JPH0581016B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to an optical recording system, and particularly to a rewritable information multilevel recording system.

[Conventional technology]

Conventionally, magneto-optical recording methods and methods utilizing phase change of inorganic materials are known as rewritable information recording methods.

In the magneto-optical recording method, the direction of spin in the medium is controlled by an external magnetic field, and a slight rotation in the polarization direction is detected using the Kerr effect or the Faraday effect.

Methods that utilize phase changes in inorganic materials produce crystals in inorganic materials through the difference between rapid cooling and slow cooling after heat application.
A crystal transition or a crystal-amorphous transition is caused, and the accompanying change in reflectance or transmittance is utilized.

In addition, Japanese Patent Application Laid-Open No. 10930/1983 discloses an information recording method that uses polymeric liquid crystal as a recording layer and uses a constant energy density for writing, with the aim of improving the S/N ratio and reducing manufacturing costs. and Japanese Patent Application Laid-Open No. 59-35989.
In this method, the reflectance of the liquid crystal groups in a uniformly oriented state and in a non-oriented state is determined by a change in transmittance.

[Problem that the invention seeks to solve]

As we become an information society, information media and systems with high recording density are desired, but the recording density is limited by the beam diameter spread of about 1 μm depending on the wavelength of the currently available light beam, and the above-mentioned polymer Even with information recording methods using liquid crystals, the maximum recording density is 10 ⁹ BPI ² .

In order to further improve the recording density, it is generally sufficient to create a plurality of different wavelength characteristics in one recording spot and record information in a multivalued manner. In this case, the magneto-optical recording method uses spin reversal, the rotation angle is small, and the S/N is
Since the ratio is small, it is difficult to apply this to multiple values. In addition, in methods that utilize phase changes in inorganic materials, the phase transition temperature region is narrow and the intermediate change in reflection spectrum characteristics in that region is small, so it is difficult to achieve multi-leveling using this change in reflection spectrum characteristics. .

SUMMARY OF THE INVENTION An object of the present invention is to provide an optical recording system that enables multilevel recording in an information recording system using a polymeric liquid crystal as a recording layer.

[Means to solve the problem]

The present invention provides an optical recording system for an optical recording medium in which information can be recorded and erased by changing the light reflectance or light transmittance of the optical recording medium through light irradiation, using a cholesteric polymer liquid crystal as the optical recording medium. , controlling the light energy irradiated to the optical recording medium in a multivalued manner, and changing the helical pitch length or the inclination of the helical axis of the cholesteric polymer liquid crystal or forming pits in accordance with the light energy; The method is characterized in that information is multivalued recorded on an optical recording medium, and the information is read out from the wavelength of the maximum or minimum value of the light amount in the optical reflection or transmission spectrum of the optical recording medium.

[Effect]

As a result of intensive research, the present inventors have found that polymer liquid crystals that have an appropriate viscosity and can control intermediate states, particularly cholesteric polymer liquid crystals (liquid crystals containing cholesteryl and chiral nematic polymer liquid crystals) that can utilize Blugg reflection. It has been found that the recording density can be improved by controlling the energy density of the irradiated light energy in a multivalued manner using at least two or more values.

FIG. 1 shows the red shift of the reflection spectrum with respect to the energy density of the irradiated light when an optical recording medium containing a cholesteric polymer liquid crystal as a recording layer is irradiated with light beam pulses having various energy densities. The threshold for selective reflection wavelength shift is 25 mJ/cm ² , and from this it shifts to the longer wavelength side within a range of 65 mJ/cm ² . In FIG. 1, indicates the initial selective reflection spectrum, indicates the selective reflection spectrum at 50 mJ/cm ² , and indicates the selective reflection spectrum at 65 mJ/cm ² . When the optical energy density is 65 mJ/cm ² or more, a uniform reflection spectrum with low reflectance is shown. It was also found that these changes in the reflection spectrum were preserved even after pulse irradiation.

