Non-linear slope background immune off-axis integral cavity output spectrum method and system

Abstract

The invention relates to the technical field of optical detection, in particular to a nonlinear slope background immune off-axis integral cavity output spectrum method and system. According to the non-linear slope background immune off-axis integral cavity output spectrum method provided by the invention, the wavelength of emergent light of a laser is locked in real time, so that the wavelength is stabilized at a set value; the optical power of the laser is adjusted in real time, so that the incident optical power of the off-axis integral cavity is stabilized at a set value; a spectrum is detected at the emergent end of the off-axis integral cavity; and complete measurement of the absorption spectrum to be measured is realized by changing the locked target wavelength. According to the invention, the problem of nonlinear inclination of an output spectrum baseline is solved through optical power locking, and the problem of nonlinear change of wavelength during continuous laser scanning is solved through wavelength locking and fixed-point scanning; according to the invention, an approximately horizontal spectrum baseline and a spectrum signal with accurate and stable wavelength are realized; the accuracy and efficiency of spectral signal fitting and background processing are improved, and the efficiency and precision of spectral baseline estimation are improved.

Description

Technical Field

[0001] The present invention relates to the field of optical detection technology, in particular to a nonlinear slope background-immune off-axis integrating cavity output spectrum method and system. Background Art

[0002] Off-axis integrated cavity output spectroscopy (OA-ICOS) is a spectral enhancement technique based on an optical resonant cavity with excellent detection sensitivity. Unlike coaxial resonant cavity systems such as cavity ring-down spectroscopy (CRDS) and cavity-enhanced absorption spectroscopy (CEAS), the probe light in off-axis systems deviates from the cavity axis, causing many different orders of spectral transverse and longitudinal modes to be excited. The goal is to obtain a very dense cavity mode spectrum and a narrow free spectral region. When it approaches or even falls below the laser linewidth, the cavity resonance is effectively suppressed. Therefore, OA-ICOS does not require strong spatiotemporal coherence between the laser and the cavity, strict mechanical stability, or complex electronics, while also providing a long effective optical path length, which has led to its application in various fields.

[0003] However, off-axis configurations also have significant disadvantages.

[0004] First, the non-resonant coupling results in only a small part of the detection light power being able to pass through the optical cavity, and as the fineness of the optical cavity and the reflectivity of the cavity mirror increase, the light power passing through the optical cavity will be lower, which places high demands on the response sensitivity and noise suppression capability of the back-end detector. Obviously, increasing the optical power of the laser source is the simplest and most direct solution to optimize this limitation. The current specific solutions include: optimizing the cavity structure (a transmission-enhanced off-axis integrating cavity structure CN106707524A, etc.), adjusting the optical path (such as "Reference paper: Zhou Zixin et al., Acta Physica Sinica, 2019, 68 (12), 129201", off-axis integrating cavity system based on adjustable re-incidence CN 118311754 A, etc.), etc.

[0005] In addition, like other technologies based on tunable semiconductor lasers to measure direct absorption spectra, OA-ICOS has also been criticized as a method with power background, and the light power background tilt caused by the laser injection current scan needs to be deducted in the subsequent signal processing. Currently, the non-absorption background is usually selected for fitting to estimate the light intensity background, or the non-absorption nitrogen spectrum is used as the background. However, the selection of the non-absorption range is subjective, and different selection ranges will lead to different processing results, especially for the measurement of complex absorption spectra, the jitter of the light power will also lead to background estimation errors. There is usually a big gap between the nitrogen spectrum and the actual light intensity background, such as "Reference paper: Shao Guodong et al., Spectroscopy and Spectral Analysis, 2021, 41 (10), 3256-3261." Another problem with OA-ICOS is the nonlinear change of frequency during continuous wavelength scanning, which usually requires real-time correction with an etalon, and accurate correction has strict requirements on the fineness of the etalon, as shown in the reference paper "Guan Linqiang et al., Acta Physica Sinica, 2019, 68 (8), 084204."

