JPS63311147A - Optical fiber type humidity sensor - Google Patents

Abstract

PURPOSE:To measure humidity highly accurately in a broad range, by providing a porous layer at the outer surface part of a core, and providing a reflecting film, which is formed at one end surface of the core, reflects incident measuring light inputted from the other end surface and returns the light to the other end surface. CONSTITUTION:The moisture sensitive part of a sensor is positioned in a gas to be measured. Measuring light is inputted into a core through a rear end surface 9 of the core 1. Then, the measuring light repeats reflection from the interface between the core 1 and a clad layer 2. The light is propagated and reaches the moisture sensitive part. A porous layer 3 adsorbs moisture in the gas to be measured. The measuring light reaching the moisture sensitive part is subjected to the effect corresponding to the amount of the adsorbed moisture in the porous layer 3 when the light is reflected from the interface between the core 1 and the porous layer 3. The scattering amount of the measuring light at the interface between the core 1 and the porous layer is changed. After the measuring light is reflected from a reflecting film 5, the light undergoes intensity change at the interface between the core 1 and the porous layer 3 again. Then, the light returns to the rear end surface 9. The outputted reflected light is separated and measured. Thus, the relative humidity of the gas to be measure is detected.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field 1] The present invention relates to an optical fiber type humidity sensor that uses an optical fiber itself as a humidity sensor and is capable of measuring the relative humidity of gas or the like.

[Prior Art] Most conventional humidity measurements utilize resistance changes and capacitance changes in materials such as ceramics and plastics due to humidity, and detect these electrically to determine humidity.

Further, as a method using an optical fiber, as shown in FIG. The light from the light source 46 is transferred to the light transmitting optical fiber 41 . The space 45 and the light receiving optical fiber 42 are transmitted and the intensity is transmitted to the light receiver 4.
There is a method of detection in 7. Furthermore, as an improvement on this method, a method has been proposed in which a moisture-sensitive resin whose absorption coefficient of a specific wavelength changes depending on humidity is interposed between the transmitting and receiving optical fibers (Japanese Patent Laid-Open No. 56-67738). ,.

[Problems to be Solved by the Invention] However, the above-mentioned method of electrically detecting changes in resistance due to humidity, etc. has many problems such as electrical noise when applied to humidity detection in a high voltage environment. Furthermore, with this method, when trying to measure multiple locations simultaneously,
It is necessary to install a sensor at each measurement point and extract the output signal from each sensor using a metal wire, and because it is necessary to install a large number of metal wires in the measurement section, it is difficult to install in terms of installation space, economy, and stability. Not practical.

The optical system shown in Figure 4 can be applied even under high voltage, but due to temperature changes etc.
The optical axes of the light-receiving optical fiber 42 and the light-receiving optical fiber 42 are easily misaligned, and stable measurements are difficult because they are easily affected by dust in the outside air, and can only be used in limited environments.

Furthermore, high-precision processing is required for the jigs and the like that support the optical lenses and optical fibers 41 and 42.

On the other hand, in a method in which a moisture-sensitive resin is interposed between optical fibers for transmitting and receiving light, the optical system can be easily fixed and is less susceptible to the effects of dust and the like. However, due to moisture absorption and swelling of the moisture-sensitive resin, displacement occurs at the joint between the moisture-sensitive resin and the optical fiber, which increases loss and makes stable measurement difficult. Furthermore, the moisture-sensitive resin deteriorates over time, making it impossible to obtain sufficient long-term performance. Furthermore, this method involves inserting a moisture-sensitive resin into the optical transmission path, and if you try to increase the sensitivity to changes in humidity by making the moisture-sensitive resin thicker, there is a problem that the attenuation of light will inevitably increase and the amount of light received will decrease. . Furthermore, since the method detects only light of a specific wavelength, the equipment is expensive.

The present invention was devised to solve the above-mentioned problems of the prior art, and the purpose of the present invention is to achieve high stability.

An object of the present invention is to provide an optical fiber type humidity sensor that can measure humidity with high precision and a wide range of applications at low cost.

[Means for Solving the Problems] In order to achieve the above object, the optical fiber type humidity sensor of the present invention detects the relative humidity of the gas to be measured from the intensity change of the measurement light propagating through the core of the optical fiber. An optical fiber type humidity sensor includes a porous layer provided on the outer periphery of the core, and a porous layer formed on one end surface of the core and reflecting measurement light incident from the other end surface to the other end surface. and a reflective film for returning the measurement light, and the other end surface is used as an input end and an output end of the measurement light.

[Function] A porous layer made of fine particles such as SiOz has the function of adsorbing moisture in the gas to be measured, and the amount of adsorption is correlated to the amount of moisture in the gas to be measured and the temperature, that is, the relative humidity It will be correlated to.

Therefore, with the above configuration, the macroscopic refractive index of the porous material changes depending on the relative humidity of the gas to be measured, and the measurement light that is reflected and propagated in the core is scattered at the interface between the core and the porous layer. The amount changes.

As a result, the intensity of this measurement light changes depending on the relative humidity of the gas to be measured. In particular, the measurement light that enters the core from the input end of the sensor is reflected by the reflective film and returns, so it is affected by the porous layer twice in both directions, and changes in relative humidity occur. You can get a big signal.

[Example] Embodiments of the present invention will be described below with reference to the accompanying drawings.

FIG. 1 is a sectional view showing the structure of an optical fiber type humidity sensor according to an embodiment of the present invention. A cladding 2 made of quartz glass, plastic, or the like is provided around the outer periphery of a core 1 made of quartz glass, multicomponent glass, or the like. The cladding 2 is peeled off over a predetermined length near the tip of the core 1, and instead of the cladding 2, a porous layer 3 made of SiOz fine particles is formed around the outer periphery of the core 1. Further, the tip end surface 4 of the core 1 is coated with a reflective film 5.

