Single-channel gas spectral measurement system based on double-optical-comb spectrum and inversion method

Abstract

The invention relates to a single-channel gas spectral measurement system and inversion method based on a double-optical-comb spectrum, and the system comprises a first optical frequency comb, a second optical frequency comb, and a first coupler, an isolator, an optical band-pass filter, a photoelectric detector, an electrical low-pass filter, an AD acquisition card and a computer which are connected in sequence, the output ends of the first optical frequency comb and the second optical frequency comb are respectively connected with the input end of the first coupler; an adopted system structure and a digital correction method are based on a double-optical-comb spectrum technology and a free induction attenuation principle, and error correction of a free operation DCS interferogram and inversion of a gas absorption spectrum can be completed only through data collected by a signal path. The problem of how to reduce high requirements on equipment consistency and digital error correction benchmark consistency between a reference path and a signal path in a free running symmetric DCS (Distributed Control System) is solved, and synchronous, real-time and high-precision measurement of multi-component trace gas can be realized through a simplified system structure and a digital correction algorithm.

Description

Technical Field

[0001] The present invention relates to the technical field of lidar, and particularly relates to a trace gas spectral detection system with a wide spectral range and high spectral resolution and a gas spectral inversion method. Background Art

[0002] The spectral measurement technology of atmospheric trace gas components utilizes the characteristic absorption of trace gas molecules on light radiation, and qualitative and quantitative analysis is carried out through emission spectra to determine the concentration of trace components in the atmosphere. As an emerging active spectral detection technology, the dual-comb spectrometer (DCS) detection technology fundamentally operates from the ultra-high spectral coverage range and ultra-high spectral resolution of the optical frequency comb. By borrowing the characteristics of the optical frequency comb, the spectral response characteristics of the sample can be reproduced in the radio frequency domain through the asynchronous optical sampling of the dual combs, and synchronous, real-time, and high-precision measurement of multi-component trace gases can be achieved without external mechanical scanning.

[0003] In terms of system structure, traditional DCS systems use a pair of optical frequency combs with a small repetition frequency difference as laser light sources. Each comb tooth mode on the light source can be expressed as: f(n) = f ceo + pf r where f ceo is the offset frequency of the optical frequency comb, f r is the comb tooth interval (i.e., the optical pulse repetition frequency), and p is the comb tooth ordinal number. After asynchronous optical sampling and multi-heterodyne coherent detection between the dual combs, the optical frequency signals that cannot be detected by the photodetector can be down-converted to the radio frequency domain, and then the information recorded on the comb teeth in the optical frequency domain is mapped to the radio frequency domain, generating a comb-like radio frequency (RF) spectrum similar to the light source and a time-domain interference pattern corresponding to the RF spectrum. Each radio frequency mode of the radio frequency comb teeth can be expressed as: is the offset frequency of the radio frequency comb, is the radio frequency comb tooth interval (i.e., the small repetition frequency difference between the dual combs).

[0004] Currently, the dual-comb spectroscopy detection technology is mainly based on symmetric systems, which pose high requirements for the equipment consistency of the measurement channel and the reference channel during the detection process. At the same time, the dual-comb spectroscopy technology requires high mutual coherence between the dual combs, that is, a high relative stability needs to be maintained between the offset frequency and the repetition frequency of the dual combs to ensure that the multiple interference pattern signals generated by the beat frequency correspond to the same set of radio frequency combs, thereby improving the phase stability of the interference pattern and the signal-to-noise ratio of the beat frequency comb teeth. However, in actual experiments, factors such as temperature drift, mechanical vibration, quantum fluctuations, and pump current noise will cause the light source parameters f ceo and f r to jitter, which is manifested as and Δfr Jitter. The hardware feedback cavity locking technology used to maintain the high mutual coherence of dual optical frequency combs for a long time locks the dual optical frequency combs to two ultra-stable narrow linewidth lasers, and controls the laser pump source and resonator through a feedback loop to make the linewidth of the comb teeth reach the Hz level, fundamentally ensuring the stability and mutual coherence of the dual optical frequency combs. In the practical application of DCS technology, the complex and costly hardware locking system limits the further field application of DCS systems. Based on this, the proposed free-running system uses a noise reference signal to digitally correct the interference pattern to restore the high mutual coherence of the signal, replacing the high-bandwidth locking system of the cavity locking scheme, and thus can eliminate the ultra-stable narrow linewidth lasers and feedback loops used for cavity locking. This scheme has a simple system structure and low cost, and restores the coherence between the two combs through a digital correction algorithm. However, in a free-running symmetric DCS system without an external reference, the digital correction benchmarks of the reference path and the signal path deviate due to the lack of a unified external reference, which will instead inversely affect the gas inversion accuracy during the spectral inversion process. Summary of the Invention

