JPS6248164B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to optical displacement detection, and more particularly to an optical displacement detection device that uses the photoelectric detector to detect displacement of a focused optical image created by focusing a light beam on a photoelectric detector. .

Conventionally, a displacement detection device has been used that guides a light beam emitted or reflected from an object onto a photoelectric detector, and uses the photoelectric detector to optically detect the displacement of the object. In particular, the displacement detection device is often used as an error detection means for controlling the position of the object. The principle of optical displacement detection will be explained below with reference to the drawings.

FIG. 1 is a conceptual diagram showing the principle of optical displacement detection. In the figure, reference numeral 1 denotes an object whose displacement is to be detected, and a reflecting mirror 2 and a convex lens 3 are fixed to the object 1. The light beam 7 is incident on the reflecting mirror 2 parallel to the direction X in which the displacement of the object is to be detected, and thereby the light beam 7 is deflected in a direction perpendicular to the X direction. The deflected light beam 7 is focused on a point S by the convex lens 3. Object 1
It is clear that when is displaced in the X direction, the focal point S of the light beam 7 is displaced in the same direction and by the same distance as said object 1.

On the other hand, the optical displacement detection device includes a condenser lens 4,
It is composed of photoelectric detectors 5a, 5b and a differential amplifier 6. The condenser lens 4 converges the light beam 7' that diverges again from the convergence point S, and
A light focused image S', which is the above-mentioned focusing point S, is formed at the boundary between the two photoelectric detectors 5a and 5b. The convex lens 3 and the condensing lens 4 constitute a light converging means. When the above-mentioned focusing point S is displaced, its light focused image
S' is displaced in the opposite direction and its magnitude is (-b/a) times the magnitude of the displacement of the focal point S. However, a is the distance between the focusing point S and the focusing lens 4, and b is the distance between the image S' and the focusing lens 4. The focused image S' is transmitted to the two photoelectric detectors 5
When displaced perpendicular to the boundary line at the boundary between a and 5b,
The difference between the outputs of the two photoelectric detectors 5a and 5b changes, and the displacement of the object 1 is detected by the output of the differential amplifier 6. At this time, in order to increase the SN ratio of the displacement detection signal and reduce the influence of temperature drift of the differential amplifier 6, it is desirable that the displacement detection sensitivity be as high as possible.

The displacement detection sensitivity is the product of the displacement amplification factor of the optical system and the image displacement detection sensitivity of the photoelectric detector. Here, the displacement amplification factor of the optical system means the displacement position of the light focused image S' with respect to a unit amount of displacement of the object 1, and the image displacement detection sensitivity of the photoelectric detector means the displacement amplification factor of the light focused image It means the amount of change in the output difference between the two photoelectric detectors 5a and 5b with respect to a unit amount of displacement of the image S'. At this time, it is clear that the displacement amplification factor of the optical system is b/a. Therefore,
By increasing the b/a ratio, it is possible to increase the displacement amplification factor of the optical system. However, the value of a is often limited to a value larger than a certain value due to other factors such as lens aberration and working distance. In this case, in order to increase the displacement amplification factor of the optical system, it is necessary to increase the value of b, which has the disadvantage that the optical path length becomes longer and the optical system becomes larger.

The present invention eliminates the above drawbacks by increasing the image displacement detection sensitivity of a photoelectric detector, and makes it possible to increase the displacement detection sensitivity without changing the optical system. Hereinafter, the present invention will be explained in detail with reference to the drawings.

