Split photo detector for detecting a central portion and a peripheral portion of a reflected light beam

What is claimed is:

1. An optical disk apparatus comprising:

a semiconductor laser;

an optical system for condensing light coming from said semiconductor laser onto an optical recording medium;

a photodetector for separately detecting the optical intensities of a central portion and a peripheral portion of the light reflected from said optical recording medium;

a signal processor for processing a plurality of signals obtained from said photodetector; and

a reproducing circuit for reproducing the signals recorded on said recording medium, from signals received from said signal processor.

2. An optical disk apparatus according to claim 1, wherein said photodetector is split into a central portion and a peripheral portion.

3. An optical disk apparatus according to claim 1, further comprising an optical element for branching said reflected light into the central portion and the peripheral portion.

4. An optical disk apparatus, comprising:

a semiconductor laser:

an optical system for condensing light coming from said semiconductor laser onto an optical recording medium;

a polarizing prism for branching light reflected from said optical recording medium into two beams;

first and second photodetectors for receiving a respective one of said two beams and detecting the optical intensities of the respective central portions and peripheral portions of the two beams;

a first signal processor for processing a plurality of outputs obtained from said first photodetector;

second signal processor for processing a plurality of outputs obtained from said second photodetector;

a differential amplifier for outputting a signal corresponding to the difference between the outputs of said first and second signal processors; and

a reproducing circuit for reproducing the signals recorded on said recording medium, from signals received from said first and second signal processors.

5. An optical disk apparatus according to claim 1, further comprising a phase filter for partially acting upon the beam incident upon said optical recording medium to change the phase thereof.

6. An optical disk apparatus according to claim 5, wherein said phase filter changes the phase of the portion of the incident light that reflects as the central portion of the reflected light from said recording medium.

7. An optical disk apparatus according to claim 1, wherein said signal processor is a differential amplifier for outputting a signal by amplifying with a predetermined gain the difference between the optical intensities of the central portion and the peripheral portion of the reflected light from said optical recording medium, detected by said photodetector.

8. An optical disk apparatus according to claim 1, wherein said photodetector is a photodetector split equivalently with a standardized split radius of 0.4 or more.

9. An optical disk apparatus comprising:

a semiconductor laser;

an optical system for condensing the light from said semiconductor laser upon a recording medium;

a photodetector for detecting light which has been diffracted by said recording medium;

means for reproducing the signal recorded on said recording medium, from an electrical signal coming from said photodetector; and

a phase filter or a light-shielding sheet for acting partially upon the light beam incident upon said recording medium,

wherein the reproduced signal is generated by detecting the optical intensity of only the sectionally peripheral portion of said diffracted light.

10. An optical information reproducing method for optically reproducing information recorded on a recording medium by irradiating said recording medium with a laser beam and by detecting the light returning from said recording medium, wherein:

when the diffracted light returning from said recording medium is detected, the optical intensity distribution of said diffracted light is detected by a plurality of detectors to generate a plurality of signals;

said plurality of signals are calculated to generate a reproduced signal for detecting said information;

the optical intensities of the central and peripheral portions of said diffracted light are detected separately to generate first and second signals having different gains; and

the difference between said first and second signals is taken to produce the reproduced signal for detecting said information.

11. An optical information reproducing method according to claim 10, wherein said central portion and said peripheral portion are separately detected with a boundary having a standardized split diameter of 0.4 or more.

12. An optical information reproducing method according to claim 10, further comprising means for controlling the phase of the light beam for illuminating said optical recording medium.

13. An optical information reproducing method for optically reproducing information recorded on a recording medium by irradiating said recording medium with a laser beam and by detecting the light returning from said recording medium, wherein:

when the diffracted light returning from said recording medium is detected, the optical intensity distribution of said diffracted light is detected by a plurality of detectors to generate a plurality of signals;

said plurality of signals are calculated to generate a reproduced signal for detecting said information;

the diffracted light returning from said recording medium is halved, the halves being individually detected by photodetectors split into a plurality of areas and having identical respective shapes;

the difference between the outputs of corresponding areas of identical shape of said photodetector are individually taken; and

said plurality of differential signals are calculated to generate the reproduced signal for detecting said information.

