JPH1125490A - Optical disk and optical disk device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical disk and an optical disk device making the reduction of coma aberration and the non-interference condition compatible and capable of miniaturizing the diameter of a beam spot by increasing the numerical aperture. SOLUTION: This device has a structure superposing two substrates 101, 102 of thickness (h), recording/reproduction is performed by irradiating an optical disk, provided with a reflection layer in which a signal pit is formed on a part separated by almost (h) from the front surface of at least one substrate, with a laser beam 210 through an objective lens 203 and the focusing error of the objective lens is detected by using a part of the reflected light beam. By representing the central wavelength of laser beam by λ0 , the half-value width of laser beam by Δλ, the numerical aperture of the objective lens by NA, the refractive index of substrate material by n2 , the reference value of coma aberration ΔW generated by the tilt θ of the substrate by ΔWc (=λ0 /14) and coefficient therein by f (n2 , θ), the thickness (h) satisfys the relation: λ0 <2> / 2.Δλ(n2 <2> -NA<2> )<0.5> }<=h<=ΔWc/ f(n2 , θ).NA<3> }.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital versatile disc (hereinafter referred to as "Digital Versatile Disc").
The present invention relates to an optical disk called “DVD” and an optical disk device using the same.

[0002]

2. Description of the Related Art In recent years, digital signal processing technology, optical disk manufacturing technology, and recording technology on optical disks have been dramatically advanced. With the advancement of such technology, recently, an optical disk having a recording capacity several times as large as that of a conventional compact disk (hereinafter abbreviated as CD) has appeared. .

[0003] This new type of optical disc has audio,
It records and reproduces information such as images, programs, and computer data using digital signals.
It is called DVD. This DVD has already been standardized on a worldwide scale (hereinafter abbreviated as DVD standard).
Has been established, and is expected to be a highly versatile recording medium suitable for the multimedia age.

According to the DVD standard, the diameter of a DVD is CD
120 (mm), which is the same as The thickness is 1.2 (m
m). However, in order to increase the recording capacity, the DVD adopts a configuration in which recording and reproduction can be performed using both sides by bonding two 0.6 (mm) disks. On the other hand, the center wavelength λ ₀ of the laser beam used for recording / reading is 650/635 (nm),
The numerical aperture (NA) of the objective lens used for reading is 0.6,
The track pitch is 0.74 (μm) and the shortest pit length is 0.
4 (μm) and the longest pit length 2.13 (μm). With such specifications, a recording capacity of 4 gigabytes or more per side is realized.

By the way, as can be seen from the above specifications, DV
In D, the depth of the recording layer, that is, the depth from the surface of the substrate to the reflective recording layer is about ２, and the shortest pit length and the track pitch are about ２ as compared with the conventional CD. Also,
Central wavelength λ _{0 of} laser beam used for recording / reading
Is also much shorter than 780 (nm) for a CD. For this reason, it is often difficult to obtain a focus error signal by a method similar to that of a conventional CD player.

That is, in this type of optical disc, a signal is recorded in the above-mentioned reflective recording layer in the form of pits. When this signal is read from an optical disk, a rotating optical disk is normally irradiated with a laser beam via an objective lens, and the irradiated light is reflected by a reflective recording layer formed in the optical disk. To obtain a recording signal. At the same time, a focus error is detected using a part of the reflected light, and focusing control of the objective lens is performed using the error signal so that the reflective recording layer is located at the focal position of the objective lens. The superiority of the focusing control depends on the optical conditions from the objective lens to the reflective recording layer, the characteristics of the laser beam used, and the like.

[0007] In the known DVD players,
Focusing control is performed in the same manner as a CD player. However, in such a method, high-precision control cannot be performed depending on the above-described optical conditions, and this causes a problem that the S / N of a reproduction signal is reduced and jitter is deteriorated.

On the other hand, recently, a high-definition and multifunctional one called High Definition-DVD, which has a larger recording capacity than the current DVD, and a Super-DVD, which is higher in all respects, are called. Development is under consideration. In order to realize these DVDs, it is necessary to further reduce the spot diameter of the laser beam. To achieve this, the wavelength of the laser beam used for recording and reproduction is shortened and the numerical aperture (NA) of the objective lens is increased. There is a need. Of these, it is expected that shortening the wavelength of the laser beam will involve difficulty in developing a light emitting device. Therefore, it is expected that the spot diameter will be reduced by increasing the numerical aperture (NA) of the objective lens.

