JPH07169056A - Photon-mode optical recording medium recording and reproducing method and device - Google Patents

Abstract

PURPOSE:To simplify, miniaturize a device and make the cost of the device low by allowing an injection current of not less than a laser oscillation threshold of a semiconductor laser to flow at the time of recording mode to emit a laser beam from a semiconductor. CONSTITUTION:A semiconductor laser 11 emits laser pulses which are modulated by an injection current of not less than an oscillation threshold corresponding to a record signal at the time of a recording mode by a semiconductor laser injection current control circuit 17, and emits constant-level LED light by a smaller injection current than the oscillation threshold at the time of a reproducing mode. Light emitted from a semiconductor laser 11 is shaped by collimator lens 12 and passes an interference filter 32 and a polarization beam splitter 13 and then is reflected by a dichromic mirror 30 and converged onto the disk surface of a photon mode optical recording body 40 by an objective lens 31.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a recording / reproducing method and a recording / reproducing apparatus for a photon mode optical recording medium.

[0002]

2. Description of the Related Art In recent years,
As a rewritable optical recording medium, research on photochromic optical recording using photon mode reaction has been actively conducted. When a photochromic material is irradiated with light of a predetermined wavelength, the structure of the molecule changes due to a photochemical reaction, which causes changes in optical characteristics such as absorbance and refractive index for light of a specific wavelength, and also light and heat of other wavelengths. Has the property that the changed molecular structure returns to its original state. Therefore, the recording of the photochromic optical recording medium is performed by the change of the molecular structure due to the irradiation of the light of the specific wavelength, and the reproduction is performed by detecting the change of the optical characteristics, especially the change of the absorbance.

In such photochromic optical recording,
Wavelength multiplex recording, in which multiple materials with different absorption spectra are used and recording is performed using laser light of multiple wavelengths corresponding to each material, and molecules having an orientation that matches the polarization plane of the recording laser are selectively reacted. By using a method such as polarization multiplexing recording, it is possible to achieve higher density recording than heat mode recording such as conventional magneto-optical recording and phase change optical recording.

By the way, in such a photon mode optical recording, since there is no threshold value in the photochemical reaction, there is a problem that the reaction gradually progresses by repeating the reproduction and eventually the recorded information is destroyed. It was In order to solve such a problem, the present inventor has disclosed in Japanese Patent Application No. 5-88854, which is not yet known, in the vicinity of P _rep (W) of the following formula (I) or P of the following formula (II). We have proposed an "ultra-low power reproduction method" in which reproduction is performed using ultra-low-power reproduction light within the range of _rep (W).

[0005]

[Equation 1]

(Here, SNR is the SN required for the system.
Power ratio (PP / rms), e is elementary electric power 1.6 × 10
^-19 (C), B is the system bandwidth (Hz), η is the sensitivity of the photodetector to a gain of 1 (A / W), γ is the pickup efficiency, R _ave is the average reflectance of the optical recording medium, and ΔR is The amount of change in reflectance between the recorded portion and the unrecorded portion of the optical recording medium is shown. )

[0007]

[Equation 2]

(Here, SNR is the SN required for the system.
Power ratio (PP / rms), e is elementary electric power 1.6 × 10
^-19 (C), B is the system bandwidth (Hz), η is the sensitivity of the photodetector to a gain of 1 (A / W), γ is the pickup efficiency, R _ave is the average reflectance of the optical recording medium, and ΔR is The amount of change in reflectance between the recorded portion and the unrecorded portion of the optical recording medium, k is Boltzmann's constant 1.38 × 10 ⁻²³ (J · K ⁻¹ ), T is absolute temperature (K), and I _{am p} is reproduction. The average noise current (A) and Z of the reproduction preamplifier indicate the impedance (Ω) of the reproduction preamplifier. )

In such "ultra-low power reproduction system",
A very low laser power of a few nW to a few μW is used to reproduce the information recorded on the photon mode optical recording medium. Therefore, when recording and reproducing using the same laser light source as a device for recording on a photochromic medium and reproducing at ultra-low power, a special attenuator such as an attenuator that can adjust the optical power inside the pickup is used. It was necessary to provide various optical elements. Further, when a reproducing light source is provided separately from the recording light source, a reproducing light source provided with means for attenuating the power to an ultra-low power level in advance is required. Therefore, the conventional recording / reproducing method for the photon mode optical recording medium using the "ultra-low power reproducing system" has a problem that the device becomes complicated and large, and the cost becomes high.

