Optical pickup apparatus and optical grating assembly therefor

An optical pickup apparatus for optically reading the data stored in a flat information carrier disc in the form of tiny pits. The pickup apparatus directs a beam of light from a light source to the disc surface. The impinging beam is modulated by the data pits and reflected back to a photodector. A novel optical grating assembly is disposed between the light source and the information disc, and includes a diffraction grating for the light beam from the source and a holographic grating for directing part of the light beam reflected by the disc to the photodetector. The diffraction and holographic gratings are formed integral with each other into the grating assembly.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an optical pickup apparatus and an optical 
grating assembly for the apparatus. More particularly, this invention 
relates to an optical pickup apparatus for reading information stored in 
an information carrier disc by directing small read-out spots of light to 
the disc and detecting the light reflected therefrom by a photosensitive 
detector, and an optical grating assembly suitable for use in such pickup 
apparatus. 
2. Description of the Related Art 
Information carrier discs are known in the art where data are stored and 
arranged along a spiral track having a succession of reflective elements. 
The reflective elements on the track generally are in the form of hollows 
called "pits", which hold audio, video or other information in a digital 
form. The information carriers, which are also dubbed as optical discs, 
are finding their way rapidly into the music, movie and computer 
industries. 
In order to read out the information stored in the carrier disc, an optical 
read-out or pickup device is used. FIG. 1 schematically illustrates the 
arrangement of a conventional optical device which employs a three-beam 
method for tracking servo control and an astigmatic detection for focusing 
servo. Before a detailed description of FIG. 1, is provided the three-beam 
method and astigmatic detection are briefly explained with reference to 
FIGS. 2 and 3. 
In FIG. 2, the three-beam method for tracking servo control uses three 
beams of light: a main beam concentrated into a main reading spot 18a and 
a pair of sub-beams focused into a pair of sub-reading spots 18b and 18c 
on the opposite sides of the main reading spot 18a. A tracking error 
signal is obtained by sensing the amount of difference between the pair of 
reflected sub-beams. 
In FIG. 3, the astigmatic detection for focusing servo control makes use of 
a cylindrical lens which acts as a lens with respect to light in one 
direction but does not act as a lens with respect to light in the opposite 
direction. A beam of light passing through the cylindrical lens forms a 
circular spot at the focal point, and distorted circular spots or 
elliptical spots on the far and near sides of the focal point. Any 
variation in shape of the reading spot of beam is electrically detected to 
thereby generate a focus error signal. 
Now referring to FIG. 1, the optical pickup or read-out device includes a 
laser source. A laser beam 16 emitted by the laser source 1 is directed to 
a diffraction grating 12 where it is diffracted into a zero-order 
diffracted main beam 17a and a pair of first-order diffracted sub-beams 
17b and 17c. The main beam 17a is for reading out the pit information 
recorded on the disc and for sensing the focusing error, while the two 
sub-beams 17b and 17c are for sensing the tracking error or failure of the 
main reading beam. The three diffracted beams 17a, 17b and 17c are 
directed splitter 13 provided with a half-silvered surface or mirror 13a 
where they are reflected toward a collimating lens 5. The three beams pass 
out of the collimating lens 5 in parallel. The collimated beams continue 
to an objective lens 6 which focuses the beams into three spots 18a, 18b 
and 18c on the surface of the information carrying disc 7 in a pattern as 
shown in FIG. 2. 
The main reading spot 18a is derived from the main beam 17a, whereas the 
sub-spots 18b and 18c are delivered from the sub-beams 17b and 17c. The 
spot forming beams 17a, 17b and 17c are modulated by the data carrying 
track on the disc and reflected back through the objective lens 6 and the 
collimating lens 5 toward the beam splitter 13, following substantially 
the identical path. The beam splitter is disposed oblique or at an angle 
with respect to the optical path of the returning beams, while the 
returning beams impinge obliquely on the beam splitter 13 and pass 
therethrough. The length of the optical paths, for example, 1 and m within 
the beam splitter for the light beams passing therethrough vary depending 
on the locations at which those beams strike the splitter. The net result 
is similar to sending the beams through a cylindrical lens, and, thus, 
astigmatism is developed. The returning beams transmitted through the beam 
splitter 13 proceed to a plane concave lens 14 by which the incident beams 
are axially or longitudinally magnified and focused into spots 19a, 19b 
and 19c onto a six-segment photosensitive detector 15. These beam spots 
19a, 19b and 19c are projected on the photodetector 15 in various patterns 
as shown in FIGS. 4A-4C depending on the positions of the disc 7 relative 
to the objective lens 6. It is noted that the spot 19a is derived from the 
main beam 17a, while the spots 19b and 19c are derived from the sub-beams 
17b and 17c, respectively. 