In view of such a technical perspective, it is desired to develop a method for obtaining a high-definition and high-quality recording / reproducing signal which can realize high-precision focusing control regardless of the standard.

[0010]

SUMMARY OF THE INVENTION Accordingly, the present invention provides an optical disk capable of contributing to high-accuracy focusing control without affecting the recording density and improving the quality of a recording / reproducing signal, and a recording medium using the same. It is an object of the present invention to provide an optical disk device used as a device.

[0011]

In order to achieve the above object, the present invention has a structure in which two substrates having a thickness of h are stacked, and at least one of the substrates has a height of approximately h from the surface. A reflection layer on which signal pits are formed, which contributes to recording or reproduction of information when light emitted from a laser light source is applied to the reflection layer via an objective lens, and reflects light from the reflection layer; In an optical disc that provides a part of laser light for focus error detection of the objective lens, a center wavelength of the laser light is λ ₀ , a half width of the laser light is Δλ, a numerical aperture of the objective lens is NA, and the substrate is n ₂ the refractive index of the material, when the coefficient of the reference value ΔWc (= λ _0/14) and therein a coma ΔW caused by the tilt of the substrate theta was f (n _2, theta), the thickness h
Satisfies the following equation (1).

Λ ₀ ² / ｛2 · Δλ (n ₂ ² −NA ² ) ^0.5 ｝ ≦ h ≦ ΔWc / ｛f (n ₂ , θ) · NA ³ … (1) where ΔW ≦ ΔWc (= λ ₀ / 14) Further, in order to achieve the above object, the present invention relates to
Light emitted from a laser light source is applied to an optical disc provided with a reflective layer in which signal pits are formed in a portion where the distance from the surface is substantially h on at least one of the substrates. Irradiation is performed through an objective lens, information is recorded or reproduced on the optical disk by the irradiation, and the focus of the objective lens is determined based on a signal obtained by detecting a part of a laser beam reflected from the reflective layer. In an optical disc device configured to detect an error, the center wavelength of the laser light is λ ₀ , the half-width of the laser light is Δλ, the numerical aperture of the objective lens is NA, the refractive index of the substrate material is n ₂ , when the coefficient of the reference value ΔWc (= λ _0/14) and therein a coma ΔW caused by the tilt of the substrate theta was f (n _2, θ), the thickness h
Satisfy the expression (1).

The laser beam may have a center wavelength λ ₀ of a power spectrum in a range of 410 (nm) to 420 (nm) and have a half width Δλ of about 0.105 (nm) or more. .

In the case of an optical disk apparatus according to the present invention, a laser beam emitted from a laser light source is irradiated on an optical disk after passing through an objective lens. Part of the irradiation light is reflected on the surface of the optical disk. The reflected light reflected by the surface of the optical disk to U _1. On the other hand, the remaining light is incident on the transparent material layer (substrate) forming the surface layer of the optical disk, is reflected by the reflective layer forming the signal mark, and passes through the transparent material layer again from the surface of the optical disk. Emit. This emitted light is defined as U ₂ .

Here, the maximum angle of the incident light beam is θ ₁ (where sin θ ₁ = numerical aperture NA), the refractive index of air is n ₁ , the refraction angle in the transparent material layer is θ _2, and the refraction of the transparent material layer is Rate n
_Then , when the optical path difference ΔL between U ₁ and U ₂ is obtained, this optical path difference ΔL is expressed by the equation (2).

ΔL = 2 · n ₂ · h · cos θ ₂ (2) From the Snell's law, n ₁ · sin θ ₁ = n ₂ · sin θ ₂ (3) holds.

Meanwhile, coherence length Lc of the laser beam is represented by ^{_{Lc = λ 0 2 / Δλ ...}} (4).

Therefore, if the optical path difference ΔL is shorter than the coherence length Lc, light wave interference occurs between the surface of the optical disk and the internal reflection layer. In the case of such a condition, for example, there is a portion where the optical path difference ΔL is shorter than the coherent distance Lc locally in the circumferential direction or the radial direction.
In the case of an optical disc having so-called uneven thickness, the focus error signal fluctuates in synchronization with the rotation of the optical disc due to the influence of the above-described interference. As a result, high-precision focusing control cannot be performed, resulting in a decrease in the S / N of the reproduction signal and a deterioration in jitter.

