JP2760848B2 - Focus position detection device and focus control device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial applications]

The present invention relates to a focus position detection device that detects a focus position of a light beam, a focus control device that controls a focus position of the light beam using the detection signal, and a polarization detection imaging optical system that detects a polarization state of light. . In particular, the present invention relates to a focus position detection device and a focus control device suitable for use in an optical information processing device that optically reads and / or writes information recorded on a moving information recording medium such as a disk or a tape.

[Prior art]

Optical reading of data recorded at high density has a technical problem in terms of accuracy, and is aimed at positioning the information recording medium and the reading / recording / reproducing apparatus relatively in the optical axis direction of the light beam. Various focus control devices have been proposed. For example, JP-A-57-108811 discloses a mask arranged so as to block a part of detection light reflected from a recording medium, and a focus of irradiation light for irradiating the recording medium with the recording medium fluctuating. There has been proposed an image rotation detection type focus position detection apparatus that detects an out-of-focus state as rotation of detection light by using a photoelectric conversion apparatus that outputs an error signal when the image data is displaced from a recording medium. As another focus position detection device, a so-called astigmatism method in which an astigmatism element is inserted into an optical path of detection light and a focus position is detected by a change in a light pattern received by a photodetector divided into four sectors. (For example, see Japanese Patent Application Laid-Open No. 50-104539).

The focus position detection by the image rotation is highly sensitive to the result of the recording medium, and in order to detect a correct error signal, a light spot formed on the photodetector by reflected light is required. Stability is essential. On the other hand, focus position detection based on astigmatism requires strict adjustment of the alignment between the 4-split photodetector and the detection light. An object of the present invention is to provide a focus position detecting device that can detect a focus shift based on a polarization state of detection light without being affected by a change in intensity distribution of the detection light. Another object of the present invention is to provide a focus control device which can detect a focus shift based on the polarization state of detection light and can easily adjust the position between the optical system and the light detection means. Another object of the present invention is to provide a polarization detection imaging optical system having different imaging characteristics depending on the polarization state of incident light.

[Means for Solving the Problems]

The focus position detecting device of the present invention comprises: an irradiation optical system for converging a radiation beam on a reflecting surface; an optical unit for extracting a reflected beam comprising at least a part of the radiation beam reflected from the reflecting surface; An imaging optical system that is disposed in the optical path of the beam and condenses the reflected beam on the first and second imaging planes according to the polarization state of the reflected beam; Detecting means disposed on the observation surface, and detecting a focus position of the radiation beam from an output of the detecting means. The focus control device according to the present invention comprises: an irradiation optical system for converging a radiation beam on a reflection surface; an optical unit for extracting a reflection beam composed of at least a part of the radiation beam reflected from the reflection surface; and the extracted reflection beam. And an imaging optical system disposed on the optical path for converging the reflected beam on the first and second image planes according to the polarization state of the reflected beam, and observing between the first and second image planes Detecting means arranged on the surface, signal processing means for obtaining a control signal for controlling the focal position of the radiation beam from the output of the detecting means, and control means for controlling the focal position of the radiation beam according to the control signal It is characterized by having. The polarization detection imaging optical system according to the present invention is characterized in that the imaging characteristics change according to the polarization state of the incident beam, and is characterized by having, for example, a lens made of an optically birefringent material.

[Action]

