JP5054931B2 - Optical sensor - Google Patents

Description

  The present invention relates to an optical sensor that detects pressure, temperature, and the like by detecting reflected light reflected by an object that is displaced (position variation) due to pressure, temperature, and the like.

  Conventionally, electric sensors have been proposed as sensors for detecting physical quantities such as temperature and pressure. In this electric sensor, when a detection signal (electric signal) is transmitted to a remote place through an electric wire, it is susceptible to electromagnetic noise, which causes a detection error.

  On the other hand, in an optical sensor, a detection signal can be converted into an optical signal and transmitted to a remote place through an optical fiber, so that the signal can be transmitted without being affected by electromagnetic noise. Therefore, the optical sensor has an advantage that detection with extremely high accuracy with few detection errors is possible.

  As such an optical sensor, a strain change body such as a Bourdon tube having a mechanism distorted by pressure is used to convert a pressure change into a strain change, and this strain is detected by a fiber grating (FBG). Proposed.

  There has also been proposed an optical sensor that detects pressure, temperature, and the like by detecting reflected light reflected by an object to be displaced (position variation) due to pressure, temperature, and the like. That is, a reflector (diaphragm, etc.) that is displaced by pressure is disposed opposite the end face of the optical fiber, and the amount of displacement of the reflector is detected by detecting the reflected light from the end face of the optical fiber. An optical sensor that detects the pressure from the amount of displacement has been proposed.

  As an optical sensor for detecting the amount of displacement of the reflecting plate, Patent Document 1 describes an interference state caused by a plurality of reflected lights and a beat frequency. Patent Document 2 describes an optical sensor in which a vibrator is fixed to a reflecting plate and a resonance frequency of the vibrator is detected. However, these optical sensors have a problem that signal processing is difficult and complicated arithmetic processing is required.

  On the other hand, there is an optical sensor that detects a change in the intensity of reflected light from the reflecting plate as an optical sensor that can detect the amount of displacement of the reflecting plate with a simple structure. In this optical sensor, the detection mechanism can be composed of inexpensive materials and parts, and signal processing after detection can be performed very easily. On the other hand, since the light intensity of the reflected light is detected, a factor other than the displacement of the reflector, for example, a change in intensity due to transmission loss or the like causes a detection error, and the error is superimposed on the detected value. There is. However, it is possible to increase the detection accuracy by making the change in transmission loss very small or compensating by taking a ratio of a plurality of reflected light intensities.

  For example, in Patent Document 3, first and second optical fibers having different parameters such as core diameter, core part refractive index distribution, and numerical aperture (NA) are fixed in parallel. An optical sensor is described in which an end face of a fiber is opposed to a reflecting plate and the optical axis direction of the optical fiber and the normal line of the reflecting surface are parallel to each other.

  In this optical sensor, the reflected light emitted from the end face of the first optical fiber and reflected by the reflecting plate is received by the second optical fiber, and the reflecting light is emitted from the end face of the second optical fiber. The light reflected by the first optical fiber can be received by the first optical fiber. If the reflected light received by each optical fiber is branched by an optical branching device and converted into an electrical signal by a photoelectric converter, this electrical signal is a signal indicating the intensity of the reflected light. By taking the ratio of these electric signals, it is possible to compensate for the effect of transmission loss. Further, in this optical sensor, light emitted from each optical fiber, reflected by a reflecting plate, and returned to the same optical fiber may be detected. Furthermore, the intensity of the reflected light of the light emitted from each optical fiber by using a total of four optical fibers, two optical fibers for emission and two optical fibers for light reception, without using an optical splitter. It can also comprise so that ratio can be taken.

  Patent Document 4 and Patent Document 5 describe a multimode in which the object to be detected is a liquid surface, the coupling with a light-emitting diode light source is easy, and the light intensity can be increased because the core diameter is large. An optical sensor using a plurality of fibers to detect displacement of the liquid level is described. The plurality of optical fibers are fixed at an angle Φ (0 ° ≦ Φ ≦ 5 °) with respect to the normal of the liquid level.

