Magneto-optical information reproducing apparatus having a polarizing beam splitter disposed with an inclination of 45 degrees

A magneto-optical information reproducing apparatus includes a device for applying a light beam polarized in a predetermined direction to a recording medium on which information is magnetically recorded, a first divider for dividing the reflected or transmitted light beam from the medium modulated into a polarized state in conformity with the information by magneto-optical effect into two light beams polarized in directions orthogonal to each other, the first divider being disposed so that the directions of polarization of the divided two light beams form an angle of 45.degree. with respect to the predetermined direction of polarization of the applied light beam, and a detector for detecting the divided two light beams.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a magneto-optical information reproducing 
apparatus which utilizes the magneto-optical effect to reproduce 
information magnetically recorded on a recording medium. 
2. Related Background Art 
In recent years, optical memories for effecting recording and reproduction 
by a laser beam have been actively studied and developed for practical use 
as high-density recording memories. Of these, magneto-optical disks 
capable of erasing and re-writing have been regarded as promising with 
optical disks exclusively for reproduction typified by compact disks and 
direct read after write (DRAW) type optical disks. Magneto-optical disks 
are such that information is magnetically recorded by the utilization of 
the localized temperature size of a magnetic thin film caused by the 
application of a laser spot thereto and the information is reproduced by 
the magneto-optical effect (particularly the Kerr effect). The Kerr effect 
refers to the phenomenon that the plane of polarization is rotated when 
light is reflected by a magnetic recording medium. 
The basic construction of a magneto-optical disk apparatus according to the 
prior art is shown in FIG. 1 of the accompanying drawings. In FIG. 1, 
reference numeral 1 designates a semiconductor laser, reference numeral 2 
denotes a collimator lens, reference numerals 11 and 12 designate 
half-mirrors, reference numeral 4 denotes an objective lens, reference 
numeral 6 designates a magneto-optical recording medium, reference 
numerals 7.sub.1 and 7.sub.2 denote analyzers, reference numeral 8 
designates a condensing lens, and reference numerals 9.sub.1 and 9.sub.2 
denote photodetectors. The direction of P-polarization is parallel to the 
plane of the drawing sheet, and the direction of S-polarization is 
perpendicular to the plane of the drawing sheet. 
Description will now be made of a case where magneto-optical information is 
reproduced in such apparatus. A light beam emitted from the semiconductor 
laser 1 as a rectilinearly polarized light in the direction of 
P-polarization is collimated by the collimator lens 2 and passes through 
the half-mirror 11. If the P-polarized component amplitude transmittance 
is t.sub.p and the S-polarized component amplitude transmittance is 
t.sub.s, .vertline.t.sub.p .vertline..sup.2 =.vertline.t.sub.s 
.vertline..sup.2 =0.5 in the half-mirror 11. The light beam is imaged as a 
minute spot on the magneto-optical recording medium 6 by the objective 
lens 4. When a magnetic section (pit) is pre-formed on the medium 6, as 
shown in FIG. 2 of the accompanying drawings, the reflected light from the 
medium 6 is subjected to the rotation of the plane of polarization of 
.+-..theta..sub.k by the Kerr effect in conformity with whether the 
direction of magnetization of the spot-applied area is upward or downward. 
Here, if the P-polarized component of the amplitude reflectance of the 
recording medium 6 is R and the S-polarized component is K, the following 
equation is established: 
##EQU1## 
The magneto-optically modulated reflected light is again collimated by the 
objective lens 4 and reflected by the half-mirror 11, whereafter it is 
made into a convergent light beam by the condensing lens 8 and divided by 
the half-mirror 12, and the divided lights pass through the analyzers 
7.sub.1 and 7.sub.2, respectively, and are detected as intensity-modulated 
light beams by the photodetectors 9.sub.1 and 9.sub.2. That is, as shown 
in FIG. 2, the angle of the optic axis of the analyzer with respect to the 
direction of P-polarization is .+-..theta..sub.A on the transmission side 
and the reflection side, and the light beam is analyzed as a regular 
projection of the amplitude thereof onto the optic axis of the analyzer. 
