An optical disk with sectional trapezoidal pits comprising a substrate having information recorded by a plurality of pit trains formed thereon at a specified track pitch, and a reflective layer formed on the substrate, wherein the information is reproduced by being irradiated with light beam via an objective lens, the track pitch is set within the range of (0.72 to 0.8) α×λ/na/1.14 μm when a wavelength of the light beam is μ nm and a numerical aperture of the objective lens is na, each of the pits is a multiplication ratio used to secure allowable disk tilt angles in an upper width within the range of (0.3 to 0.50) α×λ/na/1.14 μm, a bottom width within the range of (0.2 to 0.32) α×λ/na/1.14 μm and a depth within the range of (1/4.2×λ/n) to (1/5.2 λ/n) (n: refractive index of said substrate and λ: a wavelength) and obtained by 2.623×10-7×(d/λ)2-1.706×10-4 (ds/λ)+0.934, where ds is thickness of the substrate.
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0. 9. An optical disk comprising:
a circular substrate comprising information recorded as a plurality of pit trains formed on said circular substrate with a track pitch, each said pit train comprising a plurality of pits; and a reflecting layer formed on said substrate; wherein said information is reproduced by said pit trains being irradiated with a light beam via an objecting lens, said objective lens having a numeral aperture na, and wherein said track pitch is in the range of ( -7(ds/λ)2-1.706×10 1. An optical disk comprising:
a circular substrate having a predetermined diameter and a predetermined thickness, said substrate comprising information recorded as a plurality of pit trains formed on said circular substrate with a track pitch, each said pit train comprising a plurality of pits; and a reflecting layer formed on said substrate; wherein said information is reproduced by said pit trains being irradiated with a light beam via an objecting lens, said objective lens having a numerical aperture na, and wherein: said track pitch is in the range of (0.72 to 0.8)α×(λ/na)/1.14 μm when a wavelength of said light beam is λ μm; each of said pits is scaled by the multiplication ratio α, said multiplication ratio α being used to secure allowable tilt angles of said optical disk and being obtained by 2.623×10-7× (ds/λ)2-1.706×10-4 (ds/λ)+0.934 m , where ds is said predetermined thickness in μm and is equal to 600 μm; a radial tilt of said optical disk is not more than 9.5 mrad; and said predetermined diameter is 120 mm.
5. An optical disk apparatus comprising:
an optical disk comprising a circular substrate and a reflecting layer formed on said substrate, said substrate having a predetermined diameter and a predetermined thickness and comprising information recorded as a plurality of pit trains formed on said substrate with a track pitch, each said pit train comprising a plurality of pits; an objective lens disposed so as to face said optical disk, said objective lens having a numerical aperture na; means for projecting a light beam onto said optical disk via said objective lens; and means for sensing reflected light of said light beam projected onto said optical disk by said projecting means to reproduce said information recorded on said optical disk; wherein: said track pitch is in the range of (0.72 to 0.8)α×(λ/na)/1.14 μm when a wavelength of said light beam is λ μm; each of said pits is scaled by the multiplication ratio α, said multiplication ratio being used to secure allowable tilt angles of said optical disk and being obtained by 2.623×10-7× (ds/λ)2-1.706×10-4 (ds/λ)+0.934 m , where ds is said predetermined thickness in μm and is equal to 600 μm; a radial tilt of said optical disk is not more than 9.5 mrad; and said predetermined diameter is 120 mm.
