Magneto-optical means for reading disks by comparing signals at leading and trailing edges of laser beam

Recording medium on which information is magneto-optically recorded is irradiated with a linearly polarized light beam. A light beam reflected from or passed through the recording medium is changed in phase by a wave plate. Through an analyzer the differently polarized components of the light beam from the wave plate are combined by aligning the polarizing directions of the light beam. The light intensities of the forward and backward parts of the moving direction of the recording medium in the far field of the light beam from the analyzer are detected by a photo detector. A difference between the detected signals of the forward and backward parts is generated to reproduce the information recorded on the recording medium. Furthermore, the light beam from the wave plate is splitted into two light beams linearly polarized in mutually orthogonal directions. Two light intensities of the two light beams for the forward and backward parts of the moving direction of the recording medium in the far field of the respective light beams are detected by two photo detectors. Two differences between the outputs from the two photo detectors are determined by two subtractors. A difference or sum between the two subtractors is determined and the information is reproduced on the basis of thus determined difference or sum.

BACKGROUND OF THE INVENTION 
The present invention relates to a magneto-optical disk reading apparatus 
for magneto-optically recording and reproducing information. 
Magneto-optical disk reading apparatuses irradiate a recording medium 
consisting of magnetic material with a laser beam to thermo-magnetically 
record information in the variation form of magnetization on the medium, 
and magneto-optically read out the information by utilizing the variations 
in the polarization of the light reflected from or passed from the medium 
irradiated. They are attracting interest as highly useful filing 
apparatuses capable of not only permitting, like optical disk units, 
high-density large-capacity recording but also erasing information and 
reusing the medium for recording other information. 
Most of such conventional magneto-optical disk reading apparatuses convert 
polarization variations of the light from the medium into intensity 
variations of the light by the use of an analyzer to read out the signals 
recorded on the medium. According to such signal reading, the light 
reflected from the part where the magnetization on the medium has varied, 
i.e., the part irradiated with the recording light beam at the time of 
recording, is either bright or dark and reproduced signals are obtained by 
detecting intensity variations of the whole reflected light as similar to 
an optical disk apparatus using a reflectance varying-type recording 
carrier. As methods for such detection, there have been proposed a simple 
method which detects the light beam having passed the analyzer with a 
single photodetector (an APD for instance) and a differential detecting 
method which detects two light beams splitted through a polarizing beam 
splitter by two photodetectors and determines the difference between the 
outputs of the two photodetectors for information reading. Both detect the 
variations in the total luminous energy of the light having passed the 
analyzer or the polarizing beam splitter, and in this respect are 
essentially the same as the detection of variation between bright and 
dark. 
However, with any of the aforementioned conventional magneto-optical disk 
reading apparatuses, the light beam spot focused on the medium for the 
reading has some expanse (intensity distribution), so that the intensity 
of the reflected light does not vary steeply, resulting in the 
disadvantages that the reproduced signals tend to be inaccurate, is 
susceptible to the influences of the intensity variation of the 
irradiating light, reflectance variation of the medium and characteristics 
variation of the reproducing circuit, and readily invites reading errors 
in signal reproduction. There is the further disadvantage that, if 
information is to be recorded and reproduced in terms of the length 
variation of the region in which information is recorded in the form of 
magnetization changes (pulse width modulation) and if the region is 
detected by the intensity change of the reflected light, signal 
disturbance will be increased to make it impossible for information to be 
accurately reproduced unless the D.C. (direct current) component, or the 
low frequency component, is accurately amplified. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a magneto-optical disk 
reading apparatus which has a high recording density, excels in the 
stability of reproduced signals and is hardly susceptible to reading 
errors. 
Another object of the invention is to provide a magneto-optical disk 
reading apparatus which excels in the quality of reproduced signals, is 
relatively free from noise and therefore hardly susceptible to reading 
errors. 
