Optical pickup using split beams impinging on different photo-detector areas

An optical pickup device for producing information magnetically recorded on a recording medium includes a light source for generating a light beam polarized in a predetermined direction. An object lens is provided for focusing the light beam generated from the light source onto the recording medium. A beam splitter is arranged in an optical path extending from the light source to the recording medium, the beam splitter having a reflection plane for reflecting the light beam from the light source toward the recording medium. The beam splitter simultaneously causes the reflecting light from the recording medium to pass through the reflection plane to separate the reflected light beam from the light beam irradiated toward the recording medium. The polarization state of the reflecting light beam is modulated in accordance with the recorded information. The beam splitter splits the reflected light beam passed through the reflection plane into first and second light beams both of which are polarized in directions perpendicular to each other. The beam splitter has a bi-refractive property. First and second detection circuits are also provided for detecting the first and second light beams, respectively, and for providing first and second detection outputs respectively corresponding thereto. Circuitry is also provided for differentiating the detection outputs from the first and second detection circuits to provide information signals.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an optical pickup for irradiating a light 
beam on an information recording area of an information carrier to detect 
or record information, and more particularly to a compact, low cost 
optical pickup capable of reproducing a signal at a high S/N ratio. 
2. Related Background Art 
Research and development for a writable optical disk recording medium and 
an optical disk recording and reproducing apparatus have been vigorously 
prosecuted. 
Examples of such apparatus are shown in U.S. Pat. No. 4,451,863 and IBM 
Technical Disclosure Bulletin Vol. 19, No. 4, issued September 1976. 
However, such a prior art optical pickup has many parts and is 
disadvantageous as regards reduction in a size and cost. Further, a 
plurality of photo-detectors must be accurately positioned and adjustment 
thereof is troublesome. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an optical pickup which 
is compact and easy to adjust optically. 
An optical pickup apparatus according to the present invention includes 
photodetection means having a plurality of detection areas. Means are 
provided for converting, into an electrical signal, a light beam emitted 
from a recording medium on which information is magnetically recorded and 
modulated in intensity. These means also split the light beam into two 
beams and guide them respectively to different areas of the photodetection 
means. Means are also provided for processing outputs from each of the 
areas of the photodetection means to provide an information signal.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1A shows a first embodiment of an optical pickup according to the 
present invention. A light beam emitted from an LD 20 is collimated by a 
collimating lens 21, passes through a beam splitter 22 and is focused by 
an object lens 23 to a fine spot on a recording medium 24. A reflected 
light beam from the recording medium 24 again passes through an object 
lens 23, is reflected by the beam splitter 22, passes through a half-wave 
plate 25 with the direction of its plane of polarization being rotated by 
45 degrees, and is directed to a Wollaston prism 26. As a result, the 
light beam is split into a linearly polarized light beam which vibrates in 
the plane of the drawing and a linearly polarized light beam which 
vibrates in a plane normal to that of the drawing; and both of the split 
beams are condensed by a sensor lens 27. 
FIGS. 1B and 1C show examples of split portions or areas of a photo-sensor 
of a photo-detector 28. When the object lens 23 and the recording medium 
24 are in an in-focus position, the light beam spreads to the hatched 
areas shown in FIGS. 1B and 1C. Signal outputs I.sub.A, I.sub.B, I.sub.C, 
I.sub.D, I.sub.E, and I.sub.F, from the photo-sensors A-F, respectively, 
and a focus error signal I.sub.AF1, have the following relationship. 
EQU I.sub.AF1 =(I.sub.A +I.sub.B +I.sub.E +I.sub.F)-(I.sub.C +I.sub.D) 
In FIG. 1B, if the recording medium 24 moves away from the object lens 23, 
the light spot becomes smaller, as shown by a broken line in FIG. 1B. As a 
result, the outputs I.sub.C and I.sub.D increase and the focus error 
signal I.sub.AF1 becomes negative. When the recording medium 24 moves 
nearer to the lens 23, the light spot on the photo-sensor 29 becomes 
larger and the focus error signal I.sub.AF1 becomes positive. 
