JPH0335436A - Information recorder - Google Patents

Abstract

PURPOSE:To enable high density recording with a reproducing spot diameter being smaller than a recording spot diameter by recording information with a fundamental wave of light emitted from one laser light source and reproducing the information with the n-th higher harmonics. CONSTITUTION:The subject device is provided with an n-th higher harmonics generator 2 constituted for phase matching with a wavelength of the fundamental wave in the case of low power of the fundamental wave emitted from the laser light source 1. Then, the information is recorded by the fundamental wave of light emitted from one laser light source 1, while the information is reproduced by the n-th higher harmonics, so that since the information is reproduced by the n-th higher harmonics, the reproducing spot can be made smaller than the recording spot. By this method, even when the recording spots are close to each other, the information can be reproduced, and hence high recording densification can be achieved by a close-set amt. of the recording spots.

Description

[Detailed Description of the Invention] [Objective of the Invention] (Industrial Application Field) This invention records and records information by irradiating an information recording medium such as an optical disk with a light beam emitted from a laser light source such as a laser diode. The present invention relates to an information recording device for reproducing, or recording, erasing, and reproducing.

(Prior Art and Problems to be Solved by the Invention) In recent years, disk drives have been attracting attention as a large capacity memory. Optical discs are divided into read-only types such as compact discs, laser discs, and CD-ROMs, write-once types such as electronic document files and image files, and alternatives to hard disks and floppy disks. It is broadly classified into 3FJ types, which are rewritable type.

Among these, write-once type and rewritable type optical disks had the advantage of having a capacity about 10 times larger than that of conventional magnetic disks at the time of their development.

However, the recording capacity of optical disks is still not sufficient, and due to recent rapid advances in magnetic recording technology, magnetic disks with recording capacities close to the recording density of optical disks are being developed. There is a demand for higher recording densities.

Various methods of increasing the recording density of optical discs have been studied in the past.

For example, attempts have been made to make the recording bits smaller by reducing the spot diameter of the laser beam for recording, and to reduce the intervals between the bits. As is generally well known, if the spot diameter of the laser beam is d, the numerical aperture of the objective lens used to irradiate the optical disk with the laser beam is NA, and the laser wavelength is λ, then dcx3λ/NA Since there is a relationship between
Alternatively, λ may be made smaller.

When increasing the NA, it is possible to reduce the spot diameter of the laser beam focused on the optical disk, but since the depth of focus becomes smaller, the laser beam may There is a problem in which the beam tracking and focusing servos do not work. Considering these circumstances, the current NA is 0.
The upper limit is about 55 to 0.6, which cannot be said to be sufficient from the point of view of increasing recording density.

On the other hand, the wavelength of semiconductor lasers currently on the market is 780 nm.
It is n- or 830 na+, and research is being conducted to shorten the wavelength to 680 rv, but at present it is not available at a commercial level.

In order to reduce λ, attempts have been made to extract only the second harmonic of a semiconductor laser and use it for recording and reproducing optical discs. In this case, if the wavelength of the fundamental wave is 830n11, the wavelength of the second harmonic is 415n11.
m, and the wavelength can be made sufficiently small.

However, the power of the second harmonic is at most 1 mw, which is significantly smaller than the mw (value at the optical disk surface), which is the minimum power required for recording, so it has not yet been put to practical use. .

For non-semiconductor lasers, for example Ar, which has a wavelength of 48
There are 8na and shorter lasers than semiconductor lasers, but these are large and it is unrealistic to use such lasers in the current situation where miniaturization of laser drives is required.

This invention was made in view of such circumstances, and
It is an object of the present invention to provide an information recording device that has a relatively simple configuration and can practically achieve high recording density.

[Structure of the Invention] (Means for Solving the Problems) An information recording device according to the present invention records information by irradiating the fundamental wave onto an information recording medium, and records information by irradiating the n-th harmonic onto an information recording medium. a laser light source for reproducing the wavelength of It is characterized by comprising a harmonic generator and an optical system for condensing and irradiating a light beam from a laser light source onto the information recording medium.

(Function) In this invention, since the n-th harmonic generator is configured to phase match the wavelength of the fundamental wave when the fundamental wave emitted from the laser light source has low power, one laser Information can be recorded using the fundamental wave of light emitted from a light source, and information can be reproduced using the n-th harmonic. In this case, since information is reproduced from the inn harmonic, the reproduction spot can be made smaller compared to the recording spot, and information can be reproduced even if the recording spots are closer than before. . Therefore, the recording density can be increased to the extent that the recording spots are closer together.

