Focus error detection apparatus utilizing focusing an front and rear sides of focal planes

A focus error detection apparatus and an optical disk apparatus comprising a focusing lens for focusing reflected light coming from a light reflecting surface which is to be adjusted at the focal point of a projected light beam; a beam splitter means for dividing the output light of said focusing lens into a first and a second focused light beam; a first and a second photo-detector disposed respectively on the optical paths of said first and said second focused light beam, one of which is located behind the convergence point of said first focused light beam and the other of which is located before the convergence point of said second focused light beam, each of them generating outputs proportional to the amount of light within a predetermined region of the received light image, when it varies so that it is enlarged; and a circuit generating a focus error signal by using the difference between the outputs of said two photo-detectors.

BACKGROUND OF THE INVENTION 
This invention relates to a focus error detection apparatus and more in 
detail to a focus error detection apparatus suitable for a light beam 
application apparatus, such as optical video disk player, optical audio 
disk player, or optical digital recording apparatus, in which the focusing 
position of the illuminating light must follow variations in position of 
the reflective surface. 
In an optical disk apparatus in which a surface of a rotating recording 
medium is illuminated by laser light and information is optically 
recorded, reproduced or erased, an auto-focusing servo-system is 
necessary, which moves an objective lens in the optical head according to 
the movement of the reflective surface in the optical axis direction 
taking place in the optical disk, which is rotating, so that the data 
recording surface is always within the focus depth of the laser spot. The 
auto-focusing servo-system consists of a servo-motor, e.g. of voice coil 
type, for moving the objective lens in the optical axis direction, focus 
error detection optics, and a servo-amplifier for actuating the 
servo-motor according to focus error thus detected. However, among these 
elements the focus error detection optics are specifically important and 
in the case where it is applied to an optical disk apparatus, it is 
desirable to adopt a construction, in which variations of reflected light 
due to information pits on the information recording surface, pre-groves 
forming tracks, etc. don't influence focus error signals. Further, in 
order to obtain correct focusing control, a device construction is 
desired, in which, even if displacement in optical axis of the reflected 
light coming from the information recording surface is produced, e.g., by 
displacement in position of the optical system, it doesn't influence the 
focus error signals. 
Heretofore, various methods have been proposed for constructing a device 
for detecting the focus error described above, in one of which the 
reflected light coming from the information recording surface (reflecting 
surface) is focused by a lens and a knife-edge is disposed at the 
convergence point of the light so that only a part of the reflected light 
reaches a photo-detector located behind the knife-edge (e.g. U.S. Pat. No. 
4,450,547). According to this method, it is possible to obtain a 
semi-circular optical image on the photo-detector, which rotates according 
to the magnitude of the focus error, by disposing a cylindrical lens 
between the focusing lens and the knife-edge and to obtain a high 
precision focus error signal by means of a two-divided photo-detector by 
using its differential output. However, according to this method, since 
the focus error signal depends on the relative positional relation between 
the knife edge and the focused light, there is a problem that variations 
in position of the knife edge due to thermal expansion and deviations of 
the optical axis of the reflected light influence the focus error signal. 
Further, according to another method, as described, e.g., in Japanese 
Patent unexamined publication No. 84-77637, an optical element for 
separating the central portion and the peripheral one of the light beam 
into different directions is disposed on the optical path along which the 
reflected light is focused and the light beams thus separated are received 
by separate photo-detectors so that a focus error signal is obtained by 
using the difference between their outputs. However, since variations in 
light intensity produced by the information pits and the tracks stated 
above appear differently for the central and peripheral portions, there 
remains a problem in this method that noise components are contained in he 
error signal. 
In addition, for an optical disk device using such a focus error detection 
apparatus, since the information reproduction optical system and the 
optical system for the focus error detection and the track error detection 
are separated, there is a problem that the optical head is too big and 
includes too many parts and that as the results it is too expensive. 
SUMMARY OF THE INVENTION 
An object of this invention is to provide a focus error detection device, 
for which variations in intensity of the reflected light depending on the 
state of the reflecting surface and those in optical axis, as described 
previously, influence hardly the focus error signal. 
Another object of this invention is to provide an optical disk apparatus 
having a small and inexpensive optical head. 