Where the concentric photo-sensors 29 shown in FIG. 1C are employed, a 
difference between the outer photo-sensor and the inner photo-sensor is 
used so that the focus error signal is detected in a similar manner. 
On the other hand, a tracking error signal may be detected in the present 
invention by a known push-pull method, like that shown in U.S. Pat. No. 
3,931,459. The tracking error signal may be detected by calculating 
EQU I.sub.AT1 =(I.sub.A +I.sub.C +I.sub.E)-(I.sub.B +I.sub.D +I.sub.F) 
A principle of differential detection of the opto-magnetic information 
signal by the optical pickup of the present invention is now explained. 
In FIG. 1A, the reflected light from the recording medium 24 is directed to 
the half-wavelength plate 25 with the polarization plane thereof being 
rotated through an angle .sup..theta. K or -.sub..theta. K by the magnetic 
pattern of the recording medium 24 (direction of magnetization is either 
upward or downward) by the magneto-optical effect. The half-wavelength 
plate 25 has a crystal axis arranged at 22.5.degree. with respect to a 
crystal axis of the Wollaston prism 26. Since the Wollaston prism 26 
splits the reflected light beam into two linearly polarized light beams 
having vibration planes orthogonal to each other, those light beams are 
detected by the photo-sensor 28 as signals of opposite phases, as shown in 
FIG. 2. 
In FIG. 2, the X axis represents a compound of the crystal axis of a first 
prism of the Wollaston prism 26 and the Y axis represents a component 
normal thereto. The light beam rotated by .sup..theta. K or -.sup..theta. 
K from the polarization plane of the reflected light beam by the 
magneto-optical effect (a broken line in FIG. 2 shows the polarization 
plane) has its polarization plane rotated by 45.degree. by the 
half-wavelength plate 25. Since it is thereafter divided in two by the 
Wollaston prism 26, the resulting light beams have an equal variation 
amplitude (S1 and S1') and opposite phases. By detecting those two light 
beams by means of the photo-sensors 29, the opto-magnetic signal of the 
recording medium 24 can be produced. In the photo-detector 28 of FIG. 1B, 
the opto-magnetic information signal I.sub.S is given by 
EQU I.sub.S =(I.sub.A +I.sub.B +I.sub.C +I.sub.D +I.sub.E +I.sub.F)-(I.sub.G 
+I.sub.H +I.sub.I +I.sub.K +I.sub.L) 
and in the photo-detector 28 of FIG. 1C, it is given by 
EQU I.sub.S =(I.sub.A +I.sub.B +I.sub.C +I.sub.D)-(I.sub.E +I.sub.F +I.sub.G 
+I.sub.H) 
In accordance with the present embodiment, the size and weight of the 
apparatus are reduced and the reliability is improved. Since a set of 
photo-sensors 29 which differentially detect the light beams are arranged 
on one substrate, the photo-detector 28 is insensitive to external changes 
such as a change of environmental temperature. 
By arranging a set of amplifiers on the same substrate as the 
photo-detectors, the frequency characteristics and temperature 
characteristics of those circuits are well matched and signal quality is 
further improved. By incorporating the differential amplifier for 
differentially detecting the signals in the photo-detector, 
noise-resistant detection is attained. 
A modification of the first embodiment of the present invention is shown in 
FIG. 3. The light beam reflected by the beam splitter 22 and transmitted 
through the half-wave plate 25 (optical elements in front of the beam 
splitter 22 are omitted in FIG. 3) is condensed by a condenser lens 27, 
and the condensed beam is directed to a polarization/separation element 30 
comprising two prisms 30a and 30b having crystal axes orthogonal to each 
other (which form a Wollaston prism) and a polarization plate 30c having 
the same crystal axis as that of the prism 30b. The light beam transmitted 
through the polarization/separation element 30 propagates in different 
directions for an ordinary ray and an extraordinary ray, and an equivalent 
optical thickness changes with a difference between refractive indices No 
and Ne for the ordinary ray and the extraordinary ray. Accordingly, 
focusing positions for the ordinary ray and extraordinary ray are 
different. Thus, the photo-sensor 29 of the photo-detector 28 is arranged 
at an unfocused position for the light beam of one polarization direction 
and in the diverging light beam after focusing for the other light beam. 