[0006] It can be seen that the existing OA-ICOS spectrum faces the problem of background processing. The existing optical power enhancement scheme amplifies the power background, further increasing the demand for background processing. Figure 7 The figure shows the output spectrum of a conventional off-axis integrating cavity. The etalon signal reveals that the laser's optical signal frequency varies non-uniformly, and the absorption signal background after passing through the off-axis integrating cavity tilts, exhibiting a nonlinear trend. This makes analysis of the absorption signal more difficult, making it difficult to ensure accurate analysis. The etalon signal is the output signal of the detector after the laser light passes through the FP cavity (Perot cavity) and is focused onto the detector. Summary of the Invention

[0007] In order to overcome the defect of large error in OA-ICOS background estimation in the above-mentioned prior art, the present invention proposes an off-axis integrating cavity output spectrum method with nonlinear slope background immunity, which locks the optical power and wavelength of the incident light of the off-axis integrating cavity, so that the output light of the laser can accurately traverse multiple set optical wavelength values under the condition of stable optical power. The background light intensity of the output light spectrum is approximately a straight line, and the wavelength corresponding to the spectral signal is more accurate, which greatly improves the estimation accuracy and efficiency of the spectral background.

[0008] The present invention proposes a nonlinear slope background-immune off-axis integrating cavity output spectrum method, which adjusts the laser output wavelength so that the wavelength traverses multiple set wavelength values, where the set wavelength value is the absorption wavelength value of the molecule to be measured; adjusts the laser light power in real time so that the incident light power of the off-axis integrating cavity is stabilized at the set value; then detects the light signal corresponding to each set wavelength value at the output end of the off-axis integrating cavity, and constructs a spectrum.

[0009] Preferably, the laser linearly adjusts the output wavelength at every set time step.

[0010] Preferably, the background light signal in the form of a horizontal line is extracted by fitting the signal line in combination with the detection spectrum.

[0011] The present invention proposes an off-axis integrating cavity output spectrum system with nonlinear slope background immunity, comprising: a laser, a first beam splitter, a wavelength detection module, an off-axis integrating cavity and a detection module;

[0012] The laser's output light is divided into a main beam and a sub-beam by the first beam splitter. The main beam is received by the detection module through the off-axis integrating cavity and its spectrum is detected. The sub-beam is detected by the wavelength detection module and its wavelength is fed back to the laser. The laser adjusts the wavelength of the output light according to the feedback light wavelength.

[0013] Preferably, the laser adjusts the light wavelength according to the wavelength detection module and traverses a set wavelength set, where the wavelength set is used to collect a plurality of light wavelength values within the absorption wavelength region of the molecules to be detected.

[0014] Preferably, it also includes an optical power amplifier, a second beam splitter and an optical power detection module;

[0015] After passing through the optical power amplifier, the main beam is split by the second beam splitter. Beam 1 passes through the off-axis integrating cavity and is received by the detection module to detect the spectrum. Beam 2 passes through the optical power detection module to detect the optical power and feed it back to the optical power amplifier. The optical power amplifier adjusts its operating parameters based on the feedback optical power to stabilize the power value of beam 1.

[0016] Preferably, it also includes an optical power amplifier, a second beam splitter and an optical power detection module;

[0017] The laser's outgoing light passes through the optical power amplifier and is split by the second beam splitter. The optical power detection module detects the optical power of one beam and feeds it back to the optical power amplifier. The optical power amplifier adjusts its operating parameters based on the feedback optical power to stabilize the power value of beam one.

[0018] The other beam of light is split into a main beam and a sub-beam by the first beam splitter, and enters the off-axis integrating cavity and the wavelength detection module respectively.

[0019] Preferably, the detection module is composed of a lens and a detector, and the outgoing light of the off-axis integrating cavity is focused by the lens and then received by the detector.

[0020] The advantages of the present invention are:

[0021] (1) The present invention proposes a nonlinear slope background-immune off-axis integrating cavity output spectrum method. The method solves the problem of tilted output spectrum baseline (i.e., background light intensity trend line) by optical power locking, and realizes an approximately horizontal spectrum baseline. The horizontal spectrum baseline estimation error is small, and the spectrum baseline can be extracted more quickly and accurately when fitting the spectrum detection signal. This solves the problem that the tilted baseline causes large errors in background estimation during signal processing, and improves the efficiency and accuracy of spectrum baseline estimation.

