JP2511999B2 - Liquid detection optical fiber and liquid detection system using the same - Google Patents

Description

The present invention relates to a liquid detection optical fiber and a liquid detection optical fiber adapted to detect a liquid contact by a change in optical coupling between two cores of a dual core type optical fiber. The present invention relates to a liquid detection system using.

“Prior Art” As an optical fiber for liquid detection of distribution type, there is an eccentric core fiber in which a core 1 is eccentric in a clad 2 as shown in FIG. 7 proposed by the present inventor. (3rd Optical Fiber Sensor Workshop, Proceedings, 23 June 1986
Sun, pp. 67-73). In this eccentric core fiber, a part of the cladding mode of the core 1 is leaked to the outside as an evanescent wave, and when a liquid such as oil comes into contact with the fiber, the propagation state of the core 1 is changed and the amount of leak of the evanescent wave is changed. And the transmission loss of the core 1 increases.
By observing the increase in the transmission loss of the core 1 as a change in the intensity of the transmitted light or the backscattered light, it is possible to detect the presence or absence of contact with the liquid and the contact position.

[Problems to be Solved by the Invention] However, in the optical fiber for liquid detection composed of this eccentric core fiber, the propagation loss is large due to the scattering loss in the cladding 2 and the outside, the absorption loss due to the protective coating, and the like. However, there is a problem that it is not suitable for detecting the liquid.

"Means for Solving Problems" In the present invention, a dual core type fiber having two cores is used, the first core is arranged at the center of the cladding, and the cladding mode is prevented from leaking to the outside. , The second core is placed at a position where the optical coupling with the first core is small, and is positioned so as to be eccentric so that a part of the cladding mode leaks to the outside, and the liquid is deposited on the surface of the cladding. A liquid detecting optical fiber is one that increases the optical coupling between the first core and the second core when they come into contact with each other, and the liquid can be detected by injecting the measuring light into the first core. To solve the above problems.

FIG. 1 shows an optical fiber for liquid detection of the present invention (hereinafter,
Called the sensor fiber. ), The reference numeral 11 in the figure is a cladding. This cladding 11
Has a diameter of several tens to several hundreds of μm. The first core 12 is arranged substantially at the center of the cladding 11. The first core 12 has a higher refractive index than the cladding 11 and has a relative refractive index difference Δn of about 0.3 to 0.35% and a diameter of 8 to 10 μm. A second core is placed at an eccentric position near the surface of the cladding 11.
13 are arranged. The second core 13 has the same refractive index as the first core 12, and the diameter thereof is also the same as that of the first core 12. The distance between the first core 12 and the second core 13 is set to a distance that allows distributed coupling of guided light, and is usually 8 to 18
It is about μm. A portion of the cladding 11 around the second core 13 having the smallest thickness; the buffer region 14 has a thickness of about 0 to 8 μm, and the cladding mode of the light guided to the second core 13 A part of this is leaked to the outside.

A protective coating 15 is provided on the outside of the cladding 11. The protective coating 15 has a reinforcing effect of reinforcing the bare fiber and has a thickness of 3 to 20 μm and is made of a material such as silicone resin, urethane resin, polyamide-imide, etc., which has a property of transmitting a liquid to be detected such as oil. is there.

Further, the sensor fiber of the present invention may have a structure as shown in FIG. In the sensor fiber of this example, the cross-sectional shape of the cladding 11 has a modified cross-section in which a part of the arc is cut out in a plane, and the first core 12 is arranged at the center of the cladding 11, 2 core 13
Is located near the notched side. And
The distance between the first core 12 and the second core 13 is 8 to 18 μm, and the thickness of the buffer area 14 is about 0 to 8 μm.

In the sensor fiber having such a structure, the measurement light is incident on the first core 12 and guided. In the steady state where the liquid is not in contact with the sensor fiber, most of the measurement light is guided to the first core 12 because the coupling ratio between the first core 12 and the second core 13 is small. Waved and second core 13
Is almost not guided. FIG. 3 is a graph schematically showing the degree of exchange of the incident optical power between the first core 12 and the second core 13 in this steady state, and the horizontal axis represents the distance (Z) from the incident end, The vertical axis represents the intensity of incident light power. As shown in this graph, the incident light power is hardly transferred to the second core 13.

Also, liquid such as oil comes into contact with part of the sensor fiber,
When the surface of the cladding 11 comes into contact with the liquid, the second core
Since 13 is eccentric so that its cladding mode leaks to the outside, the propagation constant of the second core 13 changes. Now, assuming that the refractive index of the liquid is the same as the refractive index of the cladding 11, the propagation constants of the first core 12 and the second core 13 become equal, and the first core 12 and the second core 13 Becomes 1 and the power of the measuring light guided to the first core 12 is completely transferred to the second core 13,
Intensity of the measurement light of is reduced. Even if the refractive index of the liquid does not match the refractive index of the cladding 11, even if they approach each other, depending on the degree, the first core 12 to the second core
The power of the measurement light to the core 13 shifts, and the intensity of the measurement light of the first core 12 decreases. FIG. 4 is a graph schematically showing the degree of exchange of incident light power between the first core 12 and the second core 13 in this liquid contact state. It can be seen that the power is exchanged periodically depending on the distance (Z) from the incident end.

Thereby, by observing the intensity change of the transmitted light or the backscattered light of the measurement light of the first core 12, it is possible to know the presence or absence of the contact with the liquid and the position thereof.

