Optical information recording and reproducing apparatus

An optical information recording and reproducing apparatus comprises an optical disk having a reflection layer, an optical information recording layer, a super resolution layer and a protection layer on the recording surface of a substrate, a laser emission control unit for emitting the laser light to record and reproduce the optical information and changing the pulse-like emission pattern, a pickup for radiating by focusing the laser light and receiving the reflected light, a spindle and a spindle motor for supporting and rotationally driving the optical disk, and a reproduction signal processing unit for arithmetically processing the received signal.

BACKGROUND OF THE INVENTION

This invention relates to an optical information recording and reproducing apparatus, or in particular to an optical information recording and reproducing apparatus high in recording density and having a high super resolution effect.

With the recent progress of the information society using optical telecommunication, the construction of a telecommunication system capable of high-speed communication of information high in recording density has been required. An optical information recording and reproducing apparatus capable of accumulating the optical information of high recording density is an indispensable optical device for developing the high-speed optical telecommunication of high recording density. Further, with the digitization of video information such as TV images and the increased image quality such as high definition, it is of urgent necessity to develop an information recording and reproducing apparatus of high recording density capable of long-time recording while maintaining a high image quality.

Currently, a DVD having a capacity of 4.7 GB on each side finds wide applications as an optical information recording medium for handling high-density dynamic images such as computer and video information. The practical application of this DVD as a rewritable recording and reproducing medium as well as a read-only ROM (DVD-ROM) with information written directly in the substrate has been promoted. The development of these optical information recording media is aimed at a high recording density, and the laser light having a shorter wavelength of 650 nm than the laser (780 nm) for the CD is used as a means to achieve the high density information recording. For applications to the computer graphics and the digital high-definition images involving the information of large recording capacity, however, the recording density four to five times higher is required. In order to meet this requirement, an optical disk using a blue semiconductor laser still shorter in wavelength (405 nm) and having the recording density of 23.3 GB on one side has been developed and found practical applications.

As a technique to further increase the recording density of the optical disk, the development of a multilayer recording method, a multi-valued recording method and a super resolution recording method is under way. Of these next-generation methods to achieve a high recording density, the super resolution recording method is one of the most promising techniques.

In the super resolution recording method, the waist of the laser beam radiated on the recording surface is reduced using the laser focusing function or the masking function of the super resolution layer. This is one of the recording methods of high density recording realized by the reversible change in the optical constants (refractive index (n) and the extinction coefficient (k)) of the super resolution layer formed in a multilayer structure such as the recording layer, the protection layer or the reflection layer of the optical disk. The super resolution layer, upon radiation of the read/write laser thereon, is excited by the temperature rise or the absorption of photons of light. As long as the laser is radiated, therefore, the refractive index and the extinction coefficient are changed reversibly, while the original state is restored upon extinction of the laser. In the optical disk, a recording portion and a non-recording portion are determined for reproduction by the amount of the laser light returned to the pickup after being radiated and reflected on the optical disk. Due to this reversible change of the optical constants in the super resolution layer, the area of the light returned to the pickup can be reduced more than the normal radiation area of the laser light. Specifically, the resolution can be improved by reducing the readable area using the optical masking effect.

The extinction coefficient (k) is a quantity proportional to the light absorption coefficient of a material, and assumes a larger value the larger the absorption coefficient of the material. The two constants including the refractive index (n) and the absorption coefficient (k) are collectively called the optical constants.

In the prior art, as described in JP-A-10-340482, for example, a thin film material of cobalt oxide has been used as the super resolution layer. The large change in refractive index and the super resolution effect of this thin film can produce an optical disk of high recording density.

In the optical information recording and reproducing apparatus currently available, the optical information is reproduced by radiating the continuous wave (CW) light or the reproducing laser light superposed with high frequencies of about 400 MHz. In the case where the the complex refractive index of the super resolution layer is changed and the power of the reproducing light is increased until the super resolution effect is obtained, therefore, heat is accumulated on the optical information recording medium by the radiation of laser light, and a broad heat distribution occurs in the laser beam spot, thereby posing the problem that a super resolution mask high in contrast cannot be formed. Further, the heat accumulation degradate the film or the recording pit on the medium, resulting in the degradation of the repetitive reproducing operation characteristic.

A reproducing method with pulse light for improving the super resolution effect by avoiding the heat accumulation in the medium and steepening the temperature gradient in the beam spot is described, for example, in JP-A-10-40547.

In an application of the pulse reproducing method as described in JP-A-10-40547, a high response of the material is essential. Also, the beam spot in super resolution state, which is a superposition of a beam spot in ground state and a beam spot in super resolution state, is affected by the beam spot in ground state, and a satisfactory super resolution mask is difficult to obtain.

