Not more than one compound holographic optical element is provided on an optical path starting from a semiconductor laser chip and arriving at a photo detector via an optical disk. A hologram pattern of this compound holographic optical element is the superposition of a first hologram pattern, converging a laser beam emitted from the semiconductor laser chip onto the optical disk, and a second hologram pattern, diffracting the laser beam reflected at the optical disk toward the photo detector and changing it into a focusing beam. This single holographic optical element not only realizes the convergence of the laser beam onto the optical disk and/or the photo detector and the differentiation of going and returning optical paths but reduces the size and production cost of the optical pick-up apparatus.

BACKGROUND OF THE INVENTION 
1.Field of The Invention 
The present invention relates to an optical pick-up apparatus, which is 
used to record information into an optical disk and to reproduce the 
recorded information from the same. 
2. Description of The Prior Art 
An optical pick-up apparatus associated with an optical disk device 
utilizes a laser beam for record and reproduction of information. In order 
to realize the reduction of size and weight in such an optical pick-up 
apparatus, a plurality of holographic optical elements have been 
conventionally used. And, there is known a method of reflecting a laser 
beam several times in a light guide. For instance, the Japanese Unexamined 
Patent Applications Nos. 146444/1987 and 155529/1989 disclose a long and 
thin light guide made of transparent substance. In this light guide, a 
going laser beam is reflected several times and reaches an optical disk 
passing through a holographic optical element. The optical disk memorizes 
information on the surface thereof in the form of unit of spots or 
depressions. A returning laser beam reflected at the recording surface of 
the optical disk is again reflected several times in the light guide but 
is guided via a different path to an photo detector. The Japanese 
Unexamined Patent Application No. 20737/1988 discloses two holographic 
optical elements. One converges a going laser beam onto an optical disk 
and the other converges a returning laser beam reflected at the optical 
disk to a photo detector. 
Furthermore, the Japanese Unexamined Patent Applications Nos. 81335/1990 
and 220145/1989 disclose holographic optical elements integrally assembled 
with objective lenses used for causing diffraction in a returning beam 
after being reflected at an optical disk. 
Hereinafter, above-introduced conventional optical pick-up apparatus 
equipped with holographic optical elements will be explained in more 
detail with reference to FIGS. 22 to 25. FIG. 22 shows an optical pick-up 
apparatus utilizing a plurality of reflections of a laser beam, as 
represented by the Japanese Unexamined Patent Applications Nos. 
146444/1987 and 155529/1989. FIG. 23 shows an optical pick-up apparatus 
including a pair of holographic optical elements, one holographic optical 
element converging a going laser beam and the other holographic optical 
element causing diffraction of a returning laser beam, which is similar to 
that disclosed in the Japanese Unexamined Patent Application No. 
20737/1988. FIG. 24 shows an optical pick-up apparatus using an objective 
lens on the spherical surface of which a holographic optical element is 
integrally or directly formed to cause diffraction of a laser beam, the 
same type as that disclosed by the Japanese Unexamined Patent Application 
No. 81335/1990. FIG. 25 shows an optical pick-up apparatus using a complex 
objective lens whose body is split into two by a plane normal to an axis 
thereof. A holographic optical element for diffraction is sandwiched 
therebetween, as is disclosed in the Japanese Unexamined Patent 
Application No. 220145/1989. 
In FIG. 22, a reference numeral 501 represents a laser diode emitting a 
laser beam. A reference numeral 504 represents a holographic optical 
element converging the laser beam emitted from the laser diode 501 onto an 
optical disk 503 whose surface are formed with the unit of spots 
memorizing information being sensed by the laser beam. A reference numeral 
502 represents a light guide which is made of a transparent substance such 
as fused silica and causes a laser beam to reflect repeatedly at inside, 
upper and lower, surfaces thereof. A reference numeral 505 represents a 
holographic optical element of reflection type which reflects a returning 
laser beam having once reached and been reflected at the surface of the 
optical disk 503 and also diffracts this returning laser beam toward a 
photo detector 506. 
In FIG. 23, a reference numeral 507 represents a holographic optical 
element of transmission type which transmits a returning laser beam having 
been reflected at the surface of the optical disk 503 and also diffracts 
this returning laser beam toward the photo detector 506. The laser diode 
501, optical disk 503, holographic optical element 504, and photo detector 
506 are substantially the same as those explained with reference to FIG. 
18. 
In FIG. 24, a reference numeral 508 represents an objective lens converging 
a laser beam emitted from the laser diode 501 onto the optical disk 503. A 
reference numeral 509 represents a holographic optical element of 
transmission type which transmits a returning laser beam having been 
reflected at the surface of the optical disk 503 and also diffracts this 
returning laser beam toward the photo detector 506. The laser diode 501, 
the optical disk 503, and the photo detector 506 are substantially the 
same as those explained with reference to FIG. 22. 
In FIG. 25, a reference numeral 510 represents a complex objective lens 
whose body is split by a plane normal to an axis thereof into two, upper 
and lower, half bodies. This complex objective lens 510 converges a laser 
beam emitted from the laser diode 501 onto the surface of the optical disk 
503. A reference numeral 511 represents a holographic optical element of 
transmission type which transmits a returning laser beam having been 
reflected at the surface of the optical disk 503 and also diffracts this 
returning laser beam toward the photo detector 506. This holographic 
optical element 511 is sandwiched by and integrally fabricated with the 
paired half bodies of the complex objective lens 510. The laser diode 501, 
the optical disk 503, and the photo detector 506 are substantially the 
same as those explained with reference to FIG. 22. 
Operations on above introduced conventional optical pick-up apparatus will 
be explained below. 
In FIG. 22, a laser beam is emitted from the laser diode 501 and, then, 
reflected plural times at inside, upper and lower, surfaces of the light 
guide 502 so as to reach the holographic optical element 504. The 
holographic optical element 504 then converges the laser beam thus guided 
through the light guide 502 onto the surface of the optical disk 503. The 
laser beam is reflected at the surface, i.e. a recording surface, of the 
optical disk 503 and returns as a beam including information read out from 
the optical disk 502. The returning beam passes through the holographic 
optical element 504 again and, in turn, reaches the holographic optical 
element 505 of reflection type. This holographic optical element 505 not 
only reflects the returning laser beam but diffracts it toward the photo 
detector 508. The photo detector 508 receives the returning laser beam and 
detects focusing error and tracking error, as well as the read-out 
information. 
The optical pick-up apparatus shown in FIGS. 23, 24, and 25 operate in the 
same manner as that shown in FIG. 22. A laser beam emitted from the laser 
diode 501 is converged onto the recording surface of the optical disk 503 
passing through the holographic optical element 504, the objective lens 
508, or the split-type complex objective lens 510, respectively. After 
having been reflected, the returning beam comprising information read out 
from the optical disk 503 passes through the holographic optical elements 
507,509, or 511 respectively and is diffracted toward the photo detector 
506. The photo detector 506 receives the returning laser beam and detects 
focusing error and tracking error, as well as the read-out information. 
These prior art constructions of the optical pick-up apparatus are, 
however, disadvantageous in complicateness of adjusting positional 
relationship between individual optical components. One reason of 
requiring complicated positional adjustment of the optical components is 
that the conversing arrangement and the diffracting arrangement are 
independent from each other. In more detail, the conversing arrangement 
conversing a laser beam is constituted by the holographic optical element 
504, the objective lens 508, or the split-type objective lens 510. On the 
other hand, the diffracting arrangement diffracting the laser beam is 
constituted by the reflection-type holographic optical element 505 or the 
transmission-type holographic optical element 507, 509, 511. These 
conversing arrangement and the diffracting arrangement are independently 
mounted on the light guide 502 or installed into a casing. Therefore, it 
was inevitable to take a long time in an installation or fabrication of 
the pick-up apparatus because of not only positional adjustment of 
individual optical components but mutual adjustment of positional 
relationship between the conversing construction and the diffracting 
construction. This complicateness in the manufacturing process results in 
increase of production cost. 
Furthermore, in case of the optical pick-up apparatus shown in FIG. 24, the 
constructional requirement of forming the transmission-type holographic 
optical element 509 directly on the objective lens 508 further increases 
the complicateness. This will be easily understood from the spherical 
surface of the objective lens 508 which makes the formation of the 
transmission-type holographic optical element 509 thereon difficult. Still 
further, in case of the optical pick-up apparatus shown in FIG. 25, the 
structure of sandwiching the transmission-type holographic optical lens 
511 between the split half bodies of the complex objective lenses 510 is 
not only time-consuming in its assembling but tends to cause unacceptable 
deterioration in lens property. 
