JPS62144033A - Optical sensor - Google Patents

Abstract

PURPOSE:To make it possible to measure the physical quantity of an object to be measured within a wide dynamic range with high accuracy, by splitting light passing through a first Fabry-Perot interferometer into two luminous fluxes and guiding said luminous fluxes to a second reference Fabry-Perot interferometer. CONSTITUTION:The light emitted from a light emitting diode 41 passes through a first Fabry-Perot interferometer 42 changing in its characteristics by physical quantity to be measured and split into a plurality of luminous fluxes mutually having inclination by a diffraction lattice 44. After zero-order light among luminous fluxes was vertically incident to a second Fabry-Perot interferometer 43, the transmitted light thereof is received by a photodiode 45. After primary light was obliquely incident to the second Fabry-Perot interferometer 43, the transmitted light thereof is received by a photodiode 46. By comparing the signals outputted from the photodiodes 45, 46 by a signal processing circuit, all of physical quantities capable of changing the light path length of the first Fabry-Perot interferometer can be detected.

Description

[Detailed Description of the Invention] Technical Field> The present invention relates to a so-called optical sensor that detects a physical quantity using an optical method. Regarding sensors.

<Prior art> An optical sensor using a Fabry-Perot interference device has two
It is a highly sensitive sensor that creates a light interference effect by placing two parallel reflective colors facing each other, and detects a physical quantity based on this interference effect.The output changes with a displacement smaller than the wavelength of the light. Various applications are being considered.

The principle of measurement using the Fabry-Perot interferometer will be explained with reference to FIG. Light output from a monochromatic and wavelength-stable laser device 11, such as a HeNe laser oscillation device, enters a Fabry-Perot interference device 12, and the amount of transmitted light is converted into an electrical signal by a light receiving device 13. The amount of transmitted light is the wavelength of light.

It takes a specific value determined by the optical path length of the interference device 12 and the reflectance of the reflected color of the interference device 12. In a sensor with such a simple configuration, the wavelength of the light output from the light source must always be constant, and the semiconductor lasers currently on the market as small laser light sources do not have sufficient performance, so Gannu laser is usually used. etc. are used. However, the Gannu laser device increases the size and cost of the measurement system, which significantly impedes its practicality as a sensor. In order to solve this problem, a sensor shown in FIG. 7 has an improved configuration that allows the use of a small and inexpensive light source such as a light emitting diode (LED). In FIG. 7, light outputted from a light source 21 having a wide wavelength band than a laser beam is transmitted to a first Fabry-Perot interferometer 22 whose characteristics change depending on the physical quantity to be measured.
A second Fabry-Perot interference device 23 with constant characteristics from
The light enters the light receiving device 24 through the light receiving device 24, and the light intensity thereof is converted into an electrical signal by the light receiving device 24.

The operation of the optical sensor shown in FIG. 7 will be described below with reference to FIG. 8. Figure 8 CA) is the light source 210 emission spectrum intensity I (λ), Figure 8 CB) is the first Fabry
Transmittance Tl(λ) of Perot interference device 22, FIG. 8(C)
is the transmittance T2 of the second Fabry-Perot interference device 23 (
λ). When both TI(λ) and T2(λ) are represented by curve a, that is, when the peaks of T,(λ) and T2(λ) match, the first and second Fabry-Perot interferometers 22. Combined transmittance of 23 T+(λ)・T2(λ)
The integral with respect to the wavelength is maximized. On the other hand, when rt(λ) is a curve and T2(λ) is represented by curve a, that is, when the peak of TI(λ) and the valley of T2(λ) coincide, the combined transmittance TI(λ)・T2( The integral of λ) will be small. If the light emission intensity is also considered, the light reception intensity becomes the area surrounded by the curves T, , T2 shown in FIG. 8(D). Strictly speaking, it is also necessary to consider the sensitivity wavelength dependence of the light receiving element.

FIG. 9 is a waveform diagram showing the output waveform when the optical sensor shown in FIG. 7 is used as a stress sensor. The vertical axis is the light amount, and the horizontal axis is the first Fabry-Perot interference device 22.
is the amount of change in the internal optical path length of The resulting output has an oscillating waveform that corresponds to changes in the internal optical path length. When exhibiting such a vibration waveform, the internal optical path length cannot be uniquely specified even if the amount of light is determined to be a certain value. Further, when the amount of light increases or decreases, it is extremely difficult to distinguish whether the increase or decrease is due to an increase or decrease in the internal optical path length. Therefore, in conventional optical sensors, except for those with some complicated configurations, measurements are mainly performed using the following two methods.