Turning to FIGS. 4A-4C, the photodetector 15 includes photosensitive 
segments A-D arranged in a square configuration to receive the main spot 
of beam 19a, and a pair of similar photosensitive segments E and F 
disposed on both sides of the square array if segments A-D for receiving 
the sub-spots 19b and 19c of the beam. When the surface of the information 
carrying disc 7 is situated in the focal plane of the objective lens 6 and 
it is thereby made possible for the reading spots to be focused in the 
data track of the disc, the beam spots 19a-19c are projected on the 
photodetector in the shape of round circles and in the pattern as shown in 
FIG. 4B. If the disc surface is closer to the objective lens beyond its 
focal point, i.e. if the disc surface is positioned on the near side of 
the focal point of the objective lens, then the projected beam spots take 
the form of ellipses or oval circles arranged as shown in FIG. 4A. On the 
other hand, when the disc surface is farther than the focal point of the 
objective lens i.e. when the disc surface is positioned on the far side of 
the focal point of the objective lens, the beam spots 19a-19c are 
projected on the photodetector 15 in the form of oval circles and in a 
pattern as shown in FIG. 4C. 
A pit signal RF obtained by reading the pits in the data track of the disc 
with the read-out light spots is expressed by the following equation: 
EQU RF=a+b+c+d 
where a, b, c and d represent electrical signals corresponding to the 
amount of light received on the photosensitive segments A, B, C and D, 
respectively. 
A tracking error signal TES provided by the three-beam method is expressed 
as follows: 
EQU TES=e-f 
where e and f represent electrical signals proportional to the amount of 
light projected and received on the photosensitive segments E and F, 
respectively. 
A focus error signal FES generated by the astigmatic detection is expressed 
as follows: 
EQU FES=(a+d)-(b+c) 
Whenever a tracking error is sensed by a tracking error detector 41, a lens 
actuator or a driver 44 operates in response to the output signal from the 
detector 41 to move the objective lens 6 in a direction and an amount to 
correct the error. Likewise, when a focusing error is sensed by a focusing 
error detector 42, a lens actuator or a driver 43 operates responsive to 
the output signal from the detector 42 to move the objective lens 6 in a 
direction and an amount to correct the focusing error. In this manner, the 
position of the objective lens 6 relative to the disc 7 is always adjusted 
so that the disc surface is kept in the focal plane of the objective lens 
and the reading beam spots are focused right on the data track of the 
disc, so that accurate and reliable read-out of the pits in the data track 
are assured. 
In FIG. 5, there is schematically illustrated another conventional optical 
pickup device which incorporates a holographic grating. The pickup device 
relies on the push-pull method for tracking servo control and on the wedge 
prism method for focusing servo control. 
Before a description of the optical pickup device of FIG. 5, is provided 
the push-pull and wedge prism methods will be briefly explained. 
Turning to FIG. 7, the push-pull detection employs a single spot of beam 
for reading the information stored in the form of a pit on the disc. When 
the reading spot illuminated on the information track is in perfect 
registration with the data pit as represented by the center spot in FIG. 
7, two photosensitive segments of a photodetector 45 receive equal amounts 
of reflected light. However, any lateral deviation of the projected light 
spot with respect to the data pit causes a difference in the intensity of 
light impinging on the photosensitive segments. This difference sensed by 
the photodetector 45 produces a signal indicating the tracking error. 
The wedge prism method for focusing servo control employs a wedge prism 46 
as shown in FIG. 8A. One face of the prism is contoured in the form of a V 
shaped valley. A beam of light incident on the wedge prism emanates 
therefrom in two beams of light as illustrated in FIG. 8B. At its focal 
point F, the transmitted beams are converged into tiny spots, while at 
positions X and Y on the near and far sides, respectively, of the focal 
point, the beams form semicircular spots of different size and 
orientation. The varying semicircular spots of the light beams are sensed 
by a photodetector, which produces electric outputs indicating focusing 
errors. 
Now turning to FIG. 5, the optical pickup device includes a laser source 1. 
A laser beam 22 from the source of laser 1 is directed to the holographic 
grating 20. FIG. 6 illustrates the holographic grating 20 in an enlarged 
perspective view. The holographic grating 20 includes a glass or plastic 
plate member formed with a number of fine curved grooves or slits for 
diffracting the light beam passing therethrough. In order for the 
diffraction grating 20 to function like a wedge prism, the grating is 
divided into two sections 20a and 20b. 
Turning back to FIG. 5, the holographic grating 20 diffracts the laser beam 
22 from the source into several diffraction orders. Among them, only the 
zero-order diffraction beam continues to a collimating lens 6. The 
first-order diffraction beam wholly misses the collimating lens 6 because 
it has a larger diffraction angle. The zero-order diffraction beam passing 
through the collimating lens 6 is turned into parallel beams. The 
collimated beams are oriented to an objective lens 6 which focuses them 
into a spot of light on the surface of the information carrier disc 7. The 
impinging beam is re disc 7 and is returned to the holographic grating 20 
along substantially the same optical path. The return beam is diffracted 
into several orders by the diffraction grating 20. The zero-order 
diffraction beam proceeds toward the laser source 1, while the first-order 
diffraction beam is directed to a four-segment photodetector 21. As stated 
hereinabove in connection with FIG. 6, the holographic grating 20 is 
divided into grating sections 20a and 20b along a line oriented in a 
tangential direction of the disc 7 (while the dividing line is conspicuous 
in the drawing for showing the boundary between the grating sections, but 
no such line exists in an actual holographic grating). 