Figure 2 is a one-dimensional model for theoretically deriving the image displacement detection sensitivity of photoelectric detectors, and shows the light sensitivity distribution of two photoelectric detectors and the light intensity distribution of the light focused image formed on the photoelectric detector. This is what is shown. In the figure, a portion is a first photoelectric detector, a portion is a second photoelectric detector, and a portion is a boundary portion between the two photoelectric detectors. Also, in the same figure, q ₁ (x), q ₂ (x)
represent the photosensitivity distribution of the first photoelectric detector and the second photoelectric detector, respectively, and f(x) represents the light intensity distribution of the light focused image. At this time, the output Po ₁ of the first photoelectric detector and the output Po ₂ of the second photoelectric detector are respectively Po ₁ =∫ ^∞ _−∞ f(x)・g ₁ (x) dx……(1) Po ₂ =∫ ^∞ _−∞ f(x)・g ₂ (x)dx……(2). If the focused light image S′ is displaced by u along the x-axis, the output P ₁ (u) of the first photoelectric detector and the output P ₂ (u) of the second photoelectric detector are respectively
From equations (1) and (2), P ₁ (u)=∫ ^∞ _−∞ f(x-u)・g ₁ (x)dx
………(3) P ₂ (u)=∫ ^∞ _−∞ f(x−u)・g ₂ (x)dx
......(4) becomes. Therefore, the differential output of the two photoelectric detectors, that is, the difference in output Pd(u) at this time, is Pd(u)=P ₁ (u)−P ₂ (u)=∫ ^∞ _−∞ f(x− u)・{g ₁ (x)−g ₂ (x)}dx (5). The differential value of the above differential output with respect to u when u = 0, that is, the image displacement detection sensitivity Sd, is expressed by equation (5) as u
Differentiating with Sd=dPd(u)/du|u=0=∫ ^∞ _−∞ df(x)/dx{g ₂ (x) − g ₁ (x)}dx. Integrate the above equation by parts and get Sd=[f(x)・{g ₂ (x) − g ₁ (x)}] ^∞ _−∞ −∫ ^∞ _−∞ f(x)・{dg ₂ (x)/dx −dg ₁ (x)/
dx}dx……(6) The focused light reflection S′ spreads within the range of x ₁ < x < x ₂ on the x coordinate, and f(x) = 0 elsewhere. For example, the first term of equation (6) becomes zero, and Sd=∫ ^x2 _x1 f(x)・{dg ₁ (x)/dx−dg
₂ (x)/dx}dx……(7) is obtained. Equation (7), that is, image displacement detection sensitivity, is calculated by integrating the light amount of the focused image S' along the x-axis by weighting it by the difference in slope of the sensitivity distribution curves of the first and second photoelectric detectors. It shows what will happen.

The sensitive area in which a photoelectric detector is sensitive generally includes an area where the photodetection sensitivity is uniform (hereinafter referred to as a constant sensitivity area), and around this area, the photodetection sensitivity decreases as the distance from the constant sensitivity area increases. and a decreasing region (hereinafter referred to as the slope sensitivity region).

FIG. 3 shows a sensitivity distribution curve of a general photoelectric detector. Most of the light receiving part of the photoelectric detector is in a constant sensitivity region (portion A in the figure), and the light detection sensitivity is approximately constant, _S0 . There is a gradient sensitivity region (portion B in the figure) around it, and in this region, the photodetection sensitivity monotonically decreases as it moves away from the constant sensitivity region. The above-mentioned gradient sensitivity region is usually about 10 μm or more. As shown by equation (7), the image displacement detection sensitivity of the photoelectric detector is determined by the light intensity in this gradient sensitivity region.

On the other hand, the light intensity distribution of the focused light image S', when considered one-dimensionally degenerate, generally shows a distribution in which the intensity is strong at the center and weak at the periphery.

From the above relationship, in order to increase the image displacement detection sensitivity of the photoelectric detector, it is sufficient to increase the light intensity of the light focused image in the above gradient sensitivity region.
Therefore, it is clear that the boundary portion in FIG. 2 should be made as narrow as possible. For this purpose, it is preferable to integrate two light receiving cells on one photodetector instead of using two photoelectric detectors, and conventionally, photoelectric detectors that integrate multiple photodiodes have often been used. There is. However, in conventional methods, when the photodiodes of a photodetector are placed very close together, separation is applied to prevent crosstalk between each photodiode and overlapping of sensitive areas, so it is difficult to place them closer than a certain level. It was hot. Therefore, there is a limit to increasing the image displacement detection sensitivity of the photoelectric detector.

