A method for locking the length of an optical cavity based on a gas absorption spectral line and an optical cavity ring-down gas measurement system

Abstract

The present application relates to the technical field of laser absorption spectrum measurement, and discloses a light cavity length locking method based on gas absorption spectrum and a light cavity ring-down gas measurement system. A frequency band where carbon dioxide and methane absorption spectrum lines are located is scanned, an absorption spectrum is constructed according to wavelength data fed back by a wavelength detection module of a standard wavelength and a ring-down signal, and a target reference object is determined according to a signal-to-noise ratio and a hysteresis switching condition; a gas absorption peak center frequency or a standard wavelength target reference frequency corrected and saved last time through a gas absorption peak is taken as a target reference frequency, a light cavity longitudinal mode resonance point is continuously tracked, and a frequency deviation is calculated; and the cavity length is adjusted successively according to the frequency deviation, a relationship between a longitudinal mode frequency and the cavity length, and a calibration relationship of a piezoelectric ceramic actuator. The present application can compensate for slow drift of the light cavity length and reduce long-term drift of a gas concentration measurement result.

Description

Technical Field

[0001] This invention relates to the field of laser absorption spectroscopy measurement technology, specifically to a cavity length locking method based on gas absorption spectral lines and a cavity ring-down gas measurement system. It is applicable to gas analysis equipment that uses cavity ring-down spectroscopy to measure gas components such as carbon dioxide and methane and requires compensation for cavity length drift. Background Technology

[0002] Cavity ring-down spectroscopy typically employs two highly reflective cavity mirrors to form an optical resonant cavity. When the laser frequency and a certain longitudinal mode frequency of the cavity satisfy the resonance condition, the laser energy couples into the cavity. After the incident laser is turned off, the ring-down time is obtained by detecting the decay process of the light intensity within the cavity over time, and this is used to determine the absorption loss of the gas within the cavity at the corresponding frequency position. Because the laser can propagate back and forth between the two cavity mirrors multiple times, this technique can achieve a relatively long equivalent absorption optical path with a limited physical cavity length, and is therefore used for highly sensitive measurements of gaseous components such as carbon dioxide and methane.

[0003] In actual measurements, the longitudinal mode frequency of an optical cavity is closely related to its optical length. Changes in the physical cavity length, the refractive index of the gas within the cavity, or the relative positions of the cavity mirrors will cause the longitudinal mode resonance point to shift. During long-term operation, factors such as ambient temperature changes, mechanical vibration, stress release in the supporting structure, thermal expansion and contraction of the cavity materials, and device aging can all cause slow changes in the cavity length. Although existing equipment typically incorporates vibration isolation supports, active temperature control, and pressure control structures, these measures primarily reduce external disturbances and cannot completely eliminate the minute deformations and cumulative drift generated in the optical cavity during long-term operation.

[0004] When the longitudinal mode resonance point of the optical cavity shifts, the actual sampling position of the gas absorption spectrum will change accordingly. For measurement methods that acquire absorption spectra point-by-point using discrete optical cavity longitudinal mode resonance points, even if the concentration of the gas to be measured remains constant, the spectral measurement points obtained at different times may correspond to different frequency positions, resulting in slow changes in the fitted absorption peak center, peak shape parameters, and concentration inversion results. Especially in gas monitoring equipment that requires continuous operation, the aforementioned frequency sampling position drift will gradually accumulate, affecting the long-term stability of gas concentration measurement results.

[0005] Existing frequency stabilization or cavity-locking techniques typically construct error signals based on cavity reflection signals, transmission signals, or external frequency references, and then adjust the laser or cavity mirror position via servo control. Some high-precision locking methods require the setup of RF modulation sources, electro-optic modulators, high-speed detectors, mixers, filters, and servo control circuits, resulting in numerous system components, complex optical and circuit structures, and high requirements for debugging and maintenance. Furthermore, simply making the laser frequency follow the cavity resonant point does not eliminate the problem of overall resonant frequency shift when the cavity itself undergoes slow changes.

