CN113790678A - Multi-core optical fiber vector bending sensor with optical vernier effect - Google Patents
Multi-core optical fiber vector bending sensor with optical vernier effect Download PDFInfo
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Abstract
The invention provides a multi-core optical fiber vector bending sensor with an optical vernier effect, which comprises a wide-spectrum light source, a multi-mode coupler, a spectrum analyzer and a multi-core optical fiber serving as a sensing head, wherein the multi-core optical fiber comprises a plurality of fiber cores, the center of the multi-core optical fiber is provided with the fiber cores, the rest fiber cores are symmetrically arranged around the fiber core at the center, interferometers are respectively arranged in the fiber cores, and the lengths of the light paths of the interferometers in different fiber cores are different; the multimode coupler comprises an a port, a b port and a c port, the output end of the wide-spectrum light source is connected with the a port of the multimode coupler, the b port of the multimode coupler is connected with the multi-core optical fiber, the interferometers in the plurality of cores form an optical vernier effect through the parallel connection of the multimode coupler, and the c port of the multimode coupler is connected with the spectrum analyzer. The interferometer is arranged in the fiber core of the multi-core optical fiber to form the vector bending sensor with the optical vernier effect, and the method has the advantage of high sensitivity.
Description
Technical Field
The invention relates to the field of optical fiber bending sensing, in particular to a multi-core optical fiber vector bending sensor with an optical vernier effect.
Background
The optical fiber sensor has been widely used in various application scenarios due to its unique advantages, and the bending sensing measurement is an important application scenario. The bending sensing measurement has important application value and significance in the fields of large-scale engineering application, building structure quality monitoring, intelligent robots and the like, so that the development of the optical fiber bending sensor has important significance.
Common fiber optic bend sensors can be classified into fiber grating bend sensors, michelson interference type bend sensors, mach-zehnder interference type bend sensors, fabry-perot interference type bend sensors, and sagnac interference type sensors according to different principles. Most of common vector bending sensors are based on fiber grating structures, and other interference type bending sensors can realize directional bending measurement but can realize vector bending measurement rarely by constructing an asymmetric structure.
Meanwhile, the sensitivity of the fiber grating vector bending sensor is basically between dozens of picometers per curvature, and the sensitivity enhancement is relatively difficult.
The published date is 30/07/30/2021, and chinese patent publication No. CN113188468A discloses a vector bending sensing system based on a dual-core few-mode fiber tilt grating, which includes a broadband light source, a circulator, a fan-in fan-out device, a dual-core few-mode fiber and a spectrometer; the broadband light source inputs broadband light into the double-core few-mode optical fiber for reflection through the circulator and the fan-in fan-out equipment; the reflected light is output to a spectrometer through a circulator, and the reflectivity of a resonance peak before bending is recorded; bending the double-core few-mode optical fiber in a certain direction, and recording the reflectivity of a resonance peak after bending by a spectrometer; and calculating to obtain the reflectivity changes of the two fiber cores before and after bending, substituting the reflectivity changes into a matrix equation, and simultaneously obtaining the bending direction angle and the curvature. However, the patent also has the problem of low sensitivity of the sensor.
Disclosure of Invention
The invention provides a multi-core optical fiber vector bending sensor with an optical vernier effect, provides a vector bending sensor based on an interferometer, and solves the problem of low sensitivity of the bending sensor.
In order to solve the technical problems, the technical scheme of the invention is as follows:
a multi-core fiber vector bend sensor with optical vernier effect comprising a broad spectrum light source, a multimode coupler and a spectrum analyzer, the bend sensor further comprising a multi-core fiber as a sensing head, wherein:
the multicore fiber comprises a plurality of fiber cores, one fiber core is arranged at the center of the multicore fiber, the rest fiber cores are symmetrically arranged around the fiber core at the center, the plurality of fiber cores are respectively provided with an interferometer, and the lengths of optical paths of the interferometers in different fiber cores are different;
the multi-mode coupler comprises an a port, a b port and a c port, wherein the output end of the wide-spectrum light source is connected with the a port of the multi-mode coupler, the b port of the multi-mode coupler is connected with the multi-core optical fiber, the interferometers in the plurality of cores are formed by the multi-mode coupler and connected to form an optical vernier effect, and the c port of the multi-mode coupler is connected with the spectrum analyzer.
