JPH04321935A - Optical memory device and reading out method thereof - Google Patents

Abstract

PURPOSE:To allow the higher-density recording of an optical disk by incorporating at least one kind of rear earth elements into a recording layer and irradiating the layer with UV rays prior to irradiation with a laser beam for reading out. CONSTITUTION:At least one kind among the rare earth elements (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) are incorporated as impurities into the recording layer of the phase change type optical disk. The recording layer is irradiated with the UV rays from a UV lamp 23 prior to the irradiation with the laser beam for reading out from a head 22 at the time of reading out the stored information. The 4f electrons of the rare earth ions are previously excited by the irradiation with the UV rays, then the light emission spectra at the time of restoring the base state from the excitation state are measured. A measuring section discriminates whether crystal or amorphous. The spot size is effectively reduced as compared with the case of detection of a difference in reflectivity in this way.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a phase change type optical disk device that can perform recording, erasing, and reading only by modulating the intensity of laser light.

0002

2. Description of the Related Art The two most practical types of rewritable optical disks are magneto-optical disks that utilize the magneto-optical effect and phase-change optical disks that utilize phase transition between crystalline and amorphous structures. Among these, phase-change disks have the advantage that their heads can be made smaller because they do not use magnets, and that overwriting can be easily accomplished with a single beam.

FIG. 1 shows the media structure of a typical phase change optical disk. 11 is a transparent substrate made of polycarbonate or glass, 12 and 14 are ZnS, Si, etc.
Dielectric protective film made of O2, SiN, etc., 13 is GeSb
A recording film made of a chalcogenide alloy such as Te, and 15 a reflective film made of Au, Al, etc. are shown. This medium is irradiated with a semiconductor laser focused to approximately 1 μm. If the intensity of the laser beam is strong and exceeds the melting point of the recording film, the area will become amorphous during the rapid cooling process;
That part becomes a crystal. Since the refractive index is different between crystal and amorphous, the reflectance is different, so this corresponds to the information 0 and 1.

0004

[Problem to be Solved by the Invention] The size of the amorphous region formed by light irradiation is determined by the critical temperature range that exceeds or does not exceed the melting point. It is possible to write crystalline marks. However, the resolution during readout is limited by the size of the optical spot. Therefore, determining the information recording density has been limited by the size of the focused light spot.

The present invention has been made for the purpose of increasing the recording density of the above-mentioned phase change type optical disk device.

[0006]

[Means for Solving the Problems] An optical storage device according to the present invention irradiates a storage medium with light to form crystals in a recording layer of the medium.
A phase change optical storage device that records and erases information by causing a reversible phase transition between amorphous materials, characterized in that the recording layer contains at least one kind of rare earth element as an impurity.

[0007] In the reading method according to the present invention, when reading information stored in a storage medium having a recording layer containing at least one kind of rare earth element as an impurity, the storage medium is irradiated with ultraviolet rays prior to irradiation with readout light. It is characterized by

[0008]

[Operation] In the present invention, a recording film containing a rare earth element is used, and the 4f electrons of the rare earth ion are excited by irradiation with an ultraviolet lamp prior to irradiation with readout light, and then return from the excited state to the ground state. The emission spectrum is measured to determine whether the measuring section is crystalline or amorphous.

[0009] According to the present invention, the spot size can be effectively reduced compared to the case where reflectance differences are detected using the conventional configuration, and therefore the recording density of the optical disc can be increased.

0010

Embodiment FIG. 2 shows the configuration of an optical storage device according to the present invention. Reference numeral 21 denotes a phase-change optical disk, in which rare earth elements (La, Ce, Pr, Nd,
Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Y
b, Lu). 22 is an optical head for recording, erasing, and reading, 23 is an ultraviolet lamp, and 24 is a portion of the disk that is irradiated with ultraviolet light. No ultraviolet lamp is used for recording and erasing, which is exactly the same as conventional recording and erasing. Therefore, only the reading that is different from conventional devices will be described.

FIG. 3 shows the structure of the optical head according to the present invention. The light oscillated by the semiconductor laser 31 is made into parallel light by the collimating lens 32, and then shaped into a Gaussian beam by the shaping prism 33. Here, the laser beam at the time of incidence is linearly polarized light in a direction that passes through the polarizing beam splitter 34. After that, it becomes circularly polarized light by the λ/4 plate 38, and then passes through the total reflection prism 35 and the objective lens 36.
The light is focused onto the optical disk 37. Up to this point, it is the same as the optical head for a conventional phase change optical disk. Now, in the present invention, since ultraviolet rays are irradiated prior to readout, emission of ultraviolet rays or visible light occurs simultaneously with the reflected light of the laser beam for readout that has just been focused. The mechanism will be described later. Both the emitted light and the reflected light are transmitted through the objective lens 36, the total reflection prism 35, and the λ/4
It passes through the plate 38 and is reflected by the polarizing beam splitter 34,
A dichroic cube 310 is reached. Here, light having the same wavelength as that of the semiconductor laser 31 passes through and heads toward the tracking/focusing error detection optical system 39 . On the other hand, the emitted light is reflected by a dichroic cube 310, and a Fabry-Perot interferometer 312 reflects only the peak emission wavelength from the crystal, or only the light at a wavelength slightly shifted from the emission peak, and its intensity is transmitted to a light receiver 311. Detected and used as a read signal.

The principle of the regeneration process in the present invention will be explained with reference to FIGS. 4 to 6. The most characteristic feature of the present invention is that the light emitted from the portion heated by the laser beam is detected instead of detecting the difference in reflectance when reading information. 4f of rare earth ions contained in the recording film
Electrons are affected by the crystal field around the ions, and as shown in FIG. 4, the energy of the ground level and the excited level is shifted and split. Here, the energy shift in a crystal is the same for each rare earth ion, but in an amorphous state, each rare earth ion receives a different crystal field, so there is a range in the amount of energy shift.