In order to achieve the first object, the focus error detection device 
according to this invention comprises a focusing lens for focusing the 
reflected light coming from the reflecting surface on which a projected 
light beam is to be focused, a means for separating the output light beam 
of the focusing lens into a first and a second focused light beam, a first 
and a second photo-detector disposed on the light path of the first and 
the second focused light beam, respectively, and a circuit for generating 
a focus error signal at least depending on the difference between the 
outputs of the two photo-detectors, each of the photo-detectors being 
disposed on each of the light paths in such a positional relation that 
variations in magnitude of the received light images of the focused light 
beams due to focus errors appear in directions opposite to each other, the 
output stated above being generated proportionally to the light intensity 
in a certain portion of the received light image for a received light 
image changed in the enlarging direction. 
The positional relation stated above of the photo-detectors is fulfilled, 
e.g., by disposing the first photo-detector behind the convergence point 
of the first focused light beam i.e., at the rear side of the focal plane, 
and the second photo-detector before the convergence point of the second 
focused light beam i.e., at the front side of the focal plane. 
According to this invention, when the light reflecting surface deviates 
from the focal point, since the received light image is enlarged for one 
of the two photo-detectors and reduced for the other, the ratio of the 
light projected outside of the photo-electric transformation region to the 
total projected light increases and thus the output decreases at the side 
where the received light image is enlarged, if only a partial region of 
the received light image is submitted to the photo-electric 
transformation, e.g., by using a mask having a predetermined opening at 
the surface of each of the photo-detetctors. To the contrary, at the 
photo-detector where the received light image is reduced, since the ratio 
of the light projected within the photo-electric transformation region, 
the output increases. Consequently, the focus error signal can be obtained 
by forming the difference between the outputs of the two photo-detectors. 
In this case, where the reflected light intensity varies due to unevenness 
of the reflecting surface, since these noise components appear at the same 
time in the two focused beams, they are compensated by each other and thus 
influences on the focus error signal are removed. 
Further, according to this invention, since the focus error signal and 
eventually the track error signal can be detected by using the same light 
beam used also for the reproduction of information, an optical disk 
device, for which the number of parts is reduced and which has a small and 
inexpensive optical head, can be realized.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a scheme illustrating the fundamental construction of the 
apparatus for detecting the position of the focal point according to this 
invention, in which the reference numeral 1 represents a reflecting 
surface located at the position of the focal point. To the contrary, 1' 
shows a situation, where the reflecting surface deviates in position from 
the position of the focal point so that it becomes farther from the 
focusing lens, 2 and 2' illustrate the reflected light beams, when the 
reflecting surface is at each of the positions, respectively. 3 indicates 
the focusing lens for focusing the reflected light 2 at a point P.sub.1 
and 10 shows a beam splitter (e.g. half-mirror or polarizing beam 
splitter) inserted between the focusing lens 3 and the convergence point 
P.sub.1. The reflected light 2 focused by the focusing lens 3 is divided 
by the beam splitter 10 into two beams, i.e. passing beam 2A and 
reflective beam 2B, forming convergence points P.sub.1 and P.sub.2, 
respectively, which are at a same distance from the dividing surface 10'. 
According to this invention, two photo-detectors 13 and 14 are used, one 13 
of which is located at the position which is nearer to the beam splitter 
by a predetermined distance w than the convergence point P.sub.2, and to 
the contrary, the other 14 of the photo-detectors is located at the 
position which is further therefrom by the predetermined distance w than 
the convergence pint P.sub.1 so that they receive the output light of the 
beam splitter at their respective positions. 
Each of the photo-detectors 13 and 14 is so constructed that except for an 
effective photo-detector surface 16 at the central portion having a 
diameter smaller than that of the spot, when focused correctly, as 
indicated in FIG. 2D, its peripheral portion is covered by a mask 15, and 
they have a basically same form. When the reflecting surface 1 is well 
located at the position of the focal point, the two focused light beams 2A 
and 2B have identical cross-sectional areas at the same distance w from 
the convergence points P.sub.1 and P.sub.2, respectively. Received light 
images (light spots) having a same area are formed on the photo-detectors 
13 and 14, as indicated by hatching in FIG. 2B so that their ratios of the 
light received by the light receiving surface 16 to the total projected 
light are equal to each other. Consequently, if the beam splitter 10 
divides the incident beam into the passing beam 2A and the reflective beam 
2B having an identical light intensity, the outputs of the two 
photo-detectors 13 and 14 are equal to each other, when focused correctly. 
