Optical disk drive

In an optical disk drive which can reproduce signals of a high density, a laser beam emitted from a laser light source is converted to a parallel beam by a collimating lens, and is shaded by a shading member as to the light around the axis, and is converged by an objective lens on a signal plane of optical disk. The distribution of the laser beam is located on a ring belt just after the passage of an aperture plane of the optical system. The fed back light is collimated by the objective lens and shaded again by the shading member around the optical axis to reach a beam splitter. The light reflected by the beam splitter is transmitted through an optical system by a control signal detector, and focus signals and tracking signals on the signal plane of optical disk are detected. On the other hand, the light transmitted through the beam splitter is detected by a detector. A primary or secondary differential signal is derived from the detected signal, and the edges or the center of the signal marks are reproduced by using the differential signal.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an optical disk drive which can reproduce 
signals recorded on an optical disk. 
2. Description of the Prior Art 
Recently, there has been a desire to increase the signal density of optical 
disks and various approaches have been proposed. The diameter "D" of an 
optical spot converged on the signal surface of an optical disk can be 
expressed generally as follows: 
EQU D=1.21.lambda./NA, 
wherein .lambda. denotes the wavelength of a light source and NA denotes 
the aperture number of an objective lens. Therefore, a higher density can 
be attained by using a shorter wavelength light source consisting of a 
laser diode and a larger NA objective lens. At present, it is expected 
that the wavelength can be shortened down to 600 nm using a GaAs III-V 
compound, but it is difficult to shorten the wavelength further. The 
increase in NA also has problems such as the difficulty of the formation 
of the objective lens and errors which to occur due to the disk 
inclination and the defocusing. 
A ring belt aperture has been proposed to increase the density as an 
approach for obtaining a higher NA. FIG. 1 shows an example of a prior art 
optical disk drive using a ring belt aperture, illustrated in NIKKEI 
ELECTRONICS (No. 528, 129 (1991)). In FIG. 1, a laser beam emitted from a 
light source 1 consisting of a laser diode is converted to a parallel 
beam, and transmitted through a beam splitter 3 and is shaded by a 
disk-like shading member 4 arranged near the optical axis and is converged 
on the signal plane 7 of an optical disk on the rear side of an optical 
disk substrate 6. The light reflected from the signal plane 7 is 
collimated by the objective lens 5, shaded again by the shading member 4 
near the optical axis and reflected by the beam splitter 3 to reach 
another beam splitter 8. The light reflected by the beam splitter 8 is 
transmitted through an optical system 9 and is detected by a detector 10 
which detects focus error signals and tracking error signals for the beam 
spot on the signal plane 7. On the other hand, the light transmitted 
through the beam splitter 8 is collimated by a converging lens 11, and is 
transmitted through a pin hole 12 to be received by a detector 13. The 
detected signal is amplified by an amplifier 14, and is converted to a 
reproduced signal by a signal processing circuit 15. 
FIG. 2 shows a readout signal 17 detected by the detector 13 and a 
reproduced signal 18 in relation to signal marks 16a, 16b, 16c and 16d 
provided on the signal plane 7 of the optical disk. In the signal 
processing circuit 15, the readout signal 17 is compared with a detection 
level 17R, and determined to be 1 or 0 based on whether or not the readout 
signal 17 exceeds a detection level, and then the reproduced signal 18 is 
obtained. 
FIG. 3(a) displays an effect of the insertion of the shading member 4 
wherein the ordinate represents the light intensity and the abscissa 
represents the distance from the optical axis in the lateral directions 
(.epsilon.- and .eta.-axes). When the shading member 4 is inserted as 
shown in FIG. 1, the light just after the transmission of the aperture 
plane of the objective lens 5 has a shape of a ring belt. The light 
intensity distribution 19b on the focal plane (signal plane 7), shown in 
FIG. 3(a), is calculated for an NA=0.45-0.70 ring belt aperture and a 0.78 
.mu.m of wavelength. A light intensity distribution 19a with a circular 
aperture (NA=0.54) on the focal plane is shown for comparison. Because the 
aperture area is chosen to be the same, if the amount of light of the two 
cases are same, then the Strehl intensity (peak intensity) are the same. 
The distribution 19b for ring belt aperture has a smaller diameter of main 
lobe than the distribution 19a for circular aperture, while the former has 
a disadvantage that the side lobe rises. Therefore, if there are signal 
marks at the side lobe position, the effect thereof becomes large, so that 
cross talk and interference between signals arise strongly in the readout 
signals. 
