CN108931498A - Multi-pass pool gas absorption spectrum and the device and method for absorbing light path synchro measure - Google Patents

Abstract

A device and method for synchronous measurement of gas absorption spectrum and absorption optical path in a multi-pass cell relates to the simultaneous measurement technology of gas absorption spectrum and absorption optical path in a multi-pass cell, in order to solve the optical path and gas The absorption spectrum is measured separately and the measurement accuracy of the optical path is not high. The laser output from the tunable light source is respectively incident on the auxiliary interferometer and the main interferometer; the auxiliary interferometer is used to generate a clock signal and send it to the clock terminal of the data acquisition card; the optical path of the main interferometer includes intrinsic light and test light, After the test light is transmitted in the multi-pass cell, the beat frequency coherence occurs with the intrinsic light; the data acquisition card is used to collect the beat frequency coherent signal generated by the main interferometer, and sends it to the calculation module; Calculate the absorption pathlength and absorption spectrum. The invention is suitable for synchronous measurement of absorption spectrum and absorption light path.

Description

Device and method for synchronous measurement of gas absorption spectrum and absorption optical path in multi-pass cell

technical field

The invention relates to a synchronous measurement technology of gas absorption spectrum and absorption light path in a multi-pass cell, belonging to the field of laser absorption spectrum and gas sensing.

Background technique

In laser absorption spectroscopy, multi-pass cells are widely used to extend the absorption path and improve measurement sensitivity, among which the most commonly used multi-pass cells are designed based on White type and Heriott type. In order to meet different actual measurement requirements, different types of multi-pass cells have been developed, such as optical matrix gas cells and cylindrical mirror combination gas cells. To achieve highly compact devices, design types such as chaotic optical multi-pass cavities and circular multi-reflecting gas cells have emerged.

For quantitative gas detection, the absorption spectrum and the pathlength of the multipass cell must be precisely determined. In existing detection, gas absorption spectrum and absorption optical path are usually measured independently. Absorption pathlengths are calibrated by mechanical measurements, gas absorption spectroscopy, phase shifting, or interferometric techniques prior to acquiring gas absorption spectra. However, due to structural deformation, especially in long-term measurements in harsh environments, the pre-calibrated optical path of the multipass cell may change, resulting in a decrease in measurement accuracy. On the other hand, in applications that require different optical depths, although the optical path of the multipass cell can be changed by adjusting the mirror spacing or laser alignment to adjust the number of reflections, the process is complicated and time-consuming.

Contents of the invention

The purpose of the present invention is to solve the problem that the optical path and gas absorption spectrum of the multi-pass cell are measured separately in the existing quantitative gas detection and the measurement accuracy of the optical path is not high, so as to provide synchronization of the gas absorption spectrum and the absorption optical path in the multi-pass cell Measuring devices and methods.

The device for synchronous measurement of gas absorption spectrum and absorption optical path in a multi-pass cell according to the present invention includes: a tunable light source 1, an auxiliary interferometer, a main interferometer, a data acquisition card 10 and a calculation module 11;

The laser output from the tunable light source 1 is respectively incident on the auxiliary interferometer and the main interferometer;

Auxiliary interferometer, for generating clock signal, and send to the clock terminal of data acquisition card 10;

The optical path of the main interferometer includes intrinsic light and test light, and the test light is coherent with the intrinsic light after being transmitted in the multipass cell;

The data acquisition card 10 is used to collect the beat-frequency coherent signal generated by the main interferometer and send it to the calculation module;

The calculating module 11 is used for calculating the absorption optical path and the absorption spectrum according to the beat frequency coherent signal.

Preferably, the laser light output by the tunable light source 1 is divided into two paths of light by the first coupler 2-1, one path of light is incident on the auxiliary interferometer, and the other path of light is incident on the main interferometer.