The inventors derived the above equation (7) in order to investigate in detail the image displacement detection sensitivity of a photoelectric detector including two photodiodes having the above-mentioned crosstalk and overlapping of sensitive regions. It has been found that by using two superimposed photodiodes, greater image displacement detection sensitivity than before can be obtained.

The case where there is crosstalk between photodiodes or overlapping of sensitive regions will be described in detail below.

When two photodiodes are placed close to each other on the same substrate, their respective sensitive areas overlap each other in their gradient sensitive areas. This is because carriers excited by the light incident on the boundary between the two photodiodes are distributed to the electrodes of each photodiode at a predetermined ratio. Since this ratio changes depending on the distance from the electrode of each photodiode, the sensitivity changes depending on the location in this boundary region. That is, it becomes a slope sensitivity region. On the other hand, crosstalk between two photodiodes occurs when carriers are injected due to a potential difference between the photodiodes caused by photocurrent.

The above crosstalk can be reduced to an almost negligible level by reducing the load resistance of the photodiode or by using circuit techniques such as directly detecting the photocurrent with a low input impedance circuit such as a common base circuit. can.

On the other hand, it is derived from equation (7) that the superimposition of the sensitive regions does not impair the image displacement detection sensitivity at all, but rather improves it. In order to approximately obtain the relationship between the first crosstalk and image displacement detection sensitivity, consider the simplified one-dimensional model shown in FIG. 4. Generally, the electric field intensity distribution of a focused light image often exhibits a normal distribution or a distribution close to a sampling function. Here, since the generality of the present invention is not impaired in any way even if a normal distribution is assumed, the intensity distribution of the focused light image is assumed to be a normal distribution in which the light intensity at the center is 1. The optical sensitivities of the first and second photodiodes are S ₁ (x) and S ₂ (x), respectively, and are constant at S ₀ in the constant sensitivity region, and change linearly in the slope sensitivity region. Assume that its width is w. still,
The x-coordinate is normalized by the diameter of the focused light image defined at the point where the light intensity drops to 1/e ² of the center, and the center of the focused light image and the center of the boundary between the two photodiodes coincide with the origin. It was decided that When the coordinates of the constant sensitivity region and tilt sensitivity region of the first and second photodiodes are −x ₀ and x ₀ , respectively, the relationship between the width w of the tilt sensitivity region and the image displacement detection sensitivity Sd is expressed as x ₀ as a parameter. FIG. 5 shows what is shown as . As is clear from the graph in the same figure, in the range w > x ₀ , that is, in the range where the slope sensitivity regions of the first and second photodiodes overlap each other, w <
It can be seen that the image displacement detection sensitivity is improved compared to the case of x ₀ .

Based on the above facts, the present invention increases the image displacement detection sensitivity of a photoelectric detector by overlapping the gradient sensitivity regions of two photodiodes with each other. The present invention also brings about the following effects. That is, when the incident light to the photoelectric detector is modulated by an information signal and the information signal is reproduced from the summed output of each photodiode of the photoelectric detector, it is possible to increase the reproduction output of the information signal. It is possible to improve the signal-to-noise ratio. In particular, if w = 2x ₀ and the optical sensitivities of the first and second photodiodes are complementary to each other and satisfy S ₁ (x) + S ₂ (x) = S ₀ within the gradient sensitivity region, then the incident The amount of light is converted into an electric signal without wastage, and the reproduction output of the information signal is maximized.

These effects of the present invention are recognized when x ₀
The value of is large enough to be 20% of the light intensity of the focused light image.
The above is the case where the light is incident on the gradient sensitivity region of the photodiode. When the above light amount is 20% or less, sufficiently large image displacement detection sensitivity and information signal reproduction output can be obtained even with the conventional configuration.
The improvement effect of the present invention is hardly recognized.

Next, the present invention will be explained in more detail with reference to Examples.

