CN115420682B - Parallel Sagnac methane gas optical fiber sensor - Google Patents
Parallel Sagnac methane gas optical fiber sensor Download PDFInfo
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- VNWKTOKETHGBQD-UHFFFAOYSA-N methane Chemical compound C VNWKTOKETHGBQD-UHFFFAOYSA-N 0.000 title claims abstract description 156
- 239000013307 optical fiber Substances 0.000 title claims abstract description 23
- 239000000835 fiber Substances 0.000 claims abstract description 89
- 230000003287 optical effect Effects 0.000 claims abstract description 88
- 239000004038 photonic crystal Substances 0.000 claims abstract description 72
- 230000010287 polarization Effects 0.000 claims abstract description 55
- 239000007789 gas Substances 0.000 claims description 71
- 238000001228 spectrum Methods 0.000 claims description 52
- 238000000034 method Methods 0.000 claims description 11
- 239000000463 material Substances 0.000 claims description 10
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 claims description 8
- 238000005253 cladding Methods 0.000 claims description 7
- VYPSYNLAJGMNEJ-UHFFFAOYSA-N Silicium dioxide Chemical compound O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 claims description 6
- 239000012528 membrane Substances 0.000 claims description 4
- 229910052757 nitrogen Inorganic materials 0.000 claims description 4
- 239000000377 silicon dioxide Substances 0.000 claims description 3
- 235000012239 silicon dioxide Nutrition 0.000 claims 1
- 230000035945 sensitivity Effects 0.000 abstract description 12
- 230000000694 effects Effects 0.000 description 9
- 238000009826 distribution Methods 0.000 description 6
- 230000001276 controlling effect Effects 0.000 description 5
- 238000010586 diagram Methods 0.000 description 5
- 230000005684 electric field Effects 0.000 description 4
- 238000005516 engineering process Methods 0.000 description 4
- 238000009776 industrial production Methods 0.000 description 3
- 238000005259 measurement Methods 0.000 description 3
- 230000001105 regulatory effect Effects 0.000 description 2
- 239000011540 sensing material Substances 0.000 description 2
- 230000003595 spectral effect Effects 0.000 description 2
- 230000006978 adaptation Effects 0.000 description 1
- 230000003321 amplification Effects 0.000 description 1
- 230000009286 beneficial effect Effects 0.000 description 1
- 238000009529 body temperature measurement Methods 0.000 description 1
- 238000004891 communication Methods 0.000 description 1
- 238000001514 detection method Methods 0.000 description 1
- 238000007598 dipping method Methods 0.000 description 1
- 238000007654 immersion Methods 0.000 description 1
- 238000004519 manufacturing process Methods 0.000 description 1
- 239000002184 metal Substances 0.000 description 1
- 239000003345 natural gas Substances 0.000 description 1
- 238000003199 nucleic acid amplification method Methods 0.000 description 1
- 238000006467 substitution reaction Methods 0.000 description 1
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/01—Arrangements or apparatus for facilitating the optical investigation
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
- G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
- G01N21/55—Specular reflectivity
- G01N21/552—Attenuated total reflection
- G01N21/553—Attenuated total reflection and using surface plasmons
- G01N21/554—Attenuated total reflection and using surface plasmons detecting the surface plasmon resonance of nanostructured metals, e.g. localised surface plasmon resonance
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- General Health & Medical Sciences (AREA)
- General Physics & Mathematics (AREA)
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Abstract
The application provides a parallel Sagnac methane gas optical fiber sensor, which comprises: a Sagnac sensing unit; the Sagnac sensing unit includes: a coupler comprising a C3 port and a C4 port; the first optical circulator is connected with the C3 port; the second optical circulator is connected with the C4 port; the first polarization maintaining photonic crystal fiber and the first optical circulator form a first Sagnac loop; the second polarization maintaining photonic crystal fiber and the second optical circulator form a second Sagnac loop. The scheme has ultrahigh sensitivity and simple structure.
Description
Technical Field
The application relates to the field of optical fiber application, in particular to a parallel Sagnac methane gas optical fiber sensor.
Background
Methane gas is widely distributed in nature, is a main component of natural gas and methane, is inflammable, is easy to explode when mixed with air, is not easy to control in the production, transportation and use processes, and is easy to leak. Therefore, the methane gas sensor with high sensitivity, safety and reliability is manufactured, and is very important for the safe use of methane gas.