[0005] Aiming at the problem of how to reduce the high requirements for the equipment consistency and the consistency of digital error correction benchmarks between the reference path and the signal path in a free-running symmetric DCS system to better realize spectral detection and inversion, a single-channel system structure for dual optical frequency comb spectral detection and a corresponding gas spectral inversion method are proposed.

[0006] The technical solution of the present invention is as follows:

[0007] A single-channel gas spectral inversion method based on dual optical frequency comb spectra, comprising the following steps:

[0008] Step 1: Use a free-running dual optical frequency comb with a small repetition frequency difference as the light source. The two beams of light generated by the dual optical frequency comb are connected to an isolator through a first coupler to avoid echo interference with the light source, and the output end of the isolator is connected to an optical bandpass filter to avoid coherent spectral aliasing; the output end of the optical bandpass filter is connected to a gas sample to be measured. Thereafter, the beam carrying the sample spectral information sequentially enters a photodetector and an electrical low-pass filter, the output end of the electrical low-pass filter is connected to the input end of an AD acquisition card, and the output end of the AD acquisition card is connected to a computer with signal acquisition, processing, control, calculation, and display functions;

[0009] Step 2: Restore the mutual coherence of the interference pattern signal through digital correction, specifically:

[0010] Step 2.1: Correct the jitter of the repetition frequency difference on the interference pattern signal collected by the AD acquisition card through a resampling algorithm, that is, interpolate Δf r from a non-uniform phase distribution to a uniform phase distribution. Thereafter, Δf rThe interpolation method for resampling and the interpolation index are applied to the interferogram to obtain an interferogram signal without Δf r jitter;

[0011] Step 2.2: Perform offset frequency jitter correction on the resampled interferogram signal; extract characteristic parameters from the Fourier transform spectrum of each interferogram pulse, and regard the difference between the characteristic parameters as the jitter amount to be corrected.

[0012] Step 2.3: Correct the remaining offset frequency jitter and offset phase jitter between interferograms through the cross ambiguity function and the time series alignment algorithm based on non - linear distortion; after the digital correction algorithms in Steps 2.1 to 2.3, the free - running DCS system basically restores coherence.

[0013] Step 3: Perform inversion of the gas absorption spectrum: Coherently average the corrected interferogram signal to improve the signal - to - noise ratio, and at the same time extract the central pulse part of the averaged time - domain interferogram pulse signal based on the free induction decay principle and the asynchronous optical sampling process as the reference path information for spectral inversion.

[0014] Furthermore, in Step 2.2, the implementation of the offset frequency jitter correction is as follows: where S IGMs (n) and S I ′ GMs (n) are the nth interferogram pulses before and after correction respectively, is the offset frequency jitter amount to be corrected for the nth interferogram pulse, and t(n) is the generation time of the nth apodized interferogram pulse.

[0015] A single - channel gas spectrum measurement system based on dual - optical - comb spectroscopy, which is a single - channel DCS system, includes a first optical frequency comb, a second optical frequency comb, a first coupler, an isolator, an optical band - pass filter, a photodetector, an electrical low - pass filter, an AD acquisition card, and a computer connected in sequence. The output ends of the first optical frequency comb and the second optical frequency comb are respectively connected to the input ends of the first coupler.

[0016] The output beams of the first optical frequency comb and the second optical frequency comb are combined by a first coupler, and the combined beam is output to an isolator to avoid echo interference with the beam. The isolator receives the beam and outputs it to an optical bandpass filter to ensure a one-to-one correspondence between the optical frequency comb teeth and the radio frequency comb teeth. The optical bandpass filter receives the beam and outputs it to the gas sample to be measured. After passing through the gas sample to be measured, the beam carries the spectral information of the gas sample to be measured, and then successively passes through a photodetector and an electrical low-pass filter and is converted into a filtered voltage signal. Then, signal channel data acquisition is completed through an AD acquisition card. After the acquired data is processed by a computer through digital signal correction and spectral inversion algorithms, the absorption spectrum of the gas sample to be measured can be extracted.

