Optical recording medium and optical recording/reproducing apparatus

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BACKGROUND OF THE INVENTION

1. Field of the Invention

In an optical disk apparatus, information is recorded along a spiral track or concentric tracks on a recording medium, and information signals are optically reproduced by irradiating a light beam such as a laser beam. A document file system has been put into market as an early model which enabled the user to record information signals on an optical disk by a laser beam. In recent years, an optical disk apparatus serving as a peripheral recording apparatus of computers which require higher reliability has also been put to practical use. In addition, an optical disk apparatus capable of erasing and rewriting recorded information signals has been put to practical use. The technique of this type of optical disk apparatus has been applied to an optical card memory apparatus or optical tape memory apparatus which uses a card-shaped recording medium or tape-shaped recording medium.

2. Description of the Related Art

In optical recording mediums used in these optical information recording/reproducing apparatuses, a record mark with a size of about 1 micron is formed on recording tracks with a track pitch of about 1.6 microns by means of a laser beam having a beam spot diameter of about 1.2 microns. Various methods of forming the record mark have been practically used. For example, optical properties of a recording film are varied by local destruction, deformation or phase-variation. Techniques of precisely tracing record tracks by means of a laser beam include a method wherein a track guide groove is provided in advance on an optical disk and a tracking error signal is detected from a diffraction beam from the track guide groove, and a method wherein signals from marks, which are formed on both sides of a track and are slightly displaced from each other, are sampled and compared, and a tracking error signal is detected.

it is desired that these optical disk apparatuses, like other recording apparatuses, have a wider range of applications, a larger capacity and a smaller size. To meet this demand, the recording density has been increased more and more. Further, in order to realize information rewrite and overwrite, various recording materials and various film structures of recording mediums have been developed.

According to prior art, e.g. "OVERWRITE CHARACTERISTICS ANALYSIS OF PHASE-TRANSITION OPTICAL RECORDING MEDIUM", Nakamura et al., Autumn, 1990, the Digest of the 51st Applied Physics Society Symposium "Crystallizing Mechanism and Erase Ratio of Phase-variation Type Optical Disc" (The Japan Society of Applied Physics, Oct. 30, 1990), pp. 42-48, there is disclosed a recording medium wherein the thickness of a second layer and the thickness of a fourth layer in a flat multilayered structure are varied to theoretically calculate the reflectance, and the thicknesses of the layers are determined so as to obtain a desired reflectance. According to this method, the thickness of a recording material layer is set at 20 nm and the thickness of the protection layer is set at 100 nm.

According to other prior art, "CHARACTERISTICS OF REWRITE TYPE PHASE-VARIATION OPTICAL DISC", Obayashi et al., winder, 1991, the Papers of the Second Symposium of Phase-variation Recording Research "Fundamentals and Application of Phase-variation Type Optical Disc" (the Committee of Phase-variation Recording of the Japan Society of Applied Physics, Jan. 31, 1991), pp. 20-29, there is disclosed a recording medium wherein the thickness of a recording layer and the thickness of a protection layer in a flat multilayered structure model are varied to theoretically calculate the reflectance, and the thickness of the recording layer is determined so as to obtain a desired reflectance. According to this method, the thickness of the recording layer is set at 20 nm to 45 nm.

Calculation relating to a flat multilayered structure model is relatively easy, and it is convenient and effective for general estimation. In the prior art, a medium structure is designed from such estimation, and thereafter this structure is applied to a substrate with a tracking guide groove.

Even if a medium is formed on the basis of a structure determined by a conventional method, a desired reproduction signal modulation level cannot always be obtained. The reason for this is that an interference occurs between a phase variation of reflected light due to the tracking guide groove and a phase variation of reflected light from the recording mark region, thereby to deteriorate signals. Thus, a desired S/N cannot be obtained, and improvement in recording density and reliability is prevented.