With the development of optical fiber technology, an optical fiber Sagnac interferometer plays an important role in the fields of optical fiber communication and optical fiber sensing. The optical fiber Sagnac interferometer has the advantages of simple structure, electromagnetic interference resistance, short response time and the like, and the optical fiber sensor based on Sagnac interference has wide application in the fields of temperature measurement, strain measurement, twist measurement, gas concentration detection and the like.
At present, the optical fiber sensor based on interference is widely focused due to the characteristics of small occupied area, short response time, stable performance and the like. The key point of the methane gas sensor based on the surface plasma resonance technology is that the thickness of a metal film is accurately controlled, and expensive instruments and equipment are required to be used. The methane gas sensor based on the traditional Sagnac interference has higher sensitivity, but cannot meet the sensitivity requirement of industrial production.
Disclosure of Invention
Aiming at the problem that the sensitivity of the methane gas optical fiber sensor in the prior art does not meet the industrial production requirement, the application provides the parallel Sagnac methane gas optical fiber sensor so as to improve the sensitivity of the methane gas optical fiber sensor, and the parallel Sagnac methane gas optical fiber sensor has the advantage of simple structure.
The application provides a parallel Sagnac methane gas optical fiber sensor, which comprises a Sagnac sensing unit;
The Sagnac sensing unit includes:
a coupler comprising a C3 port and a C4 port;
the first optical circulator is connected with the C3 port;
the second optical circulator is connected with the C4 port;
The first polarization maintaining photonic crystal fiber and the first optical circulator form a first Sagnac loop;
The second polarization maintaining photonic crystal fiber and the second optical circulator form a second Sagnac loop.
In one embodiment, the first polarization maintaining photonic crystal fiber is a methane gas sensitive film coated polarization maintaining photonic crystal fiber; the second polarization maintaining photonic crystal fiber is a polarization maintaining photonic crystal fiber which is not coated with a methane gas sensitive film.
In one embodiment, the background material of the first photonic crystal fiber is silica, and the cladding structure of the first photonic crystal fiber includes two layers of three-sized circular voids.
In one embodiment, the cladding features include inner and outer air holes;
The inner layer of air holes are asymmetrically arranged, two first air holes with first diameters are horizontally arranged in the x-axis direction, and methane gas sensitive films are coated in the two first air holes; four second air holes with second diameters are distributed in the y-axis direction, and the four second air holes are distributed in a rectangular shape;
the outer air holes comprise 12 third air holes with third diameters which are annularly arranged, and the rotation angle between the 12 third air holes is 30 degrees;
The first diameter, the second diameter and the third diameter are all unequal.
In one embodiment, the refractive index of the methane gas sensitive membrane is adjusted by controlling the concentration of methane gas, which is adjusted by controlling the flow rates of methane and nitrogen.
In one embodiment, the first optical circulator includes a D1 port and a D2 port;
the D1 port transmits a first beam of light received by the first optical circulator, and the first beam of light is transmitted to the D2 port along the first Sagnac loop clockwise;
the D2 port transmits a second beam of light received by the first optical circulator;
The first beam light and the second beam light pass through the first polarization maintaining photonic crystal fiber to generate a phase difference, and a sensing branch Sagnac interference spectrum is formed at the first optical circulator.
In one embodiment, the second optical circulator includes an E1 port and an E2 port;
the E1 port transmits a third beam of light received by the second optical circulator, and the third beam of light is transmitted to the E2 port along the second Sagnac loop clockwise;
E2 port transmits the fourth beam of light received by the second optical circulator;
The third light beam and the fourth light beam pass through the second polarization maintaining photonic crystal fiber to generate phase difference, and a reference branch Sagnac interference spectrum is formed at the second optical circulator.
In one embodiment, the fiber optic sensor further comprises a broadband light source and an optical spectrum analyzer;
the coupler also includes a C1 port and a C2 port;
the broadband light source is connected with the C1 port and is used for emitting light signals and sending the light signals to the Sagnac sensing unit so that the Sagnac sensing unit receives and processes the light signals and sends the processed light signals to the optical spectrum analyzer;
The optical spectrum analyzer is connected with the C2 port and is used for receiving the processed optical signals output by the Sagnac sensing unit.