Preferably, the auxiliary interferometer includes a third coupler 2-3, a first Faraday rotating mirror 5-1, a second Faraday rotating mirror 5-2 and an optical fiber 6;

The third coupler 2-3 is a 2×2 coupler with a splitting ratio of 50:50;

One path of light output by the first coupler 2-1 is divided into two paths by the third coupler 2-3, one path of light is coupled into the third coupler 2-3 after being reflected by the first Faraday rotating mirror 5-1, and the other path of light passes through The optical fiber 6 is incident to the second Faraday rotating mirror 5-2, and then coupled into the third coupler 2-3 after being reflected by the second Faraday rotating mirror 5-2, and the third coupler 2-3 outputs a beat frequency coherent signal, and then A square wave signal is generated by the TTL clock generator 9, and the square wave signal is a clock signal.

Preferably, the main interferometer includes a second coupler 2-2, a fourth coupler 2-4, a polarization controller 3, a fiber circulator 4 and a balance detector 8;

The second coupler 2-2 is a 1×2 coupler, and the fourth coupler 2-4 is a 2×2 coupler with a splitting ratio of 50:50;

Another path of light output by the first coupler 2-1 is divided into two paths of light by the second coupler 2-2, and one path of light is incident to an input port of the fourth coupler 2-4 after passing through the polarization controller 3 as intrinsic light , another path of light enters from port 1 of the fiber optic circulator 4 as test light, exits from port 2 of the fiber optic circulator 4 to the multi-pass cell 7, and in the multi-pass cell 7, the reflected light re-enters the optical fiber after multi-stage reflection and gas absorption The 2-port of the circulator 4 is coupled into another input port of the fourth coupler 2-4 from the 3-port, and the intrinsic light and the test light undergo beat frequency coherence and are output from the 2 output ports of the fourth coupler 2-4 To the balance detector 8 , the output end of the balance detector 8 is connected to the acquisition signal input end of the data acquisition card 10 , and the output end of the data acquisition card 10 is connected to the input end of the calculation module 11 .

Preferably, the calculation module 11 calculates the absorption optical path and absorption spectrum according to the beat frequency coherent signal, specifically:

The calculation module 11 performs Fourier transform on the received beat-frequency coherent signal, realizes the conversion from the frequency domain spectrum to the time domain spectrum, and obtains the absorption optical path according to the optical path difference between reflection peaks;

Perform inverse Fourier transform on the information data segment of interest in the time domain spectrum, and then square the amplitude data to obtain the absorption spectrum intensity information of the gas, and then perform normalization processing to obtain a normalized absorption spectrum.

The method for synchronous measurement of gas absorption spectrum and absorption optical path in a multi-pass cell according to the present invention, the method comprises:

Step 1, filling a certain concentration of the gas to be tested into the multi-pass cell 7;

Step 2: Turn on the linear sweep output of the tunable light source 1, and at the same time, the tunable light source 1 outputs a trigger signal to trigger the data acquisition card 10 to prepare for data acquisition;

Step 3, the auxiliary interferometer generates a clock signal, which is transmitted to the data acquisition card 10, so that the data acquisition card 10 collects the beat frequency signal of the main interferometer at equal frequency intervals;

The test light of the main interferometer is introduced into the multi-pass cell 7 by the 2 ports of the optical fiber circulator 4, and the reflected light re-enters the optical fiber circulator 4 after multi-stage reflection and gas absorption in the multi-pass cell 7; adjust the polarization controller 3 so that The light derived from port 3 of the optical fiber circulator 4 is coherent with the local oscillator light, and is then transmitted to the data acquisition card 10 by the balance detector 8;

Step 4, the data acquisition card 10 collects the beat frequency signal, and the calculation module 11 saves the beat frequency signal;

Step 5, the calculation module 11 performs Fourier transform on the beat-frequency coherent signal, realizes the conversion from the frequency domain spectrum to the time domain spectrum, and obtains the absorption optical path according to the optical path difference between reflection peaks;

Step 6, select the information data segment of interest in the time-domain spectrum obtained in step 5, and carry out inverse Fourier transform to realize the transformation from the time-domain spectrum to the frequency-domain spectrum;

Step 7, performing square processing on the amplitude data in the frequency domain spectrum to obtain the absorption spectrum intensity information of the gas;

Step 8: Perform normalization processing on the obtained gas absorption spectrum intensity information to obtain a normalized absorption spectrum.

Preferably, in step five, the calculation module 11 performs Fourier transform on multiple sets of beat-frequency coherent signals to obtain multiple sets of data, and obtains the absorption optical path according to the optical path difference between reflection peaks after averaging the multiple sets of data.

