Method for initializing an optical disk

Abstract: In a method for initializing a phase-change type of optical disk, a light is irradiated onto a recording layer for initializing the optical disk. The light has a strength so that the relation between absorbing rates "Ai" of an initialized crystal region and "Ae" of an erased crystal region meets the following formula: |Ai - Ae| / Ae < 0.05

Description: FIELD OF THE INVENTION The invention relates to a method for initializing a phase-change type of optical disk, especially to a method for initializing an optical disk on which mark edge recording is carried out. BACKGROUND OF THE INVENTION These days, there are several types of optical disks in which information is recorded, erased and reproduced by a laser beam, which are an optical magnet disk, phase-change type of optical disk. In a phase-change type of optical disk, a light is irradiated onto the surface of the disk to change the phase condition to record information, erase the recorded information and reproduce the recorded information. Usually a phase-change type of optical disk includes a substrate, a lower protection layer provided on the substrate, a recording layer provided on the lower protection layer, an upper protection layer provided on the recording layer and a reflection layer provided on the upper protection layer. When the recording layer is heated to be melt and suddenly cooled down so as to become non-crystal condition, information is recorded thereon. On the other hand, when the recording layer is maintained to be at a predetermined temperature so as to become crystal condition, the recorded information is erased. The recorded information can be reproduced by using the difference in reflection rate between crystal region (non-recorded) and non-crystal region (recorded). For high density recording, a mark-edge recording by which information is recorded on the both ends of a recording mark is useful. In a phase-change type of optical disk, crystal and non-crystal regions are different in absorbing rate from each other. When overwriting information, the length of a record mark changes depending on the condition of the recording layer, crystal or non-crystal. To prevent this kind of problem, an optical disk having a reflection layer which is of Si or Au having a thickness of less than 20nm has been proposed to increase the absorbing rate of a crystal region higher than that of a non-crystal region. DC erasing can be used for the same purpose, however, overwriting can not be carried out. A phase-change type of optical disk which has been just fabricated has a recording layer of non-crystal condition, so that the disk is required to be initialized before use. For initializing such a disk, a recording layer is crystallized by irradiation laser beam or flash lamp thereto. There have been proposed some conventional methods for initializing a phase-change type of optical disk, that are described in Japanese Publication Kokai 3-278338 and 4-209317 to increase erasing rate and C/N or to shorten initializing time. In publication 3-278338, a laser beam having a larger diameter is used to maintain time for crystallization. In publication 4-209317, a recording layer has inconsistent composition. According to the conventional technology in a phase-change type of optical disk, recording characteristics change on between the beginning, which is just after initialization, and after some re-written. Especially, in mark edge recording, recording error is increased. A method for initializing an optical disk having a phase changing recording layer, with the initializing being performed by irradiating a light beam onto the recording layer, are disclosed for example in JP-A-03 216 816 and in JP-A-04 366 424, which reflect the preamble of appended claims 1 and 7. SUMMARY OF THE INVENTION Accordingly, an object of the invention is to provide an improved method for initializing a phase-change type of optical disk by which the recording characteristics of the disk do not change in many-time use. A method according to the present invention is defined in independent claims 1 and 7. The dependent claims define particular embodiments of the invention. According to an embodiment of the invention, in a method for initializing an optical disk, a light is irradiated onto a recording layer for initializing the optical disk, the light having a strength so that the relation between absorbing rates "Ai" of an initialized crystal region and "Ae" of an erased crystal region meets the following formula: Ai - Ae / Ae < 0.05 Preferably, "Ai" = "Ae". According to another embodiment of the invention, in a method for initializing an optical disk, a light is irradiated onto a recording layer for initializing the optical disk, the light having a strength so that the relation between the length of first recorded mark region "Li" and the length "Le" of a mark after second time recording meets the following formula: Li - Le < 0.05µm Preferably, "Li" = "Le". BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a conceptual view showing a phase-change type of optical disk. Fig. 2 is a cross-sectional view showing recording layers in different conditions. Fig. 3 is a graph showing the relation between initializing power and reflection rates. Fig. 4 is a graph showing the relation between initializing power and reflection and absorbing