In the construction described above, when the reflecting surface 1 is 
displaced to the position indicated by 1', the convergence points of the 
two focused light beams move to positions nearer to the beam splitter than 
the convergence points P.sub.1 and P.sub.2 when focused correctly, as 
indicated by broken lines 2A' and 2B' in FIG. 1. Therefore, on the 
photo-detector 13, the light spot 6' is reduced, as illustrated in FIG. 
2A, so that almost all the projected light is received by the light 
receiving surface 16, and on the other hand, on the other photo-detector 
14 only a part of an enlarged light spot 6" is received by the light 
receiving surface 16. In this case, the output of the photo-detector 13 is 
greater than that of the photo-detector 14 so that the difference between 
the output signals of the two photo-detectors corresponds to the magnitude 
of the focus error. In the construction described above, if the reflecting 
surface is displaced from the position indicated by 1 in such a direction 
that it becomes nearer to the lens 3, since the light spot 6'" on the 
photo-detector 13 is enlarged and the light spot 6"" on the photo-detector 
14 is narrowed, as indicated in FIG. 2C, the relation between the outputs 
of the photo-detectors is reversed. Consequently, the differential output 
V of the two photo-detectors 13 and 14 shows an S-curved characteristic 
curve with respect to the magnitude of the focus error .delta., as 
indicated in FIG. 3, and thus it is possible to construct an auto-focusing 
servo-system by utilizing this property. 
In the construction of the apparatus of the apparatus described above, in 
the case where, e.g., an optical disk having an information track in the 
form of a pre-grove is used as the reflecting surface 1, first-order 
diffractive beams appear in the light spots on the two photo-detetctors 13 
and 14, as indicated by 11a and 11b in FIG. 4. When the position of the 
light spot on the optical disk deviates from the track, the intensity of 
the first order diffractive beams 11a, 11b stated above varies. However, 
if the focus error is small, since the variations in intensity appearing 
in the two photo-detectors 13 and 14 are identical, they are compensated 
by each their by forming their differential output and therefore there are 
almost no influences on the focus error detection signal. 
FIG. 5 shows another embodiment of this invention. In this embodiment, each 
of the effective surfaces of the photo-detectors 13 and 14 is divided into 
two parts in such a manner that the first-order diffractive beams 11a, 11b 
stated above are separated and photo-detection signals A, B, C and D are 
taken out independently from the four light receiving surfaces 16a, 16b, 
16c and 16d, respectively. When independent outputs are taken out in this 
way from the four light receiving surfaces, the focus error detection 
signal AF can be obtained by forming (A+B)-(C+D) and the track position 
detection signal TR by forming (A-B)+(C-D). Furthermore, it is possible to 
obtain an information signal SN on the track by adding all the outputs. 
When each of the effective surfaces of the photo-detector is divided in 
this way into a plurality of regions, the track position detection signal 
TR can be obtained by using only one photo-detector. 
FIGS. 7A and 7B are schemes for explaining the case where the optical axis 
of the reflected light deviates from the axis of the photo-detectors 13, 
14. Such a state can take place, e.g., either when the lens is made to 
follow an off-centered track on the optical disk or when parts of the 
optical system are displaced due to temperature variations or to changes 
with the passage of time. If it is supposed that the light spot is 
displaced by .epsilon. on the reflecting surface 1 and the lens 3 is also 
displaced by .epsilon. in the same direction as the light spot, as 
indicated in FIG. 7A, the light spot 6 on the photo-detectors 13, 14 is 
displaced by .epsilon. to the position 6' indicated by broken lines, as 
indicated in FIG. 7B. Even if such a displacement of the received light 
spot 6 takes place, the outputs of the photo-detectors 13, 14 don't change 
and thus no errors are produced in the focus error signal AF, if the 
intensity distribution in the light spot 6 is uniform. Further, even if 
the intensity distribution in the light spot 6 is not uniform, since the 
displacement of the light spot 6 appears equally for the two 
photo-detectors, errors produced in the focus error signal obtained by 
using the differential output are extremely small. 
