CN110018130B - Gas sensor based on third harmonic generation in cascade micro optical fiber - Google Patents
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- 239000013307 optical fiber Substances 0.000 title claims abstract description 187
- 238000001514 detection method Methods 0.000 claims abstract description 94
- 238000010521 absorption reaction Methods 0.000 claims abstract description 40
- 239000003658 microfiber Substances 0.000 claims description 64
- VNWKTOKETHGBQD-UHFFFAOYSA-N methane Chemical compound C VNWKTOKETHGBQD-UHFFFAOYSA-N 0.000 claims description 58
- 229920001410 Microfiber Polymers 0.000 claims description 55
- 239000000835 fiber Substances 0.000 claims description 27
- 230000007704 transition Effects 0.000 claims description 12
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- 229910017000 As2Se3 Inorganic materials 0.000 claims description 5
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- VYPSYNLAJGMNEJ-UHFFFAOYSA-N Silicium dioxide Chemical compound O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 description 2
- 238000005452 bending Methods 0.000 description 2
- 229910002091 carbon monoxide Inorganic materials 0.000 description 2
- 150000004770 chalcogenides Chemical class 0.000 description 2
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- 238000005516 engineering process Methods 0.000 description 2
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- JBRZTFJDHDCESZ-UHFFFAOYSA-N AsGa Chemical compound [As]#[Ga] JBRZTFJDHDCESZ-UHFFFAOYSA-N 0.000 description 1
- 229910001218 Gallium arsenide Inorganic materials 0.000 description 1
- 229910000661 Mercury cadmium telluride Inorganic materials 0.000 description 1
- 238000000862 absorption spectrum Methods 0.000 description 1
- 150000001335 aliphatic alkanes Chemical class 0.000 description 1
- 230000005540 biological transmission Effects 0.000 description 1
- MCMSPRNYOJJPIZ-UHFFFAOYSA-N cadmium;mercury;tellurium Chemical compound [Cd]=[Te]=[Hg] MCMSPRNYOJJPIZ-UHFFFAOYSA-N 0.000 description 1
- 239000005387 chalcogenide glass Substances 0.000 description 1
- 238000005253 cladding Methods 0.000 description 1
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- 238000013461 design Methods 0.000 description 1
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- 239000002657 fibrous material Substances 0.000 description 1
- 238000011065 in-situ storage Methods 0.000 description 1
- WPYVAWXEWQSOGY-UHFFFAOYSA-N indium antimonide Chemical compound [Sb]#[In] WPYVAWXEWQSOGY-UHFFFAOYSA-N 0.000 description 1
- 238000002329 infrared spectrum Methods 0.000 description 1
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- G01N21/35—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
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Abstract
The invention provides a gas sensor based on generation of third harmonic in a cascade micro optical fiber, which comprises a first section of micro optical fiber and a second section of micro optical fiber, wherein the first section of micro optical fiber and the second section of micro optical fiber are cascaded; the first section of the micro optical fiber is arranged in a certain air chamber, two ends of the first section of the micro optical fiber penetrate out of the certain air chamber through matched drill holes and are in sealed connection with the certain air chamber, wherein gas in the certain air chamber is target detection gas, and the target detection gas in the certain air chamber is utilized to absorb mid-infrared light input from the input end of the first section of the micro optical fiber; the second section of micro optical fiber is used for generating third harmonic wave so as to convert the residual mid-infrared light into near-infrared light; the power of the output near infrared light is detected by a near infrared detector, and the concentration of the target detection gas is obtained according to the power change. The invention has the beneficial effects that: the gas sensor has the advantages of simple sensing structure, shorter gas absorption length, higher gas detection sensitivity, low detection cost, high detection speed and high detection precision, and is beneficial to integration.
Description
Technical Field
The invention relates to the field of optical fibers and the technical field of laser photoelectron, in particular to a gas sensor based on generation of third harmonic in cascade micro optical fibers.
Background
Gas sensing has always played an important role in the fields of environmental and industrial detection. Due to the large amount of gases (CO, CO) in the atmosphere2,NO,NO2,CH4Etc.) have strong absorption peaks in the mid-infrared spectrum (2.5-20 μm), making mid-infrared light a popular choice in the gas sensing field. Most remote sensing techniques are now based on optical detection, which uses light absorption measurements to determine the chemical composition of a sample. The tunable laser absorption spectroscopy technology has been proven to have high sensitivity and good selectivity for real-time in-situ trace gas sensing, and the development of commercial room-temperature mid-infrared lasers also really improves the sensitivity of trace gas detection. However, in the mid-infrared absorption spectrum, the laser signal is typically detected by a detector made of mercury cadmium telluride or indium antimonide, which has some drawbacks in the near-infrared and visible wavelength ranges compared to silicon or gallium arsenide. For example, the sensitivity of mid-infrared detectors is severely limited by thermal noise due to the small band gap, cooling is necessary to achieve better performance, and mid-infrared detectors are not cost effective. Near-infrared detectors can easily overcome these disadvantages compared to mid-infrared detectors. However, the gas absorption efficiency in this wavelength range is much lower than in the mid-infrared, thus also limiting the overall sensing performance. For example, the near infrared has a methane absorption coefficient about 100 times lower than the absorption coefficient of the mid infrared.