Japanese Patent No. 3306696 (page 15, Fig. 2) JP 2003-214966 A (4th page, Fig. 1-4) Japanese Examined Patent Publication No. 6-8724 (page 3-4, Fig. 1-2) US Pat. No. 4,996,418 (FIGS. 1, 15, 16) US Pat. No. 5,068,527 (FIG. 1)

  By the way, in the optical sensor described in Patent Document 3, since the optical axis direction of the optical fiber and the normal line of the reflecting surface are parallel, the reflected light with respect to the change in the distance between the optical fiber and the reflecting plate. When changing the intensity change amount (detection sensitivity), it is necessary to produce special optical fibers having different parameters. Therefore, in this optical sensor, it is difficult to set detection sensitivity appropriately.

  In addition, since the end face of the optical fiber is perpendicular to the optical axis direction, Fresnel reflection occurs at the boundary between the end face and the air layer, resulting in an increase in intensity loss of the emitted light and fluctuations in the received light intensity. There is a possibility that the detection accuracy is deteriorated. Furthermore, multiple reflection is likely to occur between the end face of the optical fiber and the reflecting plate, and the detection accuracy may be deteriorated due to the multiple reflection.

  And, as in the optical sensors described in Patent Document 4 and Patent Document 5, using a multimode fiber is useful in that the light intensity can be increased, but the multimode fiber is vulnerable to bending. There's a problem. That is, when a multimode fiber is used, the mode propagation state changes due to the bending of the fiber, so that there is a problem that the numerical aperture is easily changed and the loss is also changed.

  In addition, the use of a light-emitting diode (LED) as a light source is advantageous from the standpoint of manufacturing cost reduction and polarization dependence, but the emission intensity spectrum of a light-emitting diode changes with temperature. There is a problem that the emission intensity is attenuated when the temperature rises and the emission peak wavelength is shifted to the long wave side. Therefore, when a light-emitting diode is used as the light source, if an optical coupler or optical filter having wavelength dependency is used, the detection value fluctuates and the detection error increases.

  Furthermore, when a light-emitting diode is used as a light source, if an optical fiber with a large numerical aperture wavelength dependency is used, the numerical aperture of the optical fiber changes significantly due to the emission wavelength variation of the light-emitting diode, and the beam shape of the emitted light And the amount of light changes, and the coupling efficiency due to the reflecting surface changes. Such fluctuations in coupling efficiency cannot be compensated for, which leads to deterioration in detection accuracy. In the case of digital use such as optical communication, the influence of the bending loss of the optical fiber is small, but when used for analog transmission as in this optical sensor, the influence of the loss due to bending is very large and detection The accuracy will be deteriorated.

  The present invention has been made in view of the above problems, and its object is to reduce the size and improve the mechanical reliability by eliminating the bending of the bare fiber portion in the detection portion of the optical sensor. In addition, an object of the present invention is to provide an optical sensor that can suppress detection accuracy deterioration due to a change in emission wavelength spectrum of a light source and can perform highly accurate detection.

  In order to solve the above-described problems and achieve the above object, an optical sensor according to the present invention has any one of the following configurations.

[Configuration 1]
The optical sensor according to the present invention has a light source and light emitted from the light source incident on one end side, propagates the light to the other end side, emits the light from the end face on the other end side, and is relative to the end face. A first optical fiber for irradiating the detected object disposed at a distance away from the detection object; and an end surface of the first optical fiber that is directed toward the detected object and parallel to the first optical fiber at least near the end surface; A second optical fiber disposed at a predetermined distance from the fiber and reflected by the object to be detected and reflected from the end surface; and a second optical fiber having the end surface directed toward the object to be detected and at least near the end surface Output light reflected by the object to be detected and arranged parallel to the first optical fiber and spaced apart from the second optical fiber in the same direction as the second optical fiber. Reflected light from the end face A third optical fiber, first and second light receiving elements that receive the reflected light propagated by the second and third optical fibers, convert them into electrical signals, and output the first and second light receiving elements, respectively. And an arithmetic processing means for calculating the intensity ratio based on the light intensity detected by the second light receiving element and calculating the displacement amount of the detected object from the intensity ratio, and the end face of the first optical fiber is The inclined surface is an inclined surface with respect to the optical axis, and is inclined toward the opposite side of the direction in which the second and third optical fibers are disposed.
[Configuration 2]
In the optical sensor having the configuration 1, when the light intensities detected by the first and second light receiving elements are P1 and P2, the intensity ratio F (P1, P2) is expressed by the following equation: It is characterized by calculating by these.
F (P1, P2) = (P2-P1) / (P1 + P2)

[Configuration 3 ]
The present invention is characterized in that, in the optical sensor having the configuration 1 or the configuration 2 , the end surfaces of the first to third optical fibers are coated with a non-reflective coating.