According to the present invention, a recording medium on which information 
is magneto-optically recorded is irradiated with a linearly polarized 
light beam. A light beam reflected from or passed through the recording 
medium is changed in phase by a wave plate. Through an analyzer the 
differently polarized components of the light beam from the wave plate are 
combined by aligning the polarizing directions of the light beam. The 
light intensities of the forward and backward parts of the moving 
direction of the recording medium in the far field of the light beam from 
the analyzer are detected by a photo detector. A difference between the 
detected signals of the forward and backward parts is generated to 
reproduce the information recorded on the recording medium. Furthermore, 
the light beam from the wave plate is splitted into two light beams 
linearly polarized in mutually orthogonal directions. Two light 
intensities of the two light beams at their forward and backward parts 
corresponding to the forward and backward moving directions of the 
recording medium are detected by two photo detectors. Two difference 
between the outputs from the two photo detectors are determined by two 
subtractors. A difference or sum between the two subtractors is determined 
and the information is reproduced on the basis of thus determined 
difference or sum. 
Other objects and features of the present invention will be clarified from 
the following description with the accompanying drawings

PREFERRED EMBODIMENTS OF THE INVENTION 
Referring to FIG. 1, a preferred embodiment of the invention includes a 
semiconductor laser device 2, supplied with a current from a laser driving 
circuit 1, for emitting a laser beam for signal reading and a collimator 
lens 3 for collimating the divergent laser beam emitted by the device 2 
into a parallel laser beam. A polarizer 4 aligns the polarization of the 
laser beam in a single linear direction. A beam splitter 5, consisting of 
a non-polarizing half mirror for instance, is intended for separating the 
incident laser beam into two laser beams of a straight ahead direction and 
a rectangularly deviating direction. A convergent lens 6 converges the 
parallel laser beam, and irradiates the surface of a medium 7 on a 
recording carrier 8 with the converged laser beam as a tiny spot of light. 
Whereas the position of the convergent lens 6 relative to the medium 7 is 
so controlled that the tiny light spot may be well focused and formed on 
its prescribed position, this position control will not be explained 
herein as it is not directly relevant to the purpose of the present 
invention. 
The beam focused on the surface of the medium 7 is reflected (or passed) 
with its polarizing direction slightly changed according to the magnetized 
(recording) condition of the medium 7. The beam reflected by the medium 7, 
with its path deflected by the beam splitter 5, passes a wave plate 9, 
which has a character to vary in refractive index with the polarized 
direction of the light beam passing it, and functions to advance (or 
delay) the phase of a light beam whose polarizing direction is deviated by 
90.degree.. Note that, any well known wave plate can be used as the wave 
plate 9, such as one that is disclosed in the U.S. Pat. No. 4,546,463 to 
Opheij et al. The present invention, as will be described hereinafter, 
utilizes the interference between light beams different in phase, and the 
intensity variation due to interference is the most efficient (the 
greatest) when the phase difference between the differently polarized 
beams is 90.degree. (quarter-wave). Therefore, it is desirable to use a 
quarter-wave plate as the wave plate 9. In the following description, a 
region of the medium 7 where information is recorded will be referred to 
as a "recorded region". region where no information is recorded will be 
referred to as a "nonrecorded region[. The direction of magnetization of 
the recorded region is reverse so that of the non-recorded region. If the 
wave plate 9 is so positioned that a reflected light beam from the now 
recorded region of the medium 7 (this polarized beam is tentatively named 
the P-polarized beam) may suffer no phase deviation and a beam polarized a 
direction normal to the P-polarized beam (tentatively named the 
S-polarized beam) suffers a 90.degree. (quarter-wave) phase delay, the 
beam reflected from the recorded region has a component polarized in the 
rectangular direction, and therefore the beam having passed the wave plate 
9 has both a P-polarized beam component, which has no phase delay, and an 
S-polarized beam component, whose phase is delayed by 90.degree.. 
An analyzer 10 is intended for aligning different polarizing directions of 
beams and combining these beams. The P- and S-polarized beams, differing 
in phase, are combined by this analyzer 10 and interfere with each other. 
Whereas this interference occurs between every pair of beams differing in 
phase, in particular the interference between the P-polarized beam 
reflected from the non-recorded region and the S-polarized component of a 
beam reflected from the recorded region causes, when the leading or 
trailing edge of the recorded region is irradiated with a light spot, the 
beam having passed the analyzer 10 to have an intensity difference between 
forward and backward parts 1F and 1B of the reflected beam in the moving 
direction of the medium movement. A photo-detector 11, whose light 
receiving face is divided into forward and backward parts with respect to 
the moving direction of the medium, receives the beam, which has passed 
the analyzer 10, in the stage of being split into forward and backward 
directions, and supplies electric currents each corresponding to the 
intensity of one or the other of the incident beams. By detecting and 
amplifying the difference between these output currents with a 
differential amplifier 12, there is obtained a readout signal 101 forming 
a positive or negative peak when the irradiating beam hits the leading or 
trailing edge of the recorded region. By processing this readout signal 
101 with a readout signal processing circuit 13, information recorded on 
the recording medium 8 is reproduced. 