FIG. 3(b) illustrates an effect of the insertion of the pin hole 12. The 
optical intensity distribution 19c on the pin hole 12 shown in FIG. 3(b) 
corresponds to the light intensity distribution 19b for the ring belt 
aperture. Then, if the pin hole 12 allows only the main lobe of the 
converging light 19c to be transmitted through the pin hole 12, then the 
readout signal will not be affected by the signal marks located on the 
signal plane 7 on the side lobe positions of the conversion light 19b. 
Thus, cross talk and interference between signals can be decreased, and it 
is thought to be possible to reproduce high density signals. 
Another effect of the insertion of the shading member 4 is that the focal 
depth becomes deeper. For example, a defocusing amount necessary to 
decrease the Strehl intensity by 20% is 0.70 .mu.m for a circular aperture 
of NA=0.70 while is 1.14 .mu.m for a ring belt aperture of NA=0.45-0.70 if 
the wavelength is 0.78 .mu.m. Therefore, by using a ring belt aperture, 
one of the disadvantages accompanying a large aperture, that is, a shallow 
focal depth and bad effects on defocusing, can be avoided. 
However, the above-mentioned prior art optical disk drive has problems in 
the reproduction of high density signals. In order to decide if the 
reproduction of high density signals can be possible, a determination is 
made as to how the signal mark patterns shown in FIGS. 8(a)-(d) are 
reproduced. 
FIG. 4(a) shows a diagram of observation waveforms (eye patterns) of the 
readout signal 17 for the patterns shown in FIGS. 8(a) and 8(b) obtained 
by theoretical calculation, while FIG. 4(b) is a diagram of observation 
waveforms (eye patterns) of the readout signal 17 for the patterns shown 
in FIGS. 8(c) and 8(d). Further, FIG. 4(c) displays an eye pattern wherein 
a defocusing by 1.14 .mu.m (of a degree such that the Strehl intensity of 
the beam spot decreases by 20%) is added on the reproduction of the 
patterns of FIGS. 8(a) and 8(b). An eye pattern is obtained as explained 
below with reference to FIGS. 10a-10c wherein the wavelength of the light 
source is 780 nm and the NA of the ring belt aperture is 0.45-0.70. The 
ordinate denotes a signal amplitude normalized by the detected light 
quantity (100) in a case that the signal plane 7 is a mirror plane, and 
the abscissa denotes the time corresponding to the scanning position of 
the beam spot. In FIG. 4(a) and 4(c), the diamond-like areas enclosing the 
x mark at the detection level (40), as represented by a reference sign 
"a", is called an eye. Because the 1 or 0 decision on the detection of 
signals is conducted by the comparison of the readout signal with the 
detection level, it is desirable that an eye is open both in the amplitude 
direction (in the vertical direction in the graphs) and in the time 
direction (in the horizontal direction in the graphs). 
A first problem of the prior art optical disk drive is that cross talk and 
interferences between marks cannot be decreased by inserting the pin hole 
12. It is confirmed from the results of the calculation that the existence 
of the pin hole 12 or the size of the pin hole 12 does not affect the eye 
patterns shown in FIGS. 4(a) and 4(b). (FIGS. 4(a) and 4(b) show the data 
without the pin hole 12.) 
A second problem is that a large jitter "d" is generated due to the 
difference in pit patterns with adjacent tracks, as is clear from FIG. 
4(a). Especially for the patterns of FIGS. 8(c) and 8(d), cross talk is 
very large, as shown in FIG. 4(b), and an eye 22A for the pit pattern 
shown in FIG. 8(c) and an eye 22B for the pit pattern shown in FIG. 8(d) 
are separated completely and an eye formed by the overlap of the eyes 22A 
and 22B is closed. That is, the signals cannot be reproduced. 
A third problem is that as is clear from FIG. 4(c), an eye shifts largely 
upward by adding the defocusing, and jitter "d" at the detection level 40 
is increased further. This tendency is common for other error factors as 
well as defocusing. 
The generation of large jitter due to cross talk and defocusing makes it 
hard to reproduce high density signals by the prior art optical disk 
drive. 
SUMMARY OF THE INVENTION 
The present invention intends to solve the above-mentioned problems and its 
object is to provide an optical disk drive which can reproduce signals of 
a high density. 
