JPH02302932A - Optical recording and reproducing system - Google Patents

Abstract

PURPOSE:To improve a CN ratio and recording density by making the wavelength of a reproducing laser light shorter than the wavelength of a recording laser light, and reproducing a signal. CONSTITUTION:For a means to make the wavelength of the reproducing laser light shorter than the wavelength of the recording laser light and to reproduce the signal, as a small output semiconductor laser to output an irradiation energy in the degree of 1mW through an optical system, the semiconductor laser of the short wavelength small output of a wavelength 600nm order is in the course of a practical use, and when a second higher harmonics generating element 21 to utilize a non-linear effect is combined to a long wavelength laser 11, even a reproducing laser light 93 of a wavelength 400nm order can be obtained. In such a way, by shortening the wavelength of the reproducing laser light 93, the convergency limit can be contracted to <=1.2mu, at most to the degree of 0.76mum. Thus, without lowering the CN ratio, the interval mutually between recording spots and the interval mutually between information tracks can be contracted, and the recording density of an optical disk 1 and a signal transfer speed can be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for recording, reproducing, or rewriting signals on a rewritable optical recording medium by focusing a laser beam onto a minute spot.

[Conventional technology]

In recent years, there has been an increasing demand for higher density and larger capacity information recording, and research and development for this purpose has been actively conducted both domestically and internationally. has a recording density approximately 10 to 100 times that of conventional magnetic recording media, and
Since information can be recorded and reproduced without contact between the playback head and the recording medium, the recording medium is less likely to be damaged and has a long lifespan. This is promising as a large-capacity recording method.

These optical discs can be roughly classified into six types depending on their purpose: read-only type, write-once type, and rewritable type.

The read-only type is a read-only disk from which information can only be read, and the write-once type allows information to be recorded and reproduced as needed, but the recorded information cannot be erased. On the other hand, the rewritable type is capable of recording and reproducing information, as well as erasing and rewriting recorded information, and is the most promising type as it is desired to be used as a data file for computers. .

Regarding rewritable disks, two recording methods are being developed: magneto-optical and phase change, but both methods still have room for improvement in terms of recording materials, writing mechanisms, etc. . Among these methods, the phase change method generally focuses a laser beam on the recording surface of the disk, heats it, and controls the pulse output and pulse width of the laser beam, resulting in a phase change of the recording material, that is, from a crystalline state. It causes a transition to an amorphous state, and records and erases information based on the difference in reflectance in each state.

The structure of this phase change type optical disk is usually such that a protective layer made of a ceramic layer such as zinc sulfide (ZnS) is formed on the surface of a transparent substrate such as polycarbonate with many tracking grooves, and optical recording is performed on the protective layer. It has a structure in which a protective layer made of ceramic or the like and an organic surface preservation layer are sequentially deposited on the protective layer. Also,
Aluminum (AI) between the protective layer and the organic surface protective layer
Reflection and cooling layers have also been added. reflection,
The cooling layer reflects the laser beam to increase the efficiency of light utilization, and at the same time increases the cooling rate from the molten state when changing from the crystalline state to the amorphous state. acts as a layer of heat insulation 6. In order to ensure the characteristics of the medium, it is important to optimize the thickness of this heat insulating layer.

Laser light is generally applied from the opposite side of the substrate to the side on which the optical recording layer is provided. In a normal optical disc, the optical recording material is in a crystalline state in the initial state, and when recording information, it is irradiated with a recording laser beam and the irradiated area is completely melted.
It is rapidly cooled to form a recording spot in an amorphous state. During erasing, this non-crystalline recording is annealed by a sporatra laser to return it to a crystalline state. In addition, during playback, a playback laser beam with a weaker intensity than the recording laser beam is irradiated so as not to change the amorphous state of the recording spot, and a photodetector detects the different reflectance of the laser beam between the crystalline state and the amorphous state. The signal can be played back.