Considering that the Kerr rotation angle is of the order of 1.degree. and 
that the magneto-optical modulated component is of a very minute amount, 
it is necessary that the azimuth angle .theta..sub.A of the optic axis of 
the analyzer be set to such an optimum position that the C/N (the ratio 
between the carrier wave and the noise) of the detection signal becomes 
maximum. For example, in U.S. Pat. No. 4,569,035 issued on Feb. 4, 1986, 
there is shown an example of an apparatus using as a photodetector an 
avalanche photodiode (APD) or the like having a multiplying action wherein 
the azimuth of the transmission axis (the optic axis) of the analyzer is 
optimized. On the other hand, in an apparatus using as a photodetector a 
PIN photodiode or the like having no multiplying action, the azimuth angle 
.theta..sub.A of the optic axis of the analyzer has been set to 45.degree. 
so that the signal light becomes maximum. When the Kerr rotation angle is 
.+-..theta..sub.k, if the quantity of light incident on the recording 
medium is I.sub.O, the quantities of light passing through the analyzers 
on the transmission side and the reflection side and entering the 
respective photodetectors are: 
##EQU2## 
Since .theta..sub.k .about.1.degree., .vertline.R.vertline..sup.2 
&gt;&gt;.vertline.K.vertline..sup.2 is established and thus, equation (2) can be 
expressed as follows: 
##EQU3## 
In equation (3), the second term in the parentheses is the magneto-optical 
modulated component and the first term in the parentheses is the 
non-modulated component. These lights are photoelectrically converted by 
the photodetectors 9.sub.1 and 9.sub.2, respectively, and then 
differentially detected by a differential circuit, not shown, whereby a 
magneto-optical signal is obtained. 
On the other hand, a magneto-optical information reproducing apparatus 
using a polarizing beam splitter instead of the half-mirror 11 shown in 
FIG. 1 to improve the C/N of the above-mentioned reproducing signal is 
proposed in U.S. Pat. No. 4,561,032 issued on Dec. 24, 1985. Further, an 
example in which the polarizing characteristic of this polarizing beam 
splitter is set so that C/N is maximum is disclosed in U.S. Pat. No. 
4,558,440 issued on Dec. 10, 1985. 
Also, U.S. Pat. No. 4,573,149 issued on Nov. 25, 1986 discloses an example 
using a half wavelength plate and a polarizing beam splitter (hereinafter 
referred to as PBS) instead of the half-mirror 12 and the analyzers 
7.sub.1 and 7.sub.2. This example will hereinafter be described with 
reference to FIG. 3 of the accompanying drawings. 
In FIG. 3, a light beam emitted from a light source (a laser diode) 27 
enters a recording medium 31 via a beam splitter 24, a mirror 25 and an 
objective lens 26. The light beam reflected by the medium 31 then again 
passes to the beam splitter 24 via the objective lens 26 and the mirror 
25, and is reflected toward a condensing lens 23 by the beam splitter 24. 
The reference numeral 33 indicates the direction of polarization when the 
direction of polarization when the light beam is emitted from the light 
source 27 is not subjected to the Kerr effect but intactly advances toward 
the condensing lens 23. When the light beam is reflected by a medium 31, 
the light beam enters the condensing lens 23 with the plane of 
polarization rotated by an angle of Kerr rotation .+-..theta..sub.k with 
respect to the direction indicated by reference numeral 33 due to the Kerr 
effect. The plane of polarization is rotated by 45.degree. by a half 
wavelength plate 34 whose optic axis is inclined by 22.5.degree. with 
respect to the direction indicated by reference numeral 33 and is 
polarized in a direction indicated by reference numeral 35. Part of the 
light beam thus polarized is reflected by a beam splitter 22 and received 
by a servo signal detecting sensor (a photoelectric conversion element) 
28, and on the other hand, the light beam transmitted through the beam 
splitter 22 enters PBS 21. The light beam which has entered the PBS 21 is 
divided into two light beams polarized in directions orthogonal to each 
other, and these light beams are detected by magneto-optical signal 
detecting sensors (photoelectric conversion elements) 29 and 30. By taking 
the difference between the detection signals of these sensors, the 
information recorded on said medium is reproduced. 
However, in the construction shown in FIG. 3, the number of parts is great 
and the azimuths of the half wavelength plate 34 and the PBS 21 must be 
strictly adjusted relative to the direction of polarization of the light 
beam and therefore, assembly has not been easy. 
SUMMARY OF THE INVENTION 
It is the object of the present invention to solve the above-noted problems 
peculiar to the prior art and to provide a magneto-optical information 
reproducing apparatus which is easy to assemble and adjust. 
The above object of the present invention is achieved by a magneto-optical 
information reproducing apparatus comprising: 
means for applying a light beam polarized in a predetermined direction to a 
recording medium on which information is magnetically recorded; 
means for dividing the reflected or transmitted light beam from said medium 
modulated into a polarized state in conformity with said information by 
the magneto-optical effect into two light beams polarized in directions 
orthogonal to each other, said means being disposed so that the directions 
of polarization of said divided two light beams form an angle of 
45.degree. with respect to the predetermined direction of polarization of 
said applied light beam; and 
detecting means for detecting said divided two light beams.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 4 and 5 are a schematic perspective view and a side view, 
respectively, showing an embodiment of the magneto-optical information 
reproducing apparatus of the present invention. In these figures, members 
similar to those in FIG. 3 are given similar reference numerals. 