8. An optical disk comprising:
a pair of transparent circular substrates with surfaces facing one another, each of said transparent circular substrates having information recorded as a plurality of pit trains formed on a corresponding one of said transparent circular substrates with a track pitch, each said pit train comprising a plurality of pits; a pair of reflecting layers coated on said facing surfaces of said circular substrates, respectively, a pair of protective layers formed on said pair of reflecting layers, respectively; and an adhesive layer formed between said protective layers to adhere said protective layers to one another; wherein said information is reproduced by said pit trains being irradiated with a light beam via an objective lens, said objective lens having a numerical aperture na, and wherein: said track pitch is in the range of (0.72 to 0.8)α×(λ/na)/1.14 μm when a wavelength of said light beam is λ μm; each of said pits is scaled by the multiplication ratio α, said multiplication ratio α being used to secure allowable tilt angles of said optical disk and being obtained by 2.623×10-7× (ds/λ)2-1.706×10-4 (ds/λ)+0.934 m , where ds is said predetermined thickness in μm and is equal to 600 μm; a radial tilt of said optical disk is not more than 9.5 mrad; and said predetermined diameter is 120 mm.
2. The optical disk according to
a secondary substrate comprising secondary information recorded as a plurality of second pit trains formed on said secondary substrate at said track pitch, each said second pit train comprising a plurality of second pits; and a secondary reflecting layer formed on said secondary substrate, said reflecting layer and said secondary reflecting layer being disposed between said substrate and said secondary substrate.
3. The optical disk according to
4. The optical disk according to
6. The optical disk apparatus according to
7. The optical disk apparatus according to
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This is a continuation of application Ser. No. 08/778,313, filed Jan. 2, 1997 now U.S. Pat. No. 5,777,981 which is a continuation of application Ser. No. 08/541,598, filed Oct. 10, 1995, now U.S. Pat. No. 5,602,825; which is a continuation-in-part of application Ser. No. 08/475,494, filed Jun. 7, 1995, now U.S. Pat. No. 5,592,464; which is a continuation of application Ser. No. 08/304,849, filed Sep. 13, 1994, now U.S. Pat. No. 5,459,712.
1. Field of the Invention
This invention relates to an optical disk on which information is recorded in pits with high density and an optical disk apparatus containing the optical disk and a playback optical system.
2. Description of the Related Art
With the recent advances in image digital signal processing techniques and moving-picture compression techniques, the latter of which have been developed by such a standardizing organization as the MPEG (Moving Picture Image Coding Experts Group), there is a growing expectation of the advent of an optical disk capable of reproducing moving-picture information such as a movie for two hours and being the same size as a CD (compact disk) in place of a VTR or laser disk. The recording capacity required to record two hours of moving-picture information in the form of analog video signals by a standard TV system such as NTSC as on the laser disk, amounts to 80 Gbyte including sound. Use of moving-picture compression techniques prescribed by a standardized method called MPEG-2, for example, requires as small a capacity as nearly 4 Gbyte even for a picture quality as good as a high picture-quality VTR such as S-VHS. The 4-Gbyte disk has been put into practical use in the form of a 300-mm diameter write-once read-many optical disk. As more and more optical disks will be used in homes in the future, it is needed to achieve an easy-to-use 120-mm diameter disk which has the same size and almost the same capacity as the CD.
The capacity of the CD format presently available as the music CD or the CD-ROM is 790 Mbyte at the maximum (when the linear velocity is 1.2 m/s). The capacity of this order can store only 24 minutes of compressed moving-picture information by MPEG-2. Thus, to store two hours of compressed moving-picture information by MPEG-2 with the CD size, the recording density must be made five times as high as that of the CD. In the current Cd format, the substrate thickness is 1.2 mm, the track pitch is 1.6 μm, the pit pitch is 1.66 μm when the linear velocity (relative velocity between light beam and disk=disk's circumferential velocity) is 1.2 m/s, the bit length is 0.59 μm, and the modulation method is EFM (eight to fourteen modulation). In the playback optical system, the playback semiconductor laser, or the laser diode (LD) has a wavelength of 780 nm, the object lens has an NA (numerical aperture) of 0.45, and the beam spot has a diameter of 1.4 μm. The beam spot diameter is selected mainly from the standpoint of avoiding the effect of crosstalk between adjacent tracks.
