Optical pickup device

An optical pickup device used for optical information processing instruments includes a light source for emitting an outgoing light toward a recording medium, a diffraction grating for separating the outgoing light from the light source into a main beam and at least two sub-beams, an objective lens for independently focusing the main beam and sub-beams separated via the diffraction grating on the recording medium, a hologram formed by areas divided into 2.sup.n+1 for dividing a reflected light having passed through the objective lens after being reflected by the recording medium into a first diffracted beam and a second diffracted beam having different focusing distances from each other to diffract the first and second diffracted beams to one direction of the outgoing light axis of the light source, and a single photodetector having a first light receiving element divided-by-three for accepting the first diffracted beam and a second light receiving element divided-by-three for accepting the second diffracted beam to detect a focus error signal, thereby accurately detecting the focus error signal with only one photodetector without involving variation.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an optical pickup device used for optical 
information processing instruments such as CDROMs and DVDs, and more 
particularly to an optical pickup device having a structure for detecting 
a stable focus error signal free from the wavelength variation and 
positional variation of tracks of an objective lens to therefore provide a 
stable recording signal while involving less positional variation of 
optical parts and less degraded signal characteristics. 
2. Description of the Prior Art 
A hologram head which is employed as an optical head provided by 
simplifying a conventional optical system has been suggested in Japanese 
Patent Laid-open Publication No. sho63-229640. 
In the hologram head as shown in FIG. 1, once a laser beam 111 is radiated 
from a light source 101 being a semiconductor laser, beam 111 transmits 
through a hologram 103 and then is incident to an objective lens 104. At 
this time, beam 111 transmitted through hologram 103 is divided into a 
zero order beam +first order beam and -first order beam. Among these 
beams, the zero order beam is to be solely utilized. 
The beam having passed through objective lens 104 reaches an information 
recording/reproducing plane of a disc 105 to form a focus of light, and 
reflected beam reflected from disc 105 again transmits through objective 
lens 104 to be incident to hologram 103. The -first order diffracted beam 
131 and +first order diffracted beam 132 diffracted by hologram 103 are 
respectively incident to two photodetectors 106 which are arranged in the 
vicinity of light source 101. 
Photodetectors 106 are placed on a photodetector stand 107, of which 
rotation is adjustable. 
Here, the reflected beam having passed through hologram 103 is in the form 
of having two conjugate focuses 171 and 172 that are placed at the front 
and rear sides of light source 101 with respect to the optical axis 
direction. 
In other words, -first order diffracted beam 131 and +first order 
diffracted beam 132 reaching left and right photodetectors 106 are set to 
focus on spots provided further before or after respective photodetectors 
106, which will be described in more detail with reference to FIGS. 2a, 2b 
and 2c. 
When the information recording/reproducing plane of disc 105 is timely 
placed to the focus point of objective lens 104, as shown in FIG. 2b, left 
and right diffracted beams 131 and 132 exactly focus on their own 
positions, respectively. Accordingly, the diameters of diffracted beams 
131 and 132 detected by photodetectors 106 are of the same size. 
Also, when the information recording/reproducing plane, i.e., disc 105, is 
distant from objective lens 104, as shown in FIG. 2a, the diameter of left 
diffracted beam 131 detected by left photodetector 106 is increased in 
size while that of right diffracted beam 132 is decreased. 
Contrarily, when the information recording/reproducing plane, i.e., disc 
105, gets nearer to objective lens 104, the diameter of left diffracted 
beam 131 is decreased in size, and that of right diffracted beam 132 is 
increased. 
Therefore, The focusing state between objective lens 104 and disc 105 can 
be perceived in view of the quantity of light of the diffracted beams 
accepted within light receiving areas 163 to 168 of both left and right 
photodetectors 106. Assuming that such a focus error value is denoted by 
Fe, Fe can be defined by a value which is obtained by subtracting the 
quantity of light accepted by light receiving element 164 from that 
accepted by light receiving element 167. That is, it is written as 
Fe=167-164 where 167 denotes the quantity of light accepted by light 
receiving element 167 and 164 denotes that accepted by light receiving 
element 164. Otherwise, the focus error value Fe can be given as the 
equation that Fe=(163+165+167)-(164+166+168). 
Hologram 103 allows the focus of the diffracted beam to be formed onto any 
other place from that of the zero order beam when diffracting the beams 
radiated from light source 101. For this fact, unnecessary focusing on the 
information recording/reproducing plane is not performed when conducting 
the recording/reproducing operation to nor insert/record unnecessary 
playback signal. 