[0022] (2) The present invention achieves the acquisition of absorption spectrum signals at a defined and precise wavelength position through optical wavelength locking. The present invention creatively proposes point-by-point scanning under wavelength locking, that is, accurately traversing each set wavelength value to obtain a spectrum signal with accurate wavelength, thus solving the problems of nonlinear wavelength changes, large signal processing errors, and poor fitting during continuous scanning.

[0023] (3) The present invention proposes an off-axis integrating cavity output spectrum system with nonlinear slope background immunity. Through wavelength detection and feedback, the wavelength locking and point-by-point scanning of the laser are realized, the wavelength accuracy of the detected optical signal is achieved, and the efficiency and accuracy of subsequent spectrum processing and analysis are improved.

[0024] (4) The present invention improves and stabilizes the optical power through an optical power amplifier, solves the problem of spectral baseline tilt, and optimizes the problem of weak OA-ICOS output optical power, reducing the pressure on the high performance requirements of the back-end detector. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 The nonlinear slope background-immune off-axis integrating cavity output spectroscopy system proposed by the present invention;

[0026] Figure 2 This is a timing diagram of scanning wave numbers in the embodiment;

[0027] Figure 3 This is a comparison chart of signal changes before and after laser power amplification and locking;

[0028] Figure 4 The CH4 and H2O spectral signals obtained for the measurement;

[0029] Figure 5 This is a flow chart of the off-axis integrating cavity output spectroscopy method for nonlinear slope background immunity proposed in the present invention;

[0030] Figure 6 Another nonlinear slope background-immune off-axis integrating cavity output spectroscopy system proposed by the present invention;

[0031] Figure 7 It is the spectral signal obtained by the traditional off-axis integrating cavity output spectroscopy technology;

[0032] Figure 8 A signal that locks the optical power and scans continuously;

[0033] Figure 9 Scan the absorption signal point by point while locking the wavelength but not the optical power. DETAILED DESCRIPTION

[0034] The following will provide a clear and complete description of the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0035] This embodiment adopts the Figure 1 The nonlinear slope background-immune off-axis integrated cavity output spectroscopy system is shown for detecting methane (CH4) and water vapor (H2O).

[0036] The spectrum detection steps in this embodiment are as follows:

[0037] S1. Take points from the absorption wavelength range of methane (CH4) and water vapor (H2O) to construct a wavelength point value set;

[0038] S2. Turn on the system and adjust the power amplifier in real time so that the detection value of the optical power detection module is stable at the set power value;

[0039] S3. Select a wavelength value from the wavelength point value set as the target wavelength, and adjust the laser wavelength so that the detection value of the wavelength detection module is the target wavelength; the target wavelength is preferably the minimum wavelength point value;

[0040] S4, the detector detects and records the light signal value;

[0041] S5. Delete the target wavelength from the wavelength point value set, and then return to step S3.

[0042] In this way, when the laser optical power is locked, the laser wavelength is adjusted and the wavelength point value set is traversed; the detector detects the optical signal value corresponding to each optical wavelength under the locked optical power, and then fits the optical signal value according to the optical signal value. Figure 4 Spectra shown.

[0043] It is worth noting that when the laser's optical wavelength changes, the optical power also changes. Therefore, the optical power amplifier's parameter adjustment runs through the entire wavelength adjustment process to ensure that the laser's optical power is stable at the set power value.

[0044] The laser uses a tunable semiconductor laser with a central wavelength of 1.65μm to simultaneously measure methane (CH4) and water vapor (H2O) in the air. The effective optical path of the off-axis integrating cavity is 10km and the air pressure is 0.2atm.

[0045] like Figure 2 The following is a timing diagram of a wavelength scan. Wavelength-locked scanning is used to acquire complete, accurate spectra of CH4 and H2O. Because the CH4 spectrum consists of multiple overlapping absorption peaks, 15 points were measured. The H2O spectrum is relatively independent, so only 7 points were measured to minimize the time required to acquire the entire spectrum.

