Phase-change type optical disk device and optical head to be used therein

Abstract: A signal information recorded on a recording layer formed on a phase-change type optical disk (20) is reproduced as a phase change. Laser light emitted from a laser light source (1) is collimated to a reference light by a lens (7). The reference light is divided by a beam splitter (2) into a first reference light (12) and a second reference light (13). The first reference light is directed through a condenser lens (3) to the recording layer of the optical disk (20). A reflection light from the recording layer is directed through the beam splitter (2) and a second condenser lens (7) to an optical detector (6). The second reference light (13) is passed through a phase difference plate (4), a reflection mirror (5), the phase difference plate (4), the beam splitter (2) and then through the second condenser lens (7) to the optical detector (6) as a phase shifted reference light. At an input end of the optical detector (6), the reflection light and the phase-shifted reference light are interfered with each other and the signal information having substantially no phase jitter is obtained at an output of the optical detector (6).

Description: The present invention relates to a technique for reading information from an optical disk and, particularly, to a technique for reading information from a phase-change type optical disk on which the information is recorded by utilizing reversible phase-change to be caused by irradiation of the optical disk with light. It has been known that a phase-change type optical disk has an inherent advantage of easy rewriting without necessity of external magnetic field and, therefore, it has been expected that the phase-change type optical disk becomes major in the field of write/erase optical disk in the near future. In the phase-change type optical disk, an information is recorded by irradiating a recording film on the disk with a laser light spot having power corresponding to the information to locally heat the recording film to thereby change phase of the film material between crystalline state and amorphous state and reproduced by irradiating the film with laser light of low power to read difference in reflectance between a crystalline state portion and an amorphous state portion of the recording film as difference in intensity between reflected lights from the crystalline state portion and the amorphous state portion. For example, in a phase-change type optical disk utilizing a crystalline recording film whose crystallization time is relatively long, an information is recorded on a portion of the recording film formed on the optical disk by irradiating it with laser light to locally increase temperature of the recording film portion to a value higher than a melting point thereof while rotating the optical disk and by rapidly cooling the melted portion to make it in amorphous state. In erasing the information, the amorphized portion of the recording film is crystallized by heating it to a temperature in a crystallizable temperature range from its crystallization temperature to its melting point and holding the temperature for a time sufficient to crystallize the recording film portion. In this case, an erasing beam to be used has an ellipsoidal cross section whose major axis is in parallel to a moving direction of laser beam on the optical disk. In a case where a new data is to be written while erasing any previously recorded data, that is, in a case of the so-called pseudo overwriting, the recording film is irradiated with a recording beam having a circular cross section immediately following the erasing beam. On the other hand, in an optical disk utilizing an information recording film of a material capable of being crystallized at high speed, a single laser beam collimated to a circular cross section is used. In a conventional method, a recording film on an optical disk is crystallized or amorphized by switching laser light between two levels. When the recording film is irradiated with a laser light whose power is large enough to increase its temperature up to its melting point, a portion of the film which is irradiated therewith is amorphized by cooling, while it is crystallized by irradiating it with a laser light whose power is large enough to increase its temperature to a value higher than the crystallization temperature and lower than the melting point and cooling. The recording film of the phase-change type optical disk is formed of chalcogenide such as GeSbTe, InSbTe, InSe, InTe, AsTeGe, TeOx-GeSn, TeSeSn, SbSeBi or BiSeGe, etc., by using a suitable film forming technique such as resistive heating vapor deposition, electron beam vapor deposition or sputtering, etc. The recording film immediately after formation is in a sort of amorphous state and, therefore, the amorphous state recording film is pre-treated to crystallize it as a whole so that an information recording can be done by locally changing the phase of the recording film to amorphous state by laser beam irradiation. As mentioned above, in the phase-change type optical disk, data is recorded as amorphous recording mark on the crystalline recording film. A conventional method of reading the data thus recorded utilizes a difference in reflectance between the amorphous recording mark and the crystalline state portion surrounding the latter and, thus, the thickness of the recording film on the optical disk medium is selected such that the difference in reflectance becomes large enough to clearly distinguish the amorphous portion and the crystalline portion of the recording film. However, when the difference in reflectance between the crystalline portion and the amorphous portion is made large, thermal absorptance becomes different between these two portions. This means that power of laser light required for changing crystalline state to amorphous state becomes different from that required for changing previous amorphous state to new amorphous state. In such case, when the overwriting is performed by using a signal beam, there is produced a variation in the length of recording mark. Since such variation may cause jitter in a reproduced signal, the quality of the reproduced signal is degraded. In order to reduce such jitter contained in the reproduced signal, Japanese Patent Application Laid-open No. Hei 2-73537 published on March 13, 1990 proposes to reduce the difference in reflectance between an amorphous portion and a crystalline portion of a recording layer. The layers structure of the recording medium is determined so that the change in reflectance may be small and an optical phase change or difference of the reflected light may be nearly equal to (1 ± 2n)π. In such case, however, a level of a signal reproduced by using the