As can also be seen in FIG. 6, the sections 20a and 20b are formed with 
grating grooves of different designs or patterns so that the first-order 
diffracted beams emanating from these sections are converged onto 
different points. More specifically, the two first-order diffracted beams 
24a and 24b emanate from the holographic grating 20 due to its composite 
structure. The beams 24a and 24b impinge on a four-segment photodetector 
15 to form spots of light 25a and 25b. The formations of light spots 25a 
and 25b on the photodetector 21 enlarged and shown in FIGS. 9A-9C where 
the alphabetical letters A, B, C and D denote photosensitive segments 
including the photodetector 21. In an erroneous read-out situation where 
the disc 7 lies closer to the objective lens 6 beyond its focal point, 
i.e. the disc is located within the focal length of the objective lens 6, 
semicircular spots 25a and 25 are formed on the outermost photosensitive 
segments A and D as shown in FIG. 9A. When the disc 7 lies in the focal 
plane of the objective lens 6, i.e. the disc is at the focus of the 
objective lens, tiny spots of light are projected on the photodetector 21 
as shown in FIG. 9B. If the disc 7 lies farther away from the objective 
lens beyond its focal point, i.e. if it is positioned outside the focal 
length of the objective lens, semicircular spots 25a and 25b are 
illuminated on the innermost photosensitive segments B and C as shown in 
FIG. 9C. 
A pit signal RF obtained by optically reading the disc 7 is expressed as 
follows: 
EQU RF=a+b+c+d 
where a, b, c and d represent electrical signals corresponding to the 
amount of light received by the photosensitive segments A, B, C and D, 
respectively. 
A tracking error signal TES provided by the push-pull method is expressed 
as follows: 
EQU TES=(a+b)-(c+d). 
The wedge prism method provides a focusing error signal FES expressed by 
the following equation: 
EQU FES=(a+d)-(b+c). 
It should be noted that the composite holographic grating 20 acts optically 
like a wedge prism. 
Referring again to FIG. 5, when a focusing error is sensed by a focusing 
error detector 42, a lens actuator 43 operates to drive the objective lens 
6 in a direction and an amount to correct the error. Likewise, as a 
tracking error detector 41 senses a tracking error, another actuator 44 
operates to move the objective lens 6 in a direction and an amount to 
offset the focusing error. Thereby, an accurate and a reliable reading of 
the pits on the disc is achieved. 
The conventional optical pickup devices of the type described above, while 
generally satisfactory in optical read-out operation, suffer some 
drawbacks. 
The optical pickup device of FIG. 1 which employs the three-beam detection 
for the tracking servo control provides an excellent ability to detect 
tracking error. However, this device requires both the diffraction grating 
12 and the plane beam splitter 13 for the diffraction of the laser beam, 
as compared with a single holographic grating for the optical pickup 
device of FIG. 5 based on the push-pull detection method. One additional 
component part means one additional assignment of operational adjustment 
as well as additional costs. 
On the other hand, in the pickup device of FIG. 5 which relies on the 
push-pull detection technique, a single holographic grating 20 is 
sufficient for the intended diffraction of the laser beam. One fewer 
component part and one fewer assignment for the operational adjustment in 
this pickup device is used than in the device based on the three-beam 
detection which leads to a considerable cost reduction. However, the 
push-pull type of the pickup device is disadvantageous in that the 
tracking error signal fluctuates with varying pit depths and it is 
impossible to obtain a stable and constant tracking error signal. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the invention to provide an optical pickup 
apparatus for optically reading information stored in a flat information 
carrier. 
It is another object of the invention to provide an optical pickup 
apparatus for optically reading a flat information carrier which uses a 
three-beam method for tracking servo control and which is simpler in 
construction than similar conventional apparatus. 
It is still another object of the invention to provide a novel optical 
grating assembly suitable for use in the optical pickup apparatus. 
Briefly described, according to one preferred embodiment of the invention, 
there is provided an improved optical pickup apparatus for optically 
reading information recorded on a flat information carrying disc in the 
form of a spiral track of successive pits. The apparatus includes a 
radiant light source for emitting a radiation beam. The radiant beam from 
the light source is separated by a first optical grating into a main beam 
for reading the data pits on the information carrying disc, and a pair of 
sub-beams for sensing tracking errors of the read-out beam. The beams of 
radiant light are focused into three tiny reading spots on the surface of 
the information disc, which are then reflected back by the disc surface. 
