JPH0434504A - Optical fiber for detecting gas, liquid or the like - Google Patents

Abstract

PURPOSE:To prevent the light waves projecting and propagating to the outer periphery of a core from being absorbed in a primary coating by forming the off-centered core side of the primary coating to the thickness smaller than the thickness on the reflection side. CONSTITUTION:The core A having the refractive index higher than the refractive index of a clad B is disposed in the off-centered position near the outer periphery of the clad B and the primary coating C having the refractive index higher than the refractive index of the clad B is formed on the outer periphery of the clad B. The core A side of the primary coating C is formed to the thickness smaller than the thickness on the opposite side. Further, the secondary coating D having the refractive index lower than the refractive index of the clad B is formed on the outer periphery of the primary coating C. The core A side of the secondary coating D is formed to the thickness larger than the thickness on the opposite side. Of the core light, the light waves projecting and propagating from the outer periphery of the core are substantially prevented from being absorbed in the primary coating.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an optical fiber used to detect gas or liquid by utilizing light waves propagating through the optical fiber.

(Prior Art) Conventionally, a second optical fiber has been used as an optical fiber for detecting gas, liquid, etc. by utilizing a small amount of light waves propagating outside the outer periphery of the core A of the core light propagating through the core A of a single mode optical fiber. An eccentric core single mode optical fiber as shown in the figure is known. In this optical fiber, a core A having a higher refractive index than the cladding B is arranged at an eccentric position near the outer periphery of the cladding B, a primary sheathing NC is formed with a uniform thickness on the outer periphery of the cladding B, and a core A with a uniform thickness is formed on the outer periphery of the cladding B. Happi coat F! The ID is formed with a uniform thickness.

In this eccentric core single mode optical fiber, the core light propagates to the outside of the thin part of the cladding B, so gas or liquid with a higher refractive index than the core light or the same wavelength as the core light is absorbed. When a gas or liquid that easily comes into direct contact with the cladding B or diffuses or swells into the primary coating C around the cladding B, evanescent waves are absorbed and the transmission loss of the core light increases. The optical output of the same optical fiber changes. Therefore, according to the same optical fiber, the gas or liquid can be detected from this change.

On the other hand, as is already known, the light propagating within the optical fiber includes cladding mode light propagating within the cladding B in addition to the core light. The ratio of the core light to the cladding mode light is about twice to one-tenth. Moreover, although it depends on the conditions of use, the ratio of light waves that propagate beyond the outer periphery of the core to core light is about one-tenth, and furthermore, the ratio of light waves that are absorbed by gases or liquids that propagate beyond the outer periphery of the core. The ratio of light waves to light waves is about one quarter. Therefore, the amount of output change due to absorption with respect to the total optical output including cladding mode light of the eccentric core single mode optical fiber is too small to be detected. Therefore, in the past, by making the Moeki-order coating C have a higher refractive index than the cladding B, the Taloud mode light was
c, so that only the core light was detected.

Furthermore, the bimodal rfID is made to have a lower refractive index than the cladding B, so that it does not participate in the transmission and reflection of propagating light.

(Problems to be Solved by the Invention) However, the conventional eccentric core single mode optical fiber has the following problems.

■Since the primary coating C with a high refractive index is coated on the outer periphery of the cladding B with a uniform thickness, this -legal coating raC absorbs the light waves that propagate to the outer periphery of the core along with the cladding mode light. .

Therefore, conventional eccentric core single mode optical fibers have large transmission losses, cannot be used in long lengths, and cannot be used as optical transmission lines at all.

- Light waves propagating outside the outer periphery of the optical core are absorbed by the FIIC. ) must be removed from the entire circumference of the coating C, or only the primary coating near the eccentric core must be removed to detect gas or liquid. For this reason, -, - law covering C
, but this work was troublesome.

■, - In order to remove the second haptic, it is also necessary to remove the second haptic 1iID, which reduces the mechanical strength of the sensor section and shortens the life of the optical fiber. There was also the problem.

(2) Due to the above-mentioned problems such as strength and lifespan, it is not possible to make the sensor section long, so the sensor section naturally becomes short, and gas or liquid can only be detected with the short sensor section.

■Eccentric core single mode optical fiber has a large transmission loss and cannot be used as an optical transmission path, and the sensor section cannot be long. Conventionally, the sensor section was installed on a short eccentric core single mode optical fiber of several tens of centimeters. This was connected to eight long optical fibers for optical transmission and used as a detection optical fiber.However, this could only be detected by the sensor section.

It was not possible to install it over a long distance and detect it at multiple locations. In this case, conventionally, a plurality of the above-mentioned optical fibers for detection have been prepared and laid so as to shift the position of the sensor section. However, this increases the cost and limits the number of cables that can be installed, so the limited number of sensor parts is often located at locations where gas or liquid is likely to leak, such as at branch points or connections of gas pipes or liquid transport pipes. A detection optical fiber was installed so that the Even if the number of detection points is increased in this way, gases and liquids can only be detected at the sensor section, and cannot be detected between the sensor sections. It was not possible to measure the distribution state (distribution meter θ).

(Object of the Invention) The object of the present invention is that it can be used as an optical transmission path, and can detect gases and liquids without removing the target. An object of the present invention is to provide a detection optical fiber.

