JPH02172032A - Photodetector - Google Patents

Abstract

PURPOSE:To obtain a stable detection characteristic by so adopting the constitu tion that a reflected beam from an information recording medium is split into two, focusing in an isotropic way or an asymmetrical way, and moving the split light beams respectively in opposite directions on the photodetector when out of focus takes place. CONSTITUTION:The reflected light from an optical disk 18 is made incident in a collimate glass flat plate 20 via a beam splitter 14 and transmitted through two light transmission sections 20a, 20b by a light shield film 21. Then, a photodetector 26 is irradiated with the transmitted light via a detection lens 22 and a cylindrical lens 24. Through the constitution above, the beam is focused at a position H in the direction of generating line 23 and to a position G in a direction orthogonal to the generating line 23 respectively. Thus, in the case of focusing state, the light is detected nearly equally by detection cells 26a - 26d, but in the case of approaching the focal position or parting from the focal position, the beam spot is moved to a direction opposite to the axis (d) of the detector 26. Thus, the effect on the focus detection due to optical axis deviation or the like is reduced and the stable detection characteristic is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to a photodetection device that can be applied, for example, to a device for reproducing information optically recorded on an information recording medium.

(Prior Art) Recently, optical discs have been attracting attention as a type of information recording medium. The technological background lies in the advancement of laser oscillation technology, which has resulted in optical discs with superior high-density and high-speed performance. Information is recorded on this optical disc in the form of pits such as holes, and changes in the reflectance of light irradiated onto the optical disc are used to reproduce the recorded information. However, when recording or reproducing information on an optical disc, there is a focus control system that controls the light irradiated onto the optical disc to accurately focus it on the optical disc.
Servo is required. In order to perform this focus servo, various methods for detecting focus have been devised in the past.

For example, as disclosed in Japanese Unexamined Patent Publication No. 59-90237, light from an optical disk is transmitted through a mask (a light-shielding plate such as a knife wedge) that is provided on the optical path and is extracted asymmetrically with respect to the optical axis. Some devices detect focus errors by detecting them with a photodetector. In this detection method, a photodetector is placed at the focal point of the light from the optical disc. Further, the focus state is detected by utilizing the fact that the light irradiated to the photodetector moves on the photodetector, and focus servo is performed based on the detection result.

(Problem to be Solved by the Invention) However, when detecting a focus using this detection method, since the photodetector is placed at the focal point of the light from the optical disk, The size of the beam spot of light becomes smaller.

Therefore, it is susceptible to optical axis misalignment due to deformation of optical elements due to temperature changes, etc., and accurate focusing $in.
It had the disadvantage of not being able to exercise control.

Therefore, in order to reduce errors in focus detection due to the influence of optical axis deviation, etc., the present invention moves the light that has passed through two areas in opposite directions on the photodetector when an out-of-focus state occurs. , is committed to providing a photodetector with stable focus detection characteristics.

[Structure of the Invention] (Means for Solving the Problems) In order to solve the above problems, the present invention provides a first condensing means for condensing light onto an information recording medium, and a beam of light passing through the information recording medium. having first and second regions dividing the cross section into two,
an optical means for extracting a non-uniform amount of light in the first and second regions from the light irradiated to the first and second regions after passing through the information recording medium; A second condensing means that condenses light anisotropically or asymmetrically, and an information recording medium of the first condensing means by detecting the light condensed by the second condensing means. Detecting means for detecting a change in position with respect to a photodetecting device is provided.

(Function) The photodetecting device of the present invention has first and second regions that divide the beam cross section of light that has passed through the information recording medium into two, and furthermore, after passing through the information recording medium, the first and second an optical means for extracting a non-uniform amount of light in the first and second regions with respect to the light irradiated to the region, and anisotropically or asymmetrically condensing the light extracted by the optical means By including a light condensing means, light from the information recording medium is detected through the optical means and the condensing means, and defocus detection is performed.

In this detection method, by tilting the anisotropic direction of the condensing means anisotropically or asymmetrically with respect to the boundary line of the first and second regions, the first and second regions are Since the transmitted light is moved in opposite directions on the photodetector, the influence of optical axis misalignment on focus detection is reduced, detection errors are eliminated, and stable detection characteristics are obtained.

(Example) Hereinafter, an example of the present invention will be described with reference to the drawings.

