JP3461591B2 - Fiber optic plate - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fiber optical plate integrally formed by bundling a plurality of optical fibers. 2. Description of the Related Art A fiber optic plate (referred to as "FOP") as shown in FIG. 13 is conventionally known (US Pat. No. 4,932,776). The FOP 80 is obtained by bundling and integrating a large number of optical fibers 85, and has an input end face 81 cut at an angle with respect to the optical axis at one end. The finger 8 on the input end face 81
When the two are brought into close contact with each other, the concavo-convex pattern of the contact portion is displayed on the output end face 83 on the opposite side, and as a result, the image of the fingerprint 84 can be read. [0003] The tilt angle θ of the input end face 81 disclosed herein is desirably about 30 ° to 60 ° from the viewpoint of the occurrence of crosstalk and resolution. In order to keep the cutting angle of the input end face 81 in this range and to secure a space where a fingertip or the like can be in close contact with the input end face 81, it is necessary to form the FOP to a certain thickness. On the other hand, in recent years, downsizing of electronic parts and the like including optical devices has further progressed, and further downsizing of FOP has been desired. However, in order to make the FOP smaller, that is, to make it thinner, the cutting angle of the input end face 81 must be made more acute to secure a certain space in the input end face 81. However, as described above, there is a limit to the cutting angle of the input end face, which has been a major obstacle to downsizing. SUMMARY OF THE INVENTION It is an object of the present invention to provide a thinner FOP than in the prior art by eliminating the restriction on the cutting angle of the input end face in the FOP. Another object of the present invention is to provide an FOP capable of clearly outputting an image of a closely adhered portion thereof without unnecessary light such as background light being output from an output end face. . A fiber optical plate according to the present invention has a plurality of cores for transmitting incident light, a clad for covering the outer periphery of each core, and a clad for covering the outer periphery of each clad. And a light absorber that absorbs light leaked from the core and light incident on the clad. At one end of the fiber optic plate, there is provided an input end surface which is a surface for contacting the surface of the object and is inclined with respect to the optical axis of the optical fiber. The angle is set such that the light incident on the interface does not cause total reflection at the interface with the cladding. That is, the refractive index of the core is n ₀ ,
N ₁ the refractive index of the cladding, the critical angle of the refractive index of air n, the boundary surface of the core and the cladding beta, when the critical angle at the input end face and the _{γ, (1) ... n 0} sinβ = n 1 sin90
_{°, (2) ... n 0} sinγ = nsin90 °, (3) ...
The inclination angle of the input end face is set to be equal to or less than the angle α derived from the equation α + (90 ° + γ) + (90 ° −β) = 180 °. Incidentally, the fiber optical plate according to the present invention,
A configuration having an output end face parallel to the input end face may be adopted. By providing a light absorber so as to cover the outer peripheral portion of each clad, light leaked from the core and light propagating through the clad reach this light absorber. These lights are
Since the light is absorbed when the light enters the light absorber, the adjacent optical fibers are optically insulated by the light absorber. Therefore, it is possible to prevent the light propagating through each core from leaking and entering another adjacent core. This effect is independent of the angle of inclination of the input end face,
No matter how steep the inclination angle of the input end face is, the light incident into each core propagates through each core and is output from the output end face. When the inclination angle of the input end face is set to such a condition, a part of light propagating in the air and entering the core from the input end face leaks from the core to the clad and is absorbed by the absorber. Is done. Therefore, unnecessary light such as background light is gradually attenuated in the process of propagating through the core, and almost no light is output from the output end face. On the other hand, when the surface of the object is brought into close contact with the input end face, the light transmitted through the contact portion enters the core, and the incident light has such a condition that the condition of total reflection is satisfied at the boundary surface with the clad. Light incident at an angle will also be present. That is, only the light incident from the contact portion is output from the output end face. Embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 1 shows a fiber optic plate (hereinafter referred to as F
