METHOD FOR OPTICAL INFORMATION RECORDING

Abstract: In a single beam overwrite method, wherein optical film recording medium having thin recording film capable of reversible change of phase by irradiating different level laser lights, etc, the laser light is modulate to have three energy levels, i.e., a recording power level (PW), an erasing power level (PE) and a very short time of cooling power level (PC) which is provided immediately after the recording power level (PW).

Description: FIELD OF THE INVENTION AND RELATED ART STATEMENT This invention relates to method for optical information recording for recording, reproducing and erasing optical information at high speed and high density on optical information recording member, such as optical disk, by using laser light, etc. The technique for recording and reproducing information at high density utilizing laser light have been well known. This technique is applied on the document filing system, the static image filing system, etc. Applications to the rewritable type recording systems are reported. One of this rewritable type recording systems is system of phase change type optical disk, which utilizes phase change of Te compound, Se compound etc. between amorphous and crystal (amorphous-crystal phase change) or utilizes phase change between crystal and crystal (crystal-crystal phase change) which phase change is caused by volume differential between difference crystal structures. In the system utilizing the amorphous-crystal phase change complex refractive index consisting of refractive index n and extinction coefficient k is reduced (whitened), by irradiating strong and short pulse light for raising temperature of irradiated part followed by rapid quenching of the irradiated part to change it to amorphous phase. The complex refractive index consisting of refractive index n and extinction coefficient k is increased (blackened) by irradiating weak and long pulse light for making crystal phase. Recording status and erased status of signal corresponds to small complex refractive index phase and large complex refractive index phase, respectively. The small and large complex refractive index phases are detected by reflectivity difference therebetween. In the system utilizing the crystal-crystal phase change between the crystal structures are different between those rapidly cooled after heating and slowly cooled after heating. The crystal structural change causes volume change and this system utilizes such volume change. By the phase change type optical disk, it can be achieved to record new signal simultaneously with erasing the previously recorded signal (overwrite method). For such overwrite method, Japanese published unexamined patent application sho 56-145535 proposed to form plural beam spots on signal recording track of optical disk by providing plural laser diodes in optical head and to record new signal by a following beam spot simultaneously with erasing previously recorded signal by a foregoing beam spot. Because the foregoing erasing beam makes crystal phases by slow heating and slow cooling the recording film, its beam spot is designed in elliptic shape having major axis in track direction thus making irradiation time of the laser light on the same point longer than that using circle beam spot. Japanese published unexamined patent application sho 56-145530 proposed single beam overwrite method, wherein single laser spot erase the previously recorded signal and simultaneously record new signal by modulating the laser light to have two power levels, i.e., recording power level PW and erasing power level PE (PW>PE) as shown in FIG.3. In such method, the recording film having high speed crystallization characteristic is used. The above-mentioned two overwrite methods using optical disk of phase change type have difficulties as follows: In the method of providing plural laser diodes, high optical precision is necessary, because the plural laser spots should be formed on the same track simultaneously, and must be accurately adjust tracking. Therefore, the method has problem in mass-production. Further, since respective la diodes must be control separately, the system becomes complex. Furthermore, use of plural expensive laser diodes cause to render the system expensive. On the contrary, the single beam overwrite method has no such difficulties, since there is no necessity of high optical precision or complex system. However, in such method, since it is necessary to erase the previous recorded signal during passing of erasing spot, the recording film having high speed of crystallization from amorphous phase is necessary. For making amorphous phase in the recording film having the high speed of crystallization, fast cooling after melting is necessary. On the other hand, however, in such method the fast cooling for making amorphous phase is difficult to obtain, since high power erasing beam is irradiated even after irradiating of the recording beam. Therefore, in some cases, large reproducing signal fails to be produced, because the recording bit (mark) is likely to become small or the complex refractive index of recording bit is likely to become larger than that of amorphous phase owing to inclusion of microcrystal in the recording bit and thus, difference of refractive index between the crystal phase and the recording bit becomes small. When small erasing power level PE is intended to be used for fast cooling, the previously recorded signal is not always perfectly erased, and residual signal is likely to be