Preferably, the multicore fiber is a four-core fiber, one core is arranged at the center, the remaining three cores are symmetrically arranged around the center, the included angle between the remaining three cores is 120 degrees, and the optical path length of the interferometer FPI0 of the core at the center is longer than that of the interferometers in the other cores.
Preferably, the interferometer in the core is a fabry-perot interferometer.
Preferably, the reflective surface of the fabry-perot interferometer is characterized by a femtosecond laser processing technique.
Preferably, the interferometer in the core at the center FPI0 is a reference interferometer, the interferometers in the other cores FPI1, FPI2 and FPI3 are sensing interferometers, and the interferometers FPI0 and FPI1 is connected in parallel to form a first vernier evenlope1, and the interferometer FPI0 and the interferometer FPI2 are connected in parallel to form a second vernier evenlope 2; the interferometer FPI0 and the interferometer FPI3 are connected in parallel to form a third vernier evenlope3, and the amplification factor M of the optical vernier effect formed by the reference interferometer and the sensing interferometer is defined as the vernier envelope free spectral range FSRevenlopeAnd the sensing interferometer free spectral range FSR':
δ＝OPD′-OPD″
in the formula, FSR ' represents the free spectral range of the reference interferometer, OPD ' represents the optical path length of the sensing interferometer, delta is the detuning amount, and OPD ' represents the optical path length of the reference interferometer.
Preferably, the sensor measures a bending vector
The specific method comprises the following steps:
the wavelength drift amount is measured by an interferometer with three fiber cores at the outer side
n＝1,2,3；
Bending vector
Vector components projected on the outer three cores
Can be expressed as:
in the formula, KnThe maximum bending sensitivity of each fiber core in the range of 0-360 degrees of azimuth angles;
bending vector
Can be reconstructed from the amount of wavelength drift of the interferometers in any two outer cores.
Preferably, the wavelength drift bending sensitivity is obtained from the spectrum collected by the spectrum analyzer, and the interference spectrum of the interferometer in each fiber core is obtained through fast fourier transform and a filtering algorithm, so that the wavelength drift and the bending sensitivity of the interferometer in each fiber core are obtained.
Preferably, the multi-mode coupler is a large-core-diameter 2 × 2 multi-mode coupler and comprises an a port, a b port, a c port and a d port, wherein the d port is subjected to bevel cutting treatment or is immersed in an index matching fluid to prevent end surface reflection from affecting the sensor.
Preferably, the multi-core fiber bending device further comprises a first three-dimensional displacement platform and a second three-dimensional displacement platform, wherein one end of the multi-core fiber is placed on the first three-dimensional displacement platform, the other end of the multi-core fiber is placed on the second three-dimensional displacement platform, and the first three-dimensional displacement platform and the second three-dimensional displacement platform are used for adjusting the bending curvature of the multi-core fiber.
Preferably, the optical fiber bending device further comprises a first optical fiber rotating holder and a second optical fiber rotating holder, wherein one end of the multi-core optical fiber is placed on the first three-dimensional displacement platform through the first optical fiber rotating holder, the other end of the multi-core optical fiber is placed on the second three-dimensional displacement platform through the second optical fiber rotating holder, and the first optical fiber rotating holder and the second optical fiber rotating holder are used for adjusting the bending direction of the multi-core optical fiber.
Compared with the prior art, the technical scheme of the invention has the beneficial effects that:
the interferometer is arranged in the fiber core of the multi-core optical fiber, the vector bending sensor which has sensitivity amplification effect and is used for simultaneously measuring the bending size and the bending direction based on the optical vernier effect is formed, the vector bending sensor has the advantages of small structure, high integration level, high sensitivity and the like, and meanwhile, the interferometer of the middle fiber core can be used for compensating the temperature, so that the influence of temperature change on vector bending measurement is avoided.
Drawings
FIG. 1 is a schematic diagram of a multi-core fiber vector bending sensor with vernier effect according to an embodiment.
Fig. 2 is a schematic cross-sectional view of a multicore fiber.
FIG. 3 is a schematic side view of an interferometer in a multi-core fiber.
FIG. 4 is a schematic view of the bend azimuthal orientation of an embodiment.
