GB2057218A

GB2057218A - Detecting focussing error

Info

Publication number: GB2057218A
Application number: GB8020097A
Authority: GB
Original assignee: Olympus Corp; Olympus Optical Co Ltd
Current assignee: Olympus Corp
Priority date: 1979-06-25
Filing date: 1980-06-19
Publication date: 1981-03-25
Also published as: NL192406C; FR2459991A1; DE3023779A1; FR2459991B1; NL8003659A; NL192406B; GB2057218B; DE3023779C2

Abstract

An in-focus condition of an optical system 1, 2, 3, 4, 5, intended to focus a spot of light onto a surface 6, such as a video disc, is characterised in that light reflected from the surface 6 is caused to impinge upon a surface 11 set at a critical angle with respect to light reflected from the surface when the system is in-focus, whereby when the system is not in-focus reflected light impinges upon the surface 11 as either a convergent or a divergent beam, rather than as a parallel beam, and as a consequence half of the light impinging upon the surface 11 passes through it rather than being reflected by it. This loss of intensity of the light reflected by the surface 11 is detected by a split detector 12A, 12B, and used 13 to provide a signal indicating an out of focus condition. Numerous modifications are described. <IMAGE>

Description

SPECIFICATION Method and apparatus for detecting focussing error of objective lens The present invention relates to a method for detecting a focussing condition of an objective lens with respect to an object on which a light spot has to be focussed by said objective lens and to an apparatus for carrying out such a focus detecting method.

Such focus detecting method and apparatus are advantageously applied to an apparatus in which a scanning light spot is projected by an objective lens onto one or more information tracks recorded spirally or concentrically on a disc-shaped record medium to read an information recorded along the track.

In an apparatus for reproducing or picking-up an information from the above mentioned record medium, the record medium is usually called as a video disc in which encoded video and audio signals are recorded as optical information such as optical transmitting, reflection and phase properties. While the video disc is rotated at a high speed such as thirty revolutions per second, i.e. 1,800 rpm, a laser beam emitted from a laser light surce such as a helium-neon gas laser is focussed on the tracks of the disc as a light spot and the optical information is read out. One of important properties of such a record medium is a very high density of recorded information and thus a width of the information track is very narrow and a space between successive tracks is also very narrow.In a typical video disc described in, for instance, Philips Technical Review, Vol. 33, 1973, No. 7, a pitch of the tracks amounts only to 2 um. Therefore the diameter of light spot should be correspondingly small such as a 1 to 2 lim. In order to pick-up correctly the recorded information from such tracks having very narrow width and pitch, an error in a distance between the objective lens and the tracks, i.e. a focussing error should be reduced as little as possible to make a spot diameter as small as possible.

To this end, the apparatus is provided with a focussing control system in which an amount and a direction of a de-focussed condition of the objective lens with respect to the disc surface are detected to produce a focussing error signal and the objective lens is moved in a direction of the optical axis of objective lens in accordance with the detected focussing error signal.

Figure 1 is a schematic view illustrating a known focus detection system in an optical pick-up apparatus. A light source 1 is constituted by a laser and emits light which is linearly polarized in a plane of the drawing of Figure 1. The light is collimated by a collimator lens 2 into a parallel light beam which is then transmitted through a polarizing prism 3 and a quarter-wavelength plate 4. The light beam is further focussed by an objective lens 5 as a light spot on a disc 6 having one or more information tracks of crenellated pit construction. Then, the light is reflected by the information track and impinges upon the polarizing prism 3 by means of the objective lens 5 and the quarter-wavelength plate 4.The light impinging on the prism 3 is polarized in a direction perpendicular to the plane of the drawing, because it has passed the quarter-wavelength plate 4 twice and thus, is now reflected by the polarizing prism 3. The light flux reflected by the polarizing prism 3 is converged by a condenser lens 7 and a cylindrical lens 8. Since the cylindrical lens 8 has a focussing power only in one direction, the shape of the focussed beam formed by the condenser lens 7 and the cylindrical lens 8 varies as shown in Figure 1 with respect to an in-focussed condition in mutually orthogonal directions, when the disc 6 moves up and down. In the known apparatus, this variation in shape is detected by a light detector (not shown) divided into four sections and arranged at a focal plane of the lens system 7, 8 to produce a focussing error signal.The focussing error signal thus detected is supplied to a focussing mechanism such as a moving coil mechanism to move the objective lens 5 in its axial direction.

In the known focus detecting system, since a relatively long optical path is required to focus the light beam after being reflected by the polarizing prism 3, there is a drawback that an optical system is liable to be large in size. Further, since the light detector having the four sections must be arranged precisely in three axial directions, i.e. in the optical axis direction and in two orthogonal directions perpendicular to the optical axis, the adjustment in positioning the light detector is quite critical and requires a time-consuming work.

Moreover, since a dynamic range in which the accurate focussing error signal can be obtained due to the deformation of the focussed beam is relatively small, any focussing error signal could not be produced if the disc deviates from a given position only by a relatively small distance.

In the apparatus for picking-up the information recorded in the information track it is also necessary to effect the tracking control so that the light spot can always scan or trace the track precisely. There have been proposed two tracking control methods, i.e. a wobbling method and a three beam method. In the wobbling method the light spot is slightly vibrated across the track by vibrating the objective lens or a mirror inserted between the light source and the objective lens. In the three beam method three beams are simultaneously projected on the disc with being slightly separated in the track direction and in a direction perpendicular to the track direction. The three beam method is superior to the wobbling method, because the light beams need not be vibrated mechanically.In the apparatus for detecting the focussing error signal it is thus preferable that the tracking error signal can be detected by means of the three beam method.

According to the invention, a method for detecting a focussing error signal of an objective lens with respect to an object on which a light spot is to be formed by means of said objective lens comprises focussing light emitted from a light source onto the object; introducing at least a part of a light flux reflected from the object into an optical member which includes a reflection surface set to be substantially at a critical angle with respect to a light ray in the reflected light flux in an in-focussed condition; detecting a variation of light distribution of the light flux reflected by said reflection surface or a variation in amounts of the reflected light flux and a light flux transmitted through the reflection surface to produce the focussing error signal.

The present invention can provide a method for detecting a focussing error signal of an objective lens with respect to an object onto which a light spot is to be focussed, which method has an extremely high sensitivity for focus detection.

The present invention can provide a focussing detection method which can be carried out easily by means of a compact optical system.

Further the present invention can provide an apparatus for detecting a focussing error signal of an objective lens with respect to an object onto which a light spot is to be focussed by means of the objective lens, which apparatus can detect the focussing error signal at a very high sensitivity and can be made small in size and light in weight. The invention can provide an apparatus for detecting a focussing error signal, in which a light detector can be easily arranged in position without troublesome adjustment and alignment.

