KR850001652Y1 - A method for detecting a focussing error signal of an objective lens - Google Patents

Abstract

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Description

Focusing condition detection device of objective lens

1 is a schematic diagram showing an optical step of an optical capture device with a known focus detection system.

2 is a schematic diagram showing an embodiment of a focus detection apparatus proposed by the applicant.

FIG. 3 is a diagram illustrating an operation of the focus detection apparatus shown in FIG.

Figure 4 is a schematic diagram showing an embodiment of the connection error signal detection apparatus according to the present invention.

5a to 5d are schematic views illustrating the operation of the focus detection apparatus according to the present invention.

FIG. 6 shows a focus error signal obtained with the apparatus shown in FIG.

7, 8, 9 and 10 are schematic views showing various embodiments of the focusing error signal detection apparatus according to the present invention.

The present invention relates to an apparatus for detecting a focusing condition of an objective lens with respect to an object in which light spots are focused with an objective lens.

Such a focus detection device is applied to a device in which a scanning light point is projected as one or a plurality of information tracks recorded in a spiral or circular shape on a disk-shaped recording medium for reading information recorded along a track by an objective lens. It has the advantage of being.

In the embodiment of the apparatus for reproducing or capturing information from the above-described recording medium, the recording medium is called a video disk in which coded video and audio signals are recorded with optical information such as light transmission, reflection and phase characteristics. If the video disc rotates 30 revolutions per second, or at a high speed of 1800 rpm, the laser beam emitted from a laser light source, such as a helium-neon gas laser, is connected to the light spot on the disc track and the optical information is read there. do. Since one of the important characteristics of such a recording medium is high density recorded information, the width of the information track is very narrow and the interval between successive tracks is very narrow. In a typical video disc, the pinch of the track is on the order of 2 μm. Therefore, the diameter of the light spot should be a small value of 1 ~ 2㎛. In order to capture the recorded information correctly in such a track having a very narrow width and pitch, the error of the distance between the objective lens and the track, that is, the focusing error, should be reduced so that the light spot diameter is as small as possible.

Therefore, for this purpose, the light quantity and direction of the refocusing condition of the objective lens with respect to the disk surface are detected to output the focusing error signal, and the objective lens is moved in the direction of the optical axis of the objective lens according to the detected focusing error signal. The present invention provides a focusing control method.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 schematically illustrates a known focus detection system in an optical capture device, wherein the light source 1 consists of a laser and emits light polarized linearly in the drawing plane of FIG. The light is aimed at the parallel light beam transmitted by the collimating lens 2 through the polarizing prism 3 and the 1/4 lambda (1/4 wavelength) plate 4. The light beam is connected to the optical spot by the objective lens 5 on the disk 6 having one or more information tracks in the sawtooth groove structure. Then, the light is reflected by the information track and is incident on the polarizing prism 3 by means of the objective lens 5 and the 1/4? Plate 4. Light incident on the polarizing prism 3 is polarized in a direction perpendicular to the plane of the drawing. This is because it is transmitted twice through the 1/4? Plate 4 and reflected by the polarization prism 3. The light beam reflected by the polarizing prism 3 is converged by the focusing lens 7 and the cylindrical lens 8. Since the cylindrical lens 8 has a focusing force in only one direction, the shapes of the focusing beams formed by the focusing lens 7 and the cylindrical lens 8 are focused in the orthogonal direction when the disk 6 moves up and down. The conditions vary as shown in FIG. 1. In known devices this change in shape is detected by a photo detector (not shown) separated into four parts and arranged on the focal plane of the lens system 7 (8) and generates a focus error signal. Therefore, the detected focus error signal is input to a focusing mechanism such as a movable coil mechanism and moves the objective lens 5 in the axial direction.

In a known focus detection method, a relatively long optical path is required to focus the light beam after being reflected by the polarizing prism 3, so that the size of the optical system tends to be large. In addition, the adjustment of positioning the photodetector is very important and time consuming since the photo detector with four parts must be disposed in exactly three axial directions, i.e., two orthogonal directions perpendicular to the optical axis direction.