FIG. 4A shows a focus error signal produced in FIG. 3. FIGS. 4B and 4C show 
focus error signals produced by a single concentric circular 
photo-detector. They have undesirable side peaks to an ideal S-curve. In 
the embodiment of FIG. 3 which uses the combination of converging and 
diverging photo-detectors, the undesired peak is eliminated as shown in 
FIG. 4A and a desirable S-shaped focus error signal is produced. In FIG. 
3, an interval between the two light spots on the photo-detector 28 can be 
finely adjusted by moving the Wollaston prism (30a and 30b) back and 
forth. 
Another modification of the first embodiment of the present invention is 
shown in FIG. 5. The light beam from the beam splitter (not shown in FIG. 
5) is separated into an ordinary ray and an extraordinary ray by a prism 
31 formed of crystal and is focused onto the photo-detector 28 by the 
focusing lens 27. The photo-detector 28 may be arranged obliquely to the 
light beam by an appropriate angle. 
not only the prism but also the focusing lens 27 may be manufactured of 
crystal. FIG. 6 shows an example thereof. An optical element 32 has a 
prism on one side and a lens on the other side. As a result, converging 
and diverging light beams separated in accordance with the polarization 
status of the incident light are directed to the photo-detector 28. 
FIG. 7 shows a modification in which the focusing lens 27 is substituted by 
a teletype telescopic lens 33. With this arrangement, the light beams 
necessary for the differential direction are finely separated. 
The above embodiment and modifications may be implemented by optical medium 
which form a beam splitter 22' as shown in FIG. 8. 
In FIG. 8, the light beam from the laser 20 is collimating by the 
collimating lens 21 and reflected by a reflection plane of the beam 
splitter 22'. The incident light beam is a perfectly linearly polarized 
beam, and there is no difference in the effect whether the optical medium 
of the beam splitter 22' is isotropic, such as glass, or bi-refractive, 
such as quartz. The light beam reflected by the opto-magnetic medium 24 
again passes through the beam splitter 22'. If the beam splitter 22 is 
made of a uniaxial crystal having a crystal axis appropriately oriented, 
the light beam is separated into two linearly polarized beams, which are 
directed to the sensor 28 through a sensor lens 27' and are differentially 
detected thereby. The crystal axis may form an angle of 45.degree. to the 
vertical direction in the drawing and is rotated approximately 45.degree. 
with respect to the plane of the drawing the vertical plane. 
As seen from FIG. 23, the vertical plane which includes an optical axis of 
a light beam emitted from a light source and an optical axis of a light 
beam reflected by the opto-magnetic medium, is indicated by reference 
character F. Since the crystal axis of the prism is rotated by 45.degree. 
with respect to the virtual plane F, the crystal axis is parallel with a 
plane (not shown) perpendicular to a plane F', or the plane of the drawing 
in FIG. 23. The crystal axis may form an angle of 45.degree. to the 
vertical direction of the drawing in FIG. 8, namely, in the direction of 
the optical axis zero shown in FIG. 23. Accordingly, the crystal axis of 
the prism is directed to the direction D.sub.1 or the direction D.sub.2 
shown by the arrow in FIG. 23. 
While the Wollaston prism or the single prism was shown as means for 
separating the light beam by the difference between polarization status of 
the incident light beam, the embodiment of the separation means is not 
limited thereto but may be a Rochon prism or a Senarmont prism, etc. The 
crystal parallel plates or convex lens is shown as means for creating a 
converging or a diverging light beam depending on the difference in 
polarization status, although that means may instead be a combination of 
convex and concave telescopic lens or other means. 