FIG. 1 is a schematic configuration diagram showing a photodetection device of the present invention. This device includes a light source 10 composed of a semiconductor laser. Collimating lens 1 that converts light from light [10 into parallel light]
2. An objective lens 16 that focuses the light from the beam splitter 14 onto the information recording medium 18, and a parallel glass flat plate 20 that is irradiated with the light reflected from the information recording medium 18. Detection lens 2
2 and cylindrical lens 24. and a photodetector 26 that detects light reflected from the information recording medium 18.

In this photodetector during focusing, the light emitted from the light source 10 is converted into parallel light by the collimating lens 12. The light converted into parallel light passes through the beam splitter 14 and heads toward the objective lens 16, where it is further condensed and irradiated onto the information recording medium 18. This information recording medium 18 is a so-called optical disk 18, and is composed of, for example, a substrate 30 and a record 832 on which information is recorded, as shown in FIG.

The substrate 30 is made of plastic, glass, aluminum, or the like. The recording film 32 is composed of a metal film, a semimetal film, an inorganic film, a hexagonal film, or the like. This board 3
0 and the recording film 32, an uneven recording track 34 is formed in advance in a circular or spiral shape. Information bits 36 are formed along this recording track 34.

Methods for recording information on the optical disc 18 include a method in which the recording film 32 is evaporated by irradiating light to form bits, and a method in which the recording film 32 is swollen to form bubbles. Alternatively, there is a method of causing a chemical change without causing a physical change in the recording film 32.

The light irradiated onto the optical disc 18 is
8 , the beam returns to the objective lens 16 , passes through the objective lens 16 , and then irradiates the beam splitter 14 . The light irradiated onto the beam splitter 14 is guided in the direction of the parallel glass flat plate 20, passes through the parallel glass flat plate 20, passes through the detection lens 22 and the cylindrical lens 24, and is irradiated onto the photodetector 26. The light irradiated onto the parallel glass flat plate 20 is affected by interference between the pits of the optical disk 18 and the light reflected from the periphery thereof, resulting in a non-uniform light quantity distribution. That is, the light intensity distribution of the light irradiated onto the parallel glass flat plate 20 on a plane perpendicular to the optical axis 37 is as shown in FIG. As shown, a symmetrical boundary line 3 centered on the optical axis 37
The amount of light decreases and becomes dark in the inner region 38a of 8, but the outer region 38b becomes brighter than the inner region 38a. The shape of the inner region 38a formed along this boundary line 38 is the shape of the recording track 3 of the optical disc 18.
It is formed symmetrically in the direction along 4. Also, in the direction perpendicular to the direction along the recording track 34 of the optical disc 30, if the irradiated light is on the recording track 34, the inner area 38 with respect to the boundary line 38
The shape of a is symmetrical, and when it deviates from the recording track 34, it becomes asymmetrical.

Therefore, the direction along the recording track 34 is temporarily set as the a-axis, and the direction perpendicular to the recording track 34 is temporarily set as the b-axis. Detection lens 2
2 and the cylindrical lens 24 in the optical system will be explained.

The parallel glass flat plate 20 has first and second regions 19a and 19b that are divided along a boundary line parallel to the a-axis along the recording track 34 of the optical disk 18. The first and second regions 19a and 19b are light shielding portions 20c and 20d and light transmitting portions 20b and 2, respectively.
It has 0a. Moreover, the light shielding parts 20e and 20d are a
asymmetrical to each of the a-axis and the b-axis, and the first along the a-axis
and a portion is formed in each of the second regions 19a and 19b. This provides a structure in which non-uniform amounts of light are extracted in each of the first and second regions.