14. An optical information recording/reproducing method for detecting a recording mark recorded on a recording medium to reproduce information by scanning the recording mark with a light spot, wherein the light spot is sized to irradiate a plurality of recording marks simultaneously, and the spatial distribution of the diffracted light intensity by the irradiated recording mark is detected to detect the recording mark from the spatial distribution of the diffracted light intensity detected.

15. An optical information recording/reproducing method according to claim 14, wherein the information is recorded in the form of presence or absence of the recording mark at a lattice point.

16. An optical information recording/reproducing method according to claim 15, wherein a learning mark recorded at the lattice point defined on the recording medium is reproduced, the coefficient of an arithmetic equation for calculating the presence or absence of the recording mark on the lattice point from the spatial distribution of the diffracted light intensity is calculated from the reproduced signal, the calculated coefficient is stored, and a calculation is executed for reproducing the signal on the basis of the stored coefficient.

17. An optical information recording/reproducing apparatus for detecting a recording mark recorded at a predetermined lattice point on a recording medium, by scanning the recording medium with a light spot and by detecting the reproduced light by a photodetector, comprising:

a plurality of photodetectors;

calculation means; and

calculation coefficient storage means,

wherein the light spot is sized to irradiate a plurality of lattice points simultaneously, and whether or not the recording mark is at the lattice point is judged by said calculation means calculating the detection signals of the plurality of photodetectors reflecting the spatial distribution of the reproduced optical intensity, using a coefficient stored in said calculation coefficient storage means, by said calculation means.

18. An optical information recording/reproducing apparatus according to claim 17, further comprising means for updating the calculation coefficient stored in said calculation coefficient storage means, by reproducing a group of recording marks which are recorded at known lattice points on the recording medium.

Description Submit all comments and votes

BACKGROUND OF THE INVENTION

The present invention relates to an optical disk apparatus for reproducing information optically from an optical recording medium and, more particularly, to a technique for improving the resolution characteristics of an optical system.

There is known an optical disk for recording information as a string of recording marks such as phase pits or recording magnetic domains. The recording marks are arranged on the recording track, which is helically or concentrically formed on the optical disk, and the detection of the recording marks is carried out by scanning them with a light spot along the recording track and by detecting the reflected light or the transmitted light by a photodetector.

The interval between the adjoining tracks is substantially equal to the size of the light spot so as to avoid crosstalk between the tracks, and the interval between the adjoining recording marks is also substantially equal to the size of the reproduced light spot so as to avoid interference between the recording marks on the same track.

The prior art is disclosed in Japanese Patent Laid-Open No. 57134/1987, 282933/1990 or 234121/1993, for example. According to this disclosure, it is proposed to provide a dead zone in the photodetector or to insert a filter for attenuating the central portion of a detecting beam into the optical system, so as to improve the resolution in the reproduction of the optical disk.

The resolution of the optical system is determined by how much diffracted light spread due to the structure of the pits or domains on the optical disk is directed to the optical system. In these examples of the prior art, therefore, the resolution is effectively improved by increasing the weight given to the peripheral portion including more diffracted light components.

As another method for improving the resolution, there is in the prior art a technique (i.e., the super-resolution effect) for reducing the main lobe of the condensed spot by interrupting the central portion of the beam for illuminating the optical disk or by inserting a phase filter for shifting the phase of the central portion by 180 degrees from that of the peripheral portion.

In the prior art described above, the light of the central portion of the detecting beam necessarily has to be unused thereby, resulting in a decrease of the quantity of light. In the magnetooptical disk in which the signal is read out from a slight polarization rotation, for example, the loss of the quantity of light leads to lowering of the signal-to-noise ratio (S/N ratio). This is because the noise includes components such as amplifier noise, which do not depend upon the quantity of light, so that the noise components relatively increase.

When the super-resolution effect is used, the width of the main peak decreases, but the side peaks rise to cause crosstalk from the information of the adjoining tracks or the interference between the codes. In order to eliminate these phenomena, there has been proposed a method for eliminating the side peaks by focusing the beam again on the detecting system and by allowing the focused light to pass through a pin hole. However, another problem, of the difficulty of adjusting the pin hole location, arises.

In order to solve such problems, an object of the present invention is to improve the resolution without any loss of the quantity of light.