[0017] Preferably, the computer is a computer with signal acquisition, processing, control, calculation, and display functions.

[0018] Preferably, both the first optical frequency comb and the second optical frequency comb are 9-word cavity fiber mode-locked lasers operating in the 1550 nm band, in a free-running state without frequency locking; the pulse repetition frequency f r ≈100 MHz; the repetition frequency fluctuation within 1 s measured by a frequency counter is on the order of hundreds of mHz, the spectral linewidth is 300 nm, and the 10 dB linewidths are 150 nm and 100 nm respectively; the repetition frequency difference Δf between the two optical frequency combs r can be adjusted within 0 - 2 kHz.

[0019] Preferably, the first coupler is a 50:50 optical coupler.

[0020] Preferably, the optical bandpass filter is selected according to the aliasing-free spectral range calculated from the optical comb repetition frequency and the repetition frequency difference between the two optical combs: to select.

[0021] Preferably, the center frequency of the optical bandpass filter is 1572 nm, and the filtering bandwidth is 12 nm.

[0022] Preferably, the photodetector is the InGaAs biased detector DET01CFC from Thorlabs, with a bandwidth of 1 GHz.

[0023] Preferably, the passband frequency of the electrical low-pass filter is 0 - 48 MHz, and the 3 dB cut-off frequency is 50 MHz.

[0024] The beneficial effects of the present invention are as follows:

[0025] 1. Only by data processing algorithms can the long-time coherence between interference pattern signals be restored, without the need for external reference equipment for auxiliary correction.

[0026] 2. The system only samples single-channel signals and extracts reference spectral information through the signal path channel, reducing the consistency requirements for equipment and the digital jitter correction reference. Description of the Drawings

[0027] Figure 1 This is the overall structural block diagram of the single-channel gas spectral detection system for dual optical frequency comb spectroscopy provided by the embodiment of the present invention.

[0028] Reference Signs in the Drawings:

[0029] 1. First optical frequency comb; 2. Second optical frequency comb; 3. First coupler; 4. Isolator; 5. Optical band-pass filter; 6. Gas sample to be measured; 7. Photoelectric detector; 8. Electrical low-pass filter; 9. AD acquisition card; 10. Computer. Detailed Embodiment

[0030] The present invention will be described in detail below with reference to the drawings and specific embodiments. This embodiment is implemented on the premise of the technical solution of the present invention, and the detailed implementation manner and specific operation process are given, but the protection scope of the present invention is not limited to the following embodiments.

[0031] The present invention also provides a single-channel gas spectral inversion method for dual optical frequency comb spectroscopy. Using the above single-channel free-running DCS system, the digital correction algorithm and the spectral inversion algorithm only rely on the collected interferogram data of the signal path to complete. The data acquisition and spectral inversion methods are as follows:

[0032] Step 1: Use a free-running dual optical frequency comb with a small repetition frequency difference as the light source. The two beams of light generated by the dual optical frequency comb pass through the first coupler and then are connected to an isolator to avoid echo interference with the light source. The output end of the isolator is connected to an optical band-pass filter to avoid coherent spectral aliasing; the output end of the optical band-pass filter is connected to the gas sample to be measured. Then, the beam carrying the sample spectral information sequentially enters the photoelectric detector and the electrical low-pass filter. The output end of the electrical low-pass filter is connected to the input end of the AD acquisition card, and the output end of the AD acquisition card is connected to a computer with signal acquisition, processing, control, calculation, and display functions;

[0033] Step 2 is the digital correction algorithm, which restores the mutual coherence of the interferogram signal through digital correction. Specifically:

[0034] Step 2.1: Correct the repetition frequency difference jitter on the interferogram signal collected by the AD acquisition card through the resampling algorithm, that is, first interpolate Δf r from the non-uniform phase distribution to the uniform phase distribution, and then apply the interpolation method and interpolation index for resampling of Δf r to the interferogram to obtain the interferogram without Δf rJittered interferogram signal. Implementation principle: Since the jitter of the repetition frequency difference of the interferogram signal is manifested as stretching or compression of the interferogram signal in the time domain, which in turn causes inconsistent intervals between individual interferogram pulses, resampling the interferogram signal is thus performed to make each interferogram have the same pulse period and duration. And the interferogram pulse interval is determined by Δf r Therefore, the process of resampling the interferogram is equivalent to the process of resampling Δf r Since the interferogram pulse interval is equal to the period of a sine signal with Δf r as the frequency, it can be assumed that at the nth interferogram pulse, the phase of the Δf r sine signal is 2nπ, from which the phase information of the Δf r signal varying with time can be obtained. The process of locating the interferogram is completed by the cross-correlation algorithm. The phase of Δf r obtained by location shows a non-uniformly distributed phase axis due to the jitter of Δf r , while ideally the phase distribution of Δf r should be a uniformly distributed phase axis. Based on the above analysis, the process of resampling the interferogram signal is equivalent to the process of resampling Δf r , and the process of resampling Δf r is the process of interpolating Δf r from a non-uniform phase distribution to a uniform phase distribution.

