CA1248796A

CA1248796A - Focussing error detecting apparatus

Info

Publication number: CA1248796A
Application number: CA000455020A
Authority: CA
Inventors: Shigeo Kubota; Kiyoshi Ohsato
Original assignee: Sony Corp
Current assignee: Sony Corp
Priority date: 1983-05-31
Filing date: 1984-05-24
Publication date: 1989-01-17
Also published as: US4631397A; EP0127448A1; EP0127448B1; ATE30799T1; JPS59221835A; KR920001112B1; KR850000078A; DE3467447D1

Abstract

ABSTRACT OF THE DISCLOSURE

A focussing error detecting apparatus is disclosed which includes a composite lens formed of a single lens made of glass and a single lens made of plastic and a photo detector disposed behind the composite lens to oppose thereto, wherein the glass single lens has a uniform focal length, the plastic single lens is formed of a pair of lens regions divided by a flat interface including an optical axis of light incident on the composite lens, the pair of lens regions have back focal lengths different from each other and optical axes spaced apart from each other, the photo detector is placed such that its photo-detecting face is positioned at a middle point between converged points of light rays emitted from the pair of lens regions on the optical axis of the incident light when the light incident on the composite lens is in the focussed state and the photo detector is formed of first and second photo-detecting sensors for receiving light rays passed through the far-side portion and near-side portion of the first lens region of the pair of lens regions relative to the flat interface and third and fourth photo-detecting sensors for receiving light rays passed through the far-side portion and near-side portion of the second lens region of the pair of lens regions relative to the flat interface so that a focussing error signal is produced from the difference between the sum of first and fourth photo-detected signals from the first and fourth photo-detecting sensors and the sum of second and third photo-detected signals from the second and third photo-detecting sensors.

Description

BACKGROU~ OF THE INVENTION
_ Field of the Invention _ The present invention relates to a focussing error detecting apparatus suitable for being applied to an optical disc reproducing apparatus, an auto focus camera (an imayer) and so on.
BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a diagram schematically showing the arrangement of a conventional focussing error detecting apparatus;
Fig. 2 is a perspective view of a prism used therein, Fig. 3 is a diagram useful for explaining the operation of the apparatus in Fig. 1 in each focussed state thereof;
Fig. 4 is a diagram schematically showing the arrangement of another conventional focussing error detecting apparatusi Fig. 5 is a side and front view of a lens used therein;
Fig. 6 is a diagram showing the arrangement of a photo detector used therein;
Fig. 7 is a diagram useful for explaining the operation of the apparatus in Fig. 4 in each focussed state, Fig. 8 i5 a graph useful for explaining the apparatus in Fig. 4;
Fig. 9 i5 a perspective view of a ~emi-cylindrical ; ~ - 2 -` : ' .

~Zg~7~i lens used in still another conventional focussing error detecting apparatus;
~ig. 10 is a perspective view illustrating a beam spot on the lens in Fig. 9;
~igO 11 is a diagram showing a locus of a light ray incident on thP semi-cylindrical lens in Fig. 9, Figs. 12 to 14 are respectively diagrams showing beam spots formed by the lens in Fig. 9;
Fig. 15 is a diagram schematically showi~g an example of a previously proposed focussing error detecting apparatus by the same assignee of this application;
Fig. 16 is a plan view of a composite lens used in the apparatus in Fig. 15;
Fig. 17 is a plan view of a photo detector used in the apparatus in Fig. 15;
Fig. 18 is a diagram showing a positional relation of the composite lens and photo detector of the apparatus in Fig. 15;
Figs. 19 and 20 are respectively graphs useful for explaining the apparatus in Fig. 15;
Fig. 21 is a diagram schematically ~howing another example of the previously proposed focussing error detecting apparatus;
Fig. 22 is a plan view of a composite lens used in ~he apparatus in Fig. 21;
Fig. 23 is a plan view of a photo detector used in .he apparatus in Fig. 15;
Fig. 24 is a diagram showing a positional re.lation between the composite lens and photo detector of the apparatus in Fig. 21;

` ~ 3 -7~

Figs. 25 to 28 are respectively cross-sectional diagrams of examples of composite lenses used in the previously proposed focussing error detecting apparatus~
Figs. 29 to 31 are respectively plan views of examples of composite lenses used in ~he previously proposed focussing error detecting apparatus;
Figs. 32 and 33 are respectively plan views of examples of photo detectors used in the previously proposed focussing error detecting apparatus;
Fig. 34 is a diagram showing the optical system useful for explaining the function of the lens used in the previously proposed focussing error detecting apparatus; and Figs. 35 and 36 are respectively a plan view and a cross-sectional view showing an example of a composite lens used in an embodiment of a focussing error detecting apparatus acccrding to the present invention.

Descri tion of the Prior Art P
Various types of focussing error detectors have - hitherto been proposed for optical disc reproducing apparatus. One of such types having a simple optical system fox detecting a focussing error is the comblnation ~f a Foucault prism and a photo detector having 4 photo-sensors arranged in line.
~ Laser light from a laser light source (for example, - 25 a semicond~ctor laser source) 1 is made incident through a polarized beam splitter 2, a collimator lens 3, a 1/4 wavelength plate 4 and an objective lens 5 in turn to a - 4 ~

3721~

recorded surface 6 of a disc. On the recorded surace 6 of the disc are recorded pits that are modulated by audio or video information along a spiral track so as to be optically read out. The laser light reflected from the recorded surface 6 is made incident to the photo deteetor 8 through the objective lens S, the 1/4 wavelength plate 4, the collimator lens 3, the polarized beam splitter 2 and a prism 7 in turn.
The prism 7 has two rectangular refractive suraces lQ 7a and 7b making an obtuse angle as shown in Fig. 2. The prism 7 is arranged so that light emerges ~rom the re~ractive surfaces 7a and 7b and that the edge line, 7c made by the refractive surfaces 7a, 7b crossPs perpendicularly to.the optical axis, L. The photo detector 8, as shown in Fig. 3, has 4 photo-sensors 8A, 8B, 8C and 8D the photordetecting surfaces of which lie in the plane perpendicular to the optical axis L and which are arranged in a row in the X-direction perpendicular to both the optical axis ~ and the edge line 7c. In addition, the four photo sensors are ~0 disposed at a proper position along the optical axis ~ and in the X direction. The photo detector produces a diference signal, (SA + S~) - (SB ~ Sc) ~etween the sum signal, SA + SD
of the photo-detected signals SA and S~ from thY outside sensors ~A and 8D and the sum signal, ~-B ~ S~ of the photo--detected signals S~ and SC from the inside sensors RB and 8C, as a focussing error signal.
The o~ective lens 5 is moved in the optical axis direction by the current flowing ~hrough a control coil 5/