That is, since the a-axis, which is the boundary line between the first and second regions 19a and 19b, is approximately parallel to the direction along the recording track 34, the first and second regions 19a, 19b are It is divided into regions, and a non-uniform amount of light is extracted in the direction along the recording track 34 within each node 1 and the second region. A light shielding film 2 is provided in the light shielding parts 20c and 20d.
This light shielding film 21 to which 1 is added is, for example,
It is possible to use a multilayer film formed of an aluminum film, a chromium film, a carbon film, or an inorganic material such as indium oxide or nitride. Among these, carbon film or multilayer M (permeation prevention film)
The use of the light shielding film 21 is effective in that there is less reflection on the light shielding film 21 and stray light is less likely to be introduced into the optical system. When the parallel glass flat plate 20 including the light shielding parts 20c and 20d shown in FIG. 1 and FIG. The light passes through and is guided toward the detection lens 22. In this way, since the light transmitting parts 20a and 20b are formed on the parallel glass flat plates, the two lights extracted by the light transmitting parts 20a and 20b travel in the direction between the paths. The detection lens 22 has the function of uniformly focusing the irradiated light. The light that has passed through the detection lens 22 is irradiated onto a cylindrical lens 24, which is a means for condensing the light anisotropically or asymmetrically. The cylindrical lens 24 creates focal lines that converge light in its generatrix direction 23 and in a direction orthogonal to the generatrix direction 23, respectively, and introduces astigmatism into the light. This cylindrical lens 24 is arranged so that the generatrix direction 23 is substantially parallel to the a-axis. The light passing through this cylindrical lens 24 is not affected by the action of the cylindrical lens 24 in the generatrix direction 23, and the light is condensed in the same direction as before the irradiation of the cylindrical lens 24, and is condensed at the condensing position H. In the direction perpendicular to the direction 23, the light is acted upon by the cylindrical lens 24, and the light is focused at a focusing position G that is closer than the generatrix direction 23. Therefore, the light that has passed through the cylindrical lens 24 having such a condensing characteristic has a condensed color i.
A photodetector 26 placed at tH is irradiated. The photodetector 26 includes four photodetection cells 26a, 26b, 2
6c and 26d, each having similar photodetection characteristics. Photodetection cells 26a, 26
b, 26c and 26d are arranged so as to be in contact with the a-axis set parallel to the a-axis and the d-axis set parallel to the b-axis, respectively, and the photodetectors 26 are combined in a square shape. There is. The image on the photodetector 26 is located at or near the focal point of the image on the optical disk 18 in the C-axis direction, and therefore is not easily affected by light diffraction.

Next, methods for detecting focus error, detecting track error, and detecting information reproduction from the optical disc 18 in the photodetector having such a configuration will be described.

First, a focus error detection method will be explained with reference to FIGS. 5 and 6. In the photodetector having the structure described above, when the light emitted from the objective lens 16 is in focus on the first disk 18,
The shape of the beam spot 28 on the photodetector 26 is as shown in FIG. 5(a). That is, in the focused state, the light transmitted from the optical disc 18 through the light transmitting portion 20a of the parallel glass flat plate 20 is transmitted to the light detection cell 26 of the photodetector 26.
The light is irradiated onto the boundary line between a and 26b. The amount of light irradiated to these photodetection cells 26a and 26b is approximately equal, and the shape has a certain width in the d-axis direction. Similarly, the light transmitted through the light transmitting section 20b is transmitted to the photodetector 26.
The light is irradiated onto the boundary line between the photodetection cells 26c and 26d.

The amount of light irradiated onto these photodetection cells 26c and 26d is approximately equal, and the shape has a certain width in the d-axis direction. On the other hand, when the optical disc 18 is closer to the objective lens 16 than when it is in focus, as shown in FIG.
b).

That is, the light transmitted through the light transmitting portion 20a of the parallel glass flat plate 20 has a condensing position in a direction perpendicular to the generatrix direction 23 that is closer to the photodetector 26 than the condensing position G at the time of focusing. Therefore, the width of the beam spot 28 on the photodetection cell 26 is d
narrows in the axial direction. Further, in the generatrix direction 23, the beam spot 28 is irradiated onto the photodetection cell 26a of the photodetector 26 with a wider width. Similarly, the light transmitting section 20b
The beam spot 28 is also narrower in the d-axis direction because the condensing position in the direction perpendicular to the generatrix direction 23 is closer to the photodetector 26 than the condensing position G at the time of focus. Further, in the generatrix direction 23, the beam spot 28 is irradiated onto the photodetection cell 26c of the photodetector 26 with a wider width.