[0035] Step 2.2: Perform offset frequency jitter correction on the resampled interferogram signal. In Step 2, it is considered that the useful signal on the interferogram pulse only exists in the small time window around the pulse, and this time window is the apodized interferogram pulse. It is regarded that each apodized interferogram pulse is stable inside, that is, the jitter only exists between interferogram pulses. By extracting the characteristic parameters from the Fourier transform spectrum of each interferogram pulse and regarding the difference between the characteristic parameters as the jitter amount to be corrected, it is specifically manifested as: where S IGMs (n) and S I ′ GMs (n) are the nth interference pulses before and after correction respectively, is the offset frequency jitter amount to be corrected for the nth interference pulse, and t(n) is the generation time of the nth apodized interferogram pulse. Since only the jitter within the apodization window is considered in this step, there are a small amount of remaining offset frequency jitter and offset phase jitter caused by insufficient resolution of the Fourier transform spectrum within the apodization window.

[0036] Step 2.3: Correct the residual offset frequency jitter and offset phase jitter between interferograms through the cross ambiguity function and the time series alignment algorithm based on non - linear distortion. After the digital correction algorithms in Steps 2.1 to 2.3, the free - running DCS system basically restores coherence.

[0037] Step 3: Perform the inversion of the gas absorption spectrum: Coherently average the corrected interferogram signal to improve the signal - to - noise ratio. At the same time, extract the central pulse part of the averaged time - domain interferogram pulse signal as the reference path information for spectral inversion. Based on the principle of free induction decay and the asynchronous optical sampling process, extract the central pulse region signal of the signal path. The principle of free induction decay and the asynchronous optical sampling process extract the main gas absorption information on the interferogram pulse. The apodized interferogram signal contains the pulse and the small signal region beside it. The gas absorption information is mainly contained in the weak signal region far from the central pulse in the apodized interference pulse, and the central pulse region only contains background spectral information. Therefore, extracting the central pulse region signal of the signal path is equivalent to the reference path signal, and the synchronization of the signal spectrum and the reference spectrum can be ensured.

[0038] The free - running dual - comb spectroscopy system provided by the present invention only collects the data of the signal path and restores the long - time coherence of the collected signal through digital correction algorithms. Extract the reference path spectral information for spectral inversion from the signal path data. This set of system and data - processing scheme effectively reduces the high requirements for the equipment consistency between the reference path and the signal path and the consistency of the digital error correction benchmark in the free - running symmetric DCS system, and at the same time can simplify the system structure.

[0039] Figure 1 It is the overall structural block diagram of the single - channel gas spectral inversion method for dual - comb spectroscopy detection provided in this embodiment. The single - channel DCS system includes a first optical frequency comb 1, a second optical frequency comb 2, a first coupler 3, an isolator 4, an optical band - pass filter 5, a gas sample to be measured 6, a photodetector 7, an electrical low - pass filter 8, an AD acquisition card 9, and a computer 10, which are connected in sequence. The output ends of the first optical frequency comb 1 and the second optical frequency comb 2 are respectively connected to the input ends of the first coupler 3;

[0040] The output beams of the first optical frequency comb 1 and the second optical frequency comb 2 are combined by the first coupler 3, and the combined beam is output to the isolator 4 to avoid echo interference with the beam. The isolator 4 receives the beam and outputs it to the optical bandpass filter 5 to ensure a one-to-one correspondence between the optical frequency comb teeth and the radio frequency comb teeth. The optical bandpass filter 5 receives the beam and outputs it to the gas sample to be measured 6. After passing through the gas sample to be measured 6, the beam carries the spectral information of the gas sample to be measured 6 and then passes through the photodetector 7 and the electrical low-pass filter 8 in sequence and is converted into a filtered voltage signal. Then, the signal path data acquisition is completed through the AD acquisition card 9. After the acquired data is processed by the computer 10 through digital signal correction and spectral inversion algorithms, the absorption spectrum of the gas sample to be measured 6 can be extracted.