In addition, since the dynamic range in which the correct focus error signal can be obtained by the deformation of the connection beam is relatively small, the focus error signal cannot be generated at all if the disk is shifted by a relatively small distance from a given position.

The present invention can solve the above drawback and provide a method for detecting the focusing error signal of the objective lens with respect to the object to which the light spot is to be focused, which has a very high sensitivity for focus detection.

According to this invention, the following process is performed to detect the focus error signal of the objective lens with respect to the object whose light spot is to be formed by the means of the objective lens.

That is, the light emitted from the light source is focused on an object and at least a portion of the light beam reflected from the object is incident on the optical member including an optical surface that reflects the portion of the light beam, and the optical member is reflected and transmitted by the optical surface. After the light beam is made of a material having a refractive index larger than the refractive index of the incident material, at least a portion of the light beam reflected by the optical surface detects a change in the distribution of the amount of light to generate a focus error signal.

2 is a schematic view showing an optical capture device bringing the focus detection method proposed by the applicant.

In this method, the optical system for projecting the scanning light point onto the recording medium is the same as that shown in FIG. The new polarized light beam emitted from the laser light source 1 is aimed at the parallel light beam by the aiming lens 2 and penetrates through the polarizing prism 3 and the 1/4 lambda plate 4. Accordingly, the parallel light beam is incident on the objective lens 5 and focused on the information track of the disk 6 with a small light spot. The light beam reflected by the disk 6 is optically modulated in accordance with the information recorded in the track and reflected by the polarization prism 3. The configuration and operation of the optical system described so far are exactly the same as that of the known optical system shown in FIG. The light beam reflected by the polarizing prism 3 is incident on the detection prism 10 having the reflecting surface 11 and the light beam reflected by the surface 11 is received by the light detector 12. The reflecting surface 11 is arranged with respect to the incident light and thus creates a given angle with respect to the incident light such that, under unfocused conditions, the angle of incidence is equal to or slightly less than or greater than the critical angle. It is assumed that the reflective surface 11 has been set at the critical angle for the time being. The light beam totally reflected by the polarizing prism 3 under the out of focus condition is reflected by the reflecting surface 11. The substantially small amount of light is transmitted in the direction n shown in FIG. 2 by the incomplete surface condition of the reflecting surface 11.

In addition, a small amount of transmitted light can be ignored. If the disk 6 shifts in the focusing condition in the direction a of FIG. 2 and the distance between the objective lens 5 and the disk 6 becomes short, the light reflected by the polarizing prism 3 is not already a parallel beam. This results in a convergent light beam that contains the far-away rays ai ₁ and ai ₂ . On the other hand, when the disk 6 shifts in the opposite direction b, the parallel light beam changes to a converging light beam containing very far rays bi ₁ and bi ₂ . As shown in FIG. 2, the light rays from the incident optical axis OP _i to the far-away ray a i _i have an angle of incidence smaller than the critical angle and are therefore transmitted at least partially through the reflecting surface 11. In contrast, the light rays between the optical axis OP _i and the very far light ray ai ₂ are reflected by the surface 11 with an angle of incidence greater than the critical angle. Only in the deviation of the disc 6 in the direction b, the relationship is reversed and the light rays contain the incident optical axis OP _i and all the light rays below the plane of the drawing of FIG. ) And the light rays on the plane are transmitted at least partially through the reflecting surface 11. As described above, when the disk 6 shifts in the focused position, the incident angle of the light beam incident on the reflecting surface 11 changes in a continuous manner with respect to the critical angle except for the central light beam passing along the optical axis OP _i . Therefore, when the disk 6 shifts from the focusing position in the direction a or b, the density of the light reflected on the reflecting surface 11 changes abruptly near the critical angle in accordance with the above-described change in the incident angle. In this case, the sensitivity of change in the light density on both sides of the plane perpendicular to the incidence plane and containing the incident optical axis OP _i is changed in the opposite manner. On the other hand, the light beams incident on the detection prism 10 under the focusing conditions are all reflected by the reflecting surface 11, and the nonuniform light beams are incident on the light detector 12. The photo detector 12 is configured such that the lower and upper luminous fluxes enter the separation regions 12A and 12B with respect to the plane, respectively. That is, the photo detector 12 is separated along the plane perpendicular to the incident plane and including the light side OP _r of the reflected light.