Further, when the optical disk 18 is farther away from the objective lens 6 than when it is in focus, the situation becomes as shown in FIG. 5(C). The light transmitted through the light transmitting portion 20a of the parallel glass flat plate 20 has a condensing position in a direction perpendicular to the generatrix direction 23 that is farther from the photodetector 26 than the condensing position G at the time of focusing. Therefore, the width of the beam spot 28 on the photodetector 26 becomes wider in the d-axis direction. In addition, in the generatrix direction 23, since the light is focused and then irradiated onto the photodetector 26, the beam spot moves to the opposite side with respect to the d-axis when the optical disk 18 approaches, and the photodetector cell of the photodetector 26 is moved. 26b. Similarly, the light transmitted through the light transmitting portion 20b is also transmitted in the generatrix direction 23.
The light convergence position in the direction orthogonal to G is farther from the photodetector 26 than the light convergence position G at the time of focusing. Therefore, the width of the beam spot 28 on the photodetector 26 becomes wider in the d-axis direction. In addition, in the generatrix direction 23, the light is focused and then irradiated onto the photodetector 26, so when the optical disk 18 approaches, the beam spot 28 moves to the opposite side with respect to the d-axis, and the photodetector 26
The light is irradiated onto the photodetection cell 26d.

In other words, the shape of the beam spot 28 on the photodetector 26 differs between when the optical disc 18 approaches the objective lens 16 and when the beam spot 28 is away from the objective lens 16. The beam spot moves to the opposite side to the d-axis above.

As described above, the shape of the beam spot 28 irradiated onto the photodetector 26 changes depending on the positional relationship between the optical disk 18 and the objective lens 16.

A focus error signal is generated by detecting this shape. FIG. 6 shows the configuration of a signal processing circuit that generates this focus error signal.

The light irradiated onto the photodetector 26 is transmitted through the photodetection cells 26a and 2.
The output signals of 6c are added by an adder 40 and the output signals are
An adder 42 adds the output signals of b and 26d. A focus error signal can be obtained by subtracting the signals obtained by these adders 40 and 42 by a subtracter 50. This focus error signal is sent to the objective lens drive circuit 5.
6 and controlling the objective lens drive section 58, the objective lens 16 is moved in the optical axis direction and focus servo is performed. In this way, the focus signal is detected by comparing the sum signal of the photodetection cells 26a and 26c with the sum signal of the photodetection cells 26b and 26d. Even when moving in the d-axis direction, detection can be performed without error.

Further, in this photodetector, the direction along the recording track 34 of the optical disc 18, the generatrix direction 23 of the cylindrical lens 24, and the C-axis direction of the photodetector 26 are arranged to coincide with each other. Therefore, this photodetecting device can also detect tracking errors. That is, when detecting this tracking error, it is utilized that the diffraction distribution of light reflected by the optical disc 18 changes depending on the relative relationship between the irradiation light irradiated onto the optical disc 18 and the direction along the recording track 34. When the center of the luminous flux of the irradiated light is on the center line of the recording track 34, the light irradiated onto the photodetector 26 becomes symmetrical with respect to the C-axis of the photodetector 26. That is, the photodetection cells 26a, 26b, 26c, and 26d are each irradiated with the same amount of light. On the other hand, when the luminous flux of the irradiation light deviates from the center line of the recording track 34, the light irradiated onto the photodetector 26 becomes asymmetrical with respect to the C-axis of the photodetector 26. That is, the photodetection cells 26a and 26
Different amounts of light are applied to the sum of light amounts of b and the sum of light amounts of 26c and 26d, respectively. For the irradiated light, each output signal of the photodetection cells 26a and 26b is added by an adder 46, and each output signal of the photodetection cells 26c and 26d is added by an adder 48. The output signal of the adder 46.48 added by this adder 46.48 is the photodetection cell 26a.
The output signal corresponds to the amount of light irradiated to 26b, 26c, and 26d, and by taking the difference with the subtractor 54,
Therefore, the photodetection cells 26a and 26b, 26c and 26
A tracking error signal is obtained in accordance with the difference in the amount of light irradiated to d.

This tracking error signal is supplied to the objective lens drive circuit 60 to control the drive unit 62, thereby driving the objective lens 16 and performing tracking servo. In this way, the photodetection cells 26a and 26b are
By detecting the light beams 6c and 26d in an adjacent relationship, for example, even if the light that has passed through the objective lens 16 deviates from the recording track 34 on the optical disk 18, it can be detected without error.

Furthermore, when detecting a recording signal on the optical disk 18, for example, adders 46, 48, and 52 add the sums of the respective output signals of the photodetection cells 26a, 26b, 26C, and 26d. If this added signal is extracted as a recording signal, the optical disc 1
8 can be reproduced. Also, when performing focus detection, FIGS. 5(b) and (c)
As shown in FIG. 2, when the positional relationship between the optical disc 18 and the objective lens 16 is not in a focused state, the beam spot 28 is located between the photodetection cells 26a and 260. Photodetection cells 26b and 26d
It is divided into diagonals, each with a photodetector 26.
Since the spot size in the C-axis direction at the time of focusing is small, detection sensitivity is increased and stable focus detection without false detection can be obtained.