To increase the recording density of the optical disk requires techniques for forming small pits in the disk and those for making the beam spot size small on the optical disk in the playback optical system. Concerning techniques for forming pits, for example, an optical disk matrix recording technique using Kr ion laser light (ultraviolet rays) with a wavelength of 351 nm has been proposed (The 1993 Autumn National Convention of the Applied Physics Society, 28-SF-2). This technique makes it possible to form smaller pits than a conventional Ar ion laser. In the playback optical system, by making the wavelength of the playback laser beam shorter and increasing the NA, the beam spot diameter can be made smaller. Actually, however, with conventional techniques used in CD players, even if a short wavelength light source such as a red laser diode were used, the capacity would be increased by 1.5 times at most. With such an increase in the capacity, it cannot be expected to increase the capacity by five times that of an ordinary CD, which is what is required to record two hours of compressed moving-picture information.
As described above, with the conventional optical disk techniques, to avoid the problem of crosstalk between adjacent tracks, the track pitch and pit pitch are set larger than the beam spot diameter of the playback light beam. As a result, only by making the wavelength of playback light beam shorter and increasing the NA of the object lens, the recording density cannot be raised to the extent that the capacity required to store two hours of compressed moving-picture information by MPEG2 with the CD size, for example.
The object of the present invention is to provide an optical disk and an optical disk apparatus which can lessen crosstalk between adjacent tracks to the extent that there is no problem in practical use, even if the track pitch and pit pitch are smaller than the beam spot diameter of the playback light beam, and which achieves a higher density and a greater capacity than in the prior art.
According to the present invention, there is provided an optical disk comprising a substrate and a recording layer which is formed on the substrate and on which information is recorded at specific pitches in the form of pit trains, wherein the information is reproduced by projecting a light beam via an object lens, and when the wavelength of the light beam is λ μm and the numerical aperture of the objective lens is NA, the track pitch is set in the range of (0.72 to 0.8)×λ/NA/1.14 μm, and each of the pits has a trapezoidal cross section whose upper width is in the range of (0.3 to 0.5)×λ/NA/1.14 μm and whose lower width is in the range of (0.2 to 0.32)×λ/NA/1.14 μm.
According to the present invention, there is provided an optical disk apparatus comprising an optical disk comprising a substrate and a recording layer which is formed on the substrate and on which information is recorded at specific pitches in the form of pit trains, an objective lens provided so as to face the optical disk, means for projecting a light beam onto the optical disk via the objective lens, and means for sensing the reflected light of the light beam projected on the optical disk by the projecting means to reproduce the information recorded on the optical disk, wherein when the wavelength of the light beam is λ μm and the numerical aperture of the objective lens is NA, the track pitch is set in the range of (0.72 to 0.8)×λ/NA/1.14 μm, and each of the pits has a trapezoidal cross section whose upper width is in the range of (0.3 to 0.5)×λ/NA/1.14 μm and whose lower width is in the range of (0.2 to 0.32)×λ/NA/1.14 μm.
According to the present invention, there is provided an optical disk comprising a substrate and a recording layer which is formed on the substrate and on which information is recorded at specific pitches in the form of pit trains, wherein the information is reproduced by projecting a light beam via an objective lens, and when the wavelength of the light beam is λ μm and the numerical aperture of the object lens in NA, the track pitch is set in the range of (0.72 to 0.8)×/NA/1.14 μm, and each of the pits has a trapezoidal cross section whose upper width is in the range of (0.3 to 0.5)×λ/NA/1.14 μm and whose inner wall has an angle of 30°C to 60°C.
According to the present invention, there is provided an optical disk apparatus comprising an optical disk comprising a substrate and a recording layer which is formed on the substrate and on which information is recorded at specific pitches in the form of pit trains, an objective lens provided so as to face the optical disk, means for projecting a light beam onto the optical disk via the object lens, and means for sensing the reflected light of the light beam projected on the optical disk by the projecting means to reproduce the information recorded on the optical disk, wherein when the wavelength of the light beam is λ μm and the numerical aperture of the objective lens is NA, the track pitch is set in the range of (0.72 to 0.8)×λ/NA/1.14 μm, and each of the pits has a trapezoidal cross section whose upper width is in the range of (0.3 to 0.5)×λ/NA/1.14 μm and whose inner wall has an angle of 30°C to 60°C.
