JPH0371043A - Gas sensor made of optical fiber - Google Patents

Abstract

PURPOSE:To obtain an optical fiber which can detect gas distributively over a wide range with one piece by connecting the plural measuring activity fibers disposed along a material to be measured by connecting optical fibers in series. CONSTITUTION:The measuring activity fiber 11 consists of a core part 1, a clad part 2 and the air permeable coating layer 3 applied around this part. A streak of continuous recesses 4 is formed in the clad part 2 along the optical axis direction thereof. This recess 4 is nearly circular in section and the inlet part on the coating layer 3 side thereof is narrower than the diameter. The bottom part extends near to the core part 1. The gas permeates the coating layer 3 and enters the inside of the recess 4 when such optical fiber is disposed in a certain gas atmosphere. Since this gas has the absorption wavelength specific to the material, an absorption loss arises near this point at the specific characteristic thereof. The gas sensor made of the optical fiber is constituted by connecting the plural measuring activity fibers 11 in series by the connecting fibers 12. The absorption loss distribution is measured along the longitudinal direction of the sensor in this way.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to an optical fiber gas sensor that can detect a target gas in an atmosphere over a wide range.

<Conventional technology> Conventionally, as a technology for detecting gas using an optical fiber, 0PTIC3LETTERS Vol. 1.2NC
Lp, P, 437-439 (1987). That is, an optical fiber with a diameter of 1.8 μm and a length of 10 μm is exposed to a methane gas atmosphere, and the evanescent waves of the optical fiber are used to detect that light is attenuated due to absorption of a specific wavelength possessed by methane gas. be.

<Problems to be Solved by the Invention> However, the above-mentioned conventional technology only detects the gas concentration in a local atmosphere, so in order to detect it distributed over a wide range, it is necessary to use the same method as a general gas sensor. A plurality of sensors must be installed in the measurement range, and in this case, it is difficult to determine the gas detection position from a remote location. Furthermore, since the fiber itself has an extremely small diameter of 1.8 μm, there is also a strength problem.

In view of these circumstances, the present invention aims to provide an optical fiber that can detect gas distributed over a wide range with a single optical fiber.

Means for Solving the Problems> The optical fiber gas sensor according to the present invention that achieves the above object includes a measurement active fiber disposed in the vicinity of a plurality of objects to be measured or along these objects to be measured; It consists of an optical fiber that connects active fibers in series, and at least one recess is formed on the surface of at least the cladding part of the active fiber for measurement, which reduces the absorption loss of gas existing around the active fiber for measurement. It is characterized by detecting gas using .

When the detection target gas in the atmospheric gas enters the recess of the active fiber for measurement, such as by passing through a gas-permeable coating, this gas absorbs a specific wavelength of the evanescent wave that is transmitted outside the core of the optical fiber. . That is, the gas to be detected has an absorption wavelength specific to the substance, and light of that specific wavelength is attenuated there, so that the transmission loss of the fiber locally increases at the specific wavelength. Therefore, by measuring the loss distribution in the longitudinal direction of the optical fiber with a backscattered light measuring device, it is possible to detect the location where the gas to be detected has entered.

However, the measurement active fiber has a structure in which the transmission loss increases with a small amount of gas, and the transmission loss itself is high, so it is not preferable to configure the transmission IIs path using only this measurement active fiber. Additionally, gas has diffusivity. Taking these into consideration, it would be better to arrange the measurement active fibers at regular intervals and connect them with ordinary optical fibers, which would have a preferable function as an optical fiber gas sensor. In addition, if the measurement active fiber is short and has low sensitivity to loss fluctuations due to gas, it is possible to increase the sensitivity by winding each part of the measurement active fiber into a coil to make it longer. Environmental resistance can also be achieved by placing the sensor in a storage case through which at least the gas to be measured can pass.

Example of implementation Hereinafter, the present invention will be explained based on examples.

FIG. 1 shows a cross section of a measurement active fiber according to one embodiment. As shown in the figure, this measurement fiber has a core part 1.
It consists of a cladding part 2 and an air-permeable coating layer 3 surrounding the cladding part 2, and the cladding part 2 has a continuous recessed part 4 formed along the optical axis direction. The recess 4 has a substantially circular cross section, the entrance on the coating layer 3 side is narrower than the diameter, and the bottom reaches near the core 1. B concavity 4
It is necessary to form a hollow structure so that the covering layer 4 does not penetrate as much as possible, and for this purpose, Q) is preferable, in which the entrance is narrow and the inside is widened.

When such an optical fiber is placed in a certain gas atmosphere, the gas passes through the gas permeable coating layer 3 and enters the recess 4 . Since this gas has an absorption wavelength unique to the material, absorption loss occurs at that characteristic wavelength.

In other words, as shown in Figure 2, which shows the light intensity distribution of light transmitted within an optical fiber, the light seeps out from the core part 1 with a high refractive index and is transmitted, and a part of the evanescent wave that travels outside the core part 1 is Since the light propagates through the concave portion 4, if there is gas in the concave portion 4, the specific wavelength of the protruding light is attenuated at that location. therefore,
Transmission loss increases locally at that specific wavelength, and by measuring the transmission loss distribution over the longitudinal direction of the optical fiber using a backscattered light measuring device, we can identify locations where transmission loss increases, that is, locations where gas exists. can be specified.

Here, the gas-permeable coating layer 3 may be formed of a coating material that transmits at least the gas to be detected, and is not particularly limited.

