Optical head

An optical head has a light source for emitting a light beam; optical device for condensing the emitted light beam onto an information recording medium and condensing light reflected from the information recording medium; and aperture limitation device for, of the condensed reflected light, causing light of a predetermined outer region to exit in a position different from a position where light of an inner region exits, wherein the light of the outer region and the light of the inner region caused to exit by the aperture limitation devices are separated to different places.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an optical head capable of reproduction of 
different kinds of information recording media, for example, reproduction 
of high-density optical disks (DVDs) and reproduction of compact disks 
(CDs). 
2. Description of the Prior Art 
In recent years, standards for a highly densified optical disk 
(hereinafter, referred to as DVD) have been announced and some of the 
manufactures have already commercialized the DVD. FIG. 9 shows the 
construction of a conventional optical system of an optical head for the 
DVD. In FIG. 9, a laser beam 2 emitted from a semiconductor laser 1 is 
reflected at a beam splitter 4 so that its direction is bent at the right 
angle, and is converted into a parallel beam 5 by a collimator 3. The 
parallel beam 5 is converged by an objective lens 7 and applied onto an 
information recording surface of an optical disk 8. The applied light is 
reflected at the optical disk 8 and is again incident on the objective 
lens 7, converted into the parallel beam 5 and converged by the collimator 
3. The converged reflected light passes through the beam splitter 4 and is 
then detected by a photodetector 20. 
According to the standards for the DVD, the thickness of the protection 
layer on the disk signal side is 0.6 mm, the wavelength of the signal 
reading laser is 655 nm and the NA (numerical aperture) of the reading 
objective lens is 0.6. These are greatly different from those of 
conventional CDs in which the thickness of the protection layer is 1.2 mm, 
the wavelength of the signal reading laser is 790 nm and the NA of the 
reading objective lens is 0.45. Therefore, in order to reproduce a 
conventional CD by use of an optical head for DVDs as shown in FIG. 9, 
some scheme is necessary. For this reason, various methods as described 
below have been proposed. 
One method is to mechanically switch between an objective lens for the 
1.2-mm-thick protection layer and an objective lens for the 0.6-mm-thick 
protection layer by use of some devices. Another method is to provide one 
objective lens with two focal points one for DVDs and the other for CDs 
like a bifocal lens. The method in which switching between two objective 
lenses is performed includes a method in which like an axially rotating 
head, two objective lenses of different kinds are disposed about a center 
of rotation and switching therebetween is performed by rotating them. The 
method in which one objective lens is provided with two focal points is an 
excellent method being simple in the construction of the optical system. 
A still another method is to insert an aperture limitation element for both 
the light reaching the disk (go path) and the light reflected from the 
disk (return path). This method is intended to improve the quality of the 
CD reproduction signals, when a CD is reproduced by use of an objective 
lens for DVDs, by removing the light of a portion where the NA is great 
from both the light exiting from the objective lens and the light 
returning to the objective lens because reproduction signal deterioration 
is caused, particularly, in signals from the portion where the NA is 
great. 
However, according to the method in which switching between two kinds of 
objective lenses is performed, the number of mechanical parts increases, 
so that the size of the optical head increases. In addition, the 
reliability is low because the switching between the lenses is performed 
by mechanical axial rotation of the lenses. 
According to the method in which one objective lens is provided with two 
focal points, the use efficiency of the light is inferior because the 
light is inevitably divided into two parts. For this reason, the light 
quantity is insufficient for reproduction of low-reflectance RAM disks, 
etc. which will become important in the future. 
According to the method in which the aperture limitation element is used, 
since aperture limitation is provided in a go and return system in which 
the light advances in both directions, device is necessary for removing 
the aperture limitation when a DVD is reproduced and inserting the 
aperture limitation when a CD is reproduced by some method; otherwise the 
aperture limitation is always inserted and as a natural consequence, it is 
impossible to reproduce a DVD. For this reason, the aperture limitation is 
formed of liquid crystal and by applying a voltage to the liquid crystal, 
the transmittance of the aperture limitation is changed to switch between 
reproduction of a DVD and reproduction of a CD. That is, the aperture 
limitation is equivalently removed and inserted. However, according to 
this method, it is necessary to provide more electric systems than before 
in the optical head optical system, which inevitably increases the size of 
the optical head. 