Means for Solving Problems f) In the optical fiber for detecting gas, liquid, etc. of the present invention, as shown in FIG. In an optical fiber for detecting gas, liquid, etc., in which a primary covering RAC having a refractive index higher than that of the cladding B is provided on the outer periphery of the cladding B, the core A of the homogeneous wave layer C is provided. The wall thickness on one side is thinner than the wall thickness on the opposite side.

(Function) In the optical fiber for detecting gases, liquids, etc. of the present invention, as shown in Fig. 1, the thickness of the primary coating C on the core A side is formed thin, so that part of the core light protrudes from the outer periphery of the core. A light wave propagating through the core light is difficult to be absorbed by the co-order FWC, so the light wave propagating outside the core of the core light is difficult to absorb by the co-order FWC. !
It becomes easy to be absorbed by the gas or liquid that diffuses or swells within the IC, and the decrease in optical output accompanying the absorption becomes clear, making it easy to detect.

Also, - next received F! Since the thickness of the IC on the side opposite to the core A is thicker than that on the opposite side, the Talarado mode light propagating within the cladding B is absorbed by the cladding C and is sufficiently removed.

(Example) FIG. 1 shows an example of the optical fiber for detecting gas, liquid, etc. of the present invention.

In the figure, A is the core, B is the cladding, C is the primary coating, and D
is the secondary coating. In this optical fiber for detection, eight cores having a refractive index higher than that of the cladding B are arranged at an eccentric position near the outer periphery of the cladding B, and a primary wave WIC having a refractive index higher than that of the cladding B is placed on the outer periphery of the cladding B. is formed, and the wall thickness on the core A side of the same-order mrmc is formed to be thinner than the wall thickness on the opposite side. Furthermore, in this embodiment, two claddings F! having a lower refractive index than the cladding B are placed on the outer periphery of the primary cladding 1c. ID is formed, and the thickness of the secondary coating on the core A side is thicker than that on the opposite side. The specifications of each component of this embodiment are as shown in Table 1 below.

As a comparative example of the present invention, a conventional eccentric core single mode optical fiber as shown in FIG. 2 with the specifications shown in Table 1 was prepared, and the two were compared in the following manner.

■ When the transmission loss of propagating light with a wavelength of 1.33 um was measured in both optical fibers, the transmission loss of the optical fiber according to the embodiment of the present invention was 2.5 dB/km, and even if it was long enough, it could not be used as an optical transmission line. It turns out it can be done. On the other hand, the transmission loss of the comparative optical fiber is 30 dB/
km or more, and it was impossible to use it in a long length.

08 The detection sensitivity of both optical fibers to methane gas at a concentration of 50% was measured. 1. The optical fiber of the embodiment of the present invention can detect the same gas without removing both the IWD and the primary wave 1'fic. Detection sensitivity is -0.5dB
It showed an excellent value of /100 mm, and it was found that detection could be performed over the entire length of the optical fiber. In contrast, in the optical fiber of the comparative example, no methane gas could be detected without the coating being stripped off.

(Table 1) Note that because the refractive index of the secondary cladding iD is lower than that of the cladding B, it does not participate in the transmission and reflection of propagating light as in the conventional case.However, gas and liquid are diffused into the secondary cladding FIID. Alternatively, the secondary coating is detected by being swollen and then diffused or swollen into the primary target FIIc, so in order to shorten the time required and improve detection sensitivity, the secondary coating must be coated with the desired mechanical properties. It is preferable that the wall be as thin as possible while still satisfying strength.

(Effects of the Invention) The optical fiber for detecting gas, liquid, etc. of the present invention has the following effects.

■Since the wall thickness on the eccentric core A side of the second coat is thinner than that on the opposite side, light waves propagating outside the core are difficult to be absorbed by the same atrium LC. Therefore, the transmission loss of the core light is dramatically reduced, making it possible to use it in a long length, and it can also be used as an optical transmission line.

■, Primary coating C of light waves propagating outside the cocoa periphery
Since the transmission loss of the core light due to absorption in the gas or liquid becomes smaller, the decrease in optical output due to the absorption of the same light wave in gas or liquid becomes clearer, and the ratio becomes larger, making it easier to detect. There is no need to remove the coating on the parts,
Problems such as strength and lifespan are eliminated.

■The transmission loss is small, so it can be used as an optical transmission line, and gases and liquids can be detected without removing the atrium RAC. can be detected, and by observing the backscattered light of the same optical fiber, it is also possible to measure the distribution.

[Brief explanation of the drawing]

FIG. 1 is a longitudinal sectional view of an embodiment of the optical fiber for detecting gas, liquid, etc. of the present invention, and FIG. 2 is a longitudinal sectional view of a conventional optical fiber for detecting gas, liquid, etc. A is the core B is the cladding C is the primary coating

Claims (1)

[Claims] A core A having a higher refractive index than the cladding B is placed at an eccentric position near the outer periphery of the cladding B, and a primary coating C having a higher refractive index than the cladding B is placed on the outer periphery of the cladding B.
An optical fiber for detecting gas, liquid, etc., which is provided with an optical fiber for detecting gas, liquid, etc., characterized in that the thickness of the core A side of the homogeneous coating C is formed to be thinner than the thickness of the opposite side. Optical fiber for detecting liquids, etc.