In one embodiment, the sensing branch Sagnac interference spectrum and the reference branch Sagnac interference spectrum are spectrally superimposed and form an envelope on an optical spectrum analyzer.
In one embodiment, the coupler is a2 x 23 db coupler and the first optical circulator and the second optical circulator are each 1 x 2 optical circulators.
Compared with the prior art, the technical scheme provided by the application has the beneficial effects that:
1) The polarization maintaining photonic crystal fiber cladding adopted in the application has two layers of air hole distribution, and has double refraction characteristics because the air hole distribution is asymmetric;
2) In the application, as the methane gas sensitive film is only coated on the inner air holes of the first polarization maintaining photonic crystal fiber, a two-step filling technology and an immersion technology can be used to ensure that only specific air holes are coated by the methane gas sensitive film; after the internal air holes of the first polarization maintaining photonic crystal fiber are coated, the birefringence of the sensing branch Sagnac is increased, and the birefringence can be changed along with the concentration of methane gas;
3) In the application, the first Sagnac loop and the second Sagnac loop are connected in parallel through a coupler to generate vernier effect, and the maximum sensitivity can reach 216nm/%.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the application as claimed.
Drawings
FIG. 1 is a schematic diagram of a parallel Sagnac methane gas fiber sensor according to an embodiment of the application;
FIG. 2 is a cross-sectional view of a first polarization maintaining photonic crystal fiber in accordance with embodiments of the present application;
FIG. 3 (a) is a graph showing an electric field distribution of the first photonic crystal fiber in the x-axis direction according to an embodiment of the present application, and FIG. 3 (b) is a graph showing an electric field distribution of the first photonic crystal fiber in the y-axis direction according to an embodiment of the present application;
FIG. 4 is a diagram showing the Sagnac interference spectrum of the sensing branch, the Sagnac interference spectrum of the reference branch, and the parallel interference spectrum of the vernier effect when the methane gas concentration is 0.1% in the embodiment of the application;
FIG. 5 is an interference spectrum generated by using a parallel Sagnac loop based on vernier effect when the thickness t of the methane gas sensitive film is 1000nm and the concentration variation range is 0-3.5% in the embodiment of the application;
Fig. 6 is a polynomial fit of a complete envelope generated using parallel Sagnac loops based on vernier effect in an embodiment of the present application.
Detailed Description
In the following description, for purposes of explanation and not limitation, specific details are set forth such as the particular system architecture, techniques, etc., in order to provide a thorough understanding of the embodiments of the present application. It will be apparent, however, to one skilled in the art that the present application may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known systems, devices, circuits, and methods are omitted so as not to obscure the description of the present application with unnecessary detail.
The embodiment of the application provides a parallel Sagnac methane gas optical fiber sensor which can be applied to the field of optical fiber application so as to improve the sensitivity of the methane gas optical fiber sensor.
FIG. 1 is a schematic diagram of a composition structure of a parallel Sagnac methane gas fiber sensor according to an embodiment of the application.
As shown in fig. 1, in this embodiment, a parallel-connection Sagnac methane gas fiber sensor includes a broadband light source (Broadband Light Source, BBS), a Sagnac sensing unit 1, and an optical spectrum analyzer (Optical Spectrum Analyzer, OSA) connected in order. The broadband light source BBS emits an optical signal to the Sagnac sensing unit 1, the Sagnac sensing unit 1 processes the optical signal, and sends the processed optical signal to the optical spectrum analyzer OSA, and the optical spectrum analyzer OSA receives the optical signal output by the Sagnac sensing unit 1. The broadband light source BBS is connected with the Sagnac sensing unit 1 and the optical spectrum analyzer OSA through single-mode fibers.
The Sagnac sensing unit 1 includes:
A coupler 11, the coupler 11 comprising a C3 port and a C4 port;
a first optical circulator 12, the first optical circulator 12 being connected to a C3 port;
A second optical circulator 13, the second optical circulator 13 being connected to a C4 port;
A first photonic crystal fiber 14, the first photonic crystal fiber 14 and the first optical circulator 12 forming a first Sagnac loop (i.e., the sensing leg of fig. 1); the second polarization maintaining photonic crystal fiber 15, the second polarization maintaining photonic crystal fiber 15 and the second optical circulator 13 form a second Sagnac loop (i.e., the reference arm in fig. 1).