When recording a signal, the recording laser beam requires a large amount of energy in order to raise the temperature of the entire optical recording layer above its melting point temperature to form an amorphous recording spot. Therefore, a semiconductor laser with an output of several tens of mW is generally used. This type of semiconductor laser currently in practical use is a long-wavelength, high-output semiconductor laser with a wavelength of about 830 nm, and its output laser beam is focused by an optical system to obtain a recording laser spot with a distribution of irradiation intensity and The temperature distribution of the optical recording layer heated by this is as shown in FIG. position) K
Although it shows a Gaussian continuous distribution with a peak value, the spread of the temperature distribution in the in-plane direction of the optical recording layer is due to the fact that the thermal diffusion at the outer periphery of the recording spot is much larger than that at the center. Smaller width than spread 9, curve 20
0 indicates a characteristic that is sharper than the curve 100 〇 Shows this kind of irradiation intensity distribution Focusing limit (minimum spot diameter) of laser light Letting the aperture ratio of the lens be A, it is expressed by the following equation.

X=λ/A ・・・・・・・・・・・・・・・
(1) That is, assuming that the dashed line 201 in the figure is the writing level to the optical recording layer, the minimum diameter X of the recording spot.

If the wavelength of the recording laser beam is λTh83Qnm and the aperture ratio of the objective lens is A'k O,55, then Xo = 1,
The lower limit is 5 μm. However, the recording spot diameter Φ
0 is smaller than XO due to the cooling effect mentioned above, and is approximately 1μ
It can be about m.

[Problem to be solved by the invention]

In the reproducing operation for reading information recorded on an optical disk, the output of a long-wavelength, high-output semiconductor laser is irradiated at a weak level with the aperture of 9 so that the output is sufficiently lower than the melting point level of the optical recording layer, and the amorphous state is This is done by detecting the reflected light from the recording spot, but in the conventional method, one high-power semiconductor laser is used for reproduction, so the wavelength of the laser light is 83Qnm, the same as the recording laser spot, and ) Light gathering limit XO ke 1, 5
ttm is the lower limit.

FIG. 7 is an explanatory diagram showing a conventional playback situation, in which information tracks 80 of an optical disk rotating at a constant speed (for example, 1800 revolutions/minute) are spaced apart from each other by 1 μm, and Φo=
Recording spot 81 recorded at 1 μm, Xo=1.5
A state in which a reproduction laser spot 91 of .mu.m is irradiated and its reflected light is detected to obtain a reproduction signal 300 converted into an electric signal is shown. In this case, since there is a point in time when the reproduction laser spot 91 simultaneously irradiates two recording spots 81, a problem arises in that the signal-to-noise ratio (hereinafter referred to as CN ratio) of the reproduction signal 300 decreases. In addition, in order to prevent a decrease in the CN ratio, it is necessary to widen the distance between the recording spots 81 to a total of 1.58 m or more.
This causes a problem in that the recording density of the information track 80 and the transfer speed during reproduction are restricted. Furthermore, in order to suppress the mixing of signals between adjacent information tracks 80 (referred to as crosstalk noise), the groove pitch should also be set to 1.
It is necessary to set the thickness to 5 μm or more (1.6 μm according to the conventional standard), which poses the problem that the in-plane recording density of the optical disc is restricted.

An object of the present invention is to increase fi, CN ratio, and recording density by reducing the focusing limit of a reproduction laser beam.

[Means to solve all problems]

In order to solve the above problems, according to the present invention, a recording laser beam focused on a minute spot is irradiated onto an optical recording medium and a signal is recorded based on a reversible optical change,
This optically changed portion is irradiated with a reproduction laser beam weaker than the recording laser beam, and the optical change is detected to reproduce the signal, the wavelength of the reproduction laser beam being set at a wavelength lower than that of the recording laser beam. The signal will be reproduced by shortening the length.