In FIG. 2, if the disk 6 is displaced in the a direction, the lower half of the incident light beam and the light beam have an angle of incidence smaller than the critical angle. Thus, at least part of the lower half luminous flux is transmitted through the reflecting surface 11 and the amount of light incident on the light receiving region 12A is reduced. The upper half of the incident light beam has a larger incident angle than the critical angle and is totally reflected to the reflecting surface 11.

Therefore, the amount of light incident on the light receiving area 12B does not change. On the other hand, if the disk 6 shifts in the direction b, the amount of light incident on the region 12B is reduced and the amount of light incident on the region 12A does not change. In this way, the output signals of the regions 12A and 12B change in opposite ways. The focus error signal may be obtained as a difference signal of the signals of the regions 12A and 12B at the output 14 of the differential amplifier 13.

The reflective surface 11 may be set at an angle slightly smaller than the critical angle. In this case, if the disk 6 shifts in the direction a, the amount of light incident on the region 12B first increases and then becomes constant, and the amount of light incident on the region 12A decreases rapidly. On the other hand, when the disk 6 shifts in the direction b, the light incident on the region 12A first increases and then becomes constant, and the light incident on the region 12B decreases rapidly.

By detecting the difference in the output signals of the light receiving regions 12A and 12B, it is possible to have a focusing error signal having a magnitude proportional to the amount of deviation of the focusing condition and having a polarity indicating the shifting direction with respect to the focusing condition. Therefore, the obtained focusing error signal is used for focusing control of driving the objective lens in the light side direction. It is also possible to derive an information signal corresponding to the information recorded in the information track at the output 16 of the adder 15 which generates the sum signal of the output signals of the regions 12A and 12B. In addition, since the light is not transmitted through the reflecting surface 11 in the condensing condition, the light loss is very small and half of the light beam is reflected with respect to the central light in the refocusing condition, but the other half of the other half reflected by the surface 11 is reflected. The amount of luminous flux decreases considerably, but the difference in the amount of light incident on the regions 12A and 12B becomes large. Therefore, focus detection can be made very sensitive with high accuracy.

For example, using an objective lens having a cooking coefficient NA = 0.5 and a focal length f = 3 mm and a detection prism 10 having a reflectance n = 1.50 and the disk 6 is shifted by about 1 μm, the largest in incident angle is obtained. The change in the angle of incidence of the farthest light ray with a change is about 0.015 ° which can cause a sufficiently large change in the amount of light incident on the detection areas 12A and 12B.

If the optical path from the disk 6 to the detection prism 10 is very long in the focusing error signal detecting apparatus shown in FIG. 2, the convergent light beam incident on the detection prism 10 when the disk 6 is greatly shifted in the focusing condition is It is very close to the optical axis OP _i . In such a case, when the light beam is reflected by the reflecting surface 11 set at the critical angle, the reflected light beam is incident on the detector 12 after passing through the boundary plane bearing the optical axis OP _r and the positional relationship angle of the light and dark areas on the compensator 32 It is shown in reverse with respect to what has been described above with reference to the figures. This will be explained in more detail with the third aid. In FIG. 3 the disk 6 is greatly shifted in the direction a and the convergent light flux is incident on the reflecting surface 11 of the detection prism 10. In this case, the luminous flux bearing the optical axis OP _i and above the boundary plane perpendicular to the plane of the drawing is incident on the face 11 at an angle greater than the critical angle and all reflected by the face 11. On the contrary, the light beam below the boundary surface is incident on the surface 11 at an angle smaller than the critical angle, and transmitted and reflected through the surface. The light beam reflected by the surface 11 is incident on the detector 12 after passing through the light side OP _r . Therefore, the light receiving area 12A becomes bright while the area 12B becomes dark. In the device shown in FIG. 2, the condition of the light and dark areas on the detector should be obtained when the disc 6 shifts in the b direction rather than in the a direction. Therefore, focusing control in such a case cannot be the right way. That is, when the disk 6 is greatly shifted in the a direction, the focus error signal can be generated by means for driving the disk 6 in the a direction. As can be seen in FIG. 3, this undesirable phenomenon can be reduced to some extent by bringing the detector 12 closer to the objective lens 5. In addition, an optical element 3, a 1/4 lambda plate 4, and a detection prism 10 are inserted between the lens 5 and the detector 12 in practice. Therefore, it is practically difficult to bring the detector 12 close to the lens 5.