The object of the present invention is to solve the above problems of the prior art, and to record on a photon mode optical recording medium capable of recording and reproducing with the same light source without complicating and increasing the size of the apparatus. It is to provide a reproducing method and a recording / reproducing apparatus.

[0011]

According to the recording / reproducing method of the photon mode optical recording medium of the present invention, when recording information on the photon mode optical recording medium, an injection current equal to or higher than a laser oscillation threshold of a semiconductor laser is used as a recording signal. According to the semiconductor laser, the emitted light from the semiconductor laser is used as recording light, and when reproducing the information recorded in the photon mode optical recording medium, an injection current smaller than the laser oscillation threshold of the semiconductor laser is kept at a constant level. The laser light is emitted to the semiconductor laser and the emitted light from the semiconductor laser is used as the reproduction light.

Further, the recording / reproducing apparatus for the photon mode optical recording medium of the present invention records information on the photon mode optical recording medium and a semiconductor laser light source which emits light having a wavelength capable of recording / reproducing the photon mode optical recording medium. In this case, an injection current higher than the laser oscillation threshold of the semiconductor laser light source is applied to the semiconductor laser light source according to the recording signal, and the laser oscillation threshold of the semiconductor laser light source is used when reproducing the information recorded on the photon mode optical recording medium. A semiconductor laser injection current control means for flowing a smaller injection current to the semiconductor laser light source at a constant level, a lens system for condensing the emitted light from the semiconductor laser light source on the photon mode optical recording medium, and a photon mode light And a photodetector that detects reflected light from the recording medium.

[0013]

When reproducing by the ultra-low power reproducing system, the power level is several nW to several μW in the reproducing mode and several m in the recording mode.
Although it is necessary to switch the power level of W or higher, it has been conventionally considered that such control of the radiation power of the semiconductor laser is impossible. Therefore, when the conventional semiconductor laser is used as a light source for recording / reproducing, it is operated with an injection current higher than the laser oscillation threshold. In the present invention, when reproducing with ultra-low power, an injection current smaller than the laser oscillation threshold is caused to cause spontaneous emission (LED emission), and the emitted light is used as reproduction light.

In the present invention, in the recording mode, recording is performed using a semiconductor laser above the oscillation threshold as in the conventional case, and the laser power required for recording can be obtained. On the other hand, in the reproduction mode, an injection current smaller than the oscillation threshold is used and used in the LED light emitting region. By using it in the LED light emitting region as described above, it is possible to perform reproduction by using a very low laser power of several nW to several μW, and a recording mode in which light of several mW or more is emitted by changing the injection current. Thus, it is possible to easily switch the reproduction mode for emitting light of ultra low power of several μW or less.

Further, in order to enable high density recording in photon mode optical recording, it is necessary to collect the emitted light from the semiconductor laser to a minute spot determined by the diffraction limit of light during recording and reproduction. Becomes In the present invention, since the emitted light in the recording mode is laser light,
With the objective lens, it is possible to collect light up to a diffraction-limited spot. Further, in the reproduction mode, since the light is emitted from the LED, it can be condensed to a spot having the same level as the size of the light emitting point by condensing with the objective lens.

FIG. 3 is a perspective view showing a general structure of a semiconductor laser. Referring to FIG. 3, in this semiconductor laser, an n-type Al _y Ga layer is formed on an n-type GaAs substrate 1.
_1-x As clad layer 2, p-type GaAs active layer 3, p-type A
It is configured by laminating the 1 _x Ga _1-x As clad layer 4 and the p-type GaAs layer 5, and an electrode 6 is provided on the lower surface and an electrode 7 having a width w is provided on the upper surface. In such a semiconductor laser, the size of the light emitting point is a region determined by the thickness d of the active layer 3 and the width w of the electrode. Generally, the thickness d of the active layer is 0.1 to 1 μm, and the width w is about 1 to several μm. On the other hand, the spot size due to the diffraction limit is λ / NA, where NA is the numerical aperture of the objective lens and λ is the wavelength of light.
It is about 1 to 2 μm. Therefore, the width w is 1
By using a semiconductor laser of about 2 μm or less, it becomes possible to condense even the emitted light of the LED to a spot comparable to the diffraction limited spot.

