JPH0453195B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an optical recording and reading method, and particularly to a reading method for erasable optical recording media.

(Prior Art) The optical recording method, in which a laser beam is focused on a minute spot by a condensing lens and irradiated onto the surface of an optical recording medium to cause an optical change on the medium and record information, is a high-density information recording method. This method is attracting attention as a possible method. A wide variety of media have been proposed and considered for use in the optical recording. Particularly erasable optical recording methods according to the present invention include a so-called magneto-optical recording method using a magneto-optical material exhibiting magneto-optical effects such as Faraday effect and Kerr effect, and a method in which optical properties are changed by phase change of a material such as a chalcogenide compound. A recording method using a so-called phase change medium that produces a difference is known as a typical method. In addition, various media systems have been proposed, and information systems using polymeric liquid crystals, similar to the present invention, have also been proposed (Japanese Patent Laid-Open No. 59-10930, Japanese Patent Laid-open No. 59-35989).

(Problems to be solved by the invention) Magneto-optical recording is currently the most popular method.
Although it is close to practical use, the S/N is generally ~
Due to the low response speed of 40 dB and the slow response speed of the external magnetic field applied to the medium, it is not possible to partially erase each bit, which is the recording unit of information, and one track is erased all at once, making rewriting information complicated. . Furthermore, the media currently being considered as phase change media are Te-based oxides or Te-based alloys, but these have the problem of material toxicity and are feared to have an adverse effect on the human body. Furthermore, although these recording media can be made into thin films by techniques such as vapor deposition and sputtering, they have the disadvantage of high costs in terms of materials and manufacturing. There is a strong desire for an optical recording medium or optical recording system that is simple to manufacture and inexpensive. As mentioned above, a recording method using a polymer liquid crystal has been proposed as a way to solve the above-mentioned drawbacks. Since the sensor is used in the state of

On the other hand, the method of reading information is determined by the optical characteristics of the medium used and the optical system. In magneto-optical recording systems that use the Faraday effect or the Kerr effect, the highest S/N is achieved when the light beam incident on the medium to read information is linearly polarized.
There is expected. On the other hand, in the conventional phase change recording system using materials such as chalcogenite compounds, the characteristics of the light beam incident on the medium are independent of the properties of the medium itself, and are determined mainly by the constraints of the optical head optical system. That is, the optical properties of the medium itself are almost independent of the polarization state of the light beam. The readout light beam is formed to be the most suitable for the optical head optical system, but this is not restricted in any way by the characteristics of the medium.

The conventional method for reading information from an optical recording medium using the above-mentioned polymeric liquid crystal is a method of detecting scattering of a light beam, and there is a problem in that the S/N ratio is not high due to its scattering characteristics.

(Means for solving the problems) The object of the present invention is to improve the above-mentioned drawbacks. In other words, it is easy to manufacture, low cost, and has a high S/
The present invention uses an erasable polymer liquid crystal that can achieve a high S/N ratio as an optical recording medium, and provides a reading method for optical recording that can achieve a high S/N ratio.

The optical recording/reading method of the present invention uses a cholesteric polymer liquid crystal as the recording layer of an optical recording medium, and the helical pitch of the cholesteric polymer liquid crystal is such that light is selectively reflected from visible light to near-infrared wavelength range. The rotation direction of the circularly polarized light of the readout light beam is the same as the helical rotation direction of the cholesteric polymer liquid crystal.

(Operation) The basic operating principle of the present invention will be explained. A cholesteric polymer liquid crystal is used for the recording layer. It is known that the liquid crystal groups of cholesteric polymer liquid crystals have a helical arrangement, and because of the helical arrangement, they exhibit optical properties unique to cholesteric. A typical optical property is a wavelength-selective reflection phenomenon of light corresponding to the periodic pitch of the helical structure. This is a phenomenon in which, assuming that the pitch of the cholesteric polymer liquid crystal is p and the refractive index is n, only circularly polarized light having a wavelength λ=np and rotating in the same direction as the helical rotation direction is selectively reflected. The present invention is based on this selective reflection phenomenon. The light selective reflection wavelength can be changed by changing the refractive index and pitch of the polymer liquid crystal. It has been known that the pitch of low-molecular-weight cholesteric liquid crystals changes depending on temperature and other factors.

As a result of intensive studies by the present inventors, it has been found that when a cholesteric polymer liquid crystal in a cholesteric phase is rapidly heated by laser beam irradiation, a color change occurs due to a change in the light selection wavelength, which seems to correspond to a change in the helical pitch length or rotation of the helical axis. (Transmittance change) occurs, and it has been found that the color change state is preserved when rapidly cooled by removal of laser beam irradiation. This is thought to be because the changed pitch state was preserved as it was due to the rapid cooling effect. On the other hand, it was found that when the material was slowly cooled, the color change state disappeared and the product returned to its original state. When the irradiation energy of the laser beam is further increased, unlike the color change corresponding to the pitch length change, pits are formed and are preserved by rapid cooling, and the formed pits disappear by reheating and slow cooling, returning to the original state. It was confirmed that he would return.

The present invention will be explained in more detail using figures.

