JP3099275B2 - Optical information recording medium and recording method thereof - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical information recording medium and a recording method therefor, and more particularly, to an optical thickness of a light absorbing layer on lands located on the left and right of a pre-groove. The present invention relates to an optical information recording medium having improved reproduction signal characteristics such as jitter by making the thickness of a land and a pre-groove smaller as compared with a substrate, or a recording method thereof.

[Prior Art] As this type of optical information recording medium, a light-transmitting substrate having a pregroove formed as a spiral guide groove, a light-absorbing layer provided on the substrate and containing an organic dye,
A light reflecting layer provided on the light absorbing layer, and a protective layer further provided on the light reflecting layer, capable of optically writing and reading information and having a reproduction signal satisfying the CD standard. Information recording media are known.

As the shape of the pregroove of such a conventionally known optical information recording medium, the track pitch is 1.5 to 1.7 μm and the width is 0.1 μm.
The thickness is about 3 to 0.7 μm and the depth is about 60 to 100 nm. The optical information recording medium was irradiated with a laser beam having a recording power of 6 to 9 mW to deform the substrate by about 30 to 40 nm, thereby forming optical pits in the pre-groove portion and recording optical information.

However, the jitter may increase depending on the recording conditions. In addition, there are some lasers having low jitter but having a narrow power margin (allowable power for recording). Crosstalk is also greater in our products. Here, the jitter is the fluctuation or fluctuation of the digital signal in the time axis direction, and the crosstalk is a parameter indicating the influence of a signal from an adjacent track.
Specifically, it is represented by the ratio of the amplitude of the HF signal in the track portion and the inter-track portion (non-track portion).

That is, in the conventional optical information recording medium, the width of the pre-groove is set to 0.5 μm and the depth is set to 1 to secure the reflectance.
Many have a thickness of about 00 nm. In this case, a large amount of the light absorbing agent is present at the boundary between the pregroove and the lands located on the left and right sides of the pregroove due to the shape of the light absorbing layer made of a dye film or the like. In addition, since the laser power of the recording light differs depending on each recording device, recording may be performed with a power higher than the optimum power of a certain optical information recording medium depending on the recording device used. In the case where the groove is formed shallowly, the optical pits formed by the recording light extend to the above-mentioned boundary portion on both the left and right sides of the pre-groove and even the land portion, and the pit shape becomes uneven. As a result, the reproduced waveform is disturbed, that is, the jitter is increased.

Further, when the thickness of the light absorption layer is relatively large with respect to the substrate, or when the thickness of the land is relatively large with respect to the thickness of the pre-groove, optical pits easily extend to the land portion, There is a problem that the crosstalk in the reproduced waveform is increased because the pits are spread in the width direction with respect to the pit depth to be recorded.

Thus, when the pre-groove is relatively shallow as compared to the land, crosstalk may be increased. Further, the secondary decomposition reaction of the dye and the substrate material due to the heat generated in the pregroove due to the absorption of the irradiated light tends to spread to the land due to the thermal diffusion, and jitter and crosstalk increase. As a result, there is a problem in that the reproduction characteristics of recording with a power higher than the optimum power are inferior and the recording power window becomes narrow.

[Problems to be Solved by the Invention] The present invention has been made in view of the above-described problems, and is directed to an optical information recording medium that performs optical information recording by deforming a substrate or the like by recording light using a light absorbing layer. It is another object of the present invention to provide an optical information recording medium capable of clearly obtaining a reproduced signal even when recording is performed with a power higher than the optimum power, and a recording method thereof.

In particular, it is an object of the present invention to provide an optical information recording medium capable of reducing jitter in an optical information recording medium satisfying the CD standard and a recording method thereof.

[Means for Solving the Problems] That is, a first aspect of the present invention provides a light-transmitting substrate having a pregroove formed on one main surface, a light-absorbing substance provided on the substrate and absorbing recording light. An optical information recording medium, comprising: a light absorbing layer including: a light reflecting layer provided on the light absorbing layer; and recording information by irradiating the light absorbing layer with the recording light. The real part of the complex refractive index of the layer is n abs, the film thickness at the land portions located on the left and right of the pregroove is d ln, the real part of the complex refractive index of the substrate is n sub, and the wavelength of the reproduction light is Is λ, and when Ls = (n abs · d ln) / (n sub · λ), Ls <0.3.