Furthermore, the invention provides an optical disk having sectional trapezoidal pits comprising a substrate having information recorded by a plurality of pit trains formed thereon at a specified track pitch, and a reflective layer formed on the substrate, wherein the information is reproduced by being irradiated with light beam via an objective lens, the track pitch is set within the range of (0.72 to 0.8)α×
Before explanation of an embodiment, the basic concept of the present invention will be described.
To make the density of the optical disk higher, a spot diameter of the playback light beam must be made smaller. To do this, it is essential to make the wavelength of the playback laser diode shorter and increase the NA of the objective lens. Laser diodes (self-pulsation-type laser diodes) of a low-noise type whose wavelength is 0.685 μm and whose output is several milliwatts have already been put into practical use. Laser diodes whose wavelength is 0.650 μm are getting close to practical use.
The NA of the objective lens is limited by ease of making a lens and the tile angle between the lens and the disk. The smaller the lens load (the thinner the optical disk substrate, the small the lens load) and the smaller the NA, the easier it is to make the objective lens. For an objective lens whose NA is nearly 0.6, it is possible to make the beam spot diameter smaller even with a single nonspherical lens. However, with an objective lens used in the playback optical system for the optical disk, coma aberration occurs due to the tilt between the optical disk and the playback light beam caused by the tilt of the optical disk or the tilt of the optical axis of the objective lens.
Specifically, an attempt to make the NA of the object lens larger in order to make the spot size of the playback light beam smaller permits the aberration of the objective lens to increase sharply due to the tilt between the optical disk and the playback light beam. As the aberration of the objective lens becomes larger, the amount of crosstalk between adjacent tracks increases accordingly, and the playback resolving power decreases. The thinner the optical disk substrate, the smaller the effect of the tilt. In Japanese Journal of Applied Physics, Vol. 32 (1993), pp 5402-5405, changes in the shape of the spot of the playback light beam corresponding to the tilt when the substrate thickness is 1.2 mm, the same as the CD, and 0.6 mm, with a wavelength of 0.690 μm and NA=0.6 are explained. According to this description, when the substrate thickness is 1.2 mm, a tilt of 5 mrad lowers the center strength of the beam spot as much as 10% and causes a rise in the side lobe and aberration, which contributes to crosstalk. In contrast, when the substrate thickness is 0.6 mm, the substrate can withstand the tilts ranging up to 10 mrad.
However, just making the substrate thickness thinner can cause the substrate to warp substantially due to temperature or humidity. The warp of the substrate contributes mainly to the tilt. To avoid this, it is most effective to make the optical disk double-sided as the laser disk, or to give the optical disk a symmetrical structure with respect to the front and back. In that case, it is possible to record information on both sides. With a single-structure optical disk like the conventional CD, since an aluminum reflective film or protective film is formed on one side of the substrate, the substrate has an asymmetrical moisture absorption with respect to the front and back, and thus tends to warp easily. A double-sided optical disk cancels the distortion of the substrate due to moisture absorption, thereby preventing a large tilt from occurring.
The evaluation results described above show that if a combination of a laser diode with a wavelength of 0.685 μm, a 0.6 mm thick substrate, and an objective lens with an NA=0.6 are used, the wavelength will be shortened from 0.780 μm to 0.685 μm and the NA will grow larger from 0.45 to 0.6, so that the recording density can be made about 2.3 times as large as that of the conventional CD format even by conventional CD design techniques. Specifically, because the spot size is proportional to λ/NA, this gives (0.685/0.6)/(0.780/0.45), meaning that the recording density is about 2.3 times as high as that of the conventional CD. However, to achieve the capacity required to record two hours of compressed moving-picture information by MPEG2 with the CD size, it is necessary to make the recording density (capacity) about five times as large as that of the conventional CD format.