FIG. 1 is a schematic cross-sectional view of an embodiment of the optical recording medium and reading method of the present invention. In FIG. 1, a light absorbing layer is formed on a plastic or glass substrate 1 to efficiently convert the writing light beam into heat. The light absorber is preferably one that has large absorption in the wavelength range of the light beam, has a relatively high melting point, and can be made into a thin film. When a semiconductor laser (λ=0.78 to 0.83 μm) is used as the light beam 5, a phthalocyanine compound such as vanadyl phthalocyanine formed by vacuum evaporation can be used. Furthermore, soluble phthalocyanine coated with a suitable solvent and dried may also be used. Furthermore, a cholesteric polymer liquid crystal thin film 3 is formed thereon. Various materials can be used as the cholesteric polymer liquid crystal 3. One example is a siloxane polymer liquid crystal having a cholesterol derivative and nematic liquid crystal molecules added thereto, as shown by the following structural formula [I].

The cholesteric liquid crystal may contain a trace amount of additives such as a plasticizer in order to improve film forming properties. The polymer liquid crystal film can be made into a thin film by various methods. A method of solubilizing the polymer liquid crystal in a suitable solvent and applying it by spin coating, etc., a method of transferring it by gravure printing, a method of applying it with a doctor grade, etc. can be adopted, and the coating film is dried by heating. can be made into a thin film. Alternatively, it is also possible to form a thin film by molding on a substrate under heat and pressure.

The film thickness of the polymer liquid crystal thin film is 0.1 μm to several tens of μm.
is adjusted to A protective film 4 or protective layer 4 is generally formed on the polymer liquid crystal thin film. However, the protective film 4 is not an essential requirement of the present invention;
You don't need this. The helical rotation direction of the polymer liquid crystal is determined by the optically active material used. A polymer liquid crystal containing a cholesterol derivative as shown in the above structural formula [I] has a left-handed circularly polarized light λ=np.
It is selectively reflected around the wavelength indicated by .

Therefore, when the selective reflection wavelength of the pitch is adjusted from the visible range to the near-infrared wavelength range, and circularly polarized light rotated in the same direction as the cholesteric light is used as the readout light (in Fig. 1, the readout light beam 5
The polarization direction 6 is shown counterclockwise. Of course, if the medium has a clockwise helical structure, the polarization direction of the readout light beam may be clockwise. ), it shows an extremely large optical change and can achieve a high S/N. In other words, if the pitch p is adjusted in advance so that λ is approximately equal to np with respect to the reading light source wavelength λ, the area where no information is written will have an extremely high reflectance, while the mode that forms a pit will In the area where information is written, a sharp drop in reflectance occurs, resulting in a high S/N ratio. On the other hand, if the pitch p is adjusted in advance so that λ≫np with respect to the wavelength λ of the reading light source, and information is written in a light amount region where the pitch length becomes long, the non-information writing area has an extremely low reflectance. , the writing section has a high reflectance, and a high S/N can be achieved as well.

Example 1 Vanadyl phthalocyanine was formed on a glass substrate by vacuum evaporation, and a thin film of polymeric liquid crystal represented by the structural formula [I] was formed thereon by spin coating or the like. The selective reflection center wavelength of the polymer liquid crystal is approximately
Adjusted to 830nm. A laser diode with a wavelength of 830 nm was used as the light beam. Writing was performed using pulsed light of 8 mW for 80 μs, and the formation of pits was confirmed. When the above medium was reproduced with 1 mW of left-handed circularly polarized light, S/
It was possible to reproduce with a high S/N of N=60dB. On the other hand, when reproduction was performed using clockwise circularly polarized light or non-polarized light, almost no S/N ratio was obtained. By heating the medium on which the information was written to 70°C or higher, the information could be completely erased. Note that no deterioration of the medium was observed.

Example 2 Writing was performed on a medium having the same configuration as in Example 1 except that the selective reflection wavelength of the polymer liquid crystal was adjusted to 550 nm using pulsed light of 5 mW and 80 μs, and a color change based on a change in pitch length was observed. When reproduced with 1 mW of left-handed circularly polarized light, there was a noticeable increase in reflectance in the information recording section, and reproduction was possible with a high S/N of ~50 dB.
On the other hand, when reproduction was performed using clockwise circularly polarized light or non-polarized light, almost no S/N ratio was obtained.

Note that when the incident power density was continuously changed from 25 to 65 mJ/cm ² , a continuous increase in reflectance was observed corresponding to the incident energy density. This revealed the possibility of multilevel recording through multilevel control of incident energy. The recorded portion could be erased by heating at 70° C. or higher.

(Effects of the Invention) As described above, the present invention makes it possible to provide a reading method for a polymer liquid crystal optical recording medium that can achieve a high S/N ratio of 60 dB.

[Brief explanation of the drawing]

The figure is a schematic cross-sectional view of a recording medium used in an example of the present invention. In the figure, 1... Support substrate, 2... Light absorption layer, 3... Polymer liquid crystal, 4... Protective layer, 5... Light beam, 6...
Polarization direction of a light beam.

Claims (1)

[Claims] 1 An optical recording reading method in which information is recorded and erased by irradiating an optical recording medium with a light beam focused on a minute spot, and the information is read out by changing the reflectance or transmittance of the light beam. An optical recording medium having an optical recording layer made of a cholesteric polymer liquid crystal whose helical pitch has been adjusted so as to selectively reflect light in the near-infrared wavelength region is rotated in the same direction as the rotational direction of the helical pitch of the cholesteric polymer liquid crystal. 1. An optical recording and reading method characterized by irradiating circularly polarized light and detecting changes in reflected light or transmitted light.