(Implementation N) This invention will be explained in detail below.

Generally, the wavelength of light emitted from a semiconductor laser changes depending on the power of the output beam. FIG. 4 shows a specific example. That is, when the laser power is as low as about 1 mW, the wavelength of the fundamental wave is 823 nm, but as the power increases, the wavelength shifts to the longer wavelength side, and when the laser power is as high as 20 mW, the wavelength is 832 ns.

This invention utilizes the difference in the oscillation wavelength of a semiconductor laser depending on the laser power as described above, and uses the fundamental wave for recording and the 0th harmonic for reproduction from one laser light source to generate information. It is used for recording and reproducing. For example, when using second harmonics, when a light beam emitted from a semiconductor laser beam as a laser light source is input to a second harmonic generator, it is phase matched to a laser beam with a wavelength of 828 nm. By adjusting the power as follows, only the fundamental wave incident with a power of 1 mW generates a second harmonic, and when the power is larger than that, for example, 20 mW, no second harmonic is generated.

Figure 5 uses LiNbO3 for the second harmonic generator,
The relative intensity distribution of the fundamental wave and the generated second harmonic is shown when phase matching is performed using a low power laser beam (wavelength: 828 ns). In addition, in FIG. 5, the power density of the fundamental wave in LiNbO3 is ~100 mW/μm, and Δλ
-λoxlo-'. As shown in this figure, the center wavelength λ. ±Δλ, which is the range that generates the second harmonic.
-±828 x 10-'n-, so the wavelength is 83
Even if a laser with a high power of 2 is incident on the second harmonic generator, no second harmonic is generated.

Next, an information recording apparatus according to this embodiment will be explained with reference to FIG. This information recording device includes a laser light source 1 made of a semiconductor laser, and is configured to phase match the wavelength of the fundamental wave when the power of the fundamental wave emitted from the laser light source is low, and to output the second harmonic. A second harmonic generator 2 for generating the laser beam emitted from the light source 1 and an optical disk 10 as an information recording medium.
It is equipped with an optical system 20 for condensing and irradiating light, and a detector 12 for an R0F signal and a servo system for focusing and tracking.

For example, the laser light source 1 has a central oscillation wavelength of 830 nm.
i, and the power is adjusted so that the power during recording is 20 mW and the power during reproduction is 1 mW. However, when adjusted in this way, the laser power level becomes about half, 10 mW and 0.5 mW, respectively, when the light is focused on the recording layer of the optical disk 10.

The second harmonic generator 2 is made of, for example, LiNb 0°, and is adjusted so as to match the phase when a laser beam of, for example, 828 nm is incident thereon.

The optical disc 10 has a recording layer for recording information formed on a substrate, and the recording layer may be of various conventionally used types, such as a pit-forming type or a phase change type. can be applied.

During recording, a 20 mW recording laser beam is emitted from the light source 1. The wavelength of the laser light at this time is 83
2), the laser beam is transmitted to the second harmonic generator 2.
passes through as is, and no second harmonic is generated. The laser beam is then guided to optical system 20. In the optical system 20, the recording laser beam first passes through an achromatic collimator lens 3 to become a parallel beam, and then is reflected by an S-polarized beam splitter 4, and then is reflected by a half mirror.
7 and is condensed and irradiated onto an optical disk 10 by an objective lens 9. This records the information.

During reproduction, a 1 mW laser beam for reproduction is emitted from the light source 1. The wavelength of this laser beam is 828n
-, therefore, when this laser beam is incident on the second harmonic generator 2, the phase is matched, and a second harmonic with a wavelength of 414 ng+ is formed. Thereafter, the beam mainly composed of second harmonics is turned into parallel light by a collimator lens 3 and guided to a polarizing beam splitter 4. beam splitter
4 is of a transmission type for light of 414 nm, the beam passes through this, passes through the λ/4 plate 5a, and is then reflected by the reflection mirror 6a. The beam emitted from the second harmonic generator 2 contains some fundamental waves, but the mirror 6a is designed to transmit the fundamental waves, and only the second harmonics are reflected by the mirror 6a. . The second harmonic reflected by the mirror 6a passes through the λ/4 plate 5a again, so the λ/4
The beam is reflected by the beam splitter 4 because the phase is shifted by 90 degrees in total twice by 45 degrees at the plate 5a. After passing through the λ/4 plate 5b, the light is reflected by the reflection mirror 6b, returns to the original phase by passing through the λ/4 plate 5b again, and passes through the beam splitter 4. Thereafter, the second harmonic passes through the half mirror 7 and is focused onto the optical disk 10 by the objective lens 9.

