Mach-Zehnder interference optical frequency comb gas sensing system based on dispersion Fourier transform

Abstract

The invention belongs to the field of high-precision and high-selectivity optical fiber sensing and the field of optical frequency comb application, and particularly relates to a Mach-Zehnder interference optical frequency comb gas sensing system based on dispersion Fourier transform. The femtosecond mode-locked pulse laser and the Mach-Zehnder interferometer are combined, the dispersion Fourier transform technology is adopted for signal demodulation, amplification of pulse time delay information is achieved through the dispersion element, and a traditional envelope demodulation method depending on wavelength analysis is avoided; interference fringe information is utilized to directly obtain a real-time accurate relative position relation of pulses between a reference light path and a plurality of sensing light paths, so that synchronous monitoring of concentration changes of various gases is realized. The gas sensor has the advantages of real-time multi-component detection, low cost, high selectivity, high sensing precision and the like, and is particularly suitable for gas sensing requirements in extreme environments and complex working conditions.

Description

Technical Field

[0001] The present invention belongs to the field of high-precision and high-selectivity optical fiber sensing and optical frequency comb application, and specifically is a Mach-Zehnder interferometer optical frequency comb gas sensing system based on dispersion Fourier transform. Background Art

[0002] With the rapid development of information technology, sensors, as key components for information acquisition and environmental perception, play a crucial role in modern information and communication systems. Sensors can sense and transmit external environmental information in real time, providing fundamental data support for various intelligent systems. They are widely used in industrial control, environmental monitoring, healthcare, intelligent transportation, and other fields.

[0003] Among various sensor types, gas sensors have garnered widespread attention in recent years in both scientific research and industry due to their crucial role in environmental monitoring and safety early warning. Unlike biosensors, which primarily identify biomacromolecules, gas sensors rely on the sensitive detection of small gas molecule concentrations. However, due to the small size and weak chemical activity of gas molecules, their limited interaction with traditional sensing materials makes it difficult for conventional materials to produce a significant response, limiting the improvement of gas sensor sensitivity and selectivity.

[0004] In recent years, fiber-optic gas sensors, as a key branch of fiber-optic sensing technology, have gradually become a key research focus. Compared to traditional electrochemical gas sensors, fiber-optic gas sensors offer advantages such as fast response, high sensitivity, and low electromagnetic interference. In particular, gas sensors based on optical microstructures such as microfibers, fiber Bragg gratings, and microcavities, integrated with nanofunctional materials such as graphene, have provided a new development platform for high-performance optical gas sensing. These sensors offer high sensitivity, good biocompatibility, compact size, and ease of integration.

[0005] However, gas sensors based on optical microstructures currently still face two major challenges in improving their performance: first, optical fiber materials are usually made of silica, which is highly chemically inert and difficult to effectively adsorb with gas molecules, limiting further improvements in sensitivity and selectivity; second, the photoelectric response information that a single sensor structure can provide is limited, making it difficult to achieve real-time monitoring of multiple gas components at the same time, restricting its application in multi-component gas sensing scenarios.

[0006] Therefore, in order to address the above problems, it is urgent to introduce new sensitive materials and detection mechanisms to improve the sensitivity and selectivity of sensors, and at the same time combine new structural designs and signal processing technologies to develop optical gas sensing systems that can realize real-time monitoring of multiple components, so as to promote the practical application of optical gas sensing technology. Summary of the Invention

[0007] In response to the above-mentioned problems or shortcomings, and to address the common problems of poor selectivity and difficulty in achieving real-time monitoring of multi-component gases in existing fiber optic gas sensors, the present invention provides a Mach-Zehnder interferometer optical frequency comb gas sensing system based on dispersive Fourier transform to achieve high sensitivity, high selectivity and real-time sensing of multi-component gases.