The effect of such a change in the interference intensity on the focus error signal will be described in more detail.
When light wave has multiple reflection and interference between the surface and the interior of the reflective layer of the optical disk, the refractive index n than the substrate having a refractive index n ₂
The intensity Ir of the total luminous flux emitted to the air side ₁ is expressed by the following equation.

Ir = {4R · sin ² · δ / 2 · Ii} / {(1-R) 2 + 4R · sin ² · δ / 2} (5) In the equation (5), I i is an incident light intensity, R indicates the reflectance at the interface between the air (refractive index n ₁ ) and the substrate (refractive index n ₂ ). Also, [delta] is related to the phase difference between the reflected light U ₂ in the reflective layer is a reflective light U ₁ and the information recording surface at the interface is given by the following equation.

Δ = (2π / λ ₀ ) · ΔL = (4πn ₂ / λ ₀ ) h · cos θ ₂ (6) From the equation (2), the equation (6) is as follows.

[0022] [delta] = from _{(4π / λ 0) h ·} (n 2 2 over ^{_{n 1 2 · sin 2 θ 1}} ) 0.5 ... (7) (5) wherein the phase difference [delta] is changed from 0 to [pi, interference It can be seen that the intensity ranges from 0 (dark) to the maximum (bright). Therefore, from equation (7), a light-dark cycle Delta] h of the interference fringes is given by _{Δh = λ 0 / {4 (} n 2 2 over ^{_{n 1 2 · sin 2 θ 1}} ) 0.5} ... (8).

As the values of n ₁ , n ₂ and θ _{1 in the} equation (8), n ₁ = 1 (air), n ₂ = 1.5 (substrate material), θ ₁ = 0 to 36.9 deg (NA = 0.6 = sin
a theta _1, Substituting this than the maximum incident angle is obtained), the minimum and maximum values of the period Δh of interference fringes contrast,
Each Δhmin = λ _0/6, the Δhmax = λ ₀ /5.5.

For example, according to the DVD standard, the center wavelength λ ₀ of the laser beam used is λ ₀ = 650/635 (nm). Therefore, when a laser beam having a center wavelength λ ₀ = 635 (nm) is used, Δhmin = 0.105 (μm), Δhmin = 0.
hmax = 0.115 (μm). Thus, Δhmin
And Δhmax is only 0.01 (μm). For this reason, the luminous flux existing within the range of the numerical aperture NA = 0.6 has the same interference intensity for substantially the same Δh (that is, unevenness in the thickness of the substrate). For this reason, when the thickness of the substrate changes with the rotation of the optical disk, the interference fringes change from bright to dark at the maximum width, and as a result, the focus error signal also changes.

However, according to the present invention, equation (1) is established between the center wavelength λ ₀ of the laser beam to be used, the half width Δλ, the refractive index n ₂ of the substrate material, the numerical aperture NA of the objective lens and the thickness h. Since the thickness h is set in such a relationship, the optical path difference ΔL
The coherence distance Lc can be made shorter than that. As a result, multiple interference of light waves between the surface of the optical disc and the internal reflection layer can be prevented, so that a focus error can be detected with high accuracy, and high-precision focusing control can be performed, and the S / S of the reproduction signal can be improved. It is possible to prevent a decrease in N and a deterioration in jitter.

According to the present invention, the tilt θ of the optical disk is
Reference value of the coma aberration ΔW caused by ΔWc (= λ _0/1
4 = 0.07λ ₀ , Marechal evaluation standard value (Marechal C
Since the thickness h is set so that the equation (1) is established between the coefficient f (n ₂ , θ) and the numerical aperture NA of the objective lens and the thickness h, the numerical aperture NA Is increased to reduce the spot diameter of the laser beam, thereby suppressing an increase in coma when the recording density is increased.

That is, when the optical disk is inclined by θ, the coma aberration ΔW generated in the optical system forming the objective lens is given by ΔW∝f (n ₂ , θ) · h · NA ³ (9) . However, the reference value of coma aberration ΔW is ΔWc
Then, there is a relationship of ΔW ≦ ΔWc (10).

As can be seen from equation (9), the coma aberration ΔW is
It is proportional to the cube of the thickness h and the numerical aperture NA. For the coma aberration and Marechal Criterion, see "Supervision of Morio Onoe, Noboru Murayama, Hiroshi Koide, Kazusaku Yamada, Makoto Kunikane," Optical Disc Technology "Radio Technology Co., Ltd., April 10, 1990
Edition, pages 54-55, page 63 ".