As described above, while various methods have been considered to reproduce 
CDs by use of an objective lens (NA=0.6) for DVD reproduction, these 
conventional methods all use some scheme for both the go and return paths 
of the light. As mentioned previously, the heretofore contrived methods 
have problems such that the size of the apparatus increases, that the 
apparatus has a mechanical complexity and that DVD-RAM reproduction is 
impossible because the light quantity loss is great. 
SUMMARY OF THE INVENTION 
In view of the problems of the conventional optical head, an object of the 
present invention is to provide an optical head wherein the size of the 
apparatus is reduced, the apparatus has no mechanical complexity and the 
light quantity loss is small. 
The present invention is an optical head comprising: a light source for 
emitting a light beam; optical device for condensing said emitted light 
beam onto an information recording medium and condensing light reflected 
from said information recording medium; and aperture limitation device 
for, of said condensed reflected light, causing light of a predetermined 
outer region to exit in a position different from a position where light 
of a central region exits, wherein said light of the outer region and said 
light of the central region caused to exit by said aperture limitation 
devices are separated to different places.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention will hereinafter be described with reference to the 
drawings showing embodiments thereof. 
FIG. 3 shows a principle of the present invention, which will be described. 
An objective lens 7 for DVD reproduction has an NA of 0.6 and is designed 
so that the light is condensed with least aberration when an optical disk 
with a protection layer thickness of 0.6 mm is reproduced. When a CD with 
a protection layer thickness of 1.2 mm is reproduced by use of the 
objective lens 7, the light beams incident on the disk through the outer 
portion of the lens (outer beam 31) form great angles with respect to the 
disk base plate, so that these light beams are condensed at a surface 
(focal point F') inner than the disk surface. However, the light passing 
through the central portion of the lens (inner beam 32) is condensed at a 
surface (focal point F) not so inner as the surface at which the outer 
beam 31 is condensed because the inner beam 32 has a smaller incident 
angle with respect to the disk base plate than the outer beam 31. 
Therefore, when a CD is reproduced by use of the objective lens 7 for DVD 
reproduction, a parallel beam 5 is not condensed at one point within the 
disk. When a CD is reproduced under such a condition, the reproduction 
signal quality of the CD is impractical. 
In view of this, it is considered that when a CD is reproduced, by using 
only beams passing through central regions of the lens, aberration is 
comparatively small and signal reproduction can be easily performed. 
Therefore, for CD reproduction, it can be easily considered to make the 
signal condition excellent by performing aperture limitation. As mentioned 
in the description of the prior art, various such schemes have been 
contrived and previously, how the light is condensed on the disk has been 
considered important as shown in FIG. 3. Therefore, it has previously been 
considered that a practical apparatus cannot be obtained unless aperture 
limitation is inserted for the light reaching the disk (light on the go 
path). However, according to the results of the inventors' experiments, it 
has been found that it is not always necessary to insert aperture 
limitation on the go path to obtain sufficient characteristics. According 
to the experiments, it has been found that in order to obtain signal 
quality sufficient for CD reproduction, an NA of 0.375.+-.0.005 is 
excellent when the wavelength of the reproduction laser is 655 nm. That 
is, sufficient characteristics are obtained by inserting aperture 
limitation only on the return path. 