Preferably, step five is repeated to obtain multiple sets of time-domain spectra, and step six is performed after averaging the multiple sets of time-domain spectra.

In the existing gas sensing, the absorption spectrum and the absorption optical path need to be measured separately and are prone to large errors. The device and method for synchronous measurement of gas absorption spectrum and absorption optical path in a multi-pass cell of the present invention can realize high-precision measurement of absorption optical path and obtain absorption spectrum at the same time. The invention obtains absorption spectra under various optical paths without changing the physical state of the multi-pass cell, and realizes flexible expansion of the dynamic range of gas sensing measurement.

Description of drawings

Fig. 1 is a structural schematic diagram of a device for synchronous measurement of gas absorption spectrum and absorption optical path in the multi-pass cell described in Embodiment 1;

Fig. 2 is the flowchart of the method for synchronous measurement of gas absorption spectrum and absorption optical path in the multi-pass cell in the second embodiment;

Figure 3 is a graph of the collected beat-frequency coherent signal;

Fig. 4 is the time-domain spectrogram obtained by performing Fourier transform on 100 groups of beat-frequency coherent signals;

Cir corresponds to port 2 of the fiber optic circulator, Con corresponds to the position of the flange between the inlet sealing window I and the fiber optic circulator, I ₁ and I ₂ correspond to the front and rear surfaces of the inlet sealing window, and O corresponds to the outlet sealing window place;

Fig. 5 is the absorption spectrum intensity information figure that selects the signal (i.e. in the dotted line box) at the outlet sealing window O place to obtain;

Fig. 6 is the comparison chart of the normalized absorption spectrum corresponding to Fig. 5 and the simulated spectrum of the HITRAN database;

Figure 7 is the gas absorption spectrum of 1600ppm acetylene at 4 different reflection positions in the multi-pass cell. The corresponding optical paths are 4.8196m, 8.0342m, 16.0713m and 22.4972m respectively, and the lower two graphs are strong absorption lines and a zoom-in view of a relatively weak absorption line.

Detailed ways

Specific Embodiment 1: This embodiment is described in detail in conjunction with FIG. 1 . The device for synchronous measurement of gas absorption spectrum and absorption optical path in a multi-pass cell described in this embodiment includes: a tunable light source 1, an auxiliary interferometer, a main interferometer instrument, data acquisition card 10 and computing module 11;

The laser output from the tunable light source 1 is respectively incident on the auxiliary interferometer and the main interferometer;

Auxiliary interferometer, for generating clock signal, and send to the clock terminal of data acquisition card 10;

The optical path of the main interferometer includes intrinsic light and test light, and the test light is coherent with the intrinsic light after being transmitted in the multipass cell;

The data acquisition card 10 is used to collect the beat-frequency coherent signal generated by the main interferometer and send it to the calculation module;

The calculating module 11 is used for calculating the absorption optical path and the absorption spectrum according to the beat frequency coherent signal.

The linearly tunable narrow-linewidth laser output by the tunable light source 1 is divided into two paths of light by the first coupler 2-1, 1% of the light is incident on the auxiliary interferometer, and 99% of the light is incident on the main interferometer.

The auxiliary interferometer adopts a Michelson interferometer structure, including a third coupler 2-3, a first Faraday rotating mirror 5-1, a second Faraday rotating mirror 5-2 and an optical fiber 6;

The third coupler 2-3 is a 2×2 coupler with a splitting ratio of 50:50;

One path of light output by the first coupler 2-1 is divided into two paths by the third coupler 2-3, one path of light is coupled into the third coupler 2-3 after being reflected by the first Faraday rotating mirror 5-1, and the other path of light passes through The optical fiber 6 is incident to the second Faraday rotating mirror 5-2, and then coupled into the third coupler 2-3 after being reflected by the second Faraday rotating mirror 5-2, and the third coupler 2-3 outputs a beat frequency coherent signal, and then A square wave signal is generated by the TTL clock generator 9, and the square wave signal is a clock signal.

The optical fiber 6 is used for time delay, and the length is 10m-3000m.