The optical pickup apparatus also includes a second optical grating which 
directs part of the beam reflected from the information disc to a 
photosensitive detector. The photosensitive detector, then, produces 
electrical outputs indicating focusing error as well as tracking error. 
The pickup apparatus corrects focusing and tracking errors in response to 
the output signals from the photodetector while optically reading the data 
pits on the disc. The first and second optical gratings are integrally 
formed into a unitary grating assembly. 
According to one aspect of the invention, the unitary grating assembly 
comprises a first optical grating positioned to face towards the source of 
the radiant light and a second optical grating arranged to face towards 
the information disc. In a preferred embodiment of the invention, the 
first optical grating is a diffraction grating and the second optical 
grating is a holographic grating. 
These objects and other objects, features, aspects and advantages of the 
present invention will become more apparent from the following detailed 
description of the present invention when taken in conjunction with the 
accompanying drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring now to FIG. 10, there is illustrated a grating assembly for use 
in an optical pickup apparatus according to this invention. The grating 
assembly 2 has a face 202 turning toward a source of radiant light and is 
provided with a diffraction grating 4a as a first grating component. The 
diffraction grating 4a functions to distribute a light beam from the 
source into a main beam for reading a data pit formed on the information 
carrying disc, and a pair of sub-beams for sensing a tracking error of the 
main reading beam. The other face 201 of the grating assembly 2 turning 
towards the information carrying disc is provided with a holographic 
grating 3 as a second grating component. The holographic grating 3 
includes a number of curved parallel grooves arranged to deflect a part of 
the light beam impinging upon the grating. Thus, when incorporated in the 
optical pickup apparatus of the invention to be described later, the 
holographic grating 3 works to turn a portion of the light beam reflected 
from the information carrying disc to a photodetector. It should be noted 
that the diffraction grating 4a and the holographic grating 3 are made of 
plastics or glass, and formed into the grating assembly 2 of unitary or 
integral structure. However, this invention is not limited to the use of 
plastics and glass. Other suitable materials may also be used for the 
unitary grating assembly. 
Fabrication of the unitary grating assembly 2 will now be briefly described 
with reference to Figs. 11A-11G. 
FIG. 11A illustrates the grating assembly 2 for a schematic cross-section. 
The grating assembly 2 has the holographic grating 3 on one face and the 
diffraction grating 4a on the other face. The assembly is fabricated by 
introducing (thermoplastic) synthetic resin material or glass into a mold 
33 as illustrated in FIG. 11B. The mold 33 includes an upper molding half 
33a provided with a counterpart design of the desired holographic grating, 
and a lower molding half 33b ruled with a complementary design of the 
desired diffraction grating. The mold 33 also has an inlet passage 33c 
formed therein, and glass or plastic material in a molten or a fluid state 
is introduced into the mold through the inlet passage 33c. A pool of 
molten material within the mold 33 is left to set into a solid body which 
constitutes the unitary composite grating structure 2. Other grating 
structures according to the invention which will be described hereinbelow 
are all fabricated in substantially the same manner. 
The forming of the mold half 33a will be described with reference to Figs. 
11C-11G. A glass pane or a sheet of glass 30 is prepared as as illustrated 
in FIG. 11C. A coating 31 of photoresist is applied over one surface of 
the glass pane 30 (FIG. 11D). 
In the next step, a photolithographic technique is used (and, when 
necessary, a two-beam interference technique), the photoresist coating 31 
is etched away to leave a pattern of fine thread-like projections 32 (FIG. 
11E). The pattern of the fine projections 32 is covered with a coating of 
electroplated metal 33 having a desired thickness (FIG. 11f). When the 
electroplated metal coating 33 is stripped off the underlying glass sheet 
30, the coating is incorporated into the upper mold half 33a as a 
grating-forming surface layer. Similarly a grating-forming layer for the 
lower molding half 33b can be fabricated following substantially the same 
process as described above. 
The optical working of the grating assembly 2 of FIG. 10 is illustrated in 
FIG. 12 by a greatly enlarged cross-section taken along the line A-A of 
FIG. 10. Referring to FIG. 12 together with FIG. 13 which schematically 
illustrates an optical pickup apparatus according to the invention and 
having the grating assembly 2 incorporated therein, the optical pickup 
apparatus includes a laser source 1, typically a semiconductor laser 
device, and an objective lens 6 positioned close to an information carrier 
disc 7 for converging the laser light from the source 1 on the disc. A 
collimating lens 5 is disposed closer to the objective lens, while the 
optical grating assembly is disposed to the laser source 1. The pickup 
apparatus also includes a six-segment photosensor or photodetector 8 which 
produces electric outputs in response to and as a function of incident 
light radiation. 