Wherein coupler 11 further comprises a C1 port and a C2 port;
The broadband light source is connected with the C1 port and is used for emitting light signals and sending the light signals to the Sagnac sensing unit 1 so that the Sagnac sensing unit 1 receives and processes the light signals and sends the processed light signals to the optical spectrum analyzer;
and the optical spectrum analyzer is connected with the C2 port and is used for receiving the processed optical signals output by the Sagnac sensing unit 1.
It will be appreciated that the coupler 11 may be a 2 x 23 db coupler and that the first optical circulator 12 and the second optical circulator 13 may each be a1 x 2 optical circulator.
It will also be appreciated that the first photonic crystal fiber 14 is a photonic crystal fiber coated with a methane gas sensitive film; the second polarization maintaining photonic crystal fiber 15 is a polarization maintaining photonic crystal fiber not coated with a methane gas sensitive film.
As shown in fig. 2, which is a cross-sectional view of the first polarization maintaining photonic crystal fiber, in one embodiment, the background material of the first polarization maintaining photonic crystal fiber 14 is silica, and the cladding structure of the first polarization maintaining photonic crystal fiber 14 includes two layers of circular air holes with three dimensions.
Optionally, the cladding structure includes an inner layer of air holes and an outer layer of air holes;
In order to introduce higher birefringence, the inner layer air holes are asymmetrically arranged, two first air holes with first diameters are horizontally arranged in the x-axis direction, and a methane gas sensitive film is coated in each of the two first air holes; four second air holes with second diameters are distributed in the y-axis direction, and the four second air holes are distributed in a rectangular shape;
the outer air holes comprise 12 third air holes with third diameters which are annularly arranged, and the rotation angle between the 12 third air holes is 30 degrees;
The first diameter, the second diameter and the third diameter are all unequal.
It can be appreciated that the second polarization maintaining photonic crystal fiber has the same structure as the first polarization maintaining photonic crystal fiber, except that the first air hole of the first polarization maintaining photonic crystal fiber is coated with the methane gas sensitive film, and the first air hole of the second polarization maintaining photonic crystal fiber is not coated with the methane gas sensitive film.
It will be appreciated that the first diameter, the second diameter and the third diameter may be set according to practical requirements, and exemplary first diameters are d 1 =6 μm, second diameters are d 3 =1.2 μm and third diameters are d 2 =4.8 μm.
Further, the radius of the first polarization-maintaining photonic crystal fiber 14 is R 3 =15 μm; the horizontal distance of the first air hole from the center of the first polarization maintaining photonic crystal fiber 14 is R 1 =5 μm; the horizontal distance from the second air hole to the center of the first polarization-maintaining photonic crystal fiber 14 is D 1 =3 μm, and the vertical distance from the second air hole to the center of the first polarization-maintaining photonic crystal fiber 14 is D 2 =5 μm; the third air hole is located at a vertical distance R 2 =12 μm from the center of the first polarization-maintaining photonic crystal fiber 14.
Alternatively, the methane gas sensing film may employ a methane gas sensing material, and the thickness of the methane gas sensing material coated in the first air holes is 1000nm, as an example. The refractive index of the methane gas sensitive film is regulated by controlling the concentration of methane gas, which is regulated by controlling the flow rates of methane and nitrogen.
Specifically, the refractive index of the methane gas sensitive film is sensitive to the concentration of methane gas, and different methane gas concentrations can be accurately adjusted by controlling the flow of methane and nitrogen, so that the refractive index of the air holes on the left side and the right side of the fiber core of the first polarization maintaining photonic crystal fiber PMPCF is influenced, and the birefringence B (lambda, c) is influenced, so that the phase difference phi of the sensing branch and the free spectral range FSR of the interference spectrum are influenced.
Wherein the phase difference phi satisfies the following formula:
The period of the Sagnac interference spectrum can be represented by the free spectral range:
Where λ is the wavelength of the interference spectrum, B (λ, c) is the birefringence of the first polarization maintaining photonic crystal fiber 14, and L is the length of the first polarization maintaining photonic crystal fiber 14.
In this embodiment, the cladding of the polarization maintaining photonic crystal fiber PMPCF (including the first polarization maintaining photonic crystal fiber and the second polarization maintaining photonic crystal fiber) has two layers of air holes distributed, and has the characteristic of double refraction due to the larger diameters of the air holes at the left and right sides of the fiber core.