[0017]

FIG. 4 is a schematic configuration diagram showing an embodiment of a recording / reproducing apparatus according to the present invention. As the semiconductor laser 11, for example, a semiconductor laser that emits a laser in the vicinity of 630 nm and has an active layer size of about 1 to 2 μm or less is used. In the semiconductor laser 11, the semiconductor laser injection current control circuit 17 emits a laser pulse modulated by an injection current equal to or higher than the oscillation threshold value corresponding to the recording signal in the recording mode, and is constant by the injection current smaller than the oscillation threshold value in the reproduction mode. LE of level
D light is emitted.

The light emitted from the semiconductor laser 11 is
The interference filter 3 is shaped by the collimator lens 12.
2, polarization beam splitter 13, and quarter wave plate 1
4, the light is reflected by the dichroic mirror 30, and the photon mode optical recording medium 40 is reflected by the objective lens system 31.
Is focused on the disk surface.

In the reproducing mode, the LED light is emitted from the laser semiconductor 11, is reflected by the dichroic mirror 30 like the laser light in the recording mode, is transmitted through the objective lens system 31, and is collected on the photon mode recording medium 40. The light that has been emitted and reflected from the photon mode recording medium 40 is reflected by the dichroic mirror 30, passes through the quarter-wave plate 14, the optical path is changed to the photodetector 16 side by the polarization beam splitter 13, and the lens 15 is passed through. It passes through and enters the photodetector 16, and a reproduction output is obtained. As the photodetector 16, it is desirable to use a photocurrent self-amplifying device such as an avalanche photodiode. In general, LED light emitted from a semiconductor laser often has a broader spectrum than that of laser light. In such a case, if the chromatic aberration of the objective lens system 31 or the collimator lens 12 is large, there is a problem in that the focal position during recording and the focal position during reproduction are deviated, or the condensing ability during reproduction is reduced. As such, it is desirable to use lenses with such as little chromatic aberration as possible. Further, it is desirable to install an interference filter 32 that transmits only the same wavelength component as the laser oscillation wavelength in the optical system.

Further, in the recording / reproducing apparatus of the present embodiment, an optical system for servo is separately provided so that the focus / tracking servo does not become unstable at the time of reproducing ultra-low power. As the light source 21 of such a servo optical system, for example, a semiconductor laser having a wavelength of 830 nm can be used. The radiated light from the light source 21 is shaped by the collimator lens 22, and the polarized beam splitter 23
Then, the light is focused on the disk surface of the photon mode optical recording medium 40 by the objective lens system 31 through the 1/4 wavelength plate 24 and the dichroic mirror 30. Reflected light is 1/4
The optical path is converted by the action of the wave plate 24 and the polarization beam splitter 23, and enters the focus / tracking error detection optical system 25. A known optical system can be used as the focus / tracking error detection optical system 25.
Based on the tracking error signal, the focus / tracking circuit 26 drives the objective lens system 31 to execute servo. The output of the light source 21 may be constant both in the recording mode and the reproduction mode.

1 and 2 show the relationship between the light intensity of the emitted light from the semiconductor laser 11 and the amount of injected current on the disk surface of the optical recording medium 40. In FIG. 1, both the vertical axis and the horizontal axis are linear. 2 is a plot in which both the vertical axis and the horizontal axis are plotted in logarithm. As is apparent from FIG. 1, the oscillation threshold of this semiconductor laser exists at about 33 mA as an injection current. Therefore, in the case of the device of this embodiment, the injection current is 33 mA or more in the recording mode, and the injection current is less than 33 mA in the reproducing mode. As is clear from FIG. 1 and FIG. 2, a laser beam of about several mW is obtained with good linearity in the recording mode, and several tens of tens is obtained by the injection current of several mA in the reproducing mode.
LED light of several 100 nW is obtained with good linearity.

The laser spot diameter on the disk surface of the optical recording medium 40 was 1.27 μm in the recording mode (injection current 45 mA) and 1.29 μm in the reproduction mode (injection current 5 mA), which was almost unchanged. . These numerical values are the numerical aperture NA of the objective lens = 0.5 and the wavelength =
Diffraction limit value determined from 0.63 μm (λ / mA) 1.
It almost coincides with 26 μm.