remained. Therefore, appropriate setting of the two power levels PW and PE is difficult. Such difficulties that obtained reproduced signal is small and that the residual signal is remained are asserted also in the method utilizing the phase change between different crystals as signal. OBJECT AND SUMMARY OF THE INVENTION It is an object of the invention to provide an improved single beam overwrite method and to provide an improved recording method wherein large amplitude of reproduced signal can be obtained without complex system structure. The method for optical information recording in accordance with the present invention, wherein new signal is recorded simultaneously with erasing old signal by using single laser beam on optical information recording member having thin recording film capable of changing reversibly between two optically distinguishable phases, and the power level of the laser is modulated in a recording power level and an erasing power level; said method is characterised in that a low power level which is lower than the recording and the erasing power level is provided instantaneously after the recording power level. By using the irradiation light modulated in three power levels for the single beam overwrite method in accordance with the present invention, the irradiation power becomes small instantaneously after recording light irradiated and thus the parts which are irradiated by recording light can be cooled quickly. Therefore, large amorphous area can be formed or large difference of complex refractive indexes can be obtained, since the microcrystals are hardly included in the recording part, and the complex refractive index of recording part becomes small. Therefore, large reproduced signal is obtainable. Furthermore, the optical head and other system structure can be made simple and cost of the system can be reduced because the overwriting is carried out by a single laser, in the present invention. The invention will be understood from the following detailed description taken in conjunction with the drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG.1(a) is a wave form chart of output of laser diode used in the present invention. FIG.1(b) is a diagram of temperature distribution along a track of recording film. FIG.1(c) is a figure for showing the bits recorded on the recording track. FIG.2(a) is a figure for showing the optical information recording method in accordance with the present invention. FIG.2(b) is a wave form chart of inputted current to the laser diode. FIG.3(a) is a wave form chart of output of laser diode using in the prior art. FIG.3(b) is a wave form chart of reproduced output signal in the prior art. FIG.4(a) is a wave form chart of output of laser diode used in a first embodiment of the present invention. FIG.4(b) is a wave form chart of reproduced output signal in a first embodiment of the present invention. FIG.5(a) is a graph for showing a relation between C/N and a cooling power level PC. FIG.5(b) is a graph for showing a relation between an erasability and the cooling power level PC. FIG.6(a) is a graph for showing a relation between C/N and a irradiation time of the cooling power level PC. FIG.6(b) is a graph for showing a relation between an erasability and the irradiation time of the cooling power level PC. FIG.7(a) is a wave form chart of output of laser diode used in the prior art. FIG.7(b) is a wave form chart of reproduced output signal in the prior art. FIG.8(a) is a wave form chart of output of laser diode used in a second embodiment of the present invention. FIG.8(b) is a wave form chart of reproduced output signal in a second embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings in detail, the present invention is described hereafter. FIG.2(a) shows an embodiment of the method for optical information recording. An optical disk 1 has a substrate 2 whereon a recording thin film 3 is provided. As the material of the substrate 2, a resin substrate, such as the polymethylmetaacrylate (PMMA) polycarbonate (PC) or glass substrate can be used. As the material of the recording thin film 3 for recording signal by utilizing a phase change between crystal and amorphous by light irradiation, a thin film consisting of Te compound or Se compound or a thin film having TeO x (0<x<2) which is mixture of Te and TeO₂, as host material can be used. Because in such recording thin films for erasing (crystallization) circular shaped beam spot same as recording beam spot should be used, large crystallization speed is necessary. Therefore, the thin film preferable comprises Te as host material and contains at least one of Sn, Ge, In, Bi, Se, S, Sb, Au, Pd, Pb as additive, or comprises TeO x as host material and contains at least one of Sn, Ge, In, Bi, Se, S, Sb, Au, Pd, Pb as additive. In SbSe compound is suitable for material that has crystal-crystal change. In this embodiment, a laser diode 4 is used as light source. Light of 830 nm wavelength output from the laser diode 4 becomes parallel light 6 by a lens 5. The parallel light 6 is formed to have circular cross-section by a second lens 7 and is made to parallel light again by a third lens 8. The resultant parallel light is focussed into a spot 11 of approximate 1 µm, which is the diffraction limit value, on the optical disk 1 by a fourth lens 10 through a half mirror 9. Recording and erasing are carried out by the spot 11. In a reproducing operation, a photo-sensitive diode 13 detects a reflection light from the optical disk 1 through the half mirror 9 and the lens 12. The method