conventional technique which utilizes the difference in reflectance becomes too small, resulting in a new problem that an S/N ratio of the reproduced signal becomes lowered. JP-A-61 227 237 discloses an optical information reproducing device. The device comprises a source of coherent light and reflects two rays of light to branch the light by a beam splitter, a reflecting mirror to reflect one branch of the light together with reflected light from a recording medium to a detector wherein interference of the two light branches is produced. An object of the present invention is, therefore, to improve the S/N ratio of the reproduced signal of a conventional phase-change type optical disk device. Another object of the present invention is to provide a phase-change type optical disk reproducing system which is capable of improving a jitter characteristics of a signal reproduced from the phase-change type optical disk with using a simple optical system. A further object of the present invention is to provide a novel phase-change type optical disk reproducing system which makes a high density recording possible. The objects are solved by the features of claims 1 and 2. Fig. 1A illustrates a recording track on a recording layer, to or from which an optical head according to the present invention writes or read information; Fig. 1B shows a change of phase of a light reflected from an optical disk with respect to a reference light; Fig. 1C shows a phase of a phase shifted reference light with respect to the reference light; Fig. 1D shows a waveform of an output of an optical detector; and Fig. 2 shows a construction of an optical head according to the present invention. A preferred embodiment of the present invention will be described with reference to Figs. 1A to 1D which show waveforms useful in explaining the principle of the present invention and Fig. 2 which shows a construction of an optical head utilizing the present invention. First, the principle of the present invention will be described with reference to Figs. 1A to 1D. Fig. 1A illustrates a recording track on a crystalline recording layer, to or from which an optical head according to the present invention writes or reads information. Hatched portions in Fig. 1A indicate amorphized portions in the crystalline recording layer. The crystalline recording layer of an optical disk system with which the optical head of the present invention is used is designed such that reflectance of the amorphized portion is substantially equal to that of the crystalline layer while phase change of a reflected light from the amorphous layer is different from that from the crystalline layer. Fig. 1B shows a phase change of a reflected light, that is, a reproduced laser light, with respect to a reference laser light. As shown in Fig. 1B, the reflected light from the amorphous layer is different in phase from that from the crystalline layer by . As will be described later, the reflected light, that is, the reproduced light, and the reference laser light are received by an optical detector provided within the optical head of the present invention. That is, as shown in Fig. 1C, the reference laser light whose phase is different from that of the reproduced light by ψ is received, together with the reproduced laser light, by the optical detector. The laser lights are interfered with each other in the optical detector and a variation of light intensity such as shown in Fig. 1D appears at an output of the optical detector. For example, when an amplitude of the reproduced laser light is equal to that of the phase shifted reference laser light and ψ = 180°, the output level of the optical detector is substantially lowered due to interference of the reproduced laser light and the phase shifted reference laser light when the reflected light from the crystalline layer is received. In this case, the reduction of amplitude of the reflected laser light from a recording mark portion, that is, the amorphous layer, is small since phase relation between the reflected light and the phase shifted reference laser light is out of interference condition and there may be a case where the amplitude becomes larger, depending upon the value of ψ. Fig. 2 shows a construction of an embodiment of the optical head according to the present invention which is applied to the phase-change type optical disk reproducing system. The embodiment is constituted mainly with a laser light source 1, a beam splitter 2, a condenser lens 3, a phase difference plate 4, a reflection mirror 5 and an optical detector 6. A laser light emitted from the laser light source 1 is collimated by a collimator lens 7 to a parallel beam 11. The beam splitter 2 splits the parallel beam 11 into two reference lights. One of the two reference lights is condensed by the condenser lens 3 to a first reference light 12 which is fallen in a phase-change type optical disk 20. A reflected light from the optical disk 20 is passed through the condenser lens 3, the beam splitter 2 and a second condenser lens 7 to the optical detector 6. The other reference light passes through the phase difference plate 4 as a second reference light 13 and through an attenuator 8 and is reflected by the reflection mirror 5. The reflected laser light from the reflection mirror 5 passes through the attenuator 8 and the phase difference plate 4 again and through the beam splitter 2 and is directed to the optical detector 6 through the second condenser lens 7 as a phase shifted reference laser light. The laser light source 1 comprises a semiconductor laser emitting a laser light having, for example, wavelength of 830 nm and output power of 40 mW. The condenser lens 3 has numerical aperture of 0.55 and is mounted on a servo actuator which is not shown. The phase difference plate 4 is of the variable phase difference type having a similar construction to that of the well known Babinet compensator or Soleil compensator. The attenuator 8 disposed between the phase difference plate 4 and the reflection mirror 5 is provided to regulate level of the phase shifted reference laser light at the optical detector to that of the reflected light. Then, an optical disk was prepared such that difference in reflectance between crystalline portion and amorphous portion is small and difference in optical phase therebetween is large, for laser light having wavelength of 830 nm. That is, on a polycarbonate substrate having pregrooving having diameter of 130 mm and thickness of 1.2 mm, a first protective film of a mixture of ZnS and SiO 2 , a recording film of GeSbTe, a second protective film of the same mixture as that of the first protective film and a reflection film of Al were continuously formed in the order by magnetron-sputtering. The first protective film was 150 nm thick, the recording