According to the invention, an apparatus for detecting a focussing error signal of an objective lens with respect to an object onto which a light beam emitted from a light source is to be focussed as a light spot by means of said objective lens comprises a beam splitting element arranged between the light source and the objective lens for directing the light beam emitted from the light source to the objective lens and directing a light flux reflected by the object into a direction different from that to the light source; a detection prism arranged to receive at least a part of the light flux reflected from said object and including a reflection surface which is set substantially at a critical angle with respect to a light ray in the reflected light flux impinging upon the reflective surface; light detecting means having at least two light receiving regions arranged to receive a light flux reflected by the reflection surface or light fluxes reflected by and transmitted through the reflection surface, respectively to produce signals representing amounts of light fluxes impinging upon the light receiving regions; and a circuit for receiving the output signals from the light detecting means to form a difference signal as the focussing error signal.

The present invention also relates to an apparatus for detecting a tracking error signal as well as a focussing error signal. It is still another object of the invention to provide an apparatus for detecting a focussing error signal and a tracking error signal by means of the three beam method as well as the wobbling method.

According to the invention an apparatus for detecting a focussing error signal of an objective lens with respect to a disc shaped record carrier including at least one spiral or concentrical information track on which a light beam emitted from a light source is to be focussed as a light spot, and for detecting a tracking error signal of the objective lens with respect to the information track comprises a beam splitting element arranged between the light source and the objective lens for directing the light beam emitted from the light source to the objective lens and directing a light flux reflected by the record carrier into a direction different from that to the light source;; a detection prism arranged to receive at least a part of the light flux reflected from said record carrier and including a reflection surface which is set substantially at a critical angle with respect to a central light ray of the light flux impinging upon the reflection surface; a lens for converging the light flux impinging upon the reflection surface; light detecting means having at least two light receiving regions arranged substantially at a focal point of a light flux reflected by the reflection surface, the light receiving regions being divided along a plane which includes a central light ray of the light flux reflected by the reflection surface and is perpendicular to a plane of incidence; and a circuit for receiving output signals from the light receiving regions to form a difference signal as the focussing error signal and to form the tracking error signal.

The invention will now be described in detail with reference to the accompanying drawings, wherein: Figure 1 is a schematic view illustrating an optical system of an optical pick-up apparatus with a known focus detection system; Figure 2 is a schematic view showing an embodiment of a focus detection apparatus according to the invention; Figure 3 is a graph showing an intensity of reflected light having an incident angle near a critical angle; Figures 4A, 4B and 4C are graphs showing output signals from light detector regions and a focussing error signal; Figure 5 is a schematic view illustrating another embodiment of the focus detection apparatus according to the invention; Figures 6, 7, 8, and 9 are schematic views depicting modified embodiments of the focus detection apparatus according to the invention; ; Figure 10 is a schematic view illustrating an embodiment of an apparatus according to the invention; Figures 11A, 11B and 1 7C are schematic views for explaining the operation of the apparatus of Figure 10; Figure 12 is a schematic view showing a modified embodiment of the apparatus illustrated in Figure 10; Figure 13 is a schematic view depicting another embodiment of the apparatus according to the invention for detecting a focussing error signal and a tracking error signal by means of a three beam method; Figures 14A, 14B and 14C are schematic views for explaining the operation of the apparatus shown in Figure 13; and Figures 15 and 16 are schematic views illustrating still another embodiments of the focus detecting apparatus according to the invention.

Figure 2 is a schematic view illustrating an optical pick-up apparatus in which an embodiment of the focus detection apparatus of the invention is installed. In this embodiment, an optical system for projecting a scanning light spot onto a record medium is same as that shown in Figure 1. A linearly polarized light beam emitted from a laser light source 1 is collimated into a parallel light beam by a collimator lens 2 and passes through a polarizing prism 3 and a quarter-wavelength plate 4. Then, the parallel light beam impinges upon an objective lens 5 and is focussed on an information track of a disc 6 as a light spot. The light beam reflected by the disc 6 is optically modulated in accordance with information recorded in the track and is reflected by the polarizing prism 3.The construction and operation of the optical system so far explained are entirely same as those of the known optical system shown in Figure 1. The light flux reflected by the polarization prism 3 impinges upon a detection prism 10 having a reflection surface 11 and the light flux reflected by this surface 11 is received by a light detector 12. According to the invention, the reflection surface 11 is so arranged with respect to the incident light that under an in-focussed condition it makes a given angle with respect to the incident light (parallel light flux) which angle is equal to a critical angle or slightly smaller than the critical angle. Now, for the time being, it is assumed that the reflection surface 11 is set at the critical angle. In the in-focussed condition, the whole light flux reflected by the polarizing prism 3 is totally reflected by the reflection surface 11.In practice a small amount of light is transmitted into a direction n shown in Figure 2 due to incompleteness of a surface condition of the reflection surface 11. However, such a small amount of transmitted light may be ignored. If the disc 6 deviates from the in-focussed condition in a direction a in Figure 2 and a distance between the objective lens 5 and the disc 6 is shortened, the light reflected by the polarizing prism 3 is no longer the parallel beam, but changes into a diverging light beam including extreme light rays ail and aiz. On the contrary, if the disc 6 deviates in an opposite direction b, the parallel light beam is changed into a converging light beam including extreme light rays b11 and bj2. As can be seen in Figure 2, light rays from an incident optical axis OPj to the extreme light ray ail have incident angles smaller than the critical angle and thus, are transmitted through the reflection surface 11 at least partially. Contrary to this, light rays between the optical axis OPj and the extreme light ray ail have incident angles larger than the critical angle and thus are totally reflected by the surface 11.In case of deviation of the disc 6 in the direction b, the above relation becomes inversed, and light rays below a plane which includes the incident optical axis OFI and is perpendicular to the plane of the drawing of Figure 2, i.e. an incident plane, are totally reflected by the reflection surface 11, and light rays upper said plane are at least partially transmitted through the reflection surface 11.As explained above, if the disc 6 deviates from the in-focussed position, the incident angles of the light rays impinging upon the reflections surface 11 vary in a continuous manner about the critical angle except for the center light ray passing along the optical axis OPj. Therefore, when the disc 6 deviates from the in-focussed position either in the direction a orb, the intensity of the light reflected by the reflection surface 11 varies abruptly near the critical angle in accordance with the above mentioned variation in the incident angles. In this case, senses of the variations of the light intensities on both sides of said plane perpendicular to the incident plane and including the incident optical axis OP vary in mutually opposite manner.On the contrary, in the in-focussed condition, the light flux impinging upon the detection prism 10 is totally reflected by the reflection surface 11 and thus, the uniform light flux impinges upon the light detector 12. The light detector 12 is so constructed that the lower and upper light fluxes with respect to said plane are separately received by separate regions 12A and 12B, respectively. That is to say, the light detector 12 is divided along a plane which is perpendicular to the incident plane and includes an optical axis OP, of reflected light.