[Effect]

The above means has a wavelength of 600 as a low-power cow conductor laser that can output irradiation energy of about 1 mW through an optical system.
Semiconductor lasers with short wavelengths on the order of nm and low output power are being put into practical use, and when a long wavelength laser is combined with a second harmonic generation element that utilizes nonlinear effects, it is possible to achieve wavelengths of 400 nm.
This method was designed with the focus on the fact that it is possible to obtain a reproduction laser beam on the order of nm.By shortening the entire wavelength of the reproduction laser beam, the convergence limit of the reproduction laser beam can be reduced to 1.2 μm or less. Since it can be reduced to about 76 μm, it is possible to reduce the spacing between recording spots and the spacing between information tracks without reducing the CN ratio, thereby significantly improving the recording density and signal transfer speed of the optical disk. I can do it.

〔Example〕 The present invention will be explained below based on examples.

FIG. 1 is a schematic configuration diagram of an apparatus for explaining an embodiment method of the present invention, and FIG. 2 is a diagram illustrating the principle of the embodiment method. In the figure, 1 is an optical disc as an optical recording medium, and on one side of a transparent disc substrate 1A made of polycarbonate, a ceramic protective layer IB made of zinc sulfide (ZnS), germanium (Ge2) and antimony (Sb
2) Totellurium (Te, ) (D compound (Ge, 5k
)zTeB ), an optical recording layer IC consisting of ZnS ceramic protective layer 1D, aluminum reflective layer 1E, and U
Sputtered films are formed in the order of V resin-based protective layer 1F. A recording/erasing optical head 2 and a reproducing optical head 6 are arranged on the disk substrate 1A side of the optical disk 1 which rotates at a constant speed (1800 revolutions per minute) with a full gap maintained therebetween. The recording/erasing optical head 2 is equipped with a long-wavelength, high-output semiconductor laser light source that emits a laser beam with a wavelength of 830 nm, and a projection optical system that focuses this laser beam and projects it onto the information track of the optical disk 1, and forms a recording laser spot 90. Output 15
A laser beam of mW is applied to the optical recording layer 1C. The reproduction optical head 6 includes a short wavelength, low output semiconductor laser light source that emits a laser beam with a wavelength of 670 nm, its projection optical system, and a source detection section that detects reflected light, converts it into an electric signal, and outputs a reproduction signal. A reproduction laser spot 91 with a laser output of 1 mW is projected onto the optical recording layer 1C.

When information is recorded using the optical disk and optical head configured as described above, as shown in FIG. , the writing level 201 required to raise the temperature of the optical recording layer 1C to its melting point temperature.
While the focusing limit XO is 1°5 μm, the distribution of the irradiation light amount of the reproduction laser spot 91 emitted from the reproduction head 6 is at the reproduction level 220 necessary to obtain the necessary reproduction signal, as shown by the curve 120. Focusing limit X1 at
is reduced to 1.2 μm.

Therefore, the optical disc 1 is divided into layers IB, 10. ID Oyobi I
E each thickness Th120nm, 6Qnm+17
A disk with a diameter of 130 mm with information tracks formed at intervals of 1°6 μm and rotating at 1800 revolutions per minute,
15 focused to a diameter of 1.5 μm from the recording/erasing head 2
A recording laser spot 90 of mW is irradiated with a frequency of 4.
A 6 MHz pulse signal was written into the optical recording layer 1c. Next, from the reproducing head 3, a 1.2 μm diameter K focusing laser 1 is connected.
When reproduction was performed by irradiating a reproduction laser spot 91 of mW and 670 nm, a reproduction signal with a CN ratio of 47 dB was obtained. Similarly, a conventional reproducing laser spot with a wavelength of 830 nm focused to 1.5 μm from the reproducing head (Figure 2 shows the irradiation light amount distribution as curve 102)'fr: As a result of irradiation and finding the CN ratio, it was 33 dB. Data was obtained, and it became clear that in order to obtain the practically required CN ratio of 45 dB, it was necessary to lower the frequency of the recording pulse signal to 6.7 MH2 or less. From this result, by reading the signal using a reproduction laser beam that shortens the signal recorded with a laser beam with a wavelength of 830 nm to 670 nm, changing the signal frequency can improve the signal transfer speed and the recording of the signal on the C disk. The density can be increased to 1.2 times that of the conventional method.

