JPH06300521A - Distribution multiplex type optical fiber sensor - Google Patents

Abstract

PURPOSE:To continuously detect a pressure and its impressed position by an optical fiber. CONSTITUTION:In an optical fiber 2 arranged with going and returning parts, branching optical couplers C1-C3 and combining couplers C4-C6, which correspond to each other, are inserted, these are connected together by main part optical fibers 4a-4c, a Mach- Zehnder interferometer M1 is constructed of the main part optical fiber 4a and an auxiliary part optical fiber 4a', which forms an optical path rounding the main part optical fiber 4b in the following stage of the main part, optical fiber 4a, the same interferometer M2 is constructed of the main part optical fiber 4b and the auxiliary part, optical fiber 4b', and plural interferometers are constructed in the same way to be arranged in series, so that a detection signal is obtained from the optical couplers C4-C6. In this case, the relation among the coherent length W of light source light, the optical fiber optical path length difference H between the main part and the auxiliary part, in each of the interferometers, and the optical path length difference D between the optical path length for the auxiliary part, optical fiber in each of the interferometers and the optical path length rounding from the optical coupler belonging to this interferometer to the auxiliary part optical fiber belonging to the interferometer in the following stage, is H<W<D.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is capable of measuring physical quantities such as pressure, strain or temperature, and the position to which pressure is applied, and is suitable for the case of detecting landslide, etc. The present invention relates to an optical fiber sensor.

[0002]

2. Description of the Related Art An example of a conventional optical fiber sensor of this type will be briefly described with reference to FIG. The pulsed light emitted from a light source 11 such as a semiconductor laser is incident on one end of an optical fiber (optical cable) 12 installed reciprocally, and is received by a light receiver 13 provided at the other end (end). Received light. The first to the third are sequentially provided on the outward path of the optical fiber 12.
Directional optical couplers C1 to C3 are inserted, and fourth to sixth directional optical couplers C4 to C6 are sequentially inserted in the return path. These couplers C1 and C6, C2 and C5, C3 and C4 are The optical fibers 14a to 14c are optically connected to each other.

Sensing elements 15a to 15d are provided in the optical fibers 14a to 14c and at the turning points of the optical fiber 12, respectively.
Is intervening. These sensing elements 15a to 15d measure the main physical quantities (temperature, pressure, strain, current, magnetic field, etc.) from the information of the light passing through this optical fiber, and the information from each sensing element 15a to 15d. Due to
It is known to which point of the optical fiber 12 the external pressure is applied, and it is applied to the detection of, for example, a landslide. An electric signal from the light receiver 13 detects the state of the entire optical fiber, that is, whether or not an abnormality has occurred.

[0004]

In the above-described sensor, since it is necessary to emit pulsed light from the light source 1 at regular time intervals for measurement, a plurality of sensing elements 15a to 15a.
The signal from 15d is inevitably obtained intermittently, that is, there is a problem that continuous detection operation cannot be performed.
The present invention proposes a distributed multiplex optical fiber sensor that avoids this problem.

[0005]

In order to solve this problem, in the first structure of the present invention, the optical fiber is inserted into the forward path of the reciprocally arranged optical fibers forming the path of the light emitted from the light source. The multiple optical couplers for branching and the multiple optical couplers for merging inserted in the return path are optically connected by the main part optical fiber, and the output light obtained from the optical coupler for merging is received by the light receiver. Then, the main part optical fiber and the branching optical coupler and the merging optical coupler to which the main part optical fiber is connected are formed by an optical path that goes around the main part optical fiber in the subsequent stage. A plurality of Mach-Cendar interferometers are configured in series with the secondary optical fiber.

In this case, the coherent length of the light emitted from the light source is W, and the difference in optical path length between the main optical fiber and the sub optical fiber of each Mach-Cendar interferometer is H.
And the optical path length of the sub-optical fiber of each Mach-chenda interferometer and the optical path length from the branch optical coupler and the converging optical coupler belonging to this Mach-chender interferometer to the sub-optical fiber belonging to the subsequent Mach-chenda interferometer. The difference is D
Then, H <W <D is selected.