5 _ and the focussing error signal is supplied to the control coil 9 as a focussing control signal.
In this prior art apparatus, when the recorded surface 6 of the disc is properly positioned relative to the objective lens 5 to achieve just focussed state, the spot, S on the photo detector 8 appears as dots on the - boundaries between the sensors 8A and BB and between the sensors 8C and 8D as shown in Fig. 3B. Accordingly, in this case, the focussing error signal becomes zero and the objective lens 5 keeps its present position.
When the recorded surface 6 of the disc beco~es nearer to the objec~ive lens 5 to bring under-focussed state, the spot S on the photo detector 8 appears as semi-circles on the inside sensors 8B and 8C. Accordingly, in this case, the focussing error signal becomes negative, and thus the objective lens 5 is mo~ed away from the recorded surface 6.
On the contrary, when the recorded surface 6 of the disc is positioned distant from the objective lens S to bring over-focussed state, the spot S on the photo detectox 8 appears as semi-circles on the outside sensors 8A and 8D
as shown in Fig. 3C. Accordingly, in this case, the focus-sing error signal becomes positive and thus the objective : lens 5 is moved toward the recorded surface 6.
`: 25 As described above, the prism 7 and he photo detector B having 4 photo ~ensors 8A t~ 8D arranged in line are combinPd $o detect the ~ocussing error, namely, ~he distance between the disc and the objective lens 5. This error signal is then supplied to the control coil 9 as the focus control signal to automatirally control the objective lens 5 to bring always just focussed state, or to always be at a constant distance from the disc.
However, this prior art apparatus has the disad-vantage that the photo detector 8 must be disposed at a correct position relative to the optical axis L direction and to the X-direction so as to receive the spot S on the photo detector 8 under each focussed state as shown in Figs.
3A to 3C, thus the photo detector 8 being required to be adjusted in position at severely high precision. Moreover, since some light is lost at the apexes of the prism 7 and its apex forms the edge line 7c, much light is lost at the edge line. In order to reduce the loss o~ light, it is necessary to raise the precision of the finishing of the apex.
A conventi~nal focussing error detecting apparatus with the above defects obviated will be described next.
Fig. 4 shows an example of such apparatus which ~ 20 uses a lens with a conical refractive surface instead of the ; above prism 7. On the optical path from the p~larized beam splitter 2 to a photo detectox (PIN photo diode, for example3 18 is disposed a lens 17 having a conical refractive surface 17a with its refractive surface ~ at the light emerging side. In this case, as shown in Fig. 5, the lens 17 is posi-tioned with its center l lying on ~he optical axi~ L.
Further, as shown in Fig. 6, the photo detector 18 is formed of two concentric photo sensors 18A and 18B and positioned with the center 2 of the c~ncentric circles lying on the optical axis L. In this case, as shown in Fig. 7, if the basic angle of the lens 17 is taken as ~ and the radius of the dead zone (separating zone) between the sensors 18A and 18B of the photo detector 18 is taken as d, the distance, Q
between the lens 17 and the photo detector 18 is so selected as to satisfy ~ = d cot~ under which the light passed through the lens 17 at its portion quite close to the optical axis L reaches the dead zone 18C between the elements 18A and 18B. In this expression, ~ is the angle given by sin (~+ ~) = nsin 6 and n is a refracti~e index of glass forming the lens 17. Then, the difference signal SA- SB between the photo-detected signals SA and SB from the sensors 18~ and 18B is generated as a focussing error signal and this error signal is supplied to the control coil 9 as the focussing control signal.
In this apparatus, when the recorded surface 6 of the disc is at a proper position relative to the objective lens 5 to achieve the just focussed state, as shown in Fig.
7B the spot S on the photo detector 18 appears as annular shape on the dead zone 18C between the sPnsors lB~ and 18B.
Accordingly, in this case, the focussing error signal becomes zero so t~.at the objective lens 5 keeps its pr sent position .
When the recorded surface 6 of the disc is at a position closer to the objective lens 5 to bring the under-focussed state, as shown in Fig. 7A the spot S on the photo detector 18 appears as annular shape on the inside 7~

sensor 18B. Accordingly, in this case, the focussing error signal becomes negative, thus controlling the objective lens 5 to move away from the recorded surface 6.
On the contrary, when the recorded surface 6 of the disc is positioned distant from the ob~ective lens 5 to bring the over-focussed state, as shown in Fig. 7C the ~pot S on the photo detector 18 appears as annular shape on the outside sensor 18A. Accordingly, in this casel the focussing error signal becomes positive, thus controlling the objective lens ~ to move toward the recorded surface 6.
As described above, the lens 17 having the conical refractive surface 17a and the photo detector 18 formed of two concentric photo sensors 18A and 18B are combined to detect the focussed state, or th~ distance between the disc and the objective lens 5, and this error signal is supplied to the contr~l coil 9 as the control signal 50 that the : objective lens 5 is automatically controlled to present the just focussed state, or that the distance between the objective lens and the disc becomes constant at all times.
According to such apparatus as shown in Fig. 4, since the lens 17 having the conical refractive surface 17a is used in combination with the photo detector lB formed of two concentric photo sensors lBA and 18B, the alignment of the photo detector 18 becomes simple.
In other words, since in the apparatus shown in Fig. 1 the prism 7 having two rectangular refractive sur~aces 7a and 7b is combined with the photo detector 8 formed of four photo-~ensors 8A to 8D arranged in line, in Fig~ 3A or _ g _ : ` :