FIG. 6 is a schematic block diagram showing an embodiment using an optical displacement detection device according to the present invention for focus control of an optical reproducing head. Irradiation light flux 11
is focused onto an information recording carrier 13 (hereinafter referred to as a disk) by an objective lens 12 to form a light spot S.
image. The irradiation light beam 11 is reflected by the disk 13 and converged again by the objective lens 12 to obtain a reflected light beam 15. The reflected light beam 15 is separated from the incident light beam 11 by a beam splitter 14 and deflected to the right in the figure. Half of the reflected light beam 15 separated by the beam splitter 14 (the lower half in the figure) is blocked by the knife edge 16, and the remaining half (the upper half in the figure) of the reflected light beam 15' is sent to the photoelectric detector 18 by the condenser lens 17. Each photodiode 18a, 1
The light is focused on the boundary portion 18c of 8b. At this time, the boundary portion 18c is arranged parallel to the dividing line of the reflected light beam 15 (the tip of the knife edge in the figure). FIG. 7 shows how the focusing state of the reflected light beam 15' focused on the photoelectric detector 18 changes due to the focus error of the optical spot S focused on the disk 13 at this time. Figure 7 shows reflected light flux 1
FIG. 5 is an enlarged view of the vicinity of the photoelectric detector 18 to show the focused state of the photodetector 5'. Figure a shows that when the light spot S is correctly focused on the disk 13,
b indicates a case where the distance between the disk 13 and the objective lens 12 is too short, and c indicates a case where the distance between the disk 13 and the objective lens 12 is too long.
As shown in Figure a, the focused point of the reflected light, ie, the focused image S' of the light spot S, is formed on the boundary portion 18c of the photoelectric detector 18 so that the amounts of incident light on the two photodiodes are equal. Therefore, no difference occurs between the outputs of the photodiodes 18a and 18b. However, when a focus error occurs, as shown in b and c in the figure, the position of the focused light image S' moves to the front or back of the photoelectric detector 18, causing a difference in the outputs of the photodiodes 18a and 18b. It becomes like this. Therefore, a focus error can be detected from the difference in the outputs of the photodiodes 18a and 18b of the photoelectric detector 18. 7th
The figure shows a case where the focus error is very large, but in actual use, the light spot S on the disk 13 is controlled within the depth of focus by focus control, so the focused light image S on the photoelectric detector 18 The movement back and forth of ' also only occurs within the depth of focus. Therefore, a light focused image formed on the photoelectric detector 18
S' is almost never increased in size due to focus error, and it is assumed that the position of the focused light image S' on the photoelectric detector 18 is only displaced in the direction perpendicular to the boundary portion 18c. Can be done. That is, in this embodiment, the objective lens 12,
A light focusing means consisting of an optical system including a knife edge 16 and a focusing lens 17 converts the displacement of the disk 13 in the direction parallel to the optical axis into the direction perpendicular to the optical axis of the light focused image S' on the photoelectric detector 18. Convert to displacement.
The focal length of the objective lens is f ₁ , the focal length of the condensing lens 17 is f ₂ , and the irradiation light beam 11 is directed to the objective lens 12
If the intensity around the entrance pupil of is normally distributed such that it is 1/e ² of the center, then the condenser lens 1
At the center position of the weighted average of the light beams incident on the light beam 7 according to their intensity, the height from the optical axis of the focusing lens 17 is about 1/3 of the radius of the reflected light beam 15. Therefore, the displacement amplification factor g ₀ of the optical system is g ₀ ≈2(NA) f ₂ /3f ₁ (8) (where NA is the numerical aperture of the objective lens 12).

On the other hand, the photoelectric detector 18 has a sensitivity distribution (a) and a configuration (b) as shown in FIG. 8. In the figure, 21 is an n-type semiconductor substrate, 22 is an intrinsic semiconductor layer, and 23 and 24 are first and second semiconductor layers, respectively.
This is a p-type diffusion layer that becomes the anode of the photodiode. Further, S ₁ (x) and S ₂ (x) are sensitivity distribution curves of the first and second photodiodes, respectively. Since the diameter of the light focused image S' by the optical system is about 30 μm due to spherical aberration, the amount of light incident on the tilt sensitivity region of the photodiodes 18a and 18b is 1/3 of the total amount of light. At this time, the displacement detection sensitivity Sd of the photodiodes 18a and 18b with respect to the displacement of the disk 13 is Sd=2・S ₀・I ₀ /3W, since the light spot S changes substantially twice as much due to the displacement of the disk 13. ₀ ......(9) (However, I ₀ is the total light amount of the focused light image S').

