CN115420682A - 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 25
- 239000000835 fiber Substances 0.000 claims abstract description 96
- 230000003287 optical effect Effects 0.000 claims abstract description 95
- 239000004038 photonic crystal Substances 0.000 claims abstract description 74
- 230000010287 polarization Effects 0.000 claims abstract description 46
- 239000007789 gas Substances 0.000 claims description 75
- 238000001228 spectrum Methods 0.000 claims description 47
- 239000000463 material Substances 0.000 claims description 12
- 238000000034 method Methods 0.000 claims description 11
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 claims description 8
- 238000005253 cladding Methods 0.000 claims description 8
- VYPSYNLAJGMNEJ-UHFFFAOYSA-N Silicium dioxide Chemical compound O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 claims description 6
- 229910052757 nitrogen Inorganic materials 0.000 claims description 4
- 239000000377 silicon dioxide Substances 0.000 claims description 3
- 230000002999 depolarising effect Effects 0.000 claims 2
- 230000035945 sensitivity Effects 0.000 abstract description 12
- 230000000694 effects Effects 0.000 description 10
- 238000010586 diagram Methods 0.000 description 6
- 238000009826 distribution Methods 0.000 description 4
- 230000005684 electric field Effects 0.000 description 4
- 238000005516 engineering process Methods 0.000 description 4
- 230000003595 spectral effect Effects 0.000 description 4
- 238000009776 industrial production Methods 0.000 description 3
- 238000005259 measurement Methods 0.000 description 3
- 239000012528 membrane Substances 0.000 description 3
- 239000013078 crystal Substances 0.000 description 2
- 230000006978 adaptation Effects 0.000 description 1
- 150000001335 aliphatic alkanes Chemical class 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
- 238000005470 impregnation 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
- 238000002198 surface plasmon resonance spectroscopy Methods 0.000 description 1
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- G—PHYSICS
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- 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
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Abstract
The application provides a parallelly connected Sagnac methane gas optical fiber sensor includes: a Sagnac sensing unit; the Sagnac sensing unit comprises: 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 photonic crystal fiber, the first polarization photonic crystal fiber and the first optical circulator form a first Sagnac loop; and the second polarization-maintaining photonic crystal fiber, 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 the main component of natural gas and methane, is inflammable, is mixed with air to be easily exploded, and is not easy to control and easily leak in the production, transportation and use processes. Therefore, a methane gas sensor with high sensitivity, safety and reliability is manufactured, and the safe use of the methane gas is very important.
With the development of optical fiber technology, the 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 is widely used in the fields of temperature measurement, strain measurement, distortion measurement, gas concentration detection and the like.
At present, the optical fiber sensor based on interference is widely concerned due to the characteristics of small floor area, short response time, stable performance and the like. The key to the methane gas sensor based on the surface plasmon resonance technology is the precise control of the thickness of the metal film, which requires expensive equipment. The sensitivity of the traditional Sagnac interference-based methane gas sensor is high, but the sensitivity requirement of industrial production cannot be met.
Disclosure of Invention
The problem to the unsatisfied industrial production demand of methane gas optical fiber sensor sensitivity among the prior art, this application provides a parallelly connected Sagnac methane gas optical fiber sensor to improve methane gas optical fiber sensor's sensitivity, and have simple structure's advantage.
The application provides a parallel Sagnac methane gas optical fiber sensor, which comprises a Sagnac sensing unit;
the Sagnac sensing unit comprises:
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 photonic crystal fiber and the first optical circulator form a first Sagnac loop;
and the second polarization-maintaining photonic crystal fiber, 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 polarization maintaining photonic crystal fiber coated with a methane gas sensitive film; 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 polarization maintaining photonic crystal fiber is silica, and the cladding structure of the first polarization maintaining photonic crystal fiber comprises two layers of circular air holes with three sizes.
In one embodiment, the cladding structure comprises an inner layer of air holes and an outer layer of 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 layer 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 film is adjusted by controlling the concentration of methane gas, and the concentration of methane gas 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 clockwise along a first Sagnac loop;
the D2 port transmits the second beam of light received by the first optical circulator;
the first beam of light and the second beam of light generate phase difference through the first polarization-maintaining photonic crystal fiber, 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 a second Sagnac loop clockwise;
the E2 port transmits the fourth beam of light received by the second optical circulator;
and the third beam of light and the fourth beam of light generate phase difference through the second polarization-maintaining photonic crystal fiber, 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 port C1, and is used for emitting optical signals and sending the optical signals to the Sagnac sensing unit, so that the Sagnac sensing unit receives and processes the optical signals and sends the processed optical 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 signal output by the Sagnac sensing unit.
In one embodiment, the Sagnac interference spectrum of the sensing branch and the Sagnac interference spectrum of the reference branch are spectrally superimposed and form an envelope on the optical spectrum analyzer.