In the present invention, since the defocus is detected based on the polarization state of the detection light, the defocus can be detected using all outputs of the photodetector that receives the detection light, and the detection can be stably performed without being affected by a change in the intensity distribution of the detection light. . In addition, the photodetector only needs to detect the total amount of the received detection light, and the alignment between the photodetector and the detection light can be easily adjusted. Therefore, there is an advantage that it is strong against the movement of the detection light on the photodetector surface.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS To facilitate understanding of the present invention and facilitate the implementation of the present invention, a detailed description will be given with reference to the accompanying drawings. FIG. 1 is a diagram showing the principle of polarization detection according to the present invention.
FIG. 3, FIG. 3 and FIG. 4 are views showing an embodiment of the polarization detection imaging optical system according to the present invention, respectively. FIG. 5 is a view showing one application example of the polarization detection imaging optical system according to the present invention. FIG. 6 is a view showing an embodiment of a focus position detecting device according to the present invention, and FIG.
FIG. 8 is a view showing another embodiment of the focus position detecting apparatus according to the present invention, FIG. 8 is a modification of FIG. 6, and shows a case of parallel incident light, and FIG. FIG. 10 shows a modified example,
FIG. 1 is a diagram showing an embodiment of a focus control device according to the present invention.
FIG. 11 is a diagram showing another embodiment of the focus control device according to the present invention, and FIG. 12 is a diagram showing another embodiment of the focus control device according to the present invention. The principle of changing the imaging characteristics according to the polarization state of the incident light will be described with reference to FIG. In the embodiment of FIG. 1, the imaging optical system includes a lens 1 formed of a material exhibiting birefringence, for example, calcite or crystal quartz. The axis of the birefringent material is arranged such that the optical axis of the crystal constituting the lens 1 is parallel to the Oy axis with respect to the coordinates Oxyz shown in the figure.
At this time, the refractive index n ′ for light polarized parallel to the Ox axis
Which is equal to the normal refractive index of the crystal. The refractive index n ″ is shown for light polarized parallel to the Oy axis, which corresponds to the extraordinary refractive index of the crystal. The focal length f of the spherical lens is 1 / f = (n−1) ( 1 / R′−1 / R ″ + (n−1) d / (nR′R ″)) (1) where R ′ is the radius of curvature of the first surface of the lens, and R ″. Is the radius of curvature of the second surface of the lens, d is the thickness of the lens, and n is the refractive index. Therefore, the birefringent lens 1 has two focal lengths f 'and f ". If the light 2 is emitted from a point O located at a distance of g from the lens 1, the light passing through the birefringent lens 1 is An ordinary ray 4 'and an extraordinary ray 4 "are formed and formed as images 3' and 3" at positions b 'and b "from the lens 1, respectively. The distance b ', b "is determined by 1 / b' = 1 / f'-1 / g (2 ') 1 / b" = 1 / f "-1 / g (2"). Therefore, the imaging characteristics of the lens 1 having such birefringence depend on the polarization state of incident light. The imaging optical system having different imaging characteristics depending on the polarization state of the incident light is not limited to the lens 1 made of the optically birefringent material as described above, but may be an electro-optic effect, a magneto-optic effect, or a light refraction effect. Can change the focal length. The present invention is characterized in that an out-of-focus state is detected using an imaging optical system having different imaging characteristics according to the polarization state of incident light. FIG. 2 is a diagram showing an embodiment of the polarization detection imaging optical system according to the present invention. Light 5 enters a polarizing beam splitter 6. The incident light 5 follows one of the following two paths according to the polarization state. One is that the light 5 is reflected by the polarizing beam splitter 6, and the reflected light 7 'is reflected by the polarizing beam splitter 6 again around the loop formed by the polarizing beam splitter 6 and the mirrors 8, 9 and 10 clockwise. This is the route that goes out of the loop. The other is that the light 5 is transmitted through the polarizing beam splitter 6, and the transmitted light 7 ″ is turned around the loop formed by the polarizing beam splitter 6 and the mirrors 8, 9 and 10 counterclockwise, and is again turned on. The polarization beam splitter 6 and the mirrors 8, 9 and 10 pass through the loop and exit the loop.
A focusing element 11, such as a lens, is arranged at 12 positions in the loop formed by the lens. This position 12 is displaced by a distance A from a position 13 in the middle of the loop. Accordingly, the light 5 incident on the imaging optical system is imaged at two convergence points 14 'and 14 "according to the polarization direction. FIG. 3 shows another embodiment of the polarization detection imaging optical system according to the present invention. 1 is a view showing an embodiment, in which light 15 is incident on a converging lens 16. A polarizing beam splitter 18 is disposed in the optical path of a condensed beam 17, and converts the beam 17 into two beams 1 having different polarization states.
9 ', 19 ". The two beams 19', 19" split by the polarizing beam splitter 18 are spatially separated, and each of them has a quarter-wave plate 21 and a mirror 20 in its optical path.
Is arranged. The beam 19 'reflected by the polarizing beam splitter 18 is transmitted through a quarter-wave plate 21' and then reflected by a mirror 20 '.