SUMMARY OF THE INVENTION

The object of this invention is to provide an optical information recording medium formed with a super resolution film having a high response and a thermal stability and an optical information recording and reproducing apparatus capable of producing a high super resolution effect using the medium.

In order to achieve the object described above, according to this invention, there is provided an optical information recording and reproducing apparatus comprising at least a substrate, a reflection layer formed on the recording surface of the substrate, an optical information recording layer, a super resolution layer having the complex refractive index reversibly changing with temperature, an optical information recording medium having a protection layer, a laser emission control unit for emitting the laser light adapted to record or reproduce the optical information in the optical information recording medium and to change the laser emission pattern, a pickup for radiating by focusing the laser light on the surface of the optical information recording medium and receiving the light reflected from the optical information recording medium, a spindle and a spindle motor for supporting and rotationally driving the optical information recording medium, and a reproduction signal processing unit for arithmetically processing the received signal,

wherein the laser light is emitted in pulses by the laser emission control unit, and the reproduction signal processing unit acquires the reproduction signal from the pulse emitting portion and the bias emitting portion of the pulse light and produces, as a reproduction signal, the result of arithmetically processing the reproduction signal from the pulse emitting portion with reference to the reproduction signal in the bias emitting portion.

The arithmetic process described above is intended to produce the difference between the reproduction signal from the pulse emitting portion and and a constant multiple of the reproduction signal from the bias emitting portion. Also, the arithmetic process is executed after determining the reproduction signal of the bias emitting portion at the same time point as the reproduction signal of a given pulse emitting portion by interpolating the reproduction signals of the adjoining bias emitting portions temporally before and after the reproduction signal of the particular given pulse emitting portion.

The pulse emitting portion of the super resolution layer is in super resolution state but not the bias emitting portion thereof.

The super resolution state is defined as a state in which a part of the laser spot is masked and the optical resolution is increased by the change in the optical constants due to the laser radiation and the resulting temperature increase of the super resolution layer. The state not in the super resolution state, on the other hand, is called the ground state.

The super resolution layer contains Fe2O3, Co3O4, NiO, CoO, ZnO, Cr2O3, ZnS—ZnSe, GaN—InN and Ga2O3, and preferably contains Fe2O3and Ga2O3. Further, the optical information recording layer is a metal film configured of one or more metal elements selected from Ag, In, Ge, Sb and Te.

The optical information recording and reproducing apparatus using the pulse reproducing method according to this invention can reproduce, with a high efficiency, the optical information recording medium formed with a super resolution film, and an optical disk having a high resolution against a small recording mark can be fabricated. Thus, the applicability of this optical information recording and reproducing apparatus is very high.

In the optical information recording and reproducing apparatus according to this invention, the laser light shaped into pulses of the emission frequency higher than the shortest mark frequency is radiated on the optical information recording medium having a super resolution layer of high-speed response capable of following the pulse width. At the same time, the optical signals are detected from the pulse emitting portion and the bias emitting portion not emitting the pulse light, and the result of the arithmetic operation performed with these signals is used as a reproduction signal. Therefore, a super resolution mask having a large contrast can be formed. As a result, the optical information recording and reproducing apparatus higher in recording density than in the prior art can be obtained. Further, the temperature can be increased without deteriorating the super resolution layer or other optical information recording media, thereby producing an optical information recording and reproducing apparatus higher in reliability than in the prior art.

DETAILED DESCRIPTION OF THE INVENTION

An optical information recording apparatus having the recording density more than 1.5 times that of the prior art has been obtained by using an optical information recording medium containing Fe2O3or Ga2O3as a super resolution layer and, as a reproduction signal, the difference between the reproduction signal from the pulse emitting portion and the reproduction signal from the bias emitting portion, which has the pulse width of 3 ns, the pulse period of 8 ns, the emitting power of 6 mW for the pulse emitting portion and the emitting power of less than 0.8 mW for the bias emitting portion with no pulse radiation.

An optical information recording and reproducing apparatus for recording and reproducing the optical information recording medium according to the invention has been fabricated.FIG. 1is a block diagram showing the optical information recording and reproducing apparatus thus fabricated. InFIG. 1, reference numeral1designates an optical information recording medium (hereinafter referred to as the optical disk), numeral2a spindle, numeral3a spindle motor, numeral4a motor circuit control means, numeral5a pickup, numeral6a medium identifying means, numeral7a laser driver, numeral8a reproducing power DC amplifier, numeral9a reproducing peak power determining means, numeral10a reproducing bias power determining means, numeral11a recording power DC amplifier, numeral12a recording peak power determining means, numeral13a recording peak power ratio determining means, numeral14an erasing power DC amplifier, numeral15a reproduction signal detection means, numeral16a peak sample means, numeral17a bias sample means, numeral18a difference reproduction signal calculation means, numeral19an address reading means, numeral20a clock sync means, numeral21a reproduction signal demodulation means, numeral22a reproduction data supply means, numeral23a tracking error detection means, numeral24an information controller, numeral25a pickup control circuit, numeral26a recording timing correcting means, numeral27a recording data modulation means, numeral28a recording data receiving means, and numeral29a pickup motion driver.