Therefore, according to the present invention, 4f of rare earth ions
By exciting electrons by irradiating them with an ultraviolet lamp and then measuring the emission spectrum as they return from the excited state to the ground state, it is possible to distinguish whether the measurement part is crystalline or amorphous. What is important here is that after excitation, at least several tens of
The reason is that the excited state must be maintained for more than milliseconds. Here, a phosphorescence phenomenon is used, and as shown in FIG. 4, the ground state EG is excited by ultraviolet light to the excited state Eel, and then shifts to the more stable excited state Ee2. Here, direct transition from Ee2 to EG is prohibited. Next, by laser heating the optical head, Ee2
is thermally excited from Eel to Eel, and transitions from Eel to EG accompanied by light emission. FIG. 6 shows the emission spectra of crystalline and amorphous materials. In amorphous materials, the amount of energy shift varies, so the spectrum becomes broad.

In the configuration of the present invention, the emitted light is separated into spectra using a grating, and the difference in the emitted light intensity between crystal and amorphous materials or the light that cannot be emitted by the crystal is detected. The advantage of this method is that the emission intensity is an exponential function of temperature.

[0015]

[Math 1]

The advantage of this method is that the spot size can be effectively reduced compared to the conventional method of detecting reflectance differences.

FIG. 7 shows effective cross-sections of spots according to the present invention and the conventional example as curves A and B, respectively. As shown in FIG. 7, according to the present invention, 1.3 μm (1/e2)
When irradiating a spot with a diameter of

001
8]

[Math 2]

By substituting and normalizing, it can be seen that it becomes equivalent to a spot with a diameter of approximately 1 μm. As described above, by using the method of the present invention, a spot that is more effectively narrowed down than the optical limit value can be obtained.

As an example, as a phase change optical disc/
Grooved glass substrate/ZnS+SiO2 (thickness 300n
m)/Ge19Sb28Te53+Sm (0.01%)
+Ce (0.02%) (thickness 40nm)/ZnS+Si
The results are shown using a 5-inch diameter disk composed of O2 (thickness: 200 nm)/Au (thickness: 40 nm)/photopolymer/. The amounts of Sm and Ce added are shown in weight %. The wavelength of the semiconductor laser in the optical head is 8
It is 30 nm. A 250 W mercury lamp was used as the ultraviolet light source, and the light was focused in a circle with a radius of 2 cm on the disk. Linear speed during recording, erasing, and reading is 10 m/s, laser power is 1 mW during reading, 7 mW during erasing, and overwriting with base power, peak power, and pulse width fixed at 7 mW, 14 mW, and 40 ns, respectively, during writing. method was used. FIG. 8 shows the change in contrast when the recording density is increased. Normal readout (curve D)
A comparison is made between the change in contrast when the light separated by the Fabry-Perot interferometer according to the present invention is detected (curve C), and when the change in contrast is measured using an oscilloscope. As shown in the figure, it can be seen that according to the method of the present invention, the decrease in contrast is small even when the density is increased.

As the recording film, the above-mentioned Ge19Sb2
Not limited to 8Te53, any recording film commonly used as a phase change type, especially a chalcogenide alloy, can be used, and as the rare earth element, any one other than Sm and Ce can be used. The amount of rare earth elements added is approximately 0.
．． It is effective at 0.001%, but exceeding 5% is not preferable from the perspective of light emission.

[0022]

[Effects of the Invention] As explained above, according to the present invention,
It is possible to increase the recording density of an optical disc.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the medium configuration of a conventional general phase change optical disk.

FIG. 2 is a diagram showing the configuration of the present invention.

FIG. 3 is a diagram showing the configuration of an optical head according to the present invention.

FIG. 4 is a diagram showing the principle of the regeneration process according to the present invention and showing the difference between crystal and amorphous in the excitation process by ultraviolet rays.

FIG. 5 is a diagram showing the principle of the regeneration process according to the present invention and showing the difference between crystalline and amorphous light emission processes due to thermal excitation.

FIG. 6 is a diagram showing the principle of the regeneration process according to the present invention and showing the difference in emission spectra between crystal and amorphous.

FIG. 7 shows an effective cross section of a laser beam spot.
It is a figure showing a laser spot with a diameter of 1.3 μm (1/e2) and the emission intensity when the spot is irradiated.

FIG. 8 is a graph showing the difference between normal reproduction and reproduction according to the present invention during high-density recording.

[Explanation of symbols]

11 Substrate 12 Dielectric protective film 13 Recording film 14 Dielectric protective film 15 Reflective film 21 Phase change optical disk 22 Optical head 23 Ultraviolet lamp 24 Ultraviolet irradiation site 31 Semiconductor laser 32 Collimating lens 33 Shaping prism 34 Polarizing beam splitter 35 Total reflection prism 36 Objective lens 37 Optical disk 38 λ/4 plate 39 Tracking/focusing error detection optical system 310 Dichroic cube 311 Light receiver 312 Fabry-Perot interferometer

Claims (3)

[Claims] 1. A phase change optical storage device that records and erases information by irradiating a storage medium with light to cause a reversible phase transition between crystal and amorphous in a recording layer of the medium, wherein the recording An optical storage device characterized in that a layer contains at least one kind of rare earth element as an impurity. 2. When reading information stored in a storage medium having a recording layer containing at least one kind of rare earth element as an impurity, the storage medium is irradiated with ultraviolet rays prior to irradiation with readout light. reading method. 3. The readout method according to claim 2, wherein during the readout, the intensity of light having a wavelength different from that of the readout laser beam is read using a light receiving element having wavelength selectivity.