System and method for simultaneously verifying optical data writing

Abstract: The optical disk writing system disclosed is one in which a data written on an optical data writing medium is verified essentially simultaneously with the writing on the medium. The written data is verified by detecting a timing of the light irradiated on the medium and by measuring return light doses at a rise and a fall of the light irradiated on the medium and comparing the doses thereof with predetermined reference values. The medium used is one which uses a phase transition reversible between a crystal state and an amorphous state for writing/reading/erasing of data on a data writing film and includes, a first protective film (122), a phase transition type data writing film (123), a second protective film (124), a reflection film (125) formed in this order on a transparent substrate (121). The thickness of the first protective film is set such that, of three reflectivities of the data writing film respectively in crystal, amorphous and melted states, the reflectivity in the crystal state and the reflectivity in the melted state are made different from each other, and the reflectivity in the amorphous state and the reflectivity in the melted state are made different from each other. A high speed writing operation can be carried out and an overwrite operation can be completed in one time disk rotation.

Description: The present invention relates to an optical disk writing system and a method for writing, with which writing and reading of data is made by utilizing a phase transition optical disk. More particularly, the invention relates to a system and a method for simultaneously verifying a data written on an optical data writing medium, with which the data in high density is written, read and erased by using light. The invention also relates to an optical data writing medium in which there are changes in structures and optical characteristics between a crystal state and an amorphous state depending on differences in a temperature rise due to light irradiation and a thermal hysteresis due to a temperature fall. Thus, the invention relates to a phase transition optical disk and a simultaneous writing verifying method. In a prior art optical disk writing system and writing method, data is written on an optical disk by irradiating the disk with a laser beam. When writing a data into the optical disk, it is necessary to verify whether the data being written has correctly been written on the optical disk. Generally, this verification is made by reading the written area after completion of the writing. In the above prior art, a plurality of beam spots are formed. For the formation of the beam spots, diffraction grating is used as disclosed in Japanese Patent Application Kokai Publication No. Hei 2-9024 or Japanese Patent Application Kokai Publication No. Hei 3-41632, or a plurality of light sources are used. The verification is made using one or two of these beam spots. With respect to a magneto-optical disk, there is a proposal as disclosed in Japanese Patent Application Kokai Publication No. Hei 5-144112 in which a single beam verification using a single beam spot is realized by utilizing characteristics inherent to a magneto-optical disk. However, in the prior art verification methods applied to the optical disk systems described above, there have been problems that, in the case where the verification is made after the writing operation, the time required for the writing is long and that, in the case where a plurality of beam spots are used, there have been problems that the cost is high and the adjustment of optical system is difficult. An optical disk writing system using laser light makes it possible to perform a large volume data writing and a non-contact high speed access, and thus the practical use thereof as a large volume memory is increasing. The optical disks are grouped into a reading dedicated type which is known as a compact disk and a laser disk, a write once read many type in which a user can carry out writing, and a rewritable type in which a user can write and erase repeatedly. The write once read many type disk and the rewritable type disk are used as external memories of computers or document/image files. The rewritable type disk includes a phase transition type optical disk which utilizes a phase transition of a writing film and a magnet optical disk which utilizes a magnetization direction transition of a vertical magnetic film. Of these, the phase transition type optical disk does not require an external magnetic field and, moreover, since the overwrite can be made easily, the optical disk of this type is expected to become a leading type of rewritable type optical disk. Conventionally, there has been known a rewritable type optical disk, i.e., a phase transition type optical disk utilizing a writing film in which the transition is caused to occur between a crystal state and an amorphous state by the laser light irradiation. In the phase transition type optical disk, the writing is made by irradiating on the writing film a high power laser spot corresponding to the data to be written and locally raising a writing film temperature thereby causing the crystal/amorphous transition to occur, and the reading is made, using a low power laser light, by reading the changes in the resulting optical constant as an intensity difference of the reflected light or a phase transition. For example, in the phase transition optical disk utilizing a writing film in which the crystallization time is comparatively slow, the writing is made by rotating the disk, irradiating the laser light on the writing film formed on the disk, raising the temperature of the writing film to above the melting point and, after the laser light has passed, rapidly cooling the writing film so as to change it to an amorphous state. During the erasing, the writing film temperature is held for a time sufficient to allow the crystallization under a crystallization permitting temperature range which is above the crystallization temperature and below the melting point, whereby the writing film is crystallized. The known method for this purpose is a method in which the light irradiated is in an oblong shape extending along the laser advancing direction. In the case where the pseudo-over-write is carried out using two beams for writing a new data while the data already recorded is being erased, an oblong laser for erasing is positioned for irradiation ahead of a circular laser for the writing. On the other hand, in the disk utilizing the data writing film in which the quick crystallization is possible, one laser beam focused into a circular shape is used. In the conventionally known method, a change to a crystal state or an amorphous