Next, another embodiment of the present invention will be described.

First, another configuration of the parallel glass flat plate 20 shown in FIG. 1 will be explained with reference to FIGS. Focusing position G in the orthogonal direction and focusing position H in the generatrix direction 23
It is placed between. As shown in Figure 7,
The parallel glass flat plate 20' has a light transmitting portion 25 in both the first and second regions 19a and 19b except where the light shielding film 21 is provided. In addition, the first area 19
A light shielding film 21 is provided on the half region on the negative side of the b' axis and far from the b axis as viewed from the irradiation direction of the light a'. The light-shielding film 21 is also located above the b' axis as viewed from the light irradiation direction of the second region 19b, and
It is given to the half area farthest from the axis. The beam spot 28' on the photodetector 26 has a shape surrounded by a curved line, as shown in FIG. When the light passing through the objective lens 16 is focused on the optical disk 18, the shape of the beam spot 28' on the photodetector 26 is asymmetrical with respect to the b-axis of the parallel glass flat plate 20' in FIG. It is formed by light extracted in a rectangular shape from each of the first and second regions. The light transmitted through the light transmitting section 25 is directed in the generatrix direction 2.
After condensing the light in a direction perpendicular to the direction 3 and before condensing it in the generatrix direction 23, the light is irradiated onto the photodetector 26, so that it spans four areas of the photodetector 26 as shown in FIG. 8(a). This shape is set as the shape at the time of focusing when the parallel glass flat plate 20° shown in FIG. 7 is used. At this time, the amount of light irradiated by the beam spot 28' onto the photodetector 26 is determined by the photodetection cells 26a, 2
The photodetector 26 is rotated about the optical axis so as to be equal to 6b, 26c, and 26d, respectively. At this time of focusing, the optical disc 1 is compared to the objective lens 16.
8 is approaching, the shape of the beam spot 28° becomes narrower in the d-axis direction, as shown in FIG. 8(b), compared to the shape when focused in FIG. 8(a). , the shape becomes longer and wider in the C-axis direction. In addition, when the optical disk 18 is farther away from the objective lens 16 compared to when it is in focus, the beam The shape of the spot 28- is wide in the d° axis direction, but conversely narrow in the C axis direction.

Furthermore, other configurations of the parallel glass flat plate 20 shown in FIG. 1 will be explained with reference to FIGS. 9 and 10. Also, at this time, the photodetector 26 is placed at a position that is longer in the light traveling direction than the condensing position H in the generatrix direction 23 during focusing in FIG. As shown in FIG. 9, the parallel glass flat plate 20'' has first and second regions 19a and 1
At 9b, a light shielding film 21 is provided asymmetrically with respect to the a-axis and the b-axis. The shape of the light transmitting part 27 is approximately elliptical, and this approximately elliptical long sleeve is at an angle of about 45 degrees with respect to the a-axis or the b-axis.
It is tilted, and the intersection of the long sleeve and the short axis of the substantially elliptical shape almost coincides with the intersection of the a-axis and the b-axis. The shape of the beam spot 28'' on the photodetector 26 is as shown by the curved line in FIG. The shape of the beam spot 28 on the detector 26 is as follows. After condensing the light in a direction perpendicular to the generatrix direction 23, the light is condensed in the generatrix direction 23 and then irradiated onto the photodetector 26, and as shown in FIG. Detection cells 26a, 26b
, 26e and 26d.

This shape is set as the shape at the time of focusing when the parallel glass flat plate 20'' shown in FIG.
6a, 26b, 26c, and 26d, the photodetector 26 is rotated about the optical axis so that they are equal to each other.

When the optical disk 18 approaches the objective lens 16, the shape of the beam when the optical disc 18 approaches the objective lens 16 is compared to the shape when the focus is focused in FIG. The shape of the spot 28'' is such that the length of the long axis and the short axis are each small.Furthermore, when the optical disk 18 is farther away from the objective lens 16 compared to the time of focusing, as shown in FIG. 10(c) As shown in Fig. 10(a), the beam spot 28'' has a substantially elliptical shape compared to the focused shape in Fig. 10(a).
The long axis of is tilted, and the length of both the long and short axes is large.