[0008] The specific technical solutions are as follows:

[0009] A Mach-Zehnder interferometer frequency comb gas sensing system based on dispersive Fourier transform includes: a local oscillator femtosecond pulse optical path, a signal femtosecond pulse optical path, a 1×2 optical fiber coupler, a dispersion element, a pre-fiber amplifier, a second optical fiber attenuator, a photodetector, an A / D acquisition device and a computer.

[0010] The local oscillator femtosecond pulse optical path includes: a local oscillator femtosecond pulse mode-locked fiber laser, a first polarization controller, a first electric delay line and a first fiber attenuator connected in sequence; the local oscillator femtosecond pulse mode-locked laser emits a repetition frequency f r1 The local oscillator femtosecond pulse passes through the first polarization controller, the first electric delay line and the first optical fiber attenuator in sequence and is output to the 1x2 optical fiber coupler; the first polarization controller is used to adjust the polarization state of the local oscillator femtosecond pulse, and the first electric delay line is used to adjust the delay of the local oscillator femtosecond pulse.

[0011] The signal femtosecond pulse optical path includes: a signal femtosecond pulse mode-locked fiber laser, a first 1×n fiber coupler, n parallel branch sensing optical paths and a second 1×n fiber coupler, which are connected in sequence, where n≥1.

[0012] Among them, the signal femtosecond pulse mode-locked fiber laser emits a repetition frequency f r2 The signal femtosecond pulse is generated, and each branch sensing optical path includes a polarization controller, an electric delay line and a functionalized micro-fiber connected in sequence, and a functionalized micro-fiber selectively responds to a gas.

[0013] In each branch sensing optical path: the polarization controller is used to adjust the polarization state of the corresponding branch signal femtosecond pulse so that the n corresponding branch signal femtosecond pulses and the local oscillator femtosecond pulse are in the same polarization state; the electric delay line of the n branch sensing optical path is used to adjust the time delay of the corresponding branch signal femtosecond pulse so that the local oscillator femtosecond pulse and the n-way signal femtosecond pulse can both generate interference fringes without interfering with each other.

[0014] The signal femtosecond pulse is divided into n paths through the first 1×n fiber coupler. Each signal femtosecond pulse passes through the corresponding polarization controller, electric delay line and functionalized micro-fiber in the parallel branch sensing optical path. Finally, these n signal femtosecond pulses are combined into one path through the second 1×n fiber coupler and output to the 1x2 fiber coupler.

[0015] The 1x2 fiber coupler combines the received local oscillator femtosecond pulse and the n-channel signal femtosecond pulse into one channel and outputs it to the dispersion element. Its output signal includes n+1 pulses: the local oscillator femtosecond pulse and the n-channel signal femtosecond pulse passing through the n-channel functionalized micro-fiber.

[0016] The dispersion element performs a dispersion Fourier transform on the input n+1 pulses (intrinsic femtosecond pulses and signal femtosecond pulses); then a pre-fiber amplifier amplifies the optical signal; then a second fiber attenuator adjusts the optical power; finally, a photodetector is used to detect the output interference signal (the local oscillator femtosecond pulse interferes with the n signal femtosecond pulses, ultimately generating n interference signals); the obtained analog electrical signal is converted into a digital signal by an A / D acquisition device, and then transmitted to a computer for data processing to achieve gas sensing.

[0017] Furthermore, the repetition frequency f r1 and repetition frequency f r2 It cannot be too large or too small. Too large will reduce the sensing speed, and too small will cause the pulses to overlap after being stretched. Therefore, f r1 and f r2 In the range of 1 to 100 MHz, and f r1 =n*f r2 This ensures that the fringe frequencies of the n-channel signal femtosecond pulses, when interfering with the local oscillator femtosecond pulses, remain consistent across different periods. The frequency of the interference fringes after the dispersion Fourier transform must also be less than half the sampling rate of the A / D sampling device, the bandwidth of the photodetector, and the bandwidth of the A / D acquisition device.

[0018] Furthermore, n>1, so as to achieve the selective response of each functionalized micro-fiber to a variety of different gases.