FIG. 5 is an exploded view of the vector bending in the embodiment, wherein (a) the bending vector is located in the first quadrant; (b) the bending vector is positioned in the second quadrant; (c) the bending vector is positioned in the third quadrant; (d) the bending vector is located in the fourth quadrant.
In the figure, 1 is a wide-spectrum light source; 2 is a multimode coupler; 3 is a multi-core fiber, and 3-0 is a Fabry-Perot interferometer FPI0 on the middle core 0 of the multi-core fiber; 3-1 to 3-3 are Fabry-Perot interferometers FPI1, FPI2 and FPI3 on three outer fiber cores; 4 is a spectrum analyzer; 5 is a first optical fiber rotary holder, 6 is a second optical fiber rotary holder; a 7-dimensional first three-dimensional displacement platform, and 8 is a second three-dimensional displacement platform; and 9, a computer processing system.
Detailed Description
The drawings are for illustrative purposes only and are not to be construed as limiting the patent;
for the purpose of better illustrating the embodiments, certain features of the drawings may be omitted, enlarged or reduced, and do not represent the size of an actual product;
it will be understood by those skilled in the art that certain well-known structures in the drawings and descriptions thereof may be omitted.
The technical solution of the present invention is further described below with reference to the accompanying drawings and examples.
Example 1
The present embodiment provides a multi-core fiber 3 vector bending sensor with optical vernier effect, which includes a wide spectrum light source 1, a multi-mode coupler 2 and a spectrum analyzer 4, as shown in fig. 1, the bending sensor further includes a multi-core fiber 3 as a sensing head, where:
the multicore fiber 3 comprises a plurality of fiber cores, one fiber core is arranged at the center of the multicore fiber 3, the rest fiber cores are symmetrically arranged around the fiber core at the center, the plurality of fiber cores are respectively provided with interferometers, and the lengths of the light paths of the interferometers in the different fiber cores are different;
the multimode coupler 2 comprises an a port, a b port and a c port, wherein the output end of the wide spectrum light source 1 is connected with the a port of the multimode coupler 2, the b port of the multimode coupler 2 is connected with the multi-core optical fiber 3, the plurality of fiber cores form an optical vernier effect through the multimode coupler 2 and the junction structure, and the c port of the multimode coupler 2 is connected with the spectrum analyzer 4.
The spectrum obtained by the spectrum analyzer 4 is uploaded to a computer processing system 9 for processing.
The multicore fiber 3 is a four-core fiber, as shown in fig. 2 and 3, a core is disposed at the center, the remaining three cores are symmetrically disposed around the center, an included angle between the remaining three cores is 120 °, and an optical path length of an interferometer FPI0 of the core at the center is longer than that of interferometers in other cores.
The interferometer in the fiber core is a Fabry-Perot interferometer.
The reflecting surface of the Fabry-Perot interferometer is carved by a femtosecond laser processing technology.
The use of secondary scales in measuring devices and instruments can improve measurement resolution and reduce measurement uncertainty. The sensitivity and resolution of the sensor can be further improved by utilizing the optical vernier effect in the field of optical fiber sensing, and a solution is provided for improving the performance of the optical fiber sensor. Two interferometers are combined (in parallel or in series) to generate the optical vernier effect in the fiber, the interferometer in the center core FPI0 being the reference interferometer, the interferometers in the other cores FPI1, FPI2 and FPI3 being the sensing interferometers, and the stem being the dry interferometerThe interferometer FPI0 and the interferometer FPI1 are connected in parallel to form a first vernier evenlope1, and the interferometer FPI0 and the interferometer FPI2 are connected in parallel to form a second vernier evenlope 2; the interferometer FPI0 and the interferometer FPI3 are connected in parallel to form a third vernier evenlope3, and the amplification factor M of the optical vernier effect formed by the reference interferometer and the sensing interferometer is defined as the vernier envelope free spectral range FSRevenlopeAnd the sensing interferometer free spectral range FSR':
δ＝OPD′-OPD″
in the formula, FSR ' represents the free spectral range of the reference interferometer, OPD ' represents the optical path length of the sensing interferometer, delta is the detuning amount, and OPD ' represents the optical path length of the reference interferometer.