In connection with the variation of the diffraction angle resulting from 
the wavelength difference of light source 101 in the above-described 
conventional structure, photodetectors 164 and 167 parallel to light 
source 101 in the radiating direction are employed. For this reason, the 
laser beam is moved along with the division direction even in the case of 
producing the wavelength variation, so that the structure hardly induces 
variation in the focus error signal, etc. However, due to this fact, it is 
disadvantageous as below. 
First, because conventional hologram 103 is provided in a manner to have 
two conjugate focuses 171 and 172 on both sides of light source 101, the 
photodetectors are respectively required on both sides of light source 101 
to interpose it between them. 
Consequently, in the conventional structure, two photodetectors 106 are to 
be manufactured to make fixing of them onto photodetector stand 107 
greatly fastidious because of the precisely symmetrical installation on 
the identical places. According to circumstances, it may be formed such 
that a large silicon substrate is perforated to prepare the light source 
in the hole formed. But this way has a problem of significantly 
heightening unit cost of the silicon substrate and, furthermore, requiring 
a new technique (Hybrid facilities) for fixedly installing the light 
source onto the center of the integrated photodetector. 
Second, since conventional hologram 103 has two conjugate focuses 171 and 
172 by -first order diffracted beam 131 and +first order diffracted beam 
132, the optical axis-oriented distance from hologram 103 to reaching both 
photodetectors 106 should be always the same relative to the optical 
axis-oriented distance from hologram 103 to light source 101 so as to set 
two conjugate focuses 171 and 172 onto before and after the optical axis 
direction with respect to light source 101. Thus, it is very difficult to 
change in designing the distance relation among hologram 103, light source 
101 and photodetectors 106. 
On the other hand, Japanese Patent Laid-open Publication No. sho63-13134 
describes another prior art of using the hologram. 
The art is for detecting a focus error by astigmatism. As shown in FIGS. 
3a, 3b and 3c, a focus error signal Fe is detected as follows by means of 
a photodetector 206 divided-by-four, 
EQU Fe=(263+266)-(264+265) 
In this optical system, the beam is placed onto the center of photodetector 
206 as shown in FIG. 3b when the objective lens is centrally arranged. 
However, if the objective lens is moved toward the inner circumference of 
the disc while moving from the center by tracing the tracks, the position 
of the beam is moved to the lower side of the center line of photodetector 
206 as designated in FIG. 3a. Besides, when the objective lens is moved 
toward the outer circumference of the disc, the position of the beam is 
moved to the upper side of the center line of photodetector 206 as 
designated in FIG. 3c. Hence, when the objective lens is placed other than 
the center, the focus error signal involves variation as well as the 
sensitivity is changed. 
SUMMARY OF THE INVENTION 
The present invention is devised to solve the foregoing conventional 
problems. Therefore, it is an object of the present invention to provide 
an optical pickup device for diffracting two diffracted beams having 
mutually different focusing distances toward one direction of a light 
source to permit them to be accepted to a single photodetector, thereby 
being capable of detecting a focus error signal without involving 
variation. 
It is another object of the present invention to provide an optical pickup 
device having a hologram for producing two diffracted beams with 
respectively different focus powers, thereby enabling free change in 
designing within the range of causing no great difference in the relative 
positional relation among the hologram, a light source and a 
photodetector. 
To achieve the above object of the present invention, the hologram of the 
optical pickup device according to the present invention is formed by, 
after four areas are provided by being partitioned into four by means of a 
division line parallel to the train of tracks of a disc which is a 
recording medium and a division line intersecting to be perpendicular to 
the division line, making two pairs of diagonally-opposing areas have the 
same diffraction angle and diffracting focus. In this structure, it is 
characterized in that reflected light reflected from the disc is separated 
into a first diffracted beam and a second diffracted beam to allow them to 
respectively focus on the front and rear sides of a photodetector. 
In addition, the photodetector of the present invention for accepting the 
first diffracted beam and second diffracted beam is constructed by light 
receiving elements divided by two by means of a division line in parallel 
with the direction of the train of pits and then divided by at least three 
in the direction in parallel with the direction of the train of tracks. By 
this structure, even when the objective lens is horizontally moved from 
the center toward the inner circumference or outer circumference of the 
disc, the position of the beam accepted to the light receiving elements of 
the photodetector is moved along the division line to thus detect the 
focus error signal without involving variation and inhibiting the change 
of sensitivity. 