SUMMARY OF THE INVENTION 
Accordingly, the present invention has a purpose, in view of 
above-described problems or disadvantages encountered in the prior art, to 
provide an optical pick-up apparatus capable of realizing both the 
conversion of a laser beam to the optical disk and/or the photo detector 
and the differentiation of going and returning laser beam paths, by use of 
only one holographic optical element, thereby providing cheaper optical 
pick-up apparatus. 
In order to accomplish above purposes, the present invention provides not 
more than one compound holographic optical element on an optical path 
starting from a semiconductor laser chip and arriving at a photo detector 
via an optical disk. A hologram pattern of this compound holographic 
optical element is the superposition of a first hologram pattern, 
converging a laser beam emitted from the semiconductor laser chip onto the 
optical disk, and a second hologram pattern, diffracting the laser beam 
reflected at the optical disk toward the photo detector and changing it 
into a focusing beam. This single holographic optical element not only 
realizes the convergence of the laser beam onto the optical disk and/or 
the photo detector and the differentiation of going and returning optical 
paths but reduces the size and production cost of the optical pick-up 
apparatus. 
Furthermore, another aspect of the present invention provides a single 
holographic optical element having at least one hologram pattern. 
The above and other objects, features and advantages of the present 
invention will become more apparent from the following detailed 
description which is to be read in conjunction with the accompanying 
drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Hereinafter, with reference to accompanying drawings, preferred embodiments 
of the present invention will be explained in detail. 
First Embodiment 
A first embodiment of the present invention will be explained with 
reference to FIGS. 1(a) through 2 below. FIG. 1(a) is a schematic view 
Showing an optical pick-up apparatus in accordance with a first embodiment 
of the present invention. FIG. 1(b) is a view showing a hologram pattern 
of the first embodiment, which is constituted by superposing a first 
hologram pattern and a second hologram pattern. FIG. 1(c) is a view solely 
showing the first hologram pattern, and FIG. 1(d) is a view solely showing 
the second hologram pattern. 
A going optical path, starting from the semiconductor laser serving as a 
light emitting element and arriving at the optical disk, will be explained 
first of all with reference to FIGS. 1(a) through 1(d). In FIG. 1(a), a 
transparent glass substance 1 gas a bottom surface la, at a center of 
which a semiconductor laser chip 2 is mounted securely. This semiconductor 
laser chip 2 emits a laser beam 3, which diffuses or diverges in the glass 
substance 1 and reaches a compound holographic optical element 4 formed on 
a top surface 1b of the glass substance 1. The compound holographic 
optical element 4 is depicted two hologram patterns superposed as shown in 
FIG. 1(b). One hologram pattern has a concentric circles pattern, in which 
a center of these concentric circles positions at a center of the hologram 
pattern and a pitch between adjacent two circles becomes small as it goes 
outside as shown in FIG. 1(c). This hologram pattern serves as a first 
hologram pattern 4a capable of focusing the diffused laser beam 3 onto an 
information recording layer 7a of an optical disk 7 as a spot 8. Namely, 
the laser beam 3 emitted from the semiconductor laser chip 2 passes 
through the glass substance 1, diffusing symmetrically about an axis of 
the glass substance 1 and, in turn, passes through the compound 
holographic optical element 4 disposed normal to the axis of the glass 
substance 1. Then, the laser means 3 is converged along the axis of the 
glass substance 1 onto the recording layer 7a of the optical disk 7. 
The other hologram pattern has eccentric circles pattern, whose center is 
offset left in the drawing, which is different from that of the 
above-described first hologram pattern. This hologram pattern serves as a 
second hologram pattern 4b capable of converting the laser beam, being 
reflected at the recording layer 7a of the optical disk 7 and returning 
the same optical path 6 as the going laser beam, into a focusing beam 10, 
and also capable of diffracting this focusing beam 10 toward a quadrant 
photo detector 9 and conversing it onto the same. This second hologram 
pattern 4b is capable of causing astigmatic aberration as well as 
conversing the focusing beam 10. In other words, this second hologram 
pattern 4b allows the photo detector 9 to detect focusing error on the 
basis of the received beam 10. This is well known to those skilled in the 
art as the astigmatic method. 
The compound holographic optical element 4 whose hologram pattern is the 
superposition of these first and second hologram patterns 4a and 4b, can 
be fabricated through various ways such as printing and the 2P (Photo 
Polymerization) method. This 2P method utilizes a resin hardenable through 
irradiation of ultraviolet ray to transfer the matrix configuration 
thereon. 
FIG. 2 is a view illustrating change of the focusing beam 10 irradiated 
onto and sensed by the photo detector 9, the focusing beam 10 passing 
through the second hologram pattern 4b. The second hologram pattern 4b is 
designed so as to satisfy the requirement of a focusing error signal (F.E) 
described below. 
The quadrant photo sensor 9 is divided into four segments 9a, 9b, 9c, and 
9d. If assumed that I(9a), I(9b), I(9c), and I(9d) represent output 
currents detected by these four segments 9a, 9b, 9c, and 9d, respectively, 
the focusing error signal (F.E.) is defined by the following equation (1). 
EQU F.E.={I(9a)+I(9d)}-{I(9b)+I(9c)} (1) 
A differential amplifier 16a and two operational amplifiers 16b, 16c 
constitute the signal processing circuit associated with the photo 
detector 9, so that signals from the photo detector 9 are processed in 
accordance with above equation (1). 
When the spot 8 accurately focuses on the information recording layer 7a of 
the optical disk 7, the irradiation shape of the photo detector 9 becomes 
a circle 5 in FIG. 2. In this case, above equation (1) is rewritten into 
the following equation (2). 
EQU F.E.=0 (2) 
The differential amplifier 16a, serving as an output component of the 
signal processing circuit for the photo detector 9, generates an output of 
"0" in this case. 
Next, if the optical disk 7 is positioned close to the top surface 1b of 
the glass substance 1 compared with above just focused position, the 
irradiation shape of the photo detector 9 becomes an ellipse 5a in FIG. 2. 
In this case, above equation (1) is defined by the following equation (3). 
EQU F.E.&gt;0 (3) 
On the contrary, if the optical disk 7 is positioned far from the top 
surface 1b of the glass substance 1 compared with above just focused 
position, the irradiation shape of the photo detector 9 becomes an ellipse 
5b in FIG. 2. In this case, above equation (1) is defined by the following 
equation (4). 
EQU F.E.&lt;0 (4) 
As apparent from the foregoing description, the present invention requires 
no more than one holographic optical element, i.e. the compound 
holographic optical element 4, by which the present invention enables the 
optical pick-up apparatus to converge a laser beam onto the optical disk 
and/or the photo detector, differentiate the going and returning optical 
paths, and detecting the focusing error. Therefore, fabrication of the 
optical pick-up apparatus will be widely simplified compared with the 
prior art optical pick-up apparatus because positional adjustment of a 
plurality of holographic optical elements is no more required. As no 
additional holographic element is necessary, the optical path can be 
further shortened. This will be helpful to reduce the size of the optical 
pick-up apparatus. 
Second Embodiment 
Hereinafter, a second embodiment of the present invention will be explained 
with reference to FIGS. 3(a) through 8(c). FIG. 3(a) is a schematic view 
showing an optical pick-up apparatus in accordance with the second 
embodiment of the present invention. In FIG. 3(a), first photo detector 11 
and second photo detector 12 are integrally formed on an upper surface of 
a sensor substrate 13, so that their light receiving (or photo sensing) 
planes face upward. At an opposite side of the upper surface of the sensor 
substrate 13 there is provided a semiconductor laser chip 14. This 
semiconductor laser chip 14 is parallel to the upper surface of the sensor 
substrate 13. An axis of the laser beam emitted from a light emitting 
surface of the semiconductor laser chip 14 is therefore aligned parallel 
to the upper surface of the sensor substrate 13. Adjacent to the 
semiconductor laser chip 14 on the upper surface of the sensor substrate 
13, a reflection prism 15 is located. A positional relationship between 
this reflection prism 15 and the semiconductor laser chip 14 is adjusted 
in such a manner that the reflection surface of the reflection prism 15 
confronts with the light emitting surface of the semiconductor laser chip 
14 so that a laser beam reflected at the reflection surface of the 
reflection prism 15 goes upward. 