FIG. 8 is a scheme for explaining the light spot on the photo-detectors 13, 
14, in the case where deviations of the optical axis of the reflected 
light described previously take place, when the position of the track is 
detected by means of a division type photo-detector indicated in FIG. 5. 
In this case, since the light receiving surface 16a, 16b, 16c and 16d and 
the position of the first-order diffractive beam 11a, 11b are shifted from 
each other, the output decreases remarkably in 16b and 16c, where the 
boundary of the first-order diffractive beam is located. However, since 
the decrease of the output is almost identical for 16b and 16c, by 
denoting this value by p, the track position detection signal TR can be 
represented by {A-(B-p)}+{(C-p)-D}=(A-B)+(C-D), where p is eliminated. 
That is, it can be seen that variations of the optical axis don't 
influence the track position detection signal TR and that even if they do, 
errors due to them are extremely small. 
FIG. 9 shows another embodiment of the photo-detectors 13, 14. In this 
embodiment, a band-shaped mask 15, the width of which is smaller than the 
diameter of the light spot 6, when focused correctly, is so disposed that 
it divides the central portion of the photo-detectors 13, 14 into two 
parts. The parts of the light receiving surfaces divided into two parts 
16a and 16b, and 16c and 16d, respectively, are so constructed that their 
outputs are approximately equal, when focused correctly. Representing the 
output of the parts of the light receiving surfaces 16a to 16d by A, B, C 
and D, respectively, the focus error detection signal AF is given by 
(A+B)-(C+D). The output V of the error signal with respect to the movement 
of focusing position .delta. shows a characteristic curve indicated in 
FIG. 10. 
FIG. 11 shows still another embodiment, where the parts of the light 
receiving surface divided into two pats 16e and 16f, and 16g and 16h, 
respectively, are disposed for the detection of the track position at the 
portion covered by the mask 15 of the photo-detector 13, 14 in the 
embodiment represented by FIG. 9. Representing the output of the parts 
16e-16h by E-H, the track position detection signal TR is given by (E-F) 
+(G-H). Compared with the embodiment indicated in FIG. 8, since the amount 
of the first-order diffractive light 11a, 11b received by the parts of the 
light receiving surfaces 16a-16d for the focus error detection can be 
reduced, the magnitude of the errors produced by the first-order 
diffractive lights in the focus error detection signal at the moment of 
the track passage or the off-track is still smaller than that obtained in 
the preceding embodiment. In the structure in the embodiment indicated in 
FIG. 9, where the light receiving surface is divided into two parts 16a, 
16b or 16c, 16d, the outputs (A+B) and (C+D) of the photo-detectors 13 and 
14, respectively, vary slightly, when the light spots on the 
photo-detectors move in the upward and downward directions due to 
deviations of the optical axis of the reflected light, as indicated by 6' 
in FIG. 12. However, since these variations appear equally in the 
photo-detectors 13 and 14, denoting their value by q, the focusing 
position detection signal AF can be represented by 
(A+B-q)-(C+D-q)=(A+B)-(C+D) and thus the influences of these variations 
are compensated by each other. For the track position detection signal TR, 
then influences of the deviations of the optical axis are identical to 
those mentioned for the first embodiment. 