' 37~

3C not only weak light passed through the center of the prism 7 and passed through the vicinity ~hereof ~ut also intensive light passed through th~ portion distant away from the center thereof are converged around the separating zone between sensors 8A and 8B and aroun~ the dead zone (separating zone~ between the sensors 8C and 8D so as to form part of the spots of semi-circles to be formed on the inside sensor~
8B and 8C or on the outerside sensors 8A and 8D, and therefore the amount of light on those portions is relatively large.
Thus/ when the photo detector 8 is displaced in its position from the optical axis and the X-direction and as shown in Fig. 3A or 3C, the spots extend to the outside sensors 8A
and 8D or to the inside sensors BB and 8C, the focussing error signal is changed greatly as compared with the case in which the detector is at correct position and consequently misdetection may be made. Therefore, the adjustment of the position of the photo detector 8 relative to the optical axis direction and the X-direction must be ma~e with high precision.
On the other hand, since the apparatus of Fig. 4 uses the lens 17 having the conical refractive surface 17a combined with the photo detector 18 formed of two concentric photo-detecting sen~ors l~A and }8B, in Fig. 7A or 7C the portion near the dead zone 18C between the sensors 18A and 18B, of the sp:7ts to appear respectively as annular shapes on the inside sensor 18B or the outside sensor 18A has a small amount of light because of only weak light passed through the vicinity of the center o the lens 17 t while Aj - lo -37~

the portion distant from the dead zone 18C has a lArge amount of light because of only intensive light passed through the portion distant from the center of the lens 170 In other words, in Fig. 7A the amount of light of the spot is smaller at positions nearer to the outside, or to the dead zone 18C and larger at positions nearer to the inside, while in Fig. 7C, the amount of light of the spot is smaller at positions nearer to the inside, or to the dead zone 18C and larger at positions near to the outside.
Accordingly, when the photo detector 18 is slightly displaced relative to the optical axis direction and X-and Y-directions and in Fig. 7A or 7C the spot extends slightly to the outside sensor 18A or to the inside sensor 18B, the focussing error signal is hardly changed as compared with the case in which the photo detector is at correct position, with the result that there is no fear of mis-detection. Therefore, it is not necessary that the photo detector 18 be adjusted in position in the optical axis direction and X- and Y-directions with considerably high precision.
Furthermore, according to the paparatus shown in Fig. 4, since the amount of light of the spot is smaller at positions nearer to the separating zone and larger at positions more distant from the separating zone and the spot extends ~oward the opposite direction to the separating zone as the li~ht is defocussed more ~nd more, the sensitivity of detection becomes consid~rably high.
~ Although the lens 17 has a loss of light at its ; vertex portion, such loss is small because the vertex 1~

:

portion thereof is not such edge line as in the conventional prism but a dot. Therefore, in order to decrease the loss, it is not necessary that the vertex be finished with a high precision.
However, the apparatus shown in Fig. 4 has the following defects. Since the focussing error signal is obtained from the difference between the amounts of light of the center and periphery of the spot S on the photo ._ detecting surface of the photo detector 18, this focussing error signal includes a high-fre~uency signal leaked by the diffraction in pits on the disc. In addition, since the modulation-degree characteristic of light relative to the spatial frequency of pits at the central portion of spot S
is different from that at the peripheral portion thereof, the focussing error signal obtained from the apparatus in Fig. 4 is not so high in accuracy.
Furthermore, since in the apparatus in Fig. 4, the change of radius of the spot S on the photo detector 18 is detected as a change of the area thereof, the defocussing amount versus focussing error signal characteristic curve as shown in Fig. ~ is inversely proportional to the square of the radius of the spot S near the ~ust-focussed state, and thus has poor linearity.
The apparatus in Fig. 1 thus has similar defects to those of the apparatus in Fig. 4.
Another conventional ~ocussing error detecting apparatus is proposed in which the photo-detecti~g surface of the photo detector is formed circularl and the phc~o .

7~i detector is egually divided into 4 photo~detecting sensors of 4 quadrants in front of which a semi-cylindrical lens is disposed, and from which a focussing error signal is obtained by taking the difference between the sum of the photo-detected signals from the ~irst-and third-quadrant photo-detecting sensors and the sum of the photo-detected signals from the second-and fourth-guadrant photo-detecting sensors. This apparatus will hereinafter be described with reference to the drawings.
Fig. 9 shows a semi-cylindrical lens 21 having a semi-cylindrical surface 22 and a rectangular surface 23.
On the surface 23, rectangular coordinates with an origin O
at its center are provided: the X-axis parallel to the generating line of the semi-cylindrical surface 22, the Y-axis perpendicular thereto and the Z-axis vertical to the surface 23. On the Z-axis is provided an origin O' at a position separated a predetermined distance from the origin O at the semi-cylindrical surface 22 side of the lens 21 and on the plane making a right angle with the Z-axis, are formed rectangular coordinates which are formed of the x-axis making an angle of 4S~ with each positive side of the X-axis and Y-axis and the y-axis making a right ang~e therewith. ~he circular detecting surface of the photo detector is made coincidPnt with the x-y plane and the detecting surface of the photo detector is equally divided into four by the x-axis and y-axis and thus into photo-detecting sensors of 4 quadrants.
On the surface 23 of the lens 21 is incident a 7~1fi converged beam the optical axis of which is coincident with the Z-axis and which has a circular spot 24 on the surface 23 (Fig. 10 is a perspective view) as shown in Fig. 10. Axes a to d each making an angle of 45 with X-axis and Y axis are t~mporarily provided on the radius of the eircular spot 24 within the first to fourth quadrants I to IV, respectively.
The loci of light rays 26 and 27 transmitted along the XOZ cross-section and YoZ cross-section of the lens 21 when the converged beam is incident on the surface 23 of the len~ 21, will be described with reference to Fig. 11.
The light ray 26 incident along the YOZ cross-section of the lens 21 proceeds in the lens and emerges from the lens in parallel with the incident light, passing a point P on the Z-axis because the thickness of the YOZ cross=section is constant. The light ray 25 incident along the XOZ
cross~section of the lens 21 is refracted to the Z-axis side and passing a point P' on the Z-axis just before the point P because the XOZ cross-section constitutes a convex lens.
In Fig. 11, let it be assumed that when a detecting surface 25 of the photo detector is positioned at an intermediate point O' between the points P and P' ~see Fig.
9) and when a spot 24' of the beam irradiated on the detecting surface 25 becomes circular as shown in Fig. 13, the converged beam from the objective lens is focussed on the xecorded surfa-e of the optical disc. Then, if the converged beam from the objective lens i5 focussed at a _ 14 ~