FIG. 2 is a diagram for explaining the principle of recorded signal reading 
according to the invention. Information is recorded on the medium 7 in the 
form of a change (reversal) of the magnetizing direction. As the surface 
of the medium 7 is irradiated with light, the polarizing angle of the 
reflected beam is slightly deviated from that of the incident beam by the 
Kerr effect or the Faraday effect. The reflected light, restored into a 
parallel beam by the convergent lens 6, passes the wave plate 9, and the 
S-polarized component of the reflected light is delayed in phase behind 
the P-polarized component of same. If the wave plate 9 is so arranged here 
that only the beam reflected from the recorded region (the P region in the 
diagram) on the medium 7 have an S-polarized component, the beam reflected 
from the recorded region (the P region) having passed the wave plate 9 
will have a component whose phase is behind that reflected from the 
non-recorded region (the N region in the diagram), which has only a 
P-polarized component . 
This phase delay of the S-polarized component takes effect to liken the 
recorded region to a depressed pit. Therefore, combining by the analyzer 
10 the reflected beam from the N region and the S-polarized component from 
the P region the same effect as the inclination of the reflected beam at 
the leading edge of the recorded region and an intensity difference 
between the forward and backward parts 1F and 1B of the reflected beam in 
the moving direction of the medium movement, so does the formation of a 
light spot on the boundary between the P and N regions result in an 
intensity difference in the reflected light having passed the analyzer 10 
between forward and backward parts 1F and 1B. Since this intensity 
difference is proportional to the product of the light amplitude (the 
square root of intensity) of the P-polarized component from the N region 
and that of the S-polarized component from the P region, a greater level 
variation can be achieved than the intensity variation of the S-polarized 
component alone. A circle 14 shown in the upper part of FIG. 2 represents 
the distribution of brightness and darkness in the light beam having 
passed the analyzer 10. 
FIG. 3 is a diagram illustrating the relationship between magnetization of 
the medium 7 and a readout signal 101 in the embodiment of FIG. 1. In the 
figure, a waveform represents the waveform of the readout signal 101, and 
P.sub.1 and P.sub.2 denote the recorded regions. While there is no 
intensity difference in the light coming incident on the photodetector 11 
between the forward and backward parts 1F and 1B and the readout signal 
101 is at the zero level when the non-recorded region is irradiated, there 
does occur a forward-backward intensity difference in the light coming 
incident on the photodetector 11 and the readout signal 101 deviates in 
the positive direction when the light spot irradiates the leading edge 8L 
of the recorded region P.sub.1. When the center of the light spot is 
applied on the boundary between the non-recorded part and the recording 
region P.sub.1, the level of the readout signal 101 reaches its positive 
peak. When the light spot is completely within the recorded region 
P.sub.1, there will be no forward-backward intensity difference in the 
light coming incident on the photodetector 11, and the level of the 
readout signal 101 returns to zero. When the light spot is applied on the 
trailing edge 8T of the recorded region P.sub.1, there occurs the 
intensity difference in the light coming incident on the photodetector 11 
in the opposite direction to what occurs when it is applied on the leading 
edge, and the readout signal 101 deviates in the negative direction. Since 
the polarity of the readout signal 101 is reversed between the leading and 
trailing edges 8L and 8T of the recorded region, the readout signal 101 
obviously has no D.C. component, and there is no need for accurate 
amplification to the low frequency component. By detecting the timing of 
the positive and negative peaks of this readout signal 101, and the timing 
at which the light spot passes the leading and trailing edges 8L and 8T of 
the recorded region on the medium 7, information recorded on the medium 7 
can be accurately extracted. 