[Configuration 4 ]
The present invention, in the optical sensor according to any of compositions 1 through 3, the end faces of the second and third optical fibers is an inclined surface with respect to the optical axis, the first optical fiber is disposed The inclined surface is directed to the opposite side of the formed direction.

  In the optical sensor according to the present invention, by having the configuration 1, the end surface of the first optical fiber is inclined with respect to the optical axis, and the second and third optical fibers are arranged in the direction in which the second optical fiber is disposed. Since the inclined surface is directed toward the opposite side, the light emitted from the end surface of the first optical fiber is not reflected at the end surface of the first optical fiber even if the vicinity of the end surface of each optical fiber is arranged in parallel. The light is refracted in the direction in which the three optical fibers are disposed. Therefore, in this optical sensor, the incident efficiency of the reflected light to the second and third optical fibers can be improved.

  In this optical sensor, since the vicinity of the end faces of the optical fibers are all arranged in parallel, the detection unit can be reduced in size and the optical fiber does not require a bent part. Reliability can be improved. When resin is filled to protect these optical fibers, even if the resin shrinks or expands due to temperature changes, no stress is applied to the optical fiber, and loss of the propagating light occurs or the intensity distribution of the emitted light Therefore, the detection accuracy can be maintained with high accuracy.

Further, in this optical sensor, since the vicinity of the end face of each optical fiber is all arranged in parallel, the optical fiber does not require a bent portion, so that the detection accuracy deteriorates due to a change in the emission wavelength spectrum of the light source. is suppressed, also enables highly accurate detection, the optical sensor according to the present invention, by having the structure 3, since the end faces of the first to third optical fibers nonreflective coating have been made, each light Loss at the end face of the fiber can be reduced.

Furthermore, in the optical sensor according to the present invention, by having the configuration 4 , the end surfaces of the second and third optical fibers are inclined with respect to the optical axis, and the first optical fiber is disposed. Since the inclined surface is directed to the opposite side of the direction, even if the vicinity of the end face of each optical fiber is arranged in parallel, the light incident on the second and third optical fibers is the end face of these optical fibers. The incident light is refracted and the incident efficiency of the reflected light to the second and third optical fibers can be improved.

  That is, according to the present invention, it is possible to reduce the size and improve the mechanical reliability by eliminating the bending of the bare fiber portion in the detection portion of the optical sensor, and to change the emission wavelength spectrum of the light source. It is possible to provide an optical sensor that is capable of performing highly accurate detection while suppressing deterioration in detection accuracy.

  The best mode for carrying out the present invention will be described below with reference to the drawings.

  FIG. 1 is a block diagram showing a configuration of an optical sensor according to an embodiment of the present invention.

  As shown in FIG. 1, the optical sensor according to the present invention includes an LED (light emitting diode) 1 serving as a light source that emits detection light. The LED 1 emits light in a 1.3 μm band, for example. The detection light emitted from the LED 1 is incident from one end face of the first optical fiber 4, propagates in the first optical fiber 4, and is emitted from the other end face (exit end face). The first optical fiber 4 functions as an emission port. This emitted light is applied to the reflecting surface 3 of the reflecting plate (diaphragm) 2 that is to be detected.

  The reflecting plate 2 is disposed such that the relative distance D with respect to the emission end face of the first optical fiber 4 changes according to physical quantities such as pressure and temperature.

  The detection light emitted from the first optical fiber 4 and applied to the reflecting surface 3 of the reflecting plate 2 is reflected by the reflecting surface 3, and the end surfaces (incident end surfaces) of the second and third optical fibers 5 and 6 are reflected. Is incident on. These second and third optical fibers 5 and 6 are single mode optical fibers generally used for communication. The second and third optical fibers 5 and 6 function as reflection ports, and the first and second PDs (photodiodes) 7 and 8 that serve as first and second light receiving elements for the incident reflected light. Respectively.

