JP4538330B2 - Multi-photon recording method and multilayer optical recording medium - Google Patents

Description

  The present invention relates to a multiphoton recording method and a multilayer optical recording medium, and more particularly to a two-photon absorption recording method for recording information by two-photon absorption using light beams of two different wavelengths as recording materials in the optical recording medium.

  In recent years, the increase in capacity of optical discs has been achieved mainly by increasing the recording surface density by shortening the recording wavelength and improving the shortest recording wavelength by increasing the NA of the objective lens. On the other hand, attempts to achieve high-capacity recording by effectively using the entire recording medium by utilizing light transmittance originated in the hologram memory and continued to three-dimensional optical recording by bit recording. It was.

Such three-dimensional optical recording by bit recording can increase the capacity by the number of layers in the depth direction as compared with optical recording with a single recording layer, which is 100 times larger than the current recording layer. Realization of capacity recording is expected. In order to multiplexly record in the depth direction, it is basically necessary to write or read only to the intended layer of the multiplexed recording layers having the same properties. Therefore, it is necessary to introduce some non-linearity and impart selectivity. One method for imparting this nonlinearity is writing using a two-photon absorption reaction. The normal absorption process is a one-photon absorption process corresponding to the energy of the absorption band, but a similar reaction can be caused by a two-photon absorption process that simultaneously absorbs two photons having half the energy. A characteristic of this two-photon absorption process is that the amount of absorption is proportional to the square of the light intensity (energy density). As a result, absorption occurs only in the vicinity of the focus of the writing light where the energy density of light increases. Non-Patent Document 1 describes materials that can be erased and written depending on the material used for the recording layer. In the erasing process, it has been shown that only a specific write bit can be erased using a two-photon absorption process. However, since the absorption wavelength is different, a short pulse light source having a wavelength different from that of writing is required. Is done.
A.Toriumi and S.Kawata, Opt.Lett Vol.23, No.24 P.1924〜P.1926 (1998)

  However, according to Non-Patent Document 2, the recording layer can be multilayered by using the two-photon absorption process. However, the probability of the two-photon absorption process is small compared to the one-photon absorption process, and the recording layer has a very high intensity. A light source is required. Therefore, in order to achieve both the prevention of thermal damage of the recording material and the two-photon absorption reaction, an ultrashort pulse light source having a large peak power but a relatively small average output is required as a writing light source. At present, the ultra-short pulse light source is a large and expensive light source, which is a large barrier to practical use. In addition, in order to realize three-dimensional multilayer recording, it is important to develop a light source that can be replaced with a large and expensive ultrashort pulse light source, but in addition to that, a highly sensitive material with high efficiency of two-photon absorption reaction can be used. Development is essential. Various materials researches have been conducted to increase the efficiency of the two-photon absorption reaction, and one of the factors that determine practical sensitivity is the magnitude of the refractive index change. This refractive index change is caused by a volume change caused by a photoisomerization reaction of the dye. Therefore, in order to make use of the potential of the dye, photoisomerization in the medium needs to occur promptly.

  The present invention is intended to solve these problems, and is a multiphoton recording method and multilayer optical recording medium in which photoisomerization occurs quickly in the recording medium and the effective sensitivity of the recording material can be increased. The purpose is to provide.

In order to solve the above problems, the multiphoton recording method of the present invention includes a preheating step in which at least a part of the recording medium is heated simultaneously with or before the recording light irradiation, and the irradiation light amount of laser light used in the preheating step The irradiation light quantity is changed with the time from when the pit tip is irradiated until the rear end is irradiated . Therefore, recording with minimum interference between adjacent bits and adjacent recording layers is possible. Also, a highly reliable multilayer optical recording medium and multiphoton recording / reproducing apparatus capable of suppressing the aggregation of the dye in the recording layer and maintaining the S / N during recording by minimizing the input heat amount. Can be provided.

  In addition, because the heating means in the preheating process is heating by laser light irradiation, it is heated to the vicinity of the glass transition point of the binder material remotely and locally. However, since it is hardly affected by heat, the recorded contents are kept stable.

  Furthermore, the laser light used in the preheating process is introduced coaxially with the writing light and is modulated in synchronization with the recording information, so that the amount of input heat can be reduced, and variation in the writing threshold of adjacent bits can be prevented. Reliable multi-layer optical recording that is less affected by crystallization of the dye layer and reduction of recording bit contrast due to heat cycle due to a small change in temperature of recorded bits in the vicinity of writing bits such as adjacent tracks or adjacent recording layers A medium and a multiphoton recording / reproducing apparatus can be provided.