state is effected by changes between two levels of the laser light power. That is, by irradiating on the writing film a laser light having a power allowing the writing film temperature to rise above the melting point, most portions thereof are changed to an amorphous state when cooled and, on the other hand, the portions which are irradiated by the laser light having a power for the writing film temperature to rise above the crystallization temperature or to reach a temperature below the melting point are changed to a crystal state. For the writing films of the phase transition type optical disks, the materials used include GeSbTe system, InSbTe system, InSe system, InTe system, AsTeGe system, TeOx-GeSn system, TeSeSn system, SbSeBi system, and BiSeGe system which are chalcogenide type materials. Film formation methods for all these materials include a resistive heating vacuum vapor deposition method, an electron beam vacuum vapor deposition method, and a sputtering method. The state of the writing film immediately after the film formation is a kind of amorphous state and, for forming an amorphous state portion on the film upon writing, a formatting process is carried out for the entire writing film to be preset to a crystal state. The writing is achieved by forming an amorphous portion in the film of the crystal state. Conventionally, in order to carry out a verifying operation to confirm whether the erasing operation and the writing operation have been correctly effected and also whether the recorded state is correct, it requires a time corresponding to three disk rotations. Also, in the phase transition optical disk which permits overwriting, two disk rotations, one for an overwrite writing operation and the other for a verifying operation, are required. As above, when the date is rewritten, there is required a waiting time corresponding to at least two disk rotations and, for this reason, the disk transfer speed is considerably slow. As a method for compensating for the lowering of the transfer speed caused by the disk rotation waiting time during the writing, the verification carried out during the writing, that is, the simultaneous writing verifying operation is effective. For example, in the write once read many type optical disk, as disclosed in Japanese Patent Application Kokai Publication No. Sho 55-89919, Japanese Patent Application Kokai Publication No. Sho 57-92438 and Japanese Patent Application Kokai Publication No. Sho 60-145537, a change in the amount of the reflected light from the medium during the writing is monitored so as to detect the written state of the data from the reproduced wave shapes. The detection can be made because, by the writing, there occurs a shape change such as a change by formation of a hole in the write once and read many time medium, and the amount of the reflected light changes accordingly. Also, in the magneto optical disk, it has been proposed, as disclosed in the Japanese Patent Application Kokai Publication No. Hei 3-207040, that the amount of the reflected light is monitored and, from a change in the Kerr rotation angle of the optical beam caused by the Writing, the verifying signal is detected and the verifying is effected at the same time as the writing. However, in the phase transition disk with which the overwrite can easily be made, it is the present day state that no attempt has been made to effect the verifying operation during the writing and that no knowledge has been gained as to what structure of the disk is suited to the verification during the writing. U.S. Patent 4,680,594 (Bracht), which is used for the two-part from delimitation discloses a read-during-write arrangement for an optical data storage system, the arrangement comparing original and delayed versions of a write detect signal corresponding to return light from a storage medium, and indicating a successful write by the presence of an amplitude differential between the two signal versions. SUMMARY OF THE INVENTION The present invention seeks to provide an optical disk writing system and a method for writing, which utilizes an optical optical data writing medium of a type in which there is a transition in reflectivity or optical phase in a writing film when light is irradiated thereon, and which enables the writing operation and the verification to be effected concurrently by a single beam spot. According to a first aspect of the invention, there is provided an optical disk writing system in which predetermined data is written on an optical-data writing medium and the written data is verified. The system includes: a light-spot irradiation means for forming a light spot on said optical data writing medium by irradiation of light on said medium; a return-light dose measurement means for measuring a dose of return light from said light spot on said medium; and, an irradiation-light timing means for determining a first time segment that includes a period during which a light output irradiated on said medium is rising, and a second time segment that includes a period during which that light output is falling. Verification of the written data includes a measurement of the return-light dose in the first time segment as well as a measurement of the return-light dose in the second time segment. Preferably, the optical-data writing medium is constituted by a phase-transition optical disk. Preferably, the return-light dose measurement means includes two comparators with which the return light is measured by comparing the return-light dose in the first time segment with a first predetermined reference value, and by comparing the return-light dose in the second time segment with a second predetermined reference value. Preferably, the light spot irradiation means includes a laser-light emitting means, and the irradiation-light timing means includes a differentiation means for determining the first and second time segments by differentiating a driving signal to the laser light emitting means at the rising and falling of the light output. According to a second aspect of the invention, there is provided a method for writing data on an optical-data writing medium. The method includes the steps of: forming a light spot on said medium by irradiation of light on said medium; writing predetermined data on said medium using said light spot; measuring a dose of return light from said light spot on said medium; and, verifying the written data substantially simultaneously with the writing of said predetermined data on said medium. The verifying step includes: determining a first time segment that includes a period during which a light output irradiated on said medium is rising, and a second time segment that includes a period during which that light output is falling; and, measuring the