[0019] Furthermore, the signal femtosecond pulse mode-locked fiber laser and the local oscillator femtosecond pulse mode-locked fiber laser use the same type of device. This is done to ensure that the pulse characteristics (such as pulse width, pulse intensity, etc.) of the signal femtosecond pulse and the local oscillator femtosecond pulse are as consistent as possible, thereby achieving the best sensing effect.

[0020] Furthermore, the signal femtosecond pulse mode-locked fiber laser and the local oscillator femtosecond pulse mode-locked fiber laser adopt passive mode-locked fiber lasers. The passive mode-locking method is chosen because it has a simple structure, strong stability and repeatability, and low cost compared to active mode-locking. Specifically, it can be a mode-locked laser based on a true saturable absorber, a mode-locked laser based on a nonlinear amplifying ring mirror (NALM), a mode-locked laser based on the nonlinear polarization rotation effect (NPR), or a mode-locked laser based on nonlinear multimode interference (NL-MMI).

[0021] Furthermore, the dispersion element adopts an optical fiber element with a large group velocity dispersion parameter (such as a standard single-mode fiber, a dispersion-compensating fiber or a chirped Bragg grating) whose |β2| is greater than 10ps. 2 The total dispersion of the dispersion element should allow the local oscillator femtosecond pulse and the signal femtosecond pulse to be fully broadened in the time domain, while the broadening value does not exceed the period of the local oscillator femtosecond pulse itself.

[0022] Furthermore, the bandwidth of the photodetector is greater than 1G.

[0023] Furthermore, the bandwidth of the A / D acquisition device is greater than 2G, and the sampling rate is greater than 5Gs / s.

[0024] Furthermore, in the above-mentioned Mach-Zehnder interferometer frequency comb gas sensing system based on dispersion Fourier transform, the specific modulation process of the interference fringes corresponding to the n-way branch sensing optical path is as follows:

[0025] After generating stable local oscillator femtosecond pulses and signal femtosecond pulses, the first electric delay line and the electric delay line of the first branch sensing optical path are adjusted so that the local oscillator femtosecond pulse and the signal femtosecond pulse after passing through the functionalized microfiber in the first branch sensing optical path generate stable interference fringes (enhanced peak P1) after passing through the dispersion element; then, the electric delay line of the second branch sensing optical path is adjusted so that the local oscillator femtosecond pulse and the signal femtosecond pulse after passing through the functionalized microfiber in the second branch sensing optical path generate stable interference fringes (enhanced peak P2) after passing through the dispersion element.

[0026] Next, by sequentially adjusting the electric delay line of the next branch sensing optical path, the local oscillator femtosecond pulse and the signal femtosecond pulse passing through the functionalized micro-fiber in the next branch sensing optical path generate stable interference fringes (enhancement peaks) after passing through the dispersion element; until the signal femtosecond pulses corresponding to all n branch sensing optical paths and the local oscillator femtosecond pulses generate stable interference fringes after passing through the dispersion element.

[0027] In actual use: when the first target gas is selectively adsorbed on the first functionalized microfiber, the refractive index of the first functionalized microfiber is changed, thereby causing the signal femtosecond pulse passing through the first functionalized microfiber to advance or lag by Δt (if the refractive index becomes smaller, it will advance; if the refractive index becomes larger, it will lag), ultimately resulting in a change in the time delay between the signal femtosecond pulse passing through the first functionalized microfiber and the local oscillator femtosecond pulse by Δt.

[0028] According to the formula of dispersion Fourier transform:

[0029]

[0030] Where f is the interference fringe frequency, τ is the time delay between the two pulses, β2 is the group velocity dispersion parameter of the dispersive element, and L is the length of the dispersive element. When the time delay between the two pulses changes, the frequency of the interference fringes changes. Therefore, the adsorption of the first target gas causes a change in the frequency of the interference fringes of the enhanced peak P1. Similarly, the adsorption of the second target gas causes a change in the frequency of the interference fringes of the enhanced peak P2. The adsorption of the nth target gas causes a change in the frequency of the interference fringes of the enhanced peak Pn. Ultimately, selective sensing of n target gases is achieved through the corresponding changes in the frequency of the interference fringes.