Another object is to reduce the influence of the side peaks of a super-resolution spot.

In order to solve the problems, there is provided an optical disk apparatus which comprises: a semiconductor laser; an optical system for focusing the light from the semiconductor laser upon a recording medium; a photodetector for detecting the reflected light from the recording medium; and means for reproducing the signal recorded on the optical recording medium, from the electric signal from the photodetector. Further, the apparatus comprises means for separately detecting the optical intensities of the central portion and the peripheral portion of the light impinging upon the detector. These output signals are individually multiplied by constants and added to generate reproduced signals having different polarities.

Moreover, the central portion and the peripheral portion are separately detected on the photodetector.

Alternatively, the beam is divided into the center portion and the peripheral portion before the detection.

In the case of a magnetooptical disk, the magnetooptical signals are detected by using those means individually for the two split beams which are branched by a polarizing prism so as to accomplish the differential detections.

Moreover, the effect of phase super-resolution or amplitude super-resolution can be achieved by inserting a phase filter or a shielding sheet into the beam falling upon the optical recording medium.

According to another structure of the present invention, there is also provided an optical disk apparatus which comprises: a semiconductor laser; an optical system for focusing the light from the semiconductor laser upon a recording medium; a photodetector for detecting the reflected light from the recording medium; means for reproducing the signal recorded on the optical recording medium, from the electric signal of the photodetector; and a phase filter or a shielding sheet for partially acting upon the beam impinging upon the optical recording medium, wherein the light intensity of only the sectionally peripheral portion of the reflected light is detected to reproduce the signal.

The improvement in the output characteristics is prominent when the phase filter or shielding sheet acts upon the portion of the light falling on the detector, which has a standardization radius of 0.4 or less. Moreover, the quantity of light is less lost to give a higher effect when the phase filter is used. In order to detect the light intensity of only the sectionally peripheral portion of the reflected light, a split detector is used, for example, to reduce the gain of the output of the detected signal of the central portion to zero. This is effected by making the central portion of the detector a dead zone or a shielding area. Alternatively, there is provided a shielding sheet for interrupting the central portion of the beam returning from the optical recording medium.

The semiconductor laser beam is focused on the recording medium, and the circular central portion and the peripheral portion of the reflected light are separately detected. The detected signals are given gains of different polarities before they are added. As a result, the light of the central portion and the light of a part of the adjoining peripheral portion effectively offset each other so that the output obtained can approximate the output which is obtained from the light of only the outer peripheral portion. As a result, the ratio of light of the quantity of the central portion can be effectively lowered by the polarity inversion of a smaller area than that obtained merely by interrupting the central portion. Thus, it is possible to reduce the noise components that are in phase such as the laser noise irrespective of the location of the pupil plane, without wasting the light energy.

The separate detection of the central portion and the peripheral portion of the beam can be carried out by using a split photodetector. Alternatively, the beam can also be optically divided. For example, a locally reflective film is formed on the beam splitter for splitting the beam, to make the light quantity splitting ratios different between the central portion and the peripheral portion of the beam.

In the magnetooptical disk apparatus, the polarized beams may be individually detected and the signal is processed in the arrangement.

If the arrangement is used for the super-resolution spot formed by the phase filter or the amplitude filter, it is possible to effectively reduce the side lobe which has been a problem of the super-resolution. This is because the side lobe is widely spread over the disk to include large amounts of components having low spatial frequencies, so that the components slightly shift to the central portion on the pupil plane of the objective lens.

The light spot diameter cannot be smaller than .lambda./NA, where .lambda. is the wavelength of the reproduction light, and NA is the numerical aperture of the converging lens. In the recording/reproducing method of the prior art, moreover, a plurality of recording marks, if present in the reproduction light spot, cannot be separately detected, so that the optical disk has a limited recording density. Still worse, there is no effective compensation method for the signal deterioration caused by the change in the recording mark shape on the disk, the inclination of the disk, or the aberration of the illuminating optical system.

The present invention has an object to provide a method capable of recording/reproducing information at a high recording density exceeding the limit due to the light spot diameter, and a recording/reproducing method capable of optical reading even though the optical system deteriorates due to aberrations or the like.