Now to describe the operation of the illustrated pickup apparatus, a laser 
beam 9 emitted from the laser source 1 falls on the diffraction grating 4a 
provided on the face 202 of the grating assembly 2. The incident laser 
beams is diffracted by the diffraction grating into a zero-order 
diffracted beam 9a for reading the data pits on the disc 7, and a pair of 
first-order diffracted beams 10a and 11a for sensing tracking error of the 
main read-out beam 9a. These three diffracted beams 9a, 10a and 11a pass 
through the holographic grating 3 on the other face 201 of the grating 
assembly 2, which diffracts the passing beams into zero-order beams and 
first-order beams. From these beams, only the three zero-order beams 
continue to the collimating lens 5. The first-order beams miss the 
collimating lens 5 due to their greater diffraction angles. The three 
beams 9a, 10a and 11a impinging on the collimating lens 5 are in a 
parallel as they pass therethrough and are directed to the objective lens 
6, which focuses the beams into three spots 9b, 10b and 11b onto the disc 
7 for reading out the information carried therein. 
The spot 9b is derived from the major laser beam 9a, whereas the spots 10b 
and 11b are derived from the sub-beams 10a and 11a of laser, respectively. 
The three beam spots are focused on the data carrying surface of the disc 
in a relative arrangement as illustrated in FIG. 2. Thus, the spots 10b 
and 11b are displaced tangentially or in a direction of the disc with 
respect to the center spot 9b. 
The spot-forming laser beams are reflected by the disc 7 and return along 
substantially the same optical paths through the objective lens 6 and the 
collimating lens 5 to the grating assembly 2 where the return beams fall 
on the holographic grating 3. The incident beams are then diffracted into 
the zero-order diffraction beams and the first-order diffraction beams. 
While the zero-order beams continue straight back toward the laser source 
1, the first-order diffracted beams 9c, 10c and 11c are deflected 
sideways. As a consequence, the first-order beams 9c, 10c and 11c pass out 
of the grating assembly 2 through a non-grated area 4b in the lower face 
202, so that the diffraction grating 4b is bypassed. It is pointed out 
that the first-order diffracted beams which pass obliquely through the 
grating assembly 2 inevitably undergo an astigmatic effect or distortion 
which is advantageously to produce electrical signals indicating focusing 
errors as will be explained later. The first-order beams 9c, 10c and 11c 
passing out through the non-grated area 4b impinge on the six-segment 
photodetector 8 and form light spots 9d, 10d and 11d, respectively, in 
various shapes and orientations as illustrated in FIGS. 14A-14C. 
Forming of the light spots on the six-segment photodetector 8 in the pickup 
apparatus of the invention, both in their shapes and arrangements, is 
similar to the light spots formed in the conventional pickup device of 
FIG. 1 (see FIGS. 4A-4C). Note that the pickup device includes both the 
diffraction grating 12 and the plane beam splitter 13 as essential optical 
components. 
The pickup apparatus of this invention as illustrated in FIGS. 12 and 13 
makes use of the three-beam detection for tracking servo control and the 
astigmatic detection for focusing servo control in a similar manner to the 
conventional device of FIG. 1. However, the pickup apparatus of the 
invention relies only on a single grating assembly 2 for the purpose of 
beam diffraction, when compared to the two optical components in the 
conventional device. 
In an erroneous reading situation where the disc 7 is within the focal 
length of the objective lens 6 or closer to the objective lens beyond its 
focal point, the three spots 9d-11d are formed on the six-segment 
photodetector in an oblong shape and in the orientation as illustrated in 
FIG. 14A. In a correct reading situation where the disc 7 is in the focal 
plane of the objective lens 6, three circular spots 9d-11d are projected 
on the six-segment photodetector 8 as illustrated in FIG. 14B. In another 
erroneous reading situation where the disc 7 is on the far side of the 
focal point of the objective lens 6 or outside its focal length, the three 
light spots 9d-11d projected on the photodetector 8 take on oblong shapes 
oriented as illustrated in FIG. 14C. 
A pit signal RF obtained by optically reading the data pits in the disc 7 
is expressed as follows: 
EQU RF=a+b+c+d 
where a, b, c and d represent electrical outputs corresponding to the 
amount of light received by the photosensitive segments A, B, C and D. 
A tracking error signal TES provided by the three-beam method is expressed 
as follows: 
EQU TES=e-f 
where e and f represent electrical outputs in proportion to the quantity of 
light received by the photosensitive segments E and F, respectively. 
A focusing error signal FES provided by the astigmatic detection method is 
expressed as follows: 
EQU FES=(a+d)-(b+c). 
Referring again to FIG. 13, when a focus error detector 42 senses a 
focusing error through the photodetector 8, the detector 12 operates a 
lens actuator 43 so that the actuator drives the objective lens 6 in a 
direction and an amount to correct the error. Similarly, as a tracking 
error detector 41 senses a tracking error through the photodetector 8, the 
detector 12 operates a lens actuator 44 such that the actuator moves the 
objective lens 6 in a direction and an amount to correct the error. An 
accurate and reliable reading of the data pits on the disc is attained in 
this manner. 