In this embodiment, since the methane gas sensitive material is only coated on the inner walls of the air holes (i.e., the first air holes) on the left and right sides of the core of the first polarization maintaining photonic crystal fiber PMPCF of the sensing branch, two filling techniques and dipping techniques can be used to ensure that only two specific air holes are coated with the methane gas sensitive material. After the inner walls of the atmospheric holes on the left side and the right side of the sensing branch fiber core are coated with methane gas sensitive materials, the birefringence of the sensing branch is increased, and the birefringence can change along with the concentration of methane gas.
In this embodiment, the birefringence of the core is increased by distributing two large-diameter air holes (i.e., first air holes) on the left and right sides of the core of the first polarization maintaining photonic crystal fiber PMPCF. The inner walls of the two large air holes are coated with methane gas sensitive materials, the methane gas with different concentrations causes the change of refractive index of the methane gas sensitive materials, so that the change of fiber core birefringence is caused, the movement of a sensing branch Sagnac interference spectrum is further caused, the movement of an envelope spectrum generated by superposition of the sensing branch and a reference branch is finally caused, and the measurement of the concentration of the methane gas is realized by detecting the movement of the envelope spectrum.
With continued reference to fig. 1, the first optical circulator 12 includes a D1 port and a D2 port; the D1 port transmits the first light beam received by the first optical circulator 12, and the first light beam is transmitted to the D2 port along the first Sagnac loop clockwise;
The D2 port transmits the second light beam received by the first optical circulator 12;
The first light beam and the second light beam pass through the first polarization maintaining photonic crystal fiber 14 to generate a phase difference, and meet at the first optical circulator 12 to form a sensing branch Sagnac interference spectrum.
The second optical circulator 13 includes an E1 port and an E2 port;
The E1 port transmits a third beam of light received by the second optical circulator 13, and the third beam of light is transmitted to the E2 port along the second Sagnac loop clockwise;
the E2 port transmits the fourth beam of light received by the second optical circulator 13;
The third light beam and the fourth light beam pass through the second polarization maintaining photonic crystal fiber 15 to generate a phase difference, and meet at the second optical circulator 13 to form a reference branch Sagnac interference spectrum.
The sensing branch Sagnac interference spectrum and the reference branch Sagnac interference spectrum are subjected to spectrum superposition, and an envelope is formed on an optical spectrum analyzer.
Specifically, the D1 port on the right side of the 1×2 first optical circulator 12 transmits one beam of light (i.e., the first beam of light) received from the first optical circulator 12, and clockwise transmits the other beam of light (i.e., the second beam of light) received from the first optical circulator 12 along the first Sagnac loop to the D2 port, where the two beams of light (i.e., the first beam of light and the second beam of light) pass through the first polarization maintaining photonic crystal fiber PMPCF coated with a methane gas sensitive film in the air holes (i.e., the first air holes) on the left and right sides of the fiber core, and then a phase difference is generated, and finally, the two beams of light meet at the 1×2 first optical circulator 12 to form a sensing branch Sagnac interference spectrum. The E1 port on the right side of the 1×2 second optical circulator 13 transmits one beam of light (i.e., the third beam of light) received from the second optical circulator 13, and the second beam of light (i.e., the fourth beam of light) received from the second optical circulator is transmitted clockwise along the second Sagnac loop to the E2 port, and the E2 port transmits the other beam of light (i.e., the fourth beam of light) received from the second optical circulator, so that a phase difference is generated after the two beams of light (the third beam of light and the fourth beam of light) pass through the second polarization maintaining photonic crystal fiber PMPCF which is asymmetrically arranged due to air holes, and finally meet at the 1×2 second optical circulator 13 to form a reference branch Sagnac interference spectrum. The C3 port on the right side of the 2x 23 db coupler 11 transmits the fifth light beam received from the 1 x 2 first optical circulator 12, the C4 port transmits the sixth light beam received from the 1 x 2 second optical circulator 13, and the two light beams (the fifth light beam and the sixth light beam) finally meet at the 2x 23 db coupler 11, and the corresponding interference spectrums are also superimposed, thereby forming an envelope on the spectrometer OSA.