Figure 3 shows a graph representing a variation of an intensity of reflected light in accordance with an incident angle near the critical angle. Curves R, and R, indicate the lig ht intensities for P and S polarized light rays, respectively. The curves are obtained when the detection prism 10 is made of material having a refractive index of 1.50. It should be noted that an intensity of a non-polarized light ray is equal to an intermediate value of Rp + Rs 2 In Figure 2, if the disc 6 deviates in the direction a, the light rays of the lower half of the incident light flux have incident angles smaller than the incident angle.Therefore, at least a part of the lower half light flux is transmitted through the reflection surface 11 and the amount of light impinging upon the light receiving region 12A is decreased. While the upper half of the incident light flux has the incident angles larger than the critical angle and thus, is totally reflected by the surface 11. Therefore, the amount of light impinging upon the light receiving region 12B is not changed. On the contrary, if the disc 6 deviates in the direction b, the amount of light impinging upon the region 12B is decreased, but the amount of light impinging upon the region 1 2A is no, changed. In this manner, it is possible to obtain the output signals from the regions 1 2A and 12B as illust -ated in Figures 4A and 4B, respectively. A focussing error signal can be obtained at an output 14 of a d rferential amplifier 13 as a difference signal of these signals from the regions 12A and 12B, which differenc w signal is shown in Figure 4C.

According to he invention, the reflection surface 11 may be set at an angle slightly smaller than the critical angle. In such a case when the disc 6 deviates in the direction a, the amount of light impinging upon the region 12B is first increased and then becomes constant and the amount of light impinging upon the region 12A is decreased abruptly. Whereas, if the disc 6 deviates in the direction b, the amount of light impinging upon the region 12A is first increased and then becomes constant, while the amount of light impinging upon the region 12B is decreased.

In this manner by detecting a difference in output signals from the light receiving regions 12A and 12B, it is possible to obtain the focussing error signal having an amplitude which is proportional to an amount of the deviation from the in-focussed condition and a polarity which represents a direction of the deviation with respect to the in-focussed condition. The focussing error signal thus obtained is used to effect a focussing control for driving the objective lens 5 in the direction of its optical axis. Further, it is possible to derive an information signal corresponding to the pit information recorded in the information track at an output 16 of an adder 15 which produces a sum signal of the output signals from the regions 1 2A and 12B.Further, in the in-focussed condition, since the light is scarcely transmitted through the reflection surface 11, a loss of light is very small and in the defocussed condition the half of light flux with respect to the central light ray is totally reflected, but an amount of the other half of light flux reflected by the surface 11 is decreased to a great extent, the difference in the amount of light impinging upon the regions 12A and 12B becomes great.

Therefore, the very accurate focus detection can be effected with a very high sensitivity.

For instance, when use is made of the objective lens 5 having a numerical aperture NA=0.5 and a focal length f=3 mm and of the detection prism 10 having a refractive index n =1.50 and the disc 6 deviates by about 1 Zm, a variation of an incident angle for the extreme light ray which is subjected to the largest variation in incident angle is about 0.015 which can cause a sufficiently large variation in light amount impinging upon the detector regions 12A and 12B. When the disc 6 deviates in the direction a by a distance of about 0.2 mm, a virtual image is formed at 19.5 mm from the objective lens Son the side of the disc 6 with respect to the lens 5 and a diameter of light beam impinging upon the detector 12 is increased.On the other hand, when the disc 6 deviates in the direction b by the same distance of 0.2 mm, the real image is formed at 25.5 mm from the objective lens Son the side opposite to the disc 6. It is therefore preferable to arrange the detector 12 as near as possible to the objective lens 5. However, if the detector 12 is arranged at the distance of 25.5 mm from the objective lens 5, the bright and dark pattern of light impinging upon the detector 12 is reversed, when the disc 6 deviates in the direction b by a distance more than 0.2 mm, and the amounts of light impinging upon the regions 12A and 12B are decreased and increased, respectively. Therefore, the focussing signal derived under such a condition is to move the objective lens 5 towards the prism 3 and thus the objective lens 5 further departs from the disc 6.Therefore, undesired impact of the objective lens 5 against the disc 6 can be effectively avoided without providing any particular safety mechanism.

In the embodiment shown in Figure 2, the refractive index of the detection prism 10 is equal to < and thus, the light reflected by the surface 11 of detection prism 10 deviates from the incident light by 90 . If the prism 10 is made of material having a refractive index largerthan j#, the reflected light may make an angle smaller than 90 with respect to the incident light.

Figure 5 shows another embodiment of the optically reading apparatus for carrying out the focus detecting method according to the invention. In this embodiment, a part of a light flux reflected by a polarizing prism 3 impinges upon a detection prism 10 having a reflection surface 11 which is so set that in the in-focussed condition, both reflected and transmitted light fluxes are produced at a given ratio. The reflected light is received by a first light detector 17 and the transmitted or refractive light is received by a second light detector 18. The construction of the remaining portion of this apparatus is same as that of the apparatus illustrated in Figure 2. To this end, the reflection surface 11 is so arranged that the surface 11 makes an angle with respect to a certain light ray in the reflected light flux, which angle is equal to a critical angle.When the disc 6 deviates either in the direction a orb, amounts of output signals from the detectors 17 and 18 become unbalanced to produce a focussing error signal having an amplitude and a polarity which represent an amount and a direction, respectively of the deviation. It should be noted that in the present embodiment, since it is sufficient that the amounts of light fluxes impinging upon the detectors 17 and 18 have the given ratio, it is not always necessary for the light flux reflected by the disc 6 to be a parallel light flux, but may be diverged or converged.The information signal corresponding to the pit construction of the information track may be derived as a sum signal of the output signals from the detectors 17 and 18, or alternatively may be derived from a separate light detector 19 arranged to receive that part of the light flux reflected by the polarizing prism 3 which does not enter into the detection prism 10.