The optical information recording and reproducing apparatus according to the invention includes the medium identifying means6for identifying the type of the optical disk1as a recording medium which is classified into the rewritable (RW) type, the write-once (WO) type and the read-only memory (ROM). In the laser emission control system described below, only the reproducing system is driven in the case where the optical disk inserted is the ROM, the reproducing system and the recording system in the case where the optical disk inserted is of WO type, and the reproducing system, the recording system and the erasing system in the case where the optical disk inserted is of RW type. The optical disk1is fixed provisionally on a rotary mechanism connected, directly or indirectly, to the rotary shaft of the spindle motor3fixed on the spindle2and controlled by the motor circuit control means4. The information on the optical disk is read as an optical signal by the sensor for detecting the laser providing the light source of the pickup5and the reflected light. Also, the information is stored in the optical disk by the light source in the pickup. Also, the pickup is set in position along a track by the pickup motion driver29.

The laser emission control unit is classified into the reproducing system, the recording system and the erasing system. The reproducing system includes the reproducing power DC amplifier8, the reproducing peak power determining means9and the reproducing bias power determining means10for pulse reproduction according to the invention. Thus, the pulse reproduction waveform of the reproduction light is formed, and the emission pattern is sent to the laser driver7and the pickup5thereby to emit the laser in pulse form.

At the time of recording the data in the recording system, the recording data is input from the recording data receiving means28and modulated by the recording data modulation means27. The data thus modulated is input to the laser driver7through the recording timing correcting means26thereby to control the light source in the pickup5. The output of the recording peak power ratio determining means13is input to the pickup5through the recording power determining means12, the recording power DC amplifier11and the laser driver7thereby to control the light source in the pickup5. Further, in the erasing system, the recording data is input to the laser driver7through the erasing power DC amplifier14thereby to control the light source in the pickup5.

The optical signal obtained at the time of reproduction is processed in the reproduction signal processing system. The optical information detected by the reproduction signal detection means15are sampled by the peak sample means16and the bias sample means17for the pulse emitting portion and the bias emitting portion separately. These two signals are arithmetically processed by the difference reproduction signal calculation means18. The signals thus processed are output externally by the reproduction data supply means22through the address reading means19, the clock sync means20and the reproduction signal demodulation means21. The reproduction data is output by a predetermined output means such as a display unit or a speaker or processed by an information processing system such as a personal computer.

The focal point and the focal depth vary from one optical disk to another, and therefore, an optical disk having the auto focusing function was selected. Further, in keeping with the configuration in which the a focusing function layer is mounted on the disk and the tracking width is narrowed, a tracking error detection means23for high density recording was added to make possible the tracking suitable for an arbitrary medium. The information from the tracking error detection means23is transmitted to the pickup5through the information controller24and the pickup control circuit25. Also, the mechanism for identifying a medium type utilizing the reflectivity difference between media was employed to make the auto tracking possible in accordance with the difference in medium type.

The laser light source mounted on the pickup has the wavelength of 405 nm. Also, the objective lens for focusing the laser beam on the optical disk has the NA (numerical aperture) of 0.85.

The characteristic of the optical information recording medium having the super resolution effect according to the invention was evaluated using the optical information recording and reproducing apparatus shown inFIG. 1. First, the read-only ROM disk was evaluated. A sectional view of the ROM-type optical information recording medium fabricated is shown inFIG. 2. InFIG. 2, numeral31designates a substrate, numeral32a reflection layer, numerals33,35protection layers, numeral34a super resolution layer, numeral36a cover layer, and numeral37a recording pit. In the ROM-type optical information recording medium according to this embodiment, the recording pit37has the function as an optical information recording layer. According to this embodiment, a medium structure suitable for the optical system having the laser wavelength of 405 nm and the numerical aperture of 0.85 was realized by using a polycarbonate substrate31having the outer diameter of 120 mmφ, the inner diameter of 15 mmφ and 1.1 mm thick, and the cover layer36of a polycarbonate sheet having the outer diameter of 119.5 mmφ, the inner diameter of 23 mmφ and 0.1 mm thick. The reproducing laser is focused from the cover layer36side for reproduction.

The ROM disk was fabricated by the processes described below. First, a recording pit pattern having mark spaces at predetermined intervals was formed on a photo resist using a laser drawing unit. After that, the pit pattern was copied to a Ni die, and a substrate was formed by ejection molding of polycarbonate using this die. The minimum pit size was 139 nm, and the pit depth 22 nm. Also, the track pitch was 320 nm.