If f ₁ = 4 mm, f ₂ = 16 mm, and NA = 0.5, the displacement detection sensitivity g _d for the displacement of the disk 13 is (8)
From equation (9), g _d = g ₀ · Sd = (2 · S ₀ · I ₀ /9W ₀ ) · (f ₂ /f ₁ )≒8S ₀ I ₀ /9W ₀ (10). In other words, if W ₀ = 15μm, then 1μ
The difference in photocurrent of the two photodiodes caused by a focus error of m is about 6%.

FIG. 9 shows an example of the configuration of the first stage of the detection circuit in this embodiment. If current is detected by operational amplifiers 31 and 32 as shown in the figure, the input impedance will be low, and the output terminals of photodiodes 33 and 34 will be virtual grounded, so harmful crosstalk between photodiodes will be almost eliminated. .

In addition, in this example, the spacing between the p-type diffusion layers of each photodiode is made sufficiently small, so that the depletion layers of each photodiode generated in the intrinsic semiconductor layer are connected to each other. It also has the advantage that a sufficient response speed can be obtained even in the slope sensitivity region between the two photodiodes, and an information signal can be reproduced by the sum of the outputs of the two photodiodes.

[Brief explanation of the drawing]

Figure 1 is a conceptual diagram showing the principle of optical displacement detection.
Figure 2 is a one-dimensional model diagram for theoretically deriving detection sensitivity, Figure 3 is a sensitivity distribution curve diagram of a conventional photoelectric detector, Figure 4 is a simplified one-dimensional model diagram, and Figure 5 is a simplified one-dimensional model diagram.
The figure is a characteristic diagram showing the calculated value of detection sensitivity, Figure 6 is a diagram showing the main part of an embodiment of the present invention, and Figure 7 a, b, c.
8 is an enlarged view of the main part showing the principle of focus detection according to this embodiment, FIGS. 8a and 8b are diagrams showing an example of the configuration of the photoelectric detector of this embodiment, and FIG. FIG. 3 is a wiring diagram showing a configuration example. 3... Convex lens, 4, 17... Condensing lens, 5
a, 5b, 18...Photoelectric detector, 7, 7'...Light beam, 11...Irradiation light flux, 15, 15'...Reflection light flux, 16...Knife edge, 18a, 18
b, 33, 34...Photodiode, 18c...
...boundary portion, 21...n-type semiconductor substrate, 22...
Intrinsic semiconductor layer, 23, 24... p-type diffusion layer, 3
1, 32... operational amplifier.

Claims (1)

[Scope of Claims] 1. A light focusing means for focusing a light beam to form a light focused image on a photoelectric detector, and the photoelectric detector has a constant sensitivity region with constant light detection sensitivity and its surroundings. a plurality of photodiodes having a gradient sensitivity region in which the light detection sensitivity decreases as the distance from the constant sensitivity region increases, and at least two specific photodiodes among the plurality of photodiodes In the diode, the slope-sensitive regions overlap and adjoin each other at a predetermined boundary portion, and the light focusing means focuses the light spot so that at least a part of the light amount of the light-focused image is in the overlapping portion of the slope-sensitive regions. An optical displacement detection device, characterized in that the optical displacement detection device is formed such that the light is incident on the photodiode, and detects the displacement of the focused light image based on the difference in the outputs of the specific photodiodes. 2. In the description of claim 1, the specific photodiode has gradient sensitivity regions that overlap each other over almost the entire width of the gradient sensitivity region at the predetermined boundary portion, and in the gradient sensitivity region, An optical displacement detection device characterized in that the photodetection sensitivities of the respective photodiodes are configured to change approximately complementary to each other.