JPH0232570B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] This invention relates to an optical fiber sensor for detecting leaks that can detect leaks of oil, water, etc. in chemical plants and fuel pipelines using light without a power source, and its manufacture. It is about the method.

[Conventional technology]

A conventional leakage detection sensor using an electrical detection method includes one in which the outer periphery of a metal wire is covered with a Teflon porous insulating material. When oil adheres to the insulating material and becomes wet, this leakage detection sensor changes the impedance between the metal wire and the insulating material, and detects oil leakage based on this change in impedance. However, since the impedance of such leakage detection sensors changes even when water adheres to the insulating material, it is difficult to distinguish between water and oil, the sensitivity decreases due to aging, and the measurable distance is short at 1 km. Furthermore, since the detection is done electrically, there is an inherent defect that can cause false detection or fire due to induced noise.

An optical fiber sensor for detecting leakage that utilizes an optical fiber has now been invented in an attempt to eliminate this defect.

FIG. 3 is a perspective view showing a first conventional optical fiber sensor for leak detection, and FIG. 4 is an explanatory diagram showing the leak detection principle of the optical fiber sensor for leak detection.

In the figure, 1 is the cladding part of the optical fiber, 2
3 is a core portion, and 3 is a leak detection optical sensor portion formed by cutting out a predetermined length of the cladding portion 1 to expose the core portion 2.

The first conventional optical fiber sensor for detecting leakage is constructed as described above, and the principle of detecting oil leakage using this optical fiber is based on the refractive index (n=1 air, n=
For example, when oil adheres to the leakage detection optical sensor section 3, the refractive index of the leakage detection optical sensor section 3 becomes larger than that of the core section 2, and the inside of the core section 2 The light in the propagation mode is converted into a leakage mode by the leakage detection optical sensor section 3, that is, it leaks to the outside. By measuring the backscattered light using an optical pulse tester (OTDR), the time and location of leakage can be detected.

FIG. 5 is an explanatory diagram showing a second conventional optical fiber sensor for detecting leakage (Utility Model Application No. 57-22103).

In the figure, 1 is the clad part of the optical fiber,
It is made of silicone resin and also serves as a leak detection light sensor section. 2 is a normal core part.

The second conventional optical fiber sensor for detecting leakage is constructed as described above, and the principle of detecting oil leakage using this optical fiber is that oil adheres to the cladding part 1 made of silicone resin and the sensor is immersed in oil. For example, when oil adheres to a certain place in the cladding part 1, the oil immerses in the cladding part 1, and the cladding part 1 is immersed in the oil and the refractive index changes to n. =1.405 to n=1.451. Then, all the light in the propagation mode L1 in the core part ₂ is converted into the leakage mode _L2 at the part of the cladding part 1 that is immersed in oil. Note that L ₀ is light in a propagation mode that does not convert into a leaky mode even if the refractive index of the cladding portion 1 changes. In this case as well, the optical pulse tester can detect the time and location of leakage occurrence.

[Problem that the invention seeks to solve]

In the optical fiber sensor for leakage detection of the first conventional example as described above, the leakage detection optical sensor section 3 is a dot-like sensor formed by cutting out a predetermined length of the cladding section 1 to expose the core section 2, and Since the sensor is not continuous over the entire length of the fiber, if the oil leakage location and the leakage detection optical sensor section 3 do not match, detection will not be possible.If the number of the sensor sections 3 is increased, the loss will increase and the sensitivity at the rear will deteriorate, or becomes undetectable.
Furthermore, since the clad portion 1 is notched, there is a problem that it becomes mechanically weak.

In addition, in the optical fiber sensor for leakage detection of the second conventional example as described above, the cladding part 1 made of silicone resin also serves as the light leakage detection sensor part, and the part of the cladding part 1 is immersed in oil. As a result, the refractive index changes, and all of the light in the propagation mode within the core portion 2 is converted into a leakage mode at that portion. This is because the core part 2 is linear, and the light that is reflected at the core-clad interface at an angle of θ, for example, is reflected at an angle of θ no matter how far it goes because the diameter of the core part 2 is constant. When the refractive index of the cladding part 1 changes due to immersion, the light that is reflected at an angle θ at the immersed part, that is, the light in the propagation mode, is converted into light in the leakage mode at an angle θ _R , and most of it is transmitted to the outside. After leaking, the light at θ _R has already decreased at the rear, and the proportion of light that becomes leak mode in the total amount of light decreases, and the light that is reflected at another angle different from angle θ is converted to leak mode. This is because it is not done.