[0031] This invention combines a femtosecond mode-locked fiber laser with a Mach-Zehnder interferometer and innovatively employs dispersive Fourier transform technology to demodulate the signal, eliminating the traditional envelope demodulation method that relies on wavelength analysis. By introducing a dispersive element to amplify pulse delay information, the system can directly determine the real-time and precise relative position of the pulses between the reference optical path and each sensing optical path through interference fringes, thereby enabling efficient detection of changes in gas concentration.

[0032] Compared with the prior art, the present invention has the following advantages:

[0033] 1. Achieving Highly Sensitive and Selective Gas Sensing: This invention incorporates n functional materials that selectively adsorb specific gas molecules into a Mach-Zehnder interferometer system, creating n independent sensing channels. This enables highly sensitive detection of n target gases at the ppb (parts per billion) level. This effectively overcomes the chemical inertness of optical fiber materials, which hinder their ability to adsorb gas molecules, and the limited selectivity of traditional two-dimensional materials (such as graphene) for specific gases.

[0034] 2. Real-time monitoring of multi-component gases: This invention utilizes two double-femtosecond pulse mode-locked fiber lasers with multiple repetition rates as the light source. Combined with a Mach-Zehnder interferometer structure, it efficiently demodulates the interference signal through dispersive Fourier transform technology. This method directly extracts the relative temporal position information of multiple pulse signals, independent of wavelength, significantly improving the system's response speed and accuracy to multi-component gas changes. This effectively overcomes the technical bottleneck of existing single fiber optic sensors, which cannot simultaneously monitor multiple gases, and enables real-time, parallel, and accurate identification of concentration changes in multiple gases.

[0035] In summary, the present invention combines two-dimensional material optoelectronics, micro-nano fiber processing technology, and the stable generation technology of femtosecond pulse mode-locked fiber lasers to address the technical bottlenecks of existing fiber-optic gas sensors in terms of insufficient sensitivity, poor selectivity, and inability to achieve real-time measurement of multi-component gases. It provides a Mach-Zehnder interferometer frequency comb gas sensing system based on dispersive Fourier transform. This sensing system has the advantages of high sensitivity, high selectivity, and real-time measurement of multi-component gas concentrations, significantly improving the practicality and reliability of fiber-optic gas sensors in complex environments. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 Schematic diagram of the system structure of the embodiment.

[0037] Figure 2 The interference pattern and its local features collected by the gas sensing system in the embodiment.

[0038] Figure 3 3 is a response diagram of the interference fringe frequency of each enhanced peak of the gas sensing system in the embodiment changing with different gas concentrations.

[0039] Figure 1: 1-local oscillator femtosecond pulse mode-locked fiber laser, 2-signal femtosecond pulse mode-locked fiber laser, 3-first polarization controller, 4-first electric delay line, 5-first optical fiber attenuator, 6-1x2 fiber coupler, 7-first 1x3 fiber coupler, 8-second polarization controller, 9-second electric delay line, 10-first functionalized microfiber, 11-third polarization controller, 12-third electric delay line, 13-second functionalized microfiber, 14-fourth polarization controller, 15-fourth electric delay line, 16-third functionalized microfiber, 17-second 1x3 fiber coupler, 18-dispersion element, 19-pre-fiber amplifier, 20-second fiber attenuator, 21-photodetector, 22-A / D acquisition device, 23-computer. DETAILED DESCRIPTION

[0040] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0041] This embodiment provides a Mach-Zehnder interferometer frequency comb gas sensing system based on dispersive Fourier transform (DFT). This system combines a femtosecond mode-locked laser with a Mach-Zehnder interferometer and employs dispersive Fourier transform (DFT) technology for demodulation. This gas sensing system combines high sensitivity, high selectivity, and real-time multi-component measurement. In this embodiment, n = 3.