Therefore, in practical application, the sensitivity of the sensing interferometer can be amplified by designing the detuning amount between the optical paths of the sensing interferometer and the reference interferometer to determine the amplification factor. In particular, to ensure that the interferometer spectra on the outer three cores are distinguishable, the optical path lengths of interferometers FPI1, FPI2, and FPI3 are different. However, to ensure that the vernier effect can be formed with interferometer FPI0, the magnification factors M1 (vernier 1: FPI0-FPI1), M2 (vernier 2: FPI0-FPI2) and M3 (vernier 3: FPI0-FPI3) should be similar.
The sensor measures a bending vector
The specific method comprises the following steps:
the wavelength drift amount is measured by an interferometer with three fiber cores at the outer side
n＝1,2,3；
Vector of bendingMeasurement of
Vector components projected on the outer three cores
Can be expressed as:
in the formula, KnThe maximum bending sensitivity of each fiber core in the range of 0-360 degrees of azimuth angles;
bending vector
Can be reconstructed from the amount of wavelength drift of the interferometers in any two outer cores.
The wavelength drift bending sensitivity is obtained from the spectrum collected by the spectrum analyzer 4, and the interference spectrum of the interferometer in each fiber core is obtained through fast Fourier transform and a filtering algorithm, so that the wavelength drift and bending sensitivity of the interferometer in each fiber core are obtained.
The multimode coupler 2 is a large-core-diameter 2 x 2 multimode coupler 2 and comprises an a port, a b port, a c port and a d port, wherein the d port is subjected to bevel cutting treatment or is immersed in refractive index matching fluid.
The multi-core optical fiber bending device is characterized by further comprising a first three-dimensional displacement platform 7 and a second three-dimensional displacement platform 8, wherein one end of the multi-core optical fiber 3 is placed on the first three-dimensional displacement platform 7, the other end of the multi-core optical fiber 3 is placed on the second three-dimensional displacement platform 8, and the first three-dimensional displacement platform 7 and the second three-dimensional displacement platform 8 are used for adjusting the bending curvature of the multi-core optical fiber 3.
The optical fiber bending device further comprises a first optical fiber rotating holder 5 and a second optical fiber rotating holder 6, wherein one end of the multi-core optical fiber 3 is placed on the first three-dimensional displacement platform 7 through the first optical fiber rotating holder 5, the other end of the multi-core optical fiber 3 is placed on the second three-dimensional displacement platform 8 through the second optical fiber rotating holder 6, and the first optical fiber rotating holder 5 and the second optical fiber rotating holder 6 are used for adjusting the bending direction of the multi-core optical fiber 3.
Example 2
In this embodiment, a method for measuring a bending vector is provided by using the multi-core fiber vector bending sensor with an optical vernier effect provided in embodiment 1, and specifically includes:
Fabry-Perot interferometer bending sensing principle:
the individual FPI output light intensity can be expressed as:
in the above formula, I1Representing the intensity of light reflected from a first reflecting surface of a Fabry-Perot interferometer, I2Indicating the intensity of the light reflected by the second reflective surface,
representing the phase difference, which can be expressed as:
in the formula I0Is the input light intensity; r1And R2Respectively representing the reflectivity of a first reflecting surface and a second reflecting surface of the Fabry-Perot interferometer; α represents the cavity loss of the Fabry-Perot interferometer, and in this example, the cavity is a fiber core cavity, so the loss can be ignored; n represents the refractive index of the Fabry-Perot cavity medium, in the embodiment, because the femtosecond laser is used for writing a reflecting surface on the fiber core, the refractive index represents the refractive index of the fiber core; l is the cavity length, OPD is the fabry-perot interferometer optical path length written OPD as 2nL, and λ is the wavelength of the incident light.
The Free Spectral Range (FSR) is expressed as:
when the phase difference satisfies
When m is 0,1,2, the resonance valley wavelength in the reflection spectrum can be expressed as:
when the bending magnitude applied to the sensor is C, the strain caused by the bending is ∈ ═ d/R ═ d · C, d denotes the distance from the bending plane to the neutral plane of the optical fiber, R denotes the bending radius, and C denotes the bending curvature magnitude. Because of the optical fiber deformation and the stress optical effect, the optical path length OPD of the interferometer changes and can be written as follows:
ΔOPD＝2(L·Δn+n·ΔL)·Δε＝2(L·Δn+n·ΔL)·d·C
it can be seen that the sensor resonant wavelength is directly proportional to the curvature, so the amount of curvature imparted to the fiber length can be obtained by measuring the amount of resonant valley wavelength shift.