This phenomenon is thought to be due to the following mechanism. That is, the light energy generated by the irradiation of the light beam is converted into thermal energy by the light absorber, thereby increasing the temperature of the cholesteric polymer liquid crystal. Since it is known that the cholesteric pitch (helical pitch) of a cholesteric polymer liquid crystal changes with temperature, it is thought that this change in cholesteric pitch causes a shift in the selective reflection spectrum based on the cholesteric pitch. In other words, there is a relationship between the maximum selective reflection wavelength λ _nax and the cholesteric pitch P of the tube, λ _nax = n・P, and assuming that the refractive index n is 1.5 and does not change with temperature, the red shift is due to an increase in the cholesteric pitch. it is conceivable that. .

This increase in pitch is fixed by rapid cooling due to the temperature difference between the irradiation spot and its surroundings after the irradiation of the light beam is completed.

As mentioned above, in FIG. 1, a rapid decrease in reflectance is observed in the region where the energy density is 65 mJ/cm ² or more. According to microscopic observation of the obtained spot, it has been found that this spot has a pit shape and has a low reflectance. From this, in the region where the optical energy density is 65 mJ/cm2 ^or more, flow occurs due to the decrease in viscosity and pits are formed, and at the same time non-orientation occurs due to changes in the inclination of the helical axis, resulting in a decrease in selective reflectance. It is thought that a decline is occurring. Even in this region, pits and non-orientation are preserved by rapid cooling.

As described above, different states of reflection spectrum characteristics are maintained depending on each irradiation light energy density. By controlling the irradiated light energy density in a multivalued manner, the light reflection spectrum characteristics of an optical recording medium containing cholesteric polymer liquid crystal as a recording layer are changed in a multivalued manner, thereby recording information. becomes possible.

In order to erase the information recorded in this way, if the recording point of the irradiation spot created as described above is reheated (this may be the entire recording medium or just that spot) and slowly cooled, the reflection of the irradiation spot will be removed. The spectral characteristics will be the same as those of the surroundings,
It was also found that the reflection spectrum characteristics were the same as the initial reflection spectrum characteristics. This is thought to be because the cholesteric polymer liquid crystal undergoes a predetermined reorientation during reheating and slow cooling, and as a result, the reflectance returns to its initial state, meaning that the information is erased and can be rewritten or rewritten at the same time. becomes. The rewriting method can be performed in the same manner as the initial writing method described above.

The readout method utilizes the change in reflectance of the laser diode at a fixed readout wavelength with a red shift of the selected maximum reflection wavelength λ _nax . If a dye laser is used, it becomes possible to detect the red-shifted wavelength position of λ _nax and read information from it. This is done by scanning the wavelength of the dye laser to shift the selected maximum reflection wavelength. This is performed by detecting the amount of reflected light for each minute wavelength interval during wavelength scanning, and detecting the wavelength position where the difference is less than or equal to a predetermined amount (maximum value). The method of detecting the difference is to detect the amount of reflected light for each minute wavelength interval, convert it to voltage, perform analog-to-digital conversion, and digitally calculate the amount of difference.

Furthermore, the present inventors have confirmed that information can be recorded by changing the transmission spectrum characteristic in a multi-valued manner, similar to the reflection spectrum characteristic, and the information can be read from the wavelength of the transmission spectrum. When using changes in transmission spectrum characteristics, information is read out by reading information from the wavelength of the minimum light amount in the light transmission spectrum.

〔Example〕

Next, one embodiment of the present invention will be described.

In this embodiment, a cholesteric polymer liquid crystal having a molecular structure shown in FIG. 3 is used. The direction of rotation of the helix of this cholesteric polymer liquid crystal is left-handed. In this cholesteric polymer liquid crystal,
A laser diode with a wavelength of 830 nm was used to irradiate pulsed light with a duration of 80 μs (spot diameter 10 μm), and the changes in the reflection spectrum obtained with respect to the optical energy density are shown in Figure 1. The light beam energy density of 25 mJ/cm ² is the threshold of writing energy, and the reflection spectrum changes in the range from this to 65 mJ/cm ² (curve, , ). When an energy density of 65 mJ/cm ² or more is applied, the reflectance decreases uniformly (curve).