A reflection film of an alloy containing 95% Ag, 2.5% Pd and 2.5% Cu (mol %) was formed as the reflection layer32on the substrate thus fabricated. The film thickness was 20 nm. The film was formed by DC magnetron sputtering using the pure Ar gas. An amorphous film containing 80% ZnS and 20% SiO2(mol %) was used as the protection layers33,35, which were formed by RF sputtering using the pure Ar gas. A film containing 50% Fe2O3and 50% Ga2O3(mol %) was used as the super resolution layer34. This layer was formed by RF sputtering using a gas containing 95% Ar and 5% O2(flow rate %). An oxide target having the same composition as described above was used as a sputtering target to form the super resolution layer.

After forming these layers by sputtering, the cover layer36was formed. The UV-curable resin was applied, by spin coating, on the substrate 1.1 mm formed with a film, and a polycarbonate cover layer cut to a circle having the outer diameter of 119.5 mmφ, the inner diameter of 23 mmφ and the thickness of 0.085 mm was attached thereon. The resulting assembly was introduced into a vacuum chamber and while being deaired to a vacuum of about 1 Pa, the sheet and the substrate were closely attached to each other. The UV-curable resin was cured by radiating the UV light from the cover layer side. The thickness of the UV-curable resin was adjusted to the total thickness of 0.1 mm including the UV-curable resin and the cover layer.

According to this embodiment, a polycarbonate substrate 1.1 mm thick was used as the substrate31. The outer diameter of the substrate1is 120 mm, and a hole having the inner diameter of 15 mm was formed for the chuck. On this substrate, the recording pit37for the CN ratio test was formed of steps on the same track at predetermined period with the space.

The recording pits corresponding to the recording signals of2T,3T, . . . ,8T and the spaces are repeatedly recorded with the clock signal (1T=69.5 nm). Only one type of recording signal is formed on the same track, and different signals are recorded on different tracks. In this case study, the length of the recording pit of the2T signal providing the shortest mark is 139 nm, and the length of the recording pit for the8T signal providing the longest mark is 556 nm.

Further, a mirror surface portion in the shape of a ring concentric with the optical disk and having no recording pit is formed on the substrate1. This mirror surface portion is called the mirror surface. By measuring the amount of light reflected from the mirror surface, the nonlinearity of the super resolution layer34can be evaluated.

The output of the repetitive signal described above is observed by oscilloscope, and it can be concluded that the larger the ratio (resolution) of the amplitude of the fine mark such as2T and3T to the signal amplitude based on the longest mark (8T), the higher the resolution of the shortest mark. According to this invention, the resolution (Mod) was defined by Equation (1) below.

Mod=InppI8⁢pp(1)
where I8ppis the amplitude ratio based on the longest mark and Inppthe amplitude ratio of the mark nT (n: 2 to 7) to be measured.

According to this invention, the reproducing operation was performed by radiating the pulse light on the optical information recording medium shown inFIG. 2. The emission pattern of the pulse light used in this invention is shown inFIG. 3. InFIG. 3, character Pp designates the emitting power of the pulse emitting portion and character Pb the emitting power of the bias emitting portion. Character tp designates the emission time of the pulse emitting portion, and character tb the emission time of the bias emitting portion. According to this embodiment, Pp is set to not less than 8 mW but not more than 6 mW and Pb to not less than 0.3 mW but less than 0.8 mW. By doing so, the pulse emitting portion can maintain the super resolution layer in super resolution state, and the bias emitting portion can be maintained in ground state.

Also, tp is set to 1 to 5 ns, and tb to 5 to 13 ns. Further, the linear speed of rotation of the optical disk is set to 4.56 m/s. As a result, the time during which the shortest2T mark is passed is 30.5 ns, and in the case where tp is 3 ns and tb 5 ns, the sampling is possible with about 3.8 pulses.

Generally, even with the laser spot in super resolution state, the reflectivity associated with other than the super resolution state is not zero, and therefore, the signal not in super resolution state is added to the signal in super resolution state. As shown by Equation (2), the output in ground state with the bias emitting portion low in laser power and not associated with the super resolution state is arithmetically processed from the output in super resolution state with the pulse emitting portion high in emitting power and associated with the super resolution state. In this way, the signal output associated with only the super resolution state can be obtained.

The effective reproduction power Pr′ for the pulse reproducing method was determined using Eequation (2) below.

A specific example of this arithmetic operation is explained in detail. The beam spot in super resolution state is considered to be formed as the sum of the linear beam spot not in super resolution state and the nonlinear beam spot in super resolution state. Basically, therefore, only the reproduction signal in super resolution state is obtained by subtracting the portion of the normal beam spot (normally, Gaussian distribution) from the whole beam spot in super resolution state. In order to obtain the suepr resolution state, the read laser light high in power is radiated, and therefore the beam spot intensity of the linear portion also increases. As a more accurate arithmetic operation to estimate the beam spot in super resolution state correctly, therefore, the difference is taken between the beam spot of the linear portion multiplied by a constant and the beam spot in super resolution state.