In one embodiment, the coupler is a 2 × 2 3dB coupler, and the first and second optical circulators are 1 × 2 optical circulators.
Compared with the prior art, the technical scheme provided by the application has the beneficial effects that:
1) The cladding of the polarization-maintaining photonic crystal fiber adopted in the application has two layers of air holes which are distributed asymmetrically, so that the polarization-maintaining photonic crystal fiber has double refraction characteristics;
2) In the application, as the methane gas sensitive film is coated on the inner air holes of the first polarization maintaining photonic crystal fiber, a two-step filling technology and a dipping 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 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) The first Sagnac loop and the second Sagnac loop are connected in parallel through the coupler to generate vernier effect, and the maximum sensitivity of the vernier effect 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 scope of the application.
Drawings
FIG. 1 is a schematic structural diagram of a Sagnac methane gas optical fiber sensor connected in parallel in the embodiment of the application;
FIG. 2 is a cross-sectional view of a first polarization-maintaining photonic crystal fiber in an embodiment of the present application;
FIG. 3 (a) is a diagram showing an electric field distribution of the first polarization maintaining photonic crystal fiber in the x-axis direction in the embodiment of the present application, and FIG. 3 (b) is a diagram showing an electric field distribution of the first polarization maintaining photonic crystal fiber in the y-axis direction in the embodiment of the present application;
FIG. 4 is a graph of the Sagnac interference spectrum of the sensing branch, the Sagnac interference spectrum of the reference branch, and the vernier effect in parallel when the concentration of methane gas is 0.1% in the embodiment of the present application;
FIG. 5 is an interference spectrum generated by using Sagnac loops connected in parallel based on vernier effect when the thickness t of a methane gas sensitive film in the embodiment of the present application is 1000nm and the concentration variation range is 0-3.5%;
fig. 6 is a polynomial fit graph of the complete envelope generated using parallel Sagnac loops based on the vernier effect in the present embodiment.
Detailed Description
In the following description, for purposes of explanation and not limitation, specific details are set forth, such as particular system structures, 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 to improve the sensitivity of the methane gas optical fiber sensor.
Fig. 1 is a schematic structural diagram of a parallel Sagnac methane gas optical fiber sensor in the embodiment of the present application.
As shown in fig. 1, in this embodiment, a parallel Sagnac methane gas Optical fiber sensor includes a Broadband Light Source (BBS), a Sagnac sensing unit 1, and an Optical Spectrum Analyzer (OSA) connected in sequence. The broadband light source BBS sends 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 and the Sagnac sensing unit 1 as well as the Sagnac sensing unit 1 and the optical spectrum analyzer OSA are connected by single-mode optical fibers.
The Sagnac sensing unit 1 includes:
a coupler 11, the coupler 11 including a C3 port and a C4 port;
the first optical circulator 12, the first optical circulator 12 is connected with the C3 port;
the second optical circulator 13, the second optical circulator 13 is connected with port C4;
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 (i.e. a sensing branch in fig. 1); 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 (i.e., the reference branch in fig. 1).
Wherein, the coupler 11 further includes a C1 port and a C2 port;
the broadband light source is connected with a port C1, and is used for emitting optical signals and sending the optical signals to the Sagnac sensing unit 1, so that the Sagnac sensing unit 1 receives and processes the optical signals and sends the processed optical 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 signal output by the Sagnac sensing unit 1.
It is understood that the coupler 11 may be a 2 × 2 3dB coupler, and the first optical circulator 12 and the second optical circulator 13 may be 1 × 2 optical circulators.
It will also be appreciated that 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 not coated with a methane gas sensitive film.
Referring to 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 sizes.
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 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 layer 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 understood that the second polarization maintaining photonic crystal fiber has the same structure as the first polarization maintaining photonic crystal fiber, and the difference is that the first gas hole of the first polarization maintaining photonic crystal fiber is coated with the methane gas sensitive film, and the first gas hole of the second polarization maintaining photonic crystal fiber is not coated with the methane gas sensitive film.
Can understand thatThe first diameter, the second diameter and the third diameter can be set according to actual requirements, and the first diameter is d in an exemplary manner 1 =6 μm and a second diameter d 3 =1.2 μm and a third diameter d 2 ＝4.8μm。
Further, the radius of the first polarization maintaining photonic crystal fiber 14 is R 3 =15 μm; the first air hole is at a horizontal distance R from the center of the first polarization-maintaining photonic crystal fiber 14 1 =5 μm; the second air hole is at a horizontal distance D from the center of the first polarization-maintaining photonic crystal fiber 14 1 =3 μm, and the perpendicular distance of the second air hole from the center of the first polarization photonic crystal fiber 14 is D 2 =5 μm; the perpendicular distance of the third air hole from the center of the first polarization maintaining photonic crystal fiber 14 is R 2 ＝12μm。
Alternatively, the methane gas sensitive membrane may be made of a methane gas sensitive material, and the thickness of the methane gas sensitive material coated in the first gas holes is, for example, 1000nm. The refractive index of the methane gas sensitive film is adjusted by controlling the concentration of methane gas, and the concentration of methane gas is adjusted by controlling the flow of methane and nitrogen.