Embodiments of the application are further explained below using a methane gas-sensitive membrane containing cryptophane A:
the relationship between the different concentrations c and refractive indices (Index of Refraction, RI) is n methane = 1.4478-0.0038 c, when the methane gas concentration is in the sensing range of 0-3.5%, the RI of the methane gas sensitive membrane will decrease by 0.0038 every 1% increase. That is, when the methane gas concentration changes, a change in the refractive index of the first air hole of the first polarization maintaining photonic crystal fiber 14 will be caused, and the spatial light is coupled into the first branch, causing a shift in the sensing branch Sagnac interference spectrum.
In some possible embodiments, the first polarization-maintaining photonic crystal fiber 14 is set to 20cm in length and the methane gas sensitive film thickness t is 1000nm; the second polarization maintaining photonic crystal fiber 15 was set to 19cm in length, and the inside of the air hole was not coated with any material. The envelope period formed by the interference spectrum superposition expires sufficiently to be expressed as follows:
wherein FSR R＝λ2/BLR is determined by the sensing branch Sagnac interference spectrum formed by first polarization maintaining photonic crystal fiber 14, and FSR S＝λ2/BLS is determined by the reference branch Sagnac interference spectrum formed by second polarization maintaining photonic crystal fiber 15.
It should be understood that fig. 3 (a) is an electric field distribution diagram of the first polarization maintaining photonic crystal fiber 14 in the x-axis direction in the embodiment of the present application, and fig. 3 (b) is an electric field distribution diagram of the first polarization maintaining photonic crystal fiber 14 in the y-axis direction in the embodiment of the present application;
FIG. 4 shows the Sagnac interference spectrum of the sensing branch, the Sagnac interference spectrum of the reference branch and the parallel interference spectrum of the vernier effect when the methane gas concentration is 0.1% in the embodiment of the application.
Specifically, in the wavelength range of 1000nm-2000nm, the sensing branch generates 27 interference valleys, the reference branch generates 22 interference valleys, and 5 envelopes are generated after parallel connection. The FSR S of the sensing branch is 39nm, the FSR R of the reference branch is 49nm, and the FSR of the envelope after the sensing branch and the reference branch are connected in parallel is 186nm. The shift of the interference spectrum envelope based on vernier effect is several times the shift of the trough of the single Sagnac interference spectrum when the methane gas concentration varies. Thus, the amplification factor satisfies the following formula:
FIG. 5 is an interference spectrum generated by using parallel Sagnac loops based on vernier effect when the thickness t of the methane gas sensitive film is 1000nm and the concentration variation range is 0-3.5% in the embodiment of the application.
It will be appreciated that the FSR R of the reference leg is greater than the FSR S of the sensing leg, the calculated M factor is positive and the envelope is red shifted as the methane gas concentration increases.
FIG. 6 is a polynomial fitting graph of a complete envelope generated by using parallel Sagnac loops based on vernier effect in an embodiment of the application, and the maximum sensitivity of the parallel Sagnac methane gas optical fiber sensor can reach 216nm/%.
According to the application, the response to the concentration of methane gas is realized by changing the thickness of the methane gas sensitive film on the inner walls of the atmospheric holes on the left side and the right side of the fiber core of the first polarization maintaining photonic crystal fiber PMPCF of the sensing branch. When the concentration of methane gas is in the range of 0-3.5%, the length of the first polarization maintaining photonic crystal fiber PMPCF of the sensing branch is 20cm, the thickness t=1000 nm of the gas sensitive film on the inner wall of the atmospheric holes on the left side and the right side of the fiber core, the second polarization maintaining photonic crystal fiber of the reference branch has the same structure, no material is coated in the air holes, and the length is 19cm. Two Sagnac loops are connected in parallel by using a2×2-3 dB optical coupler to generate vernier effect, and the maximum sensitivity can reach 216nm/%.
In summary, the parallel Sagnac methane gas optical fiber sensor provided by the embodiment of the application has ultrahigh sensitivity, can meet the sensitivity requirement of industrial production, and has the advantage of simple structure.
It will be understood by those skilled in the art that the sequence number of each step in the above embodiment does not mean that the execution sequence of each process should be determined by the function and the internal logic, and should not limit the implementation process of the embodiment of the present application. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The application may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the unit claims enumerating several means, several of these means may be embodied by one and the same item of hardware. The use of the words first, second, third, etc. do not denote any order. These words may be interpreted as names. Other embodiments of the application will be apparent to those skilled in the art from consideration of the specification and practice of the application disclosed herein. This application is intended to cover any variations, uses, or adaptations of the application following, in general, the principles of the application and including such departures from the present disclosure as come within known or customary practice within the art to which the application pertains. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the application being indicated by the following claims.