film was 10 nm thick, the second protective film was 60 nm thick and the reflection film was 50 nm thick. In this optical disk, the difference in reflectance at wavelength of 830 nm was 2 % and phase difference was 170°. Overwriting was performed with respect to the above mentioned optical disk by using the optical head shown in Fig. 2 and characteristics of a reproduced signal was evaluated. That is, a signal of 8.4 MHz (Duty 50 %) was recorded on a circular track formed on the optical disk after initialized and having radius of 30 mm by irradiating it with a recording laser light having power of 13 mW, while rotating the optical disk at 3600 rpm, and, then, a signal of 2.2 MHz (Duty 50 %) was overwritten on the same track by irradiating it with an erasing laser beam having power of 7 mW, similarly. Then, the phase difference plate 4 was regulated such that the phase difference between the reproducing laser light and the reference laser light in a portion of the track in which there was no signal recorded becomes 180° and power levels of these laser lights were made equal by using the attenuator 8. A signal reproduced under these conditions included substantially no phase jitter caused by overwriting. As described hereinbefore, according to the present invention, a phase information recorded on an optical disk can be efficiently reproduced by using a simple optical system. According to the present invention, even small phase change can be detected by selecting interference condition, that is, selecting phase condition of the reference laser light, resulting in improved freedom of disk design. Further, the reproducing system of the present invention is particularly suitable for the mark-edge recording system in which information is recorded in edge portion of a recording mark, allowing a high density recording.

Claims: An optical disk device comprising : a) a phase-change type optical disk medium (20) including a recording film for recording data by changing phase of a portion of said recording film to either crystalline state or amorphous state; b) a light source (1) for emitting reference light; c) a beam splitter (2) for dividing the reference light (11) from said light source into first reference light (12) and second reference light (13), for supplying the first reference light to said phase-change type optical disk medium, for passing the first reference light reflected by said phase-change type optical disk medium, the first reference light reflected by said phase-change type optical disc medium being phase-shifted by data recorded thereon and for passing reflected second reference light; d) a reflection plate (5) for reflecting said second reference light to said beam splitter as said reflected second reference light; e) optical detector means (6) for receiving the reflected second reference light from said beam splitter and the first reference light reflected by said phase-change type optical disk medium and passed through said beam splitter and for outputting an electrical signal corresponding to the recorded data; f) a phase-shifting and attenuating means (4,8) for phase-shifting and attenuating said second reference light and supplying phase-shifted and attenuated second reference light to said reflecting plate and for receiving reflected light from said reflecting plate and for phase-shifting and attenuating said reflected light and supplying said second reflected reference light to said optical detector means, wherein g) said phase-shifting and attenuating means regulates the phase difference between said second reflected reference light and the first reference light, which is reflected at either a crystalline portion of said recording film or an amorphous portion of said recording film, to 180° in a portion in which there was no signal recorded, and h) said phase-shifting and attenuating means regulates a level of said second reflected reference light to that of the first reference light reflected by said phase-change type optical disk medium. An optical head for writing and reading out a recording data on a recording film of a phase-change type otpical disk medium (20) for recording the recording data by changing phase of said recording film to one of crystalline state and amorphous state corresponding to the recorded data, said recording film having a thickness selected such that reflectance of said recording film in crystalline state and reflectance of said recording film in amorphous state for a reference light are substantially the same and phase of the reflection light from said recording film in crystalline state and the reflection light from said recording film in amorphous state are different, said optical head comprising: a) a light source (1) for emitting a reference light; b) a beam splitter (2) for dividing the reference light (11) from said light source into first reference light (12) and second reference light (13), for supplying the first reference light to said phase-change type optical disk medium, for passing the first reference light reflected by said phase-change type optical disk medium, the first reference light reflected by said phase-change type optical disc medium being phase-shifted by data recorded thereon and for passing a reflected second reference light; c) a reflection plate (5) for reflecting said second reference light to said beam splitter as said reflected second reference light; d) optical detector means (6) for receiving the reflected second reference light from said beam splitter and the first reference light reflected by said phase-change type optical disk medium and passed through said beam splitter and for outputting an electrical signal corresponding to the recorded data; e) a phase-shifting and attenuating means (4,8) for phase-shifting and attenuating said second reference light and supplying phase -shifted and attenuated second reference light to said reflecting plate and for receiving reflected light from said reflecting plate and for phase-shifting and attenuating said reflected light and supplying said second reflected reference light to said optical detector means,wherein f) said phase-shifting and attenuating means regulates the phase difference between said second reflected light and the first reference light, which is reflected at either a crystalline portion of said recording film or an amorphous portion of said recording film, to 180° in a portion in which there was no signal recorded, and g) said phase-shifting and attenuating means regulates a level of said second reflected light to that of the first reference light reflected by said phase-change type optical disk medium.

Inventors: OKADA MITSUYA, OKADA, MITSUYA, Okada, Mitsuya, c/o NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1993
Main classification: G11B7
IPC codes: G11B7