[0046] Reference Figure 5 As shown, in this embodiment, the optical power of the laser is adjusted in real time by an optical power amplifier, so that the incident optical power of the off-axis integrating cavity is stabilized at a set value; the laser adjusts and locks the optical wavelength according to the feedback of the wavelength detection module, and adopts a point-by-point scanning method, that is, the laser output wavelength is adjusted according to the set value, so that the laser output wavelength accurately traverses each set wavelength value, ensuring that the detector at the output end of the off-axis integrating cavity obtains a spectral signal with accurate wavelength scanning and a signal background level.

[0047] Figure 3 The original signal in is the signal when the laser only adjusts the wavelength without adjusting the optical power, that is, Figure 1 The optical signal power value collected before the optical power amplifier in the system; the signal after the optical power amplifier is locked, that is, the wavelength is adjusted while the optical power is adjusted by the optical power amplifier to lock the power value, Figure 1 The optical signal power value collected by the system's optical power amplifier backend.

[0048] like Figure 3 As shown, when optical power amplification and locking are not performed, the optical power will change during wavelength switching. When optical power amplification and locking are performed, the locked optical powers of each wavelength are basically the same, which will result in a horizontal spectral baseline background.

[0049] In this embodiment, the measured signal is as follows: Figure 4 As shown in Figure 1, the measured signals can be inverted to obtain the complete spectral signals of CH4 and H2O through spectral linear fitting according to the Beer-Lambert absorption law.

[0050] from Figure 4The baselines corresponding to the CH4 spectral signal fitting results and the HO spectral signal fitting results are aligned, demonstrating the effectiveness of the fixed-power immune ramp background method employed in the present invention. Furthermore, in this example, both the measured CH4 and HO spectral signals lie on the corresponding fitted signal lines, demonstrating the effectiveness of the present invention in improving spectral wavelength accuracy through wavelength adjustment.

[0051] contrast Figure 2 、 Figure 4 It can be seen that in this embodiment, the wavelength detection module detects the laser light wavelength in real time and feeds it back to the laser to achieve the stable setting of the laser light wavelength. Figure 2 The set wavenumbers shown here allow the light wavelength to accurately traverse multiple absorption wavenumbers of the CH4 and H2O spectral signals. Specifically, through the collaboration of the laser and wavelength detection module, the light wavelength is locked to the specific absorption peak of the CH4 or H2O molecule being measured within each time interval. By varying the locked target wavelength, the target absorption spectrum is completely scanned point by point, achieving linear variation in wavelength and light intensity.

[0052] In this embodiment, the wavelength detection module is set before the optical power detection module, and the wavelength is fed back first, and then the optical power is adjusted. This takes into account the fact that wavelength changes will bring about optical power changes, which is conducive to ensuring the stability of the optical power incident on the off-axis integrating cavity. Figure 7 ,Placing the wavelength detection module after the optical power detection module does not affect the implementation of the solution.

[0053] In this embodiment, the effect of performing only power locking or only wavelength locking point-by-point scanning is further verified.

[0054] Off-axis integral cavity output spectroscopy technology usually uses semiconductor lasers, whose output light wavelength can change with the change of current. Molecules absorb light of specific wavelengths. However, changes in laser current will also cause changes in the output light power, resulting in Figure 7 There is a slanted background in the absorption signal, and the relationship between the laser wavelength and the current is nonlinear, which leads to nonlinear changes in the absorption signal. During continuous scanning, the laser power changes continuously, and it is almost impossible to achieve continuous scanning while locking the power. Special methods are required, such as using a special laser whose optical power does not change with wavelength and can be continuously scanned. In this embodiment, the optical signal obtained by locking the optical power and continuously scanning is as follows: Figure 8 As shown. Figure 8 It can be seen that locking the optical power adjusts the absorption signal background to a level, but the optical signal still shows non-uniform changes in wavelength.

[0055] from Figure 9It can be seen that without locking the optical power and using wavelength locking and point-by-point scanning, the wavelength linearity of the absorption signal can be improved, but the background is in a tilted state.