1 (referred to as OP). The FOP 1 is used in a fingerprint detection device or the like, which will be described later, and is integrally formed by bundling a plurality of optical fibers. One end of the FOP 1 has an input end face 2 inclined with respect to the optical axis of the fiber, and the opposite end has an output end face 3 formed substantially perpendicular to the optical axis. . As shown in FIG. 2, the FOP 1 includes a core 5 for transmitting the incident light, a clad 6 covering the outer periphery thereof, and an absorber 7 provided to cover the outer periphery of each clad 6. It is composed of three types. By providing the absorber 7 at this position, light leaked from the core 5 and light incident on the clad 6 are absorbed, so that adjacent optical fibers are optically insulated by the absorber 7. It has become. The core 5, the clad 6, and the absorber 7 are each formed of the material shown in FIG. 4. As a result, the refractive index of the core 5 is 1.621, and the refractive index of the clad 6 is 1.519. FIG. 1 shows each dimension as an example of the size of the FOP1. By forming the FOP 1 with the optical fiber in which the outer peripheral portion of the clad 6 is covered with the absorber, the light incident into each core 5 leaks from the core 5 and enters into another adjacent core 5. The incidence is prevented, the occurrence of crosstalk between adjacent fibers can be prevented, and the S / N ratio of the output image is improved. This action is independent of the inclination angle of the input end face, and no matter how steep the inclination angle of the input end face, the light incident into each core is propagated through each core and output from the output end face. Is what is done. Further, in the FOP 1 according to the present embodiment, by setting the inclination angle of the input end face 2 (the inclination angle of the optical fiber with respect to the optical axis) to a specific value, the excellent operation and effect described later can be obtained. To play. FIG. 3 shows a longitudinal section of the FOP1. In the figure,
n ₀ indicates the refractive index of the core, n ₁ indicates the refractive index of the cladding, and n indicates the refractive index of air. FOP1 of the present embodiment
Defines the inclination angle α of the input end face 2 so that the light receiving angle is 0 °. That is, the inclination angle α is set to such an angle that no total reflection occurs at the boundary surface between the core 5 and the clad 6 even when light enters the core 5 from the air at any angle. . The inclination angle α can be obtained from the following equation. N ₀ sin β = n ₁ sin 90 ° (condition of total reflection) (1) n ₀ sin γ = n sin 90 ° (condition of light receiving angle 0 °) (2) α + (90 ° + γ) + (90 °) −β) = 180 ° (3) In this case, according to FIG. 4, n ₀ = 1.621, n ₁ = 1.519, and n
= 1, it is obtained as α = 31.54 °. Therefore, when the inclination angle α in FIG.
Light that propagates in the air and enters from the input end face 2 does not cause total reflection at the interface between the core and the cladding, regardless of the angle of incidence. Accordingly, in the process of repeatedly propagating the light incident on the FOP from the input end face 2, a part of the light leaks from the core to the cladding, is absorbed by the absorber, and is gradually attenuated and disappears. When the inclination angle of the input end face 2 is set to such an angle as described above, light that propagates in the air and enters the FOP 1 from the input end face 2 is hardly output from the output end face 3. On the other hand, as shown in FIG. 5, when a specific object, for example, the surface of a finger 10 is brought into close contact with the input end face 2 in this state, the input end face 2 is brought into close contact with the input end face 2 due to the unevenness of the fingerprint. A non-contact portion 12 that forms a gap between the portion 11 and the input end face 2 is formed. In the contact portion 11, the input end face 2 and the surface of the finger 10 are in close contact with each other, and the refractive index of the finger surface is larger than the refractive index of air. Therefore, the input end face 2
The relationship between the angle of incidence of the light incident on and the condition of total reflection at the interface with the cladding changes from the relationship before adhesion,
At the interface with the cladding, there is also light that enters at an angle that satisfies the condition of total reflection. For example, it is assumed that light is emitted from behind the finger 10 and the transmitted light reaches point A (FIG. 5). Of the light that enters the FOP 1 from this point A, as shown in the figure, the light that enters within the range of 40.68 ° satisfies the condition of total reflection at the interface between the core 5 and the clad 6 ((2) From the equation), this incident light repeats total reflection at the boundary surface with the cladding, and is output from the output end face 3 side. Light incident beyond this angle range is attenuated and disappears in the process of repeatedly propagating refraction. On the other hand, in the non-contact portion 12, a gap 13 as shown is formed, in which air is present. That is, there is no difference from the state before the finger 10 is brought into close contact. Therefore, although light enters the core 5 also from the non-contact portion 12 of the input end face 2, the incident light at any angle still does not satisfy the condition of total reflection at the interface with the clad, and the core 5 Decays and disappears in the process of propagating inside. Therefore, when the surface image of the object is detected as described above, only the light incident from the projection on the surface of the object that is in close contact with the input end face 2 is output from the output end face 3 and is incident from the recess on the surface. Light attenuates and disappears during the propagation process.