The second harmonic reflected by the optical disk 10 is a half mirror.
7 and one of them is guided to the detector 12. This reproduces the information.

Therefore, when such an information recording device is used, on the optical disk surface, since the laser power is 10 mW during recording, light with a laser wavelength of 830n11 is focused, and during reproduction, the laser power is 0.5 mW, so Light with a laser wavelength of 415 nm is focused.

In this case, since the second harmonic is used for the laser beam for reproduction, the reproduction spot diameter of the optical disk 10 is
It is smaller than the recording spot diameter formed by the fundamental wave.

This will be explained with reference to FIG. Figure 2 shows
A semiconductor laser with a wavelength of 830 nm was used, and the NA was 0.
It is a diagram showing the irradiation intensity distribution at the condensed spot on the optical disk surface when an optical system using a No. 55 objective lens is incorporated into an optical disk drive, where the solid line indicates the fundamental wave and the broken line indicates the second harmonic. It is. In this case, the spot diameter corresponds to about 1.5 times the diameter (full width at half maximum) at the position where the intensity is about half. In the above example, the full width at half maximum (diameter) of the fundamental wave of the semiconductor laser is 0.57 as shown in this figure.
μm, and the full width at half maximum (diameter) of the second harmonic is 0.2
It becomes 9 μm. That is, when using the second harmonic of a semiconductor laser, the laser spot diameter can be made smaller to about 172 mm than when using the fundamental wave.

Next, the principle of increasing the recording density in this invention will be explained with reference to FIG. First, FIG. 3(a) shows the case of the conventional recording/reproducing method. Conventionally, since the same laser with different power is used for recording and reproduction, the diameter of the condensed spot on the optical disk surface for both is exactly the same. In the case of this method, when the distance between recording marks becomes narrower from the viewpoint of increasing recording density, when reproducing information is to be reproduced by irradiating the optical disc with a reproducing beam and detecting the amount of reflected light, for example, The resolution becomes extremely poor.

On the other hand, in the case of this invention, the fundamental wave of the light source is used for recording information, and the second harmonic is used for reproduction, so as mentioned above, the reproduction spot diameter is approximately 1 times the recording spot diameter.
/2. Therefore, as shown in FIG. 3(b), even if the distance between recording marks becomes narrow, reproduction can be performed with high resolution and fidelity. Therefore, it is possible to at least double the recording density compared to the conventional method.

Next, a test example in which recording and reproduction were actually performed based on the present invention will be explained. A semiconductor laser with a wavelength of 8300 square meters was used as a recording and reproducing laser oscillator, and the NA of the objective lens was set to 0.55.

The optical disk used was one with a diameter of 5 cm, and its recording layer was made of Te-C (tellurium carbon), which is a type in which information is recorded by forming holes by laser beam irradiation.

(1) The power of the recording laser on the optical disk surface is 10 m.
W, and the power of the second harmonic for reproduction is 0.8m.
I set it to W. The half width in this case was 0.57 μm for the fundamental wave and 0.29 μm for the second harmonic, similar to that shown in FIG. 2 described above.

The optical disk was rotated at 1800 rpm, and a laser beam was emitted from a semiconductor laser as a recording light source, and a laser beam of 3.7 MHz was emitted at a position 30 g + s from the center of the disk.
, recording was performed with a pulse width of 50 n5ec.

As a result, the minimum pitch (distance between adjacent pit centers) of pits (recording marks) formed in the recording layer of the optical disc was 1.5 μm. As mentioned above, the length of the pits is about 1.5 times the full width at half maximum of the irradiated fundamental wave, so the distance between the pits is 0.65 μm or less (in reality, there is a As a rim is formed, the pit spacing becomes even smaller).

The information recorded in this way is reproduced using the second harmonic as described above, and a spectroanalyzer is used to reproduce the C of the reproduced signal.
/N was measured and found to be 50 dB.

