Dynamic information storage or retrieval – Storage medium structure – Optical track structure
Reexamination Certificate
2002-03-27
2004-06-22
Edun, Muhammad (Department: 2655)
Dynamic information storage or retrieval
Storage medium structure
Optical track structure
C369S059100, C369S047100
Reexamination Certificate
active
06754166
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. The present invention allows the optical recording medium to be irradiated with a laser beam at an irradiation time and/or an irradiation power, which is changed in multi-stages in response to data made available for recording, to form a recording mark corresponding to the irradiation power and/or irradiation time and thereby perform multi-level recording of the data.
2. Description of the Prior Art
There are a number of methods for recording data on a prior-art optical recording medium by changing the length of a recording mark (the length of a reflected signal modulating portion) in multi-stages. In contrast, a number of studies have also been made on a method for recording a plurality of data on regions of the same length by changing the depth of the recording mark (the degree of modulation of a reflected signal) in multi-stages.
According to this optical recording method, a plurality of data can be recorded in the direction of depth in comparison with the recording of binary data using simply the presence and absence of pits. Thus, this makes it possible to increase the amount of signals to be assigned to a given length, thereby allowing the linear recording density to be improved. Accordingly, such optical recording methods have been suggested which make use of hologram or employ multi layers as the recording layer.
It is herein referred to as multi-level recording to record data in multi-stages using such as variations in depth of reflectivity.
It is necessary to make the recording mark short in order to improve the recording density in such multi-level recording.
However, it is made difficult to perform multi-level recording with the recording mark being made less in diameter than a focused laser beam to be used for recording and reading.
For example, described in Japanese Patent Laid-Open Publication No. Hei 10-134353 is that the amount of laser beam is adjusted to perform multi-level recording. In this publication, with dye film or phase changing film being employed as the recording medium, read signals are formed based on a difference in reflection between recorded and non-recorded portions. Therefore, the method according to Japanese Patent Laid-Open Publication No. Hei 10-134353 relates the non-recorded stage and recorded stage to the absence and presence of records, thus being not suitable for multi-level recording. More specifically, the phase changing film and the dye film provide no intermediate state between the recorded and non-recorded states.
On the other hand, for example, disclosed in Japanese Patent Laid-Open Publication No. Hei 1-182846 is an optical recording medium that allows the optical absorbance of reactants in the recording layer to vary digitally when an amount of light is incident digitally on the recording layer.
However, this optical recording medium is expected to have an extremely small absolute value of variations in optical absorbance for the amount (the number of times) of irradiation of laser beam and thus has not yet been put to actual use.
Furthermore, disclosed in Japanese Patent Laid-Open Publication No. Sho 61-211835 is an optical recording method for changing the intensity or the number of times of irradiation of beam, with which a photochromic material is irradiated, to perform recording in given different stages of coloring density states.
However, there is a problem that this optical recording method cannot read the coloring density states in five stages or more upon irradiation with laser beam for reading.
On the other hand, a double density CD (DDCD) has been suggested which is a CD with an increased storage capacity. The DDCD is 12 cm in diameter, which is the same as the CD, and formatted to provide one side thereof with a capacity of 1.3 GB and thus a recording density twice as high as that of the CD.
In addition, the DDCD makes as-is use of a laser beam of wavelength 780 nm that is used by the current CD, condensing the beam with a high NA lens to reduce the recording mark in size.
According to the DDCD specifications, the track has a pitch as narrow as 1.1 &mgr;m in comparison with 1.6 &mgr;m of the current CD and is thus provided with a higher density. In addition, as a matter of course, the recording mark is reduced in size, as the shortest recording mark was conventionally 0.833 &mgr;m in length but now is 0.623 &mgr;m while the longest recording mark is about 2 &mgr;m.
Accordingly, the DDCD can be readily implemented by making partial additions or modifications to the parts or firmware of the current CD. This allows the DDCD to feature almost no increase in fabrication costs. The DDCD is expected to find its use with personal computers as their external storage devices or the like.
However, suppose that each aforementioned type of storage medium and recording method is applied to the DDCD. As mentioned above, this would provide insufficient multi-level recording and require substantial changes to the parts and firmware of the current CD, presenting a problem of an increase in fabrication costs.
Furthermore, suppose that the binary recording method or a conventional recording method is performed on the recording medium narrower in track pitch than the conventional one. This would raise a problem that the same recording method as that of the current CD provides a so-called teardrop shaped recording mark due to heat accumulated upon recording. Thus, the recording mark having the aforementioned longest length or a length close to that is to have a teardrop shape with a tail end greater in width than the head end even when the recording laser beam spot is reduced in diameter to all manner of sizes.
As described above, the DDCD has a narrowed track pitch. This causes the teardrop shaped recording mark to produce a cross-talk signal from neighboring tracks and a cross-write phenomenon by which recording marks written on neighboring tracks are deformed. With the phase changing recording layer, this also causes a cross-erase phenomenon to increase and a so-called inter-symbol interference to increase by which the waveform of a read signal is subjected to an interference and distorted between the recording marks on neighboring tracks. Thus, the DDCD presents these new problems.
The inventor has found that multi-level recording can be performed in five stages or more by changing the time for irradiation with a laser beam even under the condition that the length of a recording mark is less than the diameter of a focused beam. The inventor filed a patent application of the invention (which has not yet been laid opened at the filing date of the application of the present invention).
The present invention made it possible to perform the aforementioned multi-level recording with the phase changing material and the dye material being employed as the material for the recording film.
In addition, the inventor made an attempt to perform multi-level recording in five stages or more by changing the time and/or power of irradiation of the laser beam.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an optical recording medium and an optical recording method that make use of an optical recording medium such as a CD-R (CD—Recordable) and CD-RW (CD—Rewritable), which are generally in wide actual use. The medium and method are adapted to perform multi-level recording in multi-stages without substantially modifying the parts-and the firmware of the recorder, thereby making it possible to provide signals of good quality.
It is another object of the present invention to provide an optical recording medium and an optical recording method which prevent the recording mark from taking a teardrop shape in the DDCD-R (DDCD—Recordable) and DDCD-RW (DDCD—Rewritable). It is thereby made possible to perform recording at a predetermined recording density without increasing cross-talk signals, cross-write, cross-erase, and inter-symbol interference.
The inventor ha
Arioka Hiroyuki
Suzawa Kazuki
Edun Muhammad
Oliff & Berridg,e PLC
TDK Corporation
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