FIG. 9 shows a second embodiment of the optical pickup of the present 
invention. The light beam emitted from the LD 20 is collimated by a 
collimating lens 21, reflected by a first beam splitter 117 and focused by 
the object lens 23 to a fine spot on the recording medium 24. The 
reflection light from the recording medium 24 again passes through the 
object lens 23 and the first beam splitter 117, is directed to a second 
beam splitter 120 and is split thereby into two light beams. The first 
split beam 121, reflected by the second beam splitter 120, passes through 
a polarization element 122 such as a polarization plate and is focused by 
the focusing lens 27 onto the photo-sensor 28. The same effect may be 
attained when the polarization element 122 is arranged between the 
focusing lens 27 and the photo-sensor 28. On the other hand, the second 
split beam 126, after passing through the second beam splitter 120, is 
reflected by a reflection area 127 at the bottom of the beam splitter 120, 
passes through the half-wavelength plate 123 and the polarization element 
122, and is focused by the focusing lens 27 onto the photo-sensor 28. 
The photo-sensor 28 is arranged at a position spaced from a focal plane on 
which the split light beams 121 and 126 are focused by the focusing lens 
27. The photo-sensor 28 has 12 photo-sensing areas as shown in FIG. 1B. 
A principle for detection of the AF and AT signal and the differential 
detection of the information signal in the apparatus of FIG. 9 is now 
explained. 
FIG. 10 shows the principle of detection of the AF signal. Only those of 
the elements shown in FIG. 9 which are necessary for the AF detection are 
shown in FIG. 10. When the recording medium 24 is located on the focal 
plane of the object lens 23, the light beam is directed as shown by solid 
lines and focused to a focal point F of the focusing lens 27. When the 
recording medium moves away from the focal plane of the object lens 23, 
the light beam is focused at forward side of the optical axis of the 
focusing lens 27 as shown by broken lines. When the recording medium moves 
closer from the focal plane of the object lens 23, the light beam is 
focused beyond the optical axis of the focusing lens 27. Accordingly, when 
the optical sensor 28 is arranged off the point F, the distribution of the 
intensity of the split light beams on the optical sensor 28 appears 
reduced or enlarged on the sensor depending on the position of the 
recording medium. 
Where the photo-sensor 28 has twelve photo-sensing areas, the operation is 
as follows. The light beam of the photo-sensor in the in-focus state of 
the object lens 23 and the recording medium 24 is shown by the hatched 
area. 
The AF error signal I.sub.AF, is given by 
EQU I.sub.AF =(I.sub.A +I.sub.B +I.sub.E +I.sub.F +I.sub.G +I.sub.H +I.sub.K 
+I.sub.L)-(I.sub.C +I.sub.D +I.sub.I +I.sub.J) 
where I.sub.A, I.sub.B, I.sub.C, I.sub.D, I.sub.E, I.sub.F, I.sub.G, 
I.sub.H, I.sub.I, I.sub.J, I.sub.K and I.sub.L are outputs from the 
photo-sensing areas. 
For the photo-sensor having concentric photo-sensing areas as shown in FIG. 
1C, the AF error signal I.sub.AF is given by 
EQU I.sub.AF =(I.sub.A +I.sub.B +I.sub.E +I.sub.F)-(I.sub.C +I.sub.D +I.sub.G 
+I.sub.H) 
where I.sub.A, I.sub.B, I.sub.C, I.sub.D, I.sub.E, I.sub.F, I.sub.G and 
I.sub.H, are outputs from the photo-sensing areas. 
The AF error signal may be obtained by the operation for only one side of 
light beams. In the example of FIG. 1B, 
EQU I.sub.AF =(I.sub.A +I.sub.B +I.sub.E +I.sub.F)-(I.sub.C +I.sub.D) 
The principle of detection of the AT signal is now explained. 