The nonlinear parametric up-conversion to gas sensing provides a promising scheme, which not only can utilize the high absorption of target gas to mid-infrared light, but also can convert mid-infrared signals into near-infrared signals and utilize the high-performance detection of near-infrared. Thus, the advantages of mid-infrared absorption and near-infrared detection are fully achieved. In the nonlinear effect of optical up-conversion, the third harmonic generation has the advantages of large wavelength interval, power scaling of third-order index, flexible platform selection and the like, so that the third harmonic generation becomes a potential method of nonlinear gas sensing.
Optical fibers are an effective medium for gas sensing and third harmonic generation. However, in the wavelength range of more than 3 μm, the conventional silica-based optical fiber has a large propagation loss. In contrast, soft glass has excellent transmittance in the mid-infrared band. In particular chalcogenide glasses have very good transmission properties in the broad band range of 1-16 μm and have a value of up to 10-17m2A non-linear refractive index of/W, which is advantageous for sensing and non-linear optics. The microfiber as a superfine fiber has excellent optical and mechanical properties, and is widely applied to gratings, active optical devices, quantum and atomic optics, nonlinear optics and the like. Although the micro optical fiber has strong constraint on the optical field, a large part of the optical field is transmitted in a vacuum or gas medium cladding in the form of an evanescent field, and the evanescent field of the micro optical fiber is easy to interact with the surrounding environment, so that the change condition of the external environment is reflected. Therefore, the micro optical fiber has great application potential in high-sensitivity optical sensing. Meanwhile, the micro optical fiber has high bending toughness and can bear a small bending radius. And the common micro optical fiber reserves the standard optical fiber as the second tail fiber, is convenient to be connected with various optical fiber components, and can realize lower insertion loss by utilizing the standard optical fiber connection technology. In addition, the transition region ensures that the optical field mode in the standard optical fiber is adiabatically converted into the optical field mode in the micro optical fiber, and the energy loss caused by mode mismatching is avoided. The micro optical fiber has small mass and small diameter, so that the micro optical fiber is used for sensing and is beneficial to integration.
Disclosure of Invention
In order to solve the above problems, the present invention provides a gas sensor based on third harmonic generation in a cascaded micro optical fiber, including a first tapered optical fiber and a second tapered optical fiber, where the first tapered optical fiber includes a first pigtail, a first transition region, a first section of micro optical fiber, a second transition region and a second pigtail, and the second tapered optical fiber includes a first pigtail, a first transition region, a second section of micro optical fiber, a second transition region and a second pigtail; the first tapered optical fiber and the second tapered optical fiber are cascaded, namely the second tail fiber of the first tapered optical fiber is butted with the first tail fiber of the second tapered optical fiber; the first section of the micro optical fiber is arranged in a certain air chamber, two ends of the first section of the micro optical fiber penetrate out of the certain air chamber through matched drill holes and are in sealed connection with the certain air chamber, wherein gas in the certain air chamber is target detection gas, and the target detection gas in the certain air chamber is utilized to absorb mid-infrared light input from the input end of the first section of the micro optical fiber; the second section of micro optical fiber is used for generating third harmonic wave so as to convert the residual mid-infrared light into near-infrared light;
inputting mid-infrared light at the input end of the first section of the micro optical fiber, and enabling part of the mid-infrared light in the first section of the micro optical fiber to be absorbed by target detection gas in the certain gas chamber by utilizing the interaction of a large evanescent field generated by the first section of the micro optical fiber and the target detection gas corresponding to the mid-infrared absorption fingerprint spectrum; then the residual mid-infrared light is output from the output end of the first section of the micro optical fiber;
the residual mid-infrared light is output from the output end of the first section of the micro-optical fiber and then enters the second section of the micro-optical fiber, and under the condition that the phase matching condition generated by the third harmonic wave is met, the mid-infrared light in the second section of the micro-optical fiber is converted into the third harmonic wave positioned in the near-infrared wave band and is output at the output end of the second section of the micro-optical fiber; the near-infrared detector detects the power P of the third harmonic wave of the near-infrared band at the output end of the second section of micro-fiber3＝▽·P1Where ▽ is the impact factor related to the solution of the coupling mode equation related to third harmonic generation;
obtaining the concentration C of the target detection gas according to the input power of mid-infrared light, the output power of the residual mid-infrared light in the first section of micro-optical fiber and the power of the third harmonic wave of the near-infrared waveband output by the second section of micro-optical fiber and further according to the absorption power of the mid-infrared light absorbed by the target detection gas;
the calculation formula of the detected target detection gas concentration C is as follows:
wherein Δ P is a power difference between the output power of the second micro-fiber when the target detected gas concentration detected by the near-infrared detector is 0 and the output power of the second micro-fiber when the target detected gas concentration is C, that is, Δ P ═ P3(C＝0)-P3(C＝C)。
Further, the materials of the first tapered optical fiber and the second tapered optical fiber are both As2Se3Glass; the diameter of the second tail fiber is the same as that of the standard optical fiber, namely 125 mu m.