That characteristics sufficient as CD signal quality are obtained by 
inserting aperture limitation only on the return path device that, as is 
apparent from embodiments described later, the optical system may be 
formed without any hindrance to DVD reproduction and it is enabled to 
easily perform DVD-RAM reproduction expected to become commercially 
practical in the future while maintaining compatibility with CDs. This is 
because when the use efficiency of light during writing is considered, the 
disk reflectance of the DVD-RAM is inevitably reduced. The normal 
reflectance which is approximately 15% is low compared with that of the 
conventional ROM disk having a reflectance of 75% or higher and when RAM 
compatibility is considered, the present invention realizes an optical 
head having high versatility. 
First Embodiment 
FIG. 1 shows the construction of an optical head according to a first 
embodiment of the present invention. In this embodiment, aperture 
limitation is performed by use of a polarization hologram 501. In FIG. 1, 
a laser beam 2 emitted from a semiconductor laser 1 serving as the light 
source is converted into a parallel beam 5 by a collimator 3. The parallel 
beam 5 is reflected at a beam splitter 4a so that its direction is bent at 
the right angle. The polarization direction of the polarization plane of 
the parallel beam 5 on the go path is one which cannot be diffracted by 
the polarization hologram 501 serving as the aperture limitation device. 
Therefore, in this case, the polarization hologram 501 does not perform 
any function and is equivalent to a mere glass plate. The light having 
passed through the polarization hologram 501 is converted into rotatory 
polarized light by a succeeding quarter .lambda. wavelength plate 51. 
Then, the light is condensed onto an optical disk 8 serving as an 
information recording medium by an objective lens 7 driven by an actuator 
100. The collimator 3, the beam splitter 4a, the quarter .lambda. 
wavelength plate 51 and the objective lens 7 constitute optical device. 
The light incident on the optical disk 8 is reflected. The light reflected 
from the optical disk 8 again passes through the objective lens 7 and 
again passes through the quarter .lambda. wavelength plate 51. 
Consequently, the polarization direction of the light differs from that of 
the incident light by 90 degrees, so that the light is diffracted by the 
polarization hologram 501 at this time. 
The pattern of the polarization hologram 501 at this time is shown in FIG. 
2(a). The polarization hologram 501 is concentrically divided into an 
inner region 501b-1 to 4 and an outer region 501a-1 to 4 and these regions 
are each subdivided into four fan-shaped regions. The diameter of the 
inner region is decided so as to correspond to an NA of 0.375 when the NA 
of the objective lens 7 is 0.6. The light incident on the outer region 
501a-1 to 4 of the polarization hologram 501 is diffracted to a region 
201a of a photodetector 20 shown in FIG. 2(b) and is converged. Naturally, 
the light beams diffracted by the polarization hologram 501 are negative 
diffracted beams coming out in positions symmetrical with respect to a 
virtual light source point 24, so that the diffration spots shown in FIG. 
2(b) are all formed in positions symmetrical with respect to the virtual 
light source point 24. The light diffracted at the inner region 501b-1 to 
4 is projected onto central portions in a region 201b of the photodetector 
20 and a region 201c symmetrical with respect to the virtual light source 
point 24. Therefore, when an RF signal is detected by use of only the 
light projected onto the region 201b, since the signal is detected by use 
of only the light of a portion located inside the portion where the NA is 
0.375, CD reproduction is performed by use of the light of only this 
region. For DVD reproduction, since it is necessary to use the light of 
all the regions, signal reproduction is performed by use of the sum of the 
light beams of the regions 201a, 201b and 201c. Alternatively, although 
the light quantity is halved, if there is no problem in noise, signal 
reproduction can be performed by use of the light of only the region 201c 
without any problem. 
As described above, according to this embodiment, while aperture limitation 
device is disposed on the optical path where the light going to the 
optical disk 8 (light on the go path) and the light reflected and 
returning from the optical disk 8 (light on the return path) both pass, as 
mentioned above, by using the polarization hologram 501 as the aperture 
limitation device, the aperture limitation function does not work for the 
light on the go path but works only for the light on the return path, so 
that the prior art problems are solved. 
In the case of this embodiment, the aperture limitation polarization 
hologram 501 is fixed to the same optical barrel as the objective lens 7. 