According to the present invention, there is provided an optical disk which enables the track pitch to be made much smaller in order to achieve a much higher density and greater capacity, while assuring the low crosstalk characteristics and the sufficient signals levels of the playback signal and the push-pull signal by optimizing the pit shape of the same beam spot size as described above. Hereinafter, the pit shape in the present invention will be described in detail.
It is assumed that RLL (Run-Length Limited) scheme are used as a modulation scheme of information recorded on the optical disk. It is necessary in this scheme to pay attention to crosstalk due to low frequency components when the longest pit is sensed. Although the crosstalk characteristics shown in
In the system design of an optical disk apparatus, if a tilt due to the warp of the optical disk itself and a tilt due to the apparatus are considered to be 5 mrad and 3 mrad, respectively, a tilt of about 8 mrad in total must be tolerated. According to the simulation in
The evaluation of
The results mentioned above show that when the pit shape across the track width is standardized with a wavelength of 0.685 μm and NA=0.6 (i.e., λ/NA=1.14), it is desirable that the upper width of a pit should be (0.3 to 0.45)×λ/NA/1.14 μm, and the lower width of the pit should be (0.2 to 0.25)×λ/NA/1.14 μm, or that the upper width Wm should be (0.3 to 0.45)×λ/NA/1.14 μm and the angle θ of the inner wall should be in the range of 50°C to 70°C. Specifically, when the track pitch Pt is selected in the range of (0.72 to 0.8)×λ/NA/1.14 μm and the track pitch is made smaller than the beam spot diameter of the playback light beam, selecting the upper width Wm and lower width Wi of the pit or the upper width Wm of the pit and the angle θ of the inner wall in the above ranges enables the amount of crosstalk to be suppressed to values less than -20 dB required in practical use in the tilt range of ±10 mrad expected in an actual optical disk apparatus, thereby achieving a remarkable improvement in the recording density. As a result, by combining these track pitch and pit shape, the laser diode with a wavelength of0.685 μm, for example, as mentioned earlier, the 0.6 mm thick substrate, the object lens with NA=0.6, the subject of recording two hours of compressed moving-picture information by MPEG with the CD size can be achieved easily.
The parameters used in the explanation of the invention are obtained through calculations on the assumption that the pit is in the form of an ideal trapezoid. Actually, however, the pit does not take the form of an accurate trapezoid, but curves at its corners as shown in FIG. 13. Therefore, the parameters for the ideally trapezoidal pit, or the model pit, differ from those for the actual pit.
Hereinafter, the structure of an optical disk according to the present invention will be described.
When the 4/9 modulation method is used as a modulation method for the information recorded on the optical disk 100, the track pitch on the optical disk 100 is 0.72 μm, and the pit pitch is 0.96 μm, it is expected that the pit density ratio is 3.84 times as high as the conventional CD format, the modulation efficiency is increased by 20%, and the formal efficiency is increased by 10%. Consequently, the capacity can be expected to increase by a total of 5.1 times. As described earlier, when moving-picture information such as a movie is reproduced with a picture quality as high as S-VHS, this requires a rate of 4.5 Mbps including sound, so that the capacity required for two hours of reproduction is 4 Gbyte. Because of the aforementioned capacity increase by 5.1 times, the 4-Gbyte capacity can be realized on one side of the disk. Furthermore, as shown in
In
An objective lens 203 is placed so as to face the optical disk 100. The objective lens 203 can be moved along the optical axis by a focus coil 204 and across the track width by a tracking coil 205. The wavelength of a laser diode 207 driven by a laser diode (LD) driver 206 is 0.685 μm. The light beam emitted from the laser diode 207 is made into parallel luminous flux by a collimate lena 208 and then enters a polarization beam splitter 209. The light beam emitted from the laser diode 207 has generally an elliptic far field pattern. Therefore, when a round pattern is needed, a beam shaping prism has only to be placed after the collimate lens 208. The light beam passed through the polarization beam splitter 209 is focused by the objective lens 203 onto the optical disk 100.