  In addition, by configuring the change in the amount of irradiation light by irradiating multiple pulse trains with different outputs and / or pulse widths, digital control becomes easier, and control at a higher speed than analog control is possible. Even when it is performed, a low-cost system can be constructed while maintaining control accuracy.

  Further, the irradiation pattern of the laser beam used in the preheating process follows a table determined in consideration of the input heat amount at the time of writing the recording bit recorded before the recording bit. Therefore, the amount of input heat can be optimized, it is less susceptible to thermal interference between adjacent bits and adjacent recording layers, and dye crystallization in the dye layer is further suppressed, so that high S / N and high It is possible to provide a multilayer optical recording medium and a multiphoton recording / reproducing apparatus that can achieve both reliability.

  In addition, the multilayer optical recording medium as another invention absorbs laser light used in a preheating process in which at least a part of the optical recording medium is heated before recording light irradiation in addition to the recording dye in the multilayer optical recording medium. It is characterized in that the pigment is dispersed. Therefore, since the amount of preheat light absorbed in the recording medium can be determined by the dye concentration, the range of choices for binders, etc. that make up the recording medium is widened. A recording medium can be provided.

  Furthermore, since the density distributions of the recording dye and the dye that absorbs the laser light used in the preheating process match, it is possible to dispose the dye only in the recording layer that requires preheating and to heat it. It is possible to provide a highly reliable multilayer optical recording medium in which the amount of heat input into the recording medium is reduced, and there is no influence of crystallization of the dye layer or reduction in contrast of recording bits due to heat cycle.

  In addition, the density of the dye that absorbs the laser light used in the preheating process has a concentration gradient depending on the depth from the surface of the multilayer optical recording medium, thereby flattening the writing and reading power for each layer and flattening the reading signal intensity. Therefore, a highly reliable multilayer optical recording medium can be provided.

  Further, since the layer containing the recording dye and the layer not containing the recording dye are made of materials having different glass transition points, the layer containing the recording dye has priority on sensitivity, and the layer not containing the recording dye has priority on stability of the recording medium. As a result, a highly reliable multilayer optical recording medium can be provided.

According to the multiphoton recording method of the present invention, the irradiation light amount of the laser beam used in the preheating process in which at least a part of the recording medium is heated at the same time or before the recording light irradiation until the rear end is irradiated after the irradiation of the pit front end. The amount of irradiation light is changed over time. Therefore, recording with minimum interference between adjacent bits and adjacent recording layers is possible. Also, a highly reliable multilayer optical recording medium and multiphoton recording / reproducing apparatus capable of suppressing the aggregation of the dye in the recording layer and maintaining the S / N during recording by minimizing the input heat amount. Can be provided.

  FIG. 1 is a sectional view showing the structure of a multilayer optical recording medium according to an embodiment of the present invention. In the multilayer optical recording medium 100 of this embodiment shown in the figure, a diarylethene as a recording material and PVB (polyvinyl butyral) as a binder are formed on a polycarbonate recording layer supporting substrate 103 in which grooves are engraved. A recording layer 104 is formed by spin-coating an equal amount dissolved in methoxyethanol and further added with an azo dye. Next, a solution containing PVB dissolved in methoxyethanol and containing no pigment was prepared, the adhesive layer 102 was formed, the layers were sequentially laminated, and then the protective substrates 101 and 105 were adhered to both surfaces to prepare a high-sensitivity multilayer substrate. Note that the configuration, material, pigment, and the like of the substrate are examples of the configuration of the present invention, and are not limited to the present embodiment.

  FIG. 2 is a cross-sectional view showing another configuration of the multilayer optical recording medium according to an embodiment of the present invention. In the multilayer optical recording medium 200 of the present embodiment shown in the figure, a recording layer supporting substrate 203 made of polycarbonate having grooves engraved with diarylethene as a recording material and PVB (polyvinyl butyral) as a binder. A recording layer 204 is formed by spin-coating an equal amount dissolved in methoxyethanol and further added with an azo dye. Next, a solution not containing a pigment obtained by dissolving PVB in methoxyethanol was prepared, and an adhesive layer 202 was formed. From the outermost recording layer 204, a solution with different dye concentrations is prepared so that the amount of absorbed light per layer is 0.2% of the amount of light on the surface of the medium at a wavelength of 780 nm. did. Finally, the protective substrates 201 and 205 were bonded to both surfaces to produce a high-sensitivity multilayer substrate. Note that the configuration, material, pigment, and the like of the substrate are examples of the configuration of the present invention, and are not limited to the present embodiment.