[0041] In terms of system structure, only one channel is used to receive the gas spectral absorption information, and the reference path spectral information for spectral inversion is extracted from the signal path interferogram. The system only requires one detection device. This system structure avoids the high requirements for device consistency and at the same time ensures the synchronization of the signal spectrum and the reference spectrum;

[0042] The system uses a free-running dual optical frequency comb with a small repetition frequency difference as the light source. The two beams generated by the dual optical frequency combs are connected to the isolator after passing through the first coupler to avoid echo interference with the light source. The output end 5 of the isolator is connected to the optical bandpass filter to avoid coherent spectral aliasing;

[0043] The output end of the optical bandpass filter is connected to the gas sample to be measured. Thereafter, the beam carrying the sample spectral information enters the photodetector and the electrical low-pass filter. The output end of the electrical low-pass filter is connected to the input end of the AD acquisition card, and the output end of the AD acquisition card is connected to a computer with signal acquisition, processing, control, calculation, and display functions;

[0044] The specific devices used in this embodiment are: both the first optical frequency comb 1 and the second optical frequency comb 2 are 9-word cavity fiber mode-locked lasers operating in the 1550 nm band, in a free-running state without frequency locking; the pulse repetition frequency f r ≈100 MHz; using a frequency counter, the repetition frequency fluctuation within 1 s is measured to be on the order of hundreds of mHz, the spectral linewidth is 300 nm, and the 10 dB linewidths are 150 nm and 100 nm respectively. The repetition frequency difference Δf between the dual optical frequency combs r can be adjusted within 0 - 2 kHz, and the experimental actual setting is around 785 Hz. The first coupler 3 is a 50:50 optical coupler. The optical bandpass filter 5 should be based on the non-aliasing spectral range calculated from the optical comb repetition frequency and the repetition frequency difference between the dual optical combs: It is selected that the center frequency is 1572 nm and the filtering bandwidth is 12 nm in this embodiment. The photodetector 7 is the InGaAs biased detector DET01CFC of Thorlabs, with a bandwidth of 1 GHz; the passband frequency of the electrical low-pass filter 8 is 0 to 48 MHz, and the 3 dB cut-off frequency is 50 MHz.

[0045] The specific process of measuring the CO2 gas concentration using the single-channel dual optical frequency comb spectroscopy system and the gas spectroscopy inversion method in the present invention is as follows:

[0046] Step 1: Pulse light sources with an output power of 40 mW are output from the first optical frequency comb 1 and the second optical frequency comb 2. After being combined by the 50:50 first coupler 3, the first coupler 3 outputs the combined light beam to the isolator 4. The isolator 4 receives the light beam and outputs it to the 12 nm bandwidth optical band-pass filter 5 to prevent aliasing. The optical filter receives the light beam and outputs it to the CO2 gas cell as the sample to be measured. After passing through the sample, the light beam enters the photodetector 7. The photodetector 7 receives the light beam and converts the optical signal into a voltage signal. Then, the photodetector 7 outputs the voltage signal to the 50 MHz electrical low-pass filter 8. The electrical low-pass filter 8 filters the voltage signal and outputs the voltage signal, that is, the interferogram signal, to the AD acquisition card 9. The AD acquisition card 9 receives the filtered voltage signal, acquires it as a digital signal, and then outputs it to the computer. The computer receives the digital signal, performs digital correction and spectral inversion algorithm processing, and then extracts the absorption spectrum of the CO2 gas cell.

[0047] Step 2: After the electrical signal enters the computer, digital calculation correction is performed with reference to Steps 2.1 - 2.3. The resampling algorithm is used for digital correction of the digital signal, and then the repetition frequency difference jitter between each interference pulse signal in the digital signal is acquired. The position of each interference pulse signal is located through the cross-correlation algorithm to estimate the phase information of Δf r and a uniform linear resampling process of the Δf r phase is used to replace the interferogram pulse resampling process; then, offset frequency jitter correction is performed on the completed resampling and interferogram signals, that is, characteristic parameters are extracted from the Fourier transform spectrum of each interferogram pulse, and the difference between the characteristic parameters is regarded as the jitter amount to be corrected to perform correction compensation. Specifically, it is manifested as: After that, the residual offset frequency jitter and offset phase jitter between interferograms are corrected through the cross-ambiguity function and the time series alignment algorithm based on machine learning and nonlinear distortion. After the digital correction algorithm of Steps 2.1 - 2.3, the free-running DCS system basically restores coherence.