This enables ideal aperture limitation because the same portion of the 
aperture is limited even if a lens shift due to tracking occurs for the 
distribution of the return light. However, although the characteristics 
are slightly inferior to the ideal state, the aperture limitation may be 
disposed on the fixed side as shown in FIG. 5 described later. 
Second Embodiment 
FIG. 4(a) shows the construction of an optical head according to a second 
embodiment of the present invention. FIG. 4(b) shows an aperture 
limitation element thereof. This embodiment is an example in which an 
aperture limitation element comprising a mirror is inserted in the optical 
system on the return path. In FIG. 4(a), in this optical system, reference 
numeral 4b represents a half mirror. A laser beam 2 emitted from a 
semiconductor laser 1 is reflected at the half mirror 4b so that the 
optical path is bent at the right angle, and is converted into a parallel 
beam 5 by a collimator 3. The construction of the optical system from the 
collimator 3 to the optical disk is similar to that of the prior art (see 
FIG. 9). The light reflected from the optical disk is converted into the 
parallel beam 5 by the objective lens and is again converted into a 
condensed beam by the collimator 3. Then, the light is partly reflected 
and partly transmitted by the half mirror 4b. As shown in FIG. 4(b), an 
aperture limitation element 502 is divided into an outer region 502a 
serving as a light reflecting portion and having an metal film formed 
thereon so that light is reflected, and a circular central region 502b 
serving as a light transmitting portion and having no metal film formed 
thereon so that light is transmitted. The light transmitted by the half 
mirror 4b is transmitted at the central region 502b of the aperture 
limitation element 502 and is reflected at the outer region 502a other 
than the central region 502b so that the optical path is bent. 
Since the half mirror 4b is obliquely inserted in the condensed beam, the 
transmitted light of the condensed beam has astigmatism. The light 
reflected at the outer region 502a of the aperture limitation element 502 
is received by a photodetector 202a. In the case of this embodiment, the 
detection area of the photodetector 202a is divided into four parts and in 
the case of DVD reproduction, tracking signals and information 
reproduction signals may be detected as well as signals of a photodetector 
202b. 
The light transmitted by the central region 502b of the aperture limiting 
element 502 is received by the photodetector 202b. The detection area of 
the photodetector 202b is also divided into four parts. The photodetector 
202b is placed in a position where the luminous beam of the condensed beam 
17 provided with astigmatism by the half mirror 4b is limited the most 
(position of the least circle of confusion). Therefore, CD focus signals 
and tracking signals may be detected by the photodetector 202b. 
While in the second embodiment, the aperture limitation element (device) is 
designed so that the light of the central region is transmitted and the 
light of the outer region is reflected, it may be designed so that the 
light of the central region is reflected and the light of the outer region 
is transmitted. 
Third Embodiment 
FIG. 5 shows the construction of an optical head according to a third 
embodiment of the present invention. FIGS. 6(a)-(b) show divided surfaces 
of a hologram and a photodetector in this embodiment. This embodiment is 
an example in which the aperture limitation that is realized with a mirror 
in the second embodiment is realized with a hologram. In the case of the 
optical head optical system of FIG. 5, since the aperture limitation 
device is not inserted in a position where the light on the go path and 
the light on the return path simultaneously pass like in the first 
embodiment (FIG. 1) but a hologram 50 serving as the aperture limitation 
device is inserted in a position where only the light passes which is on 
the return path after the light has been split into the light on the go 
path and the light on the return pass by a beam splitter 4a, any one of a 
glass hologram and a polarizing hologram may be used as the hologram 50. 
As shown in FIG. 6(a), the hologram 50 of this embodiment is 
concentrically divided into a central region 503b and an outer region 
503a. Of the light reflected at the optical disk 8 and having passed 
through the objective lens 7 and the beam splitter 4a, the light 
diffracted at the central region 503b of the hologram 50 is condensed at 
the region 203b of a photodetector 20 and the light diffracted at the 
outer region 503a is condensed at the region 203a of the photodetector 20. 