Therefore, oil immersion occurs in two or more locations, and when two or more locations are detected simultaneously, all of the propagation mode light is converted to leakage mode at the first detection location, and detection becomes impossible at the rear detection location. It was hot.

Furthermore, a clad part 1 made of silicone resin
Once immersed in oil, it is difficult to remove the oil and cannot be reused, and there are also problems in terms of durability.
This invention was made in order to solve this problem, and it is possible to detect leaking oil such as oil over the entire length of the optical fiber, and when there is a leak in two places, it is possible to detect the leak at the rear. Can be detected without reducing detection sensitivity,
The object of the present invention is to obtain an optical fiber sensor for detecting leakage that can be repeatedly detected and a method for manufacturing the same.

[Means for solving problems]

The optical fiber sensor for leakage detection according to the present invention includes at least one core portion formed in a helical shape and a clad portion surrounding the core portion,
The shortest distance between a part of the outer circumferential surface of the core portion and the outer circumferential surface of the cladding portion is formed to be less than the evanescent wavelength thickness, so that a leak detection optical sensor portion extending over the entire length is provided.

Another method of manufacturing an optical fiber sensor for detecting leakage according to the invention is to provide at least one through hole at an eccentric position of the clad material, insert a core material into the through hole, and heat the core material to form the clad material. After fixing the cladding material and the core material, the cladding material and the core material are heated to soften them, and the cladding material is drawn while rotating to form a thinned cladding part and a helical core part, and the core is The shortest distance between a part of the outer circumferential surface of the clad material and the outer circumferential surface of the cladding material is configured to form a leak detection optical sensor portion over the entire length less than or equal to the evanescent wavelength thickness.

[Effect]

In the present invention, an optical fiber sensor for leakage detection includes at least one core portion formed in a helical shape and a clad portion surrounding the core portion,
Since it is equipped with a leak detection optical sensor section that extends over the entire length and is formed so that the shortest distance between the outer circumferential surface of the core section and the outer circumferential surface of the cladding is less than the evanescent wavelength thickness, liquid leakage can be detected at any point on the cladding section. Moreover, because the core is formed in a helical shape, if oil or other liquid adheres to two or more places, the light in the propagation mode becomes the rear leakage mode, and the light in the propagation mode becomes the front leakage mode. The angle θ at which the light is reflected is different from the angle at which the light is reflected in the propagation mode, and the oil leakage detection sensitivity does not decrease.

〔Example〕

FIG. 1 is a partially omitted perspective view showing an embodiment of the present invention.

In the figure, 11 is a cladding portion of the optical fiber, 12 is a core portion formed in a helical shape, and the cladding portion 11 surrounds the core portion 12 . Reference numeral 13 denotes a leak detection optical sensor section formed over the entire length of the optical fiber, and the shortest distance between a part of the outer circumferential surface of the helical core section 12 and the outer circumferential surface of the cladding section 11 is defined as the evanescent wavelength thickness t. formed by.

Next, a method of manufacturing the optical fiber sensor for leakage detection of this embodiment will be explained.

First, a through hole with a diameter of about 1 cm is made at an eccentric position in a cladding material of, for example, 3 cm in diameter and 60 cm in length.
Next, the core material is inserted into the through hole, and the gap between the cladding material and the core material is heated and fused by drawing a vacuum while moving the heating element relatively, and then the cladding material and the core material are melted. The cladding material is heated to soften it without being heated, and the cladding material is drawn while rotating to reduce the diameter of the cladding material and the core material. When this diameter reduction is performed until the diameter of the core material becomes approximately 50 μm, the clad part 11 and core part 12 of the optical fiber of a predetermined length are formed.
is formed, and the shortest distance between a part of the outer peripheral surface of the core part 12 which is the core material and the outer peripheral surface of the clad part 1 which is the clad material is the evanescent wavelength thickness t.
A leakage detection optical sensor section 13 is formed over the entire length and has a smaller thickness than the above. Note that there is a technique similar to this called the rod tube method.
(Refer to pages 86 and 87 of “Optical Fiber Technology from the Perspective of Patents” edited by the Publisher and Japan Patent Office (Japan Institute of Invention and Innovation)).