  Next, the two-photon absorption recording apparatus according to the present invention, which uses the multilayer optical recording medium shown in FIGS. 1 and 2, performs the preheating process with laser light, and synchronizes the preheating with the laser light with the recording information, will be described. To do.

  FIG. 3 is a block diagram showing a configuration of a two-photon absorption recording / reproducing apparatus according to an embodiment of the present invention. The two-photon absorption recording / reproducing apparatus 300 of this embodiment shown in the figure has two systems of optical systems. First, the optical system of a reproducing laser and preheating light laser relating to reproduction / focus / tracking servo and preheating will be described. The light emitted from the semiconductor laser 301 for 780 nm preheating light that also serves as a reproducing light becomes parallel light by the coupling lens 302, the optical path can be changed by the non-polarizing beam splitter 303, and the polarizing beam splitter 304 and the quarter wavelength plate 305 And is converted into circularly polarized light. A part of the light condensed by the objective lens 306 on one of the recording layers of the multilayer optical recording media 100 and 200 described above is reflected by a difference in refractive index between different interfaces constituting the recording layer. In the case of the multilayer optical recording media 100 and 200 described above, it is about 0.1%. The reflected light has a reverse optical rotation direction, passes through the quarter-wave plate 305, is converted into linearly polarized light orthogonal to the incident light, and is guided to the detection system by the polarization beam splitter 304. In the detection system, the light is condensed by the condensing lens 307 and the reflected / scattered light from other than the focal point is removed by the pinhole 308. The light is condensed again by the condenser lens 309, passes through the cylindrical lens 310, and is guided to the four-divided avalanche photodiode 311. Tracking focus servo is applied by a signal detected with high sensitivity by the four-divided avalanche photodiode 311. Needless to say, the recorded information can also be read by the sum signal from the four-divided avalanche photodiode 311. The light emitted from this light source has a weak output for two-photon absorption to occur in the recording layer, and the reproduction signal cannot be deteriorated by reading. Since the recording layer contains an absorbing dye, approximately 0.2% of the recording layer is absorbed and converted to heat in this embodiment. As with CD / DVD-based recording media, only the recording portion where the photoisomerization reaction excited by two-photon absorption occurs rapidly by increasing the output at the portion where the mark is recorded and raising the recording layer above the glass transition point. To reform. However, the front and rear layers have the same spot, so the amount of heat per layer is the same, but heat is absorbed in a wide area, so the temperature does not rise, and adverse effects such as recrystallization are minimized.

  Next, the recording system will be described. The light emitted from the 780 nm recording pulse laser 312 passes through the beam expander 313, the non-polarizing beam splitter 303, the polarizing beam splitter 304, and the quarter-wave plate 305, and then into the multilayer optical recording media 100 and 200 by the objective lens 306. Focus. At this time, the emitted light of the recording pulse laser 312 needs to be adjusted to be coaxial with the emitted light of the reproducing semiconductor laser 301. At this time, since the focal points in the multilayer optical recording medium coincide, tracking / focus servo at the time of recording is also possible. With the mechanism as described above, the light emitted from the recording pulse laser 312 is focused on the multilayer optical recording media 100 and 200. In addition to the recording dye, the recording layer also contains an absorption dye for the preheating laser beam. The recording layer is also irradiated with the preheating laser in synchronism with the recording light irradiation, the recording spot periphery is preheated, and the binder resin is softened. Therefore, the isomerization reaction of the recording dye is performed promptly. Through the above process, information is written by two-photon absorption. The shape of the recording medium can take other forms such as a card in addition to the disk, and the configuration of the present embodiment, including the recording / reproducing system corresponding to the form, does not limit the configuration.