And further, if different target detection gases are filled in the certain air chamber and correspond to different absorption wavelengths on the mid-infrared absorption fingerprint spectrum, mid-infrared light with different wavelengths is input at the input end of the first section of the micro optical fiber.
Further, if the target detection gas in a certain gas chamber is different, the lengths and diameters of the first section of micro optical fiber and the second section of micro optical fiber are also different, that is, the lengths and diameters of the two sections of micro optical fibers are also correspondingly changed.
Further, after the near-infrared detector is selected, the length of the first section of the micro optical fiber is the length corresponding to the gas sensor based on the third harmonic generation in the cascade micro optical fiber when the gas sensor reaches the minimum detection limit, and the length of the second section of the micro optical fiber is the length corresponding to the near-infrared band when the third harmonic output power is maximum; the diameter of the first section of the micro optical fiber is determined by the phase matching condition generated by the evanescent field and the third harmonic, and the corresponding diameter when the evanescent field is maximum is the diameter of the first section of the micro optical fiber under the condition of avoiding the phase matching condition generated by the third harmonic; and the corresponding diameter is the diameter of the second section of the micro optical fiber when the phase matching condition generated by the third harmonic wave is met.
Further, when the target detection gas is methane gas, the certain gas chamber is a methane gas chamber; the wavelength of the mid-infrared light correspondingly input at the input end of the first section of micro optical fiber is 3300 nm; the wavelength of the near infrared light converted in the second section of the micro-fiber is 1100 nm.
Further, the minimum gas concentration which can be detected by the gas sensor based on the generation of the third harmonic in the cascade micro optical fiber is determined by the gas absorption length and the performance of the near infrared detector; the gas absorption length is the length of the first section of the micro optical fiber, for the determined near-infrared detector, the length corresponding to the minimum detection limit of the gas sensor based on the third harmonic in the cascade micro optical fiber is the optimal gas absorption length, and the parameter delta P is related to the performance of the near-infrared detector.
The technical scheme provided by the invention has the beneficial effects that: the gas sensor has the advantages of simple sensing structure, shorter gas absorption length, higher gas detection sensitivity, low detection cost, high detection speed and high detection precision, and is beneficial to integration.
Drawings
The invention will be further described with reference to the accompanying drawings and examples, in which:
FIG. 1 is a block diagram of a gas sensor based on third harmonic generation in cascaded micro optical fibers in an embodiment of the present invention;
FIG. 2 is a graph of core diameter versus effective index and confinement factor of light waves in an embodiment of the present invention;
FIG. 3 is a schematic diagram illustrating the power evolution of mid-infrared light (3300nm) and third harmonic (1100nm) in the second section of micro-fiber according to an embodiment of the present invention;
FIG. 4 is a diagram illustrating the evolution of third harmonic power with increasing propagation distance in a second section of microfiber according to an embodiment of the present invention;
FIG. 5 shows the detection limit and the first micro-fiber length L in an embodiment of the present invention1The relationship of (1);
FIG. 6 is a graph of normalized output power and sensitivity of a first section of microfiber and a second section of microfiber in accordance with an embodiment of the present invention as a function of methane gas concentration C;
FIG. 7 shows the methane gas concentration and the length L of the micro-fiber in the gas absorption section for the mid-infrared sensing sensitivity and the cascade micro-fiber sensing sensitivity of the embodiment of the present invention1The relationship (2) of (c).
Detailed Description
For a more clear understanding of the technical features, objects and effects of the present invention, embodiments of the present invention will now be described in detail with reference to the accompanying drawings.
The invention provides a gas sensor based on third harmonic generation in a cascade micro-fiber, which reflects the concentration of target detection gas in a certain gas chamber by the power change of output end near-infrared power when the concentration of the target detection gas in the certain gas chamber is 0 and non-0.