The main interferometer includes a second coupler 2-2, a fourth coupler 2-4, a polarization controller 3, a fiber circulator 4 and a balance detector 8;

The second coupler 2-2 is a 1×2 coupler, and the fourth coupler 2-4 is a 2×2 coupler with a splitting ratio of 50:50;

The other path of light output by the first coupler 2-1 is divided into two paths of light by the second coupler 2-2 of 1:99, and 1% of the path of light is used as the intrinsic light, and the polarization state of the intrinsic light passes through the polarization of the polarization controller 3 After adjustment, it is incident on an input port of the fourth coupler 2-4, and 99% of the one-way light enters through port 1 of the optical fiber circulator 4 as test light, and exits from port 2 of the optical fiber circulator 4 to the multi-pass pool 7, where the multi-pass In the pool 7, the reflected light re-enters the 2nd port of the optical fiber circulator 4 through the multi-stage reflection of the concave mirror 1 7-1, the concave mirror 2 7-2, the concave mirror 3 7-3 and the absorption of the gas, and is coupled into the 2nd port from the 3rd port. Another input port of the four-coupler 2-4, the intrinsic light and the test light have beat frequency coherence, and the beat frequency coherent signal is detected by the balance detector 8 and converted into a voltage signal, which is input to the data acquisition card 10, and the data acquisition card 10 Convert the analog voltage signal into a digital voltage signal. The role of the auxiliary interferometer is to enable the data acquisition card 10 to collect the beat frequency coherent signal detected by the balance detector 8 .

The multi-pass cell 7 is filled with the gas to be tested, and the test light enters the multi-pass cell 7. Since there is a reflection system composed of three concave mirrors inside the multi-pass cell, the test light undergoes multi-stage reflection inside the multi-pass cell, and the absorption light of the test light greatly increased. The reflected optical signal contains the absorption intensity information of the gas. Through the mathematical method of signal processing, the absorption optical path of the gas can be obtained after Fourier transform, and the absorption spectrum of the gas can be obtained after the inverse Fourier transform. The optical path and absorption spectrum information are used to demodulate the gas concentration.

In continuous wave frequency modulation technology, the reflected signal essentially contains the absorption information of the gas. The invention relates to the data processing process of two Fourier transforms. In the first Fourier transform, the beat signal recorded in the frequency domain (spectral domain) is converted into time domain (spatial domain) information as in conventional CW FM. In the space domain, the reflection intensity of each concave mirror inside the multi-pass cell distributed along the length can be clearly seen, and the multi-pass can be determined by the optical path difference of the strong reflection peak corresponding to the sealing window at the entrance and exit of the multi-pass cell The optical path of the pool. In the measurement range of tens of meters, the spatial resolution of the measurement can reach the order of submillimeters. In the second inverse Fourier transform, a reflection signal of interest in the spatial domain is selected to inverse Fourier transform it, and then converted back to the spectral domain, so as to obtain the target gas under the absorption path corresponding to the reflection signal For example, if the reflection signal at the sealing window at the outlet of the multi-pass cell is selected, the absorption spectrum corresponding to the maximum absorption optical path can be obtained. Since the reflected signal is measured, the absorption path length of the gas is twice the transmission path length of the multipass cell.

In the main interferometer, the light intensity of the test light is modulated by the spectral absorption information in the multi-pass cell, and beat frequency interference occurs with the local oscillator light, and the obtained beat frequency signal is finally collected by the data acquisition card 10, and its intensity is determined by the test Determined by the vector product of the light amplitude and the intrinsic light amplitude and the response sensitivity of the balance detector 8, since there is a square relationship between the light intensity and the amplitude, the absorption information is the light intensity information, in order to obtain the absorption intensity information of the gas to the test light , after two Fourier transform processes, square the amplitude data of the frequency domain spectrum obtained from the inversion, so as to obtain the intensity information of the test light. In order to improve the signal-to-noise ratio, after the first Fourier transform of the multiple sets of beat-frequency coherent signals collected, the average processing is performed, and then the average result is inverse Fourier transform, and the obtained inversion result is squared .