[0042] Its specific system structure is as follows Figure 1 As shown, it includes: a local oscillator femtosecond pulse mode-locked fiber laser 1, a signal femtosecond pulse mode-locked fiber laser 2, a first polarization controller 3, a first electric delay line 4, a first optical fiber attenuator 5, a 1x2 optical fiber coupler 6, a first 1x3 optical fiber coupler 7, a second polarization controller 8, a second electric delay line 9, a first functionalized micro-fiber 10, a third polarization controller 11, a third electric delay line 12, a second functionalized micro-fiber 13, a fourth polarization controller 14, a fourth electric delay line 15, a third functionalized micro-fiber 16, a second 1x3 optical fiber coupler 17, a dispersion element 18, a pre-fiber amplifier 19, a second optical fiber attenuator 20, a photodetector 21, an A / D acquisition device 22, and a computer 23.

[0043] The local oscillator femtosecond pulse mode-locked fiber laser 1 generates a repetition frequency f r1 The local oscillator femtosecond pulse passes through the first polarization controller 3, the first electric delay line 4, and the first optical fiber attenuator 5 in sequence, and is output to the 1x2 optical fiber coupler 6.

[0044] Signal femtosecond pulse mode-locked fiber laser 2 generates repetition frequency f r2 The signal femtosecond pulse is divided into three paths by the first 1x3 optical fiber coupler 7.

[0045] The first path passes through the second polarization controller 8, the second electric delay line 9 and the first functionalized micro-fiber 10 in sequence; the second path passes through the third polarization controller 11, the third electric delay line 12 and the second functionalized micro-fiber 13 in sequence; the third path passes through the fourth polarization controller 14, the fourth electric delay line 15 and the third functionalized micro-fiber 16 in sequence; finally, the three paths are combined into one path through the second 1x3 fiber coupler 17 and output to the 1x2 fiber coupler 6.

[0046] Finally, the 1x2 fiber coupler 6 combines the signal path (three paths in this embodiment) and the local oscillator path into one path; the polarization states of the four (n+1) paths of light are adjusted by polarization controllers 3, 8, 11, and 14 so that the four paths of light are in the same polarization state; and the time delays of the four (n+1) paths of light are adjusted by electric delay lines 4, 9, 12, and 15 so that the local oscillator femtosecond pulses can all generate interference fringes with the three signal femtosecond pulses without interfering with each other.

[0047] The output signal of the 1x2 fiber coupler 6 includes four pulses (a local oscillator femtosecond pulse, a signal femtosecond pulse passing through the first functionalized microfiber, a signal femtosecond pulse passing through the second functionalized microfiber, and a signal femtosecond pulse passing through the third functionalized microfiber). The dispersion element 18 performs a dispersion Fourier transform, the optical signal is then amplified by the pre-fiber amplifier 19, and the optical power is adjusted by the second fiber attenuator 20. Finally, the output interference signal is detected by the photodetector 21. The obtained analog electrical signal is converted into a digital signal by the A / D acquisition device 22 and then transmitted to the computer 23 for data processing, thereby realizing gas sensing.

[0048] In this embodiment, the signal femtosecond pulse mode-locked fiber laser and the local oscillator femtosecond pulse mode-locked fiber laser adopt a mode-locked laser based on the nonlinear polarization rotation effect. Compared with other optical frequency combs, they have the advantages of simple structure, low noise, high power, and self-starting, and are suitable for optical sensing systems with high stability and high repeatability requirements. The central wavelength of the two optical frequency combs is 1550nm, the 3dB spectrum width is 20nm, and the local oscillator femtosecond pulse mode-locked laser has a repetition frequency f r1 =64.23936MHz, the signal femtosecond pulse mode-locked laser has a repetition frequency f r2 =21.41312MHz, satisfying f r1 =3*f r2 Based on the repetition frequency and central wavelength of the two lasers, a standard single-mode optical fiber with a length of 50 km was selected as the dispersion element to provide sufficient dispersion broadening. An unbalanced photodetector with a bandwidth of 10 GHz was selected, and the A / D acquisition device used a high-speed oscilloscope with a bandwidth of 16 GHz and a maximum sampling rate of 40 Gs / s.