Furthermore, when a recorded signal is reproduced from a recording medium by a conventional optical recording/reproducing apparatus, noise due to a variation in wall shape of the guide groove formed in the recording medium, i.e. so-called groove noise, is mixed in the reproduced signal, and a desired S/N of the reproduced signal cannot be obtained, and improvement in recording density or reliability of the optical recording/reproducing apparatus is prevented.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an optical recording medium having a medium structure which can be designed relatively easily and an effectively high reproduced signal modulation level.

According to the invention, there is provided an optical recording/reproducing apparatus comprising a transparent substrate, an inner protection layer formed on the transparent substrate, a recording material layer formed on the inner protection layer, an outer protection layer formed on the recording material layer, and a reflection layer formed on the outer protection layer, wherein the ratio of the thickness of the recording material layer to the thickness of the outer protection layer in an in-groove recording mode is in a range between 0.52 and 0.95.

According to the present invention, there is provided an optical recording medium comprising a transparent substrate, an inner protection layer formed on the transparent substrate, a recording material layer formed on the inner protection layer, an outer protection layer formed on the recording material layer, and a reflection layer formed on the outer protection layer, wherein the ratio of the thickness of the recording material layer to the thickness of the outer protection layer in an on-land recording mode is in a range between 0.32 and 0.6.

According to the invention, there is provided an optical recording/reproducing apparatus employing an in-groove recording system wherein a light beam is radiated via an objective lens on a recording medium, which stores information by forming a record mark within a guide groove, thereby recording/reproducing information, the guide groove being formed on the recording medium such that the ratio of the average value of widths of upper to lower portions of the guide groove to a value (.lambda./NA) obtained by dividing the wavelength (.lambda.) of the light beam by the numerical aperture (NA) of the objective lens is set in a range of 0.25 to 0.4.

According to the invention, there is provided an optical recording/reproducing apparatus employing an inter-groove (on-land) recording system wherein a light beam is radiated via an objective lens on a recording medium, which stores information by forming a record mark between guide grooves, i.e. on a land, thereby recording/reproducing information, the guide groove being formed on the recording medium such that the ratio of a value obtained by subtracting the average value of widths of upper to lower portions of the guide groove from a track pitch to a value (.lambda./NA) obtained by dividing the wavelength (.lambda.) of the light beam by the numerical aperture (NA) of the objective lens is set in a range of 0.25 to 0.4.

Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention.

FIG. 1 shows an optical system of an optical recording/reproducing apparatus;

FIG. 2 is a partially cut-out perspective view of an optical recording medium according to a first embodiment of the present invention, which is used in the apparatus of FIG. 1;

FIG. 3 shows a cross-sectional structure of the optical recording medium shown in FIG. 2;

FIG. 4 is a graph showing calculation results of a reproduced signal level in the case where no groove is formed in the recording medium;

FIG. 5 is a graph showing calculation results of a reproduced signal level in the case where grooves are formed in the recording medium;

FIG. 6 is a graph showing the dependency of a signal amplitude upon film thickness;

FIG. 7 is a graph showing the dependency of a signal modulation level upon film thickness;

FIG. 8 is a graph showing calculation results of reproduced signal amplitude in a recording medium according to a second embodiment of the invention;

FIG. 9 is a graph in which the signal modulation level in the recording medium according to the second embodiment of the invention is plotted;

FIG. 10 is a graph showing in detail the behavior of the signal amplitude near its peak in FIG. 8;

FIG. 11 is a graph showing in detail the behavior of the signal modulation level near its peak in FIG. 9;

FIG. 12 shows conditions for setting the thickness of a recording material layer and the thickness of an outer protection layer in in-groove recording;

FIG. 13 shows conditions for setting the thickness of a recording material layer and the thickness of an inner protection layer in in-groove recording;

FIG. 14 shows conditions for setting the thickness of a recording material layer and the thickness of an outer protection layer in on-land recording;

FIG. 15 shows conditions for setting the thickness of a recording material layer and the thickness of an inner protection layer in on-land recording;

