Optical disk and optical disk apparatus

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).alpha..times..lambda./NA/1.14 .mu.m when a wavelength of the light beam is .mu. 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).alpha..times..lambda./NA/1.14 .mu.m, a bottom width within the range of (0.2 to 0.32).alpha..times..lambda./NA/1.14 .mu.m and a depth within the range of (1/4.2.times..lambda./n) to (1/5.2 .lambda./n) (n: refractive index of said substrate and .lambda.: a wavelength) and obtained by 2.623.times.10.sup.-7 .times.(d/.lambda.).sup.2 -1.706.times.10.sup.-4 (d.sub.s /.lambda.)+0.934, where d.sub.s is thickness of the substrate).

BACKGROUND OF THE INVENTION 
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 .mu.m, the pit pitch is 1.66 .mu.m 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 .mu.m, 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 .mu.m. 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. 
SUMMARY OF THE INVENTION 
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 .lambda..mu.m and the numerical aperture of the objective lens is NA, 
the track pitch is set in the range of (0.72 to 
0.8).times..lambda./NA/1.14 .mu.m, and each of the pits has a trapezoidal 
cross section whose upper width is in the range of (0.3 to 0.5 
).times..lambda./NA/1.14 .mu.m and whose lower width is in the range of 
(0.2 to 0.3 2).times..lambda./NA/1.14 .mu.m. 
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 .lambda..mu.m and 
the numerical aperture of the objective lens is NA, the track pitch is set 
in the range of (0.72 to 0.8).times..lambda./NA/1.14 .mu.m, and each of 
the pits has a trapezoidal cross section whose upper width is in the range 
of (0.3 to 0.5 ).times..lambda./NA/1.14 .mu.m and whose lower width is in 
the range of (0.2 to 0.3 2).times..lambda./NA/1.14 .mu.m. 
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 .lambda..mu.m and the numerical aperture of the object lens is NA, 
the track pitch is set in the range of (0.72 to 
0.8).times..lambda./NA/1.14 .mu.m, and each of the pits has a trapezoidal 
cross section whose upper width is in the range of (0.3 to 0.5 
).times..lambda./NA/1.14 .mu.m and whose inner wall has an angle of 
30.degree. to 60.degree.. 
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 .lambda..mu.m and 
the numerical aperture of the objective lens is NA, the track pitch is set 
in the range of (0.72 to 0.8).times..lambda./NA/1.14 .mu.m, and each of 
the pits has a trapezoidal cross section whose upper width is in the range 
of (0.3 to 0.5 ).times..lambda./NA/1.14 .mu.m and whose inner wall has an 
angle of 30.degree. to 60.degree.. 
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) 
.alpha..times..lambda./NA/1.14 .mu.m when a wavelength of the light beam 
is .lambda. nm and a numerical aperture of the objective lens is NA, each 
of the pits is magnified by a multiplication ratio .alpha. used to secure 
allowable disk tilt angles in an upper width within the range of (0.3 to 
0.50).alpha..times..lambda./NA/1.14 .mu.m, a bottom width within the range 
of (0.2 to 0.32).alpha..times..lambda./NA/1.14 .mu.m and a depth within 
the range of (1/4.2.times..lambda./n) to (1/5.2.times..lambda./n) (n: 
refractive index of said substrate and .lambda.: a wavelength 0.65 .mu.m) 
and .alpha. obtained by 2.623.times.10.sup.-7 .times.(d.sub.s 
/.lambda.).sup.2 -1.706.times.10.sup.-4 (d.sub.s /.lambda.)+0.934 m 
(d.sub.s : thickness of the substrate). 
By setting various parameters of the pit shape at the above-described 
values, the amount of crosstalk between adjacent tracks is suppressed to 
less than -20 dB, which must be met to restore the original information 
from the reproduced signal, and the playback signal level and the level of 
the push-pull signal for tracking are maintained sufficiently.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
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 .mu.m and whose output is 
several milliwatts have already been put into practical use. Laser diodes 
whose wavelength is 0.650 .mu.m are getting close to practical use. 
The NA of the objective lens is limited by ease of making a lens and the 
tilt angle between the lens and the disk. The smaller the lens load (the 
thinner the optical disk substrate, the smaller 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 .mu.m 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. 