[0049] In this embodiment, a composite microstructure sensing unit with selective gas detection capability is prepared by combining a gas-sensitive material with specific gas response characteristics with a microfiber structure. The target gases selected are carbon monoxide (CO), ammonia (NH3), and sulfur dioxide (SO2). The microfiber is prepared using a fiber fusion taper system. The tapered region is 1 cm long and the diameter of the thinnest part of the tapered region is 1 micron. This microstructure has a large surface area and light field leakage effect, which is conducive to enhancing the interaction between the gas-sensitive material and the target gas molecules, thereby improving the sensitivity and selectivity of gas sensing.

[0050] For the specific sensing of CO gas, the preparation method of the composite micro-fiber structure is as follows: 17 mg of zinc oxide (ZnO) nanoparticles and 3 mg of indium nitrate hydrate In(NO3)3·H2O are added to 20 mL of deionized water to form a dispersed solution. The prepared solution is ultrasonically treated to enhance the uniformity of particle dispersion. Subsequently, the solution is uniformly deposited on the surface of the pre-prepared micro-fiber tapered area by a droplet method and allowed to air-dry at room temperature. Finally, it is placed in an oven at 60°C and dried for 20 minutes to obtain a stable ZnO-In composite gas-sensitive film layer. The gas sensing mechanism of this structure is based on the oxygen vacancy density in the ZnO lattice and the CO gas adsorption active sites on its surface. The doping effect of In(NO3)3·H2O can be achieved by introducing In 3+ The ions break the equilibrium state of the ZnO lattice and promote the generation of oxygen vacancies, thereby significantly enhancing the adsorption and response ability of the ZnO material to CO molecules and improving the sensitivity and selectivity of the overall sensor to CO gas.

[0051] For specific sensing of NH₃ gas, a composite microfiber structure was prepared as follows: 0.3 g of polyaniline (PANI) nanofibers and 15 mg of camphorsulfonic acid (CSA) were added to 20 mL of chloroform and stirred to dissolve to form a homogeneous solution. Subsequently, the prefabricated tapered region of the microfiber was immersed in the solution for 3 seconds, removed, and allowed to air dry at room temperature. The immersion and drying steps were repeated three times to ensure a stable and uniform composite film of the gas-sensitive material formed on the microfiber surface. Finally, the coated microfiber was dried in a 60°C oven for 30 minutes to obtain a composite microstructure with NH₃-selective response. This sensing mechanism is based on the deprotonation reaction of PANI-turquoise salt upon interaction with NH₃ molecules, transforming it into PANI-turquoise base, which in turn causes a change in the material's electronic structure. This change significantly affects its refractive index and light absorption properties, resulting in a detectable optical signal change during microfiber waveguide transmission, enabling highly sensitive detection of NH₃ gas.

[0052] For specific SO2 gas sensing, a composite microfiber structure was prepared as follows: 60 mg of tin dioxide (SnO2) nanoparticles, 5 mg of reduced graphene oxide (rGO), and 13 mg of nickel oxide (NiO) nanoparticles were added to 20 mL of deionized water and ultrasonicated to ensure uniform dispersion of the three components. This uniformly dispersed solution was then deposited onto the tapered region of a prefabricated microfiber using a droplet method and allowed to air-dry. Finally, the prepared composite microfiber was dried in a 60°C oven for 20 minutes to remove residual solvent, resulting in a stable composite gas-sensing film. The sensing mechanism is based on the NiO nanoparticles reacting with SO2 to induce the dissociation of sulfur bonds, generating SO3 molecules. The SO3 molecules react with the dissociated oxygen in the SnO2, releasing electrons that are transferred through the rGO layer. The photoelectric effect of rGO effectively modulates the refractive index of the microfiber, generating a detectable optical signal change and enabling precise SO2 gas measurement.