In the known focus detection apparatus with a cylindrical lens, a fine spot has to be formed and a center of a light detector divided into four sections has to be aligned with the fine spot. On the contrary, according to the invention, such a cumbersome adjustment is not required. Further, since the light beam is not necessary to be constricted, but may impinge upon the detector as the light flux of a large diameter, an optical alignment and adjustment can be effected very easily. Moreover, since the optical system is not necessary to be adjusted with respect to two orthogonal axes, the detection prism and the light detector can be indexed mechanically in an integral body and the assembly can be rotatably arranged in the plane of the drawings of Figures 2 and 5. In the apparatus according to the invention, since the fine spot is not formed on the detector, the optical path can be shortened and thus, the whole assembly can be made small in size and light in weight. This results in that the whole optical assembly can be installed in a two-dimensional driver for driving the objective lens in a direction in parallel with the objective lens and in a direction perpendicular to the optical axis as well as to the information track. In such a system, it is desirous to use the objective lens as small as possible. To this end, the number of lens elements of the objective lens (in the drawing, for the sake of simplicity, the objective lens is illustrated as a single lens element, but in practice, it is consisting of a plurality of lens elements) is decreased and only a spherical aberration has to be taken into consideration.

Under such a circumstance, it is preferable not to use off-axis light rays and a parallel light flux has to be used. According to the invention, such requirements can be advantageously satisfied by detecting the in-focussed condition with the parallel light flux. This feature contributes to a miniaturization of the optical system to a great extent. This may be also applied to an objective lens consisting of an aspherical lens.

Further, in the embodiments explained above, the optical system is so arranged that the pits of the spiral or concentric information track of the record medium are moved in the plane of the drawings perpendicular to which the reflection surface of the detection prism is arranged. Thus, even if the light spot traverses the track to produce a variation in light distribution, the focussing error signal is not affected at all, because the variation of light distribution appears in the direction perpendicular to the plane of drawings and such a variation is cancelled in the difference signal.

It should be noted that the present invention is not limited to the embodiments explained above, but may be modified in various manners. For instance, in the embodiment shown in Figure 2, S-polarized light impinges upon the reflection surface 11 of the detection prism 10, but P-polarized light may impinge upon the reflection surface 11 by inserting a 90 rotary polarization element 20 as illustrated in Figure 6. In such a case, the intensity of reflection light changes extremely abruptly near the critical angle and thus, the sensitivity of the focussing error detection can be further increased. It is also possible to obtain the P-polarized light without the rotary polarizer 20.For instance, the detection prism 10 may be rotated by 90 about the incident axis OPi in Figure 2 with respect to the polarizing prism 3 or the transmitted light through the polarizing prism 3 may enter into the detection prism 10 as shown in Figure 7. In the latter case, the incidence light from a laser light source 1 is reflected by the polarizing prism 3. In order to further increase the detection sensitivity, the light flux may be introduced into an elongated detection prism 10' shown in Figure 8 and may be reflected several times in the detection prism 10'. In such an embodiment, the amount of light totally reflected by prism surfaces 11' is not changed at all, but the amount of light transmitted through the reflection surfaces 11' is increased by a power of the reflection times.Therefore, the sensitivity can be increased by the power of the reflection times. Further, as shown in an embodiment of Figure 9, the position of a polarizing prism 3 and a detection prism 10 may be exchanged. In this embodiment a light beam emitted from a light source 1 is reflected by the polarizing prism 3 and impinges upon the detection prism 10 as an S-polarized beam. Since a reflection surface 11 of the detection prism 10 is set at a critical angle with respect to the incident light beam, the light beam impinges upon a quarter-wavelength plate 4 and an objective lens 5 without a loss of light. The light beam reflected by an object 6 passes through the objective lens 5 and the quarter-wavelength plate 4, and impinges upon the detection prism 10 as a P-polarized light beam. Therefore, the detection sensitivity for focussing error is made extremely higher.

Further, the focus detection methods shown in Figures 6 to 9 may be effectively applied to the embodiment shown in Figure 5. In the embodiment shown in the drawings, the detection prism has the refractive index of V 2 for the sake of simplicity, but it may have any desired refractive index as long as the reflection surface is set at or near the critical angle. Further, in the above embodiments, use is made of the polarized light, but according to the invention, non-polarized light may be equally used. In the embodiment shown in Figure 5, it is sufficient for the reflection surface 11 of the detection prism to be arranged with respect to a single light ray among the light flux impinging upon the surface 11 at an angle equal to the critical angle or slightly smaller than the critical angle.Therefore, either diverging or converging light beam may be used, instead of the parallel light beam. Furthermore, the polarizing prism 3 may be replaced by a half mirror. It should be further noted that the present invention is not limited to the application to the above mentioned optically reading apparatus for the video disc, but may be applied to the focus detection in various optical instruments.

In an optical pick-up apparatus for reproducing information from a record medium such as a video disc, it is necessary not only to effect a focussing control to focus a light beam upon the disc, but also to effect a tracking control to scan or trace an information track precisely. In the embodiments explained above since the parallel light flux or substantially parallel light flux impinges upon the light detector, three beams of the three beam method for deriving the tracking error signal could not be formed separately, but the tracking error can be detected by other methods sucl as a wobbling method in which a single light spot is vibrated across the information track. Therefore a freedom of design is limited to some extent.

According to another aspect of the present invention such a problem can be effectively solved, while the various advantages of the above explained embodiments can be still attained as they are.

To this end, according to the invention the light flux reflected by the object, i.e. the disc impinges as a converging light flux upon the reflection surface which is set substantially at the critical angle with respect to a central light ray in the incident light flux, and the light detector is arranged substantially at a focal point of the converged light flux reflected by the reflection surface and has at least two light receiving regions which are divided along a boundary plane includirg an optical axis and perpendicularto a plane of incidence for the reflection surface.

Figure 10 is a schematic view showing an optical pick-up apparatus comprising an embodiment of the focussing and tracking error detecting apparatus according to the invention. A laser light source 21 emits a light beam polarized linearly in a plane perpendicular to a plane of the drawing. The light beam emitted from the light source 21 is diverged to a given extent by a lens 22 and impinges upon a polarizing prism 23 having a polarizing surface 23A. The diverged light beam is reflected by the polarizing surface 23A and is directed through a quarter-wavelength plate 24 to an objective lens 25. The lens 25 converges the light beam and projects a light spot onto a record carrier 26 such as a video disc. The light reflected by the disc 26 is converged again by the objective lens 25 and is made incident upon the polarizing prism 23 through the quarter-wavelength plate 24.Since the light is transmitted through the quarter-wavelength plate 24 twice, the polarization direction of the light is turned by 90 and the light impinging upon the polarizing surface 23A is polarized in a plane parallel to the plane of the drawing and thus, is transmitted through the polarizing surface 23A. As shown in Figure 10, on the polarizing prism 23 is arranged a detection prism 27 having a reflection surface 27A. The reflection surface 27A is set substantially at a critical angle with respect to a central light ray of the incident light flux.In this embodiment all the light flux transmitted through the polarizing prism 23 impinges upon the prism 27, so that the central light ray situates on an incident optical axis OPi. Therefore, the reflection surface 27A is arranged substantially at the critical angle with respect to the optical axis OPj. In such a construction all light rays in a light flux situated on a left hand side of a boundary plane including the optical axis OP; and perpendicular to a plane of incidence impinge upon the reflection surface 27a at incident angles largerthan the critical angle and thus, are totally reflected by this surface 27A.On the other hand, all light rays in a light flux situated on the right hand side of the boundary plane impinge upon the reflection surface 27A at angles smaller than the critical angle and thus, are almost transmitted through the reflection surface 27A. In the present embodiment it is preferable to decrease an amount of the reflected light situated on the right hand side of the boundary plane as small as possible and thus, it is possible to further enhance the advantage by increasing the number of reflections in the detection prism 10 as explained above with ference to Figure 8, A light detector 28 having two light receiving regions 28A and 28B is arranged so as to receive the light flux reflected by the reflection surface 27A.The light receiving regions 28A and 28B are divided along a plane perpendicular to the plane of incidence and including an exit optical axis OPr.