FIG. 16 is a graph showing calculation results of reproduced signal amplitude in a recording medium according to a third embodiment of the invention;

FIG. 17 is a graph in which the signal modulation level in the recording medium according to the third embodiment of the invention is plotted;

FIG. 18 is a graph showing in detail the behavior of the signal amplitude near its peak in FIG. 16;

FIG. 19 is a graph showing in detail the behavior of the signal modulation level near its peak in FIG. 17;

FIG. 20 is a graph showing calculation results of reproduced signal amplitude in a recording medium according to a fourth embodiment of the invention;

FIG. 21 is a graph in which the signal modulation level in the recording medium according to the fourth embodiment of the invention is plotted;

FIG. 22 shows signal modulation level in in-groove recording and on-land recording;

FIG. 23 shows calculation results of noise power by a diffraction light analyzing method with respect to an in-groove recording-type optical recording/reproducing apparatus; and

FIG. 24 shows an optical recording medium and an optical system according to an on-land recording-type optical recording/reproducing apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an optical system of an optical recording/reproducing apparatus. An optical head 110 is situated below an optical recording medium or optical disk 100 such that the head 110 faces the disk 100. The optical head 110 comprises a converging lens 102 facing the optical disk 100; a laser element 111 for emitting a laser beam; a collimator lens 112 for collimating the laser beam emitted from the laser element 111; a beam splitter 113 for receiving a parallel laser beam from the collimator lens 112; a reflection mirror 114 having a reflection surface for reflecting the laser beam from the beam splitter 113 towards the converging lens 102 and for reflecting the beam, which has been reflected by the optical disk 100 and has traveled through the converging lens 102, towards the beam splitter 113; a converging lens 115 for converging the reflection beam which has been deflected by the beam splitter 113; and a light detector 116 for converting the reflection beam converged by the lens 115 to an electric signal.

The optical disk 100 is constructed as shown in FIGS. 2 and 3. Specifically, a plurality of tracking guide grooves 109 are formed in a transparent substrate 103 on which a laser beam emanating from the converging lens 102 is made incident. An inner protection layer 104, a recording material layer 105, an outer protection layer 106, a metallic reflection layer 107, and a disk protection layer 108 are laminated, in this order, on the guide groove side of the transparent substrate 103. Polycarbonate resin is used as material of the transparent substrate 103, and ZnSiO.sub.2 is used as material of the inner protection layer 104 and outer protection layer 106. A GeSbTe-based phase-variation recording material is used for the recording material layer 105, Al (aluminum) is used for the reflection layer 107, and an ultraviolet-curing resin is used for the disk protection layer 108. There are limitations to the thickness of these layers in order to obtain appropriate recording characteristics of the recording medium. One condition for obtaining such characteristics is that the respective layers are formed such that an adequately high temperature rise for effecting recording in a narrow recording region can be achieved by radiating light on the recording medium in the recording mode. According to this embodiment, the thickness of the inner protection layer 104 is set at 100 nm, and that of the outer protection layer 106 is set at 20 nm. The thickness of the reflection layer 107 is set at 100 nm. Since the extinction coefficient of Al, of which the reflection layer 107 is formed, is relatively high, the thickness of the reflection layer 107 is not strictly limited. Even if the thickness of the reflection layer 107 exceeds 100 nm, the optical characteristics of the layer 107 do not greatly vary.

The variation in a reproduced signal due to the variation in thickness of the recording material layer is calculated in order to obtain a recording medium with recording characteristics in a practical range and high reproduction characteristics. This calculation is based on a method in which light propagation is analyzed in consideration of interference effect of a multilayered thin film. FIG. 4 shows the calculation results of a reproduced signal level according to this method, in the case where no groove is formed in the recording medium. In FIG. 4, the reflectance of the recording medium having the above-mentioned layers on the flat substrate is calculated with the thickness of the recording material layer being varied. This calculation is performed by use of the formulas described in M. Mansuripur, et al. "Lase-induced local heating of multilayers" Applied Optics vol. 21, No. 6, 15 Mar. 1982, H. H. Hoplins "Diffraction Theory of Laser Read-Out Systems for Optical video Disks," Janual of Optical Society of America 69, 4 (1979) or Y. Honguh, "Diffraction Analysis in Optical Disk," Optics, Vol. 20, No. 4, pp. 210-215 (1991).