Are reflected again by the wavelength plate 21 ′, return to the polarization beam splitter 18, pass through the polarization beam splitter 18 and form an image at the convergence point 22 ′. On the other hand, the beam 19 "transmitted through the polarization beam splitter 18 is transmitted through the quarter-wave plate 21", reflected by the mirror 20 ", transmitted again through the quarter-wave plate 21", and returned to the polarization beam splitter 18. Is reflected by the polarizing beam splitter 18 and forms an image at a convergence point 22 ". Therefore, the light 15 incident on this imaging optical system forms an image at two convergence points 22 'and 22" according to its polarization direction. Is done. In this imaging optical system, two beams 19 'and 19 "having different polarization states are spatially separated, so that respective convergence points 22' and 22" can be set independently of each other. In the example of FIG.
Is disposed before the polarizing beam splitter 18, and the distance between the two convergence points 22 'and 22 "is
Is given by the difference in the optical path lengths of the two beams 19 'and 19 "divided by the mirror 20', and depends on the set position of the mirrors 20 'and 20". FIG. 4 is a view showing still another embodiment of the polarization detection imaging optical system according to the present invention. Light 23 is a polarizing beam splitter
24, beams 25 ', 25 "of two different polarization states
Is divided into Mirror 2 in the optical path of beams 25 'and 25 "
6 'and 26 "are arranged respectively. Mirrors 26' and 2"
The beams respectively reflected at 6 "are directed to another polarizing beam splitter 27, where they are recombined. Beams 25 ',
Convergent lenses 62 'and 62 "are provided at two positions P' and P" which are not conjugate to each other in the optical path of 25 ", and beams 25 'and 25" are respectively connected to converging points 28' and 28 ". Also in this imaging optical system, the two converging points 28 'and 28 "can be set independently of each other because the beams 25' and 25" having two different polarization states are spatially separated. In the example, the distance lenses 62 ', 62 "between the two convergence points 28', 28"
Depends on the setting position. FIG. 5 shows an application example of the polarization detecting and imaging optical system according to the present invention, which comprises two lenses 29 and 30 made of an optically birefringent material. The material, shape, and crystal orientation of the lens 29 are set such that the focal lengths are f1 ′ and f1 ″ when the polarization of light incident on the optical system is parallel to the Ox axis and the Oy axis. Regarding the material, shape, and crystal orientation of the lens 30, the polarization of light incident on the optical system is
The focal lengths are f2 ′ and f2 ″ when parallel to the Oy axis, respectively.
Is set to be When the distance between the lens 29 and the lens 30 is B, the optical system shown in FIG. 5 is configured so that B = f1 '+ f2' = f1 "+ f2" (3). At this time, the parallel beam 31 having a beam radius C incident on the lens 29 is also a parallel beam when exiting from the lens 30. However, the beam radius is C '= Cf1' / f2 '= C (C-f2') / f2 '(4') or C "= Cf1" / f2 "= C (C-f2 ″) / F2 ″ (4 ″). Therefore, this optical system is a telescope system in which the magnification of the beam varies depending on the polarization state of the incident light. FIG. 6 shows an embodiment of a focus position detecting apparatus according to the present invention, in which the focus of a light beam is detected using the polarization detection imaging optical system shown in FIG. A light beam 33 from a light source 32 is imaged on a reflecting surface 36 by an objective lens 34.
As light. The reflection surface 36 is an information recording surface of an information recording medium such as an optical disk. The light beam 37 reflected by the reflecting surface 36 passes again through the objective lens 34 and is split into a beam splitter 38
As a result, the light beam is separated from the light beam from the light source 32 and guided to the focus detection system. That is, at least a part of the reflected beam 37 is guided to the polarization switching element 39 capable of providing two orthogonal polarization states, and then to the polarization detection imaging optical system 40 having different imaging characteristics depending on the polarization state of light. Incident. The imaging optical system 40 includes a lens formed of a material exhibiting birefringence, and, as described with reference to FIG. 1, two focusing points 41 'and 41 "according to the polarization state of incident light. That is, the polarization switching element 39 separates the polarized light from the polarized light parallel to the optical axis of the polarization detection imaging optical system 40 having birefringence and the polarized light perpendicular to the optical axis from the optical axis. Is switched to enter the polarization detection imaging optical system 40, and at least a part of the light reflected from the reflection surface 36 is condensed on one of the two condensing points 41 'and 41 ". A photodetector 42 is arranged between these two condensing points 41 'and 41 ", and the photodetector 42 converts the received light into an electric signal. Is a focus error signal. The polarization switching element 39 is, for example, a half-wave plate that can be put in and out, and a mechanically rotatable half-wave plate.
Polarized light parallel to the optical axis of the polarization detection imaging optical system 40 and perpendicular to the polarized light, such as a wave plate or a combination of a fixed quarter wave plate and a plus / minus quarter wave plate that can be moved in and out. Any device may be used as long as it is switched between the state light and the polarization detection image forming optical system 40. In the present embodiment, since the defocus is detected based on the polarization state of the detection light, the defocus can be detected using all outputs of the photodetector that receives the detection light, and the detection can be performed stably without being affected by a change in the intensity distribution of the detection light. it can. In addition, the photodetector only needs to detect the total amount of the received detection light, and the alignment between the photodetector and the detection light can be easily adjusted. FIG. 7 shows another embodiment of the focus position detecting device according to the present invention. The configuration of the optical system for condensing the light beam 33 on the reflection surface is the same as that in FIG. 6, and the description thereof will be omitted.