For different target detection gases in the certain gas chamber, the wavelengths of input mid-infrared light are different, the diameters of the first section of micro-fiber are determined by the limiting factor related to an evanescent field and the phase matching condition generated by third harmonic, the diameter of the second section of micro-fiber is determined by the phase matching condition generated by the third harmonic, and for the input mid-infrared light with different wavelengths, the diameters of the two sections of micro-fiber of the gas sensor generated based on the third harmonic in the cascade micro-fiber are different. The optimal length of the second section of the micro-fiber is the length corresponding to the maximum third harmonic output power; the optimal length of the first section of the micro optical fiber is the length corresponding to the minimum detection limit of the gas sensor generated based on the third harmonic in the cascade micro optical fiber after the near infrared detector is determined; the optimal length of the two sections of micro optical fibers is different for different target detection gases.
Referring to fig. 1, fig. 1 is a structural diagram of a gas sensor based on third harmonic generation in a cascaded micro optical fiber according to an embodiment of the present invention, including: the optical fiber comprises a first tapered optical fiber and a second tapered optical fiber, wherein the first tapered optical fiber comprises a first tail fiber, a first transition region, a first section of micro optical fiber (GA section), a second transition region and a second tail fiber, and the second tapered optical fiber comprises a first tail fiber, a first transition region, a second section of micro optical fiber (THG section), a second transition region and a second tail fiber; the first tapered optical fiber and the second tapered optical fiber are cascaded, namely the second tail fiber of the first tapered optical fiber is butted with the first tail fiber of the second tapered optical fiber; the transition region is shaped like a cone and is connected with the first tail fiber or the second tail fiber; the diameter of the second tail fiber is the same as that of the standard optical fiber, namely the diameter of the second tail fiber is the same as that of the standard optical fiber125 μm. The first conical optical fiber and the second conical optical fiber are made of As2Se3Glass, As2Se3The nonlinear index of refraction of the glass is 2.4 × 10-17m2and/W. The first section of the micro optical fiber is arranged in a certain gas chamber containing target detection gas, two ends of the first section of the micro optical fiber penetrate out of the certain gas chamber through matched drill holes and are in sealed connection with the certain gas chamber, wherein the gas in the certain gas chamber is the target detection gas, and the target detection gas in the certain gas chamber is used for absorbing mid-infrared light (MIR) input from the input end of the first section of the micro optical fiber; the second section of the micro-fiber is used for Third Harmonic Generation (THG) to convert the residual mid-infrared light into near-infrared light; the first section of the micro-optical fiber has a length L1The second section of the micro-fiber has a length L2(ii) a If the target detection gas filled in the certain gas chamber is different and corresponds to different absorption wavelengths on the mid-infrared absorption fingerprint spectrum, mid-infrared light with different wavelengths is input at the input end of the first section of the micro optical fiber. If the target detection gas in a certain gas chamber is different, the lengths and diameters of the first section of micro optical fiber and the second section of micro optical fiber are also different, namely the lengths and diameters of the two sections of micro optical fibers are also correspondingly changed when the target detection gas is different. The diameter of the first section of the micro-optical fiber is determined by the phase matching condition generated by the evanescent field and the third harmonic, and the larger the evanescent field is, the better the phase matching condition generated by the third harmonic is avoided; the diameter of the second section of the micro optical fiber is determined by the phase matching condition generated by the third harmonic wave, and because the second section of the micro optical fiber is not immersed in the target detection gas, the mid-infrared light or the near-infrared light in the second section of the micro optical fiber cannot be absorbed, namely the evanescent field cannot influence the mid-infrared light or the near-infrared light in the second section of the micro optical fiber. Therefore, the length of the first section of the micro optical fiber is the length corresponding to the gas sensor based on the third harmonic in the cascade micro optical fiber when the gas sensor reaches the minimum detection limit, and the length of the second section of the micro optical fiber is the length corresponding to the maximum output power of the third harmonic in the near-infrared band (that is, after the near-infrared detector is selected, the length of the second section of the micro optical fiber is the length corresponding to the maximum output power of the detection of the near-infrared detector); the diameter of the first section of the micro-optical fiber is subjected to the phase matching condition generated by evanescent field and third harmonicDetermining that the corresponding diameter when the evanescent field is maximum is the diameter of the first section of the micro optical fiber under the condition of avoiding phase matching generated by third harmonic; and the corresponding diameter is the diameter of the second section of the micro optical fiber when the phase matching condition generated by the third harmonic wave is met.
In the embodiment, methane gas is taken as an example, that is, a certain gas chamber is a methane gas chamber, and the target detection gas is methane gas; the corresponding absorption wavelength of methane gas on the mid-infrared absorption fingerprint spectrum is 3300nm, and the absorption coefficient is 1.6/cm, so that the input mid-infrared wavelength lambda of the first section of micro optical fiber13300nm, near infrared wavelength λ output from the output end of the second section of micro-fiber3Is 1100 nm. When the concentration of methane gas is detected, the length L of the first section of the micro optical fiber1And the diameters of the fiber cores are respectively 2.9cm and 750 nm; second length L of micro-fiber2And core diameters of 9.5cm and 545nm, respectively.