return-light dose in the first time segment as well as measuring the return-light dose in the second time segment. Preferably, the verifying step also includes: comparing the return-light dose in the first time segment with a first predetermined reference value, and comparing the return-light dose in the second time segment with a second predetermined reference value. BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, features and advantages of the present invention will be apparent from the following description of preferred embodiments of the invention explained with reference to the accompanying drawings, in which:- Fig. 1 is a diagram showing the overall structure of an optical writing system of an embodiment according to a first aspect of the invention; Fig. 2 is a timing chart showing various waveforms obtained during the verifying operation in the system shown in Fig. 1; Fig. 3 is a diagrammatic structural sectional view of an optical disk used in Example 1 and Example 2 in the system shown in Fig. 1; Fig. 4 is a graph showing a relation between the writing power and the number of writing error generation times in the system shown in Fig. 1 in comparison with that in the prior art; Fig. 5 is also a graph showing a relation between the writing power and the number of writing error generation times in the system shown in Fig. 1 in comparison with that in the prior art; Fig. 6 is a diagrammatic structural sectional view of an optical disk used in Example 3 in the system shown in Fig. 1; Fig. 7 is a diagrammatic structural sectional view of an optical disk used in Example 4 to Example 9 in an embodiment according a second aspect of the invention; Figs. 8A, 8B and 8C are diagrams for use in explaining a writing/reading method, particularly a verifying operation during writing, by using an optical disk according to the invention; Fig. 9 is a graph showing the reflectivities obtained respectively for amorphous, melted and crystal states with respect to one structure of the recording medium according to the invention; Fig. 10 is a graph showing the reflectivities obtained respectively for amorphous, melted and crystal states with respect to another structure of the recording medium according to the invention; Fig. 11 is a graph showing the reflectivities obtained respectively for amorphous, melted and crystal states with respect to still another structure of the recording medium according to the invention; Fig. 12 is circuit diagram showing a circuit for the judgment of verifying operations; Fig. 13 is a timing chart showing various waveforms obtained during the verifying operation according to the invention; and Fig. 14 is a timing chart showing various waveforms obtained during the verifying operation according to the invention. Now, an optical disk writing system and a method for writing as an embodiment of the first aspect of the invention are explained with reference to the drawings. Fig. 1 is referred to for describing a system and a method embodied in the optical disk writing system and the method for writing according to this aspect of the invention. Fig. 1 diagrammatically shows the optical disk writing system. The optical disk system shown in Fig. 1 is constituted by a laser light source 1, lenses 2 and 4, a beam splitter 3, a phase transition optical disk 5, a laser driver 6, a differentiation circuit 8, a delay circuit 9, comparators 10, 11, 15 and 16, AND circuits 17 and 18, and a pulse count circuit 21. The laser light source 1 constitutes a light source system which emits laser beam to produce a beam spot for writing/reading data on the phase transition optical disk 5. The laser light source 1 is driven by the laser driver 6. The lenses 2 and 4 constitute a collector which focuses the laser light emitted from the laser light source 1 as a laser beam spot on the phase transition optical disk 5. The beam splitter 3 constitutes a laser beam divider, which is disposed in the optical axis of the incident and divides and extracts the return light from the optical disk. In the embodiment of the invention, the light emitted from the laser light source 1 is allowed to advance linearly, and the reflected light from the phase transition optical disk 5 is divided and reflected after being refracted 90°. The phase transition optical disk 5 is an optical disk for writing data by the irradiation of the laser light. The writing of data is carried out by causing the states of the writing film of the optical disk to undergo transition in response to a local temperature rise due to light absorption of the laser light. The laser driver 6 is a circuit portion for driving the laser light source 1 and causing the laser light to be generated. The differentiation circuit 8 is a circuit for extracting a 0/1 state transition point of an output signal from the laser driver 6 that drives the laser light source 1. The delay circuit 9 is a signal delay circuit portion for timings between the driving signal and the reflection signal. The comparators 10, 11, 15 and 16 constitute a comparison device for comparing the signal to be compared and the predetermined value. In this embodiment, by using these comparators, the transition states of the driving signal and the reflection signal are represented by pulse signals. The AND circuits 17 and 18 constitute a logical product circuitry, which finds transition states of the driving signal of the laser driver 6, the laser light emitted from the laser light source 1, and the reflection light from the phase transition optical disk 5. The transition states make it possible to confirm, that is to verify, the emission of the laser light and the execution of the writing. The pulse count circuit 21 is a circuit portion for confirming whether the write confirmation signal and the read confirmation signal are correctly present. By this confirmation, the reliability of the verification can be enhanced. Fig. 2 shows, in timing charts, timings of verifying operations. The charts show respectively timings of the signals indicated in Fig. 1. Waveform 51 shows a laser light source driving signal; Waveform 52 shows a gate signal corresponding to a rise portion of the write signal; Waveform 53 shows a gate signal corresponding to a fall portion of the write signal; Waveform 54 shows an optical signal of the light returning from the phase transition optical disk during writing; Waveform 55 shows a comparator signal in which the return light dose monitor signal IL outputted when the writing signal has risen is compared with the reference voltage CLV; and Waveform 56 shows a comparator signal in which the return light dose monitor signal IT outputted when the writing signal has fallen is compared with the reference voltage. Further, by using the above mentioned signals, the