Stock material or miscellaneous articles – Circular sheet or circular blank
Reexamination Certificate
2001-07-03
2004-10-26
Mulvaney, Elizabeth (Department: 1774)
Stock material or miscellaneous articles
Circular sheet or circular blank
C428S064400, C430S270140, C369S121000
Reexamination Certificate
active
06808778
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the invention
The present invention relates to an optical recording medium that switches either the irradiation time or the irradiation power of a laser beam in many levels in correspondence to data used in the recording, irradiates the recording layer with the laser beam and then records the data in multiple levels.
2. Related Art
A great deal of research has been carried out related to methods to record multiple pieces of data in signals with identical length by switching the depth of a regeneration signal (modulation factor of reflection signal) in many levels in contrast to methods to record data by changing the length of a regeneration signal (length of the modulated part of the reflection signal) in many levels with conventional optical recording mediums such as CD-R or DVD-R which are provided with recording layers and reflecting films, in this order, on an optically transparent substrate.
According to this optical recording method, because it is possible to record multiple pieces of data in the direction of depth compared to when binary data is recorded depending on the presence or absence of only a pit, the amount of signals assigned to fixed lengths can be increased. Consequently, optical recording methods which use holographs or optical recording mediums with multiple recording layers have been proposed in order to improve the linear recording density.
Hereupon, a case wherein data is recorded in many levels using a depth variation of the reflectance is called multilevel recording.
In this type of multilevel recording the recording marks must be shortened in order to improve the recording density.
Multilevel recording is, however, difficult when attempting to reduce the recording marks smaller than the beam diameter of a converged laser used for recording and reading.
For example, Japanese Patent Laid-Open Publication No. Hei. 10-134353 describes a method in which the quantity of laser light is adjusted in order to record multiple levels. In this method a regeneration signal is formed by differences in the reflection of the recording part and the non-recording part when the recording medium is a dye film or a phase-changing film. Consequently, in the method in Japanese Patent Laid-Open Publication No. Hei. 10-134353, the non-recording level and the recording level depend on a relationship of whether or not a recording exists and are not suitable for recording in many levels. Stated more clearly, nothing exists in the intermediate state between recording and non-recording for a phase-changing film or a dye film.
Up to the present the reason why multilevel recording in many levels was possible by means of modulating the quantity of laser light using a dye film or a phase-changing film as the recording medium was mainly due to the fact that the widths of the recording marks were changed by changing the power of the laser.
A converging beam normally forms a Gaussian distribution although when the recording film is a dye film or a phase-changing film, the recording is performed on the portion that exceeds a certain threshold value. Changing the power of the laser changed the spot size of the converging beam that can record which in turn changed the width of the recording marks.
If, however, the length of the recording marks are shortened to increase the recording density, it becomes difficult to perform multilevel recording in many levels, in particular in five levels or more, using a method that modulates the power of the laser to change the recording mark width. In other words, changing the recording power makes it difficult to change the reflection level during a regeneration in five levels or more,
Normally, the diameter of the converging beam is expressed by K&lgr;/NA (K; constant, &lgr;: laser wavelength, NA: numerical aperture). Normal values for a pickup used in a CD are &lgr;=780 nm, NA=0.45 with a diameter of approximately 1.6 &mgr;m. For this case, if the recording mark length was 1.6 &mgr;m or less, it is difficult to perform multilevel recording in five levels or more using a conventional method that changes the laser power.
Further, there is an example of an optical recording medium as disclosed in Japanese Patent Laid-Open Publication No. Hei. 1-182846 wherein the absorbance of the reactive material in the recording layer changes as a digital value when a quantity of incident light is supplied as a digital value.
The absolute value of the absorbance change is presumed to be very small for this optical recording medium however and has not yet attained practical use.
Furthermore, an optical recording method is disclosed in Japanese Patent Laid-open Publication No. Sho. 61-211835 in which the intensity or irradiation frequency of the irradiation light irradiating a photochromic material is changed in order to record at different arbitrary coloring density states.
In this optical recording method there is a problem in which the coloring density state cannot be read in live levels of more when irradiating and scanning laser light.
The rapid occurrence of thermal decomposition of dyes in optical recording mediums used as a recording materials which use dye as the principal material was considered to be favorable in conventional recording methods. This is because the signal quality is better due to the clear boundary between the recording part and the non-recording part.
If, however, there is rapid decomposition of the dye material during multilevel recording, decomposition of the dye will suddenly begin when a specified laser irradiation time or irradiation power is exceeded making it difficult to control the recording in many levels required for multilevel recording.
SUMMARY OF THE INVENTION
The inventor has discovered that it is possible to perform multilevel recording in five levels or more by means of changing 7:4 the irradiation time or the irradiation power of the laser when the recording mark length is shorter than the diameter of the converging beam. The inventor has also discovered that a dye material that changes gradually is more suitable as a material for a recording film than a phase-changing material that changes quickly from non-recording to recording following temperature increases in the laser irradiation.
Furthermore, the inventor has found out it is possible to favorably perform multilevel recording by stipulating the thermal decomposition characteristics of the dye material.
At this point the thermal energy absorbed by the recording film will grow larger in line with lengthening of the laser irradiation time and/or increases in the laser irradiation power. If the thermal energy exceeds a certain threshold value, the dye will decompose and deteriorate and recording will be performed on the recording film. Excessive thermal energy that exceeds the threshold value passes through the reflecting film and diffuses around the edge. For example, if the diffusion of the thermal energy is insufficient for an optical recording medium such as a CD-R, unfavorable effects will occur such as deformation of the substrate or the guide tracks cut into the substrate.
Taking the above-mentioned issues into consideration, an object of the present invention is to provide an optical recording medium that utilizes a widely used optical recording medium, such as CD-R, to perform multilevel recording in many levels and can obtain favorable signal quality. In particular, to provide an optical recording medium that prevents deformation of the optically transparent substrate or the channels cut into the substrate used to guide the laser as well as prevent degradations in the recording signals due to deformation of the protective film on the reflecting film by making the diffusion of the thermal energy by the laser irradiation sufficient and/or by controlling the thermal decomposition characteristics of the dye material. In addition, an object of the present invention is to provide an optical recording medium that can favorably perform multi-level recording control.
The inventor has diligently conducted research on an
Arioka Hiroyuki
Sunohara Hideki
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