However, in the present invention, since the thickness h is set so as to satisfy the expression (1), "reduction of coma"
And "the condition of non-interference" can be compatible, and "the reduction of the beam spot diameter, that is, the high NA" can be achieved.

As an example, the center wavelength of the laser beam used for recording / reproduction is λ ₀ = 410 nm, the half width of the laser beam is Δλ = 0.2 nm, the refractive index of the substrate material is n ₂ = 1.5, and the numerical aperture of the objective lens is Consider the case where NA is increased from 0.6 to 0.75.

By substituting the above condition into the equation (1), the thickness h becomes 0.32 mm. In other words, the thickness is about 1/2 of the thickness of 0.6 mm specified in the DVD standard. On the other hand, the increase rate of coma is (0.32 / 0.6) × (0.75 / 0.6) ³ = 1.

As can be seen from the above, by employing the present invention, the beam spot diameter can be reduced by 25%, the increase in coma due to the tilt of the optical disk can be suppressed, and the condition of non-interference can be satisfied. You can do it.

[0033]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows an optical disk device according to an embodiment of the present invention, here, a DVD player.

Reference numeral 100 denotes a DVD as an optical disk. The DVD 100 is specifically formed as shown in FIGS. That is, DV
D100 is a thickness h = 0.32 formed in a disk shape.
(Mm) first and second information recording members 101 and 102
Are bonded to form an overall thickness of about 0.64 (mm) or an intermediate thickness of about 1.2 (mm). First and second information recording members 101, 10
2 includes light-transmitting substrates 103 and 104, respectively. On the rear surfaces of the light-transmitting substrates 103 and 104, reflection layers 105 and 106 that have pits corresponding to recording information such as compressed moving image information and reflect light are attached. On these reflective layers 105 and 106, protective layers 107 and 108 for preventing oxidation of the reflective layers are provided, respectively. The first and second information recording members 101 and 102 are attached to each other by an adhesive layer 109 made of a thermosetting adhesive inserted between the protective layers 107 and 108. A clamping hole 110 for clamping is provided at the center of the DVD 100, and a clamping zone 111 is provided around the clamping hole 110.

The material of each part constituting the DVD 100 will be described. The translucent substrates 103 and 104 are made of polycarbonate, polycarbonate, or PMMA.
(Polymethyl methacrylate). Their refractive index n ₂ is approximately 1.5. The reflection layers 105 and 106 are formed of an aluminum thin film. The protective layers 107 and 108 are formed of a photocurable resin (ultraviolet curable resin). The adhesive layer 109 is made of a hot melt adhesive (thermoplastic resin adhesive), for example, a polyvinyl ether paraffin-based material.

In the optical disk device shown in FIG.
In a state where D100 is chucked to the taper cone 200, for example, 135
It is rotated at a rotation speed of 0 (rpm). Spindle motor 20
1 is driven by a spindle motor drive circuit 202.

On the other hand, the recording / reproducing optical system is configured as follows. That is, the objective lens 203 is arranged to face one surface of the DVD 100. The objective lens 203 has a numerical aperture NA of 0.75 and is controlled to move in the optical axis direction by a focus coil 204 and in the track width direction by a tracking coil 205. A semiconductor laser diode (hereinafter abbreviated as LD) 206 is provided at a position facing the objective lens 203.
03 is arranged so as to be movable integrally therewith. This LD20
6 is the case where the center wavelength λ ₀ of the power spectrum is 410 (nm)-
In this example, the center wavelength λ ₀ is 410 (nm) and the half width Δλ is 0.105 (nm) as shown in FIG.
In this example, a laser beam having a half width Δλ of 0.2 (nm) is transmitted as shown in FIG. Note that this LD
206 is energized by the LD driver 207.

The laser beam emitted from the LD 206 is converted into a parallel light beam by the collimator lens 208 and then enters the polarization beam splitter 209. LD20
Since the laser beam emitted from 6 generally has an elliptical far field pattern, a beam shaping prism (not shown) may be arranged after the collimator lens 208 when a circular pattern is required.

The laser beam that has passed through the polarizing beam splitter 209 is converged by the objective lens 203,
As shown in FIG. 4 and FIG.
03 or 104 is incident.