Next an operation of the apparatus illustrated in Figure 10 will be explained with reference to Figures 1 1A to 11C. Figure 11A depicts an in-focussed condition which corresponds to an optical path shown by solid lines in Figure 10. When the light spot is correctly focussed on the record medium 26, an image of the light spot is formed on the detector regions 28A and 28B. As described above, since the boundary of these regions 28A and 28B situates on the optical axis OP, substantially same amounts of light fluxes impinge upon the regions 28A and 28B which produce substantially same output signals. Therefore, when a difference between these output signals is formed by a differential amplifier 29, substantially a zero output signal appears at an output terminal 30. In such a condition the apparatus can determine that the in-focussed condition has been attained.

Now if the disc 26 deviates in a direction b into a position d, the image of light spot is formed in front of the light detector 28 as illustrated by broken lines in Figure 10. Therefore, a large amount of light impinges upon the detector regions 28A, but a very small amount of light situated on the right side of the incident optical axis OPj and reflected by the reflection surface 27A impinges upon the detector region 28B. Thus the focussing error signal from the differential amplifier 29 has a great amplitude with positive polarity.

On the other hand if the disc 26 deviates in a direction a into a position e in Figure 10, the image of the light spot is formed behind the detector 28 as shown by chain lines. In this case a great amount of light impinges upon the region 28B, but the region 28A receives only a negligibly small amount of light. Therefore, at the output terminal 30 the focussing error signal having a great amplitude with negative polarity appears.

In this manner the focussing error signal of the objective lens 25 with respect to the decord medium 26 can be generated at a very high sensitivity. This focussing error signal can be applied to the focussing servo mechanism to move the objective lens 25 in its optical axis direction so as to focus always the light spot onto the record medium 26.

In this embodiment the output signals from the detector regions 28A and 28B are supplied to an adder 31 which produces at an output terminal 32 an information signal.

Further in this embodiment by vibrating the light spot to a small extent across the information track by vibrating the objective lens 25 or a reflection mirror arranged in an optical path, it is possible to derive the tracking error signal from the information signal. In this case, since the image of the light spot vibrates in a direction parallel to the boundary plane of the detector 28, the focussing error signal could not be influenced at all. It should be noted that the wobbling method for obtaining the tracking error signal can be equally applied to the embodiments illustrated in Figures 2, to 9.

Figure 12 shows a modification of the embodiment illustrated in Figure 10 and similar elements are denoted by the same reference numerals used in Figure 10. In this modified embodiment, a prism 33 is arranged on a reflection surface 27A of a detection prism 27 by means of a thin layer of air or cement. The prisms 27 and 33 are made of optical material having a same refractive index. Further a light detector 34 is so arranged to receive a light flux which is transmitted through the reflection surface 27A and the prism 33. In this embodiment, the tracking error signal can be obtained either by the wobbling method and three beam method. In case of wobbling method, the detector 34 may have a single light receiving region, but in case of three beam method, the detector 34 should have two receiving regions which can receive two images of light beams which are separated in a direction of width of the information track, and the tracking error signal can be derived as a difference between output signals of these two light receiving regions of the detector 34.

Figure 13 illustrates still another embodinent of the optical pick-up apparatus in which use is made of the three beam method for obtaining the tracking error signal. In Figure 13 similar elements as those shown in Figure 10 are denoted by the same reference numerals. In order to generate three beams, the light emitted from a light source 21 is passed through a diffraction grating 37 arranged in a parallel light flux between lenses 35 and 36. Beams of 0 order and +1 order emitted from the grating 37 are used as the three beams and are projected on a video disc 26 as three light spots by means of a polarizing prism 23, a quarter-wavelength plate 24 and an objective lens 25.The light beams reflected by the disc 26 are converged by the objective lens 25 and impinge upon a light detector 38 via the quarter-wavelength plate 24, the polarizing prism 23 and a detection prism 27 having a reflection surface 27A. Also in this embodiment the reflection surface 27A is so set that only halves of the light fluxes on one side of a boundary plane including an incident optical axis OP; impinge upon the detector 38.

Now the operation of the apparatus will be explained with reference to Figures 14A to 14C. As shown in Figure 14A the light detector 38 includes four light receiving regions 38A to 38D. The central beam impinges upon the regions 38A and 38B which are divided in a direction of the information track, and the right and left beams impinge upon the regions 38C and 38D, respectively, which regions are divided in a direction of width of the information track.

Figure 14A indicates a correct condition in which neither focussing error nor tracking error is existent. In such a condition any focussing error signal does not appear at an output of a differential amplifier 39 which produces a difference between output signals between the detector regions 38A and 38B. The information signal can be produced by an adder 40 which forms a sum of the output signals from these regions 38A and 38B. Further a differential amplifier 41 which produces a difference between output signals from the detector regions 38C and 38D does not produce the tracking error signal.

When the video disc 26 deviates in the direction b in Figure 13 and the light spots deviate in the direction of width of information track, the differential amplifier 39 produces a positive focussing error signal and the differential amplifier 41 generates a negative tracking error signal as shown in Figure 14B.

When the video disc 26 deviates in the opposite direction a and the spots deviate in the opposite direction to that of Figure 14B, the differential amplifier 39 produces a negative focussing error signal and the differential amplifier 41 generates a positive tracking error signal as illustrated in Figure 14C. In this manner the focussing error signal, the tracking error signal and the information signal can be derived effectively at a very high sensitivity.

Figure 15 illustrates still another embodiment of the focus detecting apparatus according to the invention.