The regions 203a and 203b of the photodetector 20 are each subdivided into 
four parts. The focus signal is produced from the four-part divided light 
of the region 203b. The tracking signal and the RF signal are produced 
from the sum of the light beams of the regions 203a and 203b. 
Fourth Embodiment 
FIGS. 7(a)-(b) show divided surfaces of a hologram and a photodetector in a 
fourth embodiment of the present invention. The basic construction of this 
embodiment is similar to that of the third embodiment shown in FIG. 5. 
That is, a hologram 50 is inserted in a position where only the light 
passes which is on the return path after the light has been split into the 
light on the go path and the light on the return pass by a beam splitter 
4a. As shown in FIG. 7(a), an outer region 504a of the hologram 50 
corresponds to a region 204a of a photodetector 20 and the central region 
504b of the hologram 50 corresponds to a region 204b of the photodetector 
20. 
This embodiment is different from the third embodiment in that the central 
region 504b of the hologram 50 is not circular but oval as shown in FIG. 
7(a). Consequently, tracking is performed so that the objective lens 7 
always runs on a reproduction signal groove and therefore, when the 
hologram 50 is placed in the position shown in FIG. 5, the light reflected 
from the optical disk 8 moves on the hologram 50 as the objective lens 7 
moves. Therefore, consideration is given so that the moving portion is not 
subject to the aperture limitation. Consequently, the light passing 
through the central region 504b of the hologram 50 and condensed at the 
region 204b of the photodetector 20 is also oval. 
Fifth Embodiment 
FIGS. 8(a)-(b) show divided surfaces of a hologram and a photodetector in a 
fifth embodiment of the present invention. The basic construction of this 
embodiment is similar to that of the third embodiment shown in FIG. 5. 
That is, a hologram 50 is inserted in a position where only the light 
passes which is on the return path after the light has been split into the 
light on the go path and the light on the return pass by a beam splitter 
4a. 
This embodiment is different from the third embodiment in that, in order 
that the aperture limitation is not performed only in the tracking 
direction, the shape of the aperture limitation region of the fourth 
embodiment is further changed, i.e. an outer region 505a of the hologram 
50 is of a shape such that as shown in FIG. 8(a), a central portion 
between two parallel chords is cut away from a circle. Therefore, the 
central portion of the hologram 50 corresponds to a region 205b of a 
photodetector 20 and an outer region 505a of the hologram 50 corresponds 
to a region 205a of the photodetector 20. The light passing through the 
central portion of the hologram 50 and condensed at the region 205b of the 
photodetector 20 and the light passing through the outer region 505a of 
the hologram 50 and condensed at the region 205a of the photodetector 20 
are of shapes corresponding to the shapes of the regions of the hologram 
50. 
As described above, according to the present invention, DVD-and-CD 
compatibility is enabled by inserting aperture limitation only in the 
optical system on the return path without any scheme being provided in the 
optical system on the go path. 
While in the above-described embodiments, the photodetector 20 comprises 
one detector having a detection region for detecting the light passing 
through the central region and a detection region for detecting the light 
passing through the outer region, instead thereof, separate light 
receiving elements may be provided for detecting the light beams passing 
through the regions, for example, as shown in FIG. 4. 
While bilaterally symmetrical light receiving portions are disposed in the 
above-described embodiments, only one of them is necessarily provided if 
there is no problem in the light quantity. 
As is apparent from the above description, since the present invention has 
aperture limitation device for, of light condensed by optical device, 
causing light of a predetermined outer region to exit in a position 
different from a position where light of a central region exits and a 
photodetector for detecting the light of the outer region and the light of 
the central region caused to exit by the aperture limitation device, 
advantages are produced that the size of the apparatus is reduced, that 
the apparatus has no mechanical complexity and that the light quantity 
loss is small.