Input wavelength lambda at input end of first section of micro-fiber1The infrared light is 3300nm, after passing through the first section of micro-fiber, part of the infrared light is absorbed by methane gas; then the residual mid-infrared light is output from the output end of the first section of the micro optical fiber and enters the second section of the micro optical fiber; the output power P of the rest intermediate infrared light output from the output end of the first section of the micro-optical fiber1＝P0exp(-σCL1-αL1) Wherein P is0Is the input power of the mid-infrared light, σ is the absorption coefficient of 100% methane gas, C is the concentration of methane gas in the first section of the microfiber, α is the attenuation coefficient, is the limiting factor associated with the evanescent field, L1The length of the first section of the micro optical fiber is 2.9 cm; under the condition of meeting the phase matching condition, mid-infrared light in the second section of micro-optical fiber is converted into near-infrared light through third harmonic generation and is output at the output end of the second section of micro-optical fiber; the near-infrared detector detects the power P of the third harmonic wave of the near-infrared band at the output end of the second section of micro-fiber3＝▽·P1Where ▽ is the impact factor related to the solution of the coupling mode equation related to third harmonic generation.
Obtaining the concentration C of the target detection gas according to the input power of mid-infrared light, the output power of the residual mid-infrared light in the first section of micro-optical fiber and the power of the third harmonic wave of the near-infrared band and further according to the absorption power of the mid-infrared light absorbed by the target detection gas;
the calculation formula of the detected gas concentration C of the detected object is as follows:
wherein Δ P is a power difference between the output power of the second micro-fiber when the target detected gas concentration detected by the near-infrared detector is 0 and the output power of the second micro-fiber when the target detected gas concentration is C, that is, Δ P ═ P3(C＝0)-P3(C＝C)。
The minimum gas concentration which can be detected by the gas sensor based on the generation of third harmonic in the cascade micro optical fiber is determined by the gas absorption length and the performance of the near infrared detector; the gas absorption length is the length of the first section of the micro optical fiber, for the determined near infrared detector, the length corresponding to the minimum detectable gas concentration of the gas sensor generated based on the third harmonic in the cascade micro optical fiber is the optimal gas absorption length, and the parameter delta P is related to the performance of the near infrared detector. The diameters of the two sections of micro optical fibers are respectively determined by a limiting factor related to an evanescent field and THG phase matching conditions, and the limiting factor and the phase matching conditions can respectively ensure the sufficient absorption of methane gas to mid-infrared light and the high-efficiency THG. For THG, assuming that the correction of the propagation constant by the self-phase modulation and the cross-phase is small, the phase matching condition is equivalent to the effective refractive index of the Fundamental Wave (FW) being equal to the effective refractive index of the Third Harmonic (THW). The fundamental wave in this embodiment is mid-infrared light, and the third harmonic is near-infrared light.
Referring to FIG. 2, FIG. 2 is a graph showing the relationship between the core diameter and the effective refractive index and confinement factor of a wave in an embodiment of the present invention, and FIG. 2(a) shows the refractive index at As2Se3In a micro-fiber, FW (λ)13300nm) of the fundamental mode HE11(omega) and THW (lambda)31100nm), i.e. FW (λ)13300nm) of the fundamental mode HE11(omega) and THW (lambda)31100nm) as a function of the core diameter.For the phase matching conditions of FW and THW, ideally, it should be phase matching between the fundamental mode of FW and the fundamental mode of THW. However, this is difficult to achieve due to the presence of material dispersion. In contrast, a large number of high order modes may satisfy the phase matching condition. However, these high-order modes have different mode overlap with FW. HE of THW12The (3 ω) mode provides the best mode overlap for third harmonic generation, where the corresponding fiber diameter is 545nm, so the core diameter of the second segment of microfiber is set to 545 nm. Unlike the second section of the microfiber, the first section of the microfiber needs to perform a process of absorbing mid-infrared light by methane gas without nonlinear conversion, so the first section of the microfiber should avoid satisfying the phase matching condition. However, a large evanescent field is required for methane gas to efficiently absorb mid-infrared light in the first section of microfiber. The evanescent field is characterized by a limiting factor, reflecting the degree of overlap between the gas and the mode. For mid-infrared light with 3300nm wavelength, the limiting factors corresponding to different core diameters are shown in fig. 2(b), and as can be seen from fig. 2(b), the limiting factors gradually decrease as the core diameter increases; when the diameter of the fiber core is in the range of 0.5-1 μm, the limiting factor is sharply reduced; as the core diameter continues to increase, the confinement factor is often negligible, since the evanescent field is small because the optical field is highly confined to the increasing core. Considering these factors, in order to enhance the interaction between the mid-infrared light and the methane gas, the core diameter of the first section of the micro optical fiber is set to 750nm, and the corresponding limiting factor is 0.733, so that the conditions of generating a large evanescent field and phase mismatch can be satisfied at the same time.