for optical information recording in accordance with the present invention is characterized mainly by output wave form of laser light intensity which is modulated in a special manner is irradiated onto the optical disk 1. The output wave form is controlled by current of wave form 14 (FIG.2(b)) inputted to the laser diode 4. FIG.1(a) shows the output wave form of the laser diode intensity modulated by the current of wave form 14. FIG.1(b) shows a distribution diagram of temperature of the recording film which is irradiated by the laser light in accordance with FIG.1(a). Fig.1(c) shows a recording bit on a track of the optical disk. As shown in FIG.1(a), power level of the laser light is modulated to have three power levels, i.e., a recording power level PW, an erasing power level PE which is lower than the recording power level PW, and cooling power level PC which is lower than the erasing power level PE and irradiated instantaneously after the recording power level PW. The recording power level PW is selected such that by irradiating the selected power level PW the temperature of thin film rises to the melting temperature TM of the thin film. The erasing power level PE is selected such that by irradiating the selected power level PE the temperature of thin film rises to the crystallization (blackened) temperature TX of the thin film. In genera, the erasing power level PE is selected that by irradiating the selected power level PE the temperature of thin film is not raised to the melting temperature TM of the thin film, but in some case an erasing power level PE which causes higher temperature than the melting temperature TM may be used under the condition that the irradiated part finally is crystallized. When the film is fast cooled after perfectly melted by irradiation of the recording power level PW for raising temperature of thin film enough above the melting temperature TM, the irradiated part finally becomes amorphous. However, when the film is cooled after melting by irradiation of the erasing power level PE for raising the temperature of thin film slightly above the melting temperature TM, the irradiated part does not become amorphous but becomes crystal in some cases. This is thought that the states are determined by becoming crystal, or becoming amorphous by melting state (i.e., perfectly melted, or partially melted including crystallization nucleus), and by cooling condition thereafter. The cooling power level PC is smaller than the erasing level PE for quenching the recording part irradiated by the recording power level PW. Introduction of the cooling power level PC, that does not exist in the prior art single beam rewrite method, is the main feature of the present invention. Long time irradiation by the cooling power level PC often causes undesirable residual signal. However, when the irradiating time of cooling power level PC is very short, the temperature of the irradiated part rises by overlapping irradiation of foregoing PW and following PE, and by conduction of heat from the parts irradiated by the powers PW and PE; and thus the temperature above the crystallization temperature TX is obtainable even PC=0. In fact, there is substantially no residual signal observed even in the case of PC=0 (detail will be described in examples below). Irradiating time τ of cooling power level PC is preferably: 0 < τ < λ/V where λ: wavelength of laser light V : relative speed between laser spot and disk. Since PC is smaller than PE, longtime irradiation of PC causes imperfect rise of temperature for crystallization and causes residual signal. However, when the irradiation time of PC is in a range to satisfy the above inequality, large residual signal does not exist (detail will be described in examples below). This is because in case of PC within above inequality, the temperature of irradiated part by PC rises above the crystallization temperature TX by overlapping irradiation of PW and PE and by conduction of heat from the parts irradiated by PW and PE. The recorded bit on the track is not in exact oval form, but an egg-shaped as shown in FIG.1(c). It is thought that this is because the power level downs into PC immediately after irradiation by PW, and thus more quenching condition is obtained in latter half part irradiated by PW. In the phase change type optical disk, the condition of the crystallization and making amorphous can be controlled by changing composition of recording thin film and its composition rate, disk structure, power of irradiated laser light, relative speed between the laser spot and the optical disk. The inventors discovered that in a predetermined condition, the recording thin film is crystallized by continuously irradiating the laser light for melting the thin film but the last end part at stopping of laser light of the thin film becomes amorphous phase. It is thought that the cause is that in the part continuously irradiated by the laser light the heat escapes only in vertical direction to track and thus the part is crystallized causing by slow cooling speed, but at the end part the heat escapes also in running direction of laser spot and thus the part becomes amorphous by quenching. By utilizing this principle, method for single beam overwrite is provided. When erasing the old signal, the part to be erased is continuously irradiated by the laser light for melting the recording film. When recording the new signal, the part to be recorded is partially irradiated by the laser light where the laser light is lowered (it may be zero) at a moment. In other