[0053] Figure 2 The interference pattern and its local features collected by the gas sensing system in this embodiment can be observed on an oscilloscope.

[0054] The gas sensing system of this embodiment is used to perform sensing tests on CO, NH3 and SO2 in air background. Figure 3 The graph of the frequency of interference fringes of each enhanced peak of the gas sensing system in this embodiment changes with the concentration of different gases. Figure 3 It can be seen that the three functionalized microfibers exhibit extremely high sensitivity (down to sub-ppb level) and excellent selectivity (the response to the target gas is significantly higher than that to other gases) to their respective gases.

[0055] As can be seen from the above embodiments, the present invention combines a femtosecond mode-locked pulse laser with a Mach-Zehnder interferometer, adopts dispersive Fourier transform technology for signal demodulation, and amplifies pulse time delay information through a dispersive element, avoiding the traditional envelope demodulation method that relies on wavelength analysis. The interference fringe information is used to directly obtain the real-time and accurate relative position relationship of the pulses between the reference optical path and multiple sensing optical paths, thereby realizing the synchronous monitoring of changes in the concentrations of multiple gases. The present invention has the advantages of real-time multi-component detection, low cost, high selectivity, and high sensing accuracy, and is particularly suitable for gas sensing needs in extreme environments and complex working conditions.

Claims

1. A Mach-Zehnder interferometer frequency comb gas sensing system based on dispersive Fourier transform, characterized in that: include: Local oscillator femtosecond pulse optical path, signal femtosecond pulse optical path, 1×2 fiber coupler, dispersion element, pre-fiber amplifier, second fiber attenuator, photodetector, A / D acquisition equipment and computer; The local oscillator femtosecond pulse optical path includes: a local oscillator femtosecond pulse mode-locked fiber laser, a first polarization controller, a first electric delay line and a first fiber attenuator connected in sequence; the local oscillator femtosecond pulse mode-locked laser emits a repetition frequency f r1 The local oscillator femtosecond pulse passes through the first polarization controller, the first electric delay line and the first optical fiber attenuator in sequence and is output to the 1x2 optical fiber coupler; the first polarization controller is used to adjust the polarization state of the local oscillator femtosecond pulse, and the first electric delay line is used to adjust the time delay of the local oscillator femtosecond pulse; The signal femtosecond pulse optical path includes: a signal femtosecond pulse mode-locked fiber laser, a first 1×n fiber coupler, n parallel branch sensing optical paths and a second 1×n fiber coupler, which are connected in sequence, where n≥1; Among them, the signal femtosecond pulse mode-locked fiber laser emits a repetition frequency f r2 The signal femtosecond pulse is generated, and each branch sensing optical path includes a polarization controller, an electric delay line and a functionalized micro-fiber connected in sequence, and a functionalized micro-fiber selectively responds to a gas; In each branch sensing optical path: a polarization controller is used to adjust the polarization state of the corresponding branch signal femtosecond pulse so that the n corresponding branch signal femtosecond pulses and the local oscillator femtosecond pulse are in the same polarization state; an electric delay line of the n branch sensing optical path is used to adjust the time delay of the corresponding branch signal femtosecond pulse so that the local oscillator femtosecond pulse and the n signal femtosecond pulse can both generate interference fringes without interfering with each other; The signal femtosecond pulse is divided into n paths through the first 1×n fiber coupler. Each signal femtosecond pulse passes through the corresponding polarization controller, electric delay line and functionalized micro-fiber in the parallel branch sensing optical path. Finally, these n signal femtosecond pulses are combined into one path through the second 1×n fiber coupler and output to the 1x2 fiber coupler. The 1x2 fiber coupler combines the received local oscillator femtosecond pulse and the n-channel signal femtosecond pulse into one channel and outputs it to the dispersion element. The output signal includes n+1 pulses: the local oscillator femtosecond pulse and the n-channel signal femtosecond pulses passing through the n-channel functionalized micro-fibers. The dispersion element performs a dispersion Fourier transform on the input n+1 pulses; the optical signal is then amplified by a pre-fiber amplifier; the optical power is then adjusted by a second fiber attenuator; and finally, the output interference signal is detected by a photodetector. The obtained analog electrical signal is converted into a digital signal by an A / D acquisition device and then transmitted to a computer for data processing to realize gas sensing.