As a second configuration of the present invention, the photodetector belonging to each Mach-chender interferometer in the sensor of the first configuration is omitted, and in each main-portion optical fiber and sub-portion optical-fiber of each Mach-chender interferometer, Insert a frequency modulator that frequency-modulates the intensity of light from the light source into different frequencies, and each light obtained from a plurality of Mach-Cendar interferometers is received by a common photoreceiver and converted into an electrical signal. The signal of the frequency difference in the frequency modulator belonging to each Mach-Cendar interferometer is extracted from the electrical signal, and the physical quantity applied to the optical fiber and its position are detected from these extracted signals.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An example of a distributed multiplex type optical fiber sensor according to the present invention will be described with reference to FIG. The light emitted from the light source 1 is received by the light receiver 3 through the optical fiber 2. The optical fiber 2 includes directional optical couplers C1 to C3 for branching,
The directional optical couplers C4 to C6 for merging are optically coupled, the optical couplers C1 and C6 are the optical fiber 4a, the optical couplers C2 and C5 are the optical fiber 4b, and the optical couplers C3 and C.
4 are optically coupled to each other by an optical fiber 4c, and the lights branched from the optical couplers C6 to C4 are respectively received by the light receiver 5
It is configured to be incident on a to 5c.

Hereinafter, the optical fibers 4a to 4c will be referred to as main part optical fibers. Further, the optical fiber portion (C from the optical coupler C1 to the optical coupler C2 through the optical fiber 2, the optical coupler C2, the optical fiber 4b, the optical coupler C5, the optical fiber 2).
The range from 1 to C2 to points Pb to C5 to C6) is defined as the sub optical fiber 4a '. Similarly, C2 to C3 to points Pc to C4 to C
The optical fiber portion in the range 5 is referred to as a sub optical fiber 4b ', and the optical fiber portion in the range from C3 to Pd to C4 is referred to as a sub optical fiber 4c'. As a result, the first described below
-Third Mach-Cendar interferometers M1-M3 (hereinafter simply referred to as interferometers) are configured. That is, the first interferometer M1
The second interferometer M2 is composed of the main part optical fiber 4a and the sub part optical fiber 4a ', and the third interferometer M3 is composed of the main part optical fiber 4b and the sub part optical fiber 4b'.
c and the sub optical fiber 4c '. As is clear from this, the interferometers M1 to M3 are arranged in series, and the main part optical fiber of the latter interferometer also serves as a part of the sub part optical fiber of the former interferometer. Looking at the first interferometer M1, the light from the light source 1 is split by the optical coupler C1, and the main part optical fiber 4a
And an optical coupler C6 through the sub optical fiber 4a ', and the light is incident on the light receiver 5a. Since the other interferometers M2 and M3 are similar, the description thereof will be omitted.

The optical path length of each optical fiber is determined as shown in FIG. Main part optical fiber 4a (C1-Pa-C6) = Ra Main part optical fiber 4b (C2-Pb-C5) = Rb Main part optical fiber 4c (C3-Pc-C4) = Rc Sub part optical fiber 4a '(C1) -C2-Pb-C5-C6 = L1 Sub-part optical fiber 4b '(C2-C3-Pc-C4-C5 = L2 Sub-part optical fiber 4c' (C3-Pd-C4) = L3 C1-C2-C3-Pc -C4-C5-C6 = L4 C2-C3-Pd-C4-C5 = L5 C1-C2-C3-Pd-C4-C5-C6 = L6

The difference H in the optical path length between the main optical fiber and the sub optical fiber in each of the interferometers M1 to M3 is H = L1-Ra in the first interferometer M1 and H = L2-Rb in the second interferometer M2. In the third interferometer M3, H = L3-Rc. These values H may be different for each interferometer.

Further, the difference D between the optical path length of the sub optical fiber of each interferometer and the optical path length of the sub optical fiber of the subsequent interferometer is different between the first and second interferometers M1 and M2. D = L4-L1 Between the second and third interferometers M2 and M3, D = L5-L2. Both of these values D may be different values.

In the present invention, when the coherent length of the light emitted from the light source 1 is W, each optical path length is selected so that H <W <D. As a specific example, when the coherent length W of the light emitted from the light source 1 is W = 20 m, the optical path length Ra = Rb = Rc = 490 m and the optical path length L
1 = L2 = L3 = 500 m, optical path length L4 = L5 = 100
You can choose 0m. A DFB type semiconductor laser can be used as the light source 1, and the intensity of light is constant and continuously emitted.

According to this configuration, in the first interferometer M1, the light passing through the main part optical fiber 4a (optical path length Ra),
The light that has passed through the sub optical fiber 4a '(optical path length L1) is received by the light receiver 5a, and the above two lights interfere due to the relationship of H (= L1-Ra) <W, and this interference signal is received by the light receiver 5a. Obtained from In this case, the light passing through the optical path lengths L4 and L6 is also received by the light receiver 5a, but W <D (= L4-L
Because of the relationship 1), the light passing through the optical path length L4 does not interfere with the light passing through the main part optical fiber 4a and the sub part optical fiber 4a '. The same applies to light passing through the optical path length L6.

