A sweeping laser optical path difference measurement method and an FTIR flue gas analyzer

Abstract

This invention proposes a swept-frequency laser optical path difference measurement method and an FTIR flue gas analyzer. The FTIR flue gas analyzer includes an infrared light source, a spectrometer, a moving mirror, a fixed mirror, a gas chamber, an infrared detector, a tunable swept-frequency laser light source, and a laser intensity detector. This improves the spectral stability, resampling accuracy, and resolution of the Fourier Transform Infrared Analyzer under industrial, portable, or high-vibration conditions. The swept-frequency laser optical path difference measurement method utilizes the phase change caused by the time variation (scanning) of the swept-frequency laser wavelength to directly calculate the optical path difference (OPD), obtaining continuous, real-time OPD information. This method is an OPD measurement method that does not rely on the brightness and darkness period of interference fringes, does not require constant-speed scanning, and has strong vibration resistance, avoiding calibration errors caused by fringe loss in traditional methods.

Description

Technical Field

[0001] This invention belongs to the field of Fourier transform infrared (FTIR) flue gas analyzers, and particularly relates to a sweep laser optical path difference measurement method and an FTIR flue gas analyzer. Background Technology

[0002] Fourier transform infrared (FTIR) flue gas analyzer is a high-precision flue gas monitoring device based on Fourier transform infrared spectroscopy technology. It is widely used in industrial exhaust gas, environmental monitoring, boiler emissions, and other scenarios, and can simultaneously and rapidly detect SO2 and NO. x It can detect various gaseous pollutants such as CO, CO2, VOCs, and NH3. The core components of a Fourier transform infrared (FTIR) flue gas analyzer include an infrared light source, a semi-reflective mirror, a long-path gas cell, a laser intensity detector, and an interferometer. The interferometer consists of a fixed mirror and a moving mirror. The interferogram is generated by the reciprocating movement of the moving mirror, and then the infrared spectrum is obtained through Fourier transform. To ensure the accuracy of the spectrum, the instrument must rely on a reference laser to determine the optical path difference (OPD) of the moving mirror in real time.

[0003] Current Fourier transform infrared (FTIR) flue gas analyzers generally use fixed-wavelength lasers (such as He-Ne lasers or 1550nm lasers) as reference light, providing equally spaced optical path difference markers through the bright and dark periods of interference fringes for interferogram resampling. However, in practical applications, this fixed-laser reference method has the following prominent technical problems:

[0004] (1) During operation, the moving mirror is easily affected by factors such as mechanical vibration, friction changes, and driving fluctuations, which can lead to instability of the interference fringes of the reference laser, manifested as fringe jitter, fringe non-uniformity, and decreased fringe contrast. This can cause instability in the triggering of the optical path difference markers, affecting the resampling accuracy of the interferogram, and consequently leading to spectral drift or increased noise.

[0005] (2) In portable, flue gas field or high vibration environment, due to mechanical disturbance, angle change or optical path offset, the interference fringes of fixed laser may "drop fringes", that is, some bright and dark periods cannot be stably identified, or the fringes suddenly jump. Dropped fringes will directly lead to errors in optical path difference calibration, causing the OPD axis of the interferogram to be misaligned, and ultimately resulting in obvious abnormalities in the reduced spectrum.

[0006] Since the fringe stability of the reference laser determines whether the interferogram can be accurately resampled, fixed-wavelength lasers often cannot guarantee sufficient stability under real-world conditions, especially in portable Fourier transform infrared flue gas analyzers with significant vibrations, compact structures, or frequent attitude changes. Summary of the Invention

[0007] This invention addresses the significant shortcomings of existing fixed-wavelength laser reference methods in terms of stability and fringe reliability, which can easily lead to inaccurate interferogram resampling, spectral drift, and even measurement failure. It proposes a swept-frequency laser optical path difference measurement method for FTIR flue gas analyzers and an FTIR flue gas analyzer based on this method, which can effectively improve the stability and anti-interference capability of Fourier transform infrared flue gas analyzers in various scenarios.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0009] An FTIR flue gas analyzer includes an infrared light source, a beam splitter, a moving mirror, a fixed mirror, a gas chamber, and an infrared detector. The feature is that it further includes a tunable swept-frequency laser source and a laser intensity detector. Infrared light emitted from the infrared light source is split by the beam splitter, reflected by the moving and fixed mirrors respectively, and then coherently interfering. The interfering infrared light is split again by the beam splitter and enters the gas chamber, where it is absorbed by the gas being measured and then enters the infrared detector to generate an interference pattern. The laser emitted from the tunable swept-frequency laser source is split by the beam splitter, reflected by the moving and fixed mirrors respectively, and then coherently interfering. The interfering swept-frequency laser enters the laser intensity detector to collect the interference intensity signal of the swept-frequency laser.