The experimental setup shown in fig. 3 can measure the sensor vector bending response. The multi-core fiber 3 is connected to the multi-mode coupler 2, and then connected to the spectrum analyzer 4 to measure its reflection spectrum. The multicore fiber 3 is mounted on a pair of three-dimensional displacement stages 7 and 8 using two fiber rotary clampers 5 and 6, and the bending curvature is adjusted by moving the displacement stages, and the bending direction can be achieved by rotating the two fiber rotary clampers simultaneously. As shown in FIG. 4, the bend azimuth angle θ is defined as the angle between the fiber bend plane and the axis that passes through the central core and connects to the outer core No. 1.
The amount of curvature is set by adjusting the distance between the two displacement stages, followed by changing the direction of bending by rotating both fiber rotating grippers simultaneously in steps of 15 °. The reflectance spectrum was recorded for each magnitude and direction of curvature. Due to the optical fiber strain effect, the interferometer positioned at the outer side of the bend is stretched, the optical path length is lengthened, and the reflection spectrum drifts towards the long wave direction; similarly, the interferometer positioned at the inner side of the bend is compressed, the optical path length is shortened, and the reflection spectrum shifts towards the short wave direction. However, at any angle of the fiber, the interferometer on the middle core (i.e., FPI0) is always on the neutral plane, which makes it insensitive to bending. Therefore, the interferometer can be used as a reference interferometer in vernier effect and can be used for compensating the wavelength shift caused by temperature, thereby overcoming the problem of cross sensitivity between bending and temperature in vector bending sensing application.
The spectrum collected by the spectrum analyzer in the experiment is a superposition signal of four interferometers, and because the actual physical lengths of the four interferometers are different, the interference spectrum of each interferometer can be obtained through fast Fourier transform and a filtering algorithm, so that the wavelength drift amount and the bending sensitivity of each interferometer are obtained.
For reconstructing bending vectors
Assuming that the cross-section of the multi-core fiber remains circular, the amount of each FPI wavelength shift can be measured by the apparatus of example 1. Bending vector according to the relationship between the amount of wavelength drift and the magnitude of curvature and sensitivity
Vector components projected on three outer FPIs
Can be expressed as:
in the formula KnThe maximum bending sensitivity of each FPI in the azimuth angle range of 0-360 degrees is obtained.
Bending the vector as shown in FIG. 5
Component diagrams projected into the three outer cores in four quadrants. Bending vector
Can be reconstructed according to the wavelength drift of any two outer fiber cores FPI. In this example, the three outer cores are 120 ° from each other, so there are 3 combinations (e.g.
) Is used for curvature reconstruction and three averages can be taken in the reconstruction to obtain a more accurate value.
For example, bending vectors as shown in FIG. 5(a)
In the first quadrant can be expressed as:
in the formula y1，y2And y3Comprises the following steps:
the corresponding azimuth angle can be calculated by:
when bending vector
In other image limits, the curved vector reconstruction can also be performed according to the same principles.
The same or similar reference numerals correspond to the same or similar parts;
the terms describing positional relationships in the drawings are for illustrative purposes only and are not to be construed as limiting the patent;
it should be understood that the above-described embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. Other variations and modifications will be apparent to persons skilled in the art in light of the above description. And are neither required nor exhaustive of all embodiments. Any modification, equivalent replacement, and improvement made within the spirit and principle of the present invention should be included in the protection scope of the claims of the present invention.
Claims (10)
1. A multi-core fiber vector bending sensor with optical vernier effect, comprising a broad spectrum light source, a multimode coupler and a spectrum analyzer, characterized in that the bending sensor further comprises a multi-core fiber as a sensing head, wherein:
the multicore fiber comprises a plurality of fiber cores, one fiber core is arranged at the center of the multicore fiber, the rest fiber cores are symmetrically arranged around the fiber core at the center, the plurality of fiber cores are respectively provided with an interferometer, and the lengths of optical paths of the interferometers in different fiber cores are different;
the multi-mode coupler comprises an a port, a b port and a c port, wherein the output end of the wide-spectrum light source is connected with the a port of the multi-mode coupler, the b port of the multi-mode coupler is connected with the multi-core optical fiber, the interferometers in the plurality of cores are formed by the multi-mode coupler and connected to form an optical vernier effect, and the c port of the multi-mode coupler is connected with the spectrum analyzer.