According to the absorption band of the measured gas, the linear tuning range of the tunable light source 1 can be selected. For different gases, different tuning ranges of the light source can be set to adapt to different ranges of gas absorption spectra. The linear tuning rate and acquisition card parameters can be adjusted. The output power of the light source is on the order of milliwatts and can be set according to specific needs.

Specific embodiment 2: This embodiment is described in detail in conjunction with Fig. 2 to Fig. 7, based on the gas absorption spectrum and absorption optical path synchronous measurement of the device for synchronous measurement of gas absorption spectrum and absorption optical path in the multi-pass cell described in specific embodiment 1 method, which includes:

Step 1, filling a certain concentration of the gas to be tested into the multi-pass cell 7;

Step 2: Turn on the linear sweep output of the tunable light source 1, and at the same time, the tunable light source 1 outputs a trigger signal to trigger the data acquisition card 10 to prepare for data acquisition;

Step 3, the auxiliary interferometer generates a clock signal, which is transmitted to the data acquisition card 10, so that the data acquisition card 10 collects the beat frequency signal of the main interferometer at equal frequency intervals;

The test light of the main interferometer is introduced into the multi-pass cell 7 by the 2 ports of the optical fiber circulator 4, and the reflected light re-enters the optical fiber circulator 4 after multi-stage reflection and gas absorption in the multi-pass cell 7; adjust the polarization controller 3 so that The light derived from port 3 of the optical fiber circulator 4 is coherent with the local oscillator light, and is then detected by the balance detector 8 and transmitted to the data acquisition card 10;

Step 4, the data acquisition card 10 collects the beat frequency signal, and the calculation module 11 saves the beat frequency signal;

Step 5, the calculation module 11 performs Fourier transform on the beat-frequency coherent signal, realizes the conversion from the frequency domain spectrum to the time domain spectrum, and obtains the absorption optical path according to the optical path difference between reflection peaks;

Step 6, select the information data segment of interest in the time-domain spectrum obtained in step 5, and carry out inverse Fourier transform to realize the transformation from the time-domain spectrum to the frequency-domain spectrum;

Step 7, performing square processing on the amplitude data in the frequency domain spectrum to obtain the absorption spectrum intensity information of the gas;

Step 8: Perform normalization processing on the obtained gas absorption spectrum intensity information to obtain a normalized absorption spectrum.

In Step 5, the calculation module 11 performs Fourier transform on multiple sets of beat-frequency coherent signals to obtain multiple sets of data, and obtains the absorption optical path according to the optical path difference between reflection peaks after averaging the multiple sets of data.

Repeat step five times to obtain multiple sets of time-domain spectra, and perform step six after averaging multiple sets of time-domain spectra.

In this embodiment, the White type multipass cell is taken as an example. Referring to FIG. 1 , there are three concave mirrors inside, and the light is reflected and propagated multiple times inside to form a total absorption optical path. FIG. 2 is a flow chart of the method of this embodiment. Figure 3 shows the original beat-frequency coherent signal collected by filling the multipass cell with 100ppm acetylene under standard atmospheric pressure, and digitally generating related interference fringes in the frequency domain.

Figure 4 is obtained after 100 consecutive measurement data are averaged by Fourier transform. In the space domain obtained by the first Fourier transform, reflection peaks are obtained due to the multi-level reflection of light in the three concave mirrors, and the amplitude of the reflection peaks is two orders of magnitude weaker than that at the exit sealing window.

Fig. 5 is an inverse Fourier transform result of selecting the signal at the outlet sealing window O during the inverse Fourier transform, and the absorption spectrum corresponding to the maximum absorption optical path is obtained. After the inverse Fourier transform of the data is processed, the amplitude data of the obtained frequency domain result is squared to obtain the corresponding absorption spectrum information. Corrected by baseline fitting, the normalized absorption spectrum was obtained.

The final gas absorption spectrum is shown in Fig. 6. For comparison, simulation results based on the HITRAN database are cited. The comparison shows that the residual error level between the experimental data and the simulation results of the present invention is very low, which illustrates the success of the method of this embodiment in accurately detecting the gas absorption spectrum. Based on the standard deviation of the residuals, the minimum detectable absorbance is about 0.01, which corresponds to a detection limit of 5 ppm for the measured acetylene gas.