In this embodiment a collimator lens 51 is arranged between a polarizing prism 3 and an objective lens 5, so that a parallel light flux impinges upon the objective lens 5. Thus a light beam reflected by a disc 6 passes through the polarizing prism 3 as a converging light beam. The converging light beam leaving the polarizing prism 3 is then converted into a parallel beam by means of a concave lens 52, and the parallel beam is made incident upon a detection prims 10 and a light detector 12. In general, it is preferable to make large a working distance of the t objective lens 5. To this end a numerical aperture of the objective lens 5 must be large and this results in that the parallel light beam leaving the objective lens 5 is liable to have a large diameter.Thus if the combination of the collimator lens 51 and the concave lens 52 is omitted, the parallel light beam having the large diameter would impinge upon the detection prism 10 and the detector 12. Therefore, these element 10 and 12 should have large dimensions. Contrary to this, in the embodiment shown in Figure 15, since th : combination of the collimator lens 51 and the concave lens 52 produces the parallel light beam of the sma ler diameter, the detection prism 10 and the detector 12 can be made small in size.

Figure 16 shows still another embodiment of the focus detection apparatus according to the invention. In this embodiment a convex lens 53 is arranged between a light source 1 and a polarizing prism 3 and a concave lens 54 is inserted between the polarizing prism 3 and a detection prism 10. In this construction a diverging light I eam impinges upon the objective lens 5 from the polarizing prism 3 and a converging light beam is made il cident upon the concave lens 54 and is converted into a parallel light beam. In this manner the advantage of the embodiment of Figure 15 can be substantially equally attained.

The present invention is not limited to the embodiments explained above, but many modifications can be conceived within the scope of the invention.

Claims

1. A method for detecting a focussing error signal of an objective lens with respect to an object onto which a light spot is to be formed by means of said objective lens, comprising focussing light emitted from a light source onto the object; introducing at least a part of a light flux reflected from the object into an optical member which includes a reflection surface set to be substantially at a critical angle with respect to a light ray in the reflected light flux in an in-focussed condition; and detecting a variation of light distribution of the light flux reflected by said reflection surface or a variation in amounts of the reflected light flux and a light flux transmitted through the reflection surface to produce the focussing error signal.

2. A method according to claim 1, wherein a light flux reflected by the reflection surface and situated on one side of a boundary plane which includes said light ray and is perpendicular to a plane of incidence, and a light flux reflected by the reflection surface and situated on the other side of said boundary plane are separately received.

3. A method according to claim 1, wherein a light flux reflected by the reflection surface and a light flux transmitted through the reflection surface are separately received.

4. A method according to claim 2, wherein the light flux reflected by the object impinges upon the reflection surface as a parallel light flux in the in-focussed condition and the reflection surface is set substantially at the critical angle with respect to a central light ray passing along an optical axis.

5. A method according to claim 1, wherein the light flux reflected by the object impinges upon the reflection surface as a diverging light flux in the in-focussed condition.

6. A method according to claim 1, wherein the light flux reflected by the object impinges upon the reflection surface as a converging light flux in the in-focussed condition.

7. A method according to claim 1, wherein the light flux impinging upon the reflection surface is a P-polarized light flux.

8. A method according to claim 1, wherein the light flux reflected by the object is reflected by the reflection surface by a plurality of times.

9. An apparatus for detecting a focussing error signal of an objective lens with respect to an object onto which a light beam emitted from a light source is to be focussed as a light spot by means of said objective lens comprising a beam splitting element arranged between the light source and the objective lens for directing the light beam emitted from the light source to the objective lens and directing a light flux reflected by the object into a direction different from that to the light source; a detecting prism arranged to receive at least a part of the light flux reflected from said object and including a reflection surface which is set substantially at a critical angle with respect to a light ray in the reflected light flux impinging upon the reflection surface;; light detecting means having at least two light receiving regions arranged to receive a light flux reflected by the reflection surface or light fluxes reflected by and transmitted through the reflection surface, respectively to produce output signals representing amounts of light fluxes impinging upon the light receiving regions; and a circuit for receiving the output signals from the light detecting means to form a difference signal as the focussing error signal.

10. An apparatus according to claim 9, wherein said light receiving regions are so arranged to receive separately a light flux reflected by the reflection surface and situated on one side of a boundary plane which includes the optical axis and is perpendicular to a plane of incidence, and a light flux reflected by the reflection surface and situated on the other side of said boundary plane, respectively.

11. An apparatus according to claim 9, wherein said light receiving regions are so arranged to receive separately a light flux reflected by the reflection surface, and a light flux transmitted through the reflection surface, respectively.

12. An apparatus according to claim 9, wherein said beam splitting element is constituted by a polarizing prism and a polarized light flux impinges upon the reflection surface.

13. An apparatus according to claim 12, further comprising a quarter-wavelength plate arranged between the polarizing prism and the objective lens.

14. An apparatus according to claim 12, wherein the polarized light flux is a P-polarized light flux.

15. An apparatus according to claim 9, further comprising a collimator lens arranged between the light source and the objective lens to introduce a parallel light flux to the reflection surface.

16. An apparatus according to claim 15, wherein the detection prism is so arranged that the reflection surface is set substantially at the critical angle with respect to a central light ray in the parallel light flux.

17. An apparatus according to claim 13, wherein the detection prism is arranged between the polarizing prism and the light detecting means to receive at least a part of the light flux reflected by the polarizing prism, and the light emitted from the light source is transmitted through the polarizing prim.

18. An apparatus according to claim 13, wherein the detection prism is arranged between the polarizing prism and the light detecting means to receive at least a part of the light flux transmitted through the polarizing prism, and the light emitted from the light source is reflected by the polarizing prism.

19. An apparatus according to claim 13, wherein the detection prism is arranged between the polarizng prism and the objective lens, the light emitted from the light source is reflected by the polarizing prism and then is totally reflected by the reflection surface, and the light flux reflected by the object is reflected by the reflection surface and then is transmitted through the polarizing prism.

20. An apparatus according to claim 17, further comprising a 900 rotating element arranged between the polarizing prism and the detection prism so that the P-polarized light flux impinges upon the reflection surface.

21. An apparatus according to claim 13, wherein the detection prism is arranged between the polarizng prism and the objective lens, the light emitted from the light source is transmitted through the polarizing prism and then is totally reflected by the reflection surface, and the light flux reflected by the object is reflected by the reflection surface and then is reflected by the polarizing prism.

22. An apparatus according to claim 9, further comprising a converging lens arranged between the light source and the objective lens, so that a diverging light flux impinges upon the reflection surface.

23. An apparatus according to claim 9, further comprising a diverging lens arranged between the light source and the objective lens, so that a converging light flux impinges upon the reflection surface.

24. An apparatus according to claim 9, wherein the detection prism has such a length that the light flux is reflected by the reflection surface by a plurality of times.