A groove having a depth of an approximately 1/8 of the wavelength is formed 
in a vicinity of the surface of the recording medium 24 and the signal is 
recorded or reproduced while the groove is used as a guide. A far field 
pattern formed by the light beam which is reflected by the groove and 
again passes through the object lens 23 changes with a positional 
relationship between the light spot and the groove. This is illustrated in 
FIGS. 11A, 11B and 11C. The upper part of FIGS. 11A, 11B and 11C show the 
positional relationship between the groove and the light spot, and the 
lower part shows an intensity distribution of the far field pattern. 
For the split sensor shown in FIG. 1B, if the direction T--T' is aligned to 
the direction of the groove (the direction of the signal track), the AT 
error signal I.sub.AT is given by 
EQU I.sub.AT =(I.sub.A +I.sub.C +I.sub.E +I.sub.G +I.sub.I +I.sub.K)-(I.sub.B 
+I.sub.D +I.sub.F +I.sub.H +I.sub.J +I.sub.L) 
For the split sensor shown in FIG. 1C, 
EQU I.sub.AT =(I.sub.A +I.sub.C +I.sub.E +I.sub.G)-(I.sub.B +I.sub.D +I.sub.F 
+I.sub.H) 
The AT error signal may also be obtained by the operation of the output of 
the photo-sensing areas which sense only one of the beams. 
The differential detection of the information signal by the opto-magnetic 
method is now explained. 
In FIGS. 12A and 12B, the X axis represents the direction of plane of 
polarization of the light beam directed to the recording medium 24, and 
the Y axis represents a direction orthogonal thereto. 
The light beam reflected from the recording medium 24 has the polarization 
plane thereof inclined by a small angle .sup..theta. K by the 
magneto-optical effect as shown in FIG. 12A. This inclination is clockwise 
or counterclockwise (.sup..theta. or -.sup..theta. K) relative to a 
vertical direction of magnetic domain, and the angle is approximately 
1.degree.. 
The light beam 121 reflected by the beam splitter 120 of FIG. 9 passes 
through the polarization element. If the transmission axis of the 
polarization element 122 is inclined approximately 45.degree. with respect 
to the X axis, the following signal component in the intensity is produced 
in FIG. 12 depending on the inclination of the polarization plane: 
EQU S.sub.1.sup.2 { cos (45.degree.-.sup..theta. K)}.sup.2 -{ cos 
(45.degree.-.sup..theta. K)}.sup.2 
The second split beam 126 passes through the half-wave plate 123 before it 
passes through the polarization element 122. Accordingly, the polarization 
direction of the light beam 126 makes an angle of 90.degree. with the 
light beam 121 as shown in FIG. 12B. 
Accordingly, after it passed through the polarization element 122, the 
signal component in the intensity is 
EQU S.sub.1 '.sup.2 { cos (45.degree.-.sup..theta. K)}.sup.2 -{ cos 
(45.degree.-.sup..theta. K)}.sup.2 
The signal S.sub.1.sup.2 and S.sub.1 '.sup.2 show only the P--P components 
for the sake of convenience. As seen from FIGS. 12A and 12B, the light 
beam 121 has a maximum signal amplitude and the light beam 126 has a 
minimum signal amplitude for the rotation of angle of .sup..theta. K. 
When the rotation of the polarization plane is detected by the 
opto-magnetic signal by the arrangement shown in FIG. 9, the intensity 
modulation derived from the light beams 121 and 126 is reversed in phase. 
Accordingly, for FIG. 1B, the information signal I.sub.S is given by 
EQU I.sub.S =(I.sub.A +I.sub.B +I.sub.C +I.sub.D +I.sub.E +I.sub.F)-(I.sub.G 
+I.sub.H +I.sub.I +I.sub.J +I.sub.K +I.sub.L) 
and, for FIG. 1C, 
EQU I.sub.S =(I.sub.A +I.sub.B +I.sub.C +I.sub.D)-(I.sub.E +I.sub.F +I.sub.G 
+I.sub.H) 
and the information signal is differentially detected. 