  In more detail here, an ultrashort pulse laser is used as a light source for obtaining a large electric field intensity that induces two-photon absorption, but the advantage of using an ultrashort pulse laser is that the average power is small, The thermal effect on the sample is small and no thermal damage of the sample occurs. However, since the peak power is about 3 to 5 orders of magnitude higher than the average power, it is possible to induce two-photon absorption only in the vicinity of the focused spot. Here, considering the multiphoton absorption recording by the change in refractive index from the recording dye side, the absorption wavelength of the dye that caused the two-photon absorption changes due to the change in the three-dimensional structure of the molecule by the photoisomerization reaction. Thus, the mark is written by changing the refractive index on the long wavelength side. In addition, in the application as a multi-layer memory, it is desirable that the recording dye is in an amorphous state in order to read / write using light from the recording layer multiplexed in the depth direction, and in the binder resin to stabilize the amorphous state Is distributed. However, the dispersion in the resin is constrained by restraining the dye molecule and physically inhibiting the molecule of the photoisomerization reaction that is the source of the refractive index change. For the purpose of preventing this, when using a resin that is weakly bound to dye molecules by the resin, not only the isomerization reaction is accelerated, but also the diffusion of the dye is generally fast, and the recording layer is deteriorated and recorded by crystallization. Mark contrast also decreases. Therefore, in the present invention, attention is paid to the fact that the mechanical strength changes, that is, the softening caused by applying heat to the resin, thereby weakening the constraint on the photoisomerization reaction of the dye existing in the network. Even with a light source having a small peak power, writing can be facilitated (high sensitivity), and reading resistance and storage characteristics can be ensured by setting the reading temperature and storage temperature to room temperature.

  In addition, recording is facilitated by preheating as in the present embodiment, but repeating the heat cycle also involves the diffusion of the dye in the binder resin, and the recorded information every time the number of cycles passes. Deterioration progresses. Therefore, by remotely and locally heating to the vicinity of the glass transition point of the binder material by the CW laser light, the recording threshold value is lowered only at the heated portion, but the recording is not affected by the heat in other portions. The content is kept stable.

  In addition, by heating the laser light for preheating in synchronization with the recording information (recording bits written by writing light), the input heat amount can be reduced, thereby preventing variations in the write threshold of adjacent bits. Further, the stability can be further improved by reducing the temperature change of the recorded bits in the vicinity of the write bit such as the adjacent track or the adjacent recording layer.

  Further, the advantage of optical recording is that the recording can be read and written remotely. The feature of being able to read and write remotely is that at least one side of the recording medium is made of a material that transmits light. Therefore, in the case of preheating with laser light, it is necessary to balance the utilization efficiency (that is, absorption rate) of the laser light used for preheating and the transmittance for remotely reading and writing recording. In order to achieve this balance, it is possible to change the absorptivity and transmittance of materials such as resin that constitute the recording medium, but by changing the amount of the laser light absorbing dye mixed, the physical properties of the resin ( It is possible to design the amount of absorption and transmission without greatly changing the optical properties (glass transition point).

  Furthermore, the smaller the absorption of the laser beam for preheating, the smaller the influence of thermal interference. Therefore, by matching the distribution of the recording dye and the absorption dye of the preheat light, only the region where the recording dye exists is selectively heated, and it is possible to reduce both the recording threshold and the stability of recorded bits.

  Also, in a multilayer optical recording medium having recording layers multiplexed in the depth direction, the intensity of the preheat light is reduced in the deep recording layer by the amount absorbed by the shallow recording layer compared to the recording layer having a shallow depth. . Therefore, in order to compensate for this, it is possible to increase the dye concentration in the deep recording layer and increase the amount of absorption, thereby increasing the heating efficiency and flattening the necessary amount of preheat light. Further, as described above, it is desirable that the portion heated above the glass transition point by the preheating step coincides with the writing mark site where the photoisomerization reaction is induced by the writing pulse, and the crystal of the dye around the mark. In order to suppress the formation, it is important to secure the recording characteristics of the dye layer on the medium and the storage characteristics of the recording marks so that the area around the mark is not heated as much as possible. In the configuration of the present invention, since there is a physical distance between adjacent marks at the tip of each mark, it is necessary to preheat from a temperature close to room temperature. Once it rises to the glass transition point, it is only necessary to input heat in the direction in which the circumferential bit extends. In particular, the heat diffusion is slower than that of the current disk with a metallic reflective layer, so once heated, the amount of heat to keep the temperature in the spot is small, and the amount of heat to heat the part extending in the longitudinal direction. It is only necessary to input. Therefore, the tip of the mark requires the most heat, and by controlling it as it goes backward, it becomes possible to match the shape of the recording mark with the region where the temperature rises to the glass transition point.