As is clear from the above, by injecting an injection current above the laser oscillation threshold of the semiconductor laser according to the recording signal during recording, and by injecting an injection current smaller than the laser oscillation threshold at a constant level during reproduction. , Ultra-low power reproduction system recording and reproduction can be performed with a single laser. In addition, recording light and reproducing light can be condensed with a minute spot to enable high-density recording.

FIG. 5 is a diagram showing the relationship between the injected current and the relative noise intensity of the emitted light on the disk surface of the optical recording medium. The plot shown in FIG.
It is obtained by intensity-modulating at a frequency of Hz, analyzing the output detected by the photodetector with a spectrum analyzer, and measuring the ratio of the noise level to the signal level. As is clear from FIG. 5, it is found that the noise sharply increases near the laser oscillation threshold. Therefore, even when operating the semiconductor laser with an injection current smaller than the oscillation threshold value, it is desirable to avoid operation at a value close to the oscillation threshold value from the viewpoint of the SN ratio.

[0025]

According to the present invention, laser light is emitted from the semiconductor laser as recording light by causing an injection current equal to or higher than the laser oscillation threshold of the semiconductor laser in the recording mode to be used as the recording light. A smaller injection current is passed through the semiconductor laser at a constant level to emit LED light from the semiconductor laser, which is used as reproduction light. Therefore, it is possible to perform ultra-low power reproduction with a single semiconductor laser, and it is possible to significantly improve the number of times the photon mode optical recording medium can be repeated. Moreover, since recording and reproduction can be performed with a single laser,
The device itself can be simplified, and the size and cost can be reduced.

[Brief description of drawings]

FIG. 1 is a diagram showing a linear plot of the relationship between the injection current of a semiconductor laser and the emitted light intensity.

FIG. 2 is a diagram showing the relationship between the injection current of a semiconductor laser and the emitted light intensity in a logarithmic plot.

FIG. 3 is a perspective view showing a structure of a general semiconductor laser.

FIG. 4 is a schematic configuration diagram showing a recording / reproducing apparatus of an embodiment according to the present invention.

FIG. 5 is a diagram showing a relationship between an injection current of a semiconductor laser and a relative noise level.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 ... Semiconductor laser 12 ... Collimating lens 13 ... Polarization beam splitter 14 ... 1/4 wavelength plate 15 ... Lens 16 ... Photodetector 21 ... Servo light source 22 ... Collimating lens 23 ... Polarization beam splitter 24 ... Quarter wavelength plate 25 ... Focus / tracking error detection optical system 30 ... Dichroic mirror 31 ... Objective lens system 40 ... Photon mode optical recording medium

Claims (2)

[Claims] 1. A method of recording / reproducing a photon mode optical recording medium by using a semiconductor laser, wherein when recording information on the photon mode optical recording medium, an injection current not less than a laser oscillation threshold of the semiconductor laser is used. According to a recording signal, a semiconductor laser is caused to flow, and light emitted from the semiconductor laser is used as recording light. When reproducing information recorded in the photon mode optical recording medium, an injection smaller than the laser oscillation threshold of the semiconductor laser is performed. A recording / reproducing method for a photon mode optical recording medium, characterized in that an electric current is passed through the semiconductor laser at a constant level and the light emitted from the semiconductor laser is used as reproducing light. 2. A device for recording / reproducing a photon mode optical recording medium, comprising: a semiconductor laser light source that emits light having a wavelength capable of recording / reproducing the photon mode optical recording medium; and the photon mode optical recording medium. When recording information on the semiconductor laser light source, an injection current equal to or higher than the laser oscillation threshold of the semiconductor laser light source is passed through the semiconductor laser light source according to a recording signal, and at the time of reproducing information recorded on the photon mode optical recording medium. Semiconductor laser injection current control means for causing an injection current smaller than a laser oscillation threshold of the semiconductor laser light source to flow to the semiconductor laser light source at a constant level, and light emitted from the semiconductor laser light source is condensed on the photon mode optical recording medium. And a photodetector for detecting reflected light from the photon-mode optical recording medium. Recording and reproducing apparatus of the photon-mode optical recording medium characterized Rukoto.