FIGS. 11 and 12 show the results of calculating the tilt characteristics 
when the substrate thickness (t) is 1.2 mm and 0.6 mm using the NA as a 
parameter. The abscissa represents the angle of tilt and the ordinate 
indicates the normalized peak intensity of the playback signal. The 
wavelength (.lambda.) of the playback light beam is assumed to be 0.690 
.mu.m. With the substrate thickness being 0.6 mm and the NA=0.6, the tilt 
is 9.5 mrad and the peak intensity of the playback signal decreases by 
10%. When the substrate thickness is 1.2 mm, the NA=0.49. Specifically, by 
changing the substrate thickness from 1.2 mm (the conventional CD's 
thickness) to 0.6 mm, the NA can be increased from 0.49 to 0.6, and 
consequently the surface recording density can be increased by 1.5 times. 
Because the spot size is proportional to .lambda./NA and the surface 
recording density is inversely proportional to the square of the spot 
size, this gives (0.6/0.49).sup.2, which means that the area recording 
density is 1.5 times as high as that of the conventional CD. 
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 .mu.m, 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 .mu.m to 0.685 .mu.m 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 
.lambda./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. 
FIG. 1 is an explanatory diagram of the shape of a pit in an optical disk 
according to the present invention. As shown in the figure, the shape of a 
pit 10 is approximated by a shape with a trapezoidal cross section. The 
inner wall 11 of the pit 10 is inclined downward and its bottom portion 12 
is almost flat. Numeral 13 indicates the cross section of the pit 10 along 
the radius of the optical disk (the track width direction); 14 the cross 
section along the circumference of the optical disk (the track direction); 
Wm the size of the top of pit 10 across the track width (hereinafter, the 
upper width); Wi the size of the bottom of the pit 10 across the track 
width (hereinafter, the lower width); hm the depth of the pit 10, Zm the 
length of the pit 10 along the track; and .theta. represents the angle of 
the inner wall of the pit 10 (the angle that the inner wall forms with 
respect to the surface of the optical disk). 
FIG. 2 shows a model of the playback optical system of the optical disk 
apparatus used in the analysis. The figure shows the incident light 
distribution 20 (V1 (x,y)) of playback light beam, incident light 21, a 
polarization beam splitter (or half mirror) 22 for separating the incident 
light 21 from the reflected light 26, an objective lens 23 with a 
numerical aperture of NA, the distribution 24 (V2 (x,y)) of focused light 
(beam spot) by the object lens on the recording surface (pit surface) of 
the optical disk, an optical disk 25 with a complex reflectivity of r2 
(x,y), the reflected light 26, and the distribution 27 (V3 (x,y)) of the 
reflected light 26 on a photosensor. 
FIG. 3 diagrammatically shows the pit arrangement on the optical disk used 
to calculate the levels of the playback signal and the push-pull signal, 
where the track pitch (the pitch of a pit across the track width) is Pt 
and the pit pitch (the pit pitch along the track) is Pmy. The beam spots 
30 and 31 of the playback light beam represent spot A at the center of a 
pit and spot B between pits, respectively. The amplitude of the playback 
signal is represented as .vertline.S(A)-S(B).vertline., where S(A) and 
S(B) indicate the output signals of the photosensor when the beam spot is 
positioned at (A) and (B), respectively. Lines 32 and 33 show the 
positions in which the push-pull signal (the difference signal between the 
output signals of the split photosensors arranged along the track in at 
least two sensing areas) is obtained in an area (C) with pits and an area 
(D) without pits. Those push-pull signals each have the average p--p 
values in area C and area D. 
FIG. 4 shows the results of calculating the levels of the playback signal 
and the push-pull signal using the size of a pit across the track width 
and the pit depth as parameters with a playback laser beam wavelength of 
0.685 .mu.m, NA=0.6, Zm=0.5 .mu.m, Pmy=1 .mu.m, and Pt=0.72 .mu.m. The 
beam filling factors A/W(X) and A/W(Y) of the playback light beam across 
the track width (X) and along the track (Y) are shown in the figure. As 
seen from the figure, the levels of the playback signal and the push-pull 
signal do not depend largely on the shape of a pit except the case where 
Wm=0.3 and Wi=0.2. There is no such pit depth as brings the levels of the 
playback signal and the push-pull signal to the maximum level 
simultaneously. To minimize a decrease in the push-pull signal level and 
obtain the maximum playback signal level, it is desirable from FIG. 4 that 
the pit depth should be approximately .lambda./5, preferably in the range 
of .lambda./4.2 to .lambda./5.2. More specifically, the pit has a depth in 
a range of 1/4.2.times..lambda./n to 1/5.2.times..lambda./n, where n is a 
refractive index of the substrate. 