In the optical fiber sensor for leakage detection configured as described above, the leakage detection optical sensor section 13 is continuously formed over the entire length of the optical fiber.
When a liquid with a refractive index higher than that of the core, such as leaked oil, adheres to a certain part of the cladding part 1, the light in the propagation mode that should be reflected at a certain angle and propagated when there is no oil in the part of the core part 2 where the oil leakage is attached is reflected. It is converted to leak mode and leaked to the outside. Therefore, if we measure the backscattered light using an optical pulse tester, we get
The time and location of leakage can be detected. Note that the leakage detection optical sensor section 13 is connected to the core section 12.
The reason why the shortest distance between a part of the outer circumferential surface of the cladding part 11 and the outer circumferential surface of the cladding part 11 is set to be less than the evanescent wavelength is that if the distance is longer than the evanescent wavelength, it will not be converted to the leakage mode when oil is attached.

Next, when oil adheres to two or more places on the cladding part 1, the propagation mode that would have been reflected and propagated at a certain angle in the core part 12 when there was no oil at the place where the oil adhered at the front, which is the upstream side. However, since the core part 12 is formed in a helical shape at the downstream side where the oil is attached, the light inside the core part 12 is converted into a leakage mode at a different angle than the one described above. The light in the propagation mode that is reflected and propagated and is not converted to the leakage mode at the front oil leakage point even if oil is attached to the cladding part 11, is converted to the oil leakage mode by changing its reflection angle at the rear location. Due to adhesion, it is converted to leakage mode and leaks to the outside. Therefore, even if there is oil leakage in two or more places, the oil leakage detection sensitivity will not decrease and the transmission loss will not be large. Further, unlike the second conventional example, the cladding part 11 is not immersed in oil, so that it can be used repeatedly by wiping off the oil adhering to the surface of the cladding part 11. Furthermore, since the detection sensitivity is high in that all the light in the propagation mode that is reflected at a certain angle due to the attachment of oil is converted into light in the leakage mode, only when oil is attached to the cladding part 11 in the form of oil droplets, the detection sensitivity is high. It is possible to detect oil leakage even if the oil is attached in the form of a thin film.

Although oil detection has been described in this embodiment, water detection can also be performed, and in this case, the core part 1
2 from the refractive index of the substance to be detected.
However, when detecting only oil without detecting water, n = 1.333 or more than water, n =
The refractive index of the core portion 12 may be set to about n=1.450 between the 1.458 and 1.458 oil.

FIG. 2 is a partially omitted perspective view showing a second embodiment of the invention.

This embodiment consists of two helical core portions 12 and a cladding portion 11 surrounding the core portions 12, and two leakage detection optical sensor portions 13 extending over the entire length similar to the first embodiment are formed. It is something that will be done. Therefore, per predetermined length of the cladding portion 11,
Compared to the one with one helical core part 12,
The sensor area increases and detection performance improves.

The manufacturing method of the optical fiber sensor for leakage detection of this embodiment differs from the first embodiment only in the steps of providing two parallel through holes at eccentric positions in the clad material and inserting a core material into each through hole. Since the subsequent steps are similar to those in the first embodiment, explanations of the subsequent steps will be omitted.

These two core parts 12, 12 and the clad part 11
In the so-called duffle helical type optical fiber sensor for leakage detection, the optical fiber can be made into a loop type and optical pulses can be inserted from the opposite direction to achieve redundant position detection, further improving detection accuracy. In addition to this, it is also possible to check the operation of optical fibers, such as disconnection, by detecting transmitted light.