2. The multi-core fiber vector bending sensor with optical vernier effect as claimed in claim 1, wherein the multi-core fiber is a four-core fiber, one core is disposed at the center, the remaining three cores are symmetrically disposed around the center, the remaining three cores are at an angle of 120 ° with respect to each other, and the optical path length of the interferometer FPI0 of the core at the center is longer than that of the interferometers in the other cores.
3. The multi-core fiber vector bending sensor with optical vernier effect of claim 2, wherein the interferometer in the fiber core is a fabry-perot interferometer.
4. The multi-core fiber vector bending sensor with optical vernier effect of claim 3, wherein the reflective surface of the Fabry-Perot interferometer is characterized by femtosecond laser processing technique.
5. The multi-core optical fiber vector bending sensor with optical vernier effect as claimed in claim 4, wherein the interferometer FPI0 in the core at the center is a reference interferometer, the interferometers FPI1, FPI2 and FPI3 in the other cores are sensing interferometers, the interferometer FPI0 and the interferometer FPI1 are connected in parallel to form a first vernier evenlope1, and the interferometer FPI0 and the interferometer FPI2 are connected in parallel to form a second vernier evenlope 2; the interferometer FPI0 and the interferometer FPI3 are connected in parallel to form a third vernier evenlope3, and the amplification factor M of the optical vernier effect formed by the reference interferometer and the sensing interferometer is defined as the vernier envelope free spectral range FSRevenlopeAnd the sensing interferometer free spectral range FSR':
δ＝OPD′-OPD″
in the formula, FSR ' represents the free spectral range of the reference interferometer, OPD ' represents the optical path length of the sensing interferometer, delta is the detuning amount, and OPD ' represents the optical path length of the reference interferometer.
6. The multi-core fiber vector bending sensor with optical vernier effect of claim 5, wherein the sensor measures a bending vector
The specific method comprises the following steps:
the wavelength drift amount is measured by an interferometer with three fiber cores at the outer side
Bending vector
Vector components projected on the outer three cores
Can be expressed as:
in the formula, KnThe maximum bending sensitivity of each fiber core in the range of 0-360 degrees of azimuth angles;
bending vector
Can be reconstructed from the amount of wavelength drift of the interferometers in any two outer cores.
7. The multi-core optical fiber vector bending sensor with optical vernier effect as claimed in claim 6, wherein the wavelength shift bending sensitivity is obtained from the spectrum collected by the spectrum analyzer, and the interferometer spectrum of each fiber core is obtained by fast fourier transform and filtering algorithm, so as to obtain the interferometer wavelength shift and bending sensitivity of each fiber core.
8. The multi-core fiber vector bending sensor with optical vernier effect according to any one of claims 1 to 7, wherein the multi-mode coupler is a large core diameter 2 x 2 multi-mode coupler, and comprises an a port, a b port, a c port and a d port, wherein the d port is subjected to bevel cutting treatment or is immersed in an index matching fluid.
9. The multi-core fiber vector bending sensor with optical vernier effect as claimed in any one of claims 1 to 7, further comprising a first three-dimensional displacement platform and a second three-dimensional displacement platform, wherein one end of the multi-core fiber is placed on the first three-dimensional displacement platform and the other end of the multi-core fiber is placed on the second three-dimensional displacement platform, and the first three-dimensional displacement platform and the second three-dimensional displacement platform are used for adjusting the bending curvature of the multi-core fiber.
10. The multi-core fiber vector bending sensor with optical vernier effect as claimed in claim 9, further comprising a first fiber rotating holder and a second fiber rotating holder, wherein one end of the multi-core fiber is placed on the first three-dimensional displacement platform by the first fiber rotating holder, and the other end of the multi-core fiber is placed on the second three-dimensional displacement platform by the second fiber rotating holder, and the first fiber rotating holder and the second fiber rotating holder are used for adjusting the bending direction of the multi-core fiber.
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
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