FIG. 3(b) is an enlarged view showing the semiconductor laser chip 14 and 
the reflection prism 15 shown in FIG. 3(a). The reflection prism 15 has a 
cross section of a trapezoidal shape. The reflection surface 15a of the 
reflection prism 15 is coated by a semi-transmission type film which 
transmits a part of the laser beam emitted from the semiconductor laser 
chip 14 into an inside of the reflection prism 15 without being reflected 
by the reflection surface 15a. There is formed a monitor sensor 49 on the 
upper surface of the sensor substrate 13 so as to face the bottom of the 
reflection prism 15. This monitor sensor 49 receives the laser beam 
transmits the inside of the reflection prism 15 and, then, detects change 
of light quantity of the semiconductor laser chip 14 and feeds the 
detected result back to a control circuit. 
This arrangement of the monitor sensor 49 with associated reflection prism 
15 is unique and advantageous compared with the conventional ones. 
Because, this kind of monitor sensor has been conventionally located 
behind the semiconductor laser chip 14, in order to sense a laser beam 
emitted from a rear surface 14a of the semiconductor laser chip 14. The 
behind side of the semiconductor laser chip 14 is, however, normally 
located other photo detectors, e.g. the first and second photo detectors 
11 and 12. Therefore, nevertheless its intention, the laser beam emitted 
toward the monitor sensor tends to cause stray light to these other photo 
detectors. 
On the contrary, the present invention enables the reflection prism 15 to 
partly transmit the laser beam toward the monitor sensor 49 disposed 
beneath the reflection prism 15. In other words, the present invention no 
longer emits the laser means from the rear surface 14a of the 
semiconductor laser chip 14 and therefore no stray light disturb other 
photo detectors located behind the semiconductor laser chip 14. 
The laser beam, having been emitted from the semiconductor laser chip 14, 
is partly reflected at the reflection surface 15a of the reflection prism 
15 toward a light guide 18. The light guide 18 is spaced from the 
semiconductor laser chip 14 and photo detectors 11, 12. The light guide 18 
has an incident window 17 on its second, i.e. lower, surface 18b. The 
laser beam, passing through this incident window 17, becomes a laser beam 
19 obliquely transmitting inside the light guide 18 with a predetermined 
incident angle. Although the setting of this incident angle has to be 
accurately carried out, the present invention can ensure the accurate 
setting because of the previously described parallel mounting structure of 
the semiconductor laser chip 14 with respect to the upper surface of the 
sensor substrate 13. This parallel mounting structure is also advantageous 
in facilitating wiring arrangement and heat radiation. 
The light guide 18 has a first, i.e. an upper, surface 18a, disposed in 
parallel with the second surface 18a. The first surface 18a confronts with 
an optical disk 26 located above the light guide 18. On the first surface 
18a there is provided a first reflection portion 21, which reflects the 
laser beam 19 and changes it into the laser beam 20 proceeding to the 
second surface 18b. On the second surface 18b there is provided a second 
reflection portion 23, which reflects the laser beam 20 and changes it 
into the laser beam 22 proceeding to the first surface 18a. The laser 
beam, entered from the incident window 17, is reflected two times in the 
light guide 18 by the first and second reflection portions 21, 23 and, in 
turn, reaches a compound holographic optical element 24 formed in the 
vicinity of the first reflection portion 21 on the first surface 18a. 
Although the laser beam 22 enters the compound holographic optical element 
24 with the predetermined incident angle, the light quantity distributions 
of this laser beam 22 is normally elliptic formation. To correct the light 
quantity distributions of the laser beam 22 from above elliptic formation 
to circular formation, it will be best to align a minor axis of above 
elliptic formation of the light quantity distributions on a plane 
including an optical axis of the laser beam 22 in the optical path 
starting from the semiconductor laser chip 14 and arriving at the compound 
holographic optical element 24. As the laser beam normally has a 
polarization surface in a minor axis direction of its elliptic irradiation 
beam, it will be preferable to mount the semiconductor laser chip 14 to 
coincide the optical axis plane and the polarization plane. 
FIGS. 4(a) to 4(c) are views illustrating the compound holographic optical 
element 24. This compound holographic optical element 24, shown in FIG. 
4(a), is made by superposing a first hologram pattern 24a shown in FIG. 
4(b) and a second hologram pattern 24b shown in FIG. 4(c). The laser beam 
22, entered into the compound holographic optical element 24 with the 
predetermined incident angle, is converted into a focusing beam 25 
therethrough. Subsequently, this focusing beam 25 converges onto an 
information recording surface 26a of an optical disk 26 as a spot 27. 
Next, a returning path from the optical disk 26 will be explained. A laser 
beam, having been reflected at the information recording surface 26a of 
the optical disk 26, proceeds as a diffusing or diverging beam along the 
same path as the focusing beam 25 but in an opposite direction and reaches 
the compound holographic optical element 24. The second hologram pattern 
24b of the compound holographic optical element 24 converts thus arriving 
laser beam into a diffracted focusing beam 28. The light guide 18 has a 
semi-transmission type window 29 on the second surface 18b thereof. This 
semi-transmission type window 29 transmits approximately 50% of the 
diffracted focusing beam 28 and reflects the remainder, i.e. approximately 
50%. With this function of the semi-transmission type window 29, the 
diffracted focusing beam 28 is split into two. One is a semi-transmission 
beam 30 reaching the first photo detector 11, and the other is a 
reflection beam 31 proceeding to the first surface 18a in the light guide 
18. 
On the first surface 18a there is provided a reflection film 33, which 
reflects the reflection beam 31 and changes it into the reflection beam 32 
proceeding to the second surface 18b. On the second surface 18b there is 
provided a transmission type window 34, which guides all the reflection 
beam 32 to the second photo detector 12. The diffracted focusing beam 28 
has its focal point 35 which is designed to position on an optical path 
between the semi-transmission window 29 and the second photo detector 12. 
Inputting/outputting various signals into/from the sensor substrate 13 is 
carried out through a lead frame 36. A reference numeral 37 represents a 
package which is made of non-conductive material such as resin and 
ceramics. A closed space surrounded or sealed by the light guide 18 and 
the package 37 is normally filled with inert gas such as nitrogen gas. It 
is also possible to fill this closed space with transparent resin etc. 
Next, with reference to FIG. 5, constitutions of the first and second photo 
detectors 11, 12 and their signal processing will be explained below. The 
first photo detector 11 is divided into four, 11a, 11b, 11c, and 11d, 
segments. In the same manner, the second photo detector 12 is divided into 
four, 12a, 12b, 12c, and led, segments. Here, it is assumed that I(11a), 
I(11b), I(11c), I(11d) and I(12a), I(12b), I(12c), I(12d) represent output 
currents detected by these four segments 11a, 11b, 11c, 11d, and 12a, 12b, 
12c, 12d, respectively. As can be understood from the circuit diagram of 
FIG. 5, these output currents are processed through the signal processing 
circuit consisting of numerous operational and differential amplifiers 
16d, 16e, 16f, 16g, 16h, 16i, 16j, 16k, 16l, 16m, 16n, 16p, 16q, 16r, and 
16s. This signal processing circuit is designed to fulfill the following 
equations (5) , (6), and (7) so as to obtain the focusing error signal 
(F.E.) , tracking error signal (T.E.), and recording signal (R. F.). 
EQU F.E.=[{I(11a)+I(11d)}-{I(11b)+I(11c)}]-[{I(12a)+I(12d)}-{I(12b)+I(12c)}](5) 
EQU T.E.=[{I(11a)+I(11b)}-{I(11c)+I(11d)}]-[{I(12a)+I(12b)}-{I(12c)+I(12d)}](6) 
EQU R.F.=[{I(11a)+I(11b)}+{I(11c)+I(11d)}]+[{I(12a)+I(12b)}+{I(12c)+I(12d)}](7) 
When the spot 27 accurately focuses on the information recording layer 26a 
of the optical disk 26, the irradiation shapes of the first and second 
photo detectors 11, 12 become circles 39a, 40a in FIG. 5. In this case, 
above equation (5) is rewritten into the following equation (8). 
EQU F.E.=0 (8) 
The differential amplifier 16g, serving as an focusing error signal output 
component in the signal processing circuit, generates an output of "0 " in 
this case. 
Next, if the optical disk 26 is positioned close to the first surface 18a 
of the light guide 18 compared with above just focused position, the 
irradiation shapes of the first and second photo detectors 11, 12 become 
circles 39c, 40c in FIG. 5. In this case, above equation (5) is defined by 
the following equation (9). 