position be~ore or after the position of the op~ical disc, the detecting surface 25 of the photo detector is equivalently displaced in position to a point ~ before the point o' or to a point ~ thereafter. Accordingly, the spot 24' on the detecting surface 25 of the photo detector becomes ellipses as shown in Figs. 12 and 14. Specificallv, the ellipse spot 24' in Fig. 12 has the major axis extending in the first and third quadrants I and ~I along the direction with an angle of 45D relative to the x-axis, while the spot in Fig. 14 has the major axis extending in the second and fourth quadrants ~ and IV along the direction with an angle of 45~ relative to the x-axis. In Figs. 12 to 14, axes, a' to d' respectively correspond to the axes a to d in Fig. 10.
Therefore, if the detecting surface 25 of the photo detector is larger than the spot 24', the converging state of light rays by the objective lens onto the optical lens is detected from the difference between the sum of the photo-detected signals from the photo-detecting sensors of the first and third qu~drants I and m and the sum of the photo-detected signals of the photo-detecting sensors of the second and fourth quadrants ~ and ~, of the photo-detecting sensors of 4 yuadrants. As a result, the focussing servo can be made by moving the o~ective lens along its optical axis so as to reduce the above difference to zero.
However, in the focussing error signal detecting apparatus having the combination of the semi-cylindrical lens 21 and th~ four sensors of the photo detector, when thz spot 24' on the photo-detecting surface 2S of the photo " ~ - 15 -7~

detector is ~isplaced in position from the sta~es shown in Figs. 12 to 14 in the X'-axis or Y'-axis dîrection, the accuracy of the focussing error signal is lowered ~y the amount corresponding to the above displacement.
applica~ion has previously proposed a focussing error detecting apparatus which can produce a focussin~ error signal having high precision and excellent linearity.
An example of such previously proposed Eocussing error detecting apparatus being applied to the optical disc reproducing apparatus will hereinafter-be described with reference to Fig. 15. Laser light from a laser light source (for example, a semiconductor laser light source~ 1 is passed through the collimator lens 3, polarized beam splitter 2, 1/4 wavelength plate 4 and objective lens 5 sequentially and is incident on the recorded surface 6 of the disc. On the recorded surface 6 of the disc is recorded, for example, audio or video information properly modulated on a carrier in the form of pits formed along a spiral track and which can be optically read. Then, the laser light reflected from the recorded surface 6 is passed through the objective lens 5, the 1/4 wavelength plate 4, the polarized beam splitter 2 and a composite lens 30 and is incident on the photo detector 8.
The composite lens 30 consists of a pair of~ for example, convex lens regions 30a and 30b divided by a flat interface T including the optic~l axis O of the light incident on the composite lens 30 as shown in Figs. 16 and .

` ~ - 16 -18. The paired lens regions 30a and 30b respectively have different back focal lengths fl and f2 and the optical axes, l and 2 of the lens regions 30a and 30b are separated from each other by a predetermined dlstance. As shown in Fig. 16, the composite lens 30 is positioned on the x-y plane perpendicular to ~he optical axis O of the incident light with its flat interface T made coincident with the x-axis. In this example, the optical axes l and 2 of the respective lens regions 30a and 30b pass the points on the y-axis, which are respectively separated by distances Yl and Y2 on the opposite sides of the flat interface T. This composite lens 30 may be foxmed by bonding a pair of separate lens regions 30a and 30b made of glass, plastic or the like or by integrally molding the regions with plastic or the like. Although the configuration of the lens 30 is not formed circular in the drawings or convenience sake of explanation, it is practical for the lens 30 to be formed circular. The following other examples have the similar lens 30.
Fur her, as shown in Fig. 18, the photo detector ; 8 is disposed in such a manner that the photo de~ecting surface 8' is positioned on the optical axis O of the incident light at the position intermediate between the points Pl and P2 (which are respectively positioned on the optical axes l and 2 at both sides of a plane T' extending from th~ flat interface T~ at which the light beams emerging from the p ir of lens regions 30a and 30b when the light incident on the composite lens 30 is in the .

. . ~ ' . .

::, focussed state, or in this example when the incident light rays are parallel to each other. As shown in Fig. 17, the photo detector 8 consists of irst and second photo-detecting sensors 8B and 8A for receiving light rays passed through the far-side portion and the near-side portion of the first lens region 30a of the pair of lens regions 30a and 30b relative to the flat interface T and of third and fourth photo-detecting sensors 8D and 8C for receiving light rays passed through the far-side portion and near-side portion of the second lens region 3Qb of the pair ~f lens regions 30a and 30b relative to the flat interface T.
In this example, the first ko fourth photo-detecting sensors are rectangular and sequentially disposed in one direction (for example, in the direction o~ the track on the recorded surface of the disc) in the order of 8A to 8D. In this case, the photo-detecting sensors 8B and 8C may be formed integral.
The focussing error signal (see Fig. 20) is - obtained from the difference, (SB + Sc) - (SA ~ SD~ between the sum, SB~ Sc of the first and fourth photo-detected signals SB, Sc from the first and fourth photo-detecting sensors 8B and 8C and the sum, SA ~ SD of the second and third photo-detected signals SA~ SD from the second and third photo-detecting sensors 8A and 8D. This focussing error signal is supplied to the control coil 9 as the focus control signal.
The operation of the example in Fig. 15 will be described with reference to Fig. 17. Wh~n the recorded sur~ace 6 of the disc is at a proper position relative to - lB -the objective lens 5 so that the just focussed state is brought about, as shown in Fig. 17B the collimated light rays are incident on the composite lens 30 and focussed to form the semi-circular spots Sl and S2 of the same size and orienting in the same direction on the sensors 8A, 8B and 8C, ~D of the photo detector 8. Accordingly, in this case, the focussing error signal becomes zero so that the objective lens 5 is kept at the same position.
When the recorded surface 6 of the disc is positioned nearer to the objective lens 5 to bring the under-focussed state, the light incident on the composite lens 30 is slight}y out of ~he collimated state so that as shown in Fig. 17A, the spots Sl and S2 on the photo detector 8 respectively appear as a small semi-circle on the sensors 8A and 8B and a large semi-circle orientin~
in the same direction on the sensors 8C and 8D. Ac~ordingly, in this case, the focussing error signal becomes negative to drive the objective lens 5 ~o move away from the recorded surface 6.
When the recorded surface 6 of the disc is positioned far from the objective lens 5 to bring the over-focussed state, the light incident on the composite lPns 30 is slightly out of the collimated state in the opposite direction so that as shown in Fig. 17C the spots Sl and S2 on the photo detector 8 respecti~ely appear as a large semi-circle on the sensors 8A and 8B and a small semi-circle orienting in the same direction on the sensors 8C
and 8D. Accordingly, in this case, the focussing error 7~