25. An apparatus according to claim 17 or 18, further comprising a collimator lens arranged between the polarizing prism and the objective lens for introducing a parallel light beam into the objective lens, and a concave lens arranged between the polarizing prism and the detection prism for converting the incident converging light beam into a parallel light beam.

26. An apparatus according to claim 17 or 18, further comprising a convex lens arranged between the light source and the polarizing prism for introducing the converging light beam into the objective lens, and a concave lens arranged between the polarizing prism and the detection prism for converting the converging incident light beam into the parallel light beam.

27. An apparatus according to claim 9, wherein the apparatus further comprises a lens for converging the light flux impinging upon the reflection surface, and said two light receiving regions of the light detecting means are arranged substantially at a focal point of said converging light flux, and are divided along a boundary plane which includes a central light ray of said light flux and is perpendicular to a plane of incidence of the reflection surface, said light receiving regions receiving the light flux reflected by the reflection surface and/or the light flux transmitted through the reflection surface.

28. An apparatus according to claim 27, said lens for converging the light flux impinging upon the reflection surface is formed by a diverging lens arranged in an optical path between the light source and the objective lens.

29. An apparatus according to claim 27, wherein the apparatus further comprises an auxiliary prism having one surface which faces the reflection surface via a thin layer of air or cement and made of material having a same refractive index as the detection prism, and said light detecting means further comprise an auxiliary light detector which is arranged to receive a light flux transmitted through the reflection surface and the auxiliary prism.

30. An apparatus according to claim 9 or 27, wherein in order to detect a tracking error signal of the objective lens with respect to a record carrier including at least one information track on which the light beam emitted from the light source is to be focussed as a light spot, the apparatus further comprises means for vibrating the light spot across the information track and said circuit comprises an adder for producing a sum signal of the output signals from the light receiving regions as an information signal and a circuit for deriving the tracking error signal from the information signal.

31. An apparatus according to claim 27, wherein in order to detect a tracking error signal of the objective lens with respect to a record carrier including at least one information track on which the light beam emitted from the light source is to be focussed as a light spot, the apparatus further comprises means for producing three light spots on the information track, said auxiliary light detector for receiving the light flux transmitted through the reflection surface and the auxiliary prism comprises two auxiliary light receiving regions which are divided in a direction of the information track, and said circuit comprises a differential amplifier to produce a difference signal between output signals from the two auxiliary light receiving regions of the auxiliary light detector as the tracking error signal.

32. An apparatus according to claim 27, wherein in order to detect a tracking error signal of the objective lens with respect to a record carrier including at least one information track on which the light beam emitted from the light source is to be focussed as a light spot, the apparatus further comprises means for producing three light spots on the information track, said three light spots being separated from each other in a direction of the information track as well as in a direction of a width of the information track, said light detecting means further comprises two auxiliary light receiving regions which are arranged on respective sides of the first mentioned light receiving regions viewed in the direction of the plane along which said first mentioned light receiving regions are divided, and said circuit comprises a differential amplifier for generating a difference signal between output signals from the two auxiliary light receiving regions to form the tracking error signal.

33. A method for detecting a focussing error signal of an objective lens with respect to an object onto which a light spot is to be formed by means of said objective lens, substantially as hereinbefore described with reference to Figure 2 or Figures 2 and 3 or Figures 2 to 4C, or Figure 5 or Figure 6 or Figure 7 or Figure 8 or Figure 9 or Figure 10 or Figures 10 to 1 it, or Figure 12 or Figure 13 or Figures 13to 14C, or Figure 15 or Figure 16 of the accompanying drawings.

34. An apparatus for detecting a focussing error signal of an objective lens with respect to an object onto which a light beam emitted from a light source is to be focussed as a light spot by means of said objective lens substantially as hereinbefore described with reference to Figure 2 or Figures 2 and 3 or Figures 2 to 4C, or Figure 5 or Figure 6 or Figure 7 or Figure 8 or Figure 9 or Figure 10 or Figures 10 to 11C, or Figure 12 or Figure 13 or Figures 13 to 14C, or Figure 15 or Figure 16 of the accompanying drawings.

Additional claims filed on 28.11.80.

New or amended claims:

35. A method for detecting a focussing error signal of an objective lens with respect to an object onto which a light spot is to be formed by means of said objective lens, comprising focussing light emitted from a light source onto the object; introducing at least a part of a light flux reflected from the object into an optical member including an optical surface which reflects and/or refracts said part of light flux, said optical member being made of material which has a higher refractive index than that of material into which said light flux enters after being refracted by and transmitted through said optical surface; and detecting a variation in distribution of light amount of at least a part of light flux reflected and/or refracted by said optical surface to produce the focussing error signal.

36. A method according to claim 35, wherein said optical surface is to set that a given light ray in said part of light flux is made incident upon the optical surface at an angle which is substantially equal to a critical angle when said objective lens is in an in-focussed condition.

37. A method according to claim 35, wherein said optical surface is so set that a given light ray in said part of light flux is made incident upon the optical surface at an angle which is smaller than a critical angle when said objective lens is in an in-focussed condition.

38. A method according to any one of claims 36 and 37, wherein said given light ray is a center light ray of the light flux.

39. A method according to claim 35, wherein a light flux reflected by the optical surface and situated on one side of a boundary plane which includes said light ray and is perpendicular to a plane of incidence, and a light flux reflected by the optical surface and situated on the other side of said boundary plane are separately detected.

40. A method according to claim 35, wherein a light flux reflected by the optical surface and a light flux refracted by and transmitted through the optical surface are separately detected.

41. A method according to claim 35, wherein the light flux reflected by the object impinges upon the optical surface as a parallel light flux in the in-focussed condition.

42. A method according to claim 35, wherein the light flux reflected by the object impinges upon the optical surface as a diverging light flux in an in-focussed condition.

43. A method according to claim 35, wherein the light flux reflected by the object impinges upon the optical surface as a converging light flux in an in-focussed condition.

44. A method according to claim 35, wherein the light flux impinging upon the optical surface is a P-polarized light flux.

45. A method according to claim 35, wherein the light flux reflected bythe object is reflected bythe optical surface by a plurality of times.

46. An apparatus for detecting a focussing error signal of an objective lens with respectto an object onto which a light beam emitted from a light source is to be focussed as a light spot by means of said objective lens comprising a beam splitting element arranged between the light source and the objective lens for directing the light beam emitted from the light source to the objective lens and directing a light flux reflected by the object into a direction different from that to the light source;; an optical member arranged to receive at least a part of the light flux reflected from said object and including an optical surface which reflects and/or refracts said part of light flux, said optical member being made of material which has a higher refractive index than that of material into which the light flux enters after being refracted by and transmitted through said optical surface; light detecting means having at least two light receiving regions arranged to receive at least parts of light flux reflected and/or refracted by said optical surface to produce output signals representing amounts of light impinging upon the light receiving regions; and a circuit for receiving the output signals from the light detecting means to form a difference signal as the focussing error signal.