[0006] Furthermore, gas analysis equipment may simultaneously measure multiple components such as carbon dioxide and methane. Under different operating conditions, the concentration and absorption signal intensity of each gas component are not constant. When the concentration of a certain gas is low, the absorption peak is weak, or the spectral noise is high, the center frequency obtained based on that absorption peak may not be stable enough. If a single gas absorption peak is always used as a reference, inaccurate adjustment results are easily produced when the quality of the reference signal deteriorates; if the reference object is switched only based on a single measurement result or a single threshold, frequent switching may occur due to signal fluctuations near the critical value, causing instability in the locking process.

[0007] Meanwhile, the gas absorption peak center, the wavelength data output by the etalon, the longitudinal mode resonance point of the optical cavity, and the cavity mirror displacement belong to different measurement and control stages. In existing equipment, the above information is often used for spectral measurement, wavelength calibration, or cavity length adjustment, lacking a processing mechanism that can select a reference based on the absorption spectrum quality and continuously correlate the reference frequency, the specific longitudinal mode resonance point of the optical cavity, and the cavity mirror displacement.

[0008] Therefore, it is still necessary to improve the relationship between reference selection, longitudinal mode positioning, and cavity length adjustment in the process of measuring ring-down gas in optical cavity. Summary of the Invention

[0009] The technical problem to be solved by the present invention is to reliably determine the cavity length locking reference and compensate for the offset of the longitudinal mode resonance point of the optical cavity relative to the target reference frequency when the cavity length changes during long-term operation and the absorption signal quality of different gases fluctuates.

[0010] To address the aforementioned technical problems, this invention provides a cavity length locking method based on gas absorption spectral lines, applied to an optical cavity ring-down gas measurement system, comprising the following steps:

[0011] S1. Control the laser to scan the frequency bands where the carbon dioxide absorption spectrum and the methane absorption spectrum are located. When the laser frequency of the laser resonates with the longitudinal mode of the optical cavity, the laser is turned off to collect the ring-down signal and obtain the wavelength data corresponding to the ring-down signal.

[0012] S2. Based on the ring-down signal and wavelength data, obtain the carbon dioxide absorption spectrum and methane absorption spectrum of the gas to be measured from the optical cavity, respectively.

[0013] S3. Based on the signal-to-noise ratio of the carbon dioxide absorption spectrum and the methane absorption spectrum, the current target reference object, and the preset hysteresis switching conditions, determine the target reference object from the carbon dioxide absorption peak, the methane absorption peak, and the wavelength reference provided by the pre-calibrated etalon, and determine the target reference frequency based on the target reference object; the preset hysteresis switching conditions include a preset number of consecutive frames, a first threshold, and a second threshold higher than the first threshold.

[0014] S4. From the multiple longitudinal mode resonance points of the optical cavity, determine the target optical cavity longitudinal mode resonance point whose frequency is closest to the target reference frequency, and determine the frequency deviation between the target optical cavity longitudinal mode resonance point and the target reference frequency.

[0015] S5. Based on the frequency deviation, the relationship between the longitudinal mode resonant frequency of the optical cavity and the cavity length, and the pre-calibrated correspondence between the driving voltage of the piezoelectric ceramic actuator and the change in cavity length, determine the driving voltage adjustment amount of the piezoelectric ceramic actuator, control the high-voltage driver to adjust the piezoelectric ceramic actuator according to the driving voltage adjustment amount, so that the piezoelectric ceramic actuator, which is set on the incident side of the second super-reflecting mirror and bonded to the second super-reflecting mirror along the optical cavity axis, extends and retracts along the optical cavity axis based on the inverse piezoelectric effect, thereby driving the second super-reflecting mirror to move along the optical cavity axis, and repeat steps S1 to S5.

[0016] Furthermore, before performing step S1, the driving voltage of the piezoelectric ceramic actuator is set to 75V, so that the piezoelectric ceramic actuator retains the adjustment stroke in both the elongation and shortening directions; the temperature of the optical cavity is controlled at 45±0.002℃, and the pressure fluctuation in the optical cavity is controlled within ±5Pa.