To compare the arrangement of the present invention shown in FIG. 9 with 
the arrangement of the prior art, the arrangement is advantageous to 
reduce size and cost. 
By integrating the optical elements shown in FIG. 9, the optical adjustment 
during assembly is facilitated, and misalignment of axes during use is 
prevented and a highly reliable optical pickup is provided. For example, 
in FIG. 13, the beam splitters 117 and 120, half-wave plate 123 and 
polarization element 122 are integrated by bonding material. In FIG. 14, 
the half-wave plate is arranged immediately behind the reflection plane of 
the beam splitter 120. In this case, parallelogram blocks 31 and 32 of the 
same size may be used and the preparation of the elements is simplified. 
As shown in FIG. 15, a dielectric film or metal film 23 may be formed on 
the reflection plane 127 for the second light beam 126 to rotate the 
polarization plane instead of using the half-wave plate 123. In FIG. 16, a 
desired rotation angle is attained by a number of times of reflection when 
it is difficult to rotate the polarization plane by 90.degree. by one 
reflection. 
In order to detect the AF and AT error signals and the information signal, 
it is necessary to process the signals generated by the photo-sensing 
areas. Where the differential amplifier for such processing is 
incorporated in the photo-sensor together with the photo-sensing areas, 
signal detection which is resistive to external noise is attained. 
FIG. 17 shows a third embodiment of the optical pickup of the present 
invention. The light beam emitted from the LD 26 is collimated by the 
collimater lens 21, reflected by the first beam splitter 117 and focused 
by the object lens 23 to a fine spot on the recording medium 24. The 
reflection beam from the recording medium 24 again passes through the 
object lens 23, passes through the first beam splitter 117 and the 
half-wave plate 123 with the direction of polarization plane being rotated 
by 45.degree. , and is directed to the second beam splitter 120 by which 
the beam is split into two beams. One split beam 121 reflected by the 
other beam splitter 120 is focused by the focusing lens 27 to the 
photo-sensor 28. On the other hand, the second split beam 126 is reflected 
by the reflection area 127 at the bottom of the beam splitter 120 and 
condensed by the condenser lens to the photo-sensor 28. 
The photo-sensor 28 is arranged at a position spaced from a focal plane on 
which the split beams 121 and 126 are focused by the focusing lens 27. The 
photo-sensor 28 has twelve photo-sensing areas on the photo-sensing plane 
as shown in FIG. 1B. 
The principle of the detection of the AF and AT error signals and the 
differential detection of the information signal in the arrangement of 
FIG. 17 applies equally to the second embodiment. 
The differential detection of the information signal by the opto-magnetic 
method is now explained. 
In FIG. 17, the reflection light from the recording medium 24 has the 
polarization plane thereof rotated by .sup..theta. K or -.sup..theta. K by 
the magnetic pattern (upward or downward orientation of magnetic domain) 
by the magneto-optical effect, and it is directed to the half-wave plate 
123. The half-wave plate 123 has its crystal axis inclined to the incident 
plane of the beam splitter 120 by 22.5.degree.. When the transmission and 
reflection characteristics of the beam splitter 120 are assumed as the 
polarization beam splitter characteristic (P component: 100% transmission, 
S component: 100% reflection), the transmitted and reflected light beams 
are detected by the photo-sensor as the signals having opposite phases, as 
shown in FIG. 18. 
In FIG. 18, the X axis represents the reflection component of the 
polarization beam splitter, and the Y axis represents the transmission 
component. The light beam rotated by .sup..theta. K or -.sup..theta. K 
from the polarization plane of the incident light to the recording medium 
(the broken lines in FIG. 18 show the polarization plane) has its 
polarization plane rotated by 45.degree. by the half-wave plate. 