It is an object of the present invention to obtain the above advantages of the method of using total reflection by means of the reflecting surface set at the critical angle and to develop a focusing error signal detection which can solve the above defects by the development of the positional relationship of the contrast region on the detector. And to provide a useful device.

According to the present invention, a method of detecting a focus error signal of an objective lens with respect to an object in which a light spot can be formed by the means of the objective lens is characterized in that the optical beam reflects a part of the light beam from the object and the connection light emitted from the light source to the object. Is made of a material having a refractive index greater than the refractive index of the material which enters a portion of the luminous flux reflected by the optical member bearing the face and which is transmitted and reflected by the optical surface, and by the optical surface A light detector detects a change in the distribution of the amount of light of the reflected light beam by using a photodetector to generate a focusing error signal, and guides a part of the light beam reflected by the optical surface to the relay lens, and then cuts the light beam cut from the relay lens to the photodetector. It is provided with the incident.

According to the present invention, a method of detecting a focusing error signal of an objective lens with respect to an object in which the light beam emitted from the light source can focus the light spot by means of the objective lens guides the light beam emitted from the light source to the objective lens. The luminous flux reflected from the object includes a beam dispersing element disposed between the objective lens and the light source leading from the object to the light source in a different direction from the object, and an optical surface which receives a part of the luminous flux reflected from the object and reflects the part of the luminous flux. The optical member including, the optical member is made of a material having a refractive index that is larger than the refractive index of the material entering the light beam after being reflected and transmitted through the optical surface, the relay lens receiving a portion of the light beam reflected by the optical surface And having at least two light receiving areas arranged to receive a part of the light beam passing through the relay lens, Light detecting means for generating an output signal indicative of the amount of light, and a circuit for inputting an output signal of the light detecting means to form a difference signal to generate a focus error signal.

4 is a schematic view showing an embodiment of a focusing error signal detection apparatus according to the present invention. The polarization beam emitted from the light source 21 is incident on the disk 26 by a collimating lens 22 at a small light spot through the polarizing prism 23, the 1/4 λ plate 24 and the objective lens 25. Converted to a light beam. The light reflected by the disk 26 is focused on the objective lens 25 and is incident on the detection prism 27 by means of the 1/4 lambda plate 24 and the polarizing prism 23. In the case of focusing conditions, the element is arranged such that the parallel light flux is incident on the reflecting surface 27a of the detection prism 27. Reflecting surface 27a is arranged such that parallel light flux enters surface 27a at a critical angle. The light reflected by the surface 27a is incident on the photo detector 29 by means of the relay lens 28. The photo detector 29 has first and second light receiving areas 29A and 29B connected to each input of the differential amplifier 30 having an output 30a for generating a focusing error signal.

The operation of the apparatus will be described with reference to FIGS. 5A-5D. 5A shows a focusing condition, and the light of the objective lens 25 is incident on the detection prism 27 at a parallel light flux. The parallel light beam reflected by the surface 27a is received by the relay lens forming an image of the position 31 on the detector 29. Under the condensing conditions, the light beam is totally reflected by the surface 27a, and the light receiving areas 29A and 39B receive light unevenly. Therefore, the differential amplifier 30 generates a connection error signal having a magnitude of zero. A zero magnitude focus error signal indicates that the disk 26 is in the focused position with respect to the objective lens 25.

5B shows a case where the disk 26 is shifted in the a direction. In this case, the convergent light beam is incident on the reflecting surface 27a and the left half of the incident light beam passes through the surface 27a of the prism 27, while the right half of the incident light beam is totally reflected by the surface 27a. This reflected light beam is incident independently of the second light receiving region 29B by means of the relay lens 28. Since the first light receiving area 29A does not receive light, the differential amplifier 30 generates a focusing error signal having a negative polarity. The hatched portions in FIGS. 5B-D show areas where no luminous flux is present by transmission of luminous flux through the face 27a.