For this gas sensor based on third harmonic generation in a cascaded micro-fiber, mid-infrared light at 3300nm wavelength is input into a chalcogenide fiber and then propagates along a first segment of the micro-fiber. According to the Beer-Lambert law, the output power P of the intermediate infrared light passing through the section of micro-optical fiber is obtained by the formula (1)1：
P1＝P0exp(-σCL1-αL1) (1)
Wherein P is0Is the input power, sigma (A), of mid-infrared light input into the first section of micro-fiberThe absorption coefficient of the alkane gas to the mid-infrared light with the wavelength of 3300nm is 1.6/cm) is the absorption coefficient of 100% gas, C is the concentration of the methane gas to be detected, α is αdB/4.343, α is the attenuation coefficient, αdBIs the loss of the micro-fiber and is a limiting factor associated with the evanescent field.
Third harmonic generation THG is modeled by coupling mode equation (2):
wherein A is1And A3Are amplitudes corresponding to FW and THW, respectively, and z is the propagation distance of light, α1＝αdB/4.343，α3＝αdB/4.343 are the attenuation coefficients of FW and THW, respectively, β - β3-3β1Is the propagation constant mismatch, k1＝ω1/c＝2π/λ1Is the propagation constant of FW in air, n2Is the nonlinear index of refraction coefficient of the fiber material, i represents the imaginary part, and A and J are the conjugates of A and J, respectively; j. the design is a squareiNon-linear overlap integral, for fundamental mode HE11(ω) and mode HE12(3. omega.) for J1＝0.053μm-2，J2＝0.073μm-2，J3＝0.057μm-2，J5＝0.011μm-2。
The output power of the first section of micro-fiber is the input power of the second section of micro-fiber. Thus, in the second segment of the microfiber, A1And A3Are each P1 1/2And 0. When mid-infrared light propagates in the second segment of the micro-fiber, the energy of FW is gradually converted into THW. As can be seen from fig. 3, with the input of mid-infrared light of 0.1W input power, the power of THW increases to a maximum value and then gradually decreases to 0. The generated maximum THW output power of 1.34 μ W can be detected by an efficient near infrared detector.
The detection Limit (LOD) is the minimum gas concentration (C) measurable by the gas sensor based on the third harmonic generation in the cascade micro-fibermin) From the minimum detectable power (Pmin) And (4) determining. PminThe output power P (C ═ C) when the gas concentration is 0 and the output power P (C ═ C) when the gas concentration is the minimum detectable concentration are calculatedmin) Is expressed as shown in equation (3):
Pmin≤P(C＝0)-P(C＝Cmin) (3)
wherein, PminIs determined by the performance of the photodetector.
Suppose the power resolution of mid-infrared (MIR) detection is 1 × 10-9W, and power resolution for Near Infrared (NIR) detection of 1 × 10-13W。
Another important parameter in the gas sensor based on the generation of the third harmonic in the cascaded micro optical fiber is the detection sensitivity S, which is obtained by the formula (4):
where C is the concentration of methane gas and P is the normalized power, i.e., P (C) C/P (C0), which provides comparability of detection sensitivity between MIR detection and NIR detection in fig. 1; the MIR detection in this embodiment corresponds to the output power detection of the MIR direct sensor, and the NIR detection corresponds to the output power detection of the gas sensor based on the third harmonic generation in the cascaded micro optical fiber.
Due to the small power (| A) of the generated Third Harmonic (THW)3|<<|A1|), the approximate solution of coupling mode equation (2) can be expressed as:
P3≈P1 3(k1n2|J3|)2z2(5)
wherein, P3Is the output power of the Third Harmonic (THW);
the sensitivities related to the normalized powers of MIR detection and NIR detection in the gas sensor based on third harmonic generation in the cascaded micro optical fiber are calculated using formula (6) and formula (7), respectively:
SMIR＝L1exp(-CL1) (6)
SNIR＝3L1exp(-3CL1) (7)
when the concentration of the target detection gas is extremely low, the concentration C and the length L of the first section of the micro-optical fiber are the same for the same gas1In other words, the sensitivity of NIR detection is 3 times higher than that of MIR detection, and the sensitivity of NIR detection is related to the length L of the second section of micro-fiber2Regardless, the performance of the gas sensor based on third harmonic generation in a cascade micro fiber was analyzed to fix the input power to fiber losses α at wavelengths of 0.1W, 3300nm and 1100nm, taking into account the damage threshold of chalcogenide fibersdB＝0.5dB/cm。
As shown in FIG. 4, different first segment microfiber lengths L1The output power of the near infrared light in the second section of micro optical fiber is different; along the length L of the first section of the micro-fiber1The output power of the near infrared light is reduced under the same propagation distance z value of the light. Different first segment micro-fiber lengths L1The corresponding four curves have the same change trend, and the length L of the second section of the micro-optical fiber2The second segment of micro-fiber has the maximum near infrared light output power when the length is 9.5 cm. And defining the optimal length (Lopt) as the length of the micro-optical fiber corresponding to the minimum detection limit, wherein the optimal length Lopt of the second section of the micro-optical fiber is 9.5cm when the gas sensor based on the third harmonic generation in the cascade micro-optical fiber detects the concentration of methane gas.