[0048] Step 3: Perform the inversion of the gas absorption spectrum. Specifically, it is executed as follows: Coherently average the corrected interferogram signal to improve the signal-to-noise ratio, and at the same time extract the central pulse part of the averaged time-domain pulse signal as the reference path information for spectral inversion. Based on the principle of free induction decay and the asynchronous optical sampling process, the gas absorption information is mainly contained in the weak signal region far from the central pulse in the apodized interference pulse, and the central pulse region only contains the background spectral information. Therefore, by extracting the signal in the central pulse region of the signal path, it is equivalent to the reference path signal, and the synchronization of the signal spectrum and the reference spectrum can be ensured.

[0049] The present invention provides a single-channel gas spectral inversion method for dual-comb spectroscopy. The adopted system structure and digital correction method are based on dual-comb spectroscopy technology and the principle of free induction decay. The error correction of the freely running DCS interferogram and the inversion of the gas absorption spectrum can be completed only through the data collected by the signal path. The system includes a dual-comb light source, an optical coupler, an isolator, an optical band-pass filter, a gas sample to be measured, a photodetector, an electrical low-pass filter, an AD acquisition card, and a computer connected in sequence. Through the simplified system structure and digital correction algorithm, the synchronous, real-time, and high-precision measurement of multi-component trace gases can be realized.

[0050] The basic principle of the present invention is to use two freely running optical frequency combs with a small repetition frequency difference as light sources. When the pulse sequences output by the two overlap in space, the pulse pairs overlap with each other and the time interval between every two consecutive overlapping pulse pairs increases pair by pair. Signals with different light intensities are generated by interference on the detector, so that the detector outputs a fluctuating voltage signal, which is the interferogram signal; in the frequency domain, the tooth intervals of the two optical combs also cause the frequency intervals between the tooth pairs to increase step by step due to this small repetition frequency difference. When they overlap, parallel multi-heterodyne beat frequency down-conversion occurs between the modes, mapping the information recorded on the teeth in the optical frequency domain to the radio frequency domain, generating a comb-shaped radio frequency spectrum. The radio frequency spectrum and the interferogram signal form a pair of Fourier transform pairs. The collected interferogram signal of the freely running DCS system can be regarded as an accurate interferogram signal without jitter after passing through the digital jitter correction algorithm in steps 2.1 - 2.3. Once an accurate interferogram is obtained, perform a Fourier transform on it to obtain a comb-shaped radio frequency spectrum, and then multiply it by the conversion factor to restore the optical frequency domain spectrum of the signal path that has passed through gas absorption. Thereafter, select the central pulse part of the signal path interferogram as the reference path signal, and after Fourier transform, obtain the optical frequency domain spectrum of the reference path for spectral inversion. After comparing the two signal spectra, the gas absorption information in the optical frequency domain can be restored.

[0051] The above-described embodiments merely represent one implementation mode of the present invention. The description is relatively specific and detailed, but it should not be construed as a limitation on the scope of the invention patent. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all fall within the protection scope of the present invention. Therefore, the protection scope of the present invention patent shall be subject to the appended claims.

Claims

1. A single-channel gas spectrum inversion method based on dual-comb spectroscopy, characterized in that It includes the following steps: Step 1: Use a free-running dual optical frequency comb with a small repetition frequency difference as the light source. The two beams of light generated by the dual optical frequency combs are connected to an isolator through a first coupler to avoid echo interference with the light source, and the output end of the isolator is connected to an optical bandpass filter to avoid coherent spectral aliasing; The output end of the optical bandpass filter is connected to a measurement gas sample. After that, the beam carrying the sample spectral information sequentially enters a photodetector and an electrical low-pass filter. The output end of the electrical low-pass filter is connected to the input end of an AD acquisition card, and the output end of the AD acquisition card is connected to a computer with signal acquisition, processing, control, calculation, and display functions; Step 2: Restore the mutual coherence of the interferogram signal through digital correction. Specifically: Step 2.1: Correct the jitter of the repetition frequency difference on the interferogram signal collected by the AD acquisition card through a resampling algorithm, that is, interpolate Δf r from the non-uniform phase distribution into a uniform phase distribution, and apply the interpolation method and interpolation index for resampling to the interferogram to obtain an interferogram signal without Δf r jitter; r ​ Step 2.2: Perform offset frequency jitter correction on the resampled interferogram signal; extract characteristic parameters from the Fourier transform spectrum of each interferogram pulse, and regard the difference between the characteristic parameters as the jitter amount to be corrected. Step 2.3: Correct the residual offset frequency jitter and offset phase jitter between interferograms through the cross ambiguity function and the time series alignment algorithm based on nonlinear distortion; after the digital correction algorithm in steps 2.1 to 2.3, the free-running DCS system basically restores coherence; Step 3: Invert the gas absorption spectrum: Coherently average the corrected interferogram signal to improve the signal-to-noise ratio, and at the same time extract the central pulse part of the averaged time-domain interferogram pulse signal based on the principle of free induction decay and the asynchronous optical sampling process as the reference path information for spectral inversion.