A curve 201 indicates the case where the recording material layer is in the crystalline state, and a curve 202 indicates the case where it is in the amorphous state. The horizontal axis indicates the thickness of the recording material layer, and the vertical axis indicates the reflectance of the recording medium. The curve 201 represents the reflectance in the crystalline state, and the curve 202 represents the reflectance in the amorphous state. The crystalline state is the prerecording state of the recording medium, and the amorphous state is the post-recording state of the recording medium. If the thickness of the recording material layer is set at a value at which the difference between the levels of the two curves is large, a desirable reproduced signal with a high amplitude can be obtained. In the case where a high modulation level of a reproduced signal is desired, it suffices to set the thickness of the recording material layer at a value at which the post-recording reproduced signal level (reflectance) is low. However, in these calculation results, the mark size or tracking guide grooves are not taken into account. Thus, the calculation results do not represent the actual optimal values.

FIG. 5 is a graph showing calculation results of a reproduced signal level in the case where grooves are formed in the recording medium. Specifically, FIG. 5 shows the calculation results of a reproduced signal level obtained before and after forming a mark in a medium with a groove having a track pitch of 1.2 microns and a groove width of 0.6 micron. In FIG. 5, a curve 301 indicates a pre-recording reflectance which is equal between an in-groove region and an inter-groove region. The reason for this is that the groove has a rectangular cross section with a groove width which is half the track pitch. A curve 302 indicates the calculation results regarding a post-recording reproduced signal level in the case of the in-groove recording mode in which a mark is formed in the groove by radiating a recording beam. A curve 303 indicates the calculation results in the case of the on-land recording mode in which a mark is formed in an inter-groove region by radiating a recording beam on a region between adjacent grooves. That is, FIG. 5 shows the calculation results relating to the two recording modes. In this analysis, the interference effect of the multilayered thin film, the phase shift due to the groove, and the diffraction effect relating to the converging optical system are taken into account. As can be understood from comparison between FIG. 5 and FIG. 4, the optical layered structure is different from the structure estimated from FIG. 4 because of the influence of the phase shift due to the presence of the groove. In addition, the optimal layered structure differs slightly between the two recording modes.

The behavior of the signal amplitude will now be explained in greater detail, in order to well understand the above relationship.

FIG. 6 is a graph showing the dependency of a signal amplitude upon film thickness. A curve 401 indicates the calculation results relating to the in-groove recording, and a curve 402 indicates the calculation results relating to the on-land recording. According to FIG. 5, the amplitude peak in the in-groove recording mode is in the vicinity of 23.5 nm, and the amplitude peak in the on-land recording mode is in the vicinity of 16 nm. As can be seen from FIG. 5, if the film thickness is set at 80% to 120% (the film thickness related to the amplitude peak is set at 100%), an adequately high signal amplitude can be obtained.

FIG. 7 is a graph showing the dependency of a signal modulation level upon film thickness. The signal modulation level has a value obtained by dividing the signal amplitude by a reflection light intensity in a recorded region. Curves 501 and 502 indicate the calculation results in the in-groove recording mode and on-land recording mode, respectively. According to FIG. 7, the peak of the modulation-level in the in-groove recording mode appears in the vicinity of 18 nm, and the peak of the modulation level in the on-land recording mode appears in the vicinity of 13 nm. As can be seen from FIG. 7, if the film thickness is set at 80% to 120% (the film thickness related to the amplitude peak is set at 100%), an adequately high signal modulation level can be obtained.