After determining the optimal length of the second segment of microfiber, the length L of the first segment of microfiber1The detection Limit (LOD) of the gas sensor based on third harmonic generation in the cascaded micro optical fiber, in various places, also needs to be determined. For comparison with MIR direct sensing, the minimum detection limit for MIR direct sensing was calculated. As shown in FIG. 5, for MIR direct sensing and gas sensors based on third harmonic generation in cascaded micro-fibers, the length L of the micro-fiber is measured along the first segment1Increase in LOD, both decrease and increase, indicating that there is an optimum first segment microfiber length L1Minimum detection limit for MIR direct sensing and gas sensors based on third harmonic generation in cascaded micro-fibers is 2.7 × 10, respectively-9And 3.50 × 10-9. Accordingly, the Lopt for the gas absorbing part of the MIR direct sensing is 8.7cm, while the Lopt for the first section of the micro optical fiber of the gas sensor based on the third harmonic generation in the cascaded micro optical fiber is 2.9cm, and the length of the first section of the micro optical fiber of the gas sensor based on the third harmonic generation in the cascaded micro optical fiber is 1/3 of the length of the micro optical fiber of the gas absorbing part of the MIR direct sensing. Therefore, the minimum LOD for MIR direct sensing is slightly less than the LOD for gas sensors based on third harmonic generation in the cascaded micro-fiber, but the Lopt for gas sensors based on third harmonic generation in the cascaded micro-fiber is less than the Lopt for MIR direct sensing.
When the gas sensor based on the third harmonic generation in the cascade micro optical fiber in the embodiment detects the concentration of the methane gas, the optimal length L is satisfied12.9cm and L2The sensitivity and normalized power for MIR detection and NIR detection at 9.5cm are shown in fig. 6. The normalized output power of MIR detection and NIR detection gradually decreases as the concentration C of methane gas increases, but the change in output power of gas sensing based on third harmonic generation in the cascade shimmer is more severe at low concentrations, resulting in higher sensitivity of NIR detection, as shown in fig. 6(b), which is higher at methane gas concentrations less than 0.16. When C goes to 0, the sensitivity of NIR detection is 3 times that of MIR detection, which is the same as the results of equations (6) and (7). Therefore, the proposed gas sensor based on third harmonic generation in cascaded micro-fiber is more suitable for low concentration gas detection.
As shown in FIG. 7, FIG. 7 shows the methane gas absorption length L1The critical concentration is different from the corresponding critical concentration, and the critical concentration refers to the concentration of methane gas when the sensitivities of MIR detection and NIR detection are equal; due to the length L of the second section of the micro-optical fiber2There is no effect on the normalized power sensitivity in equation (4), so the second segment of microfiber length L2Fixed at 9.5 cm. When the length L of the first section of the micro-optical fiber1When the critical concentration is increased, the critical concentration is reduced and tends to be 0, which means that the gas sensor based on the generation of third harmonic in the cascade micro optical fiber is sensitive in detection when the absorption length of methane gas is large enoughThe advantage in degree gradually disappears. However, the length of the micro-fiber is limited by the manufacturing technique. Thus, the gas sensing scheme proposed by the present invention can reduce the gas absorption length, i.e., the length of the first section of microfiber.
The invention is not limited to methane gas sensing, but is applicable to all gas sensing in the mid-infrared absorption fingerprint spectrum.
The technical scheme provided by the invention has the beneficial effects that: the gas sensor has the advantages of simple sensing structure, shorter gas absorption length, higher gas detection sensitivity, low detection cost, high detection speed and high detection precision, and is beneficial to integration.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents, improvements and the like that fall within the spirit and principle of the present invention are intended to be included therein.