FIG. 2 shows a cross-sectional view of a recording medium using this cholesteric polymer liquid crystal as a recording layer. A vanadium oxide film is deposited as a light absorber 3 on a support substrate 1, and a cholesteric polymer liquid crystal 5 is applied thereon by spin coating. Then, a spacer 7 and a transparent protection plate 11 are provided to form an air gap cell having an air gap layer 9. The cholesteric polymer liquid crystal layer is bulky at room temperature and does not need to be sealed like low molecular liquid crystals. A circularly polarizing plate 15 is disposed on the protective plate 11 and is aligned with the helical rotation direction of the cholesteric polymer liquid crystal 5.
will be established. This circularly polarizing plate is used to circularly polarize the light beam to match the direction of rotation of the cholesteric polymer liquid crystal 5 with the helical rotation direction of the light beam when an unpolarized light beam is used for writing or reading. This is because the selective reflection of the cholesteric polymer liquid crystal is effective only for light having the same rotation direction as its helical rotation direction. By providing a circularly polarizing plate, the S/N ratio during reproduction can be increased. Note that the circularly polarizing plate 15 may be provided on the lower surface of the protection plate 11. Of course, if a circularly polarized light beam is used, a circularly polarizing plate is not necessary.

To write to and read from the recording medium shown in Figure 2, the conventional DRAW
You can use a type of optical head. Information is written by controlling the light energy irradiated onto the recording medium in a multivalued manner, changing the helical pitch length or the inclination of the helical axis of the cholesteric polymer liquid crystal in response to the light energy, or forming pits. This is done by recording information on a medium in multiple values. Information is read by irradiating the recording medium with a light beam having an optical energy density of 10 mJ/cm ² and reading information from the wavelength of the maximum light intensity value of the optical reflection spectrum of the recording medium.

In the cholesteric polymer liquid crystal used in this example, the maximum peak wavelength λ _nax of the selective reflection spectrum at room temperature is 570 nm, and the half width is nm.
The half-width is a measure of the orientation order of a cholesteric polymer liquid crystal, and a half-width of 100 nm is not a very high orientation order. If a cholesteric polymer liquid crystal with high orientation order is used and the maximum peak wavelength is appropriately close to the wavelength of the writing/reading light,
It is easily estimated that the writing and reading sensitivity will be improved. Note that no deterioration of the recording medium was observed when the optical energy density of the reading light was 10 mJ/cm ² .

The transfer rate of the maximum peak wavelength is determined by the pit formation energy density from the threshold energy density of 25 mJ/ ^cm2.
50nm for every 10mJ/ ^cm2 in the range of 65mJ/ ^cm2 ,
This amount of change can be discerned even in this example, which has a dispersion of 100 nm, and this makes it possible to perform multilevel recording and readout based on wavelength. In addition,
The reproduction S/N ratio was 60 dB.

In addition, it is possible to reduce spectral dispersion by setting the selected wavelength position of the recording medium and improving the alignment film formation characteristics of the cholesteric polymer liquid crystal, making it possible to identify more reflection spectra, and further improving the S /N ratio and recording density can be expected to improve.

〔Effect of the invention〕

According to the present invention, by using a cholesteric polymer liquid crystal, it is possible to produce a plurality of different wavelength characteristics in one recording spot and record information in a multivalued manner.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the reflection spectrum characteristics of cholesteric polymer liquid crystal used to explain the operating principle of the present invention, Fig. 2 is a cross-sectional view of a recording medium used in an embodiment of the present invention, and Fig. 3 1 is a diagram showing the molecular structure of a cholesteric polymer liquid crystal used in an example of the present invention. 1... Number of support groups, 3... Light absorber, 5... Cholesteric polymer liquid crystal, 7... Spacer, 9
...Air gap layer, 11...Transparent protection plate, 1
3...Light beam, 15...Circular polarizing plate.

Claims (1)

[Claims] 1. In an optical recording method for an optical recording medium in which information can be recorded and erased by changing the light reflectance or light transmittance of the optical recording medium by light irradiation, using a cholesteric polymer liquid crystal as the optical recording medium,
controlling the light energy irradiated to the optical recording medium in a multivalued manner, and changing the helical pitch length or the inclination of the helical axis of the cholesteric polymer liquid crystal or forming pits in accordance with the light energy;
An optical recording method characterized in that information is multi-valued recorded on the optical recording medium, and the information is read from the wavelength of the maximum or minimum value of the light amount in the optical reflection or transmission spectrum of the optical recording medium.