In an optical disk drive according to the present invention, a ring belt 
aperture is combined for reproduction with the primary or secondary 
differential of the readout signal. A laser beam from a laser light source 
is converged on a signal plane of an optical disk on which signal marks 
are formed. One or more optical components are arranged in the optical 
path between the laser light source and the optical disk for converging 
the distribution of the laser beam to a ring belt just after the passage 
of an aperture plane of the optical system. The light fed back from the 
optical disk is detected, and a signal processing circuit generates a 
secondary or primary differential signal by differentiating the detected 
signals and detects the edges or the center positions of the signal marks 
by comparing the secondary or primary differential signal with a detection 
level. Because the main lobe diameter of the converging light on the 
signal plane can be decreased by adopting the ring belt aperture, the 
inflection point of the readout signal waveform can correspond with the 
edges or center position of signal marks well, and the signal marks can be 
detected correctly by detecting the cross point of the secondary or 
primary differential signal with the detection level. 
According to an optical disk drive of the present invention, signal marks 
of a high signal density of optical disk can be reproduced precisely and 
the signal density of optical disk can be increased to a large extent.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Embodiments of optical disk drives of the present invention will be 
explained below with reference to drawings, wherein like reference 
characters designate like or corresponding parts throughout the drawings. 
FIG. 5 shows the structure of an optical disk drive of a first embodiment 
of the present invention. In the optical disk drive, a laser beam emitted 
from a laser light source 41 is converted to a parallel beam by a 
collimating lens 42, and is transmitted through a beam splitter 43, and is 
shaded by a disk-like shading member 44 around the optical axis, and is 
converged by an objective lens 45 on a signal plane 47 of an optical disk 
at the rear surface of an optical disk substrate 46. The shading member 44 
is used to realize a ring belt aperture, that is, to make the optical 
distribution just after the object lens a ring belt. The light fed back 
from the signal plane 47 is collimated by the objective lens 45, and is 
shaded again by the shading member 44 around the optical axis and is 
reflected by the beam splitter 43 to reach a beam splitter 48. The light 
reflected by the beam splitter 48 is transmitted through an optical system 
49 and received by a detector 50 to detect control signals such as focus 
error signals and tracking error signals for the beam spot focused on the 
signal plane 47 of the optical disk. On the other hand, the light 
transmitted through the beam splitter 48 is collimated by a converging 
lens 51, and is transmitted through a pin hole 52 to be detected by a 
detector 53. The readout signal detected by the detector 53 is amplified 
by an amplifier 54 and a signal processing circuit 55 converts the readout 
signal 71 into a reproduced signal 78 by using the secondary differential 
of the readout signal, as explained in detail below. 
FIG. 6 is a block diagram of the signal processing circuit 55 wherein a 
primary differential circuit 61 generates a primary differential signal 72 
of the readout signal 71 by differentiating the readout signal 71, while a 
secondary differential circuit 62 generates a secondary differential 
signal 74 of the readout signal by differentiating the primary 
differential signal 72 supplied from the primary differential circuit 61. 
The primary differential signal 72 is compared with an upper level 72P and 
with a lower level 72M in a first gate 63 and in a second gate 64, 
respectively. The first and second gates 63 and 64 generate pulses 73P and 
73M when the primary differential signal 72 is higher than the upper level 
72P and lower than the lower level 72M, respectively. Next, a first cross 
detector 65 and a second cross detector 66 generate pulses 75 and 76 when 
the second differential signal 74 crosses a detection level 74R when 
pulses 73P and 73M are received from the first and second gates 63 and 64, 
respectively. Then, a signal generator 67 generates pulses (reproduced 
signal) 78 which rise at the leading edge of the pulse 75 received from 
the first cross detector 63 and decrease at the trailing edge of the 
pulses 76 received from the second cross detector 64. 
FIG. 7 shows an example of the processing of the signal processing circuit 
55 with relation to signal marks (pits) 16a, 16b, 16c and 16d on the 
signal plane 47 of the optical disk. The readout signal 71 for a signal 
mark is affected by the existence of neighboring signal marks. In this 
circuit 55, leading edges and trailing edges of signal marks can be 
detected correctly. A signal 72 is a primary differential waveform derived 
from the readout signal 71 by the primary differential circuit 61, and a 
signal 74 is a secondary differential waveform derived therefrom by the 
secondary differential circuit 62. A gate signal 73P is obtained by a 
first gate 63 by comparing the primary differential signal 72 with a 
detection level 72P, and in the gate signal 73P, a cross point of the 
secondary differential signal 74 with a detection level 74R is detected by 
the first cross detector 65, and a pulse signal 75 with a rising edge at 
the cross point is generated. The pulse signal 75 corresponds to a 
trailing edge of signal mark. Similarly, a gate signal 73M is obtained by 
the second gate 64 by comparing the primary differential signal 72 with a 
detection level 72M, and in the gate signal 73M, a cross point of the 
secondary differential signal 74 with a detection level 74R is detected by 
the second cross detector 66, and a pulse signal 76 with a rising edge at 
the cross point is generated. The pulse signal 76 corresponds to a leading 
edge of signal mark. Finally, the signal generator 67 generates a readout 
signal 78 as a signal inverting at the rising edges of the pulse signals 
75 and 76, and it corresponds to the positions of the signal marks 
16a-16d. 