As can be understood from the above calculation results, in the in-groove recording mode in which the mark is formed in the groove, the characteristics of the recording medium can be improved by increasing the thickness of the recording material layer than in the prior art. In the on-land recording mode, the characteristics can be improved by decreasing the thickness of the recording material layer than in the prior art. In this embodiment, the optimal characteristics of the recording medium can be achieved by setting the thickness of the recording material layer, d, in the range of 15 nm<d<23 nm. On the other hand, in the on-land recording mode in which the mark is formed between grooves, the optimal characteristics of the recording medium can be achieved by setting the thickness of the recording material layer, d, in the range of 10 nm<d<17 nm.

A second embodiment of the invention will now be described. The layered structure of the optical recording medium is the same as shown in FIG. 3. Specifically, the recording medium is formed such that an inner protection layer 104, a recording material layer 105, an outer protection layer 106, a metallic reflection layer 107, and a disk protection layer 108 are laminated, in this order, on the transparent substrate 103. Tracking guide grooves 109 are formed in the transparent substrate 103. Polycarbonate resin is used as material of the transparent substrate 103, and ZnS-SiO.sub.2 is used as material of the inner protection layer 104 and outer protection layer 106. A GeSbTe-based phase-variation recording material is used for the recording material layer 105, Al is used for the reflection layer 107, and an ultraviolet-curing resin is used for the disk protection layer 108. The Ge-Sb-Te ratio of the recording material layer is 2:2:5.

In the second embodiment, the thickness of the inner protection layer 104 is set at 100 nm, and that of the outer protection layer 106 is set at 20 nm. The thickness of the reflection layer 107 is set at 100 nm. Since the extinction coefficient of Al, of which the reflection layer 107 is formed, is relatively high, the thickness of the reflection layer 107 is not strictly limited. Even if the thickness of the reflection layer 107 exceeds 100 nm, the optical characteristics of the layer 107 do not greatly vary. In addition, a mark is formed on the medium with a groove having a track pitch of 1.0 micron and a groove-width of 0.5 micron. At this time, the variation of the reproduced signal due to the variation in thickness of the recording material layer 105 is calculated. The composition ratio of the recording material and optical constants in the second embodiment differ from those in the first embodiment. Thus, the optimal thickness in the second embodiment differs slightly from that in the first embodiment.

The calculation results of the reproduced signal amplitude of the recording medium according to the second embodiment correspond to those in the first embodiment as shown in FIG. 6. A curve 401 indicates the calculation results relating to the in-groove recording, and a curve 402 indicates the calculation results relating to the on-land recording. According to FIG. 8, amplitude peaks appear at a plurality of locations. This is due to the interference effect of the thin film. Since the recording material layer 105 has absorption characteristics, the amplitude peak approaches a constant value in accordance with the increase in film thickness. If the film thickness with which the maximum amplitude peak is obtained is selected, a highest-level signal can be obtained. However, if there is a problem in conditions such as recording characteristics, productivity of medium, etc., the thickness of the recording material layer 105 may be set in the vicinity of values related to the second and third amplitude peaks.

FIG. 9 is a graph in which the signal modulation level obtained with the recording medium according to the second embodiment is plotted. FIG. 9 corresponds to FIG. 7 relating to the first embodiment. The signal modulation level has a value obtained by dividing the signal amplitude by a reflection light intensity in a recorded region. In FIG. 9, curves 501 and 502 indicate the calculation results in the in-groove recording mode and on-land recording mode, respectively. According to FIG. 9, when the film thickness has values related to the second and third peaks or thereabouts, the modulation levels are low. From the viewpoint of reproduction characteristics, this film thickness is not advantageous.