According to the present invention, the information is reproduced by arranging a plurality of detectors on the light receiving face of an optical system for detecting the reproduction light from a recording medium and by calculating the signals from the individual detectors. Specifically, to the contrary of the prior art in which the light beam transmitted through the objective lens is integrated and detected, the light beam is not integrated but detected as a diffraction pattern, and the string of the recording marks contained in the light spot is determined from the shape of the diffraction pattern by the calculation.

The calculation coefficients to be used in the calculation can be calculated by reproducing the known recording mark string which is recorded in advance in the learning area of the recording medium. This learning area is preferably formed in the head of each sector.

Since the distribution of the intensity of light having passed through the objective lens is determined from the spatial distribution of the scanning light spot and the recording marks and from the function of the mark string, the recording marks contained in the light spot can be separately detected by detecting the diffraction pattern with a plurality of detectors.

The optical intensity distributions on the split detectors of the photodetectors are a superposition of a plurality of two-dimensional diffracted light intensity distributions. By calculating the outputs of the individual split detectors, however, the individual two-dimensional diffraction intensity distribution can be determined, so that the small mark string can be detected by calculating the individual diffraction intensities by using the determined two-dimensional diffraction intensity distribution and the weights determined from the light spot and the recording marks.

Accurate detection can be achieved even though aberrations are present in the optical system by forming learning areas on the recording medium to determine the calculation coefficients by the learning.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of one embodiment of the present invention;

FIG. 2 is an enlarged plan view showing a photodetector;

FIG. 3 is an enlarged side view showing a part of the optical system of the present invention;

FIG. 4 is a perspective view showing one embodiment of the present invention;

FIG. 5 is a perspective view showing an embodiment in which a super-resolution filter is used together;

FIG. 6 is a perspective view showing an embodiment of an optical disk apparatus;

FIG. 7 is a graph of the MTF by peripheral light reception;

FIG. 8 is a graph of the MTF by differential output;

FIG. 9 is a graph of the MTF by an amplitude super-resolution and the peripheral light reception;

FIG. 10 is a graph of the MTF in which a phase super-resolution and the peripheral light reception are used together;

FIG. 11 is a graph of impulse response of the prior art;

FIG. 12 is a graph of impulse response by the phase super-resolution;

FIG. 13 is a graph of impulse response of when the peripheral light reception is used together with the phase super-resolution;

FIG. 14 is a schematic diagram of a signal detecting system;

FIGS. 15a to 15c are explanatory diagrams of a magnetooptical recording medium capable of recording small marks;

FIG. 16 is a plan view for explaining the relation between the recorded marks and the reproducing spots;

FIG. 17 is a diagram for explaining the distribution of diffracted light;

FIG. 18 is a diagram showing the arrangement of a split photodetector;

FIG. 19 is a circuit diagram showing a method of connecting the detectors;

FIG. 20 is a diagram for explaining a signal detecting model;

FIGS. 21a and 21b are diagrams for explaining the addition of the vectors of diffracted light;

FIG. 22 is a diagram for explaining an arithmetic coefficient learning area;

FIG. 23 is a block diagram of a recording circuit;

FIG. 24 is a block diagram of one embodiment of the detector;

FIG. 25 is a block diagram of one example of a data selector;

FIG. 26 is a block diagram of one example of a data recorder;

FIG. 27 is a block diagram of an information reproducing circuit;

FIG. 28 is a block diagram of one example of the detector;

FIG. 29 is a block diagram of one example of a memory section;

FIG. 30 is a block diagram of one example of a calculator; and

FIGS. 31a and 31b are diagrams for explaining the timings of output signals from an area detector.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A basic embodiment of the present invention is shown in FIG. 1. The light from a semiconductor laser 1 is collimated by a collimator lens 2 into a parallel beam, which is passed through a beam splitter 3 and reflected by a riser prism 4. The reflected light is passed through an objective lens 5 and focused onto an optical disk 6. The reflected light is passed again through those elements and is reflected by the beam splitter 3 until it is focused by a condenser lens 7 onto a circular split photodetector 8. A reproduced signal is produced by amplifying the output of the central portion and the output of the peripheral portion of the reflected light beam differentially by a differential amplifier 9. The condensation by the lenses is performed for reducing the size of the detector 8 and for increasing the response speed. In the present invention, the location of the detector 8 has to be slightly shifted from the focal point. Because of the arrangement described above, the light of the central portion and a part of the light of the peripheral portion of the reflected light beam effectively offset each other so that the output is obtained as if only the light of the surrounding peripheral portion were detected. In the present embodiment, the absolute values of the gains for the outside part and inside part of the differential amplifier 9 are equal to each other. The combination of the ratio between the magnitude of the circular area and the absolute value of the gains can be freely determined so that the combination can be optimized by changing the design conditions.