An explanation follows as to how the signal mark patterns displayed in 
FIGS. 8(a)-8(d) are read out. In FIGS. 8(a)-8(d), a laser spot 20 scans a 
track line (track pitch 0.8 .mu.m) of pits 16 of mark length 0.45 .mu.m 
aligned with a pitch of 0.9 .mu.m. The signal density of the pattern is 
assumed to be four times that of a CD (compact disk). FIG. 8(a) shows a 
case of the synchronization of the pit pattern of the scanning track with 
those of adjacent tracks. FIG. 8(b) shows a case of the inverted 
synchronization or a case wherein the period of the scanning track is 
displaced by a half period compared with that of the adjacent tracks. FIG. 
8(c) shows a case of the scanning track having adjacent tracks consisting 
of sufficiently long pits 16e. FIG. 8(d) shows a case of the scanning 
track having adjacent tracks with sufficiently long pit-to-pit distance. 
FIGS. 9(a) and 9(b) show observation waveforms (eye patterns) of secondary 
differential signal, drawn based on theoretical calculation, for 0.15 
.mu.m of clock frequency T in the signal detection. The wavelength of the 
light source is set to be 780 nm and the NA of the ring belt aperture is 
set to be 0.45-0.70. The ordinate denotes signal amplitude normalized by 
the detected light quantity (100) in a case when the signal plane is a 
mirror plane, and the abscissa denotes the time corresponding to the 
scanning position of the beam spot. In the drawings, the diamond-shape 
areas enclosing the x mark are called eyes. 
FIGS. 10a-10c illustrate how eye patterns are drawn in FIGS. 9(a)-9(d). As 
shown in FIG. 10a four signal marks 31A, 31B, 31C and 31D of pit length 3T 
having a mark edge distant by T/2 in the forward and backward directions 
from a synchronization position 32 are picked up as an example. Four 
signal waveforms A, B, C and D displayed in a graph in FIG. 10b are 
obtained when a laser spot scans four patterns, respectively, and an eye 
pattern is expressed by the superposition of four readout signals. The 
ordinate denotes signal amplitude normalized by the detected light 
quantity in a case when the signal plane is a mirror plane, and the 
abscissa denotes the time. Similarly, four signals a, b, c and d displayed 
in a graph in FIG. 10c obtained by differentiating the readout signals A, 
B, C and D secondarily, respectively, and an eye pattern signal is 
expressed by the superposition of four secondary differential signals a, 
b, c and d. An eye is the diamond-like are enclosed by the four waveforms 
A-D or a-d. The opening of an eye means that the waveforms A, B, C and D 
and a, b, c and d can be distinguished or 0 and 1 can be distinguished. 
Though the waveforms observed in the experiments are obtained by 
superposing the four waveforms A-D and a-d each with a shift of position 
in the lateral direction by an integral number times T, the waveforms are 
represented here without using the above-mentioned shifts, considering 
clear representation of signal waveforms. 
FIG. 9(a) shows two eye pattern overlaps of the secondary differential 
signal of the two pit patterns shown in FIGS. 8(a) and 8(b). Similarly, 
FIG. 9(b) shows two eye pattern overlaps of the secondary differential 
signal of the two pit patterns shown in FIGS. 8(c) and 8(d). It is 
desirable for an eye to be open both in the amplitude direction (in the 
vertical direction in the graphs) and in the time direction (in the 
horizontal direction in the graphs). Further, it is desirable for the 
jitter "d" to be small. These are satisfied in FIGS. 9(a) and 9(b). 
Further, FIGS. 9(c) and 9(d) show cases wherein 1.14 .mu.m on defocusing 
(of the order of decreasing the Strehl intensity of focused spot by about 
20%) is performed in the reproduction of the mark patterns of FIGS. 8(a) 
and 8(b) and of FIGS. 8(c) and 8(d), respectively. 