FIG. 13 illustrates another embodiment, in which this invention is applied 
to an optical disk apparatus. The reference numeral 17 represents an 
optical beam source consisting of a laser diode and an optical system for 
transforming laser light to a parallel light flux and the parallel light 
flux 18 emitted by the apparatus 17 is focused through a polarizing beam 
splitter 19, a quarter wave plate 20 and an objective lens 21 on the 
information track 23 (pregroove in which header pits are disposed or 
series of uneven pits) formed on an optical disk 22. On the optical disk 
an information recording film is disposed and for the perforation 
recording it is irradiated with a light beam intensity-modulated according 
to information to be recorded so that the information is recorded by 
forming locally perforations in this recording film. The reproduction of 
the information is performed by using variations in intensity of the light 
reflected by the optical disk. For the apparatuses exclusively used for 
reproduction, such as video disk, audio disk, etc., highly reflective film 
is used and information reproduction is performed by using phase 
differences in the reflected light due to the uneven pits. The focusing 
control and the tracking control are necessary both for recording and for 
reproduction. Light reflected by the optical disk 22 passes through the 
polarizing beam splitter 19 and is led to the focus error detection 
apparatus explained, referring to FIG. 1. In this example, a half-mirror 
type beam splitter is used for the beam splitter 12. In addition, for the 
photo-detectors 13 and 14 are used those having the construction 
illustrated in FIG. 5 and the track position detection signal TR is 
obtained at the same time. Denoting the outputs of the parts of the light 
receiving surfaces 16a, 16b, 16c and 16d by A, B, C and D, respectively, 
the focus error detection signal AF, the track position detection signal 
TR and the information signal SI are given, respectively, by: 
EQU AF=(A+B)-(C+D), (1) 
EQU TR=(A-B)+(C-D), and (2) 
EQU SI=(A+B)+(C+D). (3) 
The autofocusing and the autotracking can be achieved by moving the lens 21 
in the directions indicated by the arrow 124 and by the arrow 125, 
respectively, according to the signals AF and TR. 
FIG. 14 illustrates still another embodiment, in which this invention is 
applied to an optical disk. In FIG. 14 the elements corresponding to those 
in the preceding embodiment are denoted by the same reference numerals. In 
this embodiment, a quarter wave plate 24 is disposed between the 
polarizing beam splitter 19 and the lens 3 and a polarizing beam splitter 
25 is used instead of the half-mirror type beam splitter 12. When the 
polarizing beam splitter 25 is used, by changing the mounting angle of the 
quarter wave plate 24, it is possible to adjust the ratio between the 
passing beam 2A and the reflective beam 2B and consequently regulation 
operation for equalizing the amounts of light inputted in the two 
photo-detectors 13 and 14 is easier. However, this regulation operation 
for equalizing the amounts of light can be completed by regulating the 
gain of an amplifier mounted in the output circuit for each of the 
photo-detectors. In this embodiment, for the photo-detectors 13 and 14, 
those having the structure indicated in FIG. 11 are used. Denoting the 
outputs of the parts of the light receiving surfaces 16a, 16b, 16c, 16d 
for the photo-detector 13 and those of the parts 16e, 16f, 16g, 16h for 
the photo-detector 14 by A, B, C, D, E, F, G and H, respectively, the 
focus error detection signal AF, the track position detection signal TR 
and the information signal SI can be given, respectively, by: 
EQU AF=(A+B)-(C+D) (4) 
EQU TR=(F-E)+(H-G) (5) 
EQU SI=(A+B+E+F)+(C+D+G+H) (6) 
FIG. 15 illustrates an embodiment, in which this invention is applied to a 
magneto-optical disk apparatus. Laser light emitted by a semiconductor 
laser device 31 is transformed into a collimated beam by means of a 
collimating lens 32, which passes through a beam splitter 33 and after 
having being reflected by a mirror 30, is focused by means of a focusing 
lens 21 so as to form a laser spot on the surface of the disk 22. In the 
case where a semiconductor laser emitting laser light having an 
anisotropic light intensity distribution is used, it is necessary to 
dispose an optical element for shaping the beam intensity distribution, 
e.g. a triangular prism in the optical path between the collimating lens 
32 and the beam splitter 33 (e.g. polarizing beam splitter). The reflected 
light coming from the disk is transformed again by the focusing lens 21 
into a collimated beam, which is reflected by a mirror 30 and reflected 
again by the beam splitter 33. A part of the reflected beam is reflected 
still again by another beam splitter 34 and led to an optical system for 
the focus error detection and the information reproduction consisting of a 
half wave plate 26, a focusing lens 3, a polarizing beam splitter 25 and 
two photo-detectors 35 and 36. 
At first, the principle for the focus error detection will be described. 