FIG. 5 diagrammatically shows an MTF of the playback optical system and the 
pit arrangement on the optical disk used to evaluate crosstalk between 
adjacent tracks. In the figure, the spots 50 and 51 of the playback light 
beam represent a spot at the center (A) of a pit and a spot passing 
through position (B) a distance of td away from the center of a pit. MTF 
is expressed by the power of the basic frequency component of the output 
signal from the photosensor obtained when the beam spot passes through the 
center of a pit. Crosstalk is expressed by the power of the basic 
frequency component of the output signal from the photosensor obtained 
when the beam spot passes through position B. 
FIG. 6 shows the MTF and the crosstalk characteristics when the track pitch 
Pt is fixed at 0.72 .mu.m, the depth hm of the pit 10 is fixed at 0.2 
.mu.m, and the upper width Wm and lower width Wi of the pit 10 are changed 
variously, with the spatial frequency on the abscissa and MTF and 
crosstalk on the ordinate. The values of beam packing factors A/W(X) and 
A/W(Y) of the playback light beam across the track width (X) and along the 
track (Y) are shown in the figure. In the figure, the MTF produces a 
difference of 1 to 2 dB depending on the pit shape, which is not too 
large. In contrast, it can be seen that crosstalk changes greatly with the 
pit shape. 
FIG. 7 shows the MTF and the crosstalk characteristics under the same 
conditions as in FIG. 6 except that .lambda. is set at 0.650 .mu.m. 
FIG. 8 shows the MTF and the crosstalk characteristics when the track pitch 
Pt is fixed at 0.72 .mu.m, the depth hm of the pit 10 is fixed at 0.2 
.mu.m, and the upper width Wm and the angle .theta. of the inner wall 11 
of the pit 10 are varied, with the spatial frequency on the abscissa and 
MTF and crosstalk on the ordinate. The values of beam packing factors 
A/W(X) and A/W(Y) of the playback light beam across the track width (X) 
and along the track (Y) are shown in the figure. 
FIG. 9 shows the MTF and the crosstalk characteristics under the same 
conditions as in FIG. 8 except that the upper width Wm of the pit 10 is 
fixed at 0.35 .mu.m and only the angle .theta. of the inner wall 11 of the 
pit 10 is changed variously. 
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 FIGS. 6 to 9 are for no tilt, tilts actually have to be taken 
into account. FIG. 10 shows the MTF and the crosstalk characteristics when 
tilts are taken into account. As can be seen from the figure, when tilts 
are taken into account, the MTF almost remains unchanged, but the amount 
of crosstalk increases, and the conditions for determining the parameters 
of a pit become more strict. 
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 FIG. 10, the 
amount of crosstalk can be suppressed to values less than -20 dB required 
in practical use in the tilt range of .+-.10 mrad for the same spatial 
frequency. This shows that a wavelength of 0.685 .mu.m and a track pitch 
of 0.72 .mu.m are reasonable. 
The evaluation of FIGS. 6 to 9 shows that the amount of crosstalk (the 
difference in MTF value between the MTF characteristic and the crosstalk 
characteristic) is 20 dB or less until Wm=0.45 .mu.m in the case where the 
track pitch shown in FIGS. 6 and 8 is 0.72 .mu.m. In contrast, when Wm=0.5 
.mu.m, the amount of crosstalk at low frequencies increases rapidly, 
exceeding -20 dB. The MTF characteristics are relatively good until Wm=0.3 
.mu.m, but deteriorates sharply when Wm is less than 0.3 .mu.m. Therefore, 
the range of Wm=0.3 .mu.m to 0.45 .mu.m is reasonable. 