output signal 57 of the AND circuit 17 is a logical product signal of the gate signal 52 with the waveform as shown in Fig. 2 and the comparator signal 55 with the waveform as shown in Fig. 2, and is represented by a symbol VL in the explanation given below. By this signal, it is possible to confirm that a desired laser power has been irradiated on the writing film of the phase transition optical disk 5. Also, the output signal 58 of the AND circuit 18 is a logical product signal of the gate signal 53 with the waveform as shown in Fig. 2 and the comparator signal 56 with the waveform as shown in Fig. 2, and is represented by a symbol VT in the explanation given below. By this signal, it is possible to confirm that the writing film of the phase transition optical disk 5 has melted and the writing has correctly been made. Furthermore, where the writing signal 61 delayed by the delay circuit 9 and the logical product signals VL and VT take the logical product again, the verification precision can be enhanced. In this embodiment, the delayed writing signal 61 is made a gate signal, and the number of the pulses of the output signals 57 of the AND circuit 17 and the output signals 58 of the AND circuit 18, which are generated in the gate, are counted. In this counting, when both the two pulses, namely, one'pulse of the signal VL and one pulse of the signal VT, are counted, it confirms that the writing has been correctly been made. Here, the writing signal is delayed because the time for the laser light rising and the time necessary for the writing film to melt are taken into account. EXAMPLE 1 The phase transition optical disk 5 of this example is structured as shown in Fig. 3. The structure was obtained by depositing ZnS-SiO 2 102 to 150 nm, Ge 2 Sb 2 Te 5 103 to 20 nm, ZnS-SiO 2 104 to 20 nm, and Al 105 to 60 nm in this sequence by sputtering on a polycarbonate substrate 101. On the optical disk, (2-7)-modulated random data was written by rotating this phase transition optical disk 5 at a linear speed of 7.5 m/s and setting the writing pulse width to 50 ns. The writing power Pw was set to 14 to 16 mW, the erasing power Pe was set to 7 mW, and the reading power Pr was set to 1 mW. The reflectivities of the writing film were 25% when the film was in the crystal state, and 8% when in the amorphous state. The return light dose Ic from the phase transition optical disk 5 in a crystal state of the writing film during the reading was 1.5 V, and the return light dose IA from the phase transition optical disk 5 in an amorphous state was 0.48 V. The writing error signal generating times during the writing when CLV was set to (0.9×Ic×Pw)/Pr while CTV was set to 0.5×(Ic+IA)×(PE)/Pr are shown in Fig. 4. Shown in Fig. 4 are results of the checking of writing errors by reading the written area after the writing as in the prior art. As is noted from Fig. 4, the detected number of writing error generation times in this example is in accord with that in the prior art example. In the same way as above, the verifying was carried out by varying the reference voltage CLV and the reference voltage CTV, and the checking was made as to the optimum values of the reference voltages CLV and CTV. As a result, it was found that, with respect to the reference voltage CLV, the writing error generating times detected were in accord with those in the prior art example when the condition (0.7×Ic×Pw)/Pr<CLV(Ic×Pw)/Pr was satisfied. Also, with respect to the reference voltage CTV, the writing error generating times detected were in accord with those in the prior art example when the condition (IA×PE)/Pr<CTV<(Ic×PE)/Pr was satisfied. EXAMPLE 2 The phase transition optical disk 5 of this example employs a structure which is obtained by depositing ZnS-SiO 2 102 to 100 nm, Ge 1 Sb 2 Te 4 103 to 25 nm, ZnS-SiO 2 104 to 20 nm, and Al 105 to 60 nm in this sequence by sputtering on a polycarbonate substrate 101. On this optical disk, (2-7)-modulated random data was written by rotating the disk at a linear speed of 7.5 m/s and setting the writing pulse width of 50 ns. The writing power Pw was set to 16 to 18 mW, the erasing power Pe was set to 8 mW, and the reading power Pr was set to 1 mW. The reflectivities of the writing film were 20% when the film was in the crystal state, and 6% when in the amorphous state. The return light dose Ic from the phase transition optical disk 5 in a crystal state of the writing film during the reading was 1.2 V, and the return light dose IA from the phase transition optical disk 5 in an amorphous state was 0.36 V. The writing error signal generating times with respect to the writing power during the writing when the reference voltage CLV was set to (0.8×Ic×Pw)/Pr while the reference voltage CTV was set to 0.4×(Ic+IA)×(Pe)/Pr are shown in Fig. 5. Shown in Fig. 5 are results of the checking of writing errors by reading the written area after the writing as in the prior art. As is noted from Fig. 5, the detected number of writing error generation times in this example is in accord with that in the prior art example. In the same way as above, the verifying was carried out by varying the reference voltage CLV and the reference voltage CTV, and the checking was made as to the optimum values of the reference voltages CLV and CTV. As a result, it was found that, with respect to the reference voltage CLV, the writing error generating times detected were in accord with those in the prior art example when the condition (0.7×Ic)×Pw)/Pr<CLV<(Ic×(Pw)/Pr was satisfied. Also, with respect to the reference voltage CTV, the writing error generating times detected were in accord with those in the prior art example when the condition (IA×pE)/Pr<CTV<(Ic×PE)/Pr was satisfied. EXAMPLE 3 The phase transition optical disk 5 of this example is structured as shown in Fig. 6. The structure was obtained by depositing Au 112 to 10 nm, ZnS-SiO 2 113 to 150 nm, Ge 2 Sb 2 Te 5 114 to 20 nm, ZnS-SiO 2 115 to 45 nm, and Al 116 to 100 nm in this sequence by sputtering on a polycarbonate substrate 111. On this optical disk 5, (2-7)-modulated random data was written by rotating the disk at a linear speed of 7.5 m/s and setting the writing pulse width of 50 ns. The writing power Pw was set to 18 to 20 mW, the erasing power Pe was set to 9 mW, and the reading power Pr was set to 1 mW. In the medium of this configuration, the reflectivity in the amorphous state is higher, and the reflectivity when the writing film was in the crystal state was 10% and that when the writing film was in the amorphous state was 30%. The return light dose Ic from the phase transition optical disk 5 in a crystal state of the writing film during the reading was 0.6 V, and the return light dose IA from the phase transition optical disk 5 in the amorphous state was 