According to the Beer-Lambert law of gas sensing, the shorter the absorption path, the higher the gas concentration required to produce the same absorption intensity. When the gas concentration is high, the absorption spectrum obtained from the outlet sealing window will appear absorption saturation, which is not suitable for gas sensing. At this time, the reflection signal of the concave mirror inside the multi-pass cell should be selected for inverse Fourier transform. Obtain an absorption spectrum without saturation. The inverse Fourier transformation is performed on the reflection signal of the concave mirror inside the multi-pass cell, which can greatly improve the measurement dynamic range of the method proposed by the invention. Figure 7 shows the simultaneous measurement of the gas absorption spectra at the reflection positions corresponding to different optical paths in the multipass cell. Taking advantage of the extended measurement dynamic range of this method, a relatively high concentration of 1600ppm acetylene gas was measured. Referring to Fig. 7, the absorption spectra measured at four different reflection positions are shown, and the corresponding absorption paths are 4.8196m, 8.0342m, 16.0713m and 22.4972m respectively. Each absorption spectrum is detected according to the steps described above. It can be clearly seen that at the position of 22.4972 m corresponding to the maximum absorption optical path, the strong absorption is close to saturation, so it cannot be used for gas sensing. In contrast, for relatively short absorption pathlengths, absorption lines still work. To detect higher concentrations of gases, short absorption pathlengths can be combined with weak absorption lines. Obviously, it is very beneficial to expand the effective dynamic range of measurement by selecting different optical paths.

It will be apparent to those skilled in the art that the invention is not limited to the details of the above-described exemplary embodiments, but that the invention can be embodied in other specific forms without departing from the spirit or essential characteristics of the invention. Accordingly, the embodiments should be regarded in all points of view as exemplary and not restrictive, the scope of the invention being defined by the appended claims rather than the foregoing description, and it is therefore intended that the scope of the invention be defined by the appended claims rather than by the foregoing description. All changes within the meaning and range of equivalents of the elements are embraced in the present invention.

Claims (8)

1. multi-pass pool gas absorption spectrum and the device for absorbing light path synchro measure, which is characterized in that the device includes：It is adjustable Humorous light source (1), auxiliary interferometer, main interferometer, data collecting card (10) and computing module (11)；

The laser of tunable optical source (1) output is incident to auxiliary interferometer and main interferometer respectively；

Auxiliary interferometer for generating clock signal, and is sent to the clock end of data collecting card (10)；

In the optical path of main interferometer include intrinsic light and test light, test light transmit in multi-pass pond after with intrinsic light generation beat frequency It is relevant；

Data collecting card (10), the beat frequency coherent signal for generating to main interferometer is acquired, and is sent to computing module；

Computing module (11) absorbs light path and absorption spectrum for calculating according to beat frequency coherent signal.

2. multi-pass pool gas absorption spectrum according to claim 1 and the device for absorbing light path synchro measure, feature It is, the laser of tunable optical source (1) output is divided into two-way light through the first coupler (2-1), and light is incident to auxiliary interference all the way Instrument, another way light are incident to main interferometer.

3. multi-pass pool gas absorption spectrum according to claim 2 and the device for absorbing light path synchro measure, feature It is, auxiliary interferometer includes third coupler (2-3), the first faraday rotation mirror (5-1), the second faraday rotation mirror (5- And optical fiber (6) 2)；

Third coupler (2-3) is that splitting ratio is 50:50 2 × 2 coupler；

The light all the way of first coupler (2-1) output is divided into two-way through third coupler (2-3), and light is revolved through the first faraday all the way It is coupled into third coupler (2-3) after tilting mirror (5-1) reflection, another way light is incident to the second faraday rotation after optical fiber (6) Tilting mirror (5-2), then third coupler (2-3), third coupler are coupled into after the second faraday rotation mirror (5-2) reflection (2-3) exports beat frequency coherent signal, then generates square-wave signal through TTL clock generator (9), which is clock letter Number.