Further, the second invention, d ln / when the film thickness at the land portion located on the left and right of the pre-groove is d ln, and when the film thickness at the pre-groove portion of the light absorbing layer is d gr. An optical information recording medium characterized by d gr <0.75.

The imaginary part of the complex refractive index of the light absorbing layer is k abs, the real part thereof is n abs, the thickness of the light absorbing layer is d av, the wavelength of the reproduction light is λ, and ρ = N
abs ・ d av / λ, 0.05 ≦ ρ ≦ 1.6 and ka
bs ≦ 0.3.

According to a third aspect of the present invention, there is provided an optical information recording apparatus, comprising: irradiating the recording light onto the light absorbing layer from the substrate side using the optical information recording medium according to the first aspect to perform optical information recording. This is a recording method of the information recording medium.

According to a fourth aspect of the present invention, there is provided an optical information recording apparatus comprising: using the optical information recording medium according to the second aspect described above to irradiate the recording light onto the light absorbing layer from the substrate side to perform optical information recording. This is a recording method of the information recording medium.

Next, the present invention will be described more specifically with reference to FIGS. 1 to 5.

FIG. 1 is a partially cutaway perspective view of an optical information recording medium 1 according to the present invention, FIG. 2 is a longitudinal sectional view of a main part of the optical information recording medium 1 before recording, and FIG. FIG. 3 is a longitudinal sectional view of a main part after recording No. 1;

The optical information recording medium 1 includes a light-transmitting substrate 2, a light absorbing layer 3 formed on the substrate 2, a light reflecting layer 4 formed on the light absorbing layer 3, and a light reflecting layer 4. And a protective layer 5 formed thereon. Note that an intermediate layer (not shown) may be provided between the substrate 2 and the light absorbing layer 3 and between the light absorbing layer 3 and the light reflecting layer 4 as necessary.

The substrate 2 has a pre-groove 6 formed in a spiral shape as a guide groove. On the left and right sides of the pregroove 6, portions other than the pregroove 6, that is, lands 7 are located.

Note that the substrate 2 and the light absorbing layer 3 are in contact with each other by a first layer boundary 8. The light absorbing layer 3 and the light reflecting layer 4 are in contact with each other by a second layer boundary 9. The light reflecting layer 4 and the protective layer 5 are in contact with each other by a third layer boundary 10.

As shown in FIG. 3, when the optical information recording medium 1 is irradiated with a recording light (recording laser light) L1, the light absorbing layer 3 generates heat by absorbing the energy of the laser light L1, thereby causing the substrate 2 to emit heat. The pits 11 are formed due to thermal deformation on the side. In some cases, an optical change may occur in the light absorbing layer 3.

In particular, as shown in FIG. 2, the depth from the first layer boundary 8 at the land 7 located on the left and right of the pre-groove 6 to the bottom of the first layer boundary 8 at the pre-groove 6 Is d sub.

From the second layer boundary 9 at the land 7,
The depth of the bottom of the second layer boundary 9 at the pregroove 6 (from the surface of the light absorbing layer 3 at the land 7,
(Depth of the bottom of the surface at the pregroove 6)
Is d abs.

 The real part of the complex refractive index of the substrate 2 is defined as n sub.

The imaginary part of the complex refractive index of the light absorption layer 3 is represented by k abs, and its real part is represented by n abs.

The average thickness of the light absorption layer 3 is d av. Here, the average film thickness d av is (volume of light absorbing layer 3) / (light absorbing layer 3).
Is the area of the region in which is formed).

The thickness of the light absorbing layer 3 at the pregroove 6 portion is d
gr.

The thickness of the light absorption layer 3 at the land 7 is d ln.

The wavelength of the reproduction light (reproduction laser light) L2 is λ.

Here, an optical parameter defined by Ls = (n abs · d ln) / (n sub · λ) will be described.

The present inventors have found that jitter and crosstalk can be reduced by setting Ls <0.3 for the optical parameter Ls.

That is, by decreasing d ln · n abs / n sub, the difference in recording sensitivity between the land 7 and the pre-groove 6 can be increased, and the optical pit 11 extends to the land 7. It can be prevented, and if recording power is applied excessively, or even if recording is performed with optimal power or more,
It was found that the rise of jitter and crosstalk was small.