Referring to FIGS. 15A and 15B, there are as illustrated optical grating 
assemblies according to another embodiment of the invention. The grating 
assemblies of FIGS. 15A and 15B are essentially identical in construction 
to the assemblies illustrated in FIG. 10 except for the structural feature 
to be described hereinbelow. Thus, like component parts are designated by 
like reference numerals without having any further description thereof. 
The novel structural feature of the optical grating assemblies 2 
illustrated in FIGS. 15A and 15B is the provision of a lens structure 4c 
in the lower face 202 in the area through which the laser beam deflected 
by the holographic grating 3 towards the photodetector passes. In the 
grating assembly of FIG. 15A, the lens structure 4c is provided in the 
form of a cylindrical lens, whereas in FIG. 15B, the lens structure 4c is 
in the form of a concave lens. 
The optical working of the grating assembly 2 with the lens structure 4c is 
illustrated in FIG. 16. With the cylindrical lens structure of FIG. 15A, 
the beams passing through the lens structure undergoes a greater 
astigmatic distortion, which, in turn, is effective to cause the 
photodetector to generate more distinct outputs. On the other hand, the 
concave lens structure 4c of FIG. 15B functions to magnify the transmitted 
beams in an axial or a longitudinal direction. The axially magnified beams 
are more readily picked up by the photodetector. 
While the cylindrical lens structure 4c has been separately incorporated 
for enhancing the astigmatic development, the holographic grating 3 may 
suitably be designed to have a similar function instead of providing a 
separate structure 4c. 
An optical grating assembly 2 according to still another embodiment of the 
invention is illustrated in FIG. 17. The grating assembly 2 is 
substantiallY identical in construction to the assembly of FIG. 10. Thus, 
similar parts are indicated by similar reference numerals. However, the 
grating assembly 2 of FIG. 17 differs from the assembly of FIG. 10 because 
the holographic grating 3 is divided into two sections 3a and 3b along a 
line 30 which extends in the tangential or circumferential direction of 
the disc 7. The dividing line 30 is added in the drawing for the purpose 
of indicating the boundary between the two grating sections 3a and 3b, but 
no such line is seen in an actual grating structure. The two-section 
holographic grating is fabricated following the procedure described 
hereinabove in connection with FIGS. 11A-11F. 
The operation of an optical pickup apparatus which incorporates the grating 
assembly 2 of FIG. 17 is now explained with reference to FIG. 19 together 
with FIG. 18 which illustrates the optical working of the grating 
assembly. 
A laser beam 1 emitted from a lower source 1 impinges on a diffraction 
grating 4a provided in one face 202 of a grating assembly 2 where the beam 
diffracted into a zero-order main beam 9a for reading the data pits on a 
disc 7 as well as sensing focusing error, and a pair of first-order 
sub-beams 10a and 11a for sensing the tracking error. The beams 9a, 10a 
and 11a pass through holographic grating sections 3a and 3b which diffract 
these beams again into zero- and first-order beams. The first-order beams, 
being diffracted through a far grater angle, miss a collimator lens 5. 
Thus, only the zero-order diffracted beams of the incident beams 9a, 10a 
and 11a continue to the collimator lens 5 for being cast into parallel 
rays. The collimated beams are directed to an objective lens 6 which 
functions to focus the incoming beams into three spots 9b, 10b and 11b on 
the data recorded surface of an information disc 7. The three focused 
spots assume the arrangement as illustrated in FIG. 2 with the spots 10a 
and 11a being tangentially displaced away from the center spot 9a. 
The laser beams which form the spots 9a, 10a and 11a on the disc are 
reflected by the data surface of the disc 7 and return along substantially 
the identical paths through the objective lens 6 and the collimating lens 
5 to the grating assembly 2. The returning beams fall on the grating 
sections 3a and 3b of the holographic grating 3 which function to 
independently and separately diffract the transmitted beams into zero- and 
first-order beams. As before, the zero-order diffracted beams are directed 
straight back to the laser source 1. On the other hand, the first-order 
beams 9c, 10c and 11c diffracted by the holographic grating section 3a, 
and the first-order beams 9d, 10d and 11d diffracted by the other 
holographic grating section 3b pass out of the grating assembly 2 through 
a non-grated region in the lower face 202. The holographic grating 
sections 3a and 3b are designed such that the outgoing first-order 
diffracted beams converge on different locations. Accordingly, the 
diffracted beams 9c, 10c and 11c through the grating section 3a fall on a 
six-segment photodetector 8 to form three spots 9e, 10e and 11e, while the 
diffracted beams 9d, 10d and 11d through the other grating section 3b 
impinge on the photodetector 8 to form three different spots 9f, 10f and 
11f. It is noted here that the light spots 9e and 9f derive from the main 
laser beam 9a, and that the light spots 10e-10f and 11e-11f come from the 
sub-beams 10a and 11a, respectively. With the arrangement of the pickup 
apparatus as illustrated in FIGS. 18 and 19, a laser beam from the source 
is distributed into one main beam and a pair of sub-beams by the grating 
assembly, and the three-beam tracking error detection is made possible. At 
the same time, each returning beam reflected by the disc is divided into 
two separate beams by the grating assembly, and the focusing error 
detection similar to the detection achieved by the wedge prism method is 
made possible. 