Claims (7)
1. A gas sensor based on third harmonic generation in cascade micro optical fiber is characterized in that: the optical fiber comprises a first tapered optical fiber and a second tapered optical fiber, wherein the first tapered optical fiber comprises a first tail fiber, a first transition region, a first section of micro optical fiber, a second transition region and a second tail fiber; the first tapered optical fiber and the second tapered optical fiber are cascaded, namely the second tail fiber of the first tapered optical fiber is butted with the first tail fiber of the second tapered optical fiber; the first section of the micro optical fiber is arranged in a certain air chamber, two ends of the first section of the micro optical fiber penetrate out of the certain air chamber through matched drill holes and are in sealed connection with the certain air chamber, wherein gas in the certain air chamber is target detection gas, and the target detection gas in the certain air chamber is utilized to absorb mid-infrared light input from the input end of the first section of the micro optical fiber; the second section of micro optical fiber is used for generating third harmonic wave so as to convert the residual mid-infrared light into near-infrared light;
inputting mid-infrared light at the input end of the first section of the micro optical fiber, and enabling part of the mid-infrared light in the first section of the micro optical fiber to be absorbed by target detection gas in the certain gas chamber by utilizing the interaction of a large evanescent field generated by the first section of the micro optical fiber and the target detection gas corresponding to the mid-infrared absorption fingerprint spectrum; then the residual mid-infrared light is output from the output end of the first section of the micro optical fiber;
the residual mid-infrared light is output from the output end of the first section of the micro-optical fiber and then enters the second section of the micro-optical fiber, and under the condition that the phase matching condition generated by the third harmonic wave is met, the mid-infrared light in the second section of the micro-optical fiber is converted into the third harmonic wave positioned in the near-infrared wave band and is output at the output end of the second section of the micro-optical fiber; the near-infrared detector detects the power of the third harmonic wave of the near-infrared band at the output end of the second section of the micro-optical fiber
Where ▽ is the impact factor associated with the solution of the third harmonic generation-related coupling mode equation;
obtaining the concentration C of the target detection gas according to the input power of mid-infrared light and the power of third harmonic wave of a near-infrared band output by the second section of micro-optical fiber and further according to the absorption power of the mid-infrared light absorbed by the target detection gas;
the calculation formula of the detected target detection gas concentration C is as follows:
wherein Δ P is a power difference between the output power of the second micro-fiber when the target detected gas concentration detected by the near-infrared detector is 0 and the output power of the second micro-fiber when the target detected gas concentration is C, that is, Δ P ═ P3(C＝0)-P3(C＝C)。
2. A gas sensor based on third harmonic generation in cascaded micro optical fibers as claimed in claim 1 wherein: the first conical optical fiber and the second conical optical fiber are made of As2Se3Glass; the diameter of the first tail fiber and the diameter of the second tail fiber are the same as the diameter of the standard optical fiber, namely 125 mu m.
3. A gas sensor based on third harmonic generation in cascaded micro optical fibers as claimed in claim 1 wherein: if the target detection gas filled in the certain gas chamber is different and corresponds to different absorption wavelengths on the mid-infrared absorption fingerprint spectrum, mid-infrared light with different wavelengths is input at the input end of the first section of the micro optical fiber.
4. A gas sensor based on third harmonic generation in cascaded micro optical fibers as claimed in claim 1 wherein: if the target detection gas in a certain gas chamber is different, the lengths and diameters of the first section of micro optical fiber and the second section of micro optical fiber are also different, namely the lengths and diameters of the two sections of micro optical fibers are also correspondingly changed when the target detection gas is different.
5. The gas sensor based on third harmonic generation in cascaded micro optical fibers of claim 4, wherein: after the near-infrared detector is selected, the length of the first section of the micro optical fiber is the length corresponding to the gas sensor based on the third harmonic generation in the cascade micro optical fiber when the gas sensor reaches the minimum detection limit, and the length of the second section of the micro optical fiber is the length corresponding to the near-infrared band when the third harmonic output power is maximum; the diameter of the first section of the micro optical fiber is determined by the phase matching condition generated by the evanescent field and the third harmonic, and the corresponding diameter when the evanescent field is maximum is the diameter of the first section of the micro optical fiber under the condition of avoiding the phase matching condition generated by the third harmonic; and the corresponding diameter is the diameter of the second section of the micro optical fiber when the phase matching condition generated by the third harmonic wave is met.
6. A gas sensor based on third harmonic generation in cascaded micro optical fibers as claimed in claim 1 wherein: when the target detection gas is methane gas, the certain gas chamber is a methane gas chamber; the wavelength of the mid-infrared light correspondingly input at the input end of the first section of micro optical fiber is 3300 nm; the wavelength of the near infrared light converted in the second section of the micro-fiber is 1100 nm.
7. A gas sensor based on third harmonic generation in cascaded micro optical fibers as claimed in claim 1 wherein: the minimum gas concentration which can be detected by the gas sensor based on the generation of third harmonic in the cascade micro optical fiber is determined by the gas absorption length and the performance of the near infrared detector; wherein the gas absorption length is the length of the first section of micro-fiber.
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