Here, the first and second information recording members 1
The thickness h of 01, 102 is the center wavelength λ _{0 of} the laser beam, the half-width Δλ of the laser beam, the numerical aperture NA of the objective lens 203,
Refractive index n ₂ of translucent substrates 103 and 104, DVD 100
The reference value of the coma aberration ΔW generated by the inclination θ of
(= Lambda _0/14), the coefficient f (n _2, theta) therein with respect to the (1) value that satisfies the formula, in this example h = 0.32 (m
m) is set.

At the time of reproduction, a DVD is
The laser beam incident on the 100 light-transmitting substrates 104 (103) is focused on the reflection layer 106 (105) as a fine beam spot. Then, the reflection layer 106 (10
The reflected light from 5) passes through the objective lens 203 in a direction opposite to that of the incident light, is reflected by the polarization beam splitter 209, passes through a detection optical system such as a condenser lens 211 and a cylindrical lens 212, and then becomes a photodetector. 213.

As shown in FIG. 5, the photodetector 213 has four photodetectors 214a to 214a arranged on the same plane.
4d. The four detection outputs of the photodetector 213 are output from an amplifier array 21 including an amplifier and an adder / subtractor.
5, where a focus error signal F, a tracking error signal T and a reproduction signal S are generated.

The tracking error signal T is obtained by a known method called a push-pull method.
The focus error signal F is obtained by a known astigmatism method. In this astigmatism method, the output Ia of the photodetector 214a and the output Ib of the photodetector 214b which are located on a diagonal line among the photodetectors shown in FIG. The signal obtained by adding the output Ic of the photodetector 214d to the output Ic of the photodetector 214d is subtracted.
A focus error signal is obtained from the subtraction signal.

The focus error signal F and the tracking error signal T are transmitted via the servo controller 216 to the focus coil 204 and the tracking coil 2.
05 respectively. Thereby, the objective lens 2
03 is controlled to move in the optical axis direction and the track width direction,
Focusing of the light beam on the reflective layer 106 (105), which is the recording surface of the DVD 100, and tracking on the target track are performed.

On the other hand, the reproduced signal S from the amplifier array 215 is input to a signal processing circuit 217, where the signal is equalized and binarized. In the binarization process, the reproduced signal after the waveform equalization is guided to a PLL (phase synchronization circuit) and a data identification circuit, and the reproduced signal is read from the DVD 100 by the PLL.
A channel clock, which is a basic clock when information is recorded on a channel, is extracted. Then, by identifying “0” and “1” of the reproduced signal based on the channel clock, D
Perform data identification of information recorded in VD100,
Obtain a data pulse. That is, by comparing the reproduced signal after waveform equalization with an appropriate threshold value within a predetermined time width (referred to as a detection window width or window width) based on the rising or falling timing of the channel clock, data identification is performed. I do.

The data pulse detected from the signal processing circuit 217 is input to the disk controller 218, where the format is decoded and error correction is performed, and then input to the MPEG2 decoder / controller 219 as a bit stream of moving image information. You. DVD10
In 0, data obtained by compressing and encoding moving image information in accordance with the MPEG2 standard is recorded as a pit pattern on the reflective layers 105 and 106. Thus, the MPEG2 decoder / controller 219 decodes (decompresses) the input bit stream and reproduces the original moving image information.

The reproduced moving picture information is input to a video signal generating circuit 220 and added with a blanking signal or the like to form a video signal of a predetermined television format such as the NTSC format, which is displayed on a display (not shown). Is done.

As described above, in the DVD optical disk device, the first and second information recording members 10 are used.
Since the thickness h of 1,102 is set to a value that satisfies the condition of Expression (1), the optical path difference ΔL expressed by Expression (2) is
The coherent distance Lc represented by the equation (4) can be shortened.
It is possible to prevent light wave interference from occurring between the surface 00 and the inner reflective layer 105 (106). Therefore, a focus error signal that changes linearly with respect to the focus direction displacement ΔZ of the objective lens 203 as shown in FIG. 6 can be obtained. As a result, highly accurate focusing control can be performed, and the reproduction signal can be obtained. S / N reduction and jitter deterioration can be prevented.

The first and second information recording members 10
Since the thickness h of the lenses 1, 102 is set to a value that satisfies the condition of the equation (1), the objective lens 20 is required to increase the recording density.
When trying to reduce the spot diameter of the laser beam by increasing the numerical aperture NA of 3, the thickness h must necessarily be reduced to a small value. As a result, the rate of increase in coma due to an increase in the numerical aperture NA can be sufficiently suppressed by reducing the thickness h. As a result, both “reduction of coma” and “non-interference conditions” can be achieved. In addition, it is possible to realize “reduction in beam spot diameter, that is, increase in NA”.