Specifically, the refractive index of the methane gas sensitive film is sensitive to the concentration of methane gas, and different concentrations of methane gas can be accurately adjusted by controlling the flow of methane and nitrogen, so that the refractive indexes of air holes on the left side and the right side of a fiber core of the first polarization photonic crystal fiber PMPCF14 are influenced, the birefringence B (lambda, c) is further influenced, and the phase difference phi of the sensing branch and the free spectral range FSR of an interference spectrum are further 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 polariton crystal fiber 14, and L is the length of the first polariton 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, and the air holes on the left and right sides of the fiber core have larger diameters, so that the fiber core has birefringence.
In this embodiment, since the methane gas sensitive material is only coated on the inner walls of the atmospheric holes (i.e. the first atmospheric holes) on the left and right sides of the pmcf fiber core of the first polarization photonic crystal fiber of the sensing branch, two filling techniques and impregnation techniques can be used to ensure that only two specific atmospheric holes are coated with the methane gas sensitive material. After the inner walls of the large air holes on the left side and the right side of the fiber core of the sensing branch 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, two large-diameter air holes (i.e., first air holes) are distributed on the left and right sides of the core of the first polarization photonic crystal fiber PMPCF, so that the birefringence of the core is increased. Methane gas sensitive materials are coated on the inner walls of the two large air holes, methane gas with different concentrations causes the change of the refractive index of the methane gas sensitive materials, so that the change of the double refraction of the fiber core is caused, the Sagnac interference spectrum of the sensing branch is further caused to move, the movement of the envelope spectral line generated by the superposition of the sensing branch and the reference branch is finally caused, and the measurement of the methane gas concentration is realized by detecting the movement of the envelope spectral line.
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 beam of light received by the first optical circulator 12, and the first beam of light is transmitted to the D2 port clockwise along the first Sagnac loop;
the D2 port transmits the second beam of light received by the first optical circulator 12;
the first beam of light and the second beam of light generate phase difference through the first polarization-maintaining photonic crystal fiber 14 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 the 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 beam of light and the fourth beam of light generate phase difference through the second polarization-maintaining photonic crystal fiber 15 and meet at the second optical circulator 13 to form a reference branch Sagnac interference spectrum.
And the Sagnac interference spectrum of the sensing branch and the Sagnac interference spectrum of the reference branch are subjected to spectrum superposition, and an envelope is formed on the optical spectrum analyzer.
Specifically, a D1 port on the right side of the 1 × 2 first optical circulator 12 transmits one beam of light (i.e., a first beam of light) received from the first optical circulator 12, and transmits the other beam of light (i.e., a second beam of light) clockwise along the first Sagnac loop to the D2 port, and the D2 port transmits the other beam of light (i.e., a second beam of light) received from the first optical circulator 12, and the two beams of light (the first beam of light and the second beam of light) pass through first polarization photonic crystal fibers PMPCFs 14 coated with methane gas sensitive films in air holes on the left and right sides of a fiber core (i.e., the first air holes) and then generate a phase difference, and finally 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 transmits the other beam of light (i.e., the fourth beam of light) 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, and the two beams of light (the third beam of light and the fourth beam of light) also generate phase difference after passing through the second polarization maintaining photonic crystal fiber pmpcpf 15 asymmetrically arranged due to the 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 2 x 2 3dB coupler 11 transmits the fifth light received from the 1 x 2 first optical circulator 12, the C4 port transmits the sixth light received from the 1 x 2 second optical circulator 13, the two beams (the fifth and sixth light) eventually meet at the 2 x 2 3dB coupler 11, and the corresponding interference spectra are also superimposed, forming an envelope on the spectrometer OSA.
The examples of the present application are further explained below using a methane gas sensitive membrane containing cryptophane a:
the relationship between the different concentrations c and the Refractive Index (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 sensing membrane will decrease by 0.0038 for every 1% increase. That is, when the methane gas concentration changes, the refractive index of the first air hole of the first polarization photonic crystal fiber 14 will be caused to change, and the spatial light is coupled into the first branch, causing the Sagnac interference spectrum of the sensing branch to shift.