FIG. 10 is a graph showing in detail the behavior of the signal amplitude near its peak in FIG. 8, and FIG. 11 is a graph showing in detail the behavior of the signal modulation level near its peak in FIG. 9. In FIG. 10, a curve 401 indicates the calculation results relating to the in-groove recording, and a curve 402 indicates the calculation results relating to the on-land recording. The peak in the in-groove recording is in the vicinity of 27.5 nm, and the peak in the on-land recording is in the vicinity of 15 nm. As can be seen from FIG. 10, if the film thickness is set at 50% to 150% (the film thickness related to the amplitude peak is set at 100%), an adequately high signal amplitude level can be obtained.

In FIG. 11, curves 501 and 502 indicate the calculation results in the in-groove recording mode and on-land recording mode, respectively. The peak in the in-groove recording is in the vicinity of 22 nm, and the peak in the on-land recording is in the vicinity of 14 nm. As can be seen from FIG. 11, if the film thickness is set at 50% to 150% (the film thickness related to the modulation peak is set at 100%), an adequately high signal modulation level can be obtained, and this film thickness is effective. Based on these results, the optical range of film thickness can be determined. In the second embodiment, the characteristics of the recording medium in the in-groove recording mode are further improved by setting the thickness d of the recording material layer 105 in the range of 18 nm<d<30 nm. On the other hand, the characteristics of the recording medium in the on-land recording mode in which the mark is formed between grooves are further improved by setting the thickness d of the recording material layer 105 in the range of 10 nm<d<19 nm.

Under the film thickness conditions corresponding substantially to the intersection of curves 401 and 402 in FIG. 10 and the intersection of curves 501 and 502 in FIG. 9, substantially the same signal quality can be obtained both in the in-groove recording mode and on-land recording mode. The recording medium having such structure is applicable to either the in-groove recording mode or on-land recording mode, or to the combination of both recording modes. In this case, in particular, remarkable effects can be attained by setting the thickness of the recording material layer at 14 nm. It is similarly effective, in third and fourth embodiments (described later), to find the conditions for attaining substantially the same signal quality in the in-groove recording and on-land recording modes, on the basis of the intersections of the curves.

It is thought that the in-groove recording is effective in enhancing the density of tracks. The conditions for setting the film thickness in this case will now be described. Also, an example obtained by reviwing the second embodiment from another point of view will be described. In this case, the layered structure of the optical recording medium is identical to that shown in FIG. 3.

Suppose that the thickness of the inner protection layer 104 is d1, the thickness of the recording material layer 105 is d2, and the thickness of the outer protection layer 106 is d3. FIG. 12 shows the variation of the modulation level at the time the value of d2/d3 is varied. In order to maintain a satisfactory S/N and achieve the high performance of the recording/reproducing apparatus, the modulation level must be 2 or above. For this purpose, the value of d2/d3 must be in the range of 0.52 to 0.95. In addition, when the value of d2/d1 is varied, the modulation level varies, as shown in FIG. 13. In this case, too, in order to maintain a satisfactory S/N and achieve the high performance of the recording/reproducing apparatus, the modulation level must be 2 or above. For this purpose, the value of d2/d1 must be in the range of 0.053 to 0.098.

Next, the conditions for setting the film thickness in the on-land recording mode will now be described with reference to FIGS. 14 and 15.

FIG. 14 shows the variation of the modulation level at the time the value of d2/d3 is varied. Referring to FIG. 14, in order to obtain the modulation level of 2 or above, the value of d2/d3 must be in the range of 0.32 to 0.6. In addition, when the value of d2/d1 is varied, the value of d2/d1 must be in the range of 0.033 to 0.063 as shown in FIG. 15, in order to obtain the modulation level of 2 or above.

In a modification of the second embodiment, the thickness of the inner protection layer 104 is set at 240 nm, and that of the outer protection layer 106 is set at 25 nm. The thickness of the reflection layer 107 is set at 100 nm. Since the extinction coefficient of Al, of which the reflection layer 107 is formed, is relatively high, the thickness of the reflection layer 107 is not strictly limited. Even if the thickness of the reflection layer 107 exceeds 100 nm, the optical characteristics of the layer 107 do not greatly vary. In addition, a mark is formed on the medium with a groove having a track pitch of 1.0 micron and a groove width of 0.5 micron.