2. The Mach-Zehnder interferometer frequency comb gas sensing system based on dispersive Fourier transform according to claim 1, characterized in that: The f r1 and f r2 In 1~100MHz, f r1 =n*f r2 .

3. The Mach-Zehnder interferometer frequency comb gas sensing system based on dispersive Fourier transform according to claim 1, characterized in that: Said n>1.

4. The Mach-Zehnder interferometer frequency comb gas sensing system based on dispersive Fourier transform according to claim 1, characterized in that: The signal femtosecond pulse mode-locked fiber laser and the local oscillator femtosecond pulse mode-locked fiber laser are the same type of devices.

5. The Mach-Zehnder interferometer frequency comb gas sensing system based on dispersive Fourier transform according to claim 1, characterized in that: The signal femtosecond pulse mode-locked fiber laser and the local oscillator femtosecond pulse mode-locked fiber laser are mode-locked lasers based on a real saturable absorber, mode-locked lasers based on a nonlinear amplifying ring mirror NALM, mode-locked lasers based on a nonlinear polarization rotation effect NPR, or mode-locked lasers based on nonlinear multimode interference NL-MMI.

6. The Mach-Zehnder interferometer frequency comb gas sensing system based on dispersive Fourier transform according to claim 1, characterized in that: The dispersion element adopts a standard single-mode fiber, a dispersion-compensating fiber or a fiber element with a large group velocity dispersion parameter of a chirped Bragg grating, and its |β2| should be greater than 10ps. 2 / km.

7. The Mach-Zehnder interferometer frequency comb gas sensing system based on dispersive Fourier transform according to claim 1, characterized in that: The bandwidth of the photodetector is greater than 1G.

8. The Mach-Zehnder interferometer frequency comb gas sensing system based on dispersive Fourier transform according to claim 1, characterized in that: The bandwidth of the A / D acquisition device is greater than 2G, and the sampling rate is greater than 5Gs / s.

9. The Mach-Zehnder interferometer frequency comb gas sensing system based on dispersive Fourier transform according to claim 1, characterized in that: The specific modulation process of the interference fringes corresponding to the n-branch sensing optical path is: After generating stable local oscillator femtosecond pulses and signal femtosecond pulses, the first electric delay line and the electric delay line of the first branch sensing optical path are adjusted so that the local oscillator femtosecond pulse and the signal femtosecond pulse after passing through the functionalized micro-optical fiber in the first branch sensing optical path generate a stable interference fringe enhancement peak P1 after passing through the dispersion element; then, the electric delay line of the second branch sensing optical path is adjusted so that the local oscillator femtosecond pulse and the signal femtosecond pulse after passing through the functionalized micro-optical fiber in the second branch sensing optical path generate a stable interference fringe enhancement peak P2 after passing through the dispersion element; Next, by sequentially adjusting the electric delay line of the next branch sensing optical path, the local oscillator femtosecond pulse and the signal femtosecond pulse after passing through the functionalized micro-optical fiber in the next branch sensing optical path generate stable interference fringe enhancement peaks after passing through the dispersion element; until the signal femtosecond pulses corresponding to all n branch sensing optical paths and the local oscillator femtosecond pulses generate stable interference fringe enhancement peaks after passing through the dispersion element.

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