In the case where this reproducing method with the pulse signal is used, the highly accurate reproduction with super resolution is made possible by always sampling the normal beam spot not in super resolution state and referring to the nearest beam spot in super resolution state. A specific method is described below.

FIG. 14shows an example of the waveform obtained by pulse reproduction. An example of reproducing the2T repetitive signal is taken here. Assume that an arbitrary time point is 0, an arbitrary pulse emission time point t1, the bias emission time point t2, and the pulse emission period and the sampling period δ. The pulse emission time points are given as t1, t1+δ, t1+2δ, . . . t1+Nδ, t1+(N+1)δ . . . . Also, the bias emission time points are given as t1, t1+δ, t1+2δ, . . . t1+Nδ, t1+(N+1)δ . . . . By sampling the output at each time point, the reproduction signals at the time of pulse emission and the bias emission are obtained. The waveforms obtained from only the time of pulse emission and only the time of bias emission are shown inFIG. 15A. The waveform obtained from the reproduction signal at the time of pulse emission and the waveform obtained from the reproduction signal at the time of bias emission are shifted from each other by the time {t2+Nδ)}−{t1+Nδ}=t2−t1. The waveforms with this time shift corrected after forming each reproduction signal are shown inFIG. 15B. The waveform obtained by the arithmetic process of “(reproduction signal at the time of pulse emission)−a(reproduction signal at the time of bias emission)” from the two waveforms is shown inFIG. 15C, whereais a constant. This constant is determined in such a manner as to maximize the output ofFIG. 15Cand varies with the disk type, signal type or the relation of previous and following recording marks and spaces.

In forming the waveforms ofFIG. 15Bfrom those ofFIG. 15Aby the arithmetic operation shown inFIGS. 15A,15B,15C, the reproduction signal at the time of bias emission may be inaccurate as it is not obtained from the output at the time point of t1+Nδ. In such a case, the arithmetic operation shown inFIGS. 16 and 17is performed.FIG. 16is a schematic diagram showing the read out signal at the time of pulse emission at t1+Nδ. The reproduction signal at the time of pulse emission at t1+Nδ is obtained by interpolation from the reproduction signals at the time of bias emission before and after the pulse emission at t1+Nδ, i.e. from the reproduction signals at the time of bias emission at time points t2+Nδ and t2+(N+1)δ. For example, the reproduction signal at the time of bias emission at time point t1+Nδ is determined by using the output at the time of bias emission at time point t2+Nδ and the average value or root mean value at the time of bias emission at time point t2+(N+1)δ.

FIG. 17Ashows the waveform of the reproduction signal at the time of bias emission determined in this manner and the waveform obtained from the reproduction signal at the time of pulse emission. Since the time is corrected as described above, the time shift shown inFIG. 15is eliminated. Also, the waveform obtained by the arithmetic process “(reproduction signal at the time of pulse emission)−a(reproduction signal at the time of bias emission)” from the two waveforms obtained is shown inFIG. 17B, whereais a constant. This constant is determined in such a manner as to maximize the output ofFIG. 17Band varies with the disk type, signal type or the relation between previous and following recording marks and spaces.

As described above, the aforementioned arithmetic process, which is intended to obtain the difference between the reproduction signal from the pulse emitting portion and the reproduction signal from the bias emitting portion, is executed preferably after determining, by interpolation, the reproduction signal of the bias emitting portion at the same time point as the reproduction signal of an arbitrary pulse emitting portion from the reproduction signals of the bias emitting portions immediately before and after the reproduction signal of the arbitrary pulse emitting portion. The super resolution layer is in super resolution state for the pulse reproduction emitting portion and not in super resolution state for the bias emitting portion.

First, as a preliminary study, a multilayer film similar to the film structure shown inFIG. 2was formed on a glass substrate, and the reflectivity change with the temperature increase was determined.FIG. 5shows the temperature dependency of the specific reflectivity change ΔR of the multilayer film having the film structure shown inFIG. 2. The specific reflectivity change ΔR was calculated from Equation (3) below assuming that the reflectivity at each temperature is R and the reflectivity at 30° C. is Ro.

In the film structure having the optical disk configuration shown inFIG. 2, the reflectivity was reduced with the temperature increase by heating at 350° C., and ΔR of −45.5% resulted.

FIG. 6shows the reproduction power (Pr) dependency of the reflection light intensity on the mirror surface of the optical disk having the film structure ofFIG. 2. The reflection light intensity was indicated by the output (Vout(mV)) from the photodiode making up a photo detector. Also, the rotational linear speed of the disk was set to 3 to 9 m/s for measurement.