If FIG. 9(a) is compared with FIG. 4(a), the jitter "d" is found to become 
clearly smaller. Further, in FIG. 9(b), an eye is opened clearly in 
contrast to the closed eye in FIG. 4(b), and the jitter "d" is also much 
smaller. Further, as is clear from FIGS. 9(c) and 9(d), the eye does not 
shift upward or downward when the defocusing is added, and the jitter 
remains very small as in FIGS. 9(a) and 9(b). That is, signal marks can be 
detected correctly even in the presence of defocusing. This tendency is 
common in cases of error factors other than defocusing. 
This can be explained as follows: As shown in FIG. 3(a), the beam spot 
focused, by a ring belt aperture has a large side lobe and a small main 
lobe diameter. The large side lobe intensity affects the readout signal in 
a DC component, or to the up or down as a whole, due to the interferences 
between marks. Error factors such as defocusing also affect the DC 
component of the readout signal. The DC component can be eliminated by 
using the differentiation of the readout signal. Therefore, an eye pattern 
due to secondary differential signal has a small jitter even if signal 
marks exist at the main lobe positions, and the jitter will not increase 
if other error factors are added. On the other hand, the smaller the, main 
lobe diameter is, the better the extreme point of the differential signal 
(that is, the inflection of readout signal) corresponds to edge positions 
of signal mark. Because the extreme point of the differential signal 
appears as a cross point of the secondary differential signal with a 
detection level, the cross point corresponds well with the edge position 
of signal mark. Thus, an advantage of the small side lobe diameter can be 
used in order to reproduce mark signals of high density. 
Further, it is confirmed that the results of FIGS. 9(a) and 9(b) do not 
change by the existence of the pin hole 52 or the size of the pin hole 52 
if any. (FIGS. 9(a) and 9(b) show the data without the pin hole.) The 
abovementioned effect is same even if the pin hole 52 is omitted. (The 
merit of the pin hole 52 appears at different aspects such that the 
degradation of the signal detection characteristic is small when the 
signal plane is defocused.) 
Next, another embodiment will be explained below wherein a primary 
differential signal of the readout signal is used. An optical disk drive 
according to the second embodiment is the same as that shown in FIG. 5 
except a signal processing circuit 81 used instead of the signal 
processing circuit 55. FIG. 11 shows a block diagram of the signal 
processing unit 81. A primary differential circuit 82 generates a primary 
differential signal 91 of the readout signal 71 detected by the detector 
53 by differentiating the readout signal 71. The readout signal 71 is 
compared with a detection level 92R in a gate 83. The gate 83 generates a 
pulse 92 when the readout signal 71 is lower than the detection level 92R. 
Next, a cross detector 84 generates a pulse 93 when the primary 
differential signal crosses a detection level 91R while the pulse 92 from 
the gates 83 is received. Then, a signal generator 85 generates a pulse 
which rises (or falls) at the leading edge of the pulse 93 received from 
the cross detector 84. 
FIGS. 12a-12c show the relationship of signal marks on the signal plane of 
optical disk with the readout signal 71 and the reproduced signal 94. The 
readout signal 71 is changed while the beam spot 20 is scanning over a 
signal mark 16 of uniform length. In this circuit, the centers of the 
signal marks 16 can be detected. A signal 91 (right ordinate) is a 
waveform of primary differential of the readout signal 71 (left ordinate). 
A gate signal 92 is obtained by the gate 83 by comparing the readout 
signal 71 with a detection level 92R, and when the gate signal 92 is 
supplied, a cross point between the primary differential signal 91 and a 
detection level 91R is detected by the cross detector 84 while the pulse 
signal 93 rising with a starting point of the cross point is obtained. A 
reproduced signal 94 is obtained by the signal generator 85 as a signal 
inverted at the starting point of the pulse signal 93. This detection 
method is known as PPM (Pit Position Modulation), and the signal marks 16 
have a uniform length. It is said that the effect of the errors of pit 
size on jitter in PPM is smaller than PWM (Pit Width Modulation) method 
such as EFM (Eight to Fourteen Modulation) used in CDs, and it is 
advantageous for an optical disk drive for read-out/reproduction. In this 
embodiment, it is a feature that PPM is combined with a ring belt 
aperture. 
The following is an explanation of it is explained below how the signal 
mark patterns displayed in FIGS. 8(a)-8(d) are read out. FIG. 13(a) shows 
eye patterns of primary differential signal of the mark patterns of FIGS. 