FIG. 2 is an enlarged view of the photodetector 8 of FIG. 1. Across a ring-shaped split line area 803, the outputs of the central portion 802 and the peripheral portion 801 are separately outputted. A light spot 10 for illumination is desirably larger than the split line 803. Here, the detector is circular to improve the resolution in both the linear velocity direction and the radial direction of the optical disk but may be a rectangular trisected photodetector when the resolution in the system design is required to be in only one direction. In this case, too, a differential output may be taken between the outer two detectors and one center detector.

FIG. 3 shows an embodiment in which the optical beam is optically divided into a central portion and a peripheral portion. In the optical path of the detecting beam before the condenser lens 7, there is arranged a beam splitter 11. This beam splitter 11 has a reflective film 1101 only on its central portion and may have an antireflection coating on the peripheral portion. Then, detectors 121 and 122 can be ordinary photodetectors, not split detectors. After this, effects similar to those of the embodiment of FIG. 1 can be achieved by differentiating the outputs of the detectors 121 and 122.

FIG. 4 shows the structure of the apparatus for reproducing information on the magnetooptical disk. The basic structure is similar to that of FIG. 1, and the reflected beam is polarized at first by 45 degrees by a .lambda./2 plate 13 and is condensed by the condenser lens 7. The condensed light falls upon a polarizing beam splitter 14, and the P-polarized component passing therethrough and the S-polarized component reflected thereby are detected by circular split photodetectors 81 and 82, respectively. The differences between the signals of the inner portion and the outer portion of the individual detectors are produced by differential amplifiers 91 and 92. After this, the difference between the P-polarized light and the S-polarized light is taken by a differential amplifier 93 to produce a magnetooptical signal.

FIG. 5 shows an embodiment in which a phase filter 15 is inserted to further reduce the size of a light spot incident upon the optical disk by the super-resolution effect. The basic structure is similar to that of FIG. 1, but a thin film 151 for inverting the optical phase by 180 degrees is added to the central portion of the phase filter 15. The beam can be detected by the circular split photodetector 8 to produce the differential output, reducing the influence of the side lobe.

This side lobe reducing effect can also be produced even when blocking a part of the detecting beam. This will be later described with reference to FIGS. 9 and 10.

FIG. 6 shows an embodiment of a more detailed optical system arrangement. The light of the semiconductor laser 1 is collimated by the collimator lens 2 into a parallel beam, and an elliptical beam is shaped into a circular beam by beam shaping prisms 191 and 192. These prisms may be dispensed with if the light emission intensity of the semiconductor laser is sufficiently high. The light is then passed through the beam splitter 3 and is reflected by the riser prism 4. The reflected light passes through an actuator-integrated objective lens 51 and is focused onto the optical disk 6. The reflected light is passed again through these elements, reflected by the beam splitter 3, and split into transmitted light and reflected light. The reflected light is focused onto the circular split photodetector 8 by a condenser lens 72, and a reproduced signal is obtained by taking the difference between the inner side intensity and the outer side intensity of the differential amplifier 91.

On the other hand, the light having passed through the beam splitter 9 further passes through a condenser lens 71 and is split by a beam splitter 32 into transmitted light and reflected light. Here, the reflected light is separately detected by a bisected photodetector 16, and a tracking error signal is detected by taking the difference signal of the split outputs by the differential amplifier 92. The transmitted light passes through a cylindrical lens 17 and is detected by a quadrisected photodetector 18. The difference signal of the diagonal components is outputted by the differential amplifier 93 and is used as a focus shift signal. The tracking error signal and the focus shift signal are fed back by the objective lens actuator 51 to control the light spot on the optical disk 6. The arrangement thus far described is only one example but is not necessarily fixed. Another arrangement may be basically used for achieving equivalent signals. Likewise, there can also be used another optical system for reproducing the magnetooptical signal.