signal becomes positive to drive the objecti~e lens S to move toward the recorded surface 6.
Thus, the focussing error, namely, the distance between the disc and the objective lens 5 is detected by the composite lens 30 and the photo detector 8 formed of the four phot3-detecting sensor 8A to 8D arranged in one direction. This error signal is supplied to the control coil 9 as the focus control signal so that the objective lens 5 is automatically controlled to bring the just focussed state at all times, or to be positioned at a constant distance from the disc.
Another example of such previously proposed optical disc reproducing apparatus will be described with reference to Fig. 21. As shown in Fig. 21, in this example, the composite lens 30 consists of a pair of concave lens regions 30a and 30b. Laser light from he laser light source ~for example, a semiconductor laser light source) 1 is passed through the polarized beam splitter 2, collimator lens 3, 1/4 wavelength plate 4 and objective lens 5 in turn and is incident on the recorded surface S o~ thP disc. On the recorded surface 6 of the disc i~ recorded, for Pxample, an audio or video information properly mod~lated on a carrier in the form of pits formed along a spiral track and which can be optically read. The laser light re1ected ~5 from the recorded surface 6 .is passed through the objective lens 5, the 1/4 wavelength plate 4,~the collimator lens 3, : the polarizing beam splitter 2 and the comp(,site lens 30 in turn and is incident on the photo detector 8.

, The composite lens 30 consists of a pair of, or example, concave lens regions 30a and 30b divided by ~ 1at interface T therebetween which includes the optical axis O of the light incident on the composite lens 30 as shown in Figs. 22 and 24. The paired lens regions 30a and 30b have different back focal lengths fl and f2 and the optical axes l and 2 of the pair of lens xPgions 30a and 30b are spaced from each other. In case of Fig. 22, the composite lens 30 is positioned on the x-y plane which makes a right angle with the optical axis O of the incident light with the flat interface T made coincident with the x-axis. Here, the optical axes l and 2 of the lens regions 30a and 30b respectively pass the points lying on the y-axis and separated by distances yl and y2 from the flat interface T
in the opposite directions.
The photo detector 8 is positioned in such a manner that as shown in Fig. 24, the photo-detectin~ surface ; 8' is placed on the optical axis O at a position in.termediate ~etween the points Pl and P~ (which are positioned on the optical axes 2 and l on the opposite sides of the plane T' extending from the flat interface T) where the light rays exiting from the pair of lens regions 30a and 30b are focussed when the light incident on the composite lens 30 is in the focussed state, or when the incideht light is - 25 converged toward a virtual point Q. The photo detector 8 as shown in Fig. 23 consists of first and second photo-detecting ~ensors 8C ~nd 8D for receiving the light rays respectively passed through the far side poxtion and neax-, ~ 21 -.

.

side portion of the first lens regions 30a of the pair of lens regions 30a and 30b relative to the flat interface T
and third and fourth photo-detecting sensors 8A and 8B for receiving light rays respectively passed through the far-side portion and near-side portion of the second lens regions 30b of the pair of lens regions 30a and 30b relative to the flat-interface T. EIere, the first to fourth photo-detecting sensors are rectangular and sequentially arranged in one direction (for example, in the direction of the track of the recorded surface of the disc) in the order of 8A to 8D. In this case, the photo-detecting sensors 8B and BC
may be formed integral.
Then, the focussing error signal is obtained from (SC + SB) (SD + SA) between the sum S + S
of the first and fourth photo-detected signals Sc, SB from the first and fourth light detec~ing sensors 8C, 8B and the sum SD + SA of the second and third photo-detected signals SD, SA from the second and third photo-detecting sensors 8D, 8A.
This focussing error signal is supplied to the control coil 9 as the focussing control signal.
~he operation of the example in Fig. 21 will be described with reference to Fig. 23. When the recorded surface 6 of the disc is at a proper position relative to the objective lens 5 to bring the just focussed state, light converged toward the virtual point Q is incident on the composite lens 30 so that as shown in Pig. 23B~ the spots Sl and S2 on the photo detector 8 respectively appear as semi-circles of the same size and orienting in the same :

.

7~ Ei direction on the sensors 8C, 8D and sensors 8A, 8B.
Accordingly, in this case, the focussing error signal becomes zero so that the objective lens 5 is kept at the same position.
S When the recorded surface 6 of the disc is positioned closer to the objective lens 5 to bring the under-focussed state, the light incident on the composite lens 30 is slightly out of the just focussed state so that as shown in Fig. 23A the spots Sl and S2 on the photo detector 8 respectively appear as a large semi-circle on the sensors 8C, 8D and a small semi-circle orienting in the same direction on the sensors 8A, 8B. Accordingly, in this case, the focussing error signal becomes negative to drive the objective lens 5 to move away from the recorded surface 6.
When the recorded surface 6 of the disc is posi~ioned far from the objective lens 5 to bring the over-focussed state, the light incident on the composite lens 30 is slightly out of the just focussed state in the opposite direction so that as shown in Fig. 23C the spots Sl and S2 on the photo detector 8 respectively appear as a ~mall ~: semi-circle on the sensors 8C and 8D and a lar~e semi-circle orienting in the same direction on the sensors 8A and 8B.
Accordingly, in this case, the focussing error signal becomes positive to drive the objective lens 5 to move ~: 25 toward the recorded sur~ace 6.
As set forth above, the focussing error, namely, the distance between the disc and the objective lens 5 is detected by the composite lens 30 and the photo detector - 23 - .

:, .

' 37~i 8 formed of four photo-detecting sensors 8A to RD arranged in one direction. This error signal is supplied to the control coil 9 as the focussing control signal so that the objective lens 5 is automatically controlled to bring the just focussed state, or t~ be positioned at a constant distance from the disc.
Examples of composite lens 30 will be described with reference to Figs. 25 to 31. In the composite lens 30 shown in Fig. 25, the pair of lens regions 30a and 30b are both form~d of convex lenses, while in the composite lens shown in Fig. 27 the pair of lens re~ions 30a and 30b are both formed of concave lenses. ln both examples, the optical axes l and 2 OI the paired lens xegions 30a and 30b respectively exist within the lens regions 30a and 30b themselves. In the composite lenses 30 shown in Figs. 26 and 29, the optical axes l and 2 of the pair of lens regions 30a and 30b respectively exist within the other lens regions 30b and 30a. Although not shown, both the optical axes l and 2 of the pair of lens regions 30a and 30b may be positioned in any one of the lens regions. Also as shown in Fig. 28, it is possible that the lenses of the same power are us~d as the pair of lens regions 30a and 30b and combined to have different major ~urfaces H~ ' and ~2~ ~2' Furthenmore, it is possible that when the composite lens 30 is placed on the x-y plane so that the flat interface T therebetween coincides with the x~axis, the optical axes l and o~ are positioned on the y-axis at positions on opposite sides of the flat in~erface ~ as shown :. ~