  The electric signals from the first and second PDs 7 and 8 are sent to the arithmetic processing circuit 10 via the amplifiers 9 and 9 that amplify the electric signals. The arithmetic processing circuit 10 calculates the relative distance between the incident end face of each of the optical fibers 5 and 6 and the reflecting surface 3 by taking the ratio of the electric signals amplified by the amplifiers 9 and 9, and the displacement of the reflecting plate 2. Calculate the amount.

  In this optical sensor, the vicinity of the end faces of the first, second and third optical fibers 4, 5 and 6 and the reflecting plate 2 arranged so as to oppose them constitute a detecting unit 11, and each other. The relative positional relationship between the two is maintained by an integrally formed holding member. In the detection unit 11, the first, second, and third optical fibers 4, 5, and 6 are arranged with the optical axis directions parallel to each other. Therefore, the detection unit 11 can be easily reduced in size, and the optical fibers 4, 5, and 6 do not need a bent portion, so that mechanical reliability can be improved.

  Further, in the detection unit 11, when the resin is filled to protect the optical fibers 4, 5, and 6, even if the resin contracts and expands due to a temperature change, the optical fibers 4, 5, and 6 are stressed. Therefore, no loss occurs in the propagating light and no distortion occurs in the intensity distribution of the emitted light, and the detection accuracy can be maintained with high accuracy.

Further, in this optical sensor, since the optical fibers 4, 5 and 6 are all arranged in parallel in the detection unit 11, no bent portions are required in the optical fibers 4, 5 and 6, so 2 is a side view showing the configuration of the detection unit 11. FIG. 2 is a side view showing the configuration of the detection unit 11. FIG.

  In this optical sensor, as shown in FIG. 2, the exit end face of the first optical fiber 4 is inclined with respect to the optical axis, and the second and third optical fibers 5 and 6 are provided. The inclined surface is inclined by a predetermined angle indicated by θ in FIG.

  Since the exit end face of the first optical fiber 4 has such an inclined surface, the detection light emitted from the exit end face of the first optical fiber 4 is refracted at the exit end face, and the reflector 2 When the light is reflected by the reflecting surface 3, the second and third optical fibers 5 and 6 are well coupled.

When the incident angle to the reflecting surface 3 is θ ₂ , this incident angle θ ₂ can be expressed as follows.

θ ₂ = sin ⁻¹ (n ₁ / n ₂ × sin θ) −θ
Here, n ₁ is the refractive index of the core of the first optical fiber 4, n ₂ is the refractive index of the external medium (e.g., air).

  FIG. 3 is a graph showing the change characteristics of the light intensity and the intensity ratio with respect to the change of the relative distance D between the incident end face of each of the optical fibers 5 and 6 and the reflecting surface 3.

  In this optical sensor, as shown in FIG. 3, the first and the second optical fibers 5 and 6 are changed in the first and second optical fibers 5 and 6 by the change in the relative distance D between the incident end faces and the reflecting surface 3 of the reflecting plate 2. The light intensity (P1, P2) detected in the second PDs 7, 8 and the intensity ratio F (P1, P2) thereof change. In FIG. 3, the horizontal axis indicates the relative distance D, the left vertical axis indicates the light intensity, and the right vertical axis indicates the intensity ratio.

  Here, the intensity ratio F (P1, P2) is calculated by the following equation.

F (P1, P2) = (P2-P1) / (P1 + P2)
In this optical sensor, the intensity ratio F (P1, P2) is a curve having a substantially linear slope shape. When detecting the amount of displacement of the reflecting plate 2, a curve can be used for this slope shape. In this case, the more linear is advantageous in that the correction function for converting the distance change into a physical quantity becomes simple, the arithmetic processing is easy, and the error is small.

  On the other hand, the detection sensitivity is represented by a slope slope [Δ = dF (P1, P2) / dD], and the detection sensitivity increases as this slope increases.