rates. Fig. 5 is a graph showing the relation between recording power and carrier level in a first preferred embodiment according to the invention. Fig. 6 is a graph showing the relation between the number of recording and 2nd H/C (dB) in the first preferred embodiment. Fig. 7 is a table showing test result of the first preferred embodiment. Fig. 8 is a graph showing the relation between initializing power of a laser beam and absorbing rate in a second preferred embodiment according to the invention. Fig. 9 is a table showing test result of the second preferred embodiment. Fig. 10 is a graph showing the relation between initializing energy of a flash lamp and absorbing rate in a third preferred embodiment according to the invention. Fig. 11 is a graph showing the relation between the number of recording and 2nd H/C (dB) in the third preferred embodiment. Fig. 12 is a table showing test result of the third preferred embodiment. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 shows a general structure (cross-sectional view) of a phase-change type of optical disk, which includes a substrate 1, a lower protection layer 2 provided on the substrate 1, a record layer 3 provided on the lower protection layer 2, an upper protection layer 4 provided on the record layer 3 and a reflection layer provided on the upper protection layer 4. Fig. 2 shows inside of the record layer in different conditions of just initialized, recorded and erased. Generally in a mark-edge recording, in which information to be recorded can be changed by adjusting the length thereof, the lengths of information marks should be controlled precisely. As shown in Fig. 3, the reflection rate and absorbing rate change depending on initializing power. The recording sensitivity of the first record varies depending on absorbing rate "Ai" of initialized record layer which is crystal, and the recording sensitivity of the record layer after more then twice recording varies depending on the absorbing rate "Ae" of erased crystal region. The absorbing rate "Ae" is a uniform value for the structure of the disk, however the absorbing rate changes by use of the disk, which means "Ai" and "Ae" are not equal always. For example, an optical disk is initialized by using a laser beam having a diameter of 1.5µm, the optical characteristics of the disk changes depending on the power of the laser beam, as shown in Fig. 3. If the optical disk is initialized by a laser beam having a power of "Pi1" which makes the relation of "Ai" < "Ae", the difference in reflection between crystal and non-crystal regions is large, so that C/N is high. However, erased region which has been recorded and erased some times has a higher sensitivity relative to that of an initialized region; and therefore, record marks are getting longer as time. In contrast, if the optical disk is initialized by a laser beam having a power of "Pi0" which makes the relation of "Ai" = "Ae", no difference in reflection between crystal and non-crystal regions occurs, and therefore, record marks do not change in length as time. The appropriate power "Pi0" of an initializing light, which makes the relation of Ai = Ae, is given by as follows, because the change of lengths of marks affect second harmonics distortion (2nd H/C): First, several lights having different power are irradiated onto a record region to be crystallized to make several different initialized regions on the disk, then each region is recorded and erased several times by the same power of light. If a region has a uniform second harmonics distortion, the region can be considered has to been initialized by a light having the appropriate initializing power Pi0 which makes the relation of Ai = Ae. If the disk is initialized by using the light having power Pi0, stable recording characteristics can be realized in long time. Next, preferred embodiments of the invention are described. [First Embodiment] Referring to Fig. 1 again, the substrate 1 is made of polycarbonate (PC), the lower protection layer 2 is made of ZnS-SiO 2 having a thickness of 250nm, and the record layer 3 is made of Ge 2 Sb 2 Te 5 having a thickness of 25nm, the upper protection layer 4 is made of ZnS-SiO 2 having a thickness of 20nm, and the reflection layer 5 is made of aluminum having a thickness of 60nm. These layers are fabricated by sputtering technique. The fabricated optical disk is initialized by lights having different power, then the disk is tested in length of marks and error rate responding to alternative record and erase. For initialization is performed by using a semiconductor laser having a wavelength of 830nm, a beam diameter of 1.5µm and an irradiation speed of 7.5m/s. The power of the laser beam is changed in 4, 5, 6 and 8mW. Depending on the different initializing power, the reflection rate and absorbing rate of the optical disk change as shown in Figs. 4 and 5. For each region initialized by the different power, the appropriate recording power, by which the second harmonics distortion (2nd H/C) of a recording signal is minimized in the first record processing, is given, as shown in Fig. 6. Each region is alternatively recorded and erased 100 times by the same power of light at 11.3m/s. The difference of length of marks between "Li" for first recording and "Le" after plural times recording and error rate of each region is detected, as shown in Fig. 7. As