[0017] Specifically, by controlling the constant temperature space where the optical cavity and the measurement optical path are located at 45±0.002℃, the optical cavity is kept under the corresponding constant temperature conditions; the pressure sensor is set at the air outlet of the optical cavity, and the proportional valve is set between the air outlet of the optical cavity and the air pump. By adjusting the opening of the proportional valve, the pressure fluctuation of the gas to be measured in the optical cavity is controlled within ±5Pa.

[0018] Furthermore, the frequency bands of the carbon dioxide and methane absorption spectra are scanned by changing the injection current of the laser; the free spectral range of the optical cavity is used as the frequency interval between the longitudinal mode resonance points of adjacent optical cavities, and the carbon dioxide and methane absorption spectra are measured point by point; the wavelength data fed back by the etalon when the laser is turned off each time are recorded, and the frequency coordinates of the longitudinal mode resonance points of each optical cavity are determined according to the wavelength data fed back by the etalon and the free spectral range of the optical cavity.

[0019] The lasers include a 1603 nm butterfly DFB laser for scanning the carbon dioxide absorption spectrum, a 1651 nm butterfly DFB laser for scanning the methane absorption spectrum, and a MEMS optical switch. The two butterfly DFB lasers are time-division multiplexed using the same measurement optical path and the same optical cavity via the MEMS optical switch. The center wavelength of the carbon dioxide target absorption spectrum is 1603.2266 nm, and the center wavelength of the methane target absorption spectrum is 1650.9606 nm.

[0020] The free spectral range of the optical cavity is 500 MHz, and the frequency tuning range of the laser near the corresponding target absorption line is 7.5 GHz. The algorithm control unit selects 15 complete longitudinal mode resonance points of the optical cavity from the cavity mode signals acquired within the frequency tuning range. These 15 points are located near the target absorption peak and can complete ring-down fitting. These resonance points are used to construct the absorption spectrum and perform continuous tracking. Resonance signals located at the edge of the scanning range and not fully acquired are not included in the 15 longitudinal mode resonance points.

[0021] Furthermore, exponential fitting is performed on each ring-down signal to obtain the ring-down time corresponding to the longitudinal mode resonance point of each optical cavity; the absorption signal at the corresponding frequency coordinate is determined based on the ring-down time, and carbon dioxide absorption spectrum and methane absorption spectrum are formed based on multiple frequency coordinates and corresponding absorption signals.

[0022] Furthermore, when the target reference is a carbon dioxide absorption peak or a methane absorption peak, the Voigt line-shape function, formed by the convolution of Gaussian and Lorentz line shapes, is used to fit the absorption spectrum corresponding to the target reference to obtain the center frequency of the corresponding absorption peak, and the center frequency of the absorption peak is determined as the target reference frequency.

[0023] The etalon is housed within the etalon wavelength detection module. The etalon wavelength detection module collects the transmitted and reflected light signals from the etalon, determines the etalon transmittance based on the ratio of the transmitted detection voltage to the sum of the transmitted and reflected detection voltages, and determines the wavelength data based on the pre-calibrated correspondence between the etalon transmittance and wavelength.

[0024] After the etalon wavelength detection module is initially assembled and adjusted, an external wavelength meter is used to calibrate the complete laser scanning process in the 1603nm and 1651nm operating bands, respectively, and the correspondence between etalon transmittance and wavelength is stored according to the scanning sequence. During gas measurement, the algorithm control unit determines the wavelength data corresponding to the current ring-down signal based on the operating band of the currently selected laser, the current scanning position, and the wavelength continuity between adjacent measurement points, within the calibration relationship of the corresponding operating band.

[0025] The algorithm control unit stores a backup reference record, which includes a reference operating band identifier and a target reference frequency. During the initial calibration, an initial backup reference record is established based on a pre-selected reference operating band; in this embodiment, the initial backup reference record is preferentially established using the 1603nm operating band.