In FIG. 4, reference numeral 31 designates a magneto-optical information 
recording medium. The recording medium 31 is exemplarily shown as a disk 
rotated by a spindle motor, not shown. A light beam emitted from a laser 
diode 27 which is a light source and polarized in a predetermined 
direction indicated by arrow 33 is transmitted through a beam splitter 24, 
is reflected by a deflecting mirror 25 and is condensed on the medium 31 
by an objective lens 26. The beam splitter 24 is for separating the 
applied light beam to the medium and the reflected light beam from the 
medium. This beam splitter 24, as described in the aforementioned U.S. 
Pat. No. 4,561,032, may be designed so as to have such a characteristic 
which increases a polarized component in a direction orthogonal to the 
polarized component of the reflected light beam in the direction of arrow 
33 relative to the latter polarized component. 
The light beam applied to the medium 31 is modulated into its polarized 
state in conformity with the information recorded on the medium, by the 
magneto-optical effect and is reflected. This reflected light beam again 
passes through the objective lens 26 and mirror 25 and is reflected by the 
beam splitter 24. The reflected light beam is further converged by a 
condensing lens 23, and part thereof is caused to branch off by a beam 
splitter 22 for a servo sensor and is received by a servo signal detecting 
sensor 28. The sensor 28 detects servo signals such as a focusing error 
signal and a tracking error signal from the branching-off light beam by a 
known method. 
The light beam transmitted through the beam splitter 22 enters a polarizing 
beam splitter (PBS) 21 for detecting a magneto-optical signal, and is 
divided thereby into two light beams polarized in directions orthogonal to 
each other. The divided light beams are received by photoelectric 
conversion elements (hereinafter referred to as RF sensors) 29 and 30 for 
detecting a magneto-optical signal. The PBS 21 is disposed so that the 
directions of polarization of the two light beams divided thereby form an 
angle of 45.degree. with respect to the predetermined direction of 
polarization (indicated by arrow 33) of the applied light beam. The PBS 21 
is formed by joining two pyramidal prisms together and providing an 
interference film on the joined surface thereof. By P-polarized light beam 
transmitted and S-polarized light being reflected on the joined surface, 
the light beam is divided. The thus constructed PBS 21 has a cube-like 
configuration. Accordingly, in the present embodiment, the PBS 21 is held 
in a state in which it has been rotated by 45.degree. about the optic axis 
of the reflected light beam. 
The signals detected by the RF sensors 29 and 30 are modulated into 
opposite phases and the same amplitude corresponding to the information 
recorded on the medium 31. Accordingly, by differentiating these output 
signals by a differential amplifier 38, any noise component is eliminated 
and a reproduction signal (RF signal) of a high C/N ratio is obtained. 
Each of the elements is contained in an optical head carriage 37. The 
carriage 37 is driven in the direction of arrow R (the radial direction of 
the medium 31) by a motor, not shown. The mirror 25 and the beam splitters 
22 and 24 shown in FIG. 4 are adhesively or otherwise secured to a 
mounting surface 32 provided on the frame 39 of the carriage 37. On the 
other hand, the PBS 21 is adhesively or otherwise secured to a mount 36 
provided on the frame 39 and having a V-shaped mounting surface. The 
mounting surface of the mount 36 is formed by two surfaces including the 
direction of the optic axis of the light beam entering the PBS 21 and the 
directions of polarization of the light beams divided by the PBS 21. 
Thus, according to the present invention, a half wavelength plate becomes 
unnecessary and the azimuth of polarization of the PBS may simply be 
adjusted and therefore, assembly becomes very easy. 
FIG. 6 is a schematic perspective view showing another embodiment of the 
present invention. In FIG. 6, members similar to those in FIG. 4 are given 
similar reference numerals and need not be described in detail. 
In the present embodiment, the PBS 21 is adhesively secured to and formed 
integrally with the beam splitter 22. As compared with the embodiment 
shown in FIG. 4, the present embodiment further has the advantage that the 
mount 36 is unnecessary. 
The present invention permits various applications besides the 
above-described embodiments. For example, the beam splitter 22 may be a 
polarizing beam splitter to enhance the C/N ratio of the reproduction 
signal. In such a case, the beam splitter 22 is designed so as to increase 
the polarized component in a direction orthogonal to the component of the 
applied light beam to the medium in the direction of polarization, of the 
light beam entering the beam splitter 22, relative to the latter 
component. Also, when the medium is of the transmission type, design is 
made such that information is reproduced from the light beam transmitted 
through the medium. Further, the shape of the medium is not limited to the 
disk-like shape, but may be any shape such as a tape-like shape or a 
card-like shape. The present invention covers all such applications 
without departing from the scope thereof as defined in the appended claims 
.