1.8 V. In the same way as above, the verifying was carried out by varying the reference voltage CLV and the reference voltage CTV, and the checking was made as to the optimum values of the reference voltages CLV and CTV. As a result, it was found that, with respect to the reference voltage CLV, the writing error generating times detected were in accord with those in the prior art example when the condition (0.7×Ic×Pw)/Pr<CLV<(Ic×Pw)/Pr was satisfied. Also, with respect to the reference voltage CTV, the writing error generating times detected were in accord with those in the prior art example when the condition (Ic×PE)/Pr<CTV<(IA×pE)/Pr was satisfied. As explained above, for carrying out the verification, it is necessary to confirm whether the laser light of a predetermined intensity has in fact irradiated on the writing film, or the writing film has in fact been melted. For confirming whether or not the light of the predetermined intensity has irradiated, the intensity monitor output IL of the return light from the optical disk at the rise of the writing signal may be checked. Also, for confirming whether or not the writing film has melted, the light dose monitor output IT of the return light from the optical disk at the fall of the writing signal may be checked. In the optical disk having an Rc>Ra relation, the relation among the reflectivities to be satisfied is Rc>Rm>Ra, and in the optical disk having an Rc<Ra relation, the relation among the reflectivities to be satisfied is Rc<Rm<Ra, wherein Rc is a reflectivity of the writing film when it is in a crystal state, Ra is a reflectivity when the same is in an amorphous state, and Rm is a reflectivity when the same is in a melted state. The intensity of the light dose monitor output IT may be expressed by IT=PE×Pm, wherein an erase power is represented by PE. Thus, by checking whether the relation Rc<(IT/PE)<Ra or Ra<(IT/PE)<Rc has been satisfied, it is possible to confirm whether the writing film has melted. In each of the examples explained above, since the verification can be carried out while the writing is effected using a single beam spot, it is possible to reduce the time required for the writing and also to simplify the adjustment of the optical system which is otherwise complicated, whereby the manufacturing cost is reduced. Now, a preferred embodiment of the second aspect of the invention is explained with reference to the drawings. Fig. 7 shows in a sectional view a structure of an optical data writing medium used in the invention. As shown therein, on a transparent substrate 121, there are deposited a first protective film 122, a writing film 123, a second protective film 124 and a reflection film 125. On the reflection film 125, ultraviolet-setting resin 126 is coated for protective purpose. The substrate 121 employs glass or plastic in a disk shape. The first protective film 122 and the second protective film 124 employ such dielectric materials as SiO 2 , Si 3 N 4 , AlN, TiC 2 , ZnS, and Zns-SiO 2 . The writing film 123 employs GeSbTe, InSbTe, InSe, InTe, AsTeGe, TeOx-GeSn, TeSeSn, SbSeBi, BiSeGe, etc. of chalcogenide system. In the structure where transmittive high refraction materials are used as the reflection film 125, Si and Ge are used in particular. Also, where a metal film is used as the reflection film 125, Al, Au or alloys of AlTi or alloys of AuPd, etc. can be used. The feature of this second aspect of the invention resides in that fact that, of three reflectivities of the data writing film respectively in a crystal state, in an amorphous state, and in a melted state, the reflectivity in the crystal state and the reflectivity in the melted state are different from each other, and the reflectivity in the amorphous state and the reflectivity in the melted state are different from each other. Figs. 8A, 8B and 8C are for use in explaining a writing/reading method, particularly a verifying operation during writing, by using an optical data writing medium according to the invention. By the irradiation of laser light 130 during the writing, the writing film 133 is heated to above the melting point. The numeral 131 in Fig. 8C indicates a region where the writing film has been heated to above the melting point. The monitored signals of the reflection light doses then obtained are divided into two kinds in their states as shown respectively in Fig. 8A and Fig. 8B. That is, the state in Fig. 8A is a state wherein the reflectivity of the melt region 131 of the writing film is higher than that of the region 132 that immediately precedes the melt region 131, and the state shown in Fig. 8B is a state wherein the reflectivity of the melt region 131 is lower than that of the region 132 that immediately precedes the melt region 131. For the verification operation during the writing, that is, "simultaneous writing verification", it is important that the melt region 131 described above be accurately and correctly monitored. That is, during the monitoring of the reflected light doses, it is necessary for the reflectivity from the melt region and the reflectivity from the region 132 immediately preceding the melt region 131 to be distinguishable with a sufficient S/N ratio. For this reason, it is desired that the monitored signals be in the waveforms as shown under Figs. 8A and 8B. Here, the reasons for the monitor signals resulting in the waveforms as shown in Figs. 8A and 8B are that, with the writing power irradiation, the light reflection dose becomes higher during the time period corresponding to the writing pulse irradiation, that there is a time lag between the writing power irradiation start time and the melt start time, and that the laser light 130 has a constant beam diameter. Fig. 9 is a graph showing the reflectivities obtained for an amorphous state, a crystal state and a melt state with respect to one structure of the optical data writing medium used in the invention. The structure includes a transparent substrate 121, a first protective film 122 formed on the transparent substrate 121, a phase transition type data writing film 123 formed on the first protective film 122, a second protective film 124 formed on the writing film 123, and a reflection film 125 of a high refraction dielectric formed on the second protective film 124. It also includes a layer of ultraviolet-setting resin 126. As the high refraction dielectric reflection film 125, Si is used in this structure. A wavelength is 690 nm. Here, the transparent substrate employs polycarbonate, and the first protective film employs ZnS-SiO 2 . In the layered structure, the thickness of the phase transition type data writing film of GeSbTe is 10 nm, that of the second protective film of ZnS-SiO 2 is 18 nm, and that of the reflective film of Si is 60 nm. Further, a