EQU F.E.&gt;0 (9) 
On the contrary, if the optical disk 26 is positioned far from the first 
surface 18a of the light guide 18 compared with above just focused 
position, the irradiation shapes of the first and second photo detectors 
11 and 12 become circles 39b, 40b in FIG. 5. In this case, above equation 
(5) is defined by the following equation (10). 
EQU F.E.&lt;0 (10) 
With above arrangement, the focusing error detection can be carried out. 
Such a method of detecting the focusing error is known as the 
spot-size-detection method. On the other hand, the tracking error can be 
detected by the push-pull method. 
Compared with the conventional &stigmatic method of detecting the focusing 
error, this embodiment provides a very simplified construction because no 
complicated patten normally required for generating the astigmatic 
aberration is no more required in this embodiment. The role of the second 
hologram pattern 24b of the compound holographic optical element 24 is 
only converging the beam; therefore the second hologram pattern 24b can be 
simplified. All thing necessary to effect the advantage of this embodiment 
is to design the focal point 35 of the diffracted focusing beam 28 such 
that it resides on the optical path between the semi-transmission type 
window 29 and the second photo detector 12. 
FIGS. 6(a), 6(b), 7(a) and 7(b) show unique constitutions which can more 
simply realize a focusing error detection compared with the 
spot-size-detection method. In FIG. 8(a), a layer of a semi-transmission 
type film 48 is coated directly on the second surface 18b of the light 
guide 18. On the layer of the semi-transmission type film 48, both a 
diffusion film 41 and a light shield film 42 are coated. A partly opened 
space on the layer of the semi-transmission type film 48, uncovered by the 
diffusion film 41 and light shield film 42, is a semi-transmission window 
29. The diffusion film 41 has a ring shape, as shown in FIG. 6(b), so as 
to surround the semi-transmission window 29. If assuming that inner 
diameter of the diffusion film 41 is d and an outer diameter of the 
diffusion film 41 is D, the relationship between the diffracted focusing 
beam 28 and the semi-transmission type film 48 is determined in the 
following equation (11). 
EQU d&lt;H&lt;D (11) 
wherein, H represents a diameter of the diffracted focusing beam 28 on the 
semi-transmission type film 48. 
The semi-transmission beam residing in a range from d to H is diffused by 
the diffusion film 41 so as not give adverse effect to the reflection beam 
31. In this purpose, the diffusion film 41 can be replaced by an 
absorption film which is capable of absorbing the laser beam. The 
semi-transmission beam 30, having passed through the window 29 of a 
diameter d, reaches a first photo detector 
FIG. 7(a) shows a reflection beam 32 entering a second photo detector 47. A 
light shield film 44 is coated directly on the second surface 18b of the 
light guide 18, so as to form a transmission window 43 having a diameter 
smaller than that of the reflection beam 32. It will be preferable that 
this light shield film 44 is continuously coated together with the light 
shield film 42 of FIG. 6(a). FIG. 7(b) shows largeness of the diameter of 
the reflection beam 32 in contrast with that of the transmission window 
43. Only a transmission beam 45, having passed the transmission window 43, 
reaches a second photo detector 47. These first and second photo detectors 
and 47 correspond to the first and second photo detectors 11 and 12 shown 
in FIG. 3(a); therefore positions at which these photo detectors 46, 47 
are located are substantially the same as those of the detectors 11, 12. 
In the case where such a focusing error detecting method is adopted, the 
focal point 35 of the diffracted focusing beam 28 needs to be designed to 
reside on an optical path between the semi-transmission window 29 and the 
transmission window 43. The first and second photo detectors 46 and 47 are 
not necessary to be divided into several segments in the same manner as 
the first and second photo detectors 11 and 12 of FIG. 5. If the first and 
second photo detectors 48 and 47 have wide light receiving areas 
sufficient for receiving the semi-transmission beam 30 and the 
transmission beam 45, an output difference between these first and second 
photo detectors 46, 47 can be used as the focusing error signal. FIGS. 
8(a), 8(b), and 8(c) show an output signal of the first photo detector 46, 
an output signal of the second photo detector 47, and focusing error 
signal obtained by subtracting the output of the second photo detector 47 
from the output of the first photo detector 46, respectively. 
As multi-division photo detectors, shown in FIG. 5, are not required in 
this focusing error detecting system, fine adjustment between the position 
of the first 46 and/or second 47 photo detectors and the irradiation shape 
of the laser beam is no more required. Therefore, this focusing error 
detecting system is superior in productivity and durability. 
By the way, in any of the constructions disclosed in FIG. 3(a), 6(a), and 
7(a), all the second surface 18b of the light guide 18 except the incident 
window 17, semi-transmission window 29, transmission window 34, and 
transmission window 43, can be coated by the same light shielding film 42. 
This coating can prevent various stray lights generated in the light guide 
1B from adversely effecting photo detectors, meanwhile an S/N ratio of the 
signal can be increased. Instead of this light shield film 42, the same 
material as the second reflection portion 23 can be used as a coating 
material. Furthermore, it is needless to say that the structures of FIGS. 
6(a), 6(b), 7(a), and 7(b) are not limited to this embodiment and, 
therefore, are applied to any other embodiments disclosed previously or 
later. 
Third Embodiment 
Hereinafter, a third embodiment of the present invention will be explained 
with reference to FIGS. 9(a) through 12. FIG. 9(a) is a plane view of an 
optical pick-up apparatus in accordance with the third embodiment of the 
present invention, and FIG. 9(b) is a sectional view showing the optical 
pick-up apparatus in accordance with the third embodiment, taken along a 
line X--X of FIG. 9(a). 
First of all a going optical path, starting from a semiconductor laser 
serving as a light emitting element and arriving at an optical disk, will 
be explained below. In FIG. 9(b), a semiconductor laser chip 102 is 
mounted in parallel with and on a sensor substrate 101. A laser beam 103, 
emitted horizontally from the semiconductor laser chip 102 reaches a 
reflection prism 104 mounted on the sensor substrate 101. The constitution 
of this reflection prism 104 and its vicinity is the same as the 
reflection prism 15 of the second embodiment and, therefore, will no more 
be explained in detail. This reflection prism 104, being trapezoidal 
shape, has a reflection surface confronting with the light emitting 
surface of the semiconductor laser chip 102. The laser beam, having 
reached the reflection prism 104, is reflected at the reflection surface 
and enters as a diffusing or diverging beam 107 into the inside of a 
transparent light guide 105 through an incident window 106 formed on a 
second surface 105b thereof. The light guide 105 is spaced from the 
semiconductor laser chip 104 and photo detectors 116, 122 later described. 
The light guide 105 has a first surface 105a, disposed in parallel with 
the second surface 105b. On the first surface 105a, there is provided a 
compound holographic optical element 108. The compound holographic optical 
element 108 has a compound hologram pattern shown in FIG. 10(a), which is 
the superposition of two hologram patterns shown in FIGS. 10 (b) and 
10(c). One hologram pattern, shown in FIG. 10(b), has a concentric circles 
pattern, in which a center of these concentric circles positions at a 
center of the hologram pattern and a pitch between adjacent two circles 
becomes small as it goes outside as shown in the drawing. This hologram 
pattern serves as a first hologram pattern 108a capable of focusing the 
diffused laser beam 107 onto an information recording layer 109a of an 
optical disk 109 as a spot 110 of a focusing beam 111. Namely, the laser 
beam 103 emitted from the semiconductor laser chip 102 passes through the 
transparent light guide 105, diffusing symmetrically about an axis normal 
to the first and second surfaces 105a, 105b thereof and, in turn, passes 
through the compound holographic optical element 108 disposed on the first 
surface 105a. Then, the laser means is converged along the axis normal to 
the first surface 105a of the light guide 105 onto the information 
recording layer 109a of the optical disk 109. 
The other hologram pattern, shown in FIG. 10(c), has eccentric circles 
pattern, whose center is offset lower and left in the drawing, which is 
different from that of the above-described first hologram pattern. This 
hologram pattern serves as a second hologram pattern 108b capable of 
converting the laser beam, being reflected at the information recording 
layer 109a of the optical disk 109 and returning the same optical path as 
the going laser beam, into a returning focusing beam 112, and also capable 
of diffracting this returning focusing beam 112 toward the sensor 
substrate 101 with a predetermined incident angle. 