More specifically, if Ls is larger than 0.3, the optical pits 11 spread to the lands 7 and it is difficult to suppress the jitter to 40 ns or less.

Note that Ls is desirably 0.2 or less, and by setting it to 0.2 or less, the jitter can be reduced to 30 ns or less.

In the present invention, the thickness d ln of the light absorbing layer 3 at the land 7 and the pre-groove 6
By setting d ln / d gr <0.75 with respect to the ratio of the film thickness d gr to the portion d, heat diffusion hardly occurs to the land 7 and the jitter can be reduced to 30 ns or less.

That is, when d ln / d gr is larger than 0.75, when recording is performed with the optimum power or more, the optical pit 11 spreads to the land 7 and the jitter cannot be suppressed to 40 ns or less. The reproduced waveform also becomes non-uniform.

When d ln / d gr is smaller than 0.75, since the land 7 is thinner than the pre-groove 6, the heat generated in the land 7 can easily escape from the light reflection layer 4, and the land 7 and the pre-groove 6 And the recording sensitivity is greatly different, so that even if the recording power is increased, the recording is hardly spread to the land 7 portion.

Further increase the recording power and pit up to the land 7
Even if 11 is widened, if the above Ls is 0.3 or less, the optical change recorded at the land 7 is small, and it is unlikely that jitter and crosstalk increase.

Next, an optical parameter defined by ρ = n abs · d av / λ will be described.

From the results of experiments and simulations by the present inventors, ρ = n abs · d av /
We note that λ is a very important parameter.

That is, in the optical information recording medium 1 having the configuration in which the light absorbing layer 3 and the light reflecting layer 4 are provided on the substrate 2, the reflectance specified in the CD standard is 70% or more, and the modulation of the modulation amplitude in the reproduction signal is performed. I11 / Itop indicated as a degree is 0.6 or more (however, “Itop” is the maximum reflected light amount in the CD playback signal, and “I11” is diffracted by the longest recorded pit and returned to the objective lens. This is an optical modulation component corresponding to the difference between the amount of reflected light and the amount of reflected light that is reflected by the non-pit portion and returned to the objective lens), and an output signal with a modulation degree I3 / Itop of 0.3 to 0.7 (however, "I3"
Is an optical modulation component corresponding to the difference between the reflected light amount diffracted by the recorded shortest pit and returned to the objective lens and the reflected light amount reflected by the non-pit portion and returned to the objective lens.) In order to obtain it, the real part n abs of the complex refractive index of the light absorbing layer 3 and the film thickness dg of the pregroove 6 part are required.
ρ given by the average film thickness of r and the film thickness d ln of the land 7 or the average film thickness d av and the wavelength λ of the reproduction light
By setting n abs · d av / λ within the range of 0.05 ≦ ρ ≦ 1.6, the reflectance can be easily adjusted to 70%, which conforms to the CD standard.
It is known that this can be done.

If ρ is smaller than 0.05, the film thickness d av of the light absorbing layer 3 must be considerably reduced, which is not practical for manufacturing.

Therefore, in the range of 0.05 ≦ ρ ≦ 0.6, 0.30
The range of ≦ ρ ≦ 0.6 is practical, the range of 0.1 or more is desirable for obtaining a sufficient degree of modulation, and the range of 0.45 ± 0.1 is most desirable for obtaining stable recording characteristics with a large degree of modulation. It can be said that it is a range.

Further, as shown in FIG. 4, even if ρ is in the range of 0.6 or more, if the peak point is on the graph, the reflectance is 70%.
Is possible.

In the range of 0.6 <ρ <1.6, there are two peak points, always in the range of 0.6 <ρ <1.10 and in the range of 1.10 <ρ <1.6, and it is possible to obtain a high reflectance at those peak points. I know I can.

When ρ> 1.6, the film thickness is large, so that it is difficult to control the film thickness, which is not practical for manufacturing.

The graph showing the relationship between ρ and the reflectance is expressed as a combined function of an exponential function and a periodic function, and the amplitude of the periodic function increases as ρ increases.

The amplitude of such a periodic function changes with the complex refractive index and the thickness of the layers constituting the optical information recording medium 1, their homogeneity, and the like as parameters. For example, if the refractive index of the layer on the side where light is incident from the light absorbing layer 3 is small, the reflectance shifts in the direction in which the reflectance increases in the entire graph.