words, the amorphous bit is formed only at the end of the irradiated part, and thereby the signal is recorded. This method is corresponding to the case that the erasing power level PE is increased with the recording power level PW in the single beam overwrite method where the modulation is carried out by the three power levels i.e., the erasing power level PE, the recording power level PW and the cooling power level PC. Therefore, the cooling power level may be selected in accordance with the single beam overwrite method as above-mentioned. The reproduced wave form from the optical information recording method is described hereafter, comparing with the prior art method (FIG.3). The optical disk used for comparison sample has a 1000 Å recording thin film (Te₅₀Se₃₀Au₂₀) formed by vapor evaporation method on the PMMA substrate, and another PMMA sheet is bonded on the opposite face of the PMMA substrate as a back-coat. The thin film of Te₅₀Se₃₀Au₂₀ can be used for the single beam overwrite method because its crystallization speed is fast. Since temperature of recording thin film rises high at laser irradiation, the recording thin film is put between a pair of SiO₂ film of 1000 Å for the substrate protection. The moving velocity (linear velocity) V of laser spot on the track is 4 m/sec. FIG.3(a) shows a form of irradiated laser light in accordance with the prior art method. The recording power level PW is 6 mW (on the optical disk). The erasing power level PE is 3 mW. Irradiating time of each power level is 0.5µsecond (recording frequency is 1 MHz). FIG.3(b) shows a reproducing wave form from the recorded signal in accordance with prior art method. FIG.4(a) shows a wave form of irradiated laser light in accordance with the present invention. The recording power level PW is 6 mW. The erasing power level PE is 3 mW. The cooling power level PC is 1 mW. Respective irradiating times are 0.5µsecond (for PW), 0.4µsecond (for PE) and 0.1µsecond (for PC), respectively. FIG.4(b) shows a reproducing wave form from the recorded signal in accordance with the present invention. As is obvious from comparison of the reproduced signals of FIG.3(b) and FIG.4(b), larger signal amplitude than the prior art is obtained in the present invention, and thus the large output signal is obtained in the present invention. It is thought that larger recording bit is formed in the present invention than the prior art because the cooling speed immediately after irradiating PW is fast in the present invention. In FIGs.3(b) and 4(b), carrier to noise ratios (C/N) are 46 dB and 50 dB, respectively. Effective cooling power level was examined as described hereafter. The same optical disk as described above was used and an experimentation is carried out under the same condition as in FIG.4(a), and therein only the cooling power level PC is varied. In the experimentation, signal of 1.5 MHz has been previously recorded, and then signal of 1 MHz is recorded with varying the power level PC by the single beam overwrite method. FIG.5(a) shows relation between the cooling power level PC and the reproduced signal from newly recorded signal of 1 MHz; and FIG.5(b) shows relation between the cooling power level PC and the erasability of erased signal of 1.5 MHz. As shown in FIG.5(a), C/N becomes large when the cooling power level PC is smaller than the erasing power level PE. As shown in FIG.5(b), the erasability is reduced slightly when the cooling power level PE is decreased. However, the reduction of the cooling power level PC is only 3 dB even when PC=0. Therefore, the effect of the cooling power level PC is confirmed when it is smaller than the erasing power level PE. When PC=0, the laser light may be modulated to have two levels, the erasing power level PE and recording power level PW, and thus the power control of the laser becomes simple. Effective irradiation time was examined as described hereafter. The same optical disk as described above was used and experimentation is carried out under the same condition as in FIG.4(a). Only the irradiation time of the cooling power level PC is varied. In the experimentation, signal of 1.5 MHz has been previously recorded, and then signal of 1 MHz is recorded with varying the irradiation time of the power level PC by the single beam overwrite method. FIG.6(a) shows relation between the irradiation time of the cooling power level PC and the reproduced signal from newly recorded signal of 1 MHz; and FIG.6(b) shows relation between the irradiation time of the cooling power level PC and the erasability of erased signal of 1.5 MHz. As shown in FIG.6(a), C/N becomes large as a result of introduction of the cooling power level PC. C/N increases in accordance with the irradiation time of the cooling power level PC. However, the erasability decreases with increase of the irradiation time of PC, as shown in FIG.6(b). It is thought that too long time of the cooling power level PC causes to disturb necessary temperature rise of the irradiated part, and therefore, the amorphous bit does not crystallize perfectly. It is observed that when the irradiation time of the cooling power level PC becomes long, the erasability rapidly falls at a time τth. Such irradiation time τ th is in substantially inverse proportion to the relative speed V between the disk and the laser spot. For example, the irradiation time τ th are 0.2 µsecond in 4 m/s of relative speed V, 0.1 µsecond in 8 m/s of V, 0.07 µsecond in 12 m/s of V. From the above, it is shown that the temperature of the irradiation part of PC is