A returning optical path, starting from the optical disk 109, will be 
explained below. The second surface 105b of the light guide 105 is formed 
with a polarized beam splitter 113. The polarized beam splitter 113 
includes a polarized beam splitting film coated thereon, which can 
transmits P-polarized component and reflects S-polarized component. 
It is now assumed that an arrow, shown in FIG. 9(a), represents a linearly 
polarized beam 114, which expresses the polarization state of the 
diffusion beam 107 entered into the compound holographic optical element 
108. The second hologram pattern 108b is designed in such a manner that 
the diffracted direction of the returning focusing beam 112 is inclined 
45.degree. with respect to the polarization direction of the linearly 
polarized beam 114. Accordingly, the diffracted returning focusing beam 
112 includes both P-polarized component and S-polarized component evenly, 
i.e. at a ratio of approximately 50:50. Approximately half of the 
diffracted returning focusing beam 112 transmits the polarized beam 
splitter 113; therefore, light quantity of the transmission beam 115, 
having transmitted through the polarized beam splitter 113, is reduce to 
50%. This transmission beam 115 is received by a first photo detector 116 
provided on an upper surface of the sensor substrate 101. The remainder of 
the returning focusing beam 112, after having been reflected at the 
polarized beam splitter 113, proceeds toward the first surface 105a as a 
reflection beam 117. A reflection portion 118, formed on the first surface 
105a, reflects this beam 117 so as to convert it into a reflection beam 
119 proceeding toward the second surface 105b. The reflection beam 119, in 
turn, goes through a transmission window 120 formed on the second surface 
105b and, subsequently, becomes a transmission beam 121. This transmission 
beam 121 is received by a second photo detector 122. The compound 
holographic optical element 108 and others are designed to let a focal 
point 123 of the diffracted returning focusing beam 112 reside on an 
optical path between the polarized beam splitter 113 and the second photo 
detector 122. 
Next, a method of detecting a magneto-optical signal from a magneto-optical 
disk, which is writable, will be explained with reference to FIGS. 11 and 
12. In FIG. 11, an arrow a denotes a polarization direction of the 
linearly polarized beam 114 entering the compound holographic optical 
element 108. The compound holographic optical element 108 gives no 
affection to the polarization surface. Therefore, the diffracted returning 
focusing beam 112, i.e. the reflection beam of the linearly polarized beam 
114, has the same polarization direction as the linearly polarized beam 
114 as long as the information recording layer 109a of the optical disk 
109 stores no information recorded. The information recording layer 109a 
is not magnetized when it stores no information recorded. 
The compound holographic optical element 108 is designed such that the 
diffraction direction of the returning focusing beam 112, having such a 
polarization direction, is inclined 45.degree. with respect to the 
polarization direction of the linearly polarized beam 114. The diffracted 
returning focusing beam 112 enters at 45.degree., as shown in FIG. 11, 
with respect to the polarized beam splitter 113 which transmits almost 
100% of the P-polarized component and reflects almost 100% of the 
S-polarized component. When the linearly polarized beam 114 is reflected 
at a magnetized information pit on the optical disk 109, its rotational 
direction is varied in a range of .+-..sub..THETA.k depending on polarity 
and strength of the magnetization. (Kerr effect) 
An arrow b denotes a polarization direction of the linearly polarized beam 
when the linearly polarized beam 114 rotates .THETA..sub.k from the state 
of the arrow a. An arrow c denotes a polarization direction of the 
linearly polarized beam when the linearly polarized beam 114 rotates 
-.THETA..sub.k from the state of the arrow a. If a magneto-optical signal, 
which is modulated between the linearly polarized beam shown by the arrow 
b and the linearly polarized beam shown by the arrow c, is entered into 
the polarized beam splitter 113, the P-polarized component to be detected 
by the first photo detector 116 is obtained as a signal 126 and also the 
S-polarized component to be detected by the second photo detector 122 is 
obtained as a signal 127. These signals 126 and 127 has a mutual phase 
difference of 180.degree., therefore a recording signal component is 
doubled by performing a differential amplification based on these two 
signals. Noises having the same phase component are mutually canceled; 
thus better S/N ratio is obtained. 
As can be known from the signal processing circuit shown in FIG. 12, the 
recording signal (R.F.) is obtained from signals of the first and second 
photo detectors 116 , 122. Numerous operational and differential 
amplifiers 131a, 131b, 131c, 131d, 131e, 131f, 131g, 131h, 131i, 131j, 
131k, 131l, 131m, 131n, 131p, 131q, and 131r are associated to fulfill the 
following equations (12) (13) and (14) so as to obtain the focusing error 
signal (F.E.), tracking error signal (T.E.), and recording signal (R.F.). 
EQU F.E.=[{I(116a)+I(116d)}-{I(116b)+I(116c)}]-[{I(122a)+I(122d)}-{I(122b)+I(12 
2c)}] (12) 
EQU T.E.=[{I(116a)+I(116b)}-{I(116c)+I(116d)}]-[{I(122a)+I(122b)}-{I(122c)+I(12 
2d) }] (13) 
EQU R.F.=[{I(116a)+I(116b)}+{I (116c)+I(116d) 
}]-[{I(122a)+I(122b)}+{I(122c)+I(122d)}] (14) 
Although the diffraction direction of the focusing beam is inclined 
45.degree. with respect to the polarization direction of the linearly 
polarized beam 114 in this embodiment, the inclined angle can be any of 
45.degree., 135.degree., 225.degree., and 315.degree.. 
As apparent from the foregoing description, the present invention requires 
no more than one holographic optical element, i.e. the compound 
holographic optical element 108, whose hologram pattern is the 
superposition of the first hologram pattern 108a and the second hologram 
pattern 108b. With this arrangement, the present invention enables the 
optical pick-up apparatus to converge a laser beam onto the optical disk 
and/or the photo detector, differentiate the going and returning optical 
paths, and detecting the focusing error. Furthermore, fabrication of the 
optical pick-up apparatus will be widely simplified compared with the 
prior art optical pick-up apparatus because positional adjustment of a 
plurality of holographic optical elements is no more required. As no 
additional holographic element is necessary, the optical path can be 
further shortened. This will be helpful to reduce the size of the optical 
pick-up apparatus. 
Moreover, the second hologram pattern 108b of the compound holographic 
optical element 108 functions to diffract the reflection beam returning 
from the optical disk 109 with respect to the polarization direction of 
the linearly polarized beam fed from the semiconductor laser chip 102 at a 
predetermined angle of (2n+1).pi./4 (n: integer). Accordingly, the present 
invention makes it possible to provide the polarized beam splitter 113 
with the diffracted returning focusing beam 112 including both P-polarized 
component and S-polarized component evenly, i.e. at a ratio of 
approximately 50:50. Approximately half of the diffracted returning 
focusing beam 112 transmits the polarized beam splitter 113. Half of the 
diffracted returning focusing beam 112, i.e. the transmission beam 115, is 
received by the first photo detector 116. The remainder of the diffracted 
returning focusing beam 112, after having been reflected at the polarized 
beam splitter 113, is received by the second photo detector 122. Thus the 
diffracted returning focusing beam 112 can be split evenly for the first 
and second photo detectors 116 and 122. 
Still further, performing the differential amplification of these first and 
second photo sensors 116 and 122 removes noise and brings a better quality 
recording (R.F.) signal. 
Letting the focal point of the returning beam reside on an optical path 
between the polarized beam splitter 113 and the second photo detector 122 
can facilitate obtaining the focusing error signal on the basis of the 
difference between the first and second photo detectors 116, 122 by the 
spot-size-detection method etc. 
Yet further, fabrication of the compound holographic optical element 108 
and polarized beam splitter 113 is very easy because they are patterned or 
formed on a flat, i.e. the first 105a or second 105b, surface of the light 
guide 105. Thus, it becomes possible to provide an optical pick-up 
apparatus for magneto-optical recording, which is a highly integrated with 
excellent accuracy, at a reasonable price. 
Fourth Embodiment 
Hereinafter, a fourth embodiment of the present invention will be explained 
with reference to FIGS. 13(a) through 14(c). FIG. 13(a) is a plane view of 
an optical pick-up apparatus in accordance with the fourth embodiment of 
the present invention, and FIG. 13(b) is a sectional view showing the 
optical pick-up apparatus in accordance with the fourth embodiment, taken 
along a line X--X of FIG. 13(a). 