That is, the focus error signal detection carried out by the optical pickup 
device according to the present invention is characterized by inciting no 
deviation in the focus error signal even when the photodetector has a fine 
positional variation or the hologram has a rotational position variation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The preferred embodiments of an optical pickup device according to the 
present invention will be described with reference to accompanying 
drawings. 
In FIG. 4, a light source 1 utilizes a general semiconductor laser. Beams 
11 emitted from light source 1 pass through a diffraction grating 2 for 
tracking detection to be separated into zero order beam and +first order 
beams, which then pass through a hologram 3. 
Even after passing through the hologram, the beams are also separated into 
the zero order beam and +first order beams. Among these, the zero order 
beam is used. Thereafter, beam 11 focuses on an information recording 
plane of a disc 5 by means of an objective lens 4, and a reflected beam 
reflected by the information recording plane again passes through 
objective lens 4 to be incident to hologram 3. 
Here, a first diffracted beam 31 and a second diffracted beam 32 are formed 
by hologram 3, which then reach a photodetector 6 divided-by-eight placed 
adjacently to light source 1. Photodetector 6 consists of eight-divided 
elements 61, 62, 63, 64, 65, 66, 67 and 68, which are illustrated to be 
larger than the actual size in the drawing for facilitating the 
understanding. 
Hologram 3 has two focuses by respective areas which have diffracting 
forces different from each other. 
In other words, as shown in FIGS. 4 and 5, areas 33 and 35 of hologram 3 
form a lattice pattern equivalent to interference strips of a spherical 
wave plane which proceeds light source 1 and two spherical wave planes 
which proceeds the front plane of element 67 of the photodetector 
divided-by-eight. By this lattice pattern, first diffracted beam 31 is 
generated. Similarly, remaining areas 34 and 36 of hologram 3 form a 
lattice pattern equivalent to interference strips of a spherical wave 
plane which proceeds semiconductor laser light source 1 and two spherical 
wave planes which proceed the rear plane of element 64 of the 
photodetector divided-by-eight. This lattice pattern produces second 
diffracted beam 32. These lattice patterns are illustrated to be larger 
than the actual size in FIG. 4 for facilitating the understanding. 
Among the division lines for equally partitioning hologram 3 into areas 33, 
34, 35 and 36, one division line 37 becomes in parallel with the direction 
of the train of tracks of disc 5 while going through the center point of 
hologram 3. Also, another division line 38 becomes in parallel with the 
direction of the train of tracks of disc 5 while going through the center 
point of hologram 3, and is parallel with respect to the direction of 
connecting light source 1 and photodetector 6. Thus, division line 38 
perpendicularly intersects division line 37 along the axis of light. 
Hologram 3 according to the present invention is equally partitioned into a 
plurality of areas in such a manner to be partitioned into four, eight or 
more by the plurality of division lines that go through the center point 
of hologram 3. More specifically, after equally dividing hologram 3 into 
2.sup.+1 -numbered sector areas by first division line 38 in the direction 
parallel to the train of tracks and 2.sup.n -numbered division lines which 
number at least two by including first division line 38, first diffracted 
beam 31 can be diffracted by every other 2.sup.n area alternately arranged 
among 2.sup.n+1 sector areas. Also, second diffracted beam 32 is to be 
diffracted by remaining every other 2.sup.1 area arranged between them. 
This formation can be easily understood with reference to FIG(S) 4 and/or 
6. 
For example, hologram 3 shown in FIG. 6 is obtained by dividing the 
hologram area by eight by means of four division lines including first 
division line 38, i.e., value n is set to 2 (where 2.sup.n+1 =8). Among 
these areas, first diffracted beam 31 or second diffracted beam 32 can be 
generated by partitioning into the oblique-lined areas and blank areas. 
As shown in FIG. 5, the lattice pattern of hologram 3 according to this 
embodiment is provided to form a focus 71 at the front side before first 
diffracted beam 31 diffracted nearer to light source 1 reaches 
photodetector 6 and form a focus 72 at the rear side after second 
diffracted beam 32 distant from light source 1 passes through 
photodetector 6. 
By fabricating hologram 3 to form respective focuses 71 and 72 of first 
diffracted beam 31 and second diffracted beam 32 as shown in FIG. 5, a 
focusing distance of second diffracted beam 32 remotely provided from 
light source 1 becomes naturally lengthened by the great diffraction force 
as compared with the focusing distance of first diffracted beam 31 closely 
placed to light source 1 by the relatively smaller diffraction force. 