InFIG. 6, the dashed straight line is based on the assumption that the Voutfor each Pr is proportional to Voutfor Pr of 0.3 mW. In all rotational speeds, it is understood that Voutincreases substantially linearly up to Pr of about 0.7 mW, while Voutis reduced below the linear line for 0.8 mW or more. In the film structure of this optical disk, therefore, the super resolution state is realized for Pr of 0.8 mW or more, while the ground state prevails for Pr of lower than 0.8 mW.

The smaller the rotational speed, the larger the degree of Voutreduction. Especially, for the rotational linear velocity of 3 m/s, Voutfor Pr of 2.0 mW was about 50% of the value assumed for linear Vout. With the increase in Pr or with the decrease in rotational linear velocity, the laser light radiation amount per unit time increases and therefore, the temperature on the surface of the optical disk is considered to increase. As shown inFIG. 5, in the optical disk having the film structure ofFIG. 2, the reflectivity decreases with the temperature increase, and therefore, the reflectivity is considered to decrease with the temperature increase due to laser radiation.

Next, the ROM-type optical disk was actually fabricated and the super resolution effect due to the pulse radiation was studied.

FIG. 4schematically shows the reproduction signals for the reproduction of nT marks of a single period with the pulse and the reproduction with the normal CW (continuous wave) light. In the reproduction of nT marks with the pulse light, a plurality of pulses are radiated at each portion when the marks and spaces are passed. In the reproduction with the normal CW light, on the other hand, the output changes continuously with the reflectivity of the marks and spaces.

In the pulse reproduction method according to this embodiment, as shown inFIG. 4, the low and high levels of the pulse emitting portion are designated as Vlow (pulse)and Vhigh (pulse), respectively, and the low and high levels of the bias emitting portion with no pulse radiation as Vlow (bias)and Vhigh (bias), respectively. Thus, the amplitude Inppis defined by Equation (4).
Inpp=(Vhigh(pulse)−Vhigh(bias))−a(Vlow(pulse)−Vlow(bias))   (4)

FIG. 7shows the RF signal waveform of the2T marks measured using the pulse reproduction method. The abscissa represents the time and the ordinate the output (Vout). The pulse light is radiated from time point 0, and for the time before 0, the reproduction waveform of the CW light is shown. The peak power Pwof the pulse light was set to 8 mW, and the base power Pband the reproduction power Pr with the CW light were set to 0.8 mW. Under this condition, the reproduction amplitude from the2T marks was not substantially observed from the reproduction waveform of CW light, while the signal amplitude of the2T marks was clearly observed in the pulse reproduction unit. This is considered to indicate that the pulse light radiation strongly excites the super resolution film of Fe2O3—Ga2O3into super resolution state for an improved resolution. The pulse light emission time is about 3 ns, and therefore, the response speed of the Fe2O3—Ga2O3super resolution film is not more than 3 ns, thereby indicating that the change in refractive index follows the rise of the pulse light.

A similar evaluation was conducted also for the3T and4T marks. The dependency of the resolution obtained from the pulse reproduction waveform of2T,3T and4T marks on the effective reproduction power Pr′ is shown inFIG. 8. In this embodiment, the constantashown in Equation (4) was determined for each of2T,3T and4T marks. Then, the resolution was maximized at 1.52 for2T, 1.36 for3T and 1.27 for4T. In the study below, an example of calculation using these values as the constants in Equation (4) is described.

With2T mark, the resolution for Pr′ of 2.0 mW is about 5.8 times higher than for Pr′ of 1.4 mW, and the resolution was remarkably improved as compared with the reproduction waveform for the CW light. Similarly, as for the3T mark, the resolution was improved 2.5 times. The resolution of the2T marks for Pr′ of 2.0 mW was substantially the same as that of the3T marks for Pr′ of 1.2 mW, indicating that the resolution along the linear density was improved about 1.5 times.

The resolution of the4T marks, on the other hand, was reduced with the increase in Pr′. To analyze this phenomenon, the resolution for each mark length (T) was plotted, as the result thereof is shown inFIG. 9. InFIG. 9, the ordinate represents the logarithm (dB) of the resolution and the abscissa the mark length. In the case where Pr′ is low, the resolution decreases monotonically with the decrease in mark length. With the increase in Pr′, however, the modulation degree increases in the area of the mark length of not more than 243 nm. Especially, it was found that the resolution conspicuously increases in the neighborhood of 139 nm corresponding to the2T marks. In some part of the mark length range of 243 nm to 380 nm, on the other hand, the resolution was seen to decrease with the increase in Pr.

The beam spot obtained by the super resolution effect is given as the sum of the normal laser spot not in super resolution state and the laser spot in super resolution state. The frequency dependency of the reproduction signal intensity obtained by these beams is shown in the schematic diagram ofFIG. 10. The reproduction output of the laser spot in super resolution state, though smaller than that of the normal laser beam, has a smaller diameter for an improved resolution and has a high output up to the high frequency side. Thus, the cutoff frequency increases from fo to fo′. In the front aperture method, the mask is formed by reduction in reflectivity, and therefore the contribution of the laser spot in super resolution state is negative. In the super resolution state, therefore, the output on low frequency side is lower than in the normal reproduction.