One of them is a method in which the measurement is performed by limiting the physical quantity to be measured to only rise or fall within the measurement time. However, in general, changes in the measured quantity are arbitrary, and if such limiting conditions are imposed, the applications will be significantly restricted.

The other method is to perform measurement by reducing changes in the physical quantity to be measured and making it uniquely correspond to the amount of light.

In this case, the change in light output is 1//! Since the measurement is limited to the part that undergoes periodic vibration, there are problems in that it is not possible to obtain accurate measurement data or it is difficult to obtain highly accurate measurement data.

<Object of the Invention> The present invention has been made in view of the above-mentioned problems, and provides a novel sensor configuration that can easily detect an increase or decrease in the internal optical path length of a Fabry-Perot interference device. It is an object of the present invention to provide an optical sensor that enables highly accurate measurement over a wide dynamic range.

<Example> The present invention will be described in detail below with reference to FIGS. 1 to 5.

FIG. 1 is a configuration diagram of an optical sensor showing a first embodiment of the present invention. The light output from the light emitting diode 41 passes through a first Fabry-Perot interference device 42 whose characteristics change depending on the physical quantity to be measured, and then is split by a diffraction grating 44 into a plurality of mutually tilted light beams. In this embodiment, the 0th-order light among the plurality of light beams emitted from the diffraction grating 44 is transferred to the second Fabry-Perot interference device 43.
The transmitted light is transmitted perpendicularly to the photodiode 45.
After the primary light was obliquely incident on the second Fabry-Perot interference device 43, the transmitted light was received by the photodiode 46. Each photodiode 45.46
The signals output from the two are compared and processed by a signal processing circuit (not shown), and m measured physical quantities are determined.

Here, the light emitting diode 41 may be replaced with various light sources having a relatively wide spectrum width, for example, a white light source such as a halogen lamp. Further, the diffraction grating 44 is one means for obtaining obliquely incident light and vertically incident light at the same time, and other means such as a combination of a half mirror, a prism, or a mirror may be used to obtain a plurality of light beams at angles to each other. You may get it. Furthermore, it is often more convenient for the light beam to pass through an optical fiber. Fabry-Perot interference device 42°4
The light passing through 3 may be either transmitted light or reflected light.

The reason why two beams of light with different angles of incidence are incident on the second Fabry-Perot interference device 43 is that the second Fabry-Perot interference device 43 is isometrically formed into an interference device with two different optical path lengths. This is for the purpose of Generally, the effective optical path length for interference in a Fabry-Perot interference device is ndcos
It is known that θ. However, n is the refractive index of the medium between the opposing reflecting mirrors, d is the distance between the opposing reflecting mirrors,
θ is the angle of incidence of light between the mirrors. In this example, a Fabry-Perot type interference device was used in which two transparent parallel flat plates were pasted together with a predetermined gap between them with a spacer.

In this case, the refractive index n of the medium may be regarded as 1, and the incident angle θex from the outside to the Fabry-Perot interferometer is equal to the incident angle θ between the reflecting mirrors.

In this embodiment, the angle of oblique incidence θex to the Fabry-Perot interference device 43 is the effective optical path length nd at the time of normal incidence and the effective optical path length nd at the time of oblique incidence. I set it so that it would close.

As mentioned above, n=1. θex=θ and d=12μ
m, λo=850 nm. At this time, θex near,
Since the angle is 6°, the diffraction grating 44 was manufactured so that the first-order diffracted light was at this angle.

The optical sensor having the above structure can in principle detect all physical quantities that can change the optical path length of the first Fabry-Perot interference device 42. Here, as the first Fabry-Perot interference device 42, as shown in FIG. A material bonded together with a spacer 4 in between was used. Although the reflective film 3 is a metal film here, it may be a dielectric film or a dielectric multilayer film. In this interference device, one glass plate is bent by a force F (or contact pressure or load) applied from the outside, and the internal optical path length changes, thereby changing the interference characteristics. Furthermore, the first
The Fabry-Perot interference device 42 has a reflection type structure rather than a transmission type. In other words, both the input and output of light were performed through the optical fiber 6 and microlens 5 shown in FIG. On the other hand, the second Fabry-Perot interference device 4
3 uses the same device as the first Fabry-Bello type interference device 42, but has a transmission type configuration.