As hitherto described, according to the present invention, a wave plate and 
an analyzer are arranged on the path of the reflected light from the 
recording medium for the purpose of reading out signals recorded on a 
photomagnetic disk. This arrangement results in a forward-backward 
intensity difference in the moving direction of the medium in the far 
field of the reflected light having passed the analyzer when the leading 
or trailing edge of a recorded region, formed on the medium, is irradiated 
with a reading beam. By receiving this light beam with a photodetector, 
there is obtained a readout signal which attains a positive or negative 
peak when the reading beam is applied on the leading or trailing edge, 
respectively, of the recorded region. By detecting the timing of this 
peak, recorded information can be accurately reproduced without being 
affected by variations in light intensity or the medium's reflectance, 
among other factors. 
The present invention has the further advantage that the readout signal has 
no low-frequency component, dispenses with accurate amplification of 
signals even to their low-frequency components, which is required in 
intensity variation detecting, and accordingly helps simplify the circuit 
composition. 
FIG. 4 is a block diagram illustrating another preferred embodiment of the 
invention. The first embodiment illustrated in FIG. 1 reads out a signal 
by converting a variation in polarization due to the magnetization of the 
medium into a phase delay (or advance) and detecting a change in the far 
field pattern resulting from that phase difference. However, since the 
phase difference of a light beam may also result from unevenness of the 
medium surface, there is the disadvantage that the readout signal tends to 
contain much noise due to even minute irregularity on the medium surface 
or a slight variation in the intensity distribution of the irradiating 
laser beam. 
FIG. 4 proposes an apparatus that represents a solution to this problem. 
The semiconductor laser device 2, supplied with a current from the laser 
driving circuit 1, emits a laser beam for signal reading. The collimator 
lens 3 collimates the divergent laser beam emitted by the semiconductor 
laser device 2 into a parallel laser beam. The polarizer 4, intended for 
aligning polarization of the laser beam in a single linear direction, may 
be dispensed with because a light beam emitted by the semiconductor laser 
device 2 is usually polarized linearly and therefore the prescribed 
performance characteristics can be achieved without it. The half mirror 5, 
which may consist, for instance, of a non-polarizing half mirror which 
transmits a part of a light beam coming incident irrespective of the 
polarized condition and reflects another part in the rectangular 
direction, is intended for splitting a part of the reflected light from 
the path of the incident light. The object lens 6 focuses the incident 
light beam having passed the half mirror 5, and irradiates the surface of 
the recording medium 7 on the recording carrier 8 with the focused light 
beam as a tiny spot of light. The beam irradiating the surface of the 
recording medium 7 is reflected (or transmitted) with its polarized 
direction slightly changed according to the magnetized condition (the 
recording condition) of the medium 7. 
The medium 7 is magnetized in its information recording part (recorded 
region) in a single direction normal to the medium surface, and in the 
erased part (non-recorded region) in the direction reverse to that in the 
recorded region. When the medium 7 is irradiated with a linearly polarized 
beam, the reflected or transmitted(passed) beam is inclined (rotated) by 
the Kerr effect or the Faraday effect in the polarizing direction with 
respect to the incident beam. This rotation of linearly polarized light is 
known as optical rotation, and can be explained by the phase advance and 
delay corresponding to clockwise and counterclockwise circular 
polarization, but for the sake of simplicity it may be regarded as the 
rotation of linearly polarized light. 
The beam reflected from the medium 7 is restored into a parallel beam by 
the object lens 6. The intensity distribution of this beam magnified and 
collimated by the object lens 6 is known as the far field pattern. A part 
of the reflected light having passed the object lens 6 is reflected, with 
its path turned rectangularly, by the half mirror 5, and directed toward 
the quarter-wave plate 9. The quarter-wave plate 9 has a character to 
delay, relative to a beam linearly polarized in a certain direction 
(normal beam), the phase of another beam linearly polarized in a direction 
orthogonal thereto (abnormal beam) by 90.degree. (equivalent to a quarter 
wavelength), and usually used for converting a linearly polarized beam 
into a circularly polarized beam or a circularly polarized beam into a 
linearly polarized beam. In this instance, where the reflected light from 
the medium 7 coming incident on the quarter-wave plate 9 is linearly 
polarized, it will be easier to understand if the quarter-wave plate 9 is 
supposed to be so arranged as to convert the linearly polarized beam into 
a circularly polarized beam, and therefore such an example is given, but 
the quarter-wave plate 9, as will be explained afterwards, need not be so 
arranged as to convert the linearly polarized beam reflected from the 
medium into a circularly polarized beam, and instead may be arranged at 
any desired inclination (rotational angle). 