[0026] When either the carbon dioxide or methane absorption peak meets the reference conditions, the algorithm control unit determines the center frequency of the corresponding absorption peak as the target reference frequency and updates the reference operating band identifier and target reference frequency in the backup reference record using the center frequency. When neither the carbon dioxide nor the methane absorption peak meets the reference conditions, the most recently valid backup reference record is retrieved, and the target reference frequency stored therein is determined as the current target reference frequency. The target optical cavity longitudinal mode resonance point and cavity length adjustment are selected only within the reference operating band indicated by the backup reference record. Thus, only one piezoelectric ceramic actuator drive voltage adjustment is generated within the same measurement cycle.

[0027] Furthermore, multiple spectral measurement points are selected at the baseline positions of the carbon dioxide absorption spectrum and the methane absorption spectrum, respectively; the noise amplitude of the corresponding absorption spectrum is determined based on the absorption signals of the selected multiple spectral measurement points; and the signal-to-noise ratio of the carbon dioxide absorption spectrum and the methane absorption spectrum is determined based on the ratio of the absorption signal amplitude to the noise amplitude of the corresponding absorption spectrum, respectively.

[0028] Furthermore, when the signal-to-noise ratio (SNR) of both the carbon dioxide absorption spectrum and the methane absorption spectrum is less than 5, the target reference object is switched to the wavelength reference provided by the pre-calibrated etalon. If the aforementioned conditions are not met, the current target reference object is a carbon dioxide absorption peak, and the SNR of 120 consecutive frames of carbon dioxide absorption spectra is less than 5 while the SNR of the methane absorption spectrum is not less than 5, the target reference object is switched to the methane absorption peak. If the current target reference object is a methane absorption peak, and the SNR of 120 consecutive frames of carbon dioxide absorption spectra is greater than 10, the target reference object is switched to the carbon dioxide absorption peak.

[0029] When the current target reference is the methane absorption peak and the signal-to-noise ratio of the methane absorption spectrum is less than 5, while the signal-to-noise ratio of the carbon dioxide absorption spectrum is greater than 5, the target reference is directly switched to the carbon dioxide absorption peak, without requiring the carbon dioxide absorption spectrum to be greater than 10 for 120 consecutive frames; when the signal-to-noise ratio of both absorption spectra is less than 5, the reference is switched to the etalon wavelength.

[0030] When the current target reference is an etalon wavelength reference, the system exits the etalon reference state as long as the signal-to-noise ratio (SNR) of at least one of the absorption spectra of carbon dioxide and methane is greater than 5. If the SNR of the carbon dioxide absorption spectrum is greater than 5, the carbon dioxide absorption peak is preferentially used; if the SNR of the carbon dioxide absorption spectrum is not greater than 5 but the SNR of the methane absorption spectrum is greater than 5, the methane absorption peak is used.

[0031] The priority of the carbon dioxide absorption peak, methane absorption peak, and standard etalon wavelength reference decreases in that order. The parameters of 120 consecutive frames, a signal-to-noise ratio (SNR) threshold of 5, and a SNR threshold of 10 are determined based on the equipment's operating conditions. By employing different exit and recovery thresholds, and combining this with judgment across multiple consecutive frames, frequent switching of the reference object caused by SNR fluctuations near the critical value can be reduced.

[0032] A carbon dioxide absorption spectrum refers to a carbon dioxide absorption spectrum constructed from the ring-down signal and wavelength data obtained by a 1603nm butterfly DFB laser completing a target frequency band scan; a methane absorption spectrum refers to a methane absorption spectrum constructed from the ring-down signal and wavelength data obtained by a 1651nm butterfly DFB laser completing a target frequency band scan.

[0033] When the gas absorption peak meets the reference conditions, the algorithm control unit determines the wavelength correction amount for the current working band based on the difference between the known center wavelength of the gas absorption peak and the center wavelength of the absorption peak currently determined by the etalon, and uses the wavelength correction amount to update the correspondence between the etalon transmittance and wavelength for the working band.