The light reflected by the reflecting film on the optical disk 100 passes back through the objective lens 203 in the opposite direction to the incident light beam, is reflected by the polarization beam splitter 209, and enters a photosensor 212 via the sensing optical system composed of a condenser lens 210 and a cylindrical lens 211. The photosensor 212, for example is a 4-quadrant photosensor. The four sense outputs of the photosensor are input to an amplifier array 213 containing an amplifier and an adder-subtracter, which produces a focus error signal, tracking error signal, and playback signal. The tracking error signal is obtained by, for example, a push-pull technique in the form of a push-pull signal as described earlier. The focus error signal and tracking error signal are supplied to the focus coil 204 and the tracking coil 205 via a servo controller 214. As a result of this, the objective lens 203 is moved along the optical axis and across the track width, thereby focusing the light beam onto the surface of the reflecting film serving as the recording surface of the optical disk 100, and tracking the target track.
The playback signal from the amplifier array 213 is input to a signal processing circuit 215, which binarizes the input and then senses data pulses. The sensed data pulses are inputted to a disk controller 216, which decodes the format, corrects errors, and then supplies the resulting signal as a bit stream of moving-picture information to an MPEG2 decoder/controller 217. Because the data obtained by compressing (encoding) the moving-picture information according to the MPEG2 standards is recorded on the optical disk 100, the MPEG2 decoder/controller 217 expands (decodes) the bit stream input to reproduce the original moving-picture information. The reproduced moving-picture information is supplied to a video signal generator circuit 218, which adds a blanking signal etc. to produce a video signal in a specific television format. The techniques related to MPEG2 have been disclosed in U.S. Pat. No. 5,317,397 and U.S. patent application Ser. No. 08/197,862.
As explained above, the optical disk according to the present invention has such an optimal pit shape (the upper and lower widths of a pit or the upper width of a pit and the angle of the pit's inner wall) as makes it possible to set the track pitch to a smaller value than the spot diameter of the playback light beam and decrease crosstalk between adjacent tracks to a level required for practical use. As a result, with the optical disk, the track density can be made by about 1.5 times as high as the conventional CD and the sufficient levels of the playback signal and the push-pull signal used for tracking can be assured.
Accordingly, with the present invention, as shown in the embodiments described above, the capacity about five times that of the conventional CD can be realized even using the normal CD size, for example. In addition, 4 Mbps of compressed moving-picture information with a picture quality as good as that of a high quality VTR, including sound, can be stored for two hours, which is very useful in practical use.
In the above-described embodiment, the values of a track pitch and upper and bottom widths in a pit are set as ones obtained by multiplying λ/NA (λ: wavelength (μm) and NA: numerical aperture of objective lens) by proportional coefficients within ranges respectively set in correspondence with λ/NA and the numerical aperture. This makes it possible to set a parameter suitable for recording information at high density without depending on a wavelength of light to be used or a numerical aperture of an objective lens.
However, a condition in the embodiment is set on the basis of the condition that aberration generated by inclination of a disk is equivalent to a wavelength as a reference. This will be described in detail by referring to FIG. 18.
For instance, when a wavelength is 0.65 μm and a disk tilt angle is 10 mrad, Wrms=0.0551 λ.
The condition in the above-described embodiment is a value obtained on the basis of the condition that an allowable tilt angle of 10 mrad is given when a wavelength is in the vicinity of 0.65 μm. This means, in other words, that a tilt angle of 10 mrad is allowed with respect to a wavelength in the vicinity thereof, and in case where light of a shorter wavelength is used, an allowable tilt angle is made smaller. This relationship is represented by the following expression:
To explain this by referring to
If recording density is to be improved in such a short wavelength, an allowable disk tilt angle is accordingly made smaller. Thus, a request for improvement in machine accuracy including accurate formation of a disk, accuracy of a spindle motor and a turntable, chucking accuracy of a disk, etc., will be stronger making it difficult to provide an inexpensive apparatus.