5C shows the case where the disk 26 shifts in the direction b. In this case, the first light receiving area independently receives the light beam reflected by the surface 27a, and the differential amplifier 30 generates a focusing error signal having positive polarity.

5D shows a case where the disk 26 is shifted in the b direction and is far from the objective lens 25. FIG. In this case, the luminous flux projected from the relay lens 28 reaches the detector 29 without being reversed upside down. Therefore, the light beam reflected at the surface 27a is incident on the second light receiving region 29B of the detector 29. Therefore, the differential amplifier 30 still generates a focusing error signal having a positive polarity. According to the present invention, even if the disk 26 is considerably biased, the focusing error signal can be made to the correct polarity.

As is apparent from FIG. 5D, the correct focusing error signal can be obtained from the objective lens as long as the focusing point 32 of the luminous flux is closer to the objective lens with respect to the objective surface 31 of the relay lens 28. According to the present invention, it is possible to obtain a focusing error signal of correct polarity over a wide side of the disk 26.

The solid line A of FIG. 6 shows a focus error signal generated by the differential amplifier 30 according to the present invention. The dotted line represents the focus error signal output by the differential amplifier 13 of the apparatus shown in FIG. In accordance with the present invention it is evident in the figure of FIG. 6 that the focus error signal of the correct polarity can be obtained over a very wide side of the disc. In the curve A, there is a singularity P ₃₂ corresponding to the case where the light flux in the objective lens 25 is focused on the reflective surface 27a of the detection prism 27. In such a case, the photodetector 29 is irradiated non-uniformly and the differential amplifier 30 generates zero output. Further, such a situation can occur only when the reflecting surface 27a is within the focal length of the light focused at the objective lens 25, and the surface 27a is very slightly away from the focal length, and a correct focusing error signal can be obtained. Therefore, the singularity P ₃₂ described above in the actual apparatus can be ignored without causing any accident.

7 is a schematic diagram of another embodiment of the focusing error signal detecting apparatus according to the present invention. In the embodiment of FIG. 4, since the singular point P ₃₂ is defined by the distance of the reflective surface 27a of the detection prism 27 from the objective lens 25, the relay lens 28 is connected to the reflective surface 27a and the objective lens. The effect of the singular point P ₃₂ can be eliminated by arranging the elements in such a way that the image of the objective plane in the pupil of (25) is formed only on the photodetector 29. In the embodiment shown in FIG. 7, the relay lens 28 forms an image of the pupil of the objective lens 25 on the photodetector 29.

When using a disk 26 of a phase structure and having a groove length different from 1 / 4λ, the distribution of the intensity of light in the pupil of the objective lens 25 changes asymmetrically depending on the mutual position between the groove and the beam point. Therefore, the image of the pupil of the objective lens 25 is the incident surface of the tracking error signal and the reflection surface 27a of the detection prism 27 in the embodiment shown in FIG. 7 formed by the relay lens 28 on the detector 29. It is possible to obtain the focusing error signal by using the photodetector 34 having four light receiving regions 34A 34D separated in the direction of and in the direction perpendicular to the track shown in FIG. The first sum of the outputs of the first and second light receiving zones 34A and 34B is calculated by the first adder 35A and the second of the outputs of the third and fourth light receiving zones 34C and 34D. The sum of is obtained by the second adder 35B. The focus error signal can be obtained in the first differential amplifier 36A as the difference between the first and second sums. The third sum of the outputs of the second and fourth zones 34B and 34D is obtained from the third adder 35C and the fourth sum of the outputs of the first and third zones 34A and 34C is It is calculated | required by the 4th adder 35D. The tracking error signals can be obtained from the second differential amplifier 36B as their difference.

According to the present invention, since the relay lens 28 is disposed between the detection prism 27 and the photo detector 29, the distance of the detector 29 branches from the detection prism 27 does not affect the focus detection area. Therefore, an optical element can be arranged between the relay lens and the detector 9.