47. An apparatus according to claim 46, wherein said optical member is consisting of a detection prism.

48. An apparatus according to claim 46, wherein said light receiving regions are so arranged to receive separately a light flux reflected by the optical surface and situated on one side of a boundary plane which includes an optical axis of the optical member and is perpendicular to a plane of incidence, and a light flux reflected by the optical surface and situated on the other side of said boundary plane, respectively.

49. An apparatus according to claim 46, wherein said light receiving regions are so arranged to receive separately a light flux reflected by the optical surface, and a light flux refracted by and transmitted through the optical surface, respectively.

50. An apparatus according to claim 46, wherein said beam splitting element is constituted by a polarizing prism and a polarized light flux impinges upon the optical surface.

51. An apparatus according to claim 50, further comprising a quarter-wavelength plate arranged between the polarizing prism and the objective lens.

52. An apparatus according to claim 50, wherein the polarized light flux is a P-polarized light flux.

53. An apparatus according to claim 46, further comprising a collimator lens arranged between the light source and the objective lens to introduce a parallel light flux to the optical surface.

54. An apparatus according to claim 46, wherein the optical member is so arranged that the optical surface makes with respect to a given light ray in incident light flux an angle which is substantially equal to a critical angle.

55. An apparatus according to claim 46, wherein the optical member is so arranged that the optical surface makes with respect to a given light ray in incident light flux an angle which is smaller than a critical angle.

56. An apparatus according to claim 46, wherein said light flux impinging upon the optical surface is a parallel light flux in an in-focussed condition of the objective lens.

57. An apparatus according to claim 46, wherein the light flux impinging upon the optical surface is a conveging lightflux in an in-focussed condition of the objective lens.

58. An apparatus according to claim 46, wherein the light flux impinging upon the optical surface is a diverging light flux in an in-focussed condition of the objective lens.

59. An apparatus according to claim 51, wherein the optical member is arranged between the polarizing prism and the light detecting means to receive at least a part of the light flux reflected by the polarizing prism, and the light emitted from the light source is transmitted through the polarizing prism.

60. An apparatus according to claim 51, wherein the optical member is arranged between the polarizing prism and the light detecting means to receive at least a part of the light flux transmitted through the polarizing prism, and the light emitted from the light source is reflected by the polarizing prism.

61. An apparatus according to claim 51, wherein the optical member is arranged between the polarizing prism and the objective lens, the light emitted from the light source is reflected by the polarizing prism and then istotally reflected by the optical surface, and the light flux reflected by the object is reflected by the optical surface and then is transmitted through the polarizing prism.

62. An apparatus according to claim 51, wherein the optical member is arranged between the polarizing prism and the objective lens, the light emitted from the light source is transmitted through the polarizing prism and then is totally reflected by the optical surface, and the light flux reflected by the object is reflected by the optical surface and then is reflected by the polarizing prism.

63. An apparatus according to claim 59, further comprising a 90 rotating element arranged between the polarizing prism and optical member so that the P-polarized light flux impinges upon the optical surface.

64. An apparatus according to claim 58, further comprising a converging lens arranged between the light source and the objective lens, so that the diverging light flux impinges upon the optical surface.

65. An apparatus according to claim 57, further comprising a diverging lens arranged between the light source and the objective lens, so that the converging light flux impinges upon the optical surface.

66 An apparatus according to claim 47, wherein the detection prism is a rectangular shape having such a length that the light flux is reflected by the optical surfaces by a plurality of times.

67. An apparatus according to claim 59 or 60, further comprising a collimator lens arranged between the polarizing prism and the objective lens for introducing a parallel light beam into the objective lens and a concave lens arranged between the polarizing prism and the optical member for converting the incident converging light beam into a parallel light beam.

68. An apparatus according to claim 59 or 60, further comprising a convex lens arranged between the light source and the polarizing prism for introducing the converging light beam into the objective lens, and a concave lens arranged between the polarizing prism and the detection prism for converting the converging incident light beam into a parallel light beam.

69. An apparatus according to claim 46, wherein the apparatus further comprises a lens for converging the light flux impinging upon the optical surface, and said two light receiving regions of the light detecting means are arranged substantially at a focal point of said converging light flux, and are divided along a boundary plane which includes a central ray of said light flux and is perpendicular to a plane of incidence of the optical surface, said light receiving regions receiving the light flux reflected by the optical surface and/or the light flux transmitted through the optical surface.

70. An apparatus according to claim 69, said lens for converging the light flux impinging upon the optical surface is formed by a diverging lens arranged in an optical path between the light source and the objective lens.

71. An apparatus according to claim 69, wherein the apparatus further comprises an auxiliary prism having one surface which faces the optical surface via a thin layer of air or cement and made of material having a same refractive index as the detection prism, and said light detecting means further comprise an auxiliary light detector which is arranged to receive a light flux transmitted through the optical surface and the auxiliary prism.

72. An apparatus according to claim 46 to 68, wherein in order to detect a tracking error signal of the objective lens with respect to a record carrier including at least one information track on which the light beam emitted from the light source is to be focussed as a light spot, the apparatus further comprises means for vibrating the light spot across the information track and said circuit comprises an adder for producing a sum signal of the output signals from the light receiving regions as an information signal and a circuit for deriving the tracking error signal from the information signal.

73. An apparatus according to claim 69, wherein in order to detect a tracking error signal of the objective lens with respect to a record carrier including at least one information track on which the light beam emitted from the light source is to be focussed as a light spot, the apparatus further comprises means for producing three light spots on the information track, said auxiliary light detector for receiving the light flux transmitted through the optical surface and the auxliary prism comprises two auxiliary light receiving regions which are divided in a direction of the information track, and aid circuit comprises a differential amplifier to produce a difference signal between output signals from the two auxiliary light receiving regions of the auxiliary light detector as the tracking error signal.

74. An apparatus according to claim 69, wherein in order to detect a tracking error signal of the objective lens with respect to a record carrier including at least one information track on which the light beam emitted from the light source is to be focussed as a light spot, the apparatus further comprises means for producing three light spots on the information track, said three light spots being separated from each other in a direction of the information track as well as in a direction of a width of the information track, said light detecting means further comprises two auxiliary light receiving regions which are arranged on respective sides of the first mentioned light receiving regions viewed in the direction of the plane along which said first mentioned light receiving regions are divided, and said circuit comprises a differential amplifier for generating a difference signal between output signals from the two auxiliary light receiving regions to form the tracking error signal.