An output example ft when the optical sensor of this embodiment is used as a stress sensor is shown in FIG. 3(A). In this figure, I is the output of the photodiode 45 in FIG. This is the output.

The periods of Io and 11 are different by 1/4. Therefore, if the output IQ is plotted on the horizontal axis and the output ■1 is plotted on the vertical axis, the curve shown in FIG. 3(B) can be drawn. Here, FIG. 3(B) shows the situation when the output of IO in FIG. 3(A) changes by one cycle from arrow A to arrow E. Now, if the applied force changes from C to A, 1G changes from maximum to minimum. 1 changes similarly when the applied force changes from C to E. Therefore, lot
"It is not possible to determine whether the applied force increases or decreases only by sensing it. Here, if there is an output 11 due to obliquely incident light, the applied force will increase or decrease as shown in the graph of Figure 3 (B). In other words, if it changes clockwise from C to B-A, it is a decrease, and if it changes counterclockwise from C-+D to E, it is an increase. By continuously tracking the changes in ff1Io, I+, it is possible to continuously track the applied force or, in general, the value of the physical quantity.

In the above equation (1), the effective optical path length difference of the Fabry-Perot interference device for each of the two light beams is λ.

The reason for choosing /8 is the output ■. This is to shift the period of and ■1 by '/4. At this time, the diagram shown in Figure 3 (CB) is closest to a circle, so in Figure 3 (8), the output Io,
This is because it becomes easier to track the movement of the point indicated by I+.

FIG. 4 is a configuration diagram of an optical sensor showing a second embodiment of the present invention. The light output from the light emitting diode 41 is
The first 7 applications whose characteristics change depending on the measured physical quantity.
After passing through the bellows interference device 42, the beam is split into two mutually parallel beams by a beam splitter 47 and a prism 48.

Of the two divided light fluxes, the first light flux at enters the area 49A of the second Fabry-Bello interference device 49, and then the transmitted light is received by the photodiode 45,
Further, after the second light beam bt is incident on the region 49B of the second Fabry-Perot interference device 49, the transmitted light is received by the photodiode 46. Note that each component can be replaced as described in the description of the first embodiment.

As the first Fabry-Perot interference device, in this embodiment, a reflective element shown in FIG. 2 was used, as in the first embodiment. On the other hand, the second Fabry-Perot interference device 49
The configuration shown in FIG. 5 was used. As shown in Figure 5, the water device consists of a glass plate 1 and a thickness of λO/S.
A reflection enhancing film 7 is formed on each of the glass plates 10 on which the i02 film 8 is partially formed, a Nuvesa 4 is further formed on the glass plate 101fl, and these two glass plates 1.10 are bonded together. Here, the luminous flux a is 5 on the glass plate 10.
In the area 49A where the i02 film 8 does not exist, the light flux is SiO,
+It is installed so as to pass through the region 49B where the membrane 8 exists. At this time, if the effective optical path length of the Fabry-Bello interference device 49 for the light beam a is d, then the effective optical path length for the light beam I is d-λO/S. Let 1, I be the two optical outputs that have passed through the second Fabry-Perot interference devices 49A and 49B whose internal optical path lengths differ by λO/S shown in the first embodiment. By knowing the change in the two outputs, it becomes possible to know the change in the internal optical path length of the first Fabry-Perot interference device. In fact, in the second example, almost the same output characteristics as in the first example shown in FIGS. 3(A) and 3(B) were obtained.

Note that the second Fabry-Perot interference device 49 is connected to the region 49.
It is also possible to adopt a configuration in which the device is divided into an element corresponding to region A and an element corresponding to region 49B, and arranged independently. In this case, the degree of visibility increases with respect to the alignment condition, so that the optical path converting element corresponding to the prism 48 can be omitted, for example.

In the first and second embodiments, by receiving a portion of the light flux that has passed through the first Fabry-Perot interference device and normalizing the outputs of the light receiving elements 45 and 46 with respect to that output, the amount of light from the light source can be adjusted. It is possible to obtain stable output by almost canceling out fluctuations and fluctuations in light intensity loss. Further, although both the first and second Fabry-Perot interference devices are of the hollow type, the first and second Fabry-Perot interference devices may be configured separately from each other. For example, the first Fabry-Perot interferometer has an optical path length n of the medium inside the interferometer.
The second Fabry-Perot interference device may be of a cavity type and configured so that d can change depending on the physical quantity to be detected. Further, the Fabry-Perot type interference device may be a so-called fiber Fabry-Perot device in which a single mode optical fiber is cut and both ends are subjected to reflection enhancement treatment if necessary.