The polarization direction of the incident beam to the polarizing beam 
splitter 25 can rotate by about 45.degree. with respect to the direction 
of S polarization or to the direction of P polarization of the reflecting 
film of the polarizing beam splitter 25 by rotating the half wave plate 26 
for adjustment and the incident beam is divided by the polarizing beam 
splitter 25 into two beams having approximately equal amounts of light. In 
the correct focus state, where the irradiation light spot is on the 
surface of the disk 22, a photo-detector 35 is disposed between the 
position of a convergence point P.sub.2 formed by the focusing lens 3 and 
the polarizing beam splitter 25 and another photo-detector 36 is located 
at the opposite side of the other convergence point P.sub.1 from the 
polarizing beam splitter 25. This distance between the convergence point 
P.sub.2 and the photo-detector 35 is approximately equal to that between 
the convergence point P.sub.1 and the photo-detector 36. The hatched 
portions 36a, 36b and 36c in FIGS. 16 A, 16B and 16C show the shape of 
divided photo-detector elements of the photo-detector 36 and the hatched 
portions 35a, 35b and 35c show the shape of divided photo detector 
elements of the photo-detector 35. The circles 6 in FIG. 16B show the 
shape of the light flux on the light receiving surface of the 
photo-detectors 35 and 36, respectively. In the correct focus state as 
stated above, they are approximately equally great. When the disk 22 
approaches the focusing lens 21, since the convergence point P.sub.1 
becomes closer to the photo-detector 36, the light spot 6"" on the light 
receiving surface becomes smaller, as indicated in FIG. 16C, and since the 
convergence point P.sub.2 becomes farther from the photo-detector 35, the 
light spot 6'" on the light receiving surface becomes greater. To the 
contrary, when the disk 22 becomes more distant from the focusing lens 21, 
the light spot 6" becomes greater and the light spot 6' becomes smaller. 
Therefore, by forming the difference between the sum of the outputs of the 
divided photo-detector elements 36a and 36b and the sum of the outputs of 
the divided photo-detector elements 35a and 36b, it is possible to obtain 
the focus error detection signal AF. Further, also by forming the 
difference between the divided photo-detector elements 36C and 35C, it is 
possible to obtain the focus error detection signal AF. 
Next, the magnetized information reproduction signal can be obtained by 
forming the difference between the sum of the outputs of the divided 
photo-detector elements 36a, 36b and 36c and the sum of the outputs of the 
divided photo-detector elements 35a, 35b and 35c. FIGS. 18A and 18B are 
schemes for explaining the principle of the magnetized information 
reproduction. The abscissa PX indicates the direction of P polarization 
and the ordinate SY the direction of S polarization for the reflecting 
surface of the polarizing beam splitter 25 indicated in FIG. 15. A 
perpendicular magnetic thin film is disposed on the surface of the disk 22 
and information is recorded by reversing the magnetization direction in 
this perpendicular magnetic thin film. When a laser spot is projected on 
this magnetic film, the polarization direction of the reflected light 
rotates by about 0.5 degree by the magneto-optical effect (Kerr effect) 
depending on the reverse of the magnetization direction. The 
magneto-optical disk apparatus detects this extremely small rotation of 
the polarization direction and performs the reproduction of the 
information. FIG. 18A shows the polarization direction of the light before 
the incidence to the half wave plate 26 indicated in FIG. 15, where the 
arrow 48 indicates the polarization direction of the light in the case 
where the light spot is projected in the portion of the magnetic film, 
where no information is recorded on the disk 22. When the light spot 
passes to the portion where information is recorded, the polarization 
direction rotates as indicated by the arrow 49. FIG. 18B indicates the 
polarization direction of the light which has passed through the half wave 
plate 26. Since the polarization direction can be rotated by about 45 
degrees by regulating the rotation of the half wave plate, the direction 
of the arrows 48 and 49 is rotated by about 45 degrees with respect to 
that indicated in FIG. 18A. Since the polarization beam splitter 25 has a 
property that the P polarized light indicated by the abscissa PX passes 
therethrough and the S polarized light indicated by the ordinate SY is 
reflected, when the light spot is located in the portion of the magnetic 
film, where no information is recorded, in the polarized light indicated 
by the arrow 48, the amount of light indicated by the arrow 60 passes 
through the polarizing beam splitter 25 and is received by the 
photo-detector 35; to the contrary the amount of light indicated by the 
arrow 61 is reflected by the polarizing beam splitter 25 and received by 
the photo-detector 36. On the other hand, when the light spot is located 
in the portion of the magnetic film, where information is recorded, in the 
polarized light indicated by the arrow 49, the amount of light indicated 
by the arrow 62 is received by the photo-detector 35 and the amount of 
light indicated by the arrow 63 is received by the photo-detector 36. 