Therefore, for example, when pressure is applied from the outside to the point Y in FIG. 1, the interference of the light passing through the main part optical fiber 4b and the sub part optical fiber 4b 'of the second interferometer M2 occurs. It changes from the normal state, and this state can be detected by the output from the light receiver 5b. Further, in this case, the total amount of light is reduced in the light receiver 5c of the third interferometer M3, so in this case, it can be seen that the external pressure is applied between the optical couplers C2 and C3.

In the second interferometer M2, the light passing through the main part optical fiber 4b (optical path length Rb) and the light passing through the sub part optical fiber 4b '(optical path length L2) are received by the light receiver 5b, From the relationship of H (= L2-Rb) <W, the above-mentioned two lights interfere with each other, and this interference signal is obtained from the light receiver 5b. In this case, the light passing through the optical path length L5 is also received by the light receiver 5b, but since there is a relationship of W <D (= L5-L2), the light passing through the optical path length L5 is the main part optical fiber 4b and the sub part. It does not interfere with the light passing through the optical fiber 4b '. In the third interferometer M3, the main part optical fiber 4c (optical path length R
c), and the sub optical fiber 4c '(optical path length L
3) is received by the light receiver 5c, and H (= L3
From the relationship of −Rc) <W, the above two lights interfere with each other, and this interference signal is obtained from the light receiver 5c.

In this way, it is possible to obtain the respective interference signals in the respective light receivers 5a to 5c constituting the respective interferometers, and further to detect these interference signals in the state where the light source 1 emits continuous light. Therefore, the state can always be detected continuously. In the illustrated example, three Mach-chenda interferometers M1 to M3 are configured.
It is not limited to this example.

FIG. 3 shows another embodiment of the present invention. The parts corresponding to those in FIG. 1 are designated by the same reference numerals and the description thereof will be omitted. However, the light receivers 5a to 5c are not required. In this example, with respect to the first interferometer M1 described in FIG. 1, the frequency modulator 6a is inserted in the main part optical fiber 4a, and the frequency modulator 6b is inserted in the sub part optical fiber 4a '. The main part of the second interferometer M2 is the optical fiber 4b.
The frequency modulator 6b is inserted in the sub optical fiber 4
The frequency modulator 6c is inserted in b '. Regarding the third interferometer M3, the frequency modulator 6c is inserted in the main part optical fiber 4c, and the frequency modulator 6d is inserted in the sub part optical fiber 4c '. As is clear from this, the frequency modulator 6b in the sub optical fiber 4a 'of the interferometer (eg, M1) in the former stage also serves as the frequency modulator in the main optical fiber 4b of the interferometer (eg, M2) in the latter stage. is doing.

These first to fourth frequency modulators 6a to 6
d is for modulating the light emitted from the light source 1 to frequencies f1 to f4 with respect to the intensity thereof. Also in this example, the relationship between the coherent length W of the light from the light source 1 and the optical path length of each interferometer M1, M2, M3 (H <W <
D) is similar to the case described in FIG. In this configuration, the interference light in which the intensity of the light is modulated is obtained from each of the couplers C6, C5, and C4 with the sum and difference frequencies described below.

C6: Light modulated at the sum of frequencies f1 and f2 (f1 + f2) C6: Frequency of difference between frequencies f1 and f2 (f1-f2)
Light modulated by C5: frequency of sum of frequencies f2 and f3 (f2 + f3)
Light modulated by C5: Frequency difference between frequencies f2 and f3 (f2-f3)
Light modulated by C4: Frequency of sum of frequencies f3 and f4 (f3 + f4)
Light modulated by C4: frequency difference between frequencies f3 and f4 (f3-f4)
Light modulated by

Note that the light having the frequency f1 does not interfere with the light having the frequency f3 as described with reference to FIG. The same applies to the frequency f4. Further, the light having the frequency f2 has the frequency f4.
Since it does not interfere with the light having, the light due to the sum difference between them cannot be obtained. Each of these lights is made incident on a common light receiver 3 and each electric signal (S12, S12 ', S23, S23', S34,
S34 ') and these are supplied to the low-pass filter 7 and sum signals (S12, S23,
S34) is removed and the difference signals (S12 ', S23', S34 ') are removed.
Are input to the first to third band pass filters 8a to 8c.

The first to third band pass filters 8a to 8
The signal pass bandwidths of c are f1-f2 and f2-f, respectively.
3, f3-f4. Therefore, the difference signal S12 'is obtained from the first band pass filter 8a, the difference signal S23' is obtained from the second band pass filter 8b, and the difference signal S34 'is obtained from the third band pass filter 8c. Therefore, the position of the external pressure applied to the optical fiber 2 and its physical quantity are detected from these signals. Further, since continuous light is emitted from the light source 1, continuous measurement is possible.