in Figs. 16, 22 and 29, on neither of the x-axis and y-axis at positions on opposite sides of the flat interface T as shown in Fig. 30, or on the x-axis, namely on the flat interface T as shown in Fig. 31.
In addition, the arrangement of the photo detecting sensors 8A to 8D of the photo detector 8 can be selected in response to each positional relation between the optical axes l and O~ of the composite lens 30. Fig. 32 illustrates the arranging state (substantially the same as that in Fig.
17) of the photo-detecting sensors 8A to 8D of the photo de~ector 8 when the optical axes of pair of lens regions 30a and 30b of the composite lens 30 are positioned at the points on th~ y-axis and on the opposite sides of the flat interface T as shown in Fig, 16. Such arrangement can be ~sed for the case where the optical axes l and 2 of the composite lens 30 are displaced a little from the y-axis as shown in Fig. 30. When ~he optical axes l and 2 of ~he pair of lens regions 30a and 30b of the composite lens 30 exist at points on the x-axis, namely, on the flat interface 2G T, the photo-detecting sensors 8A ~o 8D of the photo dete~tor 8 are arranged in 2 rows of two sensors each as shown in Fig. 38.
In addition, 25 shown in Fig. 32, i the radiuses of the semi-circular spots Sl and S2 on the photo-detecting sensors BA, 8B and 8C, 8D of the photo detector 8 are .
represented by Rl and R2, the lengths of the spots Sl and S2 on the photo-detecting sensors 3B and 8D by Ro and R3 (where the widths of the dead zones between the photo detecting sensors 8A to 8D are neglected), and ~he relation among these lengths Ro to R3 is selected to satisfy R0 ~ R
then the focussing error signal (SA + SD) ~ (SB+ Sc) resulting from the calculation of the photo-detected signals SA ~ SD upon just focussing can be reduced to zero even if Rl= R2 is not satisfied. When Rl= R2, the amount of ; light incident on the photo-detecting sensors 8A and ~C must be equal to the amount of light inciden~ on the photo-detecting sensors 8B and 8D and thus, regardless of Ro (= R3), it is possible to reduce the focussing error signal to zero upon just focussing.
The above focussiny error detecting apparatus in the respective examples have the following advantages. Since the focussing error signal is obtained from the sum, {(SA ~ Sc) (SD ~B) (SA+ SD) ~ (SB ~ Sc)} of the difference, SA ~ Sc ~` (or SD ~ SB) between the photo detected signals produced from the photo-detecting sensors for the light rays passed through the far-side portions of the lens regions 30a and 30b of the composite lens 30 relative to the flat interface T and ~ the diff~rence SD ~ SB (or S~ ~ Sc) between the photo-detected - signals produced from the photo-detecting sensors for the light rars passed through the near-side portions of the lens ~- regions 3~a and 30~ _elative to the flat interface ~t this focussing error signal is good in accuracy due to the fact that high-frequency signals leaked in owing to the diffraction in the pits of the disc and difference in the modulation ~` degrees of light relative to the spatial requency of.
pits at the center and periphery of the spots are .
: ~ :
~ 26 -cancelled.
Also, since the focussing error signal is obtained from the sum {(S + SD) ~ (S~ Sc)} of the difference SA SB
(or SB - SA) between the photo-detected signals produced from the pho o-detecting sensors for the light rays passed through the far-side portion and near-side portion of the lens region 30a of the composite lens 30 relative to the flat interface T and the difference, SD - Sc (or SC ~ SD) between the photo-detected signals produced from the photo-detecting sensors for the light rays passed through the near-side portion and far-side portion of the lens region 30b of the composite lens 30 relative to the flat interface T, difference signals SB ~ SA and Sc ~ SD are symmetrical (see Fig. 19), and thus the focussing error signal has excellent linearity as shown in Fig. 20.
- Furthermore, as shown in Fig. 32, if the radiuses ; of the semi-circular spots Sl and S2 on the photo-detecting sensors 8A, 8B and 8C, 8D of the photo det~ctor 8 are represented by Rl and R2, and the lengths of the spots Sl and S2 on the photo-detecting sensors 8B, 8D by Ro and R3 (the widths of the dead zones between the photo detecting sensors 8A to 8D are neglected), the amount of light incident on the photo-detecting sensors 8A and 8C ~.s equal -to the amount of 'ight incident on the photo~detecting sensors BB and BD under the condition of Rl ~ R2 and thus, it is possible to reduce the focussing error signal to zero upon just focussing irrespective of ~0 ~= R3~ . Accoxdingly, .~

even if the spots Sl and S2 on the photo detector 8 are displaced in the arranging direction of the photo-detecting sensors 8A, 8B and 8C, 8D, the precision of the focussing errGr signal is not lowered.
By the way, since the composite lens 30 used in the above focussing error detecting apparatus is complicated in shape and structure, if plastic is used as its material, the compo~ite lens 30 can be produced easily as compared with a case in which glass is used as the material of the composite lens. ~owever, the plastic lens has the large changing ratio of the focal length by the temperature as compared with that of the glass lens. Accordingly, if the composite lens 30 made of the plastic l~ns is applied to the above focussing error detecting apparatus and co~bined with the photo detector to obtain the focussing error signal, offset is caused in the focussing error signal in accordance with the change of temperatur~ so that the precision o the focussing servo for the objective lens is lowered.
Referring to Fig. 34, the way how the converged state of the converging beam from the objective lens 5 on the recorded surface 6 of the optical disc by the ~ptical disc reproducing apparatus of, for example, Fig. 21 is changed by the temperature change will be considered using mathematical formula. In Fig. 34, the objective lens 5~
the collimator lens 3 and the composite lens (concave lens) 30 (see Figs. 21, 22 and 2~) are represented by lines corresponding to the first and s~cond major suxfaces.
.~
~ 2~ -Reference characters BMll, BM21 and BM12, BM22 designate light rays which are respectively incident on the ou~ermost peripheries of the first and second lens regions 30a, 30b of the lens 30 and the flat interface T. In this case, the relation between the focal lengths fl and f2 of the first and second lens regions 30a and 30b is selected as, for example, fl ~ f2. The refxactive index of the lens 30 is made uniform and taken as n. Further, the fGcal lengths of the objective lens S and the collimator lens 3 are respectively taken as f3 and f4. The lengths from the sec~nd major surface of the lens 30 to converged points Pl and P2 along the direction of a plane T' are respectively taken as a and b. The length from the second major surface of the lens 30 to a photo-detecting surface 8' along the direction of the plane T' is taken as _. The length between the first major surface of the lens 30 and a conjugate point Q o~ a beam incident thereon (converged beam) along the direction of the plane T' is taken as d. Further, the heights of the light rays MBll, MB21 incident on the lens 3 at its outermost periphery from the plane T' are respectively taken as hl, while the heights of the light rays MBll, ~B
incident on the lens 30 from the plane T' are respectively taken as h2.
Then, the heights hl and h2 are respectively expressed by the following equations.
hl - f3 NA5 ~.o (1) h2 = f hl ... (2 ` 4 where NA5 represents the N.A. (numerical aperture) of the objective lens 5.
From the formula of lens, ~he following equations are established among a, b, d, fl and f2.