A third embodiment of the invention will now be described. The layered structure of the optical recording medium is the same as that of the second embodiment. Specifically, the recording medium is formed such that an inner protection layer 104, a recording material layer 105, an outer protection layer 106, a metallic reflection layer 107, and a disk protection layer 108 are laminated, in this order, on the transparent substrate 103. Tracking guide grooves 109 are formed in the transparent substrate 103. Polycarbonate resin is used as material of the transparent substrate 103, and ZnS-SiO.sub.2 is used as material of the inner protection layer 104 and outer protection layer 106. A GeSbTe-based phase-variation recording material is used for the recording material layer 105, Al is used for the reflection layer 107, and an ultraviolet-curing resin is used for the disk protection layer 108. The Ge-Sb-Te ratio of the recording material layer is 2:2:5.

In the third embodiment, the thickness of the inner protection layer 104 is set at 100 nm, and that of the recording material layer 105 is set at 22 nm. The thickness of the reflection layer 107 is set at 100 nm. A mark is formed on the medium with a groove having a track pitch of 1.0 micron and a groove width of 0.5 micron. At this time, the variation of the reproduced signal due to the variation in thickness of the outer protection layer 106 is calculated.

FIG. 16 shows the calculation results of the reproduced signal amplitude on the basis of the recording medium of the third embodiment. In FIG. 16, curves 401 and 402 indicate the calculation results in the in-groove recording mode and on-land recording mode, respectively. From FIG. 16, it is understood that the signal amplitude characteristics differ in the in-groove recording mode and on-land recording mode. Amplitude peaks appear at a plurality of locations as a result of the interference of the thin film. Since the material of the outer protection layer 106 has no light-absorption properties, a predetermined amplitude pattern is cyclically repeated in accordance with the increase in film thickness. The cycle of repetition corresponds to half the wavelength of light in the outer protection layer 106. In the predetermined amplitude pattern, there may be a condition in which a highest peak and another peak lower than the highest peak occur. In this embodiment, this condition can be clearly observed. If the film thickness related to the highest amplitude peak is initially selected, a maximum signal can be obtained with a least film thickness. However, considering the advantages/disadvantages in the recording characteristics, productivity of the medium and other conditions, the film thickness associated with the second peak, third peak, or an amplitude near the highest peak may be selected. In addition, in the case where a signal of an adequately high level can be obtained even if a maximum peak is not present at a thickness value close to zero, a film thickness with which the high-level signal can be obtained is selected.

FIG. 17 is a graph in which the signal modulation level based on the recording medium according to the third embodiment of the invention is plotted. Curves 501 and 502 indicate the calculation results in the in-groove recording mode and on-land recording mode. According to FIG. 17, the modulation level is low at a thickness value near the highest peak, and it is understood that this thickness value is not advantageous from the viewpoint of reproduction characteristics.

In addition, when the reproduction of the recording signal at two or more wavelengths is considered, the range of film thickness which ensures good characteristics is further limited. For example, this problem arises when the compatibility of the recording medium is desired between apparatuses employing different wavelengths. In this case, it is necessary to calculate the signal modulation level by using optical constants corresponding to the respective wavelengths and to determine the range of film thickness which ensures a satisfactory signal amplitude or signal modulation level at each of the wavelengths. Specifically, when the film thickness with which the peak can be obtained is set at 100%, the modulation levels at the respective wavelengths are calculated in the range of film thickness of 50% to 150%, and the film thickness corresponding to the overlapping calculation results is selected. In this case, since the cycle of the modulation level pattern varies depending on wavelengths, it is advantageous, from the viewpoint of characteristics of the recording medium, to set the film thickness at a value associated with the first occurring peak. Although it is possible to set the film thickness at a value associated with the second or subsequent peak, the film thickness becomes greater than in usual cases, which is a little disadvantageous in consideration of the manufacturing efficiency or other characteristics. Thus, the film thickness must be set carefully.