8(a) and 8(b), while FIG. 13(b) shows eye patterns of primary differential 
signal of the mark patterns of FIGS. 8(c) and 8(d), when the clock 
frequency T in the detection is 0.15 .mu.m. The wavelength of the light 
source is set to be 780 nm and NA of the ring belt aperture is set to be 
0.45-0.70. The ordinate denotes signal amplitude normalized by the 
detected light quantity (100) in a case when the signal plane is a mirror 
plane, and the abscissa denotes the time corresponding to the scanning 
position of the beam spot. An eye pattern is drawn similarly to in FIGS. 
9(a)-9(d), but by using primary differential signals. In the drawings, the 
diamond-shape areas enclosing the x mark are called eyes. Further, FIG. 
13(c) and 13(d) shows cases wherein 1.14 .mu.m of defocusing (on the order 
of decreasing the Strehl intensity of beam spot by about 20%) is performed 
in the readout of the mark patterns of FIGS. 8(a) and 8(b) and of FIGS. 
8(c) and 8(d), respectively. Similarly to FIGS. 9(a)-9(d), it is desirable 
for an eye to be open both in the amplitude direction (in the vertical 
direction in the graphs) and in the time direction (in the horizontal 
direction in the graphs). In FIGS. 13(a)-13(d), it is clear that eyes are 
open and the jitter "d" is very small as in FIGS. 9(a)-9(d), in contrast 
to FIGS. 4(a)-4(c). 
This can be explained as follows: As shown in FIG. 3(a), a ring belt 
aperture has a large side lobe and a small main lobe diameter. The large 
side lobe intensity affects the readout so as to add a DC component, or in 
the up or down as a whole, due to the interferences between marks. Error 
factors such as defocusing also affects the readout signal so as to add a 
DC component. The DC component can be eliminated by using the 
differentiation of the readout signal. Therefore, an eye pattern due to 
primary differential signal has a small jitter if signal marks exist at 
the side lobe positions, and the jitter will not increase if other error 
factors are added. On the other hand, the smaller the main lobe diameter 
is, the better the extreme point of the readout signal corresponds to a 
central position of signal mark. Because the extreme point of the readout 
signal appears as a cross point of the primary differential signal with a 
detection level, the cross point corresponds well with the center position 
of signal mark. Thus, an advantage of the small main lobe diameter can be 
used to reproduce signals of a high density. 
Further, similarly to in the first embodiment, it is confirmed that the 
results of FIG. 13(a) and 13(b) do not change by the existence of the pin 
hole 52 or the size of the pin hole 52 if any. (FIGS. 13(a) and 13(b) show 
the data without the pin hole 52.) The above-mentioned effect is same even 
if the pin hole is omitted. 
In the above-mentioned embodiments, the shading member 44 is used for 
realizing a ring belt aperture. However, a ring-belt-like optical 
distribution can be realized in different ways. For example, as shown in 
FIGS. 14a-14c, a transparent conical body 95 defined by a pair of conical 
planes 95A and 95B can be used. Then, when a light 96 of flat wave front 
of circular distribution (refer the cross section shown in FIG. 14b) is 
incident along the central axis of the conical body 95, the light 96 is 
refracted at the conical planes 95A and 95B to be converted to a light 97 
of flat wave front of ring-belt-like distribution (refer the cross section 
shown in FIG. 14c. 
In the above-mentioned embodiments, the shading member 44 is put between 
the beam splitter 43 and the objective lens 45. However, it may be put 
between the collimating lens 42 and the beam splitter 43. In this case, 
the conversion onto the signal plane 47 of optical disk is carried out 
with a ring belt aperture, while the detection of fed back light is 
carried out with a circular aperture. Thus, the detection characteristic 
is different from the above-mentioned embodiments. However, the principle 
is the same in a point that high density signals can be reproduced by 
using a combination of the secondary or primary differential of the 
readout signal and a ring belt aperture. 
Further, in the above-mentioned embodiments, the optical system including 
bulk optical components such as lens and prism is used. However, other 
collimating elements such as grating lens and focusing grating coupler 
make the reproduction of high density signals possible when combined with 
the secondary or primary differential. 
Although the present invention has been fully described in connection with 
the preferred embodiments thereof with reference to the accompanying 
drawings, it is to be noted that various changes and modifications are 
apparent to those skilled in the art. Such changes and modifications are 
to be understood as being included within the scope of the present 
invention as defined by the appended claims unless they depart therefrom.