The results mentioned above show that when the pit shape across the track 
width is standardized with a wavelength of 0.685 .mu.m and NA=0.6 (i.e., 
.lambda./NA=1.14), it is desirable that the upper width of a pit should be 
(0.3 to 0.45).times..lambda./NA/1.14 .mu.m, and the lower width of the pit 
should be (0.2 to 0.25).times..lambda./NA/1.14 .mu.m, or that the upper 
width Wm should be (0.3 to 0.45).times..lambda./NA/1/14 .mu.m and the 
angle .theta. of the inner wall should be in the range of 50.degree. to 
70.degree.. Specifically, when the track pitch Pt is selected in the range 
of (0.72 to 0.8).times..lambda./NA/1.14 .mu.m 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 .theta. 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 of 0.685 
.mu.m, 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. FIG. 14 shows the difference 
between the bottom groove width wi and the top groove width wg for the 
model pit and the actual pit. As seen from FIG. 14, the value of wi for 
the model pit ranges from 0.2 to 0.32 .mu.m; the value of wg for the model 
pit ranges from 0.3 to 0.45 .mu.m, whereas the value of wg for the actual 
pit varies from 0.3 to 0.5 .mu.m. Furthermore, the angle of inclination 
.theta. of the pit is as follows. As shown in FIG. 15, an angle of 
inclination of the mode pit is in the range of 50.degree. to 70.degree., 
whereas that of the actual pit in the range of 30.degree. to 60.degree.. 
Hereinafter, the structure of an optical disk according to the present 
invention will be described. FIGS. 16A and 16B are a perspective view and 
sectional view of a double-sided optical disk 100, respectively. One 
surface of each of transparent substrates 101 and 102 is embossed with 
pits made of light-transmitting resin such as polycarbonate or acrylic 
resin and is coated with a reflecting film 103 and a reflecting film 104 
(e.g., of aluminum), respectively. On these films, protective films 105 
and 106 are formed. The thickness of the transparent substrates 101 and 
102 is 0.6 mm. The transparent substrates 101 and 102, whose protective 
films 105 and 106 are allowed to face each other, are laminated together 
with an adhesion layer 107 with a thickness of several tens of .mu.m made 
of a thermoset adhesive. In the center of the optical disk 100, a hole 108 
is made for clamping. Around the hole, a clamping zone 109 is provided. A 
playback light beam 110 is emitted from a laser diode (not shown), passes 
through the playback optical system, enters an optical disk 100 via an 
object lens 111 from the transparent substrates 101 and 102, and is 
focused as a small beam spot on the reflecting films 103 and 104. 
FIG. 17 shows an example of an optical disk apparatus which reproduces 
compressed moving-picture information by using the above-mentioned optical 
disk 100. Because the optical disk 100 uses the substrates 101 and 102 
which are as thin as 0.6 mm and consequently are less immune to dust or 
dirt on their surfaces than a CD using a 1.2 mm thick substrate, the disk 
100 is housed in a cartridge 200. By housing the optical disk 100 in the 
cartridge 200, attention need not be paid to the way of holding the disk, 
dust, fingerprints, etc. as with CDs, which is helpful in handling and 
carrying. When the disk is exposed as is a CD, the ability to correct 
errors must be determined, taking into account an unexpected accident such 
as a flaw. Use of the cartridge 200, however, makes such a consideration 
unnecessary. Therefore, it is possible to use the LDC read Solomon error 
correction technique in sectors as used in a recordable optical disk. As a 
result of this, for example, when an optical disk is formatted in units of 
2 kbyte to 4 kbyte, the recording efficiency can be increased by more than 
10% as compared with the CD. 
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 .mu.m, and the pit pitch is 0.96 .mu.m, 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 format 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 FIGS. 16A and 16B, a single double-sided 
optical disk alone enables four hours of recording at a maximum. 
In FIG. 17, the optical disk 100 is chucked by a tapered cone 220 and 
rotated by a spindle motor 201. The spindle motor 201 is driven by a 
spindle motor driver circuit 202. The playback optical system is 
constructed as follows. 
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 .mu.m. 
The light beam emitted from the laser diode 207 is made into parallel 
luminous flux by a collimate lens 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. Pat. 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 
.lambda./NA (.lambda.: wavelength (.mu.m) and NA: numerical aperture of 
objective lens) by proportional coefficients within ranges respectively 
set in correspondence with .lambda./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. 
FIG. 18 shows a result of calculating aberration generated by inclination 
of a disk with respect to several wavelengths. A abscissa indicates disk 
tilt angles (mrad) in a radial direction while an ordinate indicates 
aberration rms (root mean square) values by wavelength units. The 
aberration rms values are nearly proportional to the disk tilt angles 
while inversely proportional to wavelengths. In parts in which the tilt 
angle is smaller than 20 mrad, the aberration rms value, that is, Wrms, is 
obtained by the following expression: 
EQU Wrms=3.58.times.10.sup.-3 .theta.(mrad)/.lambda.(.mu.m) 
For instance, when a wavelength is 0.65 .mu.m and a disk tilt angle is 10 
mrad, Wrms=0.0551 .lambda.. 