Next, the interval between the information tracks of the optical disc 1 described above is set to 1.
The signal recorded in the same manner as above after being shortened to 4 μm was converted to a wavelength of 67 μm.
The entire reproduction laser spot 91 of 0 nm and imW was irradiated and read out. As a result, the CN ratio of the obtained reproduced signal was 4.
It was 6dB. 'Furthermore, the crosstalk noise between tracks was -30 dB with respect to the reproduced signal. Similarly,
The CN ratio when reproduced with a laser beam with a wavelength of 830 nm is 3 QdB.

Crosstalk noise was -i 8dB. It is practically desirable that the crosstalk noise be -25 dB or less, and in the method of the embodiment, it is possible to obtain performance that meets the required level with a margin even if the track spacing is shortened to 1.4 μm. Recognize.

In this way, the method of this embodiment can increase the recording density by about 1.4 times that of the conventional method.

Next, a description will be given of the specific constituent sounds of the optical head that makes the embodiment method of the present invention possible, and the recording and reproducing methods and tables for the case where the recording/erasing head and the reproducing head are integrated.

FIG. 3 is a schematic configuration diagram of an optical head showing a different embodiment method of the present invention, in which a laser beam (wavelength 830 nm) emitted from a long wavelength semiconductor laser 11 is reflected by a polarizing beam splitter 13 and then sent to a polarizing beam splitter 15. The laser beam passes through the laser beam, is focused by the objective lens 16 with an aperture ratio A of about 0°55 into a recording laser spot 90 with a focusing limit ZO of 1°5 μm, and is projected onto the information track 80 of the optical disc 1, where it changes to an amorphous state. A recording spot 81 with a diameter of about 1 μm is formed. Further, the output laser beam (wavelength 670 nm) from the short wavelength semiconductor laser 12 is reflected by the polarizing beam splitter 16 to the opposite lens side, and when reflected by the mirror 14, the polarization plane is rotated by 90 degrees, and the polarizing beam splitter 16 and 15 ′! il-transmitted, objective lens 1
6, the reproducing laser spot 92 with a focusing limit Z of approximately 1.2 μm is focused and projected onto the recording spot 81. The reflected light from the recording spot 81 is converted into parallel light by the objective lens 16, reflected by the polarizing beam splitter 15, and converted into a reproduction signal by the photodetector 17. In this embodiment, the recording/erasing optical head and the reproducing optical head are integrated, which simplifies the configuration of the optical system.As with the previous embodiment, the CN ratio is 46 dB or more, and the crosstalk is -25 dB or more. A noise level, a transfer rate 1.2 times higher than that of the conventional method, and a recording density of 1.2 to 1.4 times that of the conventional method can be obtained.

FIG. 4 is a schematic configuration diagram of an optical head showing another embodiment of the present invention, in which long wavelength semiconductor lasers 11 are provided at two locations, and the output beam of one of the semiconductor lasers has a wavelength of 830 nm using a nonlinear effect. It is converted into a short wavelength laser beam with a wavelength of 415 nm by the second harmonic generation element 21, and is passed through the entire beam shaper 22 to the polarizing beam splitter 13. Mirror 14. Polarizing beam splitter 15. Objective lens 16
This differs from the methods of the previous embodiments in that the reproduction laser spot 93 is projected onto the optical disk 1 via a projection optical system consisting of The light collection limit determined by
The in-plane recording density can be increased four times that of the conventional method.