In both the first and second embodiments, the shortest distance between a part of the outer circumferential surface of the core section 12 and the outer circumferential surface of the cladding section 11 is made slightly smaller than the evanescent wavelength thickness in the leak detection optical sensor section 13. However, it goes without saying that the leak detection optical sensor section can also be formed even if the shortest distance is equal to or less than the evanescent wavelength (including zero). In this way, an optical fiber for leakage detection is provided with a leakage detection optical sensor section 13 in which a part of the outer circumferential surface of one or two core sections 12 is in contact with the outer circumferential surface of the cladding section 11, that is, the shortest distance is zero. When manufacturing a sensor, if a through hole is made in contact with the outer periphery of the cladding material, there is a risk of chipping. Therefore, we drilled a through hole at a certain thickness, inserted the core material into the through hole, heated it once to fix the core material to the cladding material, and then cut the outer periphery of the cladding material to create a leak detection optical sensor. Part 13
After cutting the cladding thickness to a predetermined value (below the evanescent wavelength to 0 after drawing), drawing is performed. By doing so, a simple and good optical fiber sensor for detecting leakage can be obtained.

All of the above-mentioned embodiments have been described as detecting leakage of liquid such as oil, but if the core thickness, helical pitch, helical radius, cladding thickness, etc. are set to specific values, a specific wavelength (e.g. It is possible to create a sensor that has a resonance mode at the CH ₄ absorption wavelength of 1.665 μm or 3.392 μm), and in this case, the detection sensitivity is improved and it can be used as a gas sensor that can also detect gases.

〔Effect of the invention〕

As explained above, this invention consists of at least one core portion formed in a helical shape and a cladding portion surrounding the core portion, and the shortest distance between a part of the outer circumferential surface of the core portion and the outer circumferential surface of the cladding portion. Since the distance is formed to be less than the evanescent wavelength thickness and the leak detection optical sensor section is provided over the entire length,
It can be detected even if liquid such as oil leaks adheres to any part of the leakage detection optical sensor part on the outer peripheral surface of the cladding part.Moreover, since the core part is formed in a helical shape, it is possible to detect oil leakage at two or more places on the cladding part. Even when a liquid such as the like adheres to the liquid, the light in the propagation mode that is at a different angle from the light in the propagation mode that becomes the leakage mode in the front becomes the leakage mode in the rear, and it is possible to detect liquid leakage without reducing the detection sensitivity of liquid leakage in the rear. This has the effect of being able to detect.

Another invention is to provide at least one through hole in the eccentric position of the cladding material, insert the core material into the through hole, heat the cladding material and the core material, and then remove the cladding material. The core material is heated to soften it, and the clad material is drawn while rotating to form a thinned clad portion and a helical core, and a part of the outer peripheral surface of the core portion and the clad portion are drawn. Since the shortest distance to the outer peripheral surface of the light leakage detection sensor part is formed over the entire length so that the shortest distance is less than the evanescent wavelength thickness, it is possible to detect oil leakage at any location, and also to prevent oil etc. An advantageous effect is that it is possible to easily obtain a good optical fiber sensor for leakage detection that can detect liquid adhesion without reducing sensitivity.

[Brief explanation of drawings]

Fig. 1 is a partially omitted perspective view showing a first embodiment of the present invention, Fig. 2 is a partially omitted perspective view showing a second embodiment of the invention, and Fig. 3 is a leakage diagram of the first conventional example. FIG. 4 is an explanatory diagram showing the leak detection principle of the optical fiber sensor for leak detection, and FIG. 5 is an explanatory diagram showing the second conventional optical fiber sensor for leak detection. . In the figure, 11 is a clad part, 12 is a core part, and 13 is a leakage detection optical sensor part. In each figure, the same reference numerals indicate the same or corresponding parts.

Claims (1)

[Claims] 1. Consisting of at least one helical core portion and a clad portion surrounding the core portion,
An optical fiber for leakage detection, characterized in that the shortest distance between a part of the outer peripheral surface of the core part and the outer peripheral surface of the cladding part is formed to be less than the evanescent wavelength thickness, and is provided with a leakage detection optical sensor part extending over the entire length. sensor. 2. At least one through hole is provided at an eccentric position in the cladding material, a core material is inserted into the through hole, the cladding material and the core material are fixed by heating, and then the cladding material and the core material are softened. The clad material is heated and drawn while rotating to form a thinned clad portion and a helical core, and the shortest distance between a part of the outer circumferential surface of the core portion and the outer circumferential surface of the clad portion. 1. A method for manufacturing an optical fiber sensor for leakage detection, characterized in that a leakage detection optical sensor portion is formed over an entire length less than or equal to an evanescent wavelength thickness.