1 1 _ 1 .,.
~ ~a- + a ~

1 + 1 = 1 -~ (4) From the equations (3) and (4), a and b are respectively expressed by the following equations.

df d + fl b = d t f -- (6) If the height rom a point at which the light ray MB22 is incident on the photo-detecting sur~ace 8' to a point at which the light ray MBll is similarly incident thereon is taken as h3 and the h~ight from a point at which the light ray MB12 is incident on the photo-detecting surface 8' to a point at which the light ray MB21 is similarly incident thereon is taken as h4, the heights h3 and h4 are respectively expressed by the following equations.

h2(a- c) ~- (7) 3 ~ a h2(c- b) ................................ D (8) If h3 = h4 _ h5 in the equations (7~ and (8~, c and h5 are re~pectively expressed by the following e~uations.

, 2ab ...
c a + b h2(b - a) -- (lO) a + b If the equations (5) and (6) are substituted for the equation (9), the following equation is obtained.

c = d(f ~ f2) + 2fl f2 -- (ll) In general, when the radiuses of curvature of both surfaces of a lens are taken as Rlr R2 and the refractive index thereof is taken as n, the focal length f of the lens can be expressed as follows.
f = (n- 1)( Rl - l ) -~ (12) The relation of the change of focal length f to the change of the refractive index n caused by the change of ~emperature T is expressed by the following equation.
1 df = ~ 13) Then, the changing ratio dc/d~ of the length c relative to the change of temperature T is searched for.
Modifying the equation (ll) yields the following equation Differentiating c, fl and f~ in the equation (14) ` 20 by T yields c2 ~ a~ (15) : `

~ 31 -. .

:

7~

Accordingly, dc/dT is modified into the following e~uation if the equations (13) and (14) are substituted for the equation (15) and dfl/dT = df2/dT is taken dc = C2 (-1 ~ d ) ~1 dn -- (16) The change of the length c corresponds to _he defocussed amount ~Z ~= ~c/2~2) on the recorded surface 6 of the disc where 3 is the longitudinal magnification {the composite focal length fl (or f2~ divided by f3}.aZ is expressed by the following equation.
tn~ 2~2 (c ~ T -- (17) When acryl is used as the material of the lens 30, dn/dT becomes 1.4 x 10-4 (per 1C). When the temperature change ~T is 30C, the defocussed amount ~Z is 1 ~m so that it becomes difficult to detect the focussing error with high precision wherein fl = -10 mm, f2 = -11 mm, d - 6.18 mm and n= 1.486.

OBJECTS AND SUMMARY OF THE INVENTION

_ Accordingly, it is an object of the present invention to provide a focussing error detecting apparatus which can obviate the above defects inherent in the prior art focussing error detecting apparatus.
It is another object of the present invention to provide a focussing error detecting apparatus capable of producing a focussing error signal of high precision , ~2~

and excellent linearly It is a further object of the prese~t invention to provide a focussing exror detecting appara~us suitable for use with an optical disc reproducing apparatus, an auto-focus camera and so on.
It is a still further object of the present invention t~ provide a composite lens for use with a focussing error detecting apparatus and which can easily be produced.
It is a yet further object of the present invention to provide a composite lens for use with a focussing error detecting apparatus and which can produce a focussing error signal with excellent precision regardless of the change of temperature.
According to one aspect of the present invention, there is provided a focussing error detecting apparatus comprising:
a composite lens formed of a single lens made of ylass and a single lens made of plastic; and a photo detector disposed behind said composite lens to oppose thereto, wherein said glass single lens has a uniform focal length, said plastic single lens is formed of a pair of lens regions divided by a-flat interface including an optical axis of light incident on said com-posite lens, said pair of lens reyions have back focal lengths different from each other and optical axes spaced apart from each other, said photo detector is placed such ; that its photo-detecting face is positioned at a middle ~ - 32a~

`

point between converged points o~ light rays emitted from said pair of lens regions on the optical axis of said incident light when said light incident on said composite lens is in the focussed state and s~id photo detector i5 formed of first and second photo-detecting sensors for receiving light rays passed thr~ugh the far-side portion and near-side portion of said first lens region of said pair of lens regions relative to said flat interface and third and fourth photo-detecting sensors for xeceiving light rays passed through the far-side portion and near-side portion of said second lens region of said pair of lens regions reiative to said flat interface so that a focussing error signal is produced from the difference between the sum of first and fourth photo-detected signals, from said first and fourth photo-detecting sensors and the sum of second and third photo-detected signals from said second and third photo-detecting sensors.
The other objects, features and advantages of the present invention will become apparent from the following description taken in conjunction with the accompanying dxawings through which the like references designate the same elements and parts.
DESCRIPTION OF THE PREFERRED EMBODIMENT