In such a sensor fiber, the measurement light is emitted from the first part of the central portion in the steady state section not in contact with the liquid.
Since the light is guided to the core 12 and moves to the second core 13 only in the section in contact with the liquid, the transmission loss due to the scattering loss, the absorption loss, etc. is small, and the liquid can be detected in the long distance and the long section. Become.

FIG. 5 shows an example of the liquid detection system of the present invention, which is an example of the OTDR method for measuring backscattered light.

Pulsed light generated by a light source 16 such as a light emitting diode (LED) or a laser diode (LD) is incident on a first core 12 at an incident end of a sensor fiber 18 via a directional coupler 17. The backscattered light returning from the first core 12 of the sensor fiber 18 passes through the directional coupler 17 from the incident end, is received by the light receiver 19 such as an avalanche photodiode (APD), and is converted into an electric signal. It is input to the processor 20 and subjected to arithmetic processing. By obtaining the intensity and time delay of the backscattered light, a graph as shown in Fig. 6 is obtained, and the sensor fiber can be obtained from the bending point of the intensity curve of the backscattered light.
You can see that you know where the liquid is in contact with 18.

Further, the presence or absence of liquid contact can be known by determining the intensity change of the transmitted light from the first core 12 at the emission end of the sensor fiber 18.

"Example" A sensor fiber having the following specifications was prepared.

・ Cladding; material: pure silica, diameter 125 μm ・ first core: material: germanium-doped silica,
Δn = 0.3%, diameter 9μm, placed in the center of the cladding.

・ Second core; Material: Germanium-doped silica,
Δn = 0.3%, diameter 9μm, distance from the first core is 18μ
and the buffer area is 8 μm thick.

・ Protective coating ； Material: Polyamideimide, thickness 3μ
m Using this sensor fiber 100m, the backscattered light intensity by the OTDR method was measured and a liquid detection experiment was conducted. Approximately 50.0 m and 80.5 m of crude oil from the entrance end of the sensor fiber
It was deposited over a length of 10 cm. By measuring the backscattered light, the bending points of the backscattered light intensity appear at the points of time delay corresponding to the positions of 50.2 m and 80.3 m from the incident end, and the contact with crude oil and its position can be observed almost accurately. Was confirmed.

"Effects of the Invention" As described above, the liquid detection optical fiber of the present invention includes the first core and the second core in the common cladding.
, The first core is arranged at the center of the cladding so that the cladding mode does not leak to the outside, and the second core is arranged at a position where the optical coupling with the first core is small. And the eccentricity is such that a part of the cladding mode leaks to the outside, and when the liquid comes into contact with the surface of the cladding, the optical coupling between the first core and the second core is Further, the liquid detection system of the present invention is such that the measurement light is incident on the first core of the liquid detection optical fiber, and a change in the intensity of the transmitted light or the backscattered light is obtained to make contact with the liquid. The presence of a liquid such as oil and the position thereof can be observed as a distributed optical fiber liquid detection sensor. Further, since the measurement light is less attenuated as compared with the conventional sensor using the eccentric core optical fiber, it is possible to perform a measurement in a long distance and in a high section, and a highly sensitive sensing system can be obtained.

[Brief description of drawings]

1 and 2 are schematic cross-sectional views each showing an example of the liquid-detecting optical fiber of the present invention, and FIG. 3 is a diagram showing the first core and the first core in the steady state of the liquid-detecting optical fiber of the present invention. 2 is a graph schematically showing the degree of exchange of incident light power with the core, FIG. 4 is a graph schematically showing the degree of exchange of incident light power in the liquid contact state, and FIG. 5 is
A schematic configuration diagram showing an example of a liquid detection system of the present invention,
FIG. 6 is a graph showing the intensity change of the backscattered light by the OTDR method of the liquid detection system of the present invention, and FIG. 7 is a schematic sectional view showing a conventional distributed type liquid detection optical fiber. 11 …… Cladding, 12 …… First core, 13 …… Second
Core, 18 ... Sensor fiber.

Continuation of the front page (56) Reference JP-A 63-8536 (JP, A) JP-A 61-288140 (JP, A) JP-A 62-21042 (JP, A) JP-A 62-34036 (JP , A)

Claims (2)

(57) [Claims] 1. A first core and a second core are provided in a common cladding, and the first core is arranged at the center of the cladding so that the cladding mode does not leak to the outside of the cladding. The second core is arranged at a position where the optical coupling with the first core is small, and is eccentrically positioned so that a part of the cladding mode leaks to the outside of the cladding. An optical fiber for liquid detection, wherein the optical coupling between the first core and the second core is increased when the liquid comes into contact with the surface of the liquid. 2. A first core and a second core are provided in a common cladding, and the first core is arranged at the center of the cladding so that the cladding mode does not leak to the outside of the cladding. The second core is arranged at a position where the optical coupling with the first core is small, and is eccentrically positioned so that a part of the cladding mode leaks to the outside of the cladding. An optical fiber for liquid detection is used, which increases the optical coupling between the first core and the second core when a liquid comes into contact with the surface of the liquid, and the measurement light is incident on the first core of the optical fiber for liquid detection. A liquid detection system characterized by detecting a change in intensity of the transmitted light or the backscattered light to detect contact between the liquid detection optical fiber and the liquid.