must be raised by the foregoing PW and the following PE. The relation between τ th and V was experimentally confirmed to have the following relation: τ th ≒ λ/V where λ: wavelength of laser light, and V : relative speed between laser spot and disk. Therefore, from FIGs.6(a) and 6(b) suitable irradiation time τ of cooling power level PC is given as follows: 0 < τ < λ/V In the following is described the single beam overwrite method embodying the present invention wherein the erasing power level PE is raised to the recording power level PW and the laser light is modulated to have two levels, in comparison with the prior art. Used optical disk has the same construction as above experimentation, but the composition of the recording film is Te₅₀Se₁₅Au₃₅. The recording film of Te₅₀Se₁₅Au₃₅ has faster crystallization speed he recording film of Te₅₀Se₃₀Au₂₀, which has been afore-mentioned. Therefore, the recording film is crystallized when laser light is continuously irradiated on the film with power level which can melt the film (where the linear velocity of the optical disk is 4 m/s). However, in the end of continuously irradiated part at which the laser light is off, the amorphous region is formed, because the cooling speed of this region is fast. FIG.7(a) shows wave form of laser light in the prior art. The recording power level PW is 6 mW (on the optical disk), the erasing power level PE is 3 mW and irradiation time is 0.5 µsecond (in both level). The recording frequency is 1 MHz. FIG.7(b) shows wave form of reproduced signal in the prior art. FIG.8(a) shows wave form of laser light in this embodiment. The erasing power level PE is 6 mW, the cooling power level PC is 1 mW, the irradiation time of PE for erasing is 0.85 µsecond and the irradiation time of PC for cooling of 0.15 µsecond. In this embodiment, the recording power level and the erasing power level are called the erasing power level PE together, since the level PW is the same as the level PE. FIG.8(b) shows wave form of reproduced signal in this embodiment. In comparison with the reproduced wave forms of FIG.s7(b) and 8(b), the signal amplitude is larger than the prior art and thus large output signal is obtained in the present invention. It is thought that the large recording bit is formed in the present invention, since the cooling speed after irradiation of PE in the present invention is faster than prior art. In FIG.7(b), C/N is 43 dB and in FIG.8(b), C/N is 48 dB. The effect of the cooling power level PC is larger, when the cooling power level PC is lower. The reproduced amplitude becomes maximum at PC=0. Setting of power level becomes simple at PC=0, since the laser light is modulated to have two levels, i.e., zero level and level PE. When the cooling power level PC is increased from zero, the cooling effect becomes smaller and the reproduced amplitude is rapidly decreased at the level PX. The level PX corresponds with power level wherein the recording film is crystallized at continuous irradiation. In this embodiment, the level PX is about 2.5 mW. Therefore, the cooling power level PC should be set in lower power level wherein the recording film is not crystallized at continuous irradiation. Suitable irradiation time τ of cooling power level is same as the first embodiment wherein the laser light is modulated to have three levels. As has been described in detail for various embodiments, the single beam overwrite method in accordance with the present invention gives large amplitude of reproduced signal, even in the optical disk having the recording film of fast speed crystallization characteristic. Furthermore, the optical head and other system structure can be made simple, since the present invention is achieved by one laser diode.

Claims: Method for optical information recording wherein new signal is recorded simultaneously with erasing old signal by using single laser beam on optical information recording member (1) having thin recording film (3) capable of changing reversibly between two optically distinguishable phases, and the power level of the laser is modulated in a recording power level (PW) and, an erasing power level (PE), characterised in that a low power level (PC) which is lower than said recording (PW) and said erasing power level (PE) is provided instantaneously after said recording power level (PW). The method in accordance with claim 1, wherein said thin recording film (3) comprises a material which is reversibly changeable between amorphous phase and crystal phase. The method in accordance with claim 1, wherein said erasing power level (PE) is lower than said recording power level (PW) and said lower power level (PC) which is lower than said erasing power level (PE), is momentarily applied instantaneously after recording. The method in accordance with claim 1, wherein said low power level (PC) is laser off level. The method in accordance with claim 1, wherein said recording power level (PW) is same with said erasing power level (PE). The method in accordance with claim 5, wherein in erasing operation, a laser having a predetermined power level is applied, and in recording operation, laser having zero power level is instantaneously applied. The method in accordance with claim 1, wherein irradiating time of said low power level τ is 0 < τ < λ/V where λ: wavelength of laser V : relative speed between laser spot and disk.

Inventors: NISHIUCHI, KENICHI, OHNO, EIJI, YAMADA, NOBORU
Applicants: MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1987
Main classification: G11B7
IPC codes: G11B7