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 .mu.m. 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: 
EQU .theta.(mrad)=15.4 .lambda.(.mu.m) 
To explain this by referring to FIG. 18, if there is disk inclination of 10 
mrad with a wavelength of 0.65 .mu.m, an aberration rms value is 0.0551 
.lambda.. As an example, in case where light generated by SHG (Second 
Harmonic Generation) combining YVO.sub.4 and KTP, a wavelength thereof is 
0.532 .mu.m. However, in order to limit aberration to the same rms value 
as in the case of the above-described condition, that is, 0.0551 .lambda., 
at this time, a disk tilt angle to be allowed is 8.2 mrad. When a 
wavelength is shortened to 0.42 .mu.m or 0.36 .mu.m by using materials 
such as GaN and the like, a tilt angle to be allowed is further reduced to 
6.5 mrad or 5.5 mrad. 
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 FIG. 18, if wavelengths of light are set small, e.g. 
0.532 .mu.m, 0.42 .mu.m and 0.36 .mu.m, the amount of aberration generated 
at the disk inclination of 10 mrad gradually increases to 0.0673 .lambda., 
0.0852 .lambda. and 0.0994 .lambda.. Allowing such large aberration 
correspondingly causes, if expressed in terms of an optical system of 0.65 
.mu.m, the allowable values of tilt angles to be such large ones as 12.3 
mrad, 15.5 mrad and 18.1 mrad. This conversion is represented by the 
following expression based on the above-described proportional 
relationship between the amount of aberration and an allowable tilt angle: 
EQU .theta.eq(mrad)=6.5/.lambda..times.(.mu.m) 
Here, .theta. 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 .mu.m. This angle 
.theta. 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 (.lambda./NA). 
FIG. 19 shows a relationship between disk tilt angles and window occupancy 
ratios. As typical values in the above-described embodiment, a wavelength 
is set at 0.65 .mu.m, a numerical aperture at 0.6, a track pitch at 0.7525 
.mu.m, a pit upper width at 0.35 .mu.m, a pit bottom width at 0.2 .mu.m, a 
detection window width at 0.134 .mu.m assuming that a modulated code 
scheme of d=2 (where d is the minimum run length of `0` for the RLL 
modulation code) is adopted and a pit depth at 1/5 of a value obtained by 
dividing a wavelength .lambda. by a substrate refractive index n. A 
relationship between disk tilt angles and window occupancy ratios in this 
case is represented by a curve of (1.0 times). Curves of (1.1 times) and 
(1.2 times) are obtained when a track pitch, a pit upper width, a pit 
bottom width and a detecting window width are expanded respectively by 1.1 
and 1.2 times. Though recording density declines to 1/1.1.sup.2 and 
1/1.2.sup.2, disk tilt angles to be allowed increase instead. 
In FIG. 19, window occupancy ratios indicated by an ordinate are values 
obtained by calculating reproducing signals based on the scalar 
diffraction theory, including a lower limit achievable when cross talk 
from an adjacent track is to be considered in addition to inter symbol 
interferences (ISI) by various pit patterns generated under the 
restrictions of modulated codes and in each case jitters are to be reduced 
by using an optimum equalizing circuit. It requires a great deal of time 
to calculate these window occupancy ratios and this is made possible only 
with the development of a calculation program based on high-speed 
algorithm. Also, since NA is large, even in calculation of aberration 
caused by inclination of a substrate, a more accurate evaluating method 
based on ray tracing is used rather than a usually used approximate 
expression. 
FIG. 20 is a view outgrown from FIG. 19 and obtained by plotting 
multiplication ratios necessary for allowable disk tilt angles with a 
window occupancy ratio of 80% as a reference. 