The light having passed the quarter-wave plate 9 is, for instance, 
circularly polarized and can be regarded as that resulting from the 
combination of two linearly polarized beams orthogonal to each other. A 
polarizing beam splitter 15 splits the light having passed the 
quarter-wave plate 9 into two linearly polarized and mutually orthogonal 
beams (for instance S-polarized and P-polarized beams) and substantially 
equal amplitudes (or intensities). In order to substantially equalize the 
amplitudes (or intensities) of these split light beams, it is necessary 
that the polarizing directions (S-polarization and P-polarization) of the 
two beams be approximately 45.degree. off the polarizing direction of the 
normal beam (or the abnormal beam) at the quarter-wave plate. Therefore, 
if the polarizing direction of the normal beam of the quarter-wave plate 9 
(i.e. the direction of optical axis of the quarter-wave plate) is, for 
instance 45.degree. off the direction of linear polarization of the 
reflected light (at this time the light having passed the quarter-wave 
plate 9 is substantially circularly polarized), the polarizing beam 
splitter 15 should be so arranged as to make the polarizing directions of 
the light beams split by the polarizing beam splitter 15 equal to 
directions 45.degree. further off the polarizing direction of the normal 
beam (or the abnormal beam), i.e. that of the reflected light (or the 
direction orthogonal thereto). 
If the relationship between the polarizing direction of the normal beam at 
the quarter-wave plate 9 and those of the light beams split by the 
polarized beam splitter 15 (45.degree. off each other) is satisfied, the 
inclination (rotational angle) of the quarter-wave plate 9 or the 
polarizing beam splitter 15 may be set as desired. As the polarizing beam 
splitter 15, there may also be usable, a Wollaston polarizing prism or 
some other polarizing beam splitter which slightly shifts or bends the 
paths of the two split beams with respect to each other. 
As the polarizing beam splitter 15 functions to align the polarized 
conditions of the light beams it emits in a common direction (analyzing 
action) as well as splitting of the incident beam, the beams having passed 
the polarizing beam splitter and differing in phase interfere with each 
other. 
The actions of the above described quarter-wave plate and polarizing beam 
splitter convert the change in polarization (rotation) by the 
magnetization of the recording medium 7 into a phase deviation of a light 
beam. For instance, if the magnetization of the recorded region slightly 
rotates the polarizing angle of the reflected light from the recorded 
region clockwise, the S-polarized component coming out of the polarizing 
beam splitter 15 is slightly advanced in phase. In the non-recorded region 
where the direction of magnetization is reverse, the polarizing angle of 
the reflected light from it is slightly rotated counterclockwise, contrary 
to that from the recorded region, and the S-polarized component from the 
polarizing beam splitter 15 is slightly delayed in phase. 
This relationship is reversed for the P-polarized beam from the polarizing 
beam splitter 15. Thus, immediately after the polarizing beam splitter 15, 
the difference in polarizing angle between the reflected beams from the 
recorded region and the non-recorded region results in a phase difference 
between the two reflected components. This phase relationship is reversed 
between the two beams (S-polarized and P-polarized) resulting from 
splitting by the polarizing beam splitter 15. 
For instance, viewed from the S-polarized beam (directed toward a 
photodetector 16) having come out of the polarizing beam splitter 15, the 
phase of the reflected beam from the recorded region looks ahead of that 
of the reflected light from the non-recorded region, and the recorded 
region appears to be convex. On the other hand, viewed from the 
P-polarized beam (directed toward a photodetector 17), the phase of the 
reflected beam from the recorded region looks behind, and the recorded 
region, appears to be concave. Thus, where the recording region appears to 
be convex or concave depending on how it is viewed, or in the presence of 
a phase difference between the beams reflected from the recorded region 
and from the non-recorded region, there occurs a light intensity 
difference between the forward and backward in the moving direction of the 
medium in the far field of the reflected beam (or the transmitted beam) at 
the recorded region edge of the recorded region. Recorded information can 
be detected, as was described with reference to FIG. 1, by receiving such 
beams with a divided photodetector. 