First of all a going optical path, starting from a semiconductor laser 
serving as a light emitting element and arriving at an optical disk, will 
be explained below. In FIG. 13(b), a semiconductor laser chip 202 is 
mounted in parallel with and on a sensor substrate 201. A laser beam 203, 
emitted horizontally from the semiconductor laser chip 202 reaches a 
reflection prism 204 mounted on the sensor substrate 201. The constitution 
of this reflection prism 204 and its vicinity is the same as the 
reflection prism 15 of the second embodiment and, therefore, will no more 
be explained in detail. This reflection prism 204, being trapezoidal 
shape, has a reflection surface confronting with the light emitting 
surface of the semiconductor laser chip 202. The laser beam, having 
reached the reflection prism 204, is reflected at the reflection surface 
and enters obliquely as a diffusing or diverging beam 207 into the inside 
of a transparent light guide 205 through an incident window 206 formed on 
a second, i.e. a lower, surface 205b thereof. 
The light guide 205 is spaced from the semiconductor laser chip 202 and 
photo detectors 216, 222 later described. The light guide 205 has a first, 
i.e. an upper, surface 205a, disposed in parallel with the second surface 
205a. The first surface 205a confronts with an optical disk 209 located 
above the light guide 205. On the first surface 205a there is provided a 
first reflection portion 246, which reflects the diffusion beam 207 and 
changes it into the reflection beam 247 proceeding toward the second 
surface 205b. On the second surface 205b there is provided a second 
reflection portion 248, which reflects the reflection beam 247 and changes 
it into the reflection beam 249 proceeding toward the first surface 205a. 
The laser beam, entered from the incident window 206, is reflected two 
times in the light guide 205 by the first and second reflection portions 
246, 248 and, in turn, reaches a compound holographic optical element 208 
formed in the vicinity of the first reflection portion 246 on the first 
surface 205a. 
When the laser beam is reflected at the first and second reflection 
portions 246, 248, the polarization state of the laser beam is normally 
changed upon each reflection. For example, the laser beam, entered as a 
linearly polarized beam, may be changed into an elliptic polarized beam 
after the reflection. In reading out the magneto-optical recording 
information from the optical disk 209, it is mandatory to detect a slight 
Kerr rotation angle caused by the linearly polarized beam irradiated onto 
the optical disk 209. Hence, it is very important to accurately keep the 
state of the linearly polarized beam until it reaches the optical disk 
209. In order to prevent the linearly polarization beam from being changed 
into an elliptic shape upon the reflection, there is provided a phase 
difference control film which is capable of controlling the phase 
difference between the first and second reflection portions 246, 248. 
This phase difference control film is, for example, constituted as follows. 
It is now supposed that an optical film thickness is expressed by nd, 
wherein d represents a thickness of the film and n represents a refraction 
factor. An L-layer is made of SiO.sub.2 (nd=207 nm, n=1.45). An H-layer is 
made of TiO.sub.2 (nd=199 nm, n=2.30). An incident angle of a light is 
18.35.degree. with respect to the normal of a plane. The construction of 
the phase difference control film is given as a lamination layer 
consisting of air L-layer (L-layer H-layer).sup.11. Here, a meaning of the 
expression (L-layer H-layer).sup.11 is that a combined layer of (L-layer 
H-layer) is repeatedly laminated as much as 11 times. 
The compound holographic optical element 208 has a compound hologram 
pattern shown in FIG. 14(a), which is the superposition of two hologram 
patterns shown in FIGS. 14(b) and 14(c). One hologram pattern, shown in 
FIG. 14(b), has eccentric circles pattern, in which a center of these 
concentric circles positions right of the hologram pattern as shown in the 
drawing. This hologram pattern serves as a first hologram pattern 208a 
capable of focusing the diffused laser beam 249 onto an information 
recording layer 209a of the optical disk 209 as a spot 210 of a focusing 
beam 211. Namely, the laser beam 203 emitted from the semiconductor laser 
chip 202 passes through the transparent light guide 205, diffusing 
obliquely with respect to the first and second surfaces 205a, 205b thereof 
and, in turn, passes through the compound holographic optical element 208 
disposed on the first surface 205a. Then, the laser beam is converged 
along the axis normal to the first surface 205a of the light guide 205 
onto the information recording layer 209a of the optical disk 209. 
The other hologram pattern, shown in FIG. 14(c), has eccentric circles 
pattern, whose center is offset upper and left in the drawing, which is 
different from that of the above-described first hologram pattern. This 
hologram pattern serves as a second hologram pattern 208b capable of 
converting the laser beam, being reflected at the spot 210 and returning 
the same optical path as the going laser beam, into a returning focusing 
beam 212, and also capable of diffracting this returning focusing beam 212 
toward the second surface 205b with a predetermined incident angle. 
A returning optical path, starting from the optical disk 209, will be 
explained below. The second surface 205b of the light guide 205 is formed 
with a polarized beam splitter 213. The polarized beam splitter 213 
includes a polarized beam splitting film coated thereon, which can 
transmits P-polarized component and reflects S-polarized component of the 
returning focusing beam 212. 
It is now assumed that an arrow, shown in FIG. 13(a), represents a linearly 
polarized beam 214, which expresses the polarization state of the 
reflection beam 249 entered into the compound holographic optical element 
208. The second hologram pattern 208b is designed in such a manner that 
the diffracted direction of the returning focusing beam 212 is inclined 
135.degree. with respect to the polarization direction of the linearly 
polarized beam 214. Accordingly, the diffracted returning focusing beam 
212 includes both P-polarized component and S-polarized component evenly, 
i.e. at a ratio of approximately 50:50. Approximately half of the 
diffracted returning focusing beam 212 transmits the polarized beam 
splitter 213; therefore, light quantity of the transmission beam 215, 
having transmitted through the polarized beam splitter 213, is reduce to 
50%. This transmission beam 215 is received by a first photo detector 216 
provided on an upper surface of the sensor substrate 201. The remainder of 
the returning focusing beam 212, after having been reflected at the 
polarized beam splitter 213, proceeds toward the first surface 205a as a 
reflection beam 217. A reflection portion 218, formed on the first surface 
205a, reflects this beam 217 so as to convert it into a reflection beam 
219 proceeding toward the second surface 205b. The reflection beam 219, in 
turn, goes through a transmission window 220 formed on the second surface 
205b and, subsequently, becomes a transmission beam 221. This transmission 
beam 221 is received by a second photo detector 222. The compound 
holographic optical element 208 and others are designed to let a focal 
point 223 of the diffracted returning focusing beam 212 reside on an 
optical path between the polarized beam splitter 213 and the second photo 
detector 222. 
As is apparent from the foregoing description, the present invention 
utilizes the reflections occurring inside the optical guide 205 to guide 
the laser beam 203 emitted from the semiconductor laser chip 202 to the 
compound holographic optical element 208. As a result, it becomes possible 
to use the light guide 205 having a thin width compared with the optical 
path of the laser beam 203. Hence, the optical pick-up apparatus can be 
made small in size. Furthermore, the phase difference control film, 
provided on the reflection surface, surely prevents the linearly polarized 
beam from being changed into the elliptic polarized beam upon the 
reflection. 
Although the diffraction direction of the returning focusing beam 212 is 
inclined 135.degree. with respect to the polarization direction of the 
linearly polarized beam 214 in this embodiment, the inclined angle can be 
any of 45.degree., 135.degree., 225.degree., and 315.degree., i.e. an 
angle of (2n+1).pi./4 (n: integer). 
Fifth Embodiment 
Hereinafter, a fifth embodiment of the present invention will be explained 
with reference to FIGS. 15(a) through 17(b). FIG. 15(a) is a plane view of 
an optical pick-up apparatus in accordance with the fifth embodiment of 
the present invention, and FIG. 15(b) is a sectional view showing the 
optical pick-up apparatus in accordance with the fifth embodiment, taken 
along a line X--X of FIG. 15(a). 
First of all a going optical path, starting from a semiconductor laser 
serving as a light emitting element and arriving at an optical disk, will 
be explained below. In FIG. 15(b), a semiconductor laser-chip 302 is 
mounted in parallel with and on a sensor substrate 301. A laser beam 303, 
emitted horizontally from the semiconductor laser chip 302, reaches a 
reflection prism 304 mounted on the sensor substrate 301. The constitution 
of this reflection prism 304 and its vicinity is the same as the 
reflection prism 15 of the second embodiment and, therefore, will no more 
be explained in detail. This reflection prism 304, being trapezoidal 
shape, has a reflection surface confronting with the light emitting 
surface of the semiconductor laser chip 302. The laser beam, having 
reached the reflection prism 304, is reflected at the reflection surface 
and enters obliquely as a diffusing or 20 diverging beam 307 into the 
inside of a transparent light guide 305 through an incident window 306 
formed on a second, i.e. a lower, surface 305b thereof. 