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 superposition of the interference spectrums satisfies the following formula:
wherein, FSR R ＝λ 2 /BL R FSR is determined by Sagnac interference spectrum of the sensing branch formed by first polarization maintaining photonic crystal fiber 14 S ＝λ 2 /BL S Determined by the Sagnac interference spectrum of the reference branch formed by the 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 vernier effect in parallel when the concentration of methane gas is 0.1% in the embodiment of the present 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. FSR of sensing branch S 39nm, FSR of reference arm R The wavelength is 49nm, and the FSR of the envelope after the sensing branch and the reference branch are connected in parallel is 186nm. When nailThe shift in the envelope of the interference spectrum based on the vernier effect is several times the shift in the trough of a single Sagnac interference spectrum as the concentration of alkane gas changes. Thus, the amplification factor satisfies the following formula:
FIG. 5 is an interference spectrum generated by parallel Sagnac loops based on vernier effect when the thickness t of the methane gas sensitive film in the present embodiment is 1000nm and the concentration is varied in the range of 0-3.5%.
It should be understood that the FSR of the reference leg R Larger than FSR of sensing branch S The calculated M factor is positive and the envelope is red shifted as the methane gas concentration increases.
FIG. 6 is a polynomial fit graph of the complete envelope generated by using parallel Sagnac loops based on vernier effect according to an embodiment of the present application, and the maximum sensitivity of the parallel Sagnac methane gas fiber optic sensor obtained by fitting the curve can reach 216nm/%.
The thickness of the methane gas sensitive film on the inner walls of the large holes at the left side and the right side of the PMPCF fiber core of the first polarization photonic crystal fiber of the sensing branch is changed, so that the response to the methane gas concentration is realized. 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 of the gas sensitive film on the inner walls of the large air holes at the left side and the right side of the fiber core is =1000nm, the structure of the second polarization maintaining photonic crystal fiber of the reference branch is the same, no material is coated in the air holes, and the length is 19cm. Two Sagnac loops are connected in parallel by using a 2X 2 3dB 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 requirement of industrial production on sensitivity, and has the advantage of simple structure.
It should be understood by those skilled in the art that the sequence numbers of the steps in the foregoing embodiments do not imply an execution sequence, and the execution sequence of each process should be determined by its function and its inherent logic, and should not constitute any limitation to the implementation process of the embodiments of the present application. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention 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 usage of the words first, second and third, etcetera do not indicate any ordering. These words may be interpreted as names. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. This application is intended to cover any variations, uses, or adaptations of the invention following, in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice in the art to which the invention pertains. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.
The above description is only an exemplary embodiment of the present application, but the scope of the present application is not limited thereto, and any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope of the present application should be covered within the scope of the present application. Therefore, the protection scope of the present application shall be subject to the protection scope of the claims.
Claims (10)
1. A parallel Sagnac methane gas fiber optic sensor, characterized in that the fiber optic sensor comprises 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), the first optical circulator (12) connected with the C3 port;
a second optical circulator (13), the second optical circulator (13) connected with the C4 port;
a first depolarizing photonic crystal fiber (14), the first depolarizing photonic crystal fiber (14) and the first optical circulator (12) forming a first Sagnac loop; and the second polarization-maintaining photonic crystal fiber (15), wherein the second polarization-maintaining photonic crystal fiber (15) and the second optical circulator (13) form a second Sagnac loop.
2. The fiber sensor of claim 1, wherein 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.
3. The fiber sensor of claim 1 or 2, wherein 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) comprises two layers of circular air holes of three sizes.
4. The fiber sensor of claim 3, wherein the cladding structure comprises an inner air hole and an outer air hole;
the inner-layer 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-layer 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.
5. The optical fiber sensor according to claim 4, wherein the refractive index of the methane gas sensitive film is adjusted by controlling the concentration of methane gas, which is adjusted by controlling the flow rates of methane and nitrogen.
6. The fiber 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 the second beam of light received by the first optical circulator (12);
the first beam of light and the second beam of light generate phase difference through the first polarization maintaining photonic crystal fiber (14), and a sensing branch Sagnac interference spectrum is formed at the first light circulator (12).
7. The fiber sensor of claim 6, wherein 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), the third beam of light being transmitted clockwise along the second Sagnac loop to the E2 port;
the E2 port transmits the fourth beam of light received by the second optical circulator (13);
and the third beam of light and the fourth beam of light generate 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).
8. The fiber optic sensor of claim 7, 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 optical signals and sending the optical signals to the Sagnac sensing unit (1), so that the Sagnac sensing unit (1) receives and processes the optical signals and sends the processed optical signals to the optical spectrum analyzer;
the optical spectrum analyzer is connected with the C2 port and is used for receiving the processed optical signal output by the Sagnac sensing unit (1).
9. The fiber optic sensor of claim 8, wherein the Sagnac interference spectrum of the sensing branch and the Sagnac interference spectrum of the reference branch spectrally overlap and form an envelope on the optical spectrum analyzer.
10. A fibre-optic sensor according to any of claims 1 to 9, characterized in that the coupler (11) is a 2 x 2 3dB coupler and the first (12) and second (13) optical circulators are each 1 x 2 optical circulators.
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