  Next, regarding a high-sensitivity recording method using a preheat light control sequence, the multi-layer optical recording medium shown in FIGS. 1 and 2 is used and recording is performed using the two-photon absorption recording / reproducing apparatus shown in FIG. The control sequence of the preheating light source will be described with reference to FIG. FIG. 4D is a schematic diagram of pits on the multilayer optical recording medium. 4A shows the case where the preheat light is ON / OFF controlled, FIG. 4B shows the case where the preheat light is attenuated by approximate exponential function approximation, and FIG. 4C shows the first pulse. The control sequence of the preheat light when the output is large and the remaining pulse is 60% is shown. In any case, writing by two-photon absorption was confirmed, and it was confirmed that the recording energy was lowered as compared with the case where there was no preheat light. Note that the above-described embodiment is merely an example, and it is needless to say that various conditions can be adopted for the configuration of the dye, the binder, and the recording medium, the reflectance of each layer, the absorption rate of infrared light, and the like. When the above conditions change, the control sequence also needs to be changed and is not limited to the present embodiment.

  In this way, the input heat amount can be controlled by analog control of the output of the preheating laser, so that ideal writing can be performed. However, performing analog control at high speed makes it difficult to achieve both control accuracy and cost. Therefore, the control of the input heat amount in the present invention can be performed as digital control performed by the height or width of the pulse or a combination thereof.

  In the multi-photon recording method, the irradiation pattern of the laser beam used in the preheating process follows a table determined in consideration of the input heat amount at the time of writing the recording bit recorded before the recording bit. Furthermore, as described above, according to the present invention, optimal control of the preheating process at the time of writing for each recording bit is achieved. In consideration of the fact that the multilayer optical recording medium of the present invention has poor heat conduction in the recording layer compared to current discs, and the effect of heat accumulation due to preheating tends to appear, and thermal interference between recording bits and between recording layers tends to occur. Considering the amount of heat input to the adjacent bit, and adopting an irradiation pattern of preheat light according to the table, a desired preheating effect can be obtained by inputting a minimum amount of heat.

  The present invention is not limited to the above-described embodiments, and it goes without saying that various modifications and substitutions are possible as long as they are described within the scope of the claims.

1 is a cross-sectional view illustrating a configuration of a multilayer optical recording medium according to an embodiment of the present invention. It is sectional drawing which shows another structure of the multilayer optical recording medium based on one embodiment of this invention. It is a block diagram which shows the structure of the two-photon absorption recording / reproducing apparatus based on one embodiment of this invention. It is a figure which shows the control sequence of the light source for preheating at the time of recording using a multilayer optical recording medium and a two-photon absorption recording / reproducing apparatus.

Explanation of symbols

100, 200; multilayer optical recording medium,
102, 105, 202, 205; adhesive layer,
103, 203; recording layer support substrate,
104, 204; recording layer,
300; two-photon absorption recording / reproducing apparatus,
301; semiconductor laser for heat light; 302; coupling lens;
303; non-polarizing beam splitter, 304; polarizing beam splitter,
305; 1/4 wavelength plate, 306; Objective lens,
307, 309; condenser lens, 308; pinhole,
310; cylindrical lens,
311; quadrant avalanche photodiode,
312; recording pulse laser; 313; beam expander.

Claims (9)

In a recording method for performing recording by a multiphoton absorption process on a recording material in a recording medium,
Having a preheating step of heating at least a part of the recording medium simultaneously with or before the recording light irradiation,
A multi-photon recording method characterized in that the irradiation light amount is changed with the time from the irradiation of the pit front end to the end of irradiation of the rear end of the irradiation light amount of the laser light used in the preheating step.   The multi-photon recording method according to claim 1, wherein the heating in the preheating step is heating by laser light irradiation.   3. The multiphoton recording method according to claim 2, wherein the laser beam used for heating in the preheating step is introduced coaxially with the writing beam and is modulated in synchronization with the recording information.   The multiphoton recording method according to claim 1, wherein the change in the amount of irradiation light is configured by irradiation with a plurality of pulse trains having different outputs and / or pulse widths.   5. The multiphoton recording method according to claim 1, wherein an irradiation pattern of the laser beam used in the preheating step is based on a table determined in consideration of an input heat amount at the time of writing a recording bit recorded before the recording bit.   A multilayer optical recording medium comprising a multilayer optical recording medium in which a dye that absorbs laser light used in a preheating process in which at least a part of the optical recording medium is heated before recording light irradiation is dispersed in addition to the recording dye .   The multilayer optical recording medium according to claim 6, wherein the density distribution of the recording dye and the dye absorbing the laser beam used in the preheating step match.   The multilayer optical recording medium according to claim 6 or 7, wherein the multilayer optical recording medium has a density gradient in the density of the dye that absorbs the laser light used in the preheating step depending on the depth from the surface of the multilayer optical recording medium.   The multilayer optical recording medium according to any one of claims 6 to 8, wherein the layer containing the recording dye and the layer not containing the recording dye are made of materials having different glass transition points.

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