The difference from FIG. 6 is that the light beam reflected from the reflecting surface 36
37 is that the light is directly guided from the beam splitter 38 to the polarization detection imaging optical system 40. The imaging characteristics of the polarization detection imaging optical system 40 depend on the polarization state of incident light. The polarization direction of the incident light is such that the light passing through the imaging optical system 40 has the same intensity.
Are set to be in one polarization state. If the reflected light extracted by the beam splitter 38 is linearly polarized light,
The polarization direction is tilted with respect to the optical axis of the polarization detection imaging optical system 40 and is incident on the polarization detection imaging optical system 40 to obtain an ordinary ray and an extraordinary ray having the same intensity. Circularly polarized light can be incident on the polarization detection imaging optical system 40 to obtain an ordinary ray and an extraordinary ray having the same intensity. Then, a polarization splitting element 43 is provided in the optical path, and these two polarized light beams are spatially split and condensed at two light condensing points 41 'and 41 ". For example, a Wollaston prism is provided on the observation surface between these two focal points 41 'and 41 ".
The two photodetectors 44 'and 44 "are disposed, respectively, and receive two spatially split lights in the polarization state and output two signals S' and S". The difference between these two signals is the focus error signal. The polarization splitting element 43 is not limited to a Wollaston prism, and a polarization beam splitter can also be used. In this embodiment, the polarization switching by the polarization switching element 39 is unnecessary, and two signals corresponding to the polarization state can be obtained at the same time. FIG. 8 shows another embodiment of the focus position detecting apparatus according to the present invention, which is a modification of FIG. 6 and shows an example in the case of parallel incident light. Here, the detection light 37, which is the reflected light taken out by the beam splitter 38, is parallel, and the half-wave plate 52 in which the polarization switching element 39 can be put in and out, and the imaging optical system 40 is made of an optically birefringent material. This is a lens 51. The photodetector 42 is arranged on a plane 49 orthogonal to the optical axis 50 of the light beam narrowed by the lens 51. The position of plane 49 is
Between two focal points 54 'and 54 "formed by the birefringent lens 51, a beam 55' focused on the focal point 54 'and a beam 55" focused on the focal point 54 "have the same diameter. It is set to a point 53 on the axis 50. Similarly, Fig. 9 shows another embodiment of the focus position detecting apparatus according to the present invention, which is a modification of Fig. 7. In this case, too, the detection light 37 is emitted. The lens 51 is parallel and the imaging optical system 40 is a lens 51 made of an optically birefringent material, and the polarization splitting element 43 is a Wollaston prism 56. On a plane 59 orthogonal to the optical axis 58 of the lens 51, a forastone Photodetectors 44 'and 44 "are arranged to receive beams 62' and 62", respectively, spatially separated by a prism 56. The position of plane 59 corresponds to beam 6
Between two focal points 60 ', 60 "formed by 2', 62",
The point 62 on the optical axis 58 at which the beam 62 'focused on the focal point 60' and the beam 62 "focused on the focal point 60" have the same diameter is set. FIG. 10 shows a main part of an embodiment of a focus control device according to the present invention, in which the focus position detecting device shown in FIG. 7 or 9 is used. The outputs of the two photodetectors 44 ', 44 "are guided to a differential amplifier 45, and the difference S between the two signals S', S" is obtained. When the light beam 33 is accurately converged on the reflecting surface 36,
The two photodetectors 44 ', 44 "are arranged so that the two photodetectors 44', 44" output signals S ', S "of the same level. Therefore, the focal position of the light beam 33 by the lens 34 is reflected. In the focused state coincident with the surface 36, the output S of the differential amplifier 45 is zero and the focus error signal is also zero.If the light beam 33 is not focused on the reflecting surface, the two photodetectors 44 ′, 44 ″
Light spots having different sizes are formed on the detection surface of. Which diameter is larger and which is smaller is determined by the position of the converging point 35 of the light beam 33 with respect to the reflecting surface 36. In such an out-of-focus state, the signals S ', S "obtained by the two photodetectors 44', 44" are not equal, and the difference S
Is not zero. The sign of the difference S indicates the relative position that the focal point 35 takes with respect to the reflecting surface 36. Using this difference signal S,