According to the above-described preferred embodiment, a procedure for 
designing an optical disk apparatus is as follows: 
First, by referring to FIG. 18, a determination is to be made on whether it 
is possible or not to set small an allowable value of a disk tilt angle so 
as to keep aberration within that obtained when a wavelength is 0.65 .mu.m 
and a disk tilt angle is 10 mrad. If it is possible, parameters for a 
track pitch, a pit upper width, a pit bottom width and a detection window 
width may be determined based on the above-described embodiment. On the 
other hand, if it is impossible to set an allowable disk tilt angle so 
small, the value (0.08) of the amount of aberration to be allowed is read 
from a wavelength to be used (e.g. 0.42 .mu.m) and an allowable disk tilt 
angle (e.g. 10.0 mrad) in FIG. 18, and in this amount of aberration (0.08) 
a disk tilt angle (15.0 mrad) corresponding to the amount of aberration 
obtained in an optical system in which a wavelength is 0.65 .mu.m is to be 
considered as an allowable disk tilt angle therein. Then, a magnification 
ratio (1.2) necessary to realize this allowable disk tilt angle (15.0 
mrad) is to be read from FIG. 20. Lastly, by multiplying each parameter 
value set according to the above-described embodiment by this 
multiplication ratio (1.2), it is possible to realize an optical disk 
capable of securing a practical disk tilt angle to be allowed even in a 
short wavelength. 
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 2/3 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 .lambda. means that a 
wavelength is multiplied by 1/.lambda. 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 .mu.m and a wavelength is 
.mu.m, as described above, a relationship, .theta. eq 
(mrad)=6.5/.lambda.(.mu.m), holds. In case where thickness of a substrate 
is d.sub.s .mu.m, however, the following relationship holds: 
EQU .theta.eq(mrad)=6.5/.lambda..times.(d.sub.s /600) 
(.mu.m)=1.083.times.10.sup.-2 .times.(d.sub.s /.lambda.) 
According to this relationship, a measure of an abscissa is added to the 
upper part of a graph at d.sub.s /.lambda. in FIG. 20. The graph in FIG. 
20 is made based on calculation taking a wavelength of 0.65 .mu.m as an 
example. If the measure on the upper part is used, however, without 
performing conversion via parameters of a wavelength 0.65 .mu.m scheme, it 
is possible to read necessary multiplication ratios by calculating 
proportion between a substrate and a wavelength which are actually used. 
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: 
EQU Track pitch: (0.72 to 0.8).times.(.lambda./NA)/1.14 .mu.m 
EQU Upper width: (0.3 to 0.5 ).times.(.lambda./NA)/1.14 .mu.m 
EQU Bottom width: (0.2 to 0.32).times.(.lambda./NA)/1.14 .mu.m 
EQU Depth: (1/4.2.times..lambda./n) to (1/5.2.times..lambda./n) 
These parameters are values when a wavelength is 0.65 .mu.m, and if a 
wavelength of used laser light is shorter than this, these parameters are 
multiplied by .alpha.. That is, coefficients are respectively multiplied 
by .lambda. as in the following expression: 
EQU Track pitch: (0.72 to 0.8).alpha..times.(.lambda./NA)/1.14 .mu.m 
EQU Upper width: (0.3 to 0.5 ).alpha..times.(.lambda./NA)/1.14 .mu.m 
EQU Bottom width:(0.2 to 0.32).alpha..times.(.lambda./NA)/1.14 .mu.m 
EQU Depth: (1/4.2.times..lambda./n) to (1/5.2 .lambda./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 .theta. eq 
(mrad) expressed in terms of 0.65 .mu.m and an ordinate indicates 
dimensional multiplication ratios .alpha., .alpha. is represented by the 
following expression: 
EQU .alpha.=0.002236.times..theta.eq.sup.2 -0.01575.times..theta.eq+0.934 
If by using substrate thickness d.sub.s and a wavelength .lambda. this 
expression of .alpha. is replaced by the above-described expression, that 
is, 
EQU .theta.eq(mrad)=1.083.times.10.sup.-2 .times.(d.sub.s /.lambda.) 
.alpha. is a value obtained by the following expression: 
EQU .alpha.=2.623.times.10.sup.-7 .times.(d.sub.s /.lambda.).sup.2 
-1.706.times.10.sup.-4 (d.sub.s /.lambda.)+0.934 
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, 
.theta. eq and .alpha. are represented by the following expressions. 
EQU .theta.eq(mrad)=1.083.times.10.sup.-3 .times.(.theta..sub.A d.sub.s 
/.lambda.) 
where .theta..sub.A (mrad): allowable tilt angle. 
EQU .alpha.=2.623.times.10.sup.-9 .times.(.theta..sub.A d.sub.s 
/.lambda.).sup.2 -1.706.times.10.sup.-5 (.theta..sub.A d.sub.s 
/.lambda.)+0.934.