In addition, the configuration on the Kinomoto side can be applied to mechanical sensors that detect various mechanical quantities using a hollow Fabry-Perot interference device, such as pressure/contact pressure, acoustic, vibration load sensors, etc. Additionally, a material with a high coefficient of thermal expansion is used as a medium inside the Fabry-Perot interference device to create a temperature sensor, a material that swells with humidity is used to create a humidity or condensation sensor, and one of the semi-transparent mirrors of a Fabry-Perot interference device is created. It can be applied as a variety of sensors such as a minute change sensor that is installed on a fixed object to detect changes in position.

Furthermore, if the reflector spacing is configured to vary depending on other physical quantities such as electric λ and magnetism, it is possible to detect almost any physical quantity.

<Effects of the invention> As described above, the optical sensor invented by Kiyoshi is the first Fabry sensor.
It is possible to obtain two independent signals with a very simple structure from a single beam of light that has passed through a Perot-type interference device, which allows for detection of physical quantities over a wide dynamic range that corresponds to changes in light intensity over many cycles. It has the characteristic that it can be grasped. As a result, an optical sensor with high precision and a wide dynamic range can be obtained much more easily than conventional optical sensors, and its practical value is extremely large.

[Brief explanation of drawings]

FIG. 1 is a block diagram of an optical sensor showing a first embodiment of the invention. FIG. 2 is an explanatory diagram of the first seven-applied bellows type interference device 42 in the first and second embodiments. FIG. 3 is a characteristic diagram showing the characteristics of the photodiode obtained by the example shown in FIG. FIG. 4 is a configuration diagram of an optical sensor showing a second embodiment of the present invention. FIG. 5 is a structural diagram of the second seven-applied bellows type interference device 49 in FIG. Figure 6 and Figure π7 show the I'IM configuration of a conventional optical sensor (1 factor). Figure 8 is an explanatory diagram for explaining the operation of the optical sensor shown in Figure 7m. is a characteristic diagram showing the characteristics of the optical sensor shown in No. 7. 41... Light emitting diode, 42... First Fabry-Perot interference device, 43.49... Second Fabry-Perot type interference device. Perot type interference device, 44... diffraction grating,
45.46... Photodiode, 47... Beam splinter, 48... Prism. Agent Patent attorney Aihiko Kato (and 2 others) Fig. 1 (A) Force Ckgf) I (B) Fig. 4 Fig. 5 1 Fig. 6 Fig. 7 Moat 4ζ 51 length t (D) Q/234 Force (kgf) q [゛4

Claims (1)

[Claims] 1. A light source, a first Fabry-Perot interference device whose interference characteristics change depending on the physical quantity of the object to be measured, an optical branching device that splits the beam, and a reference device whose conditions are set to be constant. the second of
a Fabry-Perot interference device, and a second Fabry-Perot interference device that is output from the light source and is split by the first Fabry-Perot interference device and the optical branching device. A second light flux that is split into a different light flux from the first light flux by a first light-receiving element that receives the first light flux that has passed through the splitter and the optical branching device, and that has passed through the second Fabry-Perot interference device. an optical sensor comprising: a second light-receiving element that receives light; and a circuit that compares output signals from the first and second light-receiving elements to obtain a physical quantity of an object to be measured. . 2. The first and second light beams have different incident angles to the second Fabry-Perot interference device.
Optical sensor described in section. 3. The second Fabry-Perot interference device is connected to the first Fabry-Perot interference device.
The optical sensor according to claim 1, wherein the optical sensor has different interference characteristics in the region through which the second light beam passes. 4. A light source, a first Fabry-Perot interference device whose interference characteristics change depending on the physical quantity of the object to be measured, a light branching device that splits the light beam, and a second reference device whose conditions are set constant.
and a third Fabry-Perot interference device; the light output from the light source passes through the first Fabry-Perot interference device and the optical branching device, and is split by the optical branching device to the second Fabry-Perot interference device; a first light receiving element that receives a first light beam that has passed through the Perot type interference device; and a light beam that is split by the light branching device into a different light beam from the first light beam that passes through the third Fabry-Perot type interference device; a second light-receiving element that receives the second light beam, and a circuit that compares output signals from the first and second light-receiving elements to obtain a physical quantity of the object to be measured. Optical sensor.