shown in Fig. 7, with each of the initializing power 5mW and 6mW, which meets a condition "|Ai - Ae| / Ae < 0.05", the disk meets a condition "|Li - Le| < 0.05µm" and error rate of less than 10 -5 . [Second Embodiment] Referring to Fig. 1 again, the substrate 1 is made of polycarbonate (PC), the lower protection layer 2 is made of ZnS-SiO 2 having a thickness of 100nm, and the record layer 3 is made of Ge 2 Sb 2 Te 5 , having a thickness of 15nm, the upper protection layer 4 is made of ZnS-SiO 2 having a thickness of 20nm, and the reflection layer 5 is made of aluminum having a thickness of 60nm. These layers are fabricated by sputtering technique. The fabricated optical disk is initialized by using a semiconductor laser having a wavelength of 830nm, a beam diameter of 100µm and an irradiation speed of 2m/s. The power of the laser beam is changed in 530, 550, 580 and 600mW. After the initializing, recording characteristics are detected using a laser beam having an irradiation speed of 11.3m/s. Depending on the different initializing power, the absorbing rate of the optical disk change as shown in Fig. 8. For each region initialized by the different power, the appropriate recording power, by which the second harmonics distortion (2nd H/C) of a recording signal is minimized in the first record processing, is given. Each region is alternatively recorded and erased 100 times by the same power of light at 11.3m/s. The difference of length of marks between "Li" for first recording and "Le" after plural times recording and error rate of each region is detected, as shown in Fig. 9. As shown in Fig. 9, with each of the initializing power 550mW and 580mW, which meets a condition "|Ai - Ae| / Ae < 0.05", the disk meets a condition "|Li - Le| < 0.05µm" and error rate of less than 10 -5 . [Third Embodiment] Referring to Fig. 1 again, the substrate 1 is made of polycarbonate (PC), the lower protection layer 2 is made of ZnS-SiO 2 having a thickness of 100nm, and the record layer 3 is made of Ge 2 Sb 2 Te 5 having a thickness of 15nm, the upper protection layer 4 is made of ZnS-SiO 2 having a thickness of 40nm, and the reflection layer 5 is made of aluminum having a thickness of 60nm. These layers are fabricated by sputtering technique. The fabricated optical disk is initialized by using a flash lamp to be changed in strength. After the initializing, recording characteristics are detected. Depending on the different energy of the flash lamp, the absorbing rate of the optical disk change as shown in Fig. 10. For each region initialized by the different energy of light, the appropriate recording power, by which the second harmonics distortion (2nd H/C) of a recording signal is minimized in the first record processing, is given, as shown in Fig. 11. Each region is alternatively recorded and erased 100 times by the same power of light at 11.3m/s. The difference of length of marks between "Li" for first recording and "Le" after plural times recording and error rate of each region is detected, as shown in Fig. 12. The test is carried out in the same condition as the first embodiment, in which the mark length of 3.39MHz signal is detected. As shown in Fig. 12, with the initializing power 2.2KJ, which meets a condition "|Ai - Ae| / Ae = 0", the disk has an error rate of less than 10 -5 . As described above, the phase-change type of optical disk according to the first to third preferred embodiments has a small variation of mark length in a mark-edge recording, so that stable recording can be realized.

Claims: A method for initializing an optical disk, on which information is recorded, erased and reproduced by changing phase condition between crystal and non-crystal, comprising the steps of: providing a recording layer; and irradiating a light onto the recording layer for initializing the optical disk, said method being characterized by the light having a strength so that the relation between absorbing rates "Ai" of an initialized crystal region and "Ae" of an erased crystal region meets the following formula: Ai - Ae / Ae < 0.05 The method according to claim 1, wherein: said "Ai" and "Ae" are equal to each other. The method according to claim 1, wherein: said information is recorded by a mark-edge recording technique. The method according to claim 1, wherein: said light for initializing is generated by a semiconductor laser. The method according to claim 1, wherein: said light for initializing is generated by a flash lamp. A method according to claim 1, further characterized by the step of: providing a reflection layer on the recording layer. A method for initializing an optical disk, on which information is recorded, erased and reproduced by changing phase condition between crystal and non-crystal, comprising the steps of: providing a recording layer; and irradiating a light onto the recording layer for initializing the optical disk, said method being characterized by the light having a strength so that the relation between the length of first recorded mark region "Li" and the length "Le" of a mark after second time recording meets the following formula: Li - Le < 0.05µm The method according to claim 7, wherein: said "Li" and "Le" are equal to each other.

Inventors: OHKUBO SHUICHI, OHKUBO, SHUICHI, OHKUBO, SHUICHI, C/O NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1995
Main classification: G11B7
IPC codes: G11B7