[0010] Preferably, the beam splitter is a semi-reflective semi-transparent lens.

[0011] Preferably, the tunable sweep laser has a sweep frequency of fsweep and a moving mirror scanning frequency of fmirror, where... .

[0012] This invention also proposes a method for measuring the optical path difference of a swept-frequency laser in an FTIR flue gas analyzer, comprising the following steps:

[0013] Step 1: Set the sweep frequency of the tunable sweep laser to f. sweep The moving mirror scanning frequency is f mirror ,in ;

[0014] Step 2: Obtain the current-wavelength function of the tunable sweeping laser through pre-calibration, i.e., λ=f(I). During the sweeping process, the controller outputs the current sequence I(t), and the wavelength sequence λ(t) is obtained according to the current-wavelength function.

[0015] Step 3: Perform envelope phase analysis or local frequency estimation on the swept laser interference signal to obtain the brightness-darkness variation frequency f(t);

[0016] Step 4: Using the formula OPD(t)=k·f(t)·λ(t), where k is the structure constant, combine λ(t) and f(t) to obtain the complete OPD(t) curve.

[0017] Preferably, the following steps are also included:

[0018] Step 5: Establish equally spaced sampling points along the optical path difference axis, OPD n =n·ΔOPD, where OPD n Let ΔOPD be the nth sampling position on the optical path difference axis, and ΔOPD be the fixed step size of the optical path difference. The time-domain infrared interferogram x(t) is mapped to X(OPD) with equal optical path difference sampling using interpolation. n );

[0019] Step 6: Perform a Fourier transform on the resampled interferogram to obtain a stable infrared spectrum.

[0020] Preferably, the moving mirror scanning frequency f mirror The sweep frequency f of the tunable sweep laser is 5Hz~50Hz. swee p is 5kHz~50kHz.

[0021] Compared with the prior art, the advantages and positive effects of the present invention are as follows:

[0022] The FTIR flue gas analyzer of this invention uses a tunable sweep laser to replace the fixed wavelength laser stripe counting method, avoiding the instability caused by vibration and optical path drift of fixed laser stripes, and improving the spectral stability, resampling accuracy and resolution of the Fourier transform infrared analyzer under industrial, portable or high vibration conditions.

[0023] This invention relates to a swept-frequency laser optical path difference measurement method for FTIR flue gas analyzers. This method utilizes the phase change caused by the time-varying (scanning) wavelength of the swept-frequency laser to directly calculate the optical path difference (OPD), obtaining continuous, real-time OPD information. It is an OPD measurement method that does not rely on the brightness and darkness period of interference fringes, does not require constant-speed scanning, and has strong vibration resistance. This avoids calibration errors caused by fringe loss in traditional methods. It also enhances the interferometer's ability to maintain optical path difference information in dynamic environments, enabling the recovery of the optical path difference calculation process even if fringe instability or short-term loss occurs during scanning. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the structure of the sweeping laser optical path difference measurement system of the present invention;

[0025] In the above figures: 1. Infrared light source; 2. Semi-reflective mirror; 3. Fixed mirror; 4. Moving mirror; 5. Long optical path gas cell; 6. Tunable sweeping laser light source; 7. Laser intensity detector; 8. Interferometric laser beam; 9. Interferometric infrared beam. Detailed Implementation

[0026] To better understand the present invention, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. Example 1

[0027] like Figure 1 As shown, a Fourier transform infrared analyzer includes an infrared light source, a semi-reflective mirror, a moving mirror, a fixed mirror, a long-path gas chamber, an infrared detector, a tunable sweep laser light source, and a laser intensity detector. The infrared light source emits broadband infrared light. A collimating lens can be placed in front of the infrared light source to ensure that the infrared light is emitted horizontally. The fixed mirror is fixedly installed on the right side of the infrared light source and does not change over time; it is used to reflect the light. The moving mirror is installed perpendicular to the direction of infrared light emission and is driven by a drive mechanism to move back and forth along the optical axis to generate interference fringes and change the optical path difference. The semi-reflective mirror is installed between the infrared light source and the fixed mirror at a 45-degree angle to the infrared light; it is used for beam splitting and combining. The infrared light emitted by the infrared light source is split by the semi-reflective mirror, reflected by the moving mirror and the fixed mirror respectively, and then coherently interfering. The interfering infrared light is split again by the semi-reflective mirror and enters the gas chamber, where it is absorbed by the gas being measured before entering the infrared detector to generate an interference pattern.