The two light beams (S-polarized and P-polarized) resulting from splitting 
by the polarizing beam splitter 15 come incident on the photodetectors 16 
and 17, respectively, whose light receiving faces are divided into at 
least two forward and backward parts 1F and 1B each in the moving 
direction of the medium (indicated by an arrow 30 in FIG. 4), and which 
receive, divided into forward and backward parts 1F and 1B the far field 
patterns of the light beam reflected from the medium 7 and coming out of 
the polarizing beam splitter 15 and supply currents, each corresponding to 
the intensity of one or the other of the incident beams. 
When the leading edge of the recorded region is irradiated with a reading 
beam, the area ahead of the far field of the reflected beam will become 
bright if the recorded region is regarded as convex, or that behind the 
far field will, if the recorded region is regarded as concave. In the 
structure of this embodiment, therefore, if the leading edge of the 
recorded region is irradiated, the part ahead (the lower part in FIG. 4) 
of the photodetector 16 will receive a greater quantity of light, and so 
will the part behind (the right hand side in FIG. 4) of the photodetector 
17. 
A differential amplifier 18 receives output currents from the forward and 
backward parts 1F and 1B into which the photodetector 16 are split, 
amplifies the difference between them, and outputs it as an intensity 
differential signal 102. In the above described instance, this intensity 
differential signal 102 forms a positive peak at the leading edge and a 
negative peak at the trailing edge of the recorded region. Meanwhile, 
another differential amplifier 19 takes a similar action on the 
photodetector 17, and outputs another intensity differential signal 103. 
In the above described instance, this intensity differential signal 103, 
forming a negative peak at the leading edge of the recorded region, is 
reverse in polarity to the intensity differential signal 102. A 
subtracting circuit 20 adds the information contents of these two 
intensity differential signals 102 and 103 (the peaks at the leading and 
trailing edges of the recorded region) in the form of taking the 
difference between the two signals, amplifies the sum and outputs it as a 
readout signal 104. As is obviously understood, if the connection of the 
differential amplifier 18 or 19 to the photodetector 16 or 17 is inverse, 
an adding circuit can be used in place of the subtracting circuit 20. By 
reading the readout signal 104 supplied by this subtracting circuit 20 and 
processing it with a signal processing circuit 21, information recorded on 
the recording medium 7 is reproduced. 
As intensity variations in the forward and backward region due to the 
magnetization of the medium 7, in the reflected beams coming incident on 
the photodetectors 16 and 17 are reverse in polarity with each other, and 
the readout signal 104 is obtained in the form of taking the difference 
between these variations, noise of the same phase resulting from 
variations in the medium's reflectance, the power of the irradiating laser 
beam and so forth is kept to the minimum. Meanwhile, intensity variations 
in the forward and backward directions in the reflected light due to the 
surface irregularities of the medium 7 convexes or concaves on the medium 
surface or variations in the light intensity distribution of the 
irradiating laser beam, unlike those due to magnetization, are the same in 
phase between the photodetectors 16 and 17. Accordingly, by figuring out 
the difference between the intensity differential signals 102 and 103, the 
intensity variations in the forward and backward directions in the 
reflected light due to these non-magnetization factors is canceled, and 
only the variations in the polarization of the reflected light due to 
magnetization, or the recorded information, are accurately taken out, 
resulting in low-noise high-quality reproduction of desired signals. 
FIG. 5, intended for explaining the principle of recorded signal reading 
according to the invention, is a vector diagram schematically illustrating 
how the optical activity (rotation of a linearly polarized beam) due to 
the Faraday effect or Kerr effect is converted into a phase advance or 
delay as the beam passes the quarter-wave plate 9 and the polarizing beam 
splitter 15. In FIG. 5(A), the V direction represents the polarization 
vector of the linearly polarized beam, to which corresponds, for instance, 
the polarization of the reflected beam from the non-recorded part of the 
recording medium 7. The amplitude vector of this beam is represented by 
E.sub.o =cos.omega.t. 