FIGS. 7 and 8 are the results of calculation with an MTF (Modulation Transfer Function) when the difference between the inner side and the outer side is taken and when only the peripheral portion of the beam is merely received. The horizontal axis is marked with the space frequency of the structure on the optical disk, and the vertical axis is marked with the amplitude of the space frequency component normalized with a DC component. The space frequency is normalized with NA/.lambda. of the resolving power determined by the optical system. Here, NA is the numerical aperture of the optical system, and .lambda. is the wavelength of the light. Moreover, the resolution limit of the optical system is 2NA/.lambda..

Here will be briefly described the method of calculating the MTF and the conditions for the calculations. The MTF is calculated by assuming a magnetooptical domain having a width of 0.4 .mu.m and a length of 3 .mu.m, by making the response of one domain edge a step response, when the magnetic domain is scanned with a light spot formed by collimating the light from a light source having a full width at half maximum of emission angle of 23.degree. and a wavelength .lambda. of 0.68 .mu.m and by focusing the light beam with an optical system having a numeral aperture NA of 0.55, by spline-interpolating and then differentiating the waveform, by making the result of the differentiation an impulse response, and by subjecting the impulse response to a one-dimensional Fourier transformation.

In FIG. 7, the standardized light-shielding radius means the radius of a circular light-shielding area of the central portion when the radius of the detected beam is 1. The graph shows the results of MTF calculation when the beam is "Normal" (no light-shielding) and when the standardized light-shielding radii is 0.5, 0.7 and 0.8. It will be seen from the graph that the resolution is improved by the light-shielding and more improved by a larger light-shielding radius.

FIG. 8 shows the results of MTF calculation when the differential output is produced by the present invention. The standardized split radius is shown when the radius of the circular split line of the circular split photodetector and the radius of the incident beam is 1. It will be understood that the differential output of FIG. 8 is more effective in the improvement of the resolution than that of FIG. 7. The improvement of the standardization amplitude is prominent when the standardized split radius is larger than 0.4 and the graph show the effect in the area where the space frequency is higher than 0.5. Especially for the standardized split radius of 0.5, the standardization amplitude is doubled for the space frequency of 1.0 compared to when the beam is simply interrupted, as shown in FIG. 7.

FIG. 9 shows, for comparison, the results of the MTF calculation of the peripheral light reception when the gains for the amplitude super-resolution and the central portion are zero (or when the central portion is shielded with a light-shielding sheet) and when the optical system is an ordinary one. For both of these cases, the standardized shielding diameter is 0.7. The light reception of the peripheral portion, indicated by A, shows excellent response characteristics all over the area, although it is inferior to the amplitude super-resolution, indicated by B, in the high frequency region. In this case, moreover, there is no difference between the super-resolution alone and the combination of the peripheral light reception and the super-resolution. When the super-resolution effect of the light-shielding type is used, little light returns to the center, so that the effect of the peripheral light reception is not achieved when both the standardized shielding radii are 0.7.

FIG. 10 shows, for comparison, the results of MTF calculation in the case (.alpha.) in which the super-resolution effect is used when the phase of 40% of the central portion of the optical beam is shifted by 180 degrees, and in the case (.beta.) in which the peripheral light reception of the standardized shielding radius of 0.4 and the super-resolution effect of the case (.alpha.) are used together. In both the cases (.alpha.) and (.beta.), an ordinary (Normal) optical system is used. FIG. 10 teaches that the deterioration of the low frequency region due to the use of only the super-resolution is considerably prevented by the combined use of the peripheral light reception. At this time, the change in the high frequency region is little.

FIGS. 11, 12 and 13 shows the impulse responses which are obtained in the course of determining FIG. 10. Here, the differentiated values of the original step response waveform, as they are, are plotted vertically. FIG. 11 shows the normal case; FIG. 12 shows the super-resolution case; and FIG. 13 shows the case in which the peripheral light reception is combined in use. From these graphs, it will been seen that the heights of the side peaks are considerably suppressed while the main peak at the center is hardly changed.