Therefore, it is easier to be fabricated when the pitch of the lattice 
pattern of hologram 3 shown in FIG. 5 is relatively widened that the 
contrary case of respective focusing positions of FIG. 5; that is, than 
the case of forming the lattice pattern of hologram 3 as shown in FIG. 7 
in which focus 71 is formed at the rear side after first diffracted beam 
31 diffracted toward the nearer side of light source 1 passes through 
photodetector 6 and focus 72 is formed at the front side before 
remotely-provided second diffracted beam 32 reaches photodetector 6. 
In more detail, as shown in FIG. 7, the present invention may be embodied 
as focus 72 of second diffracted beam 32 is formed on the place before 
reaching photodetector 6 where focus 71 of first diffracted beam 31 passes 
through photodetector 6. However, the case illustrated in FIG. 5 is more 
preferable in forming the hologram pattern. 
Again referring to FIGS. 4 and 5, first diffracted beam 31 diffracted from 
hologram areas 33 and 35 which are of the diagonally-opposite areas 
sectioned by two division lines 37 and 38 forms focus 71 at the front side 
of element 67 of photodetector 6 of FIG. 5, and then diverges again to 
reach over photodetectors 66, 67 and 68. 
Since second diffracted beam 32 diffracted from hologram areas 34 and 36 
forms focus 72 at the rear side of photodetector 11 /2 element 64, it 
reaches over photodetector elements 63, 64 and 65 while converging. 
Photodetector elements 61 and 62 are for detecting 3-beam tracking 
sub-beams. That is, photodetector elements 61 and 62 are for track error 
detecting elements, and photodetector elements 63, 64, 65, 66, 67 and 68 
are used for detecting the focus error. 
Hereinbelow, the focus error signal detection will be described. 
FIG. 8b shows the light receiving states of first diffracted beam 31 and 
second diffracted beam 32 on photodetector 6 in the left and right when 
the focus of the beam is precisely formed on disc 5. Here, two diffracted 
beams are symmetrical with respect to the division line that divides 
photodetector 6 into a first light receiving element consisting of 
3-divided light receiving elements 63, 64 and 65 and a second light 
receiving element consisting of 3-divided light receiving elements 66, 67 
and 68. Besides, the states of first diffracted beam 31 and second 
diffracted beam 32 accepted to photodetector 6 diffracted by hologram 3 
are respectively illustrated when the focus is blurred due to the 
approaching between disc 5 and objective lens 4 as shown in FIG. 8a and 
when the focus is blurred due to the distancing between disc 5 and 
objective lens 4 as shown in FIG. 8c. 
Accordingly, a focus error signal Fe can be given by a value obtained by 
subtracting the quantity of light of light receiving element 67 from the 
quantity of light of light receiving element 64. In other words, it is 
written that Fe=64-67 where 64 denotes the quantity of light of light 
receiving element 64 and 67 denotes the quantity of light of light 
receiving element 67. Otherwise, focus error value Fe can be defined by an 
equation that Fe=(64+66+68)-(63+65+67). 
In this embodiment, it is preferable that hologram 3 is formed to have a 
structure of which rotation is adjustable as consistently distributing the 
quantity of the light of beam to respective photodetector elements so as 
to allow the focus error signal to be a zero cross point when the focus of 
the beam is formed. 
Now, the track error signal detection will be described. 
A track error signal Te is detected via a 3-beam method typically employed 
in the optical pickup field. 
Beam 11 emitted from light source 1 is incident to diffraction grating 2, 
and then diffracted as zero order beam and +first order beams. Here, 
+first order beams are used for track error signal. 
After having passed through hologram 3 and objective lens 4, the .+-. first 
order beams focusing on another place of being interposed with the zero 
order beam on the information recording plane of disc 5 is reflected to be 
diffracted from hologram 3 after passing through objective lens 4, thereby 
reaching elements 61 and 62 of photodetector 6. 
At this time, track error signal Te can be obtained by subtracting 62 from 
61 in such a manner that the focuses of the +first order beam and -first 
order beam on the information recording plane of disc 5 are placed on 
spots of the tracks of respectively forming +90.degree. and -90.degree. 
with respect to the tracing tracks. 
Furthermore, a playback signal Rf can be detected by the total sum of six 
elements for detecting the focus error signal of photodetector 6. That is, 
Rf=63+64+65+66+67+68. 