The light guide 305 is spaced from the semiconductor laser chip 302 and 
photo detectors 316, 322 later described. The light guide 305 has a first, 
i.e. an upper, surface 305a, disposed in parallel with the second surface 
305a. The first surface 305a confronts with an optical disk 309 located 
above the light guide 305. On the first surface 305a there is provided a 
going-path reflection portion 346, which reflects the diffusion beam 307 
and changes it into the reflection beam 347 proceeding toward the second 
surface 305b. On the second surface 305b there is provided a going-path 
polarized beam splitter 348, which includes a polarized beam splitting 
film capable of transmitting P-polarized component and reflecting 
S-polarized component. The laser beam, having been reflected by the 
going-path polarized beam splitter 348, proceeds toward the first surface 
305a as a reflection beam 349. Thus, the laser beam, entered from the 
incident window 306, is reflected two times in the light guide 305 by the 
going-path reflection portion 346 and the going-path polarized beam 
splitter 348 and, in turn, reaches a compound holographic optical element 
308 formed in the vicinity of the going-path reflection portion 346 on the 
first surface 305a. 
Described hereinafter is the reason why the polarized beam splitting film 
is used in the going-path polarized beam splitter 348. When the laser beam 
is reflected at the reflection portions 346, the polarization state of the 
laser beam is normally changed upon reflection. For example, the laser 
beam, entered as a linearly polarized beam, may be changed into an 
elliptic polarized beam after the reflection. In reading out the 
magneto-optical recording information from the optical disk 309, it is 
mandatory to detect a slight Kerr rotation angle caused by the linearly 
polarized beam irradiated onto the optical disk 309. Hence, it is very 
important to accurately keep the state of the linearly polarized beam 
until it reaches the optical disk 309. 
In order to prevent the linearly polarized beam from being changed into an 
elliptic shape upon the reflection, the above-described fourth embodiment 
adopts the phase difference control film which is capable of controlling 
the phase difference of the polarized components. The phase difference 
control film is, however, constituted in a multi-layer film structure. 
This may be somewhat complicated in view of manufacturing process and 
therefore will raise production cost. 
The fifth embodiment however allows the reflection beam 347 to be changed 
into an elliptic polarized beam. Accordingly, the going-path reflection 
portion 346 can be made in a simple reflection film structure. Instead, 
the P-polarized component, generated by the reflection, transmit the 
going-path polarized beam splitter 348. Therefore, the reflection beam 349 
again becomes a linearly polarized beam having S-polarized beam only. This 
going-path polarized beam splitter 348 is formed on the same surface (i.e. 
the second surface 305b) as a returning-path polarized beam splitter 313 
which will be described later. Hence, these polarized beam splitter 348 
and 313 can be coated continuously by the same material at the same 
process, so as to suppress the manufacturing cost. 
The compound holographic optical element 308 has a compound hologram 
pattern shown in FIG. 16(a), which is the superposition of two hologram 
patterns shown in FIGS. 16 (b) and 16(c). One hologram pattern, shown in 
FIG. 16(b), has a concentric circles pattern, in which a center of these 
concentric circles positions in the center of the hologram pattern as 
shown in the drawing. This hologram pattern serves as a first hologram 
pattern 308a capable of focusing the diffused laser beam 349 onto an 
information recording layer 309a of the optical disk 309 as a spot 310 of 
a focusing beam 311. Namely, the laser beam 303 emitted from the 
semiconductor laser chip 302 passes through the transparent light guide 
305, diffusing obliquely with respect to the first and second surfaces 
305a, 305b thereof and, in turn, passes through the compound holographic 
optical element 308 disposed on the first surface 305a. Then, the laser 
beam is converged along the axis normal to the first surface 305a of the 
light guide 305 onto the information recording layer 309a of the optical 
disk 309. 
The other hologram pattern, shown in FIG. 16(c), has eccentric circles 
pattern, whose center is offset upper and left in the drawing, which is 
different from that of the above-described first hologram pattern. This 
hologram pattern serves as a second hologram pattern 308b capable of 
converting the laser beam, being reflected at the spot 310 and returning 
the same optical path as the going laser beam, into a returning focusing 
beam 312, and also capable of diffracting this returning focusing beam 312 
toward the second surface 305b with a predetermined incident angle. 
A returning optical path, starting from the optical disk 309, will be 
explained below. The second surface 305b of the light guide 305 is formed 
with the returning-path polarized beam splitter 313. This returning-path 
polarized beam splitter 313 includes a polarized beam splitting film 
coated thereon, which can transmits P-polarized component and reflects 
S-polarized component of the returning focusing beam 312. 
It is now assumed that an arrow, shown in FIG. 15(a), represents a linearly 
polarized beam 314, which expresses the polarization state of the 
reflection beam 349 entered into the compound holographic optical element 
308. The second hologram pattern 308b is designed in such a manner that 
the diffracted direction of the returning focusing beam 312 is inclined 
45.degree. with respect to the polarization direction of the linearly 
polarized beam 314. Accordingly, the diffracted returning focusing beam 
312 includes both P-polarized component and S-polarized component evenly, 
i.e. at a ratio of approximately 50:50. Approximately half of the 
diffracted returning focusing beam 312 transmits the returning-path 
polarized beam splitter 313; therefore, light quantity of the transmission 
beam 315, having transmitted through the returning-path polarized beam 
splitter 313, is reduce to 50%. This transmission beam 315 is received by 
a first photo detector 316 provided on an upper surface of the sensor 
substrate 301. The remainder of the returning focusing beam 312, after 
having been reflected at the returning-path polarized beam splitter 313, 
proceeds toward the first surface 305a as a reflection beam 317. A 
returning-path reflection portion 318, formed on the first surface 305a, 
reflects this beam 317 so as to convert it into a reflection beam 319 
proceeding toward the second surface 305b. The reflection beam 319, in 
turn, goes through a transmission window 320 formed on the second surface 
305b and, subsequently, becomes a transmission beam 321. This transmission 
beam 321 is received by a second photo detector 322. The compound 
holographic optical element 308 and others are designed to let a focal 
point 323 of the diffracted returning focusing beam 312 reside on an 
optical path between the returning-path polarized beam splitter 313 and 
the second photo detector 322. 
FIGS. 17(a) and 17(b) show the construction applied in a case where a laser 
beam emitted from the semiconductor laser chip 302 is a P-polarized beam 
with respect to the second surface 305b. FIG. 17(a) is an enlarged view 
showing a construction of the semiconductor laser chip 302 and an optical 
rotator 356 (for example, a halfwave plate). This optical rotator 356, 
mounted on the second surface 305b, transmits a P-polarized beam 350 
emitted from the semiconductor laser chip 302 and changes it into an 
S-polarized beam 351. FIG. 17(b) is an enlarged view showing a 
construction of a semiconductor laser chip 302 and a quarterwave plate 353 
which includes a reflection film 352 coated on a backside thereof. The 
P-polarized beam 354, emitted from the semiconductor laser chip 302, goes 
and returns in this quarterwave plate 353 being reflected at and changed 
into an S-polarized beam 533 by the quarterwave plate 353. An advantage of 
converting the P-polarized beam emitted from the semiconductor laser chip 
302 into the S-polarized beam will be explained below. 
The laser beam 303 emitted from the semiconductor laser chip 302 normally 
has light quantity distributions of elliptic formation. Furthermore, the 
polarization surface resides in a direction of the minor axis of the 
ellipse. In order to correct the light quantity distributions of the laser 
beam 303 entering into the compound holographic optical element 308 from 
above elliptic formation to circular formation, it will be best to align 
the minor axis of above elliptic formation on a plane including an optical 
axis of the laser beam 303 in the optical path starting from the 
semiconductor laser chip 302 and arriving at the compound holographic 
optical element 308 and to enter the S-polarized beam into the going-path 
polarized beam splitter 348. With this alignment, it becomes possible to 
provide the compound holographic optical element 308 with a linearly 
polarized beam including only S-polarized beam of better-quality. 
Although the diffraction direction of the returning focusing beam 312 is 
inclined 45.degree. with respect to the polarization direction of the 
linearly polarized beam 314 in this embodiment, the inclined angle can be 
any of 45.degree., 135.degree., 225.degree., and 315.degree., i.e. an 
angle of (2n+1).pi./4 (n: integer). 