As has been described hereinbefore, according to the present invention, it is possible to improve the resolving power of the optical system without any optical loss thereby to highly precisely reproduce the information on the optical disk which is highly densely recorded, with little crosstalk between the adjacent tracks.

Here will be described an embodiment for the highly dense recording and reproducing of the recording marks by arranging them closer together than a spot size, not only in the track direction but also in a track radius direction.

A method of recording smaller marks than the light spot size will be described with reference to FIG. 15. These details are disclosed in Japanese Patent Application No. 65547/1995 (or PCT patent application No. PCTJP95/00542).

The magnetooptical recording medium to be used is exemplified by a medium of which the recording temperature characteristics are locally lowered on the disk surface, as shown by the solid line 20 in FIG. 15a. The central portion of the light spot is located in the portion where the recording temperature is lowered, to make a temperature distribution, shown by the solid line 21. Then, the portion to be heated to a recording temperature 20 is restricted to the peak portion of the temperature distribution 21, and therefore it is smaller than the light spot diameter. In the recording using the peak portion of the temperature distribution of the prior art, the size of the recording marks so seriously changes due to the variation of the temperature distribution that the recording marks cannot be stably formed. If this recording medium is used, the gradient of the recording temperature with respect to the position is polarized oppositely to the gradient of the temperature distribution with respect to the position so that the variation of the intersection of the two can be suppressed against the variation of the temperature distribution. As a result, small recording marks can be stably formed with a lower recording energy than that of the prior art.

There are two methods for changing the recording temperature. One is a method for changing the recording magnetic field locally: the other is a method for changing the coercive characteristics of the medium locally. FIG. 15a shows one example in which the magnetic field is locally changed. A buried layer 24 having fine magnetizations 23 buried therein in advance is formed in contact with a recording film 22. The fine magnetization marks 23 are two-dimensionally arranged and the external magnetic field over the recording layer 22 in contact with the buried layer 24 is increased by the intensity of the magnetic field generated by the magnetization marks 23, to change the effective external magnetic field. An effectively intense external magnetic field is applied to the portion of the recording layer 22 located over the buried magnetic domains 23, lowering the recording temperature below that of the recording layer around the former. As a result, when the recording medium is irradiated with the recording light, recording marks 25 that are smaller than the recording spots are formed in the area over the buried domains 23 at a lowered recording temperature.

FIGS. 15b and 15c are explanatory diagrams of examples in which the coercive characteristics of the recording medium are locally changed. FIG. 15b shows an example in which the surface of the area 27, other than the area 26 where the recording marks are to be formed, is roughened to enhance the coercive force. With the rough surface, the surface energy for stopping the magnetic domain wall is increased to increase the apparent coercive force. In FIG. 15b, therefore, the recording temperature is relatively lowered in the recording mark area 26. According to the surface roughening method, the surface excepting the two-dimensional lattice points is roughed by applying resist which is cross-linked when irradiated with light and does not dissolve in developer, irradiating the small mark portions on the two-dimensional lattice points with light, and etching them with a dense developer. The surface around the small marks can be roughed by preparing a stamper from an original thus fabricated and by stamping a plastic sheet with its rough surface.

FIG. 15c shows an example in which the recording marks are located on small rough marks 28. These rough marks 28 may be formed by the aforementioned lithography and by an ordinary stamping process. The portion 280 other than the rough portions has a smooth surface of a low surface energy having no nucleus for locating the marks formed as a result of the temperature rise. Therefore, if there exist needle-like recesses 28, the recording marks are trapped by the small roughnesses 28. The mark shape at this time is circular and consequently the surface energy is the smallest. This phenomenon occurs in the case of TbFeCo, an ordinary magnetooptical recording material, and often occurs particularly in a medium having a low coercive force. This tendency is higher in Pt/Co which is frequently used in a short wavelength range, compared to TbFeCo.

As shown in FIG. 16, the recording marks are two-dimensionally arranged and information is recorded. For the mark recording method, the medium structure and the recording/reproducing system as disclosed in PCT patent application No. PCTJP95/00542 are used to arrange the marks on a two-dimensional lattice. In this arrangement, at least three marks are provided in advance at regular intervals in the spot movement direction indicated by an arrow. Of these marks, two are a mark set 31 of transversely displaced marks, and one mark 33 is provided on the center line of spot movement. From the two mar