provided at a high density with difficulty, thus resulting in mutually contradictory circumstances.
The concept of a conventional optical recording method for performing multi-level recording by switching the power of a laser beam in stages is based on the premise that the length of a recording mark is greater than the diameter of a converging beam (beam waist) available at the time of recording. That is, the concept aims to realize multi-level recording by modulating the optical reflectivity of the recording mark itself in multi-stages and then directly reading the reflectively.
In general, the diameter of a converging beam is expressed by K&lgr;/NA (where K is a constant, &lgr; is the wavelength of the laser, and NA is the numerical aperture of the lens). In general, &lgr;=780 nm and NA=0.45 are employed in pickups for use with CDs with the converging beam being about 1.6 &mgr;m in diameter. In this case, with the recording mark being around 1.6 &mgr;m in length, the aforementioned problem of signal degradation become apparent, making it difficult to perform multi-level recording in five stages or more. According to a close study made by the inventor, this was presumably in part because of the fact that the prior art was based on the precondition that the length of the recording mark was greater than the diameter of the converging beam (beam waist) available at the time of recording, then modulated the optical reflectivity of the recording mark itself in multi-stages, and directly read the reflectivity to thereby realize multi-level recording.
The problems mentioned above are conceivably resulted from every factor, intertwined in a complicated manner, such as the power setting of the laser beam or the characteristics of the recording medium. However, as far as known to the inventor, the causes of the problem have not yet been made clear until now. Thus, the real picture is that the multi-level recording of a high density as well as its recording method have not yet been accomplished.
SUMMARY OF THE INVENTION
The present invention has been developed in view of the aforementioned problems. It is therefore the object of the present invention to suggest a new multi-level recording method by setting the characteristics of a recording medium to a predetermined condition, thereby accomplishing the multi-level recording of a high density.
The inventor has made intensive studies on optical recording media, and found a method for assuming virtual recording cells of a given size on the recording layer, where recording marks of different sizes were formed, to modulate the reflectivity of the entire virtual recording cells in multi-stages for multi-stage recording. The inventor confirmed that the method made it possible to perform multi-level recording of high densities in five stages or more on an optical recording medium. That is, the invention described below makes it possible to achieve the aforementioned object.
(1) An optical recording medium in which on an optical transparent substrate having predetermined grooves, a recording layer is formed at least to cover the grooves and irradiated with a laser beam to form recording marks at least on the recording layer to thereby record information on the recording layer, the optical recording medium wherein on the recording layer, contiguously defined along a feed direction of the laser irradiation are virtual recording cells which have a given unit length in the feed direction along the groove and a given unit width in the direction orthogonal thereto; the groove width W is set so that 0.20×(&lgr;/NA)<W<0.50×(&lgr;/NA), where &lgr; (nm) is the wavelength of the laser beam and NA is the numerical aperture of the objective lens for the laser beam in a irradiation optical system; at least either the irradiation time or irradiation power is set in five stages or more to radiate the virtual recording cell with the laser beam, thereby allowing for forming recording marks having different sizes of five types or more on the virtual recording cell; and optical reflectivity is modulated in accordance with the area ratio of the recording mark to the virtual recording cell, thereby allowing for recording information in multi-levels.
(2) The optical recording medium according to (1) in the foregoing, characterized in that the groove width W is set so that 0.25×(&lgr;/NA)<W<0.45×(&lgr;/NA).
(3) The optical recording medium according to (1) or (2) in the foregoing, characterized in that a plurality of the grooves are formed generally parallel to each other, and the pitch P between the grooves adjacent to each other is set so that 0.65×(&lgr;/NA)<P, more preferably 0.7×(&lgr;/NA)<P<1.2×(&lgr;/NA).
(4) The optical recording medium according to (1), (2), or (3) in the foregoing, characterized in that the recording layer is adapted to contain an organic dye.
(5) The optical recording medium according to any one of (1) to (4) in the foregoing, characterized in that the recording layer is adapted to contain a cyanine base dye.
(6) An optical recording medium in which on an optical transparent substra
Arioka Hiroyuki
Horai Takashi
Ootsuki Shirou
Tsukamoto Syuji
Blackwell-Rudasill G. A.
Jones Deborah
LandOfFree
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