FIG. 5(B) illustrates the polarization vector of the linearly polarized 
beam shown in FIG. 5(A) after having passed the quarter-wave plate 9. With 
.alpha. representing the angle at which the direction O in which the 
quarter-wave plate 9 polarizes the normal light is changed into the 
S-polarizing direction, a linearly polarized beam E.sub.o coming incident 
with .alpha. is 45.degree. passes it, converted into a counterclockwise 
circularly polarized beam. The amplitude of this passing beam being 
represented by .phi..sub.o, this .phi..sub.o delays by 
##EQU1## 
the phase of only the abnormal light component cos.omega.t of E.sub.o, 
but not its normal light component E.sub.oo =cos.alpha.e.sup.i.alpha. 
cos.omega.t, so that and this can be rearranged into 
##EQU2## 
which means counterclockwise circular polarization, so that it is 
demonstrated that the quarter-wave plate 9, whose direction of polarizing 
the normal light is 45.degree. inclined, converts a linearly polarized 
beam into a circularly polarized beam. 
Meanwhile FIG. 5 (C) illustrates the polarization of a beam whose 
polarizing angle is inclined by .theta. with respect to the V direction, 
to which corresponds, for instance, the reflected beam from the magnetized 
pit (recording part) of the recording medium 7. The vector of this 
linearly polarized beam is represented by E.sub.p =e.sup.i.theta. 
.multidot.cos.omega.t. FIG. 5 (D) illustrates the polarization vector of 
the linearly polarized beam shown in (C) after having passed the 
quarter-wave plate 9, whose direction of polarizing the normal light is 
inclined, like in (B), by .alpha. with respect to the V direction. This 
amplitude vector being represented by .phi..sub.p, the phase of only the 
abnormal light component 
##EQU3## 
but not its normal light component E.sub.oo 
=cos(.alpha.-.theta.).multidot.e.sup.i.alpha. .multidot.cos.omega.t is 
delayed by so that and this can be rearranged into 
##EQU4## 
Supposing .alpha.=(45.degree.) and Q is small here, .phi..sub.p will also 
means counterclockwise circular polarization substantially like 
.phi..sub.o. FIG. 5 (E) illustrates the amplitude vector of what has an 
S-polarized component out of the two light beams into which the polarizing 
beam splitter 10 splits the incident beam thereon. 
FIG. 5 (F) illustrates the amplitude vector of the other split beam which 
has a P-polarized component. The S-polarized beam from the polarizing beam 
splitter 15 forms an angle of 
##EQU5## 
with respect to the normal light direction (O direction) of the 
quarter-wave plate 9. Therefore the S-polarized beam from the polarizing 
beam splitter 15 forms an angle of with respect to the V direction in 
which the reflected beam from the non-recorded part, shown in FIG. 5 (A), 
is polarized. As will be readily understood, where the V direction is the 
same as the S direction. When a light beam having an amplitude 
(polarization) vector .phi. comes incident on the polarizing beam splitter 
15, which splits an incident light beam into two beams, one S-polarized in 
the direction of the angle and the other P-polarized in the direction, 
orthogonal to it, of the angle, the amplitude vector of the S-polarized 
component is (where Re(Z) represents the real number component of Z), and 
that of the P-polarized component Therefore, after the light beam with 
an amplitude vector of .phi..sub.o, resulting from the passage of the 
quarter-wave plate 9 by the reflected beam from the non-recorded part, 
passes the polarizing beam splitter 15, the amplitude of its S-polarized 
component is 
##EQU6## 
and that of its P-polarized component is 
##EQU7## 
On the other hand, for the light beam with an amplitude vector of 
.phi..sub.p resulting from the passage of the quarter-wave plate 9 by the 
reflected beam from the recorded region (magnetized recording part), the 
amplitude of the S-polarized beam, one of the two beams into which it is 
split by the polarizing beam splitter 15, is 
##EQU8## 
and that of P-polarized beam is 
##EQU9## 
These equations reveal that, out of the beams resulting from splitting by 
the polarizing beam splitter 15, the S-polarized beam has its component 
E.sub.ap, attributable to the reflected beam from the recorded region, 
ahead of the component E.sub.ao, attributable to that from the 
non-recorded part, by .theta. in phase. It is seen that, in the 
P-polarized beam on the other hand, the component E.sub.bp, attributable 
to the reflected beam from the recorded region, is behind the component 
E.sub.bo, attributable to that from the non-recorded part, by .theta. in 
phase. Thus, viewed from the S-polarized beam, the recorded region appears 
to be convex, while it appears to be concave viewed from the P-polarized 
beam. It may also be readily understood that this relationship holds 
irrespective of the degree of .alpha., the diagrammatic angle of the 
quarter-wave plate 9. Thus in each of the two light beams resulting from 
splitting by the polarizing beam splitter 15, there occurs a phase 
difference between the reflected beam from the recorded region and that 
from the non-recording part, with the result that there arises a light 
intensity difference between the forward and backward parts, in the moving 
direction of the medium, of the far field of the reflected beam at the 
(leading or trailing) edge of the recorded region. Recorded information 
can be detected by receiving these beams with photodetectors, which are 
divided as described above, and thereby detecting the forward-backward 
variation of light intensity. 