4. multi-pass pool gas absorption spectrum according to claim 3 and the device for absorbing light path synchro measure, feature It is, main interferometer includes the second coupler (2-2), the 4th coupler (2-4), Polarization Controller (3), optical fiber circulator (4) With balanced detector (8)；

Second coupler (2-2) is 1 × 2 coupler, and the 4th coupler (2-4) is that splitting ratio is 50:50 2 × 2 coupling Device；

The another way light of first coupler (2-1) output is divided into two-way light through the second coupler (2-2), and light is as intrinsic light all the way An input port of the 4th coupler (2-4) is incident to after Polarization Controller (3), another way light is as test light by optical fiber 1 port of circulator (4) enters, and is emitted to multi-pass pond (7) by 2 ports of optical fiber circulator (4), through more in multi-pass pond (7) The absorption back reflection light of grade reflection and gas reenters 2 ports of fiber optical circulator (4) and is coupled into the 4th from 3 ports Beat frequency phase dry doubling occurs for another input port of coupler (2-4), intrinsic light by the 2 of the 4th coupler (2-4) with test light A output end is exported to balanced detector (8), the acquisition signal of output end connection data collecting card (10) of balanced detector (8) Input terminal, the input terminal of output end connection computing module (11) of data collecting card (10).

5. multi-pass pool gas absorption spectrum according to claim 1 and the device for absorbing light path synchro measure, feature It is, computing module (11) is calculated according to beat frequency coherent signal absorbs light path and absorption spectrum, specially：

Computing module (11) carries out Fourier transformation to the beat frequency coherent signal received, realizes the change by frequency domain spectra to Time Domain Spectrum It changes, is absorbed light path according to peak-to-peak optical path difference is reflected；

Inverse Fourier transform is carried out to information data section interested on Time Domain Spectrum, then a square processing is made to amplitude data, is obtained The absorption spectrum strength information of gas, then be normalized, obtain normalized absorption spectrum.

6. multi-pass pool gas absorption spectrum and the method for absorbing light path synchro measure, which is characterized in that this method includes：

Step 1: certain density under test gas is filled in multi-pass pond (7)；

Step 2: opening the output of tunable optical source (1) linear frequency sweep, while tunable optical source (1) output trigger signal triggers number Make data acquisition according to capture card (10) to prepare；

Step 3: auxiliary interferometer generates clock signal, data collecting card (10) are transmitted to, make data collecting card (10) to trunk The beat signal of interferometer carries out equal frequency intervals acquisition；

The test light of main interferometer imports multi-pass pond (7) by 2 ports of fiber optical circulator (4), through multistage anti-in multi-pass pond (7) It penetrates and the absorption back reflection light of gas reenters fiber optical circulator (4)；Bat Polarization Controller (3) is adjusted to make from fiber optic loop Light derived from (4) 3 port of row device and local oscillator light occur beat frequency and are concerned with, and are then transmitted to data collecting card by balanced detector (8) (10)；

Step 4: data collecting card (10) acquires beat signal, computing module (11) saves beat signal；

Step 5: computing module (11) carries out Fourier transformation to beat frequency coherent signal, the change by frequency domain spectra to Time Domain Spectrum is realized It changes, is absorbed light path according to peak-to-peak optical path difference is reflected；

Step 6: choosing interested information data section in the Time Domain Spectrum that step 5 obtains, inverse Fourier transform is carried out, is realized By the transformation of Time Domain Spectrum to frequency domain spectra；

Step 7: making a square processing to the amplitude data in frequency domain spectra, the absorption spectrum strength information of gas is obtained；

Step 8: the gas absorption spectra strength information to acquisition is normalized, normalized absorption spectrum is obtained；

Device of this method based on multi-pass pool gas absorption spectrum described in claim 4 or 5 and absorption light path synchro measure It realizes.

7. multi-pass pool gas absorption spectrum according to claim 6 and the method for absorbing light path synchro measure, feature It is, computing module (11) carries out Fourier transformation to multiple groups beat frequency coherent signal in step 5, multi-group data is obtained, to multiple groups Data are absorbed light path after being averaged according to reflecting peak-to-peak optical path difference.

8. multi-pass pool gas absorption spectrum according to claim 6 or 7 and the method for absorbing light path synchro measure, special Sign is that repetition step 5 is multiple, obtains multiple groups Time Domain Spectrum, carries out step 6 after being averaged to multiple groups Time Domain Spectrum again.