ZnS-SiO 2 layer of 120 nm thick as an interference layer and an ultraviolet-setting resin layer of 10 µm are formed on the resulting structure. Where the thicknesses of the first protective film are between 70 and 130 nm and between 230 and 280 nm, the reflectivity of the crystal state and that of the melt state become essentially the same as each other so that it is difficult to distinguish one from the other. However, in the remaining regions of the first protective film, the reflectivities respectively of the three states, namely, the crystal, amorphous and melt states, are distinctly different from one another, thereby making it possible to distinguish the melt state from the other states. Fig. 10 is a graph showing the reflectivities obtained for an amorphous state, a crystal state and a melt state with respect to another structure of the optical data writing medium. The structure includes a transparent substrate 121, a first protective film 122 formed on the transparent substrate 121, a phase transition type data writing film 123 formed on the first protective film 122, a second protective film 124 formed on the writing film, and a reflection film 125 of a metal formed on the second protective film 124. It also includes a layer of ultraviolet-setting resin 126. As the metal reflection film 125, Al is used in this structure. A wavelength is 690 nm. Here, the transparent substrate employs polycarbonate, and the first protective film employs ZnS-SiO 2 . In the layered structure, the thickness of the phase transition type data writing film of GeSbTe is 12 nm, that of the second protective film of ZnS-SiO 2 is 20 nm, and that of the reflective film of Al is 60 nm. Further, an ultraviolet-setting resin layer of 10 µm is formed on the resulting structure. As shown in Fig. 10, within the extent of the thicknesses of the first protective film up to 300 nm, the reflectivities in the three states, that is, crystal, amorphous and melt states, are distinctly different one another in the neighborhood of 130 nm, of 210 nm and of 300 nm of the thicknesses of the first protective film, and this enables the confirmation of the melt state. Fig. 11 is a graph showing the reflectivities obtained for an amorphous state, a crystal state and a melt state with respect to still another structure of the optical data writing medium according to the invention. The structure includes a transparent substrate 121, a first protective film 122 formed on the transparent substrate 121, a phase transition type data writing film 123 formed on the first protective film 122, a second protective film 124 formed on the writing film 123, and a reflection film 125 of a metal formed on the second protective film 124. It also includes a layer of ultraviolet-setting resin 126. As the metal reflection film 125, a thin film of Au is used in this structure. A wavelength is 690 nm. Here, the transparent substrate employs polycarbonate, and the first protective film employs ZnS-SiO 2 . In the layered structure, the thickness of the phase transition type data writing film of GeSbTe is 40 nm, that of the second protective film of ZnS-SiO 2 is 140 nm, and that of the reflective film of Au is 10 nm. Further, an ultraviolet-setting resin layer of 10 µm is formed on the resulting structure. As shown in Fig. 11, the reflectivities in the amorphous and melt states are approximately the same and not distinguishable in the neighborhood of 100 nm and 260 nm of the thicknesses of the first protective film, but the reflectivities in the three states are significantly different from one another where the first protective film has thicknesses other than above thicknesses, thus enabling the confirmation of the melt state. In all of the above structures, it is the setting of the thicknesses of the medium that has made it possible to realize desired three reflectivity characteristics, namely, the reflectivity characteristics under the crystal state, those under the amorphous state and those under the melt state of the data writing film. For confirming the effects of this aspect, various tests have been conducted as explained below. EXAMPLE 4 The optical data writing medium for this test is prepared as follows: The transparent substrate employs a polycarbonate substrate having preformed grooves and having a diameter of 130 mm and a thickness of 1.2 mm. By sputtering, there are formed a ZnS-SiO 2 film of 150 nm thick as a first protective film, a Ge 2 Sb 2 Te 5 film of 10 nm thick as a writing film, a ZnS-SiO 2 film of 18 nm thick as a second protective film, and an Si reflection film of 60 nm thick as a reflection film. Also, a ZnS-SiO 2 film of 120 nm thick is formed as an interference layer. Further, by spin coating, an ultraviolet-setting resin film of 10 µm thick is deposited. Then, by using an optical head mounted with a laser diode with a wavelength of 690 nm, the overwrite is carried out and the reproduced waveforms during the writing are monitored. The reproduced waveforms during the writing have shown changes in the reflectivity in the melt portion, thus enabling the confirmation of the writing state as being in order. COMPARISON EXAMPLE 1 The optical data writing medium for this comparison test is prepared as follows: The transparent substrate employs a polycarbonate substrate having preformed-grooves and having a diameter of 130 mm and a thickness of 1.2 mm. By sputtering, there are formed a ZnS-SiO 2 film of 100 nm thick as a first protective film, a Ge 2 Sb 2 Te 5 film of 10 nm thick as a writing film, a ZnS-SiO 2 film of 18 nm thick as a second protective film, and an Si film of 60 nm thick as a reflection film. Also, a ZnS-SiO 2 film of 120 nm thick is formed as an interference layer. Further, by spin coating, an ultraviolet-setting resin film of 10 µm thick is deposited. Then, by using an optical head mounted with a laser diode having a wavelength of 690 nm, the overwrite is carried out and the reproduced waveforms during the writing are monitored. From these reproduced waveforms, it has not been possible to observe a distinction between the melt portion and the crystal portion, that is, it has not been possible to confirm the propriety of the writing state. EXAMPLE 5 The optical data writing medium for this test is prepared as follows: The transparent substrate employs a polycarbonate substrate having preformed-grooves and having a diameter of 130 mm and a thickness of 1.2 mm. By sputtering, there are formed a ZnS-SiO 2 film of 130 nm thick as a first protective film, a Ge 2 Sb 2 Te 5 film of 12 nm thick as a writing film, a ZnS-SiO 2 film of 20 nm thick as a second protective film, and an Al film of 60 nm thick as a reflection film. Also, by spin coating, an ultraviolet-setting resin film of 10 (µm thick is deposited. Then, by