As is apparent from the foregoing description, the present invention 
utilizes the reflections occurring inside the optical guide 305 to guide 
the laser beam 303 emitted from the semiconductor laser chip 302 to the 
compound holographic optical element 308. 
Furthermore, the polarized beam splitter 348 surely prevents the linearly 
polarized beam from being changed to the elliptic polarized beam upon the 
reflection in the optical guide 305. 
Moreover, if the polarized beam splitting film of the going-path polarized 
beam splitter 348 and the polarized beam splitting film of the 
returning-path polarized beam splitter 313 are coated continuously by the 
same material at the same process, it becomes possible to suppress the 
manufacturing cost. 
Sixth Embodiment 
Hereinafter, a sixth embodiment of the present invention will be explained 
with reference to FIGS. 18(a) through 21 (b). FIG. 18(a) is a plane view 
of an optical pick-up apparatus in accordance with the sixth embodiment of 
the present invention, and FIG. 18(b) is a sectional view showing the 
optical pick-up apparatus in accordance with the sixth embodiment, taken 
along a line X--X of FIG. 18(a). 
First of all a going optical path, starting from a semiconductor laser 
serving as a light emitting element and arriving at an optical disk, will 
be explained below. In FIG. 18(b), a semiconductor laser chip 602 is 
mounted in parallel with and on a sensor substrate 601. A laser beam 603, 
emitted horizontally from the semiconductor laser chip 802, reaches a 
reflection prism 604 mounted on the sensor substrate 601. The constitution 
of this reflection prism 604 and its vicinity is the same as the 
reflection prism 15 of the second embodiment and, therefore, will no more 
be explained in detail. This reflection prism 604, being trapezoidal 
shape, has a reflection surface confronting with the light emitting 
surface of the semiconductor laser chip 602. The laser beam, having 
reached the reflection prism 604, is reflected at the reflection surface 
and enters as a diffusing or diverging beam 607 into the inside of a 
transparent light guide 605 through an incident window 606 formed on a 
second, i.e. a lower, surface 605b thereof. 
The light guide 605 is spaced from the semiconductor laser chip 602 and 
photo detectors 623, 629 later described. The light guide 605 has a first, 
i.e. an upper, surface 605a, disposed in parallel with the second surface 
605b. There is formed a holographic optical element 608 on the first 
surface 605a. The diffusing or diverging beam 607 reaches the holographic 
optical element 608, in which the diffusing or diverging beam 607 is 
diffracted. A diffracted beam goes out of the light guide 605 and becomes 
a diffusing or diverging beam 609 consisting of 0-order diffraction beam. 
This diffusing or diverging beam 609 reaches a collimator lens 610 and is 
converted into a parallel beam 631 therethrough. This parallel beam 631 
reaches an objective lens 630. Then, the beam passing through the 
objective lens 630 becomes a focusing beam 613 and is focused on an 
information recording layer 611a of the optical disk 611 as a spot 612. 
Namely, the laser beam 603 emitted from the semiconductor laser chip 602 
passes through the transparent light guide 605, diffusing normal to the 
first and second surfaces 605a, 605b thereof and, in turn, passes through 
the holographic optical element 608 disposed on the first surface 605a. 
Then, the laser beam is converged along the axis normal to the first 
surface 605a of the light guide 605 onto the information recording layer 
611a of the optical disk 611 after passing through the collimator lens 610 
and the objective lens 630. 
The holographic optical element 608 has at least one hologram pattern shown 
in FIG. 19. This hologram pattern, shown in FIG. 19, has eccentric circles 
pattern, in which a center of these concentric circles positions at the 
center of the hologram pattern as shown in the drawing. This hologram 
pattern converts the laser beam, being reflected at the spot 612 and 
returning the same optical path as the going laser beam, into a returning 
diffraction beam 614 diffracted toward the second surface 605b with a 
predetermined incident angle. 
A returning optical path, starting from the optical disk 611, will be 
explained below. The laser beam, having been reflected at the optical disk 
611, reaches the objective lens 630 and is converted into a parallel beam, 
which advances toward the collimator lens 610. The leaser beam is, in 
turn, converted into a focusing beam by the collimator lens 610. The laser 
beam is subsequently converted into the returning diffraction beam 614 by 
the holographic optical element 608. The second surface 605b of the light 
guide 605 is formed with the returning-path polarized beam splitter 615. 
This returning-path polarized beam splitter 615 includes a polarized beam 
splitting film coated thereon, which can transmits P-polarized component 
and reflects S-polarized component of the returning diffraction beam 614. 
It is now assumed that an arrow, shown in FIG. 18(a), represents a linearly 
polarized beam 621, which expresses the polarization state of the 
diffusing beam 607 entered into the holographic optical element 608. The 
hologram pattern of the holographic optical element 608 is designed in 
such a manner that the diffraction direction of the returning diffraction 
beam 614 is inclined 45.degree. with respect to the polarization direction 
of the linearly polarized beam 621. Accordingly, the returning diffraction 
beam 614 includes both P-polarized component and S-polarized component 
evenly, i.e. at a ratio of approximately 50:50. Approximately half of the 
returning diffraction beam 614 transmits the returning-path polarized beam 
splitter 615; therefore, light quantity of the transmission beam 622, 
having transmitted through the returning-path polarized beam splitter 615, 
is reduce to 50%. This transmission beam 622 is received by a first photo 
detector 623 provided on an upper surface of the sensor substrate 601. The 
remainder of the returning diffraction beam 614, after having been 
reflected at the returning-path polarized beam splitter 615, proceeds 
toward the first surface 605a as a reflection beam 624. A returning-path 
reflection portion 625, formed on the first surface 605a, reflects this 
beam 624 so as to convert it into a reflection beam 626 proceeding toward 
the second surface 605b. The reflection beam 626, in turn, goes through a 
transmission window 627 formed on the second surface 605b and, 
subsequently, becomes a transmission beam 628. This transmission beam 628 
is received by a second photo detector 629. The holographic optical 
element 608 and others are designed to let a focal point 630 of the 
returning diffraction beam 614 reside on an optical path between the 
returning-path polarized beam splitter 615 and the second photo detector 
629. 
FIGS. 20(a) and 20(b) show a modified construction of the sixth embodiment. 
As shown in FIG. 20(b), there is provided a polarized beamsplitter 
auxiliary member 716 beneath a light guide 705. This auxiliary member 716 
is transparent and directly connected to a second surface 705b of the 
light guide 705. A refractive index of this auxiliary member 716 is 
substantially identical with that of the light guide 705. This auxiliary 
member 716 increases a polarized beam splitting efficiency of the 
polarized beam splitter 715. A polarized beam splitter 715 can be 
completely surrounded by the light guide 705 and the auxiliary member 716. 
The application of this auxiliary member 716 brings a remarkable 
improvement in productivity. Because, it is no longer necessary to fill a 
space 718 with a transparent resin material or the like having 
substantially the same refractive index as the light guide 705. This space 
718 is surrounded by the light guide 705 and a package 717. The package 
717 is usually made of a nonconductive material such as ceramic. In FIGS. 
20(a) and 20(b), the remainder of components disclosed in the drawings are 
substantially the same as those of FIGS. 18(a) and 18(b) and therefore 
will no more be explained. 
FIGS. 21(a) and 21(b) show still another modification of the sixth 
embodiment. In FIG. 21(b), there is provided a objective lens 810 of 
finite system. The collimator lens 610 and the objective lens 630 of the 
sixth embodiment can be replaced by this infinite-system objective lens 
810. In FIGS. 21(a) and 21(b), the remainder of components disclosed in 
the drawings are substantially the same as those of FIGS. 18(a) and 18(b) 
and therefore will no more be explained. 
Although the diffraction direction of the returning diffraction beam 614 is 
inclined 45.degree. with respect to the polarization direction of the 
linearly polarized beam 621 in this embodiment, the inclined angle can be 
any of 45.degree., 135.degree., 225.degree., and 315.degree., i.e. an 
angle of (2n+1).pi./4 (n: integer). 
As this invention may be embodied in several forms without departing from 
the spirit of essential characteristics thereof, the present embodiments 
are therefore illustrative and not restrictive, since the scope of the 
invention is defined by the appending claims rather than by the 
description preceding them, and all changes that fall within meets and 
bounds of the claims, or equivalence of such meets and bounds are 
therefore intended to embraced by the claims.