For example, by driving an actuator 47 that moves the objective lens 34 in the optical axis direction, a focus control servo system is configured by controlling the focal position of the light beam 33 so as to always coincide with the reflection surface 36. By connecting the outputs S 'and S "of the two photodetectors 44' and 44" to the adder 48, the sum can be detected. This sum signal L is proportional to the total intensity of the light reflected from the reflecting surface 36. When the reflecting surface 36 is a moving information recording medium such as an optical disk, signals other than the focus error signal can be obtained. With the sum signal L, it is possible to detect a reproduction signal corresponding to uneven pits and light and dark pits recorded on the information recording medium, and it is also used as a monitor for monitoring the recording state of data recorded on the information recording medium. be able to. Further, it can be used for detecting a track shift signal. On the other hand, the difference signal S can be used for detecting a magneto-optical signal corresponding to a magnetization domain recorded on a magneto-optical recording medium in addition to a focus error signal. FIG. 11 shows another embodiment of the focus control device according to the present invention, and shows a signal processing mechanism when an information recording medium for obtaining a reproduction signal from reflected light intensity-modulated by recording information is used for the reflection surface 36. Things. The output S of the differential amplifier 45 becomes a focus error signal. For example, by driving an actuator 47 that moves the objective lens 34 in the optical axis direction,
The point that the focus position of the light beam 33 is controlled to always coincide with the reflection surface 36 to form a focus control servo system is the same as the embodiment of FIG. The output L of the adder 48 is connected to a decoder 63, which demodulates a reproduction signal 65 corresponding to data recorded on the information recording medium in the form of uneven pits or light and shade pits. Further, a pre-wobble pit for detecting a tracking error signal is provided intermittently, and a sample-type recording medium that records and reproduces data in a data area between the pre-wobble pits while detecting the tracking error signal in a sample manner is used. , The output L of the adder 48 is also used for detecting a tracking error signal. In such a recording medium of the sample type, the servo area provided with the pits for detecting track deviation and the data area in which data is recorded are divided into areas along the track, so that tracking information and data information occur simultaneously. Therefore, discrimination can be easily performed using a clock obtained from synchronization information such as synchronization pits recorded on a medium. This discrimination and detection of the tracking error signal can be performed in the decoder 63. The detected tracking error signal 64 is driven by an actuator 66 for moving the objective lens 34 in a direction crossing the tracking or a galvano mirror provided in the irradiation light path, so that the irradiation position of the light beam 33 is aligned with the center line of the track. A tracking servo system is configured by performing control so as to always follow. FIG. 12 shows another embodiment of the focus control device according to the present invention, and an information recording medium for obtaining a reproduction signal from reflected light that is polarization-modulated by recording information, such as a magneto-optical disk medium,
5 shows a signal processing mechanism when used for the reflection surface 36. The point that the output S of the differential amplifier 45 becomes a focus error signal is as follows.
This is the same as the embodiment of FIGS. 10 and 11. In this embodiment, the signal S is also used for reproducing data information. The focus error signal is a low-frequency signal, and the data signal is a signal having a considerably higher frequency than the focus error signal. Therefore, the focus error signal can be separated by the frequency filters 68 'and 68 ".
The low-frequency component 69 extracted by the frequency filter 68 'becomes a focus error signal.For example, by driving an actuator 47 for moving the objective lens 34 in the optical axis direction, the focal position of the light beam 33 is shifted to the reflection surface 36. A focus control servo system is configured by performing control so as to always match. Frequency filter 6
The high-frequency component 65 extracted at 8 ″ is connected to a decoder (not shown), and a reproduction signal 65 corresponding to data recorded as magnetization information on the perpendicular magnetic recording medium is demodulated. In a sample-type recording medium in which pre-wobble pits are provided intermittently and data is recorded and reproduced in a data area between the pre-wobble pits while detecting a tracking error signal as a sample, the output L of the adder 48 is Are used for detecting a tracking error signal.The tracking error signal is detected by the decoder 67, and the detected tracking error signal 64 is detected.
By controlling the actuator 66 for moving the objective lens 34 in the direction traversing the track or the galvanomirror provided in the irradiation light path, the irradiation position of the light beam 33 is controlled so as to always follow the center line of the track. To form a tracking servo system. Also, track address,
When address information such as a sector address is recorded on the information recording medium in the form of concave and convex pits and light and dark pits, the output L is used for detecting a reproduction signal corresponding to the address information, and is demodulated by the decoder 67. As described above, in the focus control device of the present invention, all of the focus error signal, the tracking error signal, the data signal, and the preformat signal can be detected by the detection device including the optical system as shown in FIG. As a result, the configuration of an optical system (optical head, optical pickup) in an optical information storage device such as an optical disk device or a magneto-optical disk device can be extremely simplified, and thus the size can be reduced. In particular, according to the optical pickup of the present invention, in order to detect a focus error signal, a tracking error signal, a data signal, a preformat signal, and the like, the detection light that is reflected light from the recording medium is divided, and each light path There is no need to provide a detection optical system, and all of the focus error signal, tracking error signal, data signal, and preformat signal can be detected by the same detection optical system. It goes without saying that the invention is not limited to the embodiments described above, but these embodiments are only given by way of example.