9 is a schematic view showing another embodiment of the focus error signal detection according to the present invention. In this embodiment half mirror 37 is incident on detector 29 and differential amplifier 30 generates a focus error signal. A photo detector 38 having two light receiving zones 38A and 38B is disposed at the focal position of the light beam reflected by the half mirror 37. The tracking error signal can be obtained in the differential amplifier 39 by the difference of the outputs in the two light receiving areas 38A and 38B.

10 is a schematic view showing another embodiment of a focusing error signal detecting apparatus according to the present invention. In this embodiment, the aiming lens 22 and the relay lens 28 shown in FIG. 4 are formed by the common lens 40. As shown in FIG. That is, the laser beam emitted from the laser light source 21 is reflected by the polarization prism 23 and converted into a parallel beam by means of the lens 40. The parallel beam is incident on the disk 26 by means of the detection prism 27, the quarter-lambda plate 24, and the reflecting surface 27a of the objective lens 25. The object reflected by the disk 26 is focused by the objective lens 25 and is incident on the detection prism 27 by means of the quarter-lambda plate 24. Light reflected by the surface 27a is incident on the lens 40 and passes through the polarizing prism 23. The transmitted light is incident on the photo detector 29. Similar to the embodiment shown in FIG. 7, the lens 40 forms an image of the pupil of the objective lens 25 on the detector 29. Therefore, the light detector 34 can be replaced.

Detection prism in the above embodiment The reflective surface reflects incident light in a direction of 90 ° with respect to incident light. The angle may be smaller than 90 ° when a material having a relatively large refractive index is used.

The present invention is not limited to the above embodiments, and may be variously modified within the scope of the present invention. For example, in the embodiment of FIG. 4, S-polarized light is incident on the reflecting surface 27a of the detection prism 27, and P-polarized light inserts a 90 ° rotor between the polarizing prism 23 and the detection prism 27. Therefore, it can be incident on the detection prism 27. In such a variation, the detection sensitivity can be further increased because the intensity of the light reflected by the face 27a changes rapidly near the critical angle. This can be obtained without arranging a 90 ° rotor in the embodiment shown in FIG. 10 where P polarized light transmitted through the polarizing prism 23 is incident on the detection prism 27. Detection sensitivity can be increased by having an elongated detection prism with parallel reflecting surfaces and light is reflected several times between these surfaces. In such cases, the sensitivity can be increased by the number of reflections.

In the embodiment shown in the figure the detection prism It may have any predetermined refractive index as long as the reflective surface is set at or near the critical angle.

In addition, although the polarization is used in the above embodiment, unpolarized light according to the present invention may be used together. In the embodiment of FIG. 4, it is sufficient that the reflecting surface 27a of the detection prism 27 is smaller than the critical angle or at an angle equal to the critical angle so that the surface 11 is arranged with respect to a single ray of the incident light beams.

Therefore, converging or converting light beams can be used instead of parallel light beams. In addition, the polarizing prism 23 may be replaced by a half mirror. Also, in the above embodiment, the optical member is made of a detection prism having a suitable refractive index and may be composed of any other element such as a flat glass plate.

The present invention is not limited to the application of the optical reading device of the video disk described above, it can be seen that it can be applied to the focus detection of various optical instruments.

Claims (1)

An apparatus for detecting a focusing error signal of an objective lens with respect to a target object focused on a light spot by an objective lens, wherein the light beam is disposed between the light source and the objective lens and emitted from the light source to the objective lens. A non-dispersion element for guiding and guiding the luminous flux reflected from the object in a direction different from the direction from the object to the light source, and an optical surface for receiving at least a portion of the luminous flux reflected from the object and reflecting and / or refracting a portion of the luminous flux. An optical member made of a material having a higher index of refraction than the material entering the back light beam refracted and transmitted through the optical surface, and a relay lens arranged to receive at least a portion of the light beam reflected or refracted by the optical surface, The relay to generate output signals indicative of the amount of light impinging on the light receiving areas. And light detecting means having at least two light receiving regions arranged to receive the light beam transmitted through the lens, and a circuit for receiving output signals from the light detecting means to form a difference signal as a focused error signal. Focusing condition detection device for an objective lens.