Forming the light spots on the photodetector 8 under some typical reading 
situations are illustrated in FIGS. 20A-20C. 
In an erroneous reading situation where the information disc 7 is located 
off the focal point of the objective lens 6 on the near side thereof, i.e. 
the disc is positioned within the focal length of the objective 20 lens, 
light spots 9e-11e and 9f-11f are formed on the photodetector in a 
semicircular shape and in an arrangement as illustrated in FIG. 20A. In a 
correct reading situation where the disc 7 is located on the focal point 
of the objective lens 6, pinpoint spots 9e-11e and 9f-11f are projected on 
the photodetector in a pattern as illustrated in FIG. 20B. In another 
erroneous reading situation where the disc 7 is located off the focal 
point of the objective lens on the far side thereof, i.e., the disc is 
positioned outside the focal length of the objective lens, semicircular 
light spots 9e-11e and 9f-11f are formed on the photodetector in an 
arrangement and an orientation as illustrated in FIG. 20C. 
A pit signal RF obtained by reading the pits in the data track of the disc 
7 with the read-out light spots is expressed by the following equation: 
EQU RF=a+b+c+d 
where a, b, c and d represent electrical signals corresponding to the 
amount of light received on the photosensitive segments A, B, C and D, 
respectively. 
A tracking error signal TES provided by the three-beam method is expressed 
as follows: 
EQU TES=e-f 
where e and f represent electrical signals proportional to the amount of 
light projected and received on the photosensitive segments E and F, 
respectively. 
A focus error signal FES generated by the wedge prism type detection is 
expressed as follows: 
EQU FES=(a+d)-(b+c) 
Whenever a tracking error is sensed by a tracking error detector 41, a lens 
actuator or a driver 44 operates in response to the output signal from the 
detector 41 to move the objective lens 6 in a direction and an amount to 
correct the error. Likewise, when a focusing error is sensed by a focusing 
error detector 42, a lens actuator or a driver 43 operates responsive to 
the output signal from the detector 42 to move the objective lens 6 in a 
direction and quantity to correct the focusing error. In this manner, the 
position of the objective lens 6 relative to the disc 7 is always adjusted 
so that the disc surface is kept in the focal plane of the objective lens 
and the reading beam spots are focused right on the data track of the 
disc. Thereby, and reliable read-out of the pits in the data track is 
assured. 
In FIG. 21, there is illustrated a grating assembly according to still 
another embodiment of the invention and suitable for use in an optical 
pickup apparatus of the invention. The grating assembly of FIG. 21 is 
substantially identical in construction to the assembly illustrated in 
FIG. 17, and like parts are designated by like reference numerals. A 
distinct feature of the grating assembly 2 of FIG. 21 not found in the 
grating assembly of FIG. 17 is that a holographic device for relieving the 
influence of astigmatism is provided over the holographic grating sections 
3a and 3b. The holographic grating may be made to have an additional 
function of relieving the influence of the coma. In FIG. 21, the provision 
of the anti-astigmatism holographic device is represented by drawing the 
grating sections 3a and 3b to have a grating configuration or a pattern 
different from the getting information or patter illustrated in FIG. 17. 
The particular holographic device may be fabricated by a photolithographic 
technique which employs two-beam interference (FIG. 11E). As can be seen 
in FIG. 18, the returning light beams are transmitted obliquely through 
the grating assembly 2 toward the photodetector 8, which inevitably 
develops astigmatic distortion in the transmitted light beams. As a 
result, these beams form on the photodetector light spots of oblong 
circles or ellipses instead of regular circles. By providing the 
anti-astigmatism holographic device over the holographic grating sections 
3a and 3b, the development of astigmatic effects on the light beams 
transmitted through the grating assembly 2 is prevented. Thereby, the 
formation of fully round light spots on the photodetector result. 
It is also possible to design the holographic grating to have an image 
magnifying function for enlarging the light spots formed on the 
photodetector. With the enlarged light spots, the photodetector works to 
generate electrical outputs of a greater intensity. 
There is illustrated in FIG. 22 an optical grating assembly 2 according to 
still another embodiment of the invention, which is essentially identical 
in structure to the assembly illustrated in FIG. 17. Thus, like component 
parts are designated by like reference numerals. 