Meantime, because the focus error detecting optical system using hologram 3 
has an diffraction angle differed with respect to the variation of the 
wavelength of the light source, the position of the diffracted beam 
reaching the photodetector may be deviated due to the differed diffraction 
angle. As the result, the ratio of quantity of light or area ratio over 
respective photodetector elements may be changed to induce a focus 
variation, which, however, can be solved as below in the present 
invention. 
More specifically, the diffracting direction of first diffracted beam 31 
and second diffracted beam 32 of hologram 3 has the diffraction angle 
diffracted in the direction parallel to the train of tracks at one side of 
light source 1, and the element division lines in photodetector 6 are also 
parallel to the train of tracks as the diffracting direction. Due to this 
fact, even if the positions of diffracted beams 31 and 32 are changed, 
they are unavoidably moved along the division lines of photodetector 6. 
Consequently, the ratio of quantity of light or area ratio reaching 
respective elements of photodetector 6 is not changed to cause no problem. 
When the wavelength is changed toward the long wavelength rather than that 
of the reference light as stated above, the pattern of the diffracted 
beams moved to the direction remotely distanced from the light source 
along the division lines of the photodetector is illustrated in FIG. 9. 
Also, if objective lens 4 is deviated from the center of disc 5 while 
tracing the information recording track on disc 5, the positions of the 
diffracted beams reaching respective elements of the photodetector may be 
moved or deviated. 
As the result, a problem of changing the ratio of quantity of light or area 
ratio presented over respective elements of the photodetector to produce 
the focus variation liably occurs, which, however, can be solved in the 
present invention as follows. 
That is, the movement of beam over hologram 3 incited by the deviation of 
objective lens 4 is conducted along division line 38. Accordingly, the 
light-use area of hologram areas 33 and 35 for generating first diffracted 
beam 31 is not changed even after the movement. The light-use area of 
hologram areas 34 and 36 for generating second diffracted beam 32 in the 
same way is unchanged, either. The positional deviation of respective 
diffracted beams reaching respective elements of photodetector 6 is 
conducted along the division line. Therefore, since the ratio of quantity 
of light or area ratio reaching respective elements of photodetector 6 is 
unchanged to cause no trouble. The foregoing movement of the diffracted 
beams over photodetector 6 resulting from the changed position of 
objective lens 4 is illustrated in FIG. 10. 
On the other hand, FIG. 11 shows another embodiment of the present 
invention, in which a hologram head module including the hologram is 
formed to be installed to the optical pickup device according to the 
present invention. 
Referring to FIG. 11 showing one embodiment of a hologram head module 8, 
there is provided a light source 1 for emitting an outgoing light toward a 
recording medium 5, and a diffraction grating 2 for dividing the outgoing 
light emitted from light source 1 into a main beam and at least two 
sub-beams. In addition to these, a hologram 3 is formed such that, after 
being equally divided into 2.sup.n+1 sector areas by a first division line 
38 parallel to the train of tracks for dividing the reflected light 
reflected from recording medium 5 from an outgoing light axis and 2.sup.n 
division lines numbering at least two by including first division line 38, 
first diffracted beam 31 is diffracted by every other 2.sup.n area 
alternately arranged among the 2.sup.n+1 sector areas and second 
diffracted beam 32 is diffracted by remaining every other 2.sup.n area. 
Hologram head module 8 further has a photodetector 6 including first light 
receiving elements 66, 67 and 68 divided by at least three in the 
direction of the train of tracks for accepting first diffracted beam 31 
diffracted from hologram 3 and second light receiving elements 63, 64 and 
65 divided by at least three in the direction of the train of tracks for 
receiving second diffracted beam 32. 
An objective lens 4 for focusing the main beam and sub-beams separated by 
diffraction grating 2 onto recording medium 5 independently from one 
another is installed between hologram head module 8 and recording medium 
5, of which construction is the same as that shown in FIG. 5. 
As described above in detail, the optical pickup device according to the 
present invention employs only one small photodetector to be advantageous 
of lowering manufacturing cost, unrestraining the designing and involving 
less change in signal characteristics free from the liable wavelength 
variation or deviated tracking position of the objective lens, while being 
easy to be adjusted and presenting stable quality. 
While the present invention has been particularly shown and described with 
reference to particular embodiment thereof, it will be understood by those 
skilled in the art that various changes in form and details may be 
effected therein without departing from the spirit and scope of the 
invention as defined by the appended claims.