[0056] from Figure 8 、 Figure 9 It can be seen that only by locking the optical power and adopting wavelength-locked point-by-point scanning can an absorption signal with a background baseline level and accurate and stable wavelength be achieved, thereby improving the analysis efficiency and accuracy of the absorption signal.

[0057] Of course, it will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, but also encompasses the same or similar structures that can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as illustrative and non-restrictive, and the scope of the invention is defined by the appended claims, not the foregoing description, and it is intended that all variations that fall within the meaning and range of equivalents of the claims be encompassed within the present invention. Any reference signs in the claims should not be construed as limiting the claim to which they relate.

[0058] In addition, it should be understood that although this specification is described in terms of implementation methods, not every implementation method contains only one independent technical solution. This narrative method of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.

[0059] The technology, shape, and structure not described in detail in the present invention are all well-known technologies.

Claims

1. A nonlinear slope background immune off-axis integral cavity output spectroscopy method, characterized in that: Adjust the laser's output wavelength so that it traverses multiple set wavelength values, where the set wavelength value is the absorption wavelength value of the molecule to be measured; adjust the laser's optical power in real time so that the incident light power of the off-axis integrating cavity is stabilized at the set value; then detect the optical signal corresponding to each set wavelength value at the output end of the off-axis integrating cavity and construct a spectrum.

2. The off-axis integrating cavity output spectroscopy method with nonlinear slope background immunity according to claim 1, characterized in that: The laser linearly adjusts the output wavelength at every set time step.

3. The off-axis integral cavity output spectroscopy method with nonlinear slope background immunity according to claim 1 or 2, characterized in that: The background light signal that appears as a horizontal line is extracted by fitting the signal line in combination with the detection spectrum.

4. A nonlinear slope background immune off-axis integrating cavity output spectroscopy system, characterized in that: include: A laser, a first beam splitter (1), a wavelength detection module, an off-axis integrating cavity (3), and a detection module; The output light of the laser is divided into a main beam and a sub-beam through a first beam splitter (1); the main beam is received by a detection module through an off-axis integrating cavity (3) and its spectrum is detected; the sub-beam is detected by a wavelength detection module and its wavelength is fed back to the laser; the laser adjusts the wavelength of the output light according to the feedback light wavelength.

5. The off-axis integrating cavity output spectroscopy system with nonlinear slope background immunity according to claim 4, characterized in that: The laser adjusts the light wavelength according to the wavelength detection module and traverses the set wavelength set, which is used to collect multiple light wavelength values located in the absorption wavelength region of the molecules to be detected.

6. The off-axis integrating cavity output spectroscopy system with nonlinear slope background immunity according to claim 4, characterized in that: It also includes an optical power amplifier, a second beam splitter (2) and an optical power detection module; The main light beam is split by the second beam splitter (2) after passing through the optical power amplifier. The first light beam passes through the off-axis integrating cavity (3) and is received by the detection module to detect the spectrum. The second light beam passes through the optical power detection module to detect the optical power and feeds it back to the optical power amplifier. The optical power amplifier adjusts the working parameters according to the fed-back optical power to stabilize the power value of the first light beam.

7. The off-axis integrating cavity output spectroscopy system with nonlinear slope background immunity according to claim 4, characterized in that: It also includes an optical power amplifier, a second beam splitter (2) and an optical power detection module; The output light of the laser passes through the optical power amplifier and is then split by the second beam splitter (2). The optical power of one beam is detected by the optical power detection module and fed back to the optical power amplifier. The optical power amplifier adjusts the working parameters according to the fed-back optical power to stabilize the power value of the first beam. The other beam of light passes through a first beam splitter (1) and is split into a main beam and a sub-beam, which enter the off-axis integrating cavity (3) and the wavelength detection module respectively.

8. The off-axis integrating cavity output spectroscopy system with nonlinear slope background immunity according to claim 4, 5, 6 or 7, characterized in that: The detection module is composed of a lens (4) and a detector (5); the outgoing light of the off-axis integrating cavity is focused by the lens (4) and then received by the detector (5).

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