On the high frequency side, on the other hand, the output from the normal laser beam is 0 on the frequency side higher than the cutoff frequency fo. The output due to the super resolution spot, however, becomes conspicuous and improved. The boundary between the output decrease on the low frequency side and the output improvement on the high frequency side is considered fc.

The result of the experiment shown inFIG. 9indicates that the mark length corresponding to fc is 243 nm or substantially equal to one half of the laser beam spot diameter. In the case where the mark size is not less than one half of the laser spot diameter, the signal can be separated even with the normal spot diameter having no super resolution effect, and therefore the amount of the reflected light is conspicuously reduced by the decrease in the laser spot size due to the super resolution phenomenon, thereby probably resulting in a reduced resolution. In the case where the mark size is smaller than one half of the diameter of the normal spot, on the other hand, more than two marks can be contained in the spot, and therefore, the resolution would be reduced. In view of the fact that the super resolution effect reduces the laser spot size and the number of marks in the spots is reduced to less than two, thereby probably resulting in an improved resolution.

Next, the reason why the reproduction method using the pulse light is higher in super resolution effect than the CW reproduction is explained.FIG. 11is a schematic diagram showing a temperature profile on the disk film surface and an optical mask formed in the reproduction of the optical disk having the super resolution effect with the CW light and the pulse light. In the schematic diagram showing the temperature profile in the upper part ofFIG. 11, the abscissa represents the position on the circumference of the disk, and the ordinate the temperature on the film surface. The laser spot is moved to positive from negative side in the drawing. When viewed in an arbitrary time section as shown in the drawing, the light intensity distribution of the laser spot constitutes a Gaussian distribution with the center at position ro. Also, the schematic diagram of the optical mask at the position corresponding to the temperature profile is shown in the lower part ofFIG. 11. This diagram shows a case in which the higher the temperature, the lower the reflectivity. The method shown in this diagram, in which the rear part of the laser beam is masked while the front window is opened in the shape of a mask, is called the “front aperture detection method”.

In the case where the CW light is radiated, the fact that the laser spot position is moved from negative to positive side increases the temperature of the film surface due to the continuous laser light radiation and decreases the temperature with the decrease in the laser light intensity. The resulting shape has a tail on negative side. In accordance with this temperature distribution, the optical constants of the super resolution film change and the reflectivity decreases. Thus, the negative area of the laser spot is masked thereby to reduce the effective diameter of the laser spot.

The pulse laser light, on the other hand, is not radiated continuously as long as the pulse light emission period is sufficiently long as compared with the pulse moving time, and therefore, the tail on negative side is very small as compared with the CW light, thereby producing a temperature profile approximate to the Gaussian distribution. Also, the laser light is radiated for so short a length of time that the specimen is thermally damaged only slightly, and therefore, the peak intensity of the laser light can be increased. As a result, the local temperature at the pulse peak position can be increased. Generally, the higher the temperature, the larger the amount of change in the optical constants of the super resolution material, and therefore, as compared with the CW reproduction, the reflectivity change is considered large. Thus, the reflectivity of the mask portion can be reduced and a super resolution laser spot larger in contrast is considered possible to form.

The foregoing description concerns an embodiment implemented using the arithmetic method shown inFIGS. 15A,15B,15C. The arithmetic process using the method shown inFIGS. 16,17A,17B in similar fashion could produce as high a super resolution effect as inFIGS. 15A,15B,15C.

Next, the recording-type disk was similarly studied. A sectional view of the recording-type optical information recording medium thus fabricated is shown inFIG. 12. InFIG. 12, numeral41designates a substrate, numeral42a reflection layer, numerals43,45,48protection layers, numeral44a super resolution layer, numeral46a cover layer, numeral47a recording layer, numeral49a land, and numeral50a groove. In the recording-type optical information recording medium according to this embodiment, the recording layer47has the function as an optical information recording layer.

According to this embodiment, a medium structure suitable for the optical system having the laser wavelength of 405 nm and the numerical aperture of 0.85 was realized by using the polycarbonate substrate41having the outer diameter of 120 mmφ, the inner diameter of 15 mmφ and the thickness of 1.1 mm and the cover layer46formed of a polycarbonate sheet having the outer diameter of 119.5 mmφ, the inner diameter of 23 mmφ and the thickness of 0.1 mm. The reproducing laser is focused from the cover layer46side for reproduction.