For this reason, unnecessary light such as background light is removed, and only the image of the portion in close contact with the input end face is transmitted and output. The above-described example is based on NA (numerical aperture).
= 0.55, and Table 1 shows the inclination angles α with respect to other main NAs. [Table 1] Further, the shape of the input end face 2 of the FOP 1 can be made as shown in FIG. The shape in FIG. 7 shows a cross section obtained by cutting the input end face 2 along the line I-I ', and the arrow y in the figure shows the optical axis of each fiber constituting the FOP 1 (FIG. 6). FIG. 7A shows the input end face 2 having a planar shape as shown in FIG.
In (e), the input end surface 2 is concave, and the contact surface with the finger is enlarged. Further, in FIGS. 6F and 6G, the input end face 2 is formed in a slanted shape, so that the image of the part pressed by the input end face 2 can be enlarged and output as compared with FIG. Can be. The FOP 40 may be formed as shown in FIG. In the FOP 40 shown in FIG.
To the through hole 4
No. 1 was formed and its inner surface was polished. When a finger is inserted into the through hole 41 along the arrow c, the finger is fixed in the through hole 41, and fingerprint detection with good reproducibility can be performed. In this case, the arrow d
Light for illumination is emitted from the direction. In the example of FIG.
Although the P40 main body is machined to be integrally formed, for example, the same applies to the case where a pressing portion for pressing the inserted finger toward the input end surface 2 is provided at a position facing the input end surface 2 in FIG. The effect of is obtained. Here, a specific configuration of an apparatus provided with such an FOP 1 will be described. FIG. 9 shows a fingerprint detection device, which comprises the above-described FOP1, the CCD 20, the computer 21, and the like. The input end face 2 of FOP1 is
It is formed so as to be inclined with respect to the optical axis, and the inclination angle is set to be equal to or less than the inclination angle α obtained by the above-described equations (1) to (3). Press the finger 22 against this surface to bring it into close contact,
Light is emitted from a light source 23 provided behind or on the side surface. When the contact is performed in this manner, a concavo-convex pattern (fingerprint image) of the contact portion is transmitted to the output end face 3, the image data is given to the CCD 20, and the signal is subjected to image processing in the computer 21. . FIG. 10 shows another configuration example. In this example, a lens 30 is arranged on the output end face 3 side of the FOP 1, an image output to the output end face 3 via the lens 30 is read by a line sensor 31, and the signal is subjected to image processing in a computer 32. Has become. In the illustrated devices of FIGS. 9 and 10,
It is desirable that the angle of the input end face 2 of the FOP 1 is set to be equal to or smaller than the inclination angle α such that the light incident from the input end face 2 does not satisfy the condition of total reflection at the interface with the clad. In this case, since the FOP 1 can be formed thin, the surface image of the object output from the output end face 3 can be reduced, and the CCD 20, the lens 30, the line sensor 31, and the like can be reduced in size. It is possible to proceed. Further, the FOP may be shaped as shown in FIG. This FOP plate 50 is similar to the FOP 1 of FIG.
The output end face 3 is cut in parallel with the input end face 2, and the inclination angle θ between the optical axis of each optical fiber constituting the FOP plate 50 and the input end face 52 is equal to the inclination angle α shown in FIG. The conditions are met. The output end face of the FOP plate 50 has a CC for detecting an image obtained from the output end face.
D53 is arranged, and FO is applied to the input surface 53a of the CCD 53.
The output end face of the P plate 50 is fixed and integrally formed.
With this configuration, a fingerprint image can be detected with a compact configuration. As an example of the use of the FOP, as shown in FIG. 12, two FOPs can be connected in cascade. In this case, FOP1a is
The OP1b is disposed at an angle of 90 °, and the input end face of the FOP1b is fixed to the output end face of the FOP1a. In this case, the inclination angles θ ₁ and θ ₂ of one of the input end faces of the two FOPs _{1 a} and ₁ b need only satisfy the above-described condition of the inclination angle α. And, for example, F
By fixing the CCD 53 to the output end face of the OP 1b, it is also possible to detect a fingerprint image or the like of the finger. In each of the embodiments described above, an example is shown in which a finger is brought into close contact with an input end face such as FOP1 for detecting a fingerprint. However, the present invention is not limited to this example. , Palm, nose, etc.