【The invention's effect】

According to the present invention, the defocus can be detected by using all outputs of the photodetector that receives the detection light, and the detection can be stably performed without being affected by a change in the intensity distribution of the detection light. And as a photodetector,
It suffices to detect the total light amount of the detection light to be received, and it is easy to adjust the alignment between the photodetector and the detection light. Therefore,
There is an advantage that it is strong against the movement of the detection light on the photodetector surface. Further, the configuration of an optical system (optical head, optical pickup) in an optical information storage device such as an optical disk device or a magneto-optical disk device can be extremely simplified, and thus the size can be reduced. In particular, according to the present invention, in order to detect a focus error signal, a tracking error signal, a data signal, a preformat signal, and the like, a detection light which is reflected light from a recording medium is divided, and a detection optical system is provided in each optical path. Need not be provided, and all of the focus error signal, tracking error signal, data signal and preformat signal can be detected by the same detection optical system.

[Brief description of the drawings]

FIG. 1 is a diagram showing the principle of polarization detection according to the present invention, FIG.
FIGS. 3 and 4 show an embodiment of the polarization detection imaging optical system according to the present invention, respectively. FIG. 5 shows an application example of the polarization detection imaging optical system according to the present invention. FIG. 7 is a diagram showing an embodiment of a focal position detecting device according to the present invention; FIG. 7 is a diagram showing another embodiment of a focal position detecting device according to the present invention;
8 shows a modification of FIG. 6, showing a case of parallel incident light, FIG. 9 shows a modification of FIG. 7, and FIG. 10 shows a modification of the focus control device according to the present invention. FIG. 11 is a view showing an embodiment, and FIG. 11 is a view showing another embodiment of the focus control device according to the present invention.
FIG. 12 is a diagram showing another embodiment of the focus control device according to the present invention.