using an optical head mounted with a laser diode having a wavelength of 690 nm, the overwrite is carried out and the reproduced waveforms during the writing are monitored. The reproduced waveforms during the writing have shown an increase in the reflectivity in the melt portion as compared with that in the crystal portion, thus enabling the confirmation of the writing state as being in order. COMPARISON EXAMPLE 2 The optical data writing medium for this comparison test is prepared as follows: The transparent substrate employs a polycarbonate substrate having preformed-grooves and having a diameter of 130 mm and a thickness of 1.2 mm. By sputtering, there are formed a ZnS-SiO 2 film of 180 nm thick as a first protective film, a Ge 2 Sb 2 Te 5 film of 12 nm thick as a writing film, a ZnS-SiO 2 film of 20 nm thick as a second protective film, and an Al film of 60 nm thick as a reflection film. Further, by spin coating, an ultraviolet-setting resin film of 10 µm thick is deposited. Then, by using an optical head mounted with a laser diode having a wavelength of 690 nm, the overwrite is carried out and the reproduced waveforms during the writing are monitored. From these reproduced waveforms, it has not been possible to observe a distinction between the melt portion and the crystal portion, that is, it has not been possible to confirm the propriety of the writing state. EXAMPLE 6 The optical data writing medium for this test is prepared as follows: The transparent substrate employs a polycarbonate substrate having preformed-grooves and having a diameter of 130 mm and a thickness of 1.2 mm. By sputtering, there are formed a ZnS-SiO 2 film of 180 nm thick as a first protective film, a Ge 2 Sb 2 Te 5 film of 40 nm thick as a writing film, a ZnS-SiO 2 film of 140 nm thick as a second protective film, and an Au film of 10 nm thick as a reflection film. Further, by spin coating, an ultraviolet-setting resin film of 10 µm thick is deposited. Then, by using an optical head mounted with a laser diode having a wavelength of 690 nm, the overwrite is carried out and the reproduced waveforms during the writing are monitored. The reproduced waveforms during the writing have shown a decrease in the reflectivity in the melt portion as compared with that in the amorphous portion, thus enabling the confirmation of the writing state as being in order. COMPARISON EXAMPLE 3 The optical data writing medium for this comparison test is prepared as follows: The transparent substrate employs a polycarbonate substrate having preformed-grooves and having a diameter of 130 mm and a thickness of 1.2 mm. By sputtering, there are formed a ZnS-SiO 2 film of 110 nm thick as a first protective film, a Ge 2 Sb 2 Te 5 film of 40 nm thick as a writing film, a ZnS-SiO 2 film of 140 nm thick as a second protective film, and an Au film of 10 nm thick as a reflection film. Further, by spin coating, an ultraviolet-setting resin film of 10 µm thick is deposited. Then, by using an optical head mounted with a laser diode having a wavelength of 690 nm, the overwrite is carried out and the reproduced waveforms during the writing are monitored. From these reproduced waveforms, it has not been possible to observe a distinction between the melt portion and the amorphous portion, that is, it has not been possible to confirm the propriety of the writing state. EXAMPLE 7 The optical data writing medium for this test is prepared by using Ge for the reflection film instead of Si used in Example 4 with the remaining being the same as in Example 4. Then, by using an optical head mounted with a laser diode having a wavelength of 690 nm, the overwrite is carried out and the reproduced waveforms during the writing are monitored. The reproduced waveforms during the writing have shown changes in the reflectivity in the melt portion, thus enabling the confirmation of the writing state as being in order. EXAMPLE 8 The optical data writing medium for this test is prepared by using Al-Ti for the reflection film instead of Al used in Example 5 with the remaining being the same as in Example 5. By using an optical head mounted with a laser diode having a wavelength of 690 nm, the overwrite is carried out and the reproduced waveforms during the writing are monitored. The reproduced waveforms during the writing have shown an increase in the reflectivity in the melt portion as compared with that in the crystal portion, thus enabling the confirmation of the writing state as being in order. EXAMPLE 9 In order to confirm the effect of the writing/reading method according to the invention, by using the optical data writing medium described with reference to Example 4, the verifying operation during the writing has been evaluated for confirming the effect of the writing/reading method according to the invention. For the judgment of the verifying operation, a judgment circuit illustrated in Fig. 12 has been used. Here, a comparator 144 receives a reading signal 141 during the writing and a comparator level 142, and it outputs a comparator output signal 143. In the arrangement shown in Fig. 12, the comparator input level with respect to the reading signal during the writing has been set as shown by 142 in Fig. 13. Fig. 13 shows the waveforms obtained when the writing is made under the crystal state, the waveform 140 showing the writing signal, the waveform 141 showing the reading signal during the writing, and the waveform 142 showing the verifying comparator level. In this example, since the reflectivity in the melt state is lower than that in the crystal state, the comparator output pulses are as shown by the waveform 143. Thus, the crystal state and the melt state can be distinguished from each other whereby a desired verifying output can be obtained. Fig. 14 shows the waveforms obtained when the writing is made under the amorphous state, the waveform 140 showing the writing signal, the waveform 141 showing the reading signal during the writing, and the waveform 142 showing the verifying comparator level. In this example, since the reflectivity in the melt state is lower than that in the crystal state, the comparator output pulses are as shown by the waveform 143. Thus, the crystal state and the melt state can be distinguished from each other whereby a desired verifying output can be obtained. As explained above, the invention provides the structure of the medium which is suited for the simultaneous writing verifying operations of the phase transition type disk, and it also provides a writing/reading method which is featured in simultaneous writing verifying operations. According to the invention, since it is possible to realize the structure of the medium in which the reflectivities in the melt, crystal and amorphous states are distinctly different from one