As described above, even when the photodetector 26 is placed before and after the light focusing position H in the generatrix direction 23 during focusing in FIG. A similar detection effect can be obtained. In other words, the objective lens 16
The shape of the beam spot 28 on the photodetector 26 is the same as that of the light 1 output cell 2 when the optical disc 18 and the optical disc 18 are in the focused position and when they are approaching and when they are moving away from each other.
6a and 26c and the photodetection cells 26b and 26d move in a shape that increases detection sensitivity when detecting.

That is, the signal processing shown in FIG. 6 can be applied even when the photodetector 26 is placed before and after the condensing position H.

First and second regions 19a and 19 are divided such that the boundary line is a direction parallel to the a-axis along the recording track 34 of the light-shielding film 21 applied to the parallel glass flat plate 20.
b, a non-uniform amount of light is extracted, and the first and second regions 19a and 19b are applied so as to extract an asymmetric amount of light along the recording track 34, respectively. In this optical means, the shape of the beam spot 28 that is irradiated onto the photodetector 26 at the time of focusing is used as a reference, and the beam spot 28 on the photodetector 26, the amount of light irradiated onto the photodetector 26 is determined by the amount of light irradiated onto the photodetector 26.
The number of photodetecting cells increases from a and 26c to photodetecting cells 26b and 26d, or from photodetecting cells 26b and 26d to photodetecting cells 26a and 26e. As a result, the photodetection cell 2
The difference in light amount detection between the photodetection cells 6a and 26e and the photodetection cells 26c and 26d becomes large, and stable detection without false detection can be performed. Further, even if the optical axis is shifted in the direction of the recording track 34 or in a direction perpendicular to the direction of the recording track 34, the focus can be detected accurately by detecting in this manner.

In the embodiment of the present invention, the detection lens 22 is provided, but this detection lens 22 is used to obtain a small beam spot 28 with the photodetector 26, and the photodetector 26 is used to obtain a small beam spot 28. This detection lens 22 is unnecessary if it has a wide detection area. Furthermore, the position where the photodetector 26 is arranged is as follows in this embodiment:
Although it is arranged at the condensing position H and before and after it, the same effect can be obtained even if it is at the condensing position G and before and after it.
Although the light from the optical disk 18 is treated as reflected light, the same can be said of transmitted light. Further, an axis parallel to the direction along the recording track 34 is referred to as the a-axis, and an axis perpendicular to the a-axis is referred to as the b-axis, but it is sufficient that they are in a substantially orthogonal relationship. Also, the axis parallel to the a-axis is the a-axis, and the axis parallel to the b-axis is a
Although the axes are used, it is sufficient if they are approximately parallel. Furthermore, although the light shielding film 21 may be applied to the parallel glass flat plate 20, it is not necessary to shield light, and any member that extracts a non-uniform amount of light in brightness and darkness may be used.

[Effects of the Invention] As described above, according to the present invention, it is possible to reduce errors in detection such as optical axis deviation, and to obtain a photodetecting device with stable detection characteristics when performing focusing control.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram showing an embodiment of the present invention, FIG. 2 is an example of recording track formation, FIG. 3 is a distribution diagram of the amount of reflected light from an optical disk, and FIGS. FIG. 9 is a diagram of the parallel glass flat plate including the light shielding part of this embodiment, FIGS. 5, 8, and 10 are the shapes of the beam spot at the photodetector in the implementation, and FIG. FIG. 3 is a diagram showing signal processing of an output signal of a photodetector. 16... Objective lens, 20... Parallel glass flat plate, 2
4... Cylindrical lens, 18... Optical disk, 26...
・Photodetector diagram (a) C(ζ) d diagram diagram diagram diagram IYI2 diagram diagram diagram

Claims (1)

[Scope of Claims] A first condensing means for condensing light onto an information recording medium; a first and a second region dividing a beam cross section of the light that has passed through the information recording medium into two; After passing through the recording medium,
optical means for extracting a non-uniform amount of light in the first and second regions with respect to the light irradiated to the first and second regions; and an optical means for extracting a non-uniform amount of light in the first and second regions; A second light condensing means that focuses the light symmetrically or asymmetrically, and by detecting the light collected by the second light condensing means, the position of the first light condensing means with respect to the information recording medium can be determined. A photodetection device comprising: detection means for detecting a change.