[0023]

According to the present invention described above, there is an effect that the signals of a plurality of detection points can be taken out individually and continuously. Further, there is an effect that the states of a plurality of points can be continuously detected by the output from the common light receiver 3.

[Brief description of drawings]

FIG. 1 is an optical fiber connection diagram showing an example of a distributed multiplexing optical fiber sensor according to the present invention.

FIG. 2 is an explanatory diagram illustrating a length between specific points in FIG.

FIG. 3 is an optical fiber connection diagram showing another example of the distributed multiplex type optical fiber sensor according to the present invention.

FIG. 4 is an explanatory diagram of an optical path using a conventional optical fiber and showing a sensor for measuring a pressure, a physical quantity such as strain or temperature, and a position to which a pressure is applied.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hiroyuki Sasaki 1-21-6 Dogenzaka, Shibuya-ku, Tokyo Inside Japan Aviation Electronics Industry, Ltd. (72) Shinji Motohara 1-21-6 Dogenzaka, Shibuya-ku, Tokyo No. Japan Aviation Electronics Industry Co., Ltd. (72) Inventor Shuichi Aihara 1-21-6 Dogenzaka, Shibuya-ku, Tokyo Japan Aviation Electronics Industry Co., Ltd. (72) Yuzo Takasaka 1-26-1 Dogenzaka, Shibuya-ku, Tokyo No. Japan Aviation Electronics Industry Co., Ltd. (72) Inventor Masaaki Ujikawa 1-21-6 Dogenzaka, Shibuya-ku, Tokyo Japan Aviation Electronics Industry Co., Ltd.

Claims (2)

[Claims] 1. A reciprocally arranged optical fiber forming a path of light emitted from a light source, a plurality of branching optical couplers inserted into the forward path of the optical fiber, and a return path of the optical fiber. A plurality of merging optical couplers inserted corresponding to the branching optical coupler, and a main part optical fiber optically connecting between each of the corresponding branching optical couplers and merging optical couplers. A plurality of light receivers for receiving the output light obtained from the merging optical coupler and converting it into an electric signal, the main optical fiber, and a branching optical coupler in which the main optical fiber is connected, A plurality of Mach-Cendar interferometers are configured in series with a sub-optical fiber formed by an optical path around the main optical fiber located one stage after the converging optical coupler, and the coherent length of the light emitted from the light source is W
Let H be the difference in optical path length between the main optical fiber and the sub optical fiber of each Mach-chender interferometer, and the optical path length of the sub optical fiber of each Mach-chender interferometer and the branching light belonging to this Mach-chender interferometer. When the difference between the coupler and the optical coupler for merging and the optical path length around the secondary optical fiber belonging to the subsequent Mach-Cendar interferometer is D, H
A distributed multiplex type optical fiber sensor characterized in that <W <D. 2. A reciprocally arranged optical fiber forming a path of light emitted from a light source, a plurality of branching optical couplers inserted in the forward path of the optical fiber, and in the return path of the optical fiber. A plurality of merging optical couplers inserted corresponding to the branching optical coupler, and a main part optical fiber optically connecting between each of the corresponding branching optical couplers and merging optical couplers. A plurality of main part optical fibers and a sub part optical fiber formed by an optical path around the main part optical fiber one stage after the branching optical coupler and the merging optical coupler to which the main part optical fibers are connected. Of Mach-Cendar interferometers are connected in series, and the coherent length of the light emitted from the light source is W,
Let H be the difference in optical path length between the main optical fiber and the sub optical fiber of each Mach-chender interferometer, and the optical path length of the sub optical fiber of each Mach-chender interferometer and the branching light belonging to this Mach-chender interferometer. When the difference between the coupler and the optical coupler for merging and the optical path length around the secondary optical fiber belonging to the subsequent Mach-Cendar interferometer is D, H
<W <D, and a frequency modulator for frequency-modulating the intensity of light from the light source into different frequencies is inserted into the main part optical fiber and the sub part optical fiber of each Mach-chenda interferometer. Then, each light obtained from each Mach-chenda interferometer is received by a common photo-receiver and converted into an electric signal, and from this electric signal, the signal of the difference in frequency in the frequency modulator belonging to each Mach-chenda interferometer is detected. A distributed multiplex optical fiber sensor, which is characterized by detecting the physical quantity applied to an optical fiber and the position thereof extracted from these extracted signals.