  FIG. 4 is a graph showing the change characteristic of the intensity ratio with respect to the change of the relative distance D when the inclination angle θ of the emission end face of the first optical fiber is changed.

  In this optical sensor, when the inclination angle θ of the emission end face of the first optical fiber 4 is changed, the slope of the slope indicating the intensity ratio F (P1, P2) changes as shown in FIG. That is, by increasing the inclination angle θ of the emission end face of the first optical fiber 4, the slope of the slope indicating the intensity ratio F (P1, P2) is increased, and the detection sensitivity can be increased.

  The characteristics shown in FIG. 4 are the distance between the central axes d1 of the first optical fiber 4 and the second optical fiber 5, and the distance d2 between the central axes of the second optical fiber 5 and the third optical fiber 6. These are for the case of 127 μm.

  The end surfaces of the first to third optical fibers 4, 5, and 6 may be provided with a non-reflective coating in order to prevent deterioration in detection accuracy due to multiple reflection between the reflection surfaces 2 of the reflection plate 2. desirable.

  Further, the incident end surfaces of the second and third optical fibers 5 and 6 may be inclined surfaces with respect to the optical axis, and may be inclined surfaces opposite to the direction in which the first optical fiber 4 is disposed. It is effective in improving detection sensitivity. In this case, the reflected light incident on the second and third optical fibers 5 and 6 is refracted and incident on the incident end faces of these optical fibers, and is reflected on the second and third optical fibers 5 and 6. The incident efficiency of light can be improved.

It is a block diagram which shows the structure of the optical sensor which concerns on embodiment of this invention. It is a side view which shows the structure of the detection part in the said optical sensor. It is a graph which shows the change characteristic of the light intensity and intensity ratio with respect to the change of the relative distance between the incident end face of the 2nd and 3rd optical fiber in the said optical sensor, and a reflective surface. It is a graph which shows the change characteristic of the intensity ratio with respect to the change of a relative distance at the time of changing the inclination | tilt angle of the output end surface of a 1st optical fiber in the said optical sensor.

Explanation of symbols

1 LED
2 Reflecting plate 3 Reflecting surface 4 First optical fiber 5 Second optical fiber 6 Third optical fiber 7 First PD
8 Second PD
9 Amplifier 10 Arithmetic Processing Circuit 11 Detector

Claims (4)

A light source;
The light emitted from the light source is incident on one end side, propagates the light to the other end side, emits the light from the end face on the other end side, and is disposed at a relative distance from the end face. A first optical fiber for irradiating the detection object with emitted light;
The end surface is directed to the object to be detected, and at least in the vicinity of the end surface is parallel to the first optical fiber and is disposed at a predetermined distance from the first optical fiber, and is reflected by the object to be detected. A second optical fiber into which the reflected light of the emitted light is incident from an end surface;
The end face is directed to the object to be detected and at least near the end face, the second light is parallel to the first optical fiber and in the same direction as the second optical fiber with respect to the first optical fiber. A third optical fiber that is disposed away from the fiber and receives the reflected light of the emitted light reflected by the object to be detected from an end surface;
First and second light receiving elements that receive reflected light propagated by the second and third optical fibers, respectively, convert the light into electrical signals, and output the electrical signals;
Calculation means for calculating the intensity ratio of these based on the light intensity detected in the first and second light receiving elements , and calculating the displacement amount of the detected object from the intensity ratio ,
The end surface of the first optical fiber is an inclined surface with respect to the optical axis, and is an inclined surface directed to the opposite side of the direction in which the second and third optical fibers are disposed. An optical sensor.   When the light intensities detected in the first and second light receiving elements are P1 and P2, the intensity ratio F (P1, P2) is calculated by the following equation.
  The optical sensor according to claim 1.
  F (P1, P2) = (P2-P1) / (P1 + P2) The end faces of the first to third optical fibers, according to claim 1, characterized in that no reflective coating is being made, or, an optical sensor according to claim 2, wherein. The end surfaces of the second and third optical fibers are inclined surfaces with respect to the optical axis, and are inclined surfaces on the opposite side of the direction in which the first optical fibers are disposed. optical sensor according to any one of claims 1 to 3,.

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