In the above example, a DVD player is shown as an example of an optical disk, but a ROM (D) for recording / reproducing a program, computer data, etc.
VD-ROM) and RAM (DVD-RAM). Further, the present invention is not limited to the above-described embodiment, and can be, of course, variously modified and implemented.

[0051]

As described above, according to the present invention, it is possible to achieve both a reduction in coma aberration and a condition of non-interference, and a reduction in beam spot diameter, that is, a higher NA.

[Brief description of the drawings]

FIG. 1 is a block diagram of an optical disk device according to an embodiment of the present invention.

FIG. 2 is a perspective view of a DVD used as a recording medium in the apparatus shown in FIG.

FIG. 3 is a diagram partially showing a cross section of the DVD shown in FIG. 2;

FIG. 4 is a diagram showing an optical system of the apparatus shown in FIG.

FIG. 5 is a front view of an optical sensor incorporated in the optical system shown in FIG. 4 and used for obtaining a focus signal;

FIG. 6 is a diagram showing a focus error signal detected in the device shown in FIG. 1;

FIG. 7 is a diagram showing an example of a power spectrum of a laser diode used in the optical disk device.

[Explanation of symbols]

100 ... Digital Versatar Disc (DV
D) 101, 102: Information recording member 103, 104: Transparent substrate 105, 106: Reflective layer 107, 108: Protective layer 109: Adhesive layer 201: Spindle motor 202: Spindle motor drive circuit 203: Objective lens 204: Focus Coil 205 Tracking coil 206 Laser diode (LD) 207 LD driver 208 Collimating lens 209 Polarizing beam splitter 210 Incident light flux 212 Cylindrical lens 213 Photodetector 214a to 214d Photodetector 215 Amplifier array 216 ... Servo controller 217 ... Signal processing circuit 218 ... Disk controller 219 ... MPEG2 decoder / controller 220 ... Video signal generation circuit

Claims (3)

[Claims] 1. A structure in which two substrates having a thickness of h are stacked, at least one of the substrates is provided with a reflective layer in which signal pits are formed at a position substantially at a distance h from the surface, and is provided from a laser light source. When light is irradiated toward the reflective layer through the objective lens, the light contributes to recording or reproduction of information, and provides a part of the reflected laser light from the reflective layer for detecting a focus error of the objective lens. In the optical disc, the center wavelength of the laser light is λ ₀ , the half width of the laser light is Δλ, the numerical aperture of the objective lens is NA, the refractive index of the substrate material is n ₂ , and the inclination θ of the substrate is generated. when the coefficient of the reference value ΔWc (= λ _0/14) and therein a coma ΔW was f (n _2, θ), an optical disk, wherein the thickness h satisfies the following equation. λ 0 ² / {2 · Δλ (n ₂ ² −NA ² ) ^0.5 } ≦ h ≦ ΔW
_{c / {f (n 2,} θ) · NA 3} _{However, ΔW ≦ ΔWc (= λ 0} /14) 2. A structure in which two substrates having a thickness h are stacked, and at least one of the substrates has a distance from the surface of approximately h.
A light emitted from a laser light source is irradiated through an objective lens onto an optical disc provided with a reflection layer in which signal pits are formed in a portion of the optical disc, and information is recorded or reproduced on the optical disc by the irradiation. An optical disc device configured to detect a focus error of the objective lens based on a signal obtained by detecting a part of a laser beam reflected from a reflective layer, wherein a center wavelength of the laser beam is λ ₀ , the half-width [Delta] [lambda], NA the numerical aperture of the objective lens, the refractive index of the substrate material n _2, .DELTA.WC the reference value of the coma aberration ΔW caused by the tilt of the substrate θ (= λ _0/14) and of An optical disk device, wherein the thickness h satisfies the following equation, where f (n ₂ , θ) is a coefficient inside. λ 0 ² / {2 · Δλ (n ₂ ² −NA ² ) ^0.5 } ≦ h ≦ ΔW
_{c / {f (n 2,} θ) · NA 3} _{However, ΔW ≦ ΔWc (= λ 0} /14) 3. The laser light according to claim 1, wherein the center wavelength λ ₀ of the power spectrum is in a range of 410 (nm) to 420 (nm).
3. The optical disk device according to claim 2, wherein the optical disk device has a half width Δλ of about 105 (nm) or more.