This graph shows the imaginary part k of the complex refractive index of the light absorption layer 3.
It is expressed by an exponential function using abs and d av as parameters. As shown in FIG. 5, it is also known that as k abs increases, the attenuation of the reflectance of the entire graph increases.

The light absorbing layer 3 is homogeneous and the real part n ab of its complex refractive index
It is known from the simulations of the present inventors that there is no change in the period of the point showing the peak in the graph unless there is an uneven distribution in s and the film thickness dav.

In addition, depending on the conditions, the reflectance at the bottom point of the graph in FIG. 4 can be increased by controlling the above parameter conditions. However, when ρ is set near the bottom point, It is difficult to increase the degree of modulation, and in some cases, the reflectance may be higher than before recording. Therefore, it is desirable that ρ is set near the peak point.

 The above-mentioned k abs will also be mentioned.

In order to obtain a high reflectance, it is necessary that k abs is 0.3 or less.

The present inventors have found that the numerical setting of k abs is an important parameter. Ie this k ab
If s is 0.3 or less, the reflectance increases as it approaches 0. Therefore, this range is most desirable. However, as the recording sensitivity approaches 0, the recording sensitivity becomes worse. Specifically, a range of 0.01 or more is desirable, and in practice, about 0.05 is desirable.

When ρ is in the range of 0.05 to 0.6, the imaginary part k abs of the complex refractive index of the same layer is desirably 0.3 or less. And ρ
Is in the range of 0.6 to 1.6, k abs is desirably 0.2 or less.

 Next, the material and physical properties of each layer will be described.

First, the light-transmitting substrate 2 is made of a highly transparent material having a refractive index to laser light in the range of 1.4 to 1.6 and mainly formed of a resin having excellent impact resistance, such as a glass plate, an acrylic plate, and an epoxy resin. Use a plate or the like. Note that it is desirable to mold a resin such as polycarbonate by injection molding. Further, another layer on the substrate 2, for example, a solvent-resistant layer such as SiO2 or an enhancement layer may be coated.

These materials are molded by means such as injection molding.
The thickness of the substrate 2 should be 1.1 mm to 1.5 mm so as to conform to the CD standard.
mm is desirable.

As the tracking guide means formed on the surface of the substrate 2 on the light absorption layer 3 side, the pre-groove 6 (FIGS. 2 and 3) formed in a spiral shape is desirable. The pre-groove 6 is used to guide tracking when recording a data signal. The pre-groove 6 is not limited to a spiral shape and may meander.

The so-called track pitch between the pregroove 6 and the adjacent pregroove 6 is desirably 1.6 μm.

Next, the light absorbing layer 3 is a layer made of a light absorbing material formed on the tracking guide means of the substrate 2, and is irradiated with a laser to generate heat, melt, sublime, deform or modify. It is. This light absorbing layer 3 is preferably a layer containing a light absorbing organic dye such as a cyanine dye. That is, for example, a cyanine-based dye or the like dissolved in a solvent is uniformly coated on the surface of the substrate 2 by means such as a spin coating method to form the substrate.

Next, the light reflection layer 4 is formed as needed, and is a metal film. For example, gold, silver, copper, aluminum, or an alloy containing these is formed by means such as a vapor deposition method and a sputtering method. To form Since it is necessary to have a reflectance of 70% or more, a metal film mainly composed of gold or an alloy containing gold is preferable among them.

Further, in order to prevent the light reflecting layer 4 from being oxidized, the light reflecting layer 4
Another layer such as an oxidation-resistant layer may be provided on the substrate.

Next, the protective layer 5 is also formed as needed, and is formed of a resin having excellent impact resistance similar to that of the substrate 2. For example, an ultraviolet curing resin is applied by a spin coating method, and is irradiated with ultraviolet rays to be cured to form the resin. In addition, epoxy resin,
An acrylic resin, a silicone-based hard coat resin, or the like may be used.

[Operation] The optical information recording medium according to the present invention can be recorded by a known optical information recording device. The optical information recording medium 1 is arranged such that the surface of the light-transmitting substrate 2 faces the laser irradiation means of the optical information recording apparatus, that is, the side where the pickup is provided. While the optical information recording medium 1 is rotated by a spindle motor, a laser spot modulated into a signal conforming to the CD standard is tracked according to the tracking guide means, and is picked up from the substrate 2 side of the optical information recording medium 1 by a pickup. By irradiating the light absorbing layer 3, the light absorbing layer generates heat, and the heat deforms the substrate 2, thereby forming pits 11.