FIG. 6 is a diagram illustrating the relationship between the phases of the 
recorded regions and the readout signal 104 in the embodiment illustrated 
in FIG. 4. In the diagram, P.sub.1 and P.sub.2 represent pits (recording 
parts), and the waveforms of the intensity differential signal 102, the 
other intensity differential signal 103 and the readout signal 104, 
respectively. 
When a light spot 40 is irradiating a non-recorded region, the beams coming 
incident on the photodetectors 16 and 17 have no forward-backward 
intensity differences and both intensity differential signals 102 and 103 
are at the zero level, but when the light spot 40 hits the leading edge of 
the recorded region P.sub.1, the beams coming incident on the 
photodetectors 16 and 17 come to have the intensity differences, and the 
intensity differential signals 102 and 103 deviate in the positive and 
negative directions, respectively. After that, when the light spot 40 
fully enters the recorded region P.sub.1, the beams coming incident on the 
photodetectors 16 and 17 no longer have intensity differences, and both 
intensity differential signals 102 and 103 return to the zero level. 
When the light spot 40 hits the trailing edge of the recorded region, the 
beams coming incident on the photodetectors 16 and 17 come to have the 
intensity differences in the reverse direction to what occurred when the 
light spot 40 was on the leading edge, and the intensity differential 
signals 102 and 103 deviate in the negative and positive directions, 
respectively. As the polarities of the intensity differential signals 102 
and 103 are contrary between the leading and trailing edges of the 
recorded region, these signals have no D.C. components, and therefore need 
not be accurately amplified to the low frequency region. By taking and 
amplifying the difference between these two intensity differential signals 
102 and 103, there is obtained the readout signal 104 which attains a 
positive peak at the leading edge and a negative peak at the trailing edge 
of the recorded region. Nor does this readout signal 104, which 
essentially has no D.C. component, need be accurately amplified to the low 
frequency region. 
Forward-backward intensity variations of the reflected beams due to other 
factors than the recorded region (magnetization), including slight 
convexes and concaves on the medium surface or variations in intensity 
distribution of the irradiation laser beam, occur in a common direction to 
the beams coming incident on the photodetectors 16 and 17, and therefore 
these variations are reflected on the intensity differential signals 102 
and 103 as variations in the same direction (polarity). As the readout 
signal 104 is obtained in the form of taking the difference between the 
intensity differential signals 102 and 103, these variations in the same 
direction cancel each other. Therefore the readout signal 104 accurately 
catches only the variations of the reflected beam due to the recorded 
region (magnetization), and variations due to other factors (noise) are 
kept to the minimum. 
As hitherto described, according to the present invention, there are 
arranged on the path of the light beam reflected or transmitted by a 
recording medium, for the purpose of reading out signals recorded on a 
photomagnetic disk, a quarter-wave plate, a polarizing beam splitter, and 
divided photodetectors, one for each of the two light beams resulting from 
splitting by the polarizing beam splitter. By detecting the 
forward-backward intensity difference of the beam coming incident on each 
photodetector and further by subjecting the intensity difference signals 
detected by the two photodetectors to subtraction or addition, there is 
obtained a readout signal which attains a positive or negative peak when 
the beam hits the leading or trailing edge of the recorded region. This 
arrangement has the benefit of accurately reproducing recorded information 
without being affected by variations in light intensity or the medium's 
reflectance, which posed problems to prior art apparatuses. The apparatus 
according to the present invention has the additional advantage that the 
readout signal has no low frequency component and there is no need for 
accurate signal amplification to the low frequency region. The invention 
also makes it possible to eliminate noise due to other factors than 
recorded information including slight unevenness of the medium surface and 
changes in the intensity distribution of the irradiating laser beam, and 
thereby to provide low-noise high-quality readout signals.