another and the simultaneous writing verifying procedures which are suited for such medium, there is a great advantage that the data rewriting operation can be effected at a high speed. The great advantage is that the overwrite operation can be completed within one disk rotation. While the invention has been described in its preferred embodiments, it is to be understood that the words which have been used are words of description rather than limitation and that changes within the purview of the appended claims may be made without departing from the true scope of the invention as defined by the claims. In summary the optical disk writing system disclosed is one in which a data written on an optical data writing medium is verified essentially simultaneously with the writing on the medium. The written data is verified by detecting a timing of the light irradiated on the medium and by measuring return light doses at a rise and a fall of the light irradiated on the medium and comparing the doses thereof with predetermined reference values. The medium used is one which uses a phase transition reversible between a crystal state and an amorphous state for writing/reading/erasing of data on a data writing film includes, a first protective film (122), a phase transition type data writing film (123), a second protective film (124), a reflection film (125) formed in this order on a transparent substrate (121). The thickness of the first protective film is set such that, of three reflectivities of the data writing film respectively in crystal, amorphous and melted states, the reflectivity in the crystal state and the reflectivity in the melted state are made different from each other, and the reflectivity in the amorphous state and the reflectivity in the melted state are made different from each other, and the reflectivity in the amorphous state and the reflectivity in the melted state are made different from each other. A high speed writing operation can be carried out and an overwrite operation can be completed in one time disk rotation.

Claims: An optical disk writing system in which predetermined data is written on an optical-data writing medium (5) and the written data is verified, the system comprises: a light-spot irradiation means (1,2,4,6) for forming a light spot on said medium by irradiation of light on said medium; a return-light dose measurement means (15,16,17,18) for measuring a dose of return light from said light spot on said medium; said system being characterized by comprising: an irradiation-light timing means (8,10,11) for determining a first time segment that includes a period during which a light output irradiated on said medium is rising, and a second time segment that includes a period during which that light output is falling; and, verification of said written data includes a measurement of the return-light dose in the first time segment as well as a measurement of the return-light dose in the second time segment. The optical disk writing system according to claim 1, wherein said optical-data writing medium is constituted by a phase-transition optical disk. The optical disk writing system according to claim 1, wherein said return-light dose measurement means includes two comparators (15,16) with which the return light is measured by comparing the return-light dose in the first time segment with a first predetermined reference value, and by comparing the return-light dose in the second time segment with a second predetermined reference value. The optical disk writing system according to claim 1, wherein said light-spot irradiation means includes a laser-light emitting means (1), and said irradiation-light timing means includes a differentiation means (8) for determining the first and second time segments by differentiating a driving signal to said laser-light emitting means at the rising and falling of the light output. A method for writing data on an optical data writing medium (5), comprising the steps of: forming a light spot on said medium by irradiation of light on said medium; writing predetermined data on said medium using said light spot; and, measuring a dose of return light from said light spot on said medium; wherein the method is characterized by also comprising the step of: verifying the written data substantially simultaneously with the writing of said predetermined data on said medium, the verifying step including: determining a first time segment that includes a period during which a light output irradiated on said medium is rising, and a second time segment that includes a period during which that light output is falling; and, measuring the return-light dose in the first time segment as well as measuring the return-light dose in the second time segment. The method according to claim 5, wherein said optical data writing medium is constituted by a phase-transition optical disk. The method according to claim 5, wherein the verifying step also comprises: comparing the return-light dose in the first time segment with a first predetermined reference value, and comparing the return-light dose in the second time segment with a second predetermined reference value. The method of writing data on an optical disk according to claim 7, wherein the first predetermined reference value (CLV) is obtained based on the following relation: (0.7xIcxPw)/Pr < CLV < (IcxPw)/Pr, wherein: Pw is a writing laser power; Pr is a reading power; and, Ic is a return light does monitor signal outputted upon reading when a writing film of the optical disk is in a crystal state. The method of writing data on an optical disk according to claim 7, in which the second predetermined reference value (CTV) is obtained based on the following relation: (IAxPE)/Pr < CTV < (IcxPE)/Pr, wherein: PE is an erasing laser power; Pr is a reading power; Ic is a return light dose monitor signal outputted upon reading when a writing film of the optical disk is in a crystal state; and, IA is a return light dose monitor signal outputted upon reading when the writing film is in an amorphous state. The method of writing data on an optical disk according to claim 7, in which the second predetermined reference value (CTV) is obtained based on the following relation: (IcxPE)/Pr < CTV < (IAxPE)/Pr, wherein: PE is an erasing laser power; Pr is a reading power; Ic is a return light dose monitor signal outputted upon reading when a writing film of the optical disk is in a crystal state; and, IA is a return light dose monitor signal outputted upon reading when the writing film is in an amorphous state.

Inventors: ITOU MASAKI, OHKUBO SHUICHI, OKADA MITSUYA, ITOU, MASAKI, OHKUBO, SHUICHI, OKADA, MITSUYA, OHKUBO, SHUICHI,
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1996
Main classification: G11B7
IPC codes: G11B7