In this way, a moisture sensing section of the sensor is constructed, and a cover 6 that encloses and protects this moisture sensing section is attached to the outer circumference of the cladding 2 with an adhesive 7. This cover 6
A large number of ventilation holes 8 are provided to improve ventilation.

Next, the operation of this embodiment will be described.

First, the moisture sensitive part of the sensor is positioned in the gas to be measured, and measurement light is made to enter the core 1 from the rear end surface 9 of the core 1. Then, the measurement light propagates while being repeatedly reflected at the interface between the core 1 and the cladding 2, and reaches the moisture sensing section. Here, the porous layer 3 adsorbs moisture in the gas to be measured, and the measurement light that reaches the moisture sensing section is absorbed by the porous layer 3 when reflected at the interface between the core 1 and the porous layer 3. The effect will depend on the amount of water. That is, the macroscopic refractive index of the porous layer 3 changes in accordance with the amount of adsorbed water, and the amount of scattering of measurement light at the interface between the core 1 and the porous layer 3 changes.

In this way, the measurement light whose intensity has changed depending on the relative humidity of the gas to be measured is reflected by the reflective film 5 and then undergoes an intensity change again at the interface between the core 1 and the porous layer 3, and then reaches the rear end surface 9. and return.

Therefore, by separately measuring the reflected light emitted from the rear end surface 9, the relative humidity of the gas to be measured is detected.

When the relative humidity of various gases was measured using the sensor of this example, the intensity change of reflected light showed characteristics as shown in FIG. 2 depending on the value of relative humidity. In other words, the intensity of reflected light decreases as the relative humidity increases, and when the relative humidity changes from 20% to 80%, the change in the intensity of reflected light is approximately 10 d.
It will be as big as B.

The porous layer 3 made of 5iOz fine particles can be formed by coating the outer periphery of the core 1 with a silicone polymer and then heat-treating it.

That is, a silicone polymer such as silicone resin or silicone rubber is applied to the outer circumferential portion of the core 1, and then burned and oxidized, and then the surface is cleaned with alcohol. This way,
5iOz silicone polymer oxide on the surface of core 1
The fine particles are left in a porous state, forming a porous layer 3.

In addition, the porous layer 3 can also be formed by directly depositing Si 02 soot on the outer circumference of the core 1 .

Next, FIG. 3 shows an embodiment in which a plurality of moisture sensing sections are provided to perform multi-point detection. Branch portions 32 are provided at a plurality of locations along the longitudinal direction of one optical fiber 31, and a moisture sensing portion 33 as shown in FIG. 1 is formed at the tip of each branch portion 32. Then, the proximal end of the optical fiber 31 is connected to an optical pulse tester (OT).
DR device) 34.

When a light pulse is inputted into the optical fiber 31 from this light pulse test device 34, a time delay corresponding to the distance from the light pulse test device 34 to each humidity sensing portion 33 occurs, and
A reflected light pulse returns with an intensity corresponding to the relative humidity at the temperature. Therefore, by measuring this reflected light pulse, it becomes possible to simultaneously measure the relative humidity of the gas to be measured at multiple points.

[Effects of the Invention] As explained above, according to the present invention, the following excellent effects are exhibited.

(1) No optical elements other than optical fibers are required, and there is no need to measure at specific wavelengths such as infrared rays. Therefore, the measurement system can be simplified, handling is easy, and costs can be reduced.

(2) The measurement light that is reflected by the reflective film and returned is influenced by the porous layer twice in both directions, resulting in highly sensitive measurements.

(3) Since the optical fiber itself is used as a sensor, there is no loss caused by deterioration over time or the intervention of optical lenses, etc., and it has excellent long-term reliability and is not affected by electromagnetic induction.

(4) The humidity sensing section is small and lightweight, making it possible to measure humidity in minute spaces.

(5) Since changes in optical constants accompanying water adsorption in the porous layer are utilized, measurements with high sensitivity and excellent responsiveness can be performed over a wide range of humidity.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view showing the configuration of an optical fiber type humidity sensor according to an embodiment of the present invention, FIG. 2 is a graph showing measurement results according to the embodiment, and FIG. Fig. 4 is a block diagram showing a conventional example. In the figure, 1 is a core, 2 is a cladding, 3 is a porous layer, 4 is a front end surface, 5 is a reflective film, and 9 is a rear end surface. Patent applicant: Tokyo Electric Power Company, Hitachi Cable, Ltd. Representative Patent Attorney Nobuo Kinutani 1...Core 4...Tip surface 2...
Cladding 5... Reflective film 3... Porous layer 9... Rear end surface relative humidity (0 μm) Figure 2

Claims (4)

[Claims] (1) In an optical fiber humidity sensor that detects the relative humidity of a gas to be measured from changes in the intensity of measurement light propagating through the core of the optical fiber, a porous layer provided on the outer periphery of the core and a portion of the core and a reflective film formed on the end surface and for reflecting the measurement light incident from the other end surface and returning it to the other end surface, and the other end surface is used as an input end and an output end of the measurement light. Features: Optical fiber type humidity sensor. (2) The optical fiber type humidity sensor according to claim 1, wherein the porous layer is made of SiO_2 fine particles. (3) The optical fiber type humidity sensor according to claim 2, wherein the porous layer is formed by coating the outer periphery of the core with a silicone polymer and then heat-treating it. . (4) The optical fiber type humidity sensor according to claim 2, wherein the porous layer is formed by attaching SiO_2 soot to the outer periphery of the core.