Thereafter, it is reflected, transmitted and split by the polarization 
beam splitter 120. Thus, the two beams have the same variation amplitude 
(S.sub.2 ', S.sub.2) and opposite phases. By detecting those two light 
beams using the photo-sensor 28, the information recorded on the recording 
medium can be read. In the photo-sensor of FIG. 1B, the opto-magnetic 
information signal I.sub.S is given by 
EQU I.sub.S =(I.sub.A +I.sub.B +I.sub.C +I.sub.D +I.sub.E +I.sub.F)-(I.sub.G 
+I.sub.H +I.sub.I +I.sub.J +I.sub.K +I.sub.L) 
For the photo-sensor of FIG. 1C, 
EQU I.sub.S =(I.sub.A +I.sub.B +I.sub.C +I.sub.D)-(I.sub.E +I.sub.F +I.sub.F 
+I.sub.G) 
By integrating the optical elements of FIG. 17, the optical adjustment 
during the assembly is facilitated, misalignment of axes during use is 
prevented, and a highly reliable optical pickup is provided. For example, 
in FIG. 19, the beam splitter 117, half-wave plate 123 and beam splitter 
120 are integrated by bonding material. In FIG. 20, the half-wave plate 
123 is arranged immediately behind the reflection plane of the beam 
splitter 117. In this case, parallelogram blocks 130 and 131 of the same 
size may be used and the preparation of the elements is simplified. 
In order to detect the AF and AT error signals and the information signal, 
it is necessary to process the signals generated by the photo-sensing 
areas. By incorporating the processing differential amplifier in the 
photo-sensor together with the photo-sensing areas, signal detection which 
is resistive to external noise is attained. 
In the present embodiment, the variation of the signals generated by the 
photo-sensing areas of the sensor is detected to improve the precision. 
The advantage of the differential detection is explained with reference to 
a third embodiment. FIGS. 21A and 21B show signal amplitudes generated by 
the sensing areas of the sensor. The light beam reflected by the recording 
medium 24 having a longitudinal axis along the polarization direction of 
the incident light beam has its polarization plane rotated by .sup..theta. 
K or -.sup..theta. K depending on the orientation (up or down) of the axis 
of the opto-magnetic pattern. Since the combination of the half-wave plate 
123 and the polarization beam splitter 120 is equivalent to a system 
having a polarization plate whose transmission axis is inclined by 
45.degree., difference S.sub.1 and S'.sub.1 of the projection component to 
a virtual transmission axis (broken line axis inclined by 45.degree.) are 
signal amplitudes. Since .sup..theta. K and -.sup..theta. K vary with time 
depending on the opto-magnetic pattern, the signal intensity changes are 
represented by the light beams shown in FIGS. 22A and 22B, which are 
shifted by 180.degree. in phase from each other. The opto-magnetic signal 
generated by the sensing areas of the photo-sensor is reversed in phase 
but noise components (noise from the recording medium and swing noise of 
the LD light) usually ride on those signals and the noise components are 
of the same phase. 
Accordingly, when the signals from the sensing areas of the sensor are 
differentially detected, the signal components are added and the noise 
components are cancelled. If the optical system is precisely arranged, the 
signal components S.sub.1.sup.2 and S'.sub.1.sup.2 from the respective 
sensors are equal and the noise amplitude are also equal. Accordingly, the 
signal is doubled and the noise is cancelled. Thus, the differential 
detection provides a high S/N signal. 
As described hereinabove, in accordance with the present invention, the 
light beam from the recording medium is split and they are differentially 
detected by the single photo-detector having the split photo-sensing 
plane. Accordingly, 
1. The optical pickup can be constructed in a compact size. 
2. The S/N ratio of the signal detection is improved by the differential 
detection. 
3. The number of components is reduced and the cost is reduced. 
4. By integrating the components, a more reliable pickup is provided. 
5. Because only one photo-sensor is used, no consideration of the variation 
of the characteristics between the sensors is required, and the position 
adjustment is simple. 
6. By arranging the signal processing circuit on the same chip as the 
photo-detector, signal detection which is resistive to external noise is 
attained.