Consequently, the magnetized information reproduction signal can be 
obtained by forming the difference between the amounts of light received 
by the photo-detectors, respectively, depending on the presence or absence 
of the recorded information, because the amount of light received by the 
photo-detector 35 decreases by that indicated by the arrow 64 and the 
amount of light received by the photo-detector 36 increases by that 
indicated by the arrow 65. 
Since there are common parts in the operations of the outputs of the 
divided photo-detector elements for obtaining the focus error signal and 
the information reproduction signal, problems concerning mutual 
interference between the signals can take place. However, taking into 
account the fact that the frequency of the information reproduction signal 
is several Megaherz, because the interval of the information recording is 
several .mu.m, while the variable frequency of the focus error signal is 
several hundreds of Herz, because the number of rotation of the disk is 
several tens of Herz, the two signals can be satisfactorily separated, 
e.g., by using a filter circuit and thus there is no problem. 
For example, as indicated in FIG. 19, the outputs of the divided 
photo-detector elements 36a and 36b are added in an adding circuit 81 and 
the outputs of the divided photo-detector elements 35a and 35b are added 
in an adding circuit 82. Then, the focus error detection signal can be 
obtained by forming the difference between the outputs of the adding 
circuits 81 and 82, which have passed through low pass filters 85 and 86, 
respectively, whose cut-off frequency is, e.g., 1 kHz in an adding circuit 
90. Furthermore, the outputs of the divided photo-detector elements 36a, 
36b and 36c are added in an adding circuit 83 and the outputs of the 
photo-detector elements 35a, 35b and 35c are added in an adding circuit 
84. Then, the information reproduction signal can be obtained by forming 
the difference between the outputs of the adding circuits 83 and 84, which 
have passed through high pass filters 87 and 88, respectively, whose 
cut-off frequency is, e.g., 100 kHz, in a differential circuit 91. 
The photo-detector 39 is a detector, which detects deviations of the light 
spot from the track on the surface of the disk, and has, e.g., divided 
photo-detector elements 39a, 39b and 39c, hatched in FIG. 17. The 
principle of the track error detection is described in detail in U.S. Pat. 
No. 4,525,826 and therefore explanation therefor is omitted. The pattern 
of the light flux on the light receiving surface of the photo-detector 39 
is constructed by interference between 0th order diffractive beam 40a and 
.+-. 1st order diffractive beams 40b and 40c. Since light intensities in 
interference areas 40ab and 40ac vary depending on the deviations from the 
track, the track error signal TR can be obtained by forming the difference 
between the outputs of the photo-detectors 39a and 39b located in the 
interference areas 40ab and 40ac, respectively. 
In addition, an electro-magnet 41 is used, which produces an external 
magnetic field applied to the magnetization film on the disk 22, in order 
to record or erase information. Further, the optical head consisting of 
the optical element described above and the electro-magnet 41 are mounted, 
e.g., on a moving base so that they are movable in a radial direction of 
the disk 22 and located at a desired track position by means of a driving 
motor, such as linear motor, etc. Furthermore, the tracking control is 
effected by using, e.g., a rotating mirror as the mirror 30 and the 
tracking signal TR detected from the output of the photo-detector 39 
representing deviations of this rotating mirror. 
On the other hand, the autofocusing control is effected, e.g., by disposing 
an actuator such as voice coils, etc. around the focusing lens 21, driving 
this actuator depending on the focus error detection signal AF detected 
from the outputs of the photo-detectors 35 and 36, and moving the focusing 
lens 21 along the optical axis. Further, it is also possible that the 
tracking control and the autofocusing control can be effected at the same 
time by disposing a two-dimensional actuator around the focusing lens 21, 
which is movable also in a radial direction of the disk and driving this 
actuator depending on the tracking signal TR and the focus error detection 
signal AF. 