The recording-type disk was fabricated through the process described below. First, a reflection layer42of an alloy having the contents of 95% Ag, 2.5% Pd and 2.5% Cu (mol %) was formed on the polycarbonate substrate having spiral lands and grooves on the recording surface thereof. This layer was formed to the thickness of 200 nm by DC magnetron sputtering using the pure Ar gas. The protection films43,45were formed of amorphus having the contents including 80% ZnS and 20% SiO2(mol %). These films were formed by RF sputtering using the pure Ar gas. Also, the recording film47was formed as a phase change recording film of GeSbTe. This film was formed by RF sputtering using the pure Ar gas.

The recording layer47constituting the optical information recording layer can be a phase change recording film of AgInSbTe or a metal film of at least selected one of the metal elements including Ag, In, Ge, Sb and Tb as well as a phase change recording film of GeSbTe used in this embodiment. Then, a rewritable recording medium reversibly changeable between amorphus and crystal can be fabricated. Also, in the case where a multilayer recording film with a stack of Si and an alloy containing Cu is used, the two elements are mixed by the radiation of the recording laser. and therefore, a stable WORM (write-once-read-many) recording medium can be fabricated.

After sputtering the aforementioned layers, the cover layer46was formed. The UV-curable resin was formed by spin coating on the substrate 1.1 mm thick formed with a film, and a polycarbonate cover layer 0.085 mm thick cut into a circle having the outer diameter of 119.5 mmφ and the inner diameter of 23 mmφ was attached thereon. The resulting assembly was introduced into a vacuum chamber and while being dearing up to about 1 Pa, the sheet and the substrate were closely attached to each other. The UV-curable resin was cured by radiating the UV light from the cover layer side. The thickness of the UV-curable resin was adjusted to the total thickness of 0.1 mm including the UV-curable resin and the cover layer.

According to this embodiment, the polycarbonate substrate 1.1 mm thick was used as the substrate41. This substrate1has the outer diameter of 120 mm and is formed with an inner hole having the inner diameter of 15 mm for a chuck. A guide groove having lands49and grooves50is formed spirally on this substrate. According to this embodiment, data was recorded only in the grooves50. The track pitch was set to 320 nm and the groove depth to about 22 nm.

The signal to be recorded is a repetition of recording pits and spaces corresponding to the recording signals of2T,3T, . . . ,8T for the clock signal (1T=69.5 nm). Only one type of recording signal is formed on the same track, and different signals are recorded on different tracks. In this case study, the recording pit length of the signal2T providing the shortest mark is 139 nm, while the recording pit length of the signal8T providing the longest mark is 556 nm.

FIG. 13shows an example of the recording waveform used for recording. In order to record a mark, a plurality of pulses are radiated for recording by what is called the multi-pulse method. According to this embodiment, in order to record nT marks, the output of the recording power Pw(mW) for τwseconds and the low output of Pr(mW) for τrseconds were repeatedly radiated in (n−1) pairs to form one recording mark.FIG. 13shows the case to form4T marks. With4T as τm, the light of power Pe(mW) was radiated for the same time length τsas τmthereby to record a space. This recording operation was performed for one round of the track having the same radius. According to this embodiment, Pwwas set to 7.2 mW, and Prto 0.1 mW. Also, Pewas set to 4.0 mW.

According to this embodiment, various materials shown in Table 1 were evaluated as the super resolution layer44.

Table 1 shows the composition of the super resolution film materials studied in this embodiment and the the result of studying the effect of improving, by the pulse reproduction method, the resolution of the2T mark as compared with the8T mark. Table 1 shows the resolution at the time of reproduction with the CW light having the reproduction power of 0.5 mW, the resolution at the time of pulse reproduction with the emission power of 6 mW of the pulse emitting portion lower than the recording power and the emission power of 0.5 mW of the bias emitting portion, and the ratio of the resolution for pulse reproduction to the resolution for CW reproduction. A comparative example in which a SiO2film exhibiting no nonlinearity is formed in place of the super resolution layer44is also shown. The resolution is defined as shown by Equation (1).

The first to ninth embodiments show the cases in which Fe2O3, NiO, CoO, CO3O4, ZnO, 78% GaN and 22% InN, 50% Fe2O3and 50% Ga2O3, Cr2O3, 49% ZnS and 51% ZnSe and Ga2O3are formed, respectively, as the super resolution layer44. In all the cases, the resolution for pulse reproduction is very high as compared with that for the CW reproduction. Also, the resolution ratio of the pulse reproduction to the CW reproduction was as large as 4.0 to 7.0. Especially, the resolution ratio for the thin film of 50% Fe2O3and 50% Ga2O3formed as a super resolution layer was a very large 7.0. In the case where SiO2is formed as a comparative example, on the other hand, the resolution substantially remains the same for CW and pulse reproduction, although the resolution for pulse reproduction was slightly lower in the case under consideration.

As described above, an optical disk having a high resolution can be obtained by reproducing, using the pulse reproduction method, the optical information recording medium formed with the super resolution film material according to the invention.