Further, the shape, number, depth, size, position, etc. of the recessed portions are not particularly limited. For example, the shape may be parabolic, semicircular, rectangular, triangular, etc. For example, the shape may be hollow as described above. In order to ensure this, as shown in FIG. 3, the entrance portion of the recess may be a narrow groove with a recess 4A having a circular cross section. Moreover, the number may be two, for example, provided symmetrically with the core part 1 in between. Further, the depth and size may be selected to be most appropriate considering the conditions under which the optical fiber is installed. For example, increasing the depth and size increases the detection sensitivity. Furthermore, the effect of increasing the depth of the recess and increasing the sensitivity can also be obtained by moving the core part from the center toward the recess. However, arranging the core portion at the center of the cladding portion has the advantage that outer diameter alignment becomes possible and connection becomes easier.

Next, an example of manufacturing the optical fiber shown in FIG. 1 will be described.

First, a base material for a normal single-mode optical fiber is manufactured by a conventional method, and this is cut, processed, and polished to form a base material having grooves that will become the recesses 4. For example, if this base material is heated at a temperature of 2
By heating and melting at 050° C. and drawing at a drawing speed of 50 m for 7 minutes and a tension of 40 g, the optical fiber of Example #1 above is formed.

Here, the diameter of the cladding part 2 of this measurement active fiber is 1
25μm1 spot size is 5.1μm1 cutoff wavelength is 1
．． The distance from the center of the core portion 1 to the bottom of the recess 4 is 7 μm, and the depth of the recess 4 is 50 μm. Further, as the coating layer 3, an ultraviolet curing resin having a refractive index substantially equal to that of quartz was used, and the finished diameter was set to 250 μm.

Next, an optical fiber gas sensor 10 as shown in FIG. 4 was manufactured using such a measurement active fiber. That is, a plurality of measurement active fibers 11 are connected in series by a connecting fiber 12, and this is connected to a backscattered light measuring device (hereinafter referred to as 0TD) via a dummy fiber 13 consisting of an 8M fiber.
(abbreviated as R) 14 to form an optical fiber gas sensor 10. This 0TDR14 generally has a wavelength of 0.85 μm.
, 1.3 μm.

A wavelength of 1.55 μm is widely used, but in this example, for the purpose of detecting methane gas, a wavelength of 1.33 μm, which is the absorption wavelength of methane gas, was used. This 0TDR14
For example, it includes means for inputting pulse-modulated signal light into an optical fiber, and means for receiving the backscattered light output generated within the optical fiber in the time domain and performing signal processing such as averaging processing and differentiation processing. The loss distribution over the longitudinal direction of an optical fiber in which the active fiber 11 and the connecting fiber 12 are connected in series can be measured in real time.

The connecting fiber 12 has a core made of quartz glass with a diameter of 8 μm and a core made of F-doped quartz glass with a diameter of 125 μm.
A fiber consisting of a cladding with a diameter of m is coated with silicone resin.
A material coated with a diameter of 00 μm was used.

Using such an optical fiber gas sensor 1o, one of the measuring active fibers 11 was surrounded by a container, and the container was filled with methane gas at a concentration of 5%. As a result, 1.
A transmission loss variation of (L80 dB) was detected in the 33 μm band.

Therefore, by arranging a fiber in which the measurement active fiber 11 and the connection fiber 12 of the optical fiber gas sensor 10 are connected, it is possible to detect the gas at the location where the measurement active fiber 11 is arranged, and conversely, The length of each connecting fiber 12 may be adjusted so that the measurement active fiber 11 can be placed at the desired location. Furthermore, in order to improve the detection sensitivity of each measurement location, the measurement active fiber may be wound into a coil and placed at the detection location. Furthermore, in this case, the above-mentioned air-permeable coating layer may be omitted or the coiled active fiber for measurement may be housed in an air-permeable case, thereby improving environmental resistance.

<Effects of the Invention> As explained above, the optical fiber gas sensor of the present invention uses a single fiber cable consisting of a plurality of measurement active fibers and an optical fiber connecting these fibers in series, which is arranged along the target object. For example, a gas detection system that would otherwise require hundreds of spot-type gas sensors and hundreds of copper cables can be replaced with just one fiber cable. It is only necessary to install the optical fiber, which has a great economical effect, and since the active fiber for measurement can be connected with a low-loss optical fiber, it can be made long and can be used over a wide range of areas. Furthermore, in order to increase the sensitivity, the measurement active fiber can be lengthened and made into a coil shape, and the sensor section can be protected from the external environment by storing this coiled measurement active fiber in a storage case. I can do it.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view of an optical fiber according to one embodiment of the present invention, FIG. 2 is an explanatory diagram showing its light intensity distribution, FIG. 3 is a cross-sectional view of an optical fiber according to another embodiment, and FIG. 1 is an external view showing an optical fiber gas sensor according to one embodiment. In the drawing, 1 is a core part, 2 is a cladding part, 3 is an air-permeable coating layer, 4 is a recessed part, 10 is an optical fiber gas sensor, 11 is a measurement active fiber, 12 is a connecting fiber, 13 is a dummy fiber, = 11- 14 is a backscattered light measuring device. 2-

Claims (3)

[Claims] (1) Consisting of a measurement active fiber disposed near or along a plurality of objects to be measured, and an optical fiber connecting these measurement active fibers in series, and at least a cladding of the measurement active fiber. 1. An optical fiber gas sensor, characterized in that at least one recess is formed on the surface of the fiber, and gas is detected by utilizing absorption loss of gas existing around the measurement active fiber. (2) In the optical fiber gas sensor according to claim 1,
The recessed cladding surface of the measurement active fiber is
An optical fiber gas sensor having an air-permeable coating that transmits at least a gas to be detected. (3) The optical fiber gas sensor according to claim 1 or 2, wherein the recess has a narrow entrance and a wide interior.