In contrast to the grating assembly of FIG. 17, the grating assembly 2 of 
FIG. 22 has been divided into two holographic grating sections 3a and 3b 
along a line 30 which extends parallel to the fine grooves in the 
diffraction grating 4a on the other face 202 of the assembly 2. When the 
dividing line 30 extends orthogonally with respect to the grated grooves, 
the following problems arise. 
Turning back to FIG. 18, the wavelength of the laser beam 9 emitted by the 
semiconductor source 1 undergoes a very slight change corresponding 
varying temperature. As the wavelength of the emanating laser beam 
changes, so does the return beam reflected by the disc 7. The relation 
expressed by the following equation exists between the wavelength .lambda. 
of the reflected beam and the diffraction angle .THETA. through which the 
reflected beam is diffracted into the first-order beam: 
##EQU1## 
where d denotes the pitch spacing of the holographic grating. 
As will be seen from the equation, any variation in the wavelength of the 
light beam causes a change in the diffraction angle .THETA.. If the 
diffraction angle with respect to the first-order diffracted beam 
undergoes fluctuation responsive to the varying wavelength .lambda., the 
light spots 9e-11e and 9f-11f focused on the photodetector 8 are displaced 
laterally in the direction toward or away from the source 1 as illustrated 
in FIG. 18. As a result, the spots 9e-11e are positioned much closer to 
one another, making it difficult to obtain clear and distinct RF signals, 
tracking error signals and focusing error signals. This is true of the 
light spots 9f-11f. 
In order to make allowance for the possible fluctuation in the wavelength 
of the laser beam caused by varying temperature conditions or different 
semiconductor devices used as the laser source 1, it has been necessary to 
design the optical system of the pickup apparatus such that the light 
spots 10e and 10f are displaced on the photodetector 8, as much as 
possible, away from the light spots 11e and 11f, respectively. To this 
end, the beams 10b and 11b on the disc 7 must be made to lie farthest 
apart from each other, or alternatively additional optical component parts 
should be incorporated. These propositions are disadvantageous because 
they impose stringent requirements on the mounting accuracy of the whole 
apparatus, and also involve higher manufacturing costs and lower operating 
stability. 
The aforementioned problems relating to the fluctuating wavelength of the 
laser beam is effectively overcome by the novel grating assembly 2 of FIG. 
22 in which the holographic grating 3 is divided into the grating sections 
3a and 3b along the line 30 extending parallel to the fine grooves or 
slits of the diffraction grating 4a. FIG. 23, which illustrates the 
optical working of the grating assembly 2 of FIG. 22 is and FIG. 24, which 
illustrates an optical pickup apparatus employing the grating assembly of 
FIG. 22, are referred to next. As the wavelength of the laser beam 9 from 
the source 1 fluctuates, the light spots focused on the photodetector 8 
are displaced laterally in a direction away or toward the laser source 1. 
However, neither the between the light spot 10e and 11e nor is the 
distance between the spots 10f and 11f are diminished. Hence, it is not 
necessary to set the spacing between the light spots 10e and 11e, and the 
spacing between the light spots 10f and 11f broader than usual in 
anticipation of possible shrinking. 
Fluctuations of the spacing between the spots 10e and 10f, and the spacing 
between the spots 11e and 11f in the direction toward or away from the 
laser source 1 are effectively compensated by adjusting the pitch of the 
grating grooves in the grating sections 3a and 3b. 
As has been described in detail hereinabove, the optical pickup apparatus 
of the invention includes the diffraction grating which distributes a 
light beam from a source into a main beam for reading the data pits on the 
information carrier disc, and a pair of sub-beams for sensing tracking 
error of the main reading beam. The diffraction grating ensures a 
generation of stable tracking error signals. The pickup apparatus also 
includes a holographic grating for directing and converging part of the 
light beam reflected by the disc onto the photodetector. The holographic 
grating is integrally formed with the diffraction grating into a unitary 
grating assembly, which eliminates the need for two separate grating 
components. A reduction of the requisite grating components leads to a 
reduction of assignments of delicate operational adjustments. The overall 
result is the provision of an inexpensive optical pickup apparatus which 
possesses an excellent ability to sense the tracking error of the read-out 
beam. 
Further, in the novel grating assembly of the invention, the diffraction 
grating for diffracting the beam from the source into zero- and 
first-order beams, and the holographic grating for orienting part of the 
light beams reflected by the disc toward the photodetector are formed 
integral with each other. This unitary structure makes it possible to 
fabricate both the diffraction grating and the holographic grating 
simultaneously in an economical manner as illustrated in FIG. 11B. The 
unitary grating assembly also makes putting component parts together into 
a pickup apparatus easier and less troublesome. 
Although the present invention has been described and illustrated in 
detail, it is clearly understood that the same is by way of illustration 
and example only and is not to be taken by way of limitation, the spirit 
and scope of the present invention being limited only by the terms of the 
appended claims.