Continuation of front page (58) Field surveyed (Int.Cl. ⁶ , DB name) G02B 7/11

Claims (17)

(57) [Claims] An irradiation optical system for converging an irradiation beam on a reflection surface; separation means for extracting at least a part of the irradiation beam reflected from the reflection surface; A first and a second light source disposed in an optical path according to a polarization state of the reflected beam;
An imaging optical system for converging the reflected beam on the imaging surface of (a), and detection means arranged corresponding to the first and second imaging surfaces, and the output from the detection means A focal position detecting device for detecting a focal position of an irradiation beam. 2. The image forming optical system according to claim 1, wherein the image forming optical system is configured to convert reflected light from the reflecting surface into a beam parallel to an optical axis of the image forming optical system according to a polarization state of the reflected light. Deflection switching means for switching to a beam perpendicular to the polarization switching means, and polarization detection imaging means for forming first and second imaging planes in accordance with the state of deflection of the light emitted from the polarization switching means. Focus position detecting device. 3. The method according to claim 2, wherein
And a detection element for receiving a beam from the polarization detection imaging means between the second imaging plane and the second imaging plane. 4. A focus position detecting device according to claim 2, wherein said polarization switching means is a half-wave plate. 5. The image forming optical system according to claim 1, wherein the image forming optical system forms first and second image forming surfaces of the light reflected from the reflecting surface according to a polarization state of the reflected light. Polarization detection imaging means; and a polarization splitting element for spatially separating and condensing beams in mutually orthogonal polarization states from the polarization detection imaging means in accordance with the polarization state of the beam. Focus position detecting device. 6. A focus position detecting apparatus according to claim 5, further comprising a detecting element for receiving each of the two polarized light beams separated by said polarization splitting element. 7. A focus position detecting device according to claim 5, wherein said polarization splitting element comprises a Wollaston prism. 8. A focus position detecting device according to claim 2, wherein said polarization detecting and imaging means is a lens made of an optically birefringent material. 9. An irradiation optical system for converging an irradiation beam on a reflection surface, an optical unit for extracting a reflection beam comprising at least a part of the irradiation beam reflected from the reflection surface, and an optical path for the reflection beam. An imaging optical system for converging the reflected beam on the first and second imaging planes in accordance with the polarization state of the provided reflected beam; and a first optical system disposed on the first imaging plane. Detecting means, a second detecting means disposed on the second imaging surface, and controlling a focal position of the radiation beam from a difference output between the first detecting means and the second detecting means. A focus control device comprising: signal processing means for obtaining a control signal; and control means for controlling a focal position of the irradiation beam on the reflection surface according to the control signal. 10. The recording medium according to claim 9, wherein said reflection surface is made of an information recording medium, and a reproduction signal recorded on said recording medium is obtained from a difference output between said first detecting means and said second detecting means. A focus control device characterized by obtaining. 11. A recording medium according to claim 9, wherein said reflection surface is made of an information recording medium, and a reproduction signal recorded on said recording medium is obtained from a sum output of said first detection means and said second detection means. A focus control device characterized by obtaining. 12. A high-pass filter according to claim 9, wherein a high-pass filter is provided at an output destination of a difference between said first detecting means and said second detecting means, and an output of said high-pass filter is provided. A focus control device characterized in that: 13. A recording medium according to claim 9, wherein at least the servo area provided with the track shift detection bit and the data area in which data is recorded have a track shape. A focus control device comprising an information recording medium divided into regions along the line, and detecting a tracking error signal as a sample from the sum output of the first detection means and the second detection means. 14. A recording medium according to claim 9, wherein said reflecting surface is formed of an information recording medium in which at least address information such as a track address or a sector address is recorded in the form of uneven pits or light and shade pits. A focus control device for detecting the address information from the sum output of the first detection means and the second detection means. 15. The image forming optical system according to claim 9, wherein the image forming optical system forms first and second image forming surfaces according to a polarization state of the reflected light. Polarization detection imaging means; and a polarization splitting element for spatially separating beams in mutually orthogonal polarization states from the polarization detection imaging means and condensing the beams on the first and second imaging planes. A focus position detecting device. 16. A focal position detecting apparatus according to claim 15, wherein said polarization detecting and imaging means is a lens made of an optically birefringent material. 17. A focus position detecting device according to claim 15, wherein said polarization splitting element comprises a Wollaston prism.