[0028] A tunable swept-frequency laser is positioned parallel to the infrared light source to output narrowband light. Its wavelength varies with time according to a set function (linear or sawtooth wave) and serves as a reference light for optical path difference. The laser emitted from the tunable swept-frequency laser source is split by a beam splitter and then reflected by a moving mirror and a fixed mirror, resulting in coherent interference. The resulting swept-frequency laser enters a laser intensity detector to collect the interference intensity signal. This interference signal contains phase change information related to the optical path difference and serves as the basis for subsequent OPD calculations. Example 2

[0029] A method for measuring optical path difference using a swept-frequency laser for FTIR flue gas analyzers is proposed. This method utilizes the change in interference brightness and darkness frequency caused by the change in wavelength of a tunable swept-frequency laser over time to directly calculate the optical path difference (OPD) of the interferometer. Then, based on the continuous optical path difference (OPD), the infrared interferogram is resampled with equal optical path difference to obtain a stable Fourier infrared spectrum.

[0030] Unlike traditional fixed-wavelength lasers that rely on "strip counts", this method relies on the functional relationship between brightness and darkness frequencies and wavelength, enabling continuous, stable, and highly vibration-resistant real-time measurement of optical path difference (OPD).

[0031] Specifically, the following steps are included:

[0032] Step 1: Set the sweep laser parameters

[0033] Set the sweep frequency of the tunable sweep laser to f. sweep The moving mirror scanning frequency is f mirror To ensure that the changes in brightness and darkness of the interference are mainly caused by wavelength scanning and are not affected by the instantaneous movement of the moving mirror, the sweep frequency f of the tunable sweep laser is set. sweepMuch greater than the moving mirror scanning frequency f mirror ,Right now:

[0034] ,

[0035] In this embodiment, the moving mirror scanning frequency f is set. mirror The sweep frequency f of the tunable sweep laser is 5Hz~50Hz. swee p is 5kHz~50kHz.

[0036] Under these conditions, the displacement of the moving mirror within one sweep cycle is much smaller than the working wavelength. It can be assumed that the optical path difference (OPD) remains constant within a single sweep cycle. This parameter is set as the basis for establishing a stable correspondence between the sweep brightness and darkness frequency and the OPD.

[0037] Step 2: Calibrate the wavelength of the swept laser

[0038] Due to the wavelength of the swept frequency laser Controlled by the driving current I, the current-wavelength function can be obtained through pre-calibration. ,Right now:

[0039] ,

[0040] During the frequency sweep process, the controller outputs the drive current sequence as follows: The wavelength sequence is obtained from the current-wavelength function. This method of obtaining the equivalent wavelength through driving current can accurately obtain the real-time wavelength without the need for a grating or wavelength meter. , As input parameters for OPD solution, their determinism affects the repeatability of λ(t) and the stability of OPD solution.

[0041] Step 3: Extract brightness and darkness frequencies

[0042] The basic principle of the correspondence between the brightness and darkness frequency of a swept laser and the optical path difference is as follows:

[0043] Under the premise that the single-cycle OPD remains approximately constant, the brightness changes of the interference signal are caused by wavelength scanning.

[0044] When OPD is constant, within a certain wavelength scanning range, the number of brightness changes N caused by scanning λ(t) is proportional to the optical path difference:

[0045] ,

[0046] Equivalently, the frequency of brightness changes Similarly, it satisfies the condition that it is proportional to the optical path difference:

[0047] ,

[0048] Specifically, a large optical path difference (OPD) results in dense fringes and high brightness / dark frequencies; a small OPD results in sparse fringes and low brightness / dark frequencies. Therefore, the corresponding optical path difference can be deduced from the brightness / dark frequencies or the number of brightness / dark cycles within a frequency sweep period. Continuous frequency sweeping yields the complete OPD(t) curve.

[0049] This principle avoids problems such as "stripes loss" and "count jumps" in fixed-wavelength lasers, and greatly improves vibration resistance and stability.

[0050] Step 4: Real-time calculation of continuous OPD

[0051] During the frequency sweep process, the wavelength change λ(t) caused by the driving current I(t) results in brightness and darkness fluctuations in the interference signal corresponding to the optical path difference. By employing methods such as envelope phase tracking and instantaneous frequency estimation, the brightness and darkness frequencies f(t) can be extracted from the interference signal in real time. Combined with λ(t), the continuous optical path difference can be calculated in real time. :

[0052] ,

[0053] Where k is the structure constant.

[0054] Since λ(t) has been obtained in step 2, f(t) can be reliably extracted, thus enabling continuous acquisition of OPD(t) throughout the entire scan.