FIG. 18 is a graph showing in detail the behavior of the signal amplitude near its peak in FIG. 16, and FIG. 19 is a graph showing in detail the behavior of the signal modulation level near its peak in FIG. 17. In FIG. 18, a curve 401 indicates the calculation results relating to the in-groove recording, and a curve 402 indicates the calculation results relating to the on-land recording. The peak in the in-groove recording appears when the thickness of the outer protection layer 106 is in the vicinity of 22 nm, and the peak in the on-land recording appears when the thickness of the outer protection layer 106 is in the vicinity of 2 nm. As can be seen from FIG. 18, the amplitude peak in relation to the variation in film thickness of the outer protection layer 106 is not conspicuous, as compared to the peak in relation to the variation in film thickness of the recording material layer 105. In each case, however, if the film thickness is set at 50% to 150% (the film thickness related to the amplitude peak is set at 100%), an adequately high signal amplitude level can be obtained, and the film thickness in this range is effective.

In FIG. 19, curves 501 and 502 indicate the calculation results in the in-groove recording mode and on-land recording mode, respectively. The peak in the in-groove recording appears when the film thickness is in the vicinity of 28 nm, and the peak in the on-land recording appears when the film thickness is in the vicinity of 8 nm. As can be seen from FIG. 19, if the film thickness is set at 50% to 150% (the film thickness related to the modulation peak is set at 100%), an adequately high signal modulation level can be obtained, and this film thickness is effective. Based on these results, the optical range of film thickness can be determined. In the third embodiment, the characteristics of the recording medium in the in-groove recording mode are further improved by setting the thickness d of the outer protection layer 106 in the range of 18 nm<d<35 nm. On the other hand, the characteristics of the recording medium in the on-land recording mode in which the mark is formed between grooves are further improved by setting the thickness d of the outer protection layer 106 in the range of 1 nm<d<12 nm.

A fourth embodiment of the invention will now be described. The layered structure and material of the optical recording medium are the same as those of the second or third embodiment, and a description thereof may be omitted.

In the fourth embodiment, the thickness of the outer protection layer 106 is set at 20 nm, and that of the recording material layer 105 is see at 22 nm. The thickness of the reflection layer 107 is set at 100 nm. A mark is formed on the medium with a groove having a track pitch of 1.0 micron and a groove width of 0.5 micron. At this time, the variation of the reproduced signal due to the variation in thickness of the inner protection layer 104 is calculated.

FIG. 20 shows the calculation results of the reproduced signal amplitude on the basis of the recording medium of the fourth embodiment. In FIG. 20, curves 401 and 402 indicate the calculation results in the in-groove recording mode and on-land recording mode, respectively. From FIG. 20, it is understood that the signal amplitude characteristics differ in the in-groove recording mode and on-land recording mode. Amplitude peaks appear at a plurality of locations as a result of the interference of the thin film. Since the difference between the refractive index of the inner protection layer 104 and that of the substrate material is smaller than in the case of the third embodiment, the variation in amplitude in FIG. 20 is gentle, as compared to FIGS. 16 and 17. However, the signal quality can be varied by the manner of setting the film thickness. In the fourth embodiment, like the third embodiment, an amplitude pattern is repeated at a cycle corresponding to half the wavelength of light in the inner protection layer 104. Thus, even if the film thickness is set in the vicinity of the maximum peak, the degree of freedom of an integer-number of times of the cycle is provided. The curve 401 in FIG. 20 does not have a maximum peak in the vicinity of the zero of the film thickness, but a signal of an adequately high level can be obtained. The film thickness may be set at this value, or near 180 nm associated with the peak. However, considering the advantages/disadvantages in the recording characteristics, productivity of the medium and other conditions, the film thickness can be set in the vicinity of the second or third peak.

FIG. 21 is a graph in which the signal modulation level in the recordi