On the other hand, when reproducing using the fundamental wave as in the past, the C/N of the reproduced signal was 49 dB, which was almost the same as when reproducing using the second harmonic.

(2) For the optical disc configured as described above, (1
) using the same fundamental wave and changing only the recording frequency,
The distance between recording pits is 0.4 μm, o, 3 μm, 0.2
5μm, 0.20μm.

An experiment was conducted in which recording was performed with a length of 0.15 μm and the recorded information was reproduced using the second harmonic and the fundamental wave as described above. The results are shown in Table 1.

Table 1 As shown in Table 1, the C/N decreases as the bit interval becomes smaller in both cases of reproduction using the fundamental wave and reproduction using the second harmonic. In the case of reproduction, the decrease is large, whereas in the case of reproduction with the second harmonic, this decrease is small.

For digital records such as electronic document files, C/
For practical purposes, N only needs to be 45 dB or more, so when reproducing using the second harmonic, the pit interval can be reduced to 0.2 μm. When using the fundamental wave, the bit interval must be 0.4 or more, so it has been confirmed that when reproducing using the second harmonic, the bit interval can be reduced to 1/2 of the conventional value. In other words, the pit density could be increased to twice as high.

In the above example, the recording method is write-once type,
Although an optical disk has been shown in which Te-C is used as the recording layer and information is recorded by forming holes with a pulse laser, the present invention is not limited thereto and can be modified in various ways.

For example, instead of a recording layer that forms holes, a diffusion film or an alloy film that forms recording marks by laminating two metals or semiconductors and causing atomic diffusion in the laser irradiated area, or a film that is non-recording in the non-recording state. A similar effect can be obtained using a phase change type recording layer which is crystalline and is crystallized by laser beam irradiation to form recording marks.

Alternatively, it may be of a writeable type so that the recorded information can be erased. In this case as well, by using the second harmonic for reproduction, it is possible to obtain exactly the same effect as in the write-once type. In the rewritable type, the above-mentioned recording light source may also be used for erasing. Examples of the recording layer used in such a write-pull type include a variable type that changes reversibly between crystal and amorphous, and a magneto-optical recording type that records and erases information by changing the direction of magnetization. There is something.

Although we have explained an example of so-called pit position recording in which the recording pits are made circular and information is processed according to the pit position, this is not limited to this, but it is also possible to use so-called length recording in which the pits are made oval and signal processing is performed according to the length of the pit. However, the same effect can be obtained.

Furthermore, in the above example, the second harmonic was used as the reproduction beam, but third and subsequent harmonics may also be used as long as a reproducible power can be obtained.

Furthermore, information recording media are not limited to optical discs.
A plain card such as an optical card may also be used.

[Effects of the Invention] According to the present invention, since the zero-order harmonic generator is configured to phase match the wavelength of the fundamental wave when the fundamental wave emitted from the laser light source has low power, Information is recorded using the fundamental wave of light emitted from one laser light source, and a second
Information can be reproduced using the harmonics, thereby making it possible to make the reproduction spot diameter smaller than the recording spot diameter.

Therefore, high recording density can be achieved with a relatively simple device configuration.

[Brief explanation of drawings]

FIG. 1 is a schematic configuration diagram showing an information recording device according to an embodiment of the present invention, FIG. 2 is a diagram showing the intensity distribution of the fundamental wave and second harmonic of a semiconductor laser as a light source, and FIG. 3 is a diagram showing a conventional information recording device. 4 is a diagram for explaining the relationship between the recording spot and the reproducing spot in this invention, FIG. 4 is a diagram for explaining the relationship between the power and wavelength of the semiconductor laser, and FIG. 5 is a diagram for explaining the relationship between the power and the wavelength of the semiconductor laser. FIG. 3 is a diagram showing the relative intensity distribution of the second harmonic and the fundamental wave generated from the . 1; Laser light source, 2: Second harmonic generator, 10; Information recording medium (optical disk) 12; Detector, 20; Optical system.

Claims (1)

[Claims] A laser light source is used to record information by irradiating the fundamental wave onto an information recording medium, and to reproduce the information by irradiating the n-th harmonic thereof. an n-th harmonic generator configured to phase-match the wavelength of the fundamental wave and generate an n-th harmonic; and a laser light source for condensing and irradiating a light beam onto the information recording medium. An information recording device characterized by having an optical system for.