The foregoing is merely illustrative of the embodiments of the present application, and the scope of the present application is not limited thereto, and any changes or substitutions easily contemplated by those skilled in the art within the technical scope of the present application should be included in the scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.
Claims (7)
1. A parallel-connection Sagnac methane gas optical fiber sensor, which is characterized by comprising a Sagnac sensing unit (1);
the Sagnac sensing unit (1) comprises:
-a coupler (11), the coupler (11) comprising a C3 port and a C4 port;
-a first optical circulator (12), said first optical circulator (12) being connected to said C3 port;
a second optical circulator (13), the second optical circulator (13) being connected to the C4 port;
a first polarization maintaining photonic crystal fiber (14), wherein the first polarization maintaining photonic crystal fiber (14) and the first optical circulator (12) form a first Sagnac loop; a second polarization maintaining photonic crystal fiber (15), the second polarization maintaining photonic crystal fiber (15) and the second optical circulator (13) form a second Sagnac loop; the first polarization-maintaining photonic crystal fiber (14) is a polarization-maintaining photonic crystal fiber coated with a methane gas sensitive film; the second polarization maintaining photonic crystal fiber (15) is a polarization maintaining photonic crystal fiber which is not coated with a methane gas sensitive film;
the background material of the first polarization-maintaining photonic crystal fiber (14) is silicon dioxide, and the cladding structure of the first polarization-maintaining photonic crystal fiber (14) comprises an inner layer of air holes and an outer layer of air holes;
the inner layer air holes are asymmetrically arranged, two first air holes with first diameters are horizontally arranged in the x-axis direction, and a methane gas sensitive film is coated in each of the two first air holes; four second air holes with second diameters are distributed in the y-axis direction, and the four second air holes are distributed in a rectangular shape;
the outer air holes comprise 12 third air holes with third diameters which are annularly arranged, and the rotation angle among the 12 third air holes is 30 degrees;
the first diameter, the second diameter, and the third diameter are all unequal.
2. The fiber optic sensor of claim 1, wherein the refractive index of the methane gas sensitive membrane is adjusted by controlling the concentration of methane gas, which is adjusted by controlling the flow rates of methane and nitrogen.
3. The fiber optic sensor of claim 1, wherein the first optical circulator (12) includes a D1 port and a D2 port;
The D1 port transmits a first beam of light received by the first optical circulator (12), the first beam of light being transmitted clockwise along the first Sagnac loop to the D2 port;
the D2 port transmits a second beam of light received by the first optical circulator (12);
The first beam of light and the second beam of light pass through the first polarization maintaining photonic crystal fiber (14) to generate a phase difference, and a sensing branch Sagnac interference spectrum is formed at the first optical circulator (12).
4. A fibre-optic sensor according to claim 3, characterized in that the second optical circulator (13) comprises an E1 port and an E2 port;
the E1 port transmits a third beam of light received by the second optical circulator (13), and the third beam of light is transmitted to the E2 port along the second Sagnac loop clockwise;
the E2 port transmits a fourth beam of light received by the second optical circulator (13);
The third light beam and the fourth light beam generate a phase difference through the second polarization maintaining photonic crystal fiber (15), and a reference branch Sagnac interference spectrum is formed at the second optical circulator (13).
5. The fiber optic sensor of claim 4, further comprising a broadband light source and an optical spectrum analyzer;
The coupler (11) further comprises a C1 port and a C2 port;
the broadband light source is connected with the C1 port, and is used for emitting light signals and sending the light signals to the Sagnac sensing unit (1) so that the Sagnac sensing unit (1) receives and processes the light signals and sends the processed light signals to the optical spectrum analyzer;
the optical spectrum analyzer is connected with the C2 port and is used for receiving the processed optical signals output by the Sagnac sensing unit (1).
6. The fiber optic sensor of claim 5, wherein the sensing branch Sagnac interference spectrum and the reference branch Sagnac interference spectrum are spectrally superimposed and form an envelope on the optical spectrum analyzer.
7. The fiber optic sensor according to any of claims 1-6, wherein the coupler (11) is a2 x 23 db coupler, and the first optical circulator (12) and the second optical circulator (13) are each 1 x2 optical circulators.
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