It can be used for detecting palm squeezing, nose squeezing, and the like. Also,
Although the output end face 3 has been described as being inclined at right angles to the optical axis, the present invention is not limited to this example. The output end face 3 may be arbitrarily inclined with respect to the optical axis. According to the fiber optical plate of the present invention, since the light absorber is provided so as to cover the outer periphery of each clad, the adjacent optical fibers are optically separated by the light absorber. In this state, light propagating through each core can be prevented from leaking and entering another adjacent core. Since this effect is achieved irrespective of the inclination angle of the input end face, the inclination angle (cutting angle) of the input end face is not restricted as in the related art. For this reason, the fiber optic plate is formed thinner than before,
Even when the input end face is inclined at an acute angle, a surface image of the object brought into close contact with the input end face can be accurately transmitted to the output end face. At this time, the inclination angle of the input end face is set to an angle such that light incident on the core from the air does not cause total reflection at the interface with the clad, that is, the refractive index of the core is n ₀ , n ₁ the refractive index of the cladding, the critical angle of the refractive index of air n, the boundary surface of the core and the cladding beta, when the critical angle at the input end face and the _{γ, (1) ... n 0} sinβ = n 1
sin 90 °, (2)... n ₀ sin γ = nsin 90
°, (3) ... α + (90 ° + γ) + (90 ° −β) = 1
Since the angle is set to be equal to or less than α derived by the equation of 80 °, when the surface of the object is brought into close contact with the input end face, unnecessary light such as background light is not output from the output end face,
It is possible to output an image of only the contact portion.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view showing a fiber optic plate according to an embodiment of the present invention. FIG. 2 is a partial cross-sectional view of a fiber optic plate. FIG. 3 is a partial longitudinal sectional view of a fiber optic plate. FIG. 4 shows the composition of each material of the core, clad and absorber in% by weight.
5 is a table showing the refractive indexes of the core and the clad. FIG. 5 is a longitudinal sectional view of the fiber optic plate, showing a state in which the surface of a finger is brought into close contact with the input end face of the fiber optic plate, in a partially enlarged manner. FIG. 6 is a schematic view showing a side surface of a fiber optic plate. FIG. 7 shows the fiber optic plate of FIG.
It is a figure which shows the cross section cut | disconnected by l 'line, and figure (a)-figure (g) is sectional drawing which illustrates the shape of an input end surface. FIG. 8 is a schematic configuration diagram showing a fingerprint detection device provided with a fiber optical plate. FIG. 9 is a diagram schematically showing another device configuration including a fiber optic plate. FIG. 10 is a schematic diagram illustrating a side surface of a fiber optic plate. FIG. 11 is a diagram schematically showing another device configuration including a fiber optic plate. FIG. 12 is a diagram schematically showing another device configuration including a fiber optic plate. FIG. 13 is a perspective view showing a conventional fiber optical plate. [Description of Signs] 1 ... Fiber optical plate, 2 ... Input end face, 3 ... Output end face, 5 ... Core, 6 ... Clad, 7 ... Absorber (light absorber).

────────────────────────────────────────────────── (5) References JP-A-3-228464 (JP, A) US Patent 4,932,776 (US, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G02B 6 / 42 G02B 6/04

Claims (1)

(57) [Claim 1] A fiber optic plate integrally formed by bundling a plurality of optical fibers, wherein the fiber optic plate has a plurality of cores for propagating incident light. And a clad that covers an outer peripheral portion of each of the cores,
A light absorber that is provided so as to cover an outer peripheral portion of each of the claddings and absorbs light leaked from the core and light incident on the cladding; the fiber optical plate has irregularities on its surface. an input end surface inclined with respect to the optical axis of the optical fiber , the input end surface being parallel to the input end surface;
The input end face has an inclination angle of n ₀ , a refractive index of the cladding of n ₁ , a refractive index of air of n, and a critical angle at a boundary surface between the core and the cladding. A fiber optic plate wherein the angle is set to be equal to or less than the angle α derived by the following equations (1) to (3), where β is the angle and γ is the critical angle at the input end face. (1)... N ₀ sin β = n ₁ sin 90 ° (2)... N ₀ sin γ = nsin 90 ° (3)... Α + (90 ° + γ) + (90 ° −β) = 18
0 °