In recording, it is desirable to irradiate a laser spot having a wavelength λ of around 780 nm. Also, in relation to the CD standard, the linear velocity must be 1.2 to 1.4 m / sec,
The recording power may be about 6 to 9 mW.

In the present invention, the shape of the light absorption layer 3 or the light reflection layer 4 above the pre-groove 6 is Ls = (n abs
(D ln) / (n sub · λ) <0.3, the difference in recording sensitivity between the land 7 and the pre-groove 6 can be increased.

In addition, since d ln / d gr <0.75 is satisfied, even if the part is recorded with power higher than the optimum power,
Excess heat is dissipated from the light absorbing layer 3 to the light reflecting layer 4. However, since the land 7 is thinner than the pre-groove 6, the ratio of heat dissipation is relatively high in the land 7, It is possible to prevent the pit 11 from extending to seven portions. In other words, the absorbed heat is confined in the pre-groove 6, and a clean pit 11 with a good edge can be formed. That is, the rate of diffusion of heat based on the absorption of the recording laser beam L1 of the light absorbing layer 3 to the outside is suppressed, and the heat is confined in the pregroove 6 to form a clear deformed portion (pit 11) on the substrate 2 surface. be able to.

Furthermore, a parameter ρ = about the thickness of the light absorbing layer 3
By setting n absd av / λ in the range of 0.05 to 1.6 and setting the imaginary part k abs of the complex refractive index of the light absorption layer to 0.3 or less, the CD standard is satisfied and the block error rate and the jitter are low. It can be an optical information recording medium.

Examples Next, examples of the optical information recording medium according to the present invention will be described below.

(Example 1) Spiral pregroove having a width of 600 nm, a depth of d sub90 nm, and a pitch of 1.6 μm, a thickness of 1.2 mm, and an outer diameter of 1
A disc-shaped polycarbonate substrate having a diameter of 20 mm and an inner diameter of 15 mm was formed by injection molding. On this substrate, as a cyanine dye, 1,1′dibutyl 3,3,3 ′, 3′tetramethyl 4,
5,4 ', 5'-Dibenzoindodicarbocyanine perchlorate was dissolved in diacetone alcohol to a concentration of 60 g / liter, and the solution was applied by a spin coating method with an appropriate rotation pattern, and then further sputtered. A light-reflecting layer was formed by the above-mentioned method, and a UV-cured protective layer was formed on the light-reflecting layer to obtain a CD.

Recording was performed on the CD thus manufactured by changing the recording power from 5.0 mw to 10.0 mw, and reproduction was performed using a 780 nm laser beam.

At this time, n abs = 2.70, n sub = 1.58, λ = 780 nm, dl
n = 114 nm and d gr = 168 nm. Therefore, Ls
= 0.25, dln / dgr = 0.68.

Under these conditions, the jitter at the optimum power of 6.8mw is 3T
(T is the reference time width) was the largest, 28 ns. This value complies with the CD standard. Power windows with a block error rate of 220 cps or less are 6.0 mw to 9.5
mw and wide. The crosstalk is 0.23, which also meets the CD standard.

(Comparative Example) A cyanine dye was similarly dissolved in diacetone alcohol on a polycarbonate substrate similar to that in the above-described example, and the concentration was 75%.
g / liter was applied to obtain a CD.

Recording was performed on the CD thus manufactured by changing the recording power from 5.0 mw to 10.0 mw, and reproduction was performed using a 780 nm laser beam.

At this time, n abs = 2.7, n sub = 1.58, λ = 780 nm, d
ln = 135 nm and d gr = 180 nm. Therefore, Ls = 0.
3, d ln / d gr = 0.75.

Under these conditions, the jitter at the optimum power of 6.8mw is 3T
(T is the reference time width) was the largest at 42 ns. This value does not conform to the CD standard. Power windows with a block error rate of 220cps or less are 5.8mw ~
7.0mw and narrow. At a recording power of 7.0 mw or more, both jitter and crosstalk do not satisfy the CD standard.

The present inventors have found that similar effects can be obtained not only in this embodiment but also in other ranges.