2. The single-channel gas spectrum inversion method based on dual-comb spectroscopy according to claim 1, characterized in that In Step 2.2, the implementation of the offset frequency jitter correction is as follows: where S IGMs (n) and S I ′ GMs (n) are the nth interference pulses before and after correction respectively, is the offset frequency jitter amount to be corrected for the nth interference pulse, and t(n) is the generation time of the nth apodized interferogram pulse.

3. A single-channel gas spectrum measurement system based on dual-comb spectroscopy, characterized in that, It is a single-channel DCS system, including a first optical frequency comb, a second optical frequency comb, a first coupler, an isolator, an optical bandpass filter, a photodetector, an electrical low-pass filter, an AD acquisition card, and a computer connected in sequence. The output ends of the first optical frequency comb and the second optical frequency comb are respectively connected to the input end of the first coupler; The output beams of the first optical frequency comb and the second optical frequency comb are combined by the first coupler, and the combined beam is output to the isolator to avoid echo interference with the beam. The isolator receives the beam and outputs it to the optical bandpass filter to ensure one-to-one correspondence between the optical frequency comb teeth and the radio frequency comb teeth. The optical bandpass filter receives the beam and outputs it to the gas sample to be measured. After passing through the gas sample to be measured, the beam carries the spectral information of the gas sample to be measured and then sequentially passes through the photodetector and the electrical low-pass filter and is converted into a filtered voltage signal. Then, the signal path data acquisition is completed through the AD acquisition card. After the acquired data is processed by the computer through digital signal correction and spectral inversion algorithms, the absorption spectrum of the gas sample to be measured can be extracted.

4. The single-channel gas spectrum inversion method based on dual-comb spectroscopy according to claim 3, wherein The computer is a computer with signal acquisition, processing, control, calculation, and display functions.

5. The single-channel gas spectrum inversion method based on dual-comb spectroscopy according to claim 3, wherein Both the first optical frequency comb and the second optical frequency comb are 9 - cavity fiber mode - locked lasers operating in the 1550 nm band, in a free - running state without frequency locking; the pulse repetition frequency f r ≈100 MHz; The repetition frequency fluctuation within 1 s measured by the usage frequency counter is on the order of hundreds of mHz, the spectral linewidth is 300 nm, and the 10 dB linewidths are 150 nm and 100 nm respectively; the repetition frequency difference Δf between the two optical frequency combs r can be adjusted within 0 - 2 kHz.

6. The single-channel gas spectrum inversion method based on dual-comb spectroscopy according to claim 3, wherein The first coupler is a 50:50 optical coupler.

7. The single-channel gas spectrum inversion method based on dual-comb spectroscopy according to claim 3, wherein The optical bandpass filter is selected according to the aliasing-free spectral range calculated from the optical comb repetition frequency and the repetition frequency difference between the two optical combs: and is selected accordingly.

8. The single-channel gas spectrum inversion method based on dual-comb spectroscopy according to claim 3, wherein The central frequency of the optical bandpass filter is 1572 nm, and the filtering bandwidth is 12 nm.

9. The single-channel gas spectrum inversion method based on dual-comb spectroscopy according to claim 3, wherein, The photodetector is the InGaAs biased detector DET01CFC from Thorlabs, with a bandwidth of 1 GHz.

10. The single-channel gas spectrum inversion method based on dual-comb spectroscopy according to claim 3, wherein, The passband frequency of the electrical low-pass filter is 0 - 48 MHz, and the 3 dB cut-off frequency is 50 MHz.

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