The preferred embodiment was devised in view of this situation and an optical disk apparatus having recording density as high as possible is provided without increasing a demand on machine accuracy. This optical disk apparatus achieves high recording density by keeping an allowable disk tilt angle constant, e.g. 10 mrad. In this case, since the amount of aberration generated in each wavelength at 10 mrad increases, in order to allow such an increase in aberration it is necessary to set a larger parameter used to set recording density for a track pitch and a detection window width. In
Here, θ eq indicates an angle at which aberration equivalent to that generated when there is inclination of 10 mrad in each wavelength is generated in the optical system of a wavelength 0.65 μm. This angle θ eq increases inversely proportional to the wavelength.
To allow such a large tilt angle, it is necessary to set the value of parameters for determining recording density, e.g., a track pitch, a detection window width and so on, at a value larger than that set proportionally to (λ/NA).
In
According to the above-described preferred embodiment, a procedure for designing an optical disk apparatus is as follows:
First, by referring to
Furthermore, in the description of the embodiment, thickness of a substrate was 0.6 mm. However, since aberration is proportional to thickness of a substrate, it is possible to set a parameter by calculating proportion when thickness of a substrate is one other than the above. For instance, if a substrate has thickness of 0.4 mm, a value of aberration is ⅔ times as that in FIG. 18 and thus aberration may be set according to this multiplication. In other words, as described above, since the amount of aberration is inversely proportional to a wavelength, if this condition is used, multiplication of substrate thickness by λ means that a wavelength is multiplied by 1/λ in terms of an aberration amount. Therefore, by performing conversion under this condition, it is possible to determine multiplication ratios with respect to parameters for a track pitch, a pit upper width and bottom width set according to the above-described embodiment as in the case of the procedure.
In case where thickness of a substrate is 600 μm and a wavelength is μm, as described above, a relationship, θ eq (mrad)=6.5/λ (μm), holds. In case where thickness of a substrate is ds μm, however, the following relationship holds:
According to this relationship, a measure of an abscissa is added to the upper part of a graph at ds/λ in FIG. 20. The graph in
Further in detail, in the above-described embodiment, ranges of a track pitch, upper and bottom pit widths and depth are set in the following way:
Track pitch: (0.72 to 0.8)×(λ/NA)/1.14 μm
Upper width: (0.3 to 0.5)×(λ/NA)/1.14 μm
Bottom width: (0.2 to 0.32)×(λ/NA)/1.14 μm
Depth: (1/4.2×λ/n) to (1/5.2×λ/n)
These parameters are values when a wavelength is 0.65 μm, and if a wavelength of used laser light is shorter than this, these parameters are multiplied by α. That is, coefficients are respectively multiplied by α as in the following expression:
Track pitch: (0.72 to 0.8)α×(λ/NA)/1.14 μm
Upper width: (0.3 to 0.5)α×(λ/NA)/1.14 μm
Bottom width:(0.2 to 0.32)α×(λ/NA)/1.14 μm
Depth: (1/4.2×α/n) to (1/5.2 λ/n).
This multiple can be read from the graph in FIG. 20. However, an approximate expression like the following may be applied to designing. That is, if an abscissa indicates allowable disk tilt angles θ eq (mrad) expressed in terms of 0.65 μm and an ordinate indicates dimensional multiplication ratios α, α is represented by the following expression:
If by using substrate thickness ds and a wavelength λ this expression of α is replaced by the above-described expression, that is,
α is a value obtained by the following expression:
According to this expression, an optimum multiplication ratio when substrate thickness and a wavelength are both changed can be obtained as a function therebetween.
The above embodiment sets the allowable tilt angle at 10 mrad. However, the present invention can be applied to another tilt angle. In this case, θ eq and α are represented by the following expressions.
where θA (mrad): allowable tilt angle.
Honguh, Yoshinori, Sugaya, Toshihiro
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