[Effect of the Invention] As described above, according to the present invention, the optical parameter Ls
Or the ratio of land and pre-groove film thickness d ln / dg
Since the value of r is equal to or less than a predetermined value, pits can be formed along the shape of the pre-groove, and the deformation of the substrate is limited within the pre-groove. Records can be made.

Therefore, jitter and crosstalk are small,
In addition, a wide power window can be used for recording, and good recording can be performed even if the power of the recording light is strong, and waveform distortion during reproduction is less likely to occur, so the jitter standard value specified in the CD standard (30 ns or less) It is possible to provide an optical information recording medium capable of satisfying the requirement (1).

[Brief description of the drawings]

FIG. 1 is a partially cutaway perspective view of an optical information recording medium 1 according to the present invention, and FIG. 2 is a view for explaining an optical information recording medium 1 and a method for recording optical information on the optical information recording medium 1. FIG. 3 is a main part longitudinal sectional view showing a state in which pits 11 are formed in the pregroove 6, and FIG. 4 is a graph showing a relationship between ρ (= n abs · d av / λ) and reflectance. FIG. 5 is a graph showing the relationship between the complex refractive index k abs of the light absorbing layer 3 and the reflectance. DESCRIPTION OF SYMBOLS 1 ... Optical information recording medium 2 ... Translucent board 3 ... Light absorption layer 4 ... Light reflection layer 5 ... Protective layer 6 ... Pregroove (guide groove) 7 ... Land 8 ... First 9: second layer 10: third layer 11: pit d sub: light absorption layer 3 and substrate 2 at land 7
From the first layer boundary 8 to the depth d abs at the bottom of the first layer boundary 8 at the pre-groove 6. The second between the light absorbing layer 3 and the light reflecting layer 4 at the land 7 From the layer boundary 9 to the depth n sub at the bottom of the second layer boundary 9 in the portion of the pregroove 6... The real part n abs of the complex refractive index of the substrate 2. Real part k abs… Imaginary part of complex refractive index of light absorbing layer 3 d av… Average thickness of light absorbing layer 3 d gr… Film thickness d ln… of light absorbing layer 3 in pregroove 6 portion Film thickness at the land 7 of the light absorbing layer 3 .lambda .: wavelength of reproduction light L1: laser beam for recording L2: laser beam for reproduction

──────────────────────────────────────────────────続 き Continuation of front page (72) Inventor Emiko Hamada 6-16-20 Ueno, Taito-ku, Tokyo Taiyo Yuden Co., Ltd. (56) References JP-A-2-132656 (JP, A) JP-A-60 -50641 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G11B 7/24

Claims (3)

(57) [Claims] 1. A light-transmitting substrate having a pre-groove formed on one main surface, a light-absorbing layer provided on the substrate and containing a light-absorbing substance that absorbs recording light, An optical information recording medium for recording information by irradiating the light absorbing layer with the recording light, wherein the real part of the complex refractive index of the light absorbing layer is n abs Ls = (n abs · d), where d ln is the film thickness at the lands located on the left and right sides of the pregroove, n is the real part of the complex refractive index of the substrate, and λ is the wavelength of the reproduction light. When ln) / (n sub · λ), Ls <0.3 and the thickness of the light absorbing layer in the pregroove portion is
An optical information recording medium, wherein dgr / dgr <0.75, where dgr. 2. An imaginary part of a complex refractive index of the light absorbing layer is represented by k abs.
When the real part is n abs, the thickness of the light absorbing layer is d av, the wavelength of the reproduction light is λ, and ρ = n abs · d av / λ, 0.05 ≦ ρ The optical information recording medium according to claim 1, wherein ≤ 1.6 and k abs ≤ 0.3. 3. A light-transmitting substrate having a pre-groove formed on one main surface, a light-absorbing layer provided on the substrate and containing a light-absorbing substance that absorbs recording light, A light reflection layer provided in the optical information recording method of recording information by irradiating the light absorbing layer with the recording light, wherein the real part of the complex refractive index of the light absorbing layer is n abs Ls = (n abs · d), where d ln is the film thickness at the lands located on the left and right sides of the pregroove, n is the real part of the complex refractive index of the substrate, and λ is the wavelength of the reproduction light. When ln) / (n sub · λ), Ls <0.3 and the thickness of the light absorbing layer in the pregroove portion is
When d gr, d ln / d gr <0.75, and optical information recording is performed by irradiating the recording light to the light absorbing layer from the substrate side. .