In the embodiment shown in FIG. 15 the track error detection signal is 
obtained by using the photo-detector 39 of exclusive use, but the 
diffraction interference pattern due to the track is produced also in the 
light flux 6 in FIG. 16B. Consequently, when divided photo-detector 
elements are formed symmetrically in the region of the interference of the 
0th order diffraction beam and the .+-.1st order diffraction beam within 
the central photo-detector elements 35c and 36c of the photo-detectors 35 
and 36, respectively, the track error signal can be obtained also by 
forming the difference between their outputs. Therefore, the beam splitter 
34 and the photo-detector 39 can be omitted in this way. 
FIG. 20 is a scheme showing another embodiment of this invention, for which 
the embodiment indicated in FIG. 15 is ameliorated on the basis of the 
idea described above. The photo-detector 51 has divided photo-detector 
elements indicated by hatched parts 51a, 51b, 51c, 51d and 51e in FIG. 21A 
and the photo-detector 52 has divided photo-detector elements indicated by 
hatched parts 52a, 52b, 52c, 52d and 52e. Since the function of the other 
constructional parts in FIG. 20 is identical to that in FIG. 15, their 
explanation is omitted. The focus error detection signal AF can be 
obtained by forming the difference between the sum of the outputs of the 
divided photo-detector elements 51a and 51c and the sum of the outputs of 
the divided photo-detector elements 52a and 52b. The track error detection 
signal TF can be obtained by forming the difference between the outputs of 
the divided photo-detector elements 51d and 51e located in the region of 
interference of the 0th order diffraction beam 53a and the .+-.1st order 
diffraction beams 53b and 53c or by forming the difference between the 
outputs of the elements 52d and 52elocated in the region of interference 
of the 0th order diffraction beam 54a and the .+-.1st order diffraction 
beam 54b and 54c, or by adding the two results of subtraction stated 
above. Furthermore, the information reproduction signal can be obtained by 
forming the difference between the total output of the photo-detector 51 
and that of the photo-detector 52. 
In addition, the tracking control and the autofocusing control can be 
effected in the same manner as for the embodiment indicated in FIG. 15. 
It is evident that the optical head according to the embodiments indicated 
in FIGS. 15 and 20 is not limited to that in which information 
reproduction is effected by the magneto-optical effect, but it is also 
possible to reproduce information recorded by adding the total outputs of 
the photo-detector 35 and those of the photo-detector 36 indicated in FIG. 
15, by adding the total outputs of the photo-detector 51 and those of the 
photo-detector 52 indicated in FIG. 20, or by the uneven pit type, the 
hole type or the crystal phase change type recording. 
In the above, some preferred embodiments have been described, but in the 
realization of this invention, for the form of the effective light 
receiving surface of the photo-detector, various modifications other than 
the forms indicated in the above mentioned embodiments are possible and 
any form can be used, for which the size of the light spot projected on 
the photo-detector varies depending on the magnitude of the focus error 
and only a predetermined part of the light spot is reflected on the 
received light signal. Further, although two photo-detectors 13, 14 or 35, 
36 or 51, 52 are located at positions, which are equally distant by w from 
the convergence points in the embodiments described above, their positions 
can be regulated, depending on the amount of light of the passing beam 2A 
and that of the reflective beam 2B. Furthermore, it is also possible that 
the passing beam 2A is received before the convergence point and the 
reflective beam 2B is received behind the convergence point, the two 
photo-detectors being replaced mutually in the positional relation, and 
that the light receiving surfaces of the two photo-detectors have 
different areas, depending on the characteristics of the beam splitter 12, 
25. 
As it is evident from the above explanation, according to this invention, 
since light coming from the reflecting surface which is to be adjusted at 
the focal point of a projected light beam is divided into two beams which 
are directed in two directions; photo-detectors are disposed on the 
optical paths of the divided light beams in such a positional relation 
that variations in size of the received light images (spots) due to focus 
errors appear in directions opposite to each other; each of the 
photo-detectors transform always only a part of light of each of the 
received light images; and the difference between the outputs of the two 
photo-detectors described above is used as a focus error detection signal, 
even if variations in the reflected light are produced by variation in the 
form of the reflecting surface, it is possible to obtain a focus error 
detection signal, which is influenced only slightly by these variations. 
Further, since the focus error signal can be detected by using a same light 
flux as for the information reproduction, the number of optical parts 
constituting an optical head and thus it is possible to realize a small, 
light and inexpensive optical head.