FIG. 5 is a configuration diagram of an optical head showing another different embodiment method, in which one long-wavelength high-output semiconductor laser light source 11
The emitted laser beam is switched by a beam switcher 31 into an optical path for recording/erasing and an optical path for reproducing, and when recording or erasing, it is made incident on the polarizing beam splitter 16 of the projection optical system by a mirror 32, and when reproducing, it is made to enter the polarizing beam splitter 16 of the projection optical system. The laser beam whose wavelength has been shortened from 33 Q nm to 415 nm by the harmonic generation element 21 is made to enter the polarizing beam splitter 13 of the projection optical system in the opposite direction to that during recording via the beam shaper 22 and mirror 36. This point is different from the method of the above-mentioned embodiment. As the beam switch 61, the normally used acoustic port 1? It is possible to apply methods such as using a grating or using mechanical rotation of a mirror, and it is possible to reduce the focusing limit to X'kO076 μm during reproduction using a single long-wavelength high-output semiconductor laser light source. can.
- [Effects of the Invention] As described above, the present invention is configured so that the wavelength of the laser beam during reproduction is shorter than that during recording. the result,
A conventional method in which a large-capacity semiconductor laser, which is required to write or erase information by heating the optical recording layer of an optical disc to its melting point temperature, is configured to reduce its output during playback to form a weak playback laser spolatra. Then, the wavelength of the output laser light of the large capacity semiconductor laser is 830n.
However, it is possible to generate laser light with a wavelength of 670 nm or 415 nm by providing a small-capacity semiconductor laser for reproduction only or by using a second harmonic generation element. As a result, it is possible to reduce the focusing limit to 1.2 μm or 0.76 μm, and based on this, it is possible to improve the in-plane recording density of optical disks to about 1.4 or 4 times that of conventional methods, and to improve recording performance. The transfer speed of the transmitted signal has been reduced to 1
．． It can be improved by a factor of two or two. Further, since the recording density and transfer speed can be improved using the same rewritable optical disk as in the past, there is an advantage that it can contribute to increasing the recording capacity and high performance of computers, for example.

[Brief explanation of drawings]

FIG. 1 is a schematic configuration diagram of an apparatus for explaining an embodiment method of the present invention, FIG. 2 is a diagram explaining the principle of the embodiment method, and FIG.
The figure is a schematic configuration diagram of an optical head showing different embodiment methods;
Fig. 4 is a block diagram of an optical head showing another embodiment method, Fig. 5 is a schematic block diagram of an optical head showing another different embodiment method, and Fig. 6 is a recording laser beam irradiation intensity showing a conventional method. and a distribution diagram of recording layer temperature, and FIG. 7 is an explanatory diagram of a conventional reproduction method. 1... Optical recording medium (optical disc), 2... Recording/erasing optical head (including long wavelength high output semiconductor laser),
6... Reproduction optical head (including short wavelength low output semiconductor laser), 11... Long wavelength semiconductor laser, 12...
・Short wavelength semiconductor laser, 13.15...Polarizing beam splitter, 14.32.33...Mirror, 16...
- Objective lens, 17... Photodetector, 21... Second harmonic generation element, 22... Beam shaper, 31... Beam switcher, 80... Information track, 81... Recording spot, 90...recording laser spot), 91°9
2.93...Reproduction laser spot, XO...Diameter of recording laser spot, Xl...Diameter of reproduction laser spot. □°”・, 1 Representative Patent Attorney Iwao Yamaguchi ″・ 1,'I...Mino? L door・ ←Tsuka Hironobu C Iiwahi wife Tsubo amount Δ Ko-suke 6 rebellion trap Inashu Ki Rinjo Misaki 6 Nephew sword

Claims (1)

[Claims] 1) Irradiate the optical recording medium with a recording laser beam focused on a minute spot, record a signal based on a reversible optical change, and irradiate this optically changed part with a reproduction laser beam that is weaker than the recording laser beam. An optical recording and reproducing method which detects the optical change and reproduces a signal, wherein the wavelength of the reproducing laser beam is made shorter than the wavelength of the recording laser beam to reproduce the signal.