Now, an embodiment of the focussing error detecting apparatuc according to the present invention will hereinafter be described with reference to Figs. 35 and 36. In this case, however, only a composite lens used therein will be _ 32b ~

described and the construction of the whole of the focussing error detecting apparatus of the invention will be omitted because the whole of the focussing error detecting apparatus 32c -37~

is constructed same as those of Fig. 15 and the followings.
In Figs. 35 and 36, reference numeral 30 designates the whole of a composite lens (concave lens corresponding to the composite lens 30 shown in Figs. 22 and 24 and used in the apparatus shown in, for example, Fig. 21). The composite lens 30 consists of a glass single lens 30~ and a plastic single lens 30~ which are bonded together along their peripheries, for example. The glass single lens 30~ is a uniform lens having its optical axis on a flat interface T' and the focal length of which is fO (positive or negative).
In this embodiment, the glass single lens 30~ is a concave lens of flat concave type (in this embodiment, the focal length fO is -10.5 mm, for example)~- T-he plastic single lens 30~ is, for e~ample, a concave and convex type lens.
l~ Reference numeral 30y designates a space defined between the single lenses 30 and 30~.
As shown in Fig. 35, the plastic single lens 30 includes lens regions 30'a and 30'b which are vertically halved by the boundary flat interface T (x-axis) including an optical axis O of incident light. The focal length (back focal length) of the first lens region 30'a is taken as f'l and the focal length (back focal length) of the second lens region 30'b is taken as f'2 The optical axes l and 2 of the respective lens regions 30'a and 30'b are spaced apart on the y-axis by predetermined lengths Yl and Y2 relative to the optical axis O (boundary flat interface T) of the incident light as in the same way as in F~gs. 22 and 24. The focal lengths f'l and f'2 may be either positive or negative in general if ~ f'2 Also it is possi~le that any one of the focal lengths f'l and f'2 is ~. In this ~2~

embodiment, the focal lengths f'l and fl2 are respectively selected to be -210 mm and 231 mm. The outline of the composite lens 30 may be arbitrary but is practical when formed circular as shown in Fig. 35.
The lens regions 30'a and 301b of the single lens 30~ and the corresponding lens regions of the single lens 30~ constitute lens regions 30a and 30b of the composite lens (flat concave lens) 30.
Such single lens 30~ can be produced in two ways, one of which is to make lens regions made of plastic lens materials of two kinds having different re~ractive indexes such as acrylic material integral by bonding or the like and to mold and abrade the ~onded lens regions to thereby produce the single lens 30~.
lS The other way is to mold, abrade lenses made of the same plastic material such as acrylic material but with different curvatures, namely, different focal lengths, divide the lenses two and to make the lens regions thus obtained integral by bonding or the like to thereby produce the sinyle lens 30~.
The changing ratio of the focal length or the like of the composite lens 30 thus formed relative to the change of the temperature will be discussed. If the refractive indexes of the glass single lens 30~ and the plastic single lens 30~ are respectively taken as n~ and n~, the chanying ratios of the focal lengths fl and f2 ~ the lens regions 30a and 30b relative to the temperature T will be expressed by the following equations with reference to the equation (13).

... .

7~6 1 dfl fl -l dn~
l dT fO n~ - l dT

+ fl -l . dn~ -- (18) l n~- l dT

1 df2 f2 -l dna f2 ~~~ fo n~ - 1 dT

f'2 n~- l dT ~O (19) However, dn~/dT of the glass single lens 30a is 1.7 x 10 -6 (per 1C) in the case of BK7 glass (substantially the same in the case of other glass), while dn~/dT of the ~ plastic single lens 30~ is -1.4 x lO -4 (per 1C~ in the case of acryl (substantially the same in the case of other plastics).
Consequently, as will be clear from the above, the condition of dna/dT dn~/dT is established between dn~/dT and dn~/dT.
Thus, the above equations (18) and (19) can respectively be approximated to as l d ~ . 1 O dn~ .O- (20) f2 dT f'2 n~- 1 dn~ ............................. (21) Moreover, dc/dT when the composite lens 30 in Figs. 35 and 36 is applied to the optical system in Fig. 34 can be expressed by the following equation with reference to the equations (15) and (16).

dc c2 1 dfl + 1 df2 dT 2 ( fl~ f22 dT ) c~ --l - dn~ ~ t fl ) ... (22) ~,. . :

' 7~

Accordingly, in order to reduce dc/dT, it is sufficient to make (l/f'1 + 1/f'2) small. In other words, if f1 and f2 are determined, it is sufficient to make fl/f' and f2/f'2 small, respectively. Also, if f'l and f'2 are selected to satisfy the condition of f'l/f'2 = -1, dc/dT
can be reduced approximately to zero.
- According to the complex lens 30 described just as above, if as the glass single lens 30~ such one is used which has one optical axis and uniform focal length, as the single lens 30~ is formed of plastic lens and both the lenses are bonded together to be the complex lens, the manufacturing process thereof can be made simple and the changing ratios of the focal lengths f~_and f2 of the lens regions 30a and 3~b relative to the change of the temperature T can be made small.
Moreover, if the above composite lens 30 is combined wikh the photo detector described abo~e, the offset appearing in the focussing error signal from the focussing error signal detecting apparatus caused by the temperature change can be reduced and hence the focussing error signal with excellent precision can be produced.
It is needless to say that the composite lens 30 used in the present invention can be applied to the examples of the modified complex lenses used in the examples of the previousl~ proposed focussing error signal detecting apparatus shown in Fig. 15 and the followings.
As set forth above, according to the present invention, the composite lens used in the focussing error detecting apparatus, can be made easily and the focussing error signal with excellent precision regardless of change - 3~ -~%~l~37~
of the temperature can be produced.
The above description is given on a single preferred embodiment of the inventionl but it will be apparent that many modifications and variations could be effected by one skilled in the art without departing from the spirits or scope of the novel concepts of the inve~tion, so that the scope of the invention should be determined by the appended claims only.

,'''~ ' ..

Claims

WE CLAIM AS OUR INVENTION

1. A focussing error detecting apparatus comprising:
a composite lens formed of a single lens made of glass and a single lens made of plastic; and a photo detector disposed behind said composite lens to oppose thereto, wherein said glass single lens has a uniform focal length, said plastic single lens is formed of a pair of lens regions divided by a flat interface including an optical axis of light incident on said composite lens, said pair of lens regions have back focal lengths different from each other and optical axes spaced apart from each other, said photo detector is placed such that its photo-detecting face is positioned at a middle point between converged points of light rays emitted from said pair of lens regions on the optical axis of said incident light when said light incident on said composite lens is in the focussed state, and said photo detector is formed of first and second photo-detecting sensors for receiving light rays passed through the far-side portion and near-side portion of said first lens region of said pair of lens regions relative to said flat interface and third and fourth photo-detecting sensors for receiving light rays passed through the far-side portion and near-side portion of said second lens region of said pair of lens regions relative to said flat interface so that a focussing error signal is produced from the difference between the sum of first and fourth photo-detected signals from said first and fourth photo-detecting sensors and the sum of second and third photo-detected signals from said second and third photo-detecting sensors.

2. A focussing error detecting apparatus according to claim 1, wherein a relation between the focal lengths of said pair of lens regions is selected so as to satisfy f'1/f'2 = -1 where f'1 : the back focal length of said first lens region f'2 : the back focal length of said second lens region.