[0055] Step 5: Resampling with equal optical path difference:

[0056] The infrared interferogram x(t) is sampled in the time domain, while the Fourier transform requires equal OPD sampling. Therefore, equally spaced points are established on the optical path difference axis using OPD(t).

[0057] ,

[0058] Where n is an integer index (…, −2, −1, 0, 1, 2, …) used to identify discrete points of the interferogram on the optical path difference axis; OPD n ΔOPD represents the nth sampling position on the optical path difference axis, used to construct an equally spaced optical path difference sequence; ΔOPD is the fixed step size of the optical path difference, which is a constant, and its size is determined by the system's spectral resolution requirements.

[0059] Furthermore, an interpolation method is used to map x(t) to X(OPD). n ).

[0060] Step 6: Generate high-quality spectra

[0061] A Fourier transform was performed on the resampled interferogram to obtain a stable infrared spectrum that is vibration-resistant, has no fringe loss, and is in phase correct.

[0062] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any other way. Any person skilled in the art may make changes or modifications to the above-disclosed technical content to create equivalent embodiments for application in other fields. However, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the protection scope of the present invention.

Claims

1. An FTIR flue gas analyzer, comprising an infrared light source, a spectrometer, a moving mirror, a fixed mirror, a gas chamber, and an infrared detector, characterized in that: It also includes a tunable sweep laser source and a laser intensity detector. The infrared source emits infrared light from a horizontal direction. A fixed mirror is fixedly installed on the right side of the infrared source and does not change over time. It is used to reflect the light. A moving mirror is installed in a position perpendicular to the direction of infrared light emission and is driven by a drive mechanism to move back and forth along the optical axis. It is used to generate interference fringes and change the optical path difference. A beam splitter is installed between the infrared source and the fixed mirror and at a 45-degree angle to the infrared light. It is used for beam splitting and beam combining. The infrared light emitted by the infrared source is split by the beam splitter and then reflected by the moving mirror and the fixed mirror respectively. After interference, the infrared light is split by the beam splitter and enters the gas chamber. After being absorbed by the gas being measured, it enters the infrared detector to generate an interference pattern. A tunable swept laser is set parallel to an infrared light source to output narrowband light. Its wavelength changes with time according to a linear or sawtooth wave function and is used as a reference light for optical path difference. The laser emitted by the tunable swept laser source is reflected by a moving mirror and a fixed mirror after passing through a beam splitter and then coherently. The swept laser after interference enters a laser intensity detector to collect the interference intensity signal of the swept laser. This interference intensity signal contains phase change information related to the optical path difference. The sweep frequency of a tunable sweep laser is f sweep The moving mirror scanning frequency is f mirror ,in .

2. The FTIR flue gas analyzer according to claim 1, characterized in that: The beam splitter is a semi-reflective semi-transparent lens.

3. A method for measuring the optical path difference of a swept-frequency laser in an FTIR flue gas analyzer as described in claim 1 or 2, characterized in that, Includes the following steps: Step 1: Set the sweep frequency of the tunable sweep laser to f. sweep The moving mirror scanning frequency is f mirror ,in ; Step 2: Obtain the current-wavelength function of the tunable swept-frequency laser through pre-calibration, i.e. The wavelength of the swept-frequency laser is I is the driving current. During the frequency sweep process, the controller outputs a current sequence I(t), and the wavelength sequence λ(t) is obtained according to the current-wavelength function. Step 3: Perform envelope phase analysis or local frequency estimation on the swept laser interference signal to obtain the brightness-darkness variation frequency f(t); Step 4: Using the formula OPD(t)=k·f(t)·λ(t), where k is the structure constant, combine λ(t) and f(t) to obtain the continuous optical path difference OPD(t) curve.

4. The method for measuring the optical path difference of a swept-frequency laser according to claim 3, characterized in that, It also includes the following steps: Step 5: Establish equally spaced sampling points along the optical path difference axis, OPD n =n·ΔOPD, where OPD n Let ΔOPD be the nth sampling position on the optical path difference axis, and ΔOPD be the fixed step size of the optical path difference. The time-domain infrared interferogram x(t) is mapped to X(OPD) with equal optical path difference sampling using interpolation. n ); Step 6: Perform a Fourier transform on the resampled interferogram to obtain a stable infrared spectrum.

5. The method for measuring optical path difference using swept-frequency lasers according to claim 4, characterized in that: Moving mirror scanning frequency f mirror The sweep frequency f of the tunable sweep laser is 5Hz~50Hz. sweep The frequency range is 5kHz to 50kHz.

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