[0034] Furthermore, the correspondence between the piezoelectric ceramic actuator driving voltage and the cavity length change is pre-calibrated in the following way: keeping the laser output frequency constant, a triangular wave scanning voltage is applied to the piezoelectric ceramic actuator, causing the piezoelectric ceramic actuator to reciprocate along the optical cavity axis based on the inverse piezoelectric effect, and driving the second supermirror to reciprocate along the optical cavity axis through the bonding interface between the actuator and the second supermirror; multiple optical cavity resonance peaks formed during the movement of the second supermirror are collected by a photodetector, and the piezoelectric ceramic actuator driving voltage corresponding to each optical cavity resonance peak is determined; based on the frequency interval between two adjacent optical cavity resonance peaks as the free spectral range of the optical cavity, the correspondence between the piezoelectric ceramic actuator driving voltage and the change of the optical cavity longitudinal mode resonance frequency is established; based on the relationship between the optical cavity longitudinal mode resonance frequency and the physical cavity length, the correspondence between the piezoelectric ceramic actuator driving voltage and the change of the optical cavity longitudinal mode resonance frequency is converted into the correspondence between the piezoelectric ceramic actuator driving voltage and the cavity length change.

[0035] In another embodiment, the calibration can also be performed using a sawtooth wave scanning voltage with a linear scanning segment. The calibration coefficient between the piezoelectric ceramic actuator drive voltage and the axial displacement is the equivalent average coefficient within the corresponding calibration voltage range; during the cavity length locking process, the effects of nonlinearity and hysteresis of the piezoelectric ceramic actuator are reduced by limiting the single adjustment amount and performing multi-cycle successive approximation.

[0036] After completing the cavity length adjustment for this measurement cycle, the algorithm control unit determines whether the updated drive voltage has reached the travel boundary. If the travel boundary has not been reached, the saved longitudinal mode resonance point position of the optical cavity is retained and the next measurement cycle is directly entered; if the travel boundary is reached, the drive voltage is restored to 75V, the longitudinal mode position saved in the previous measurement cycle is cleared, the longitudinal mode resonance point of the optical cavity is rescanned and initialized within the complete frequency tuning range, and then the next measurement cycle is entered.

[0037] The present invention also provides a cavity ring-down gas measurement system with cavity length locking function, including a laser, a mode matching lens group, an isolator, a first mirror and a second mirror are sequentially arranged on the output optical path of the laser; an optical cavity is arranged on the reflected optical path of the second mirror, the optical cavity includes a first super-reflecting mirror and a second super-reflecting mirror arranged opposite to each other along the optical cavity axis, and a resonant space for accommodating the gas to be measured is formed between the first super-reflecting mirror and the second super-reflecting mirror.

[0038] The second supermirror has a detection component on its exit side and a piezoelectric ceramic actuator on its incident side. The piezoelectric ceramic actuator is bonded to the second supermirror along the optical cavity axis, and its extension and retraction direction is consistent with the optical cavity axis. The piezoelectric ceramic actuator is connected to a high-voltage driver. When the high-voltage driver changes the driving voltage applied to the piezoelectric ceramic actuator, the piezoelectric ceramic actuator extends or shortens along the optical cavity axis based on the inverse piezoelectric effect, and transmits the axial displacement to the second supermirror through the bonding interface.

[0039] The system also includes a etalon and an algorithm control unit. The algorithm control unit includes a processor, a memory, and signal interfaces that are connected to the laser, the detection component, the etalon, and the high-voltage driver, respectively. The memory stores control programs, spectral data, reference switching parameters, and calibration relationships of the piezoelectric ceramic actuators that can be executed by the processor.

[0040] The connection between the algorithm control unit and the etalon, as referred to in this specification, means that the algorithm control unit establishes a signal connection with the transmission photodetector and the reflection photodetector used to collect the transmitted light signal and reflected light signal of the etalon, rather than that the algorithm control unit directly forms an electrical connection with the passive etalon body.

[0041] The processor executes the control program to control the laser to perform frequency scanning and shutdown, receives the ring-down signal collected by the detection component and the wavelength data obtained by the etalon, performs ring-down time fitting, absorption spectrum construction, signal-to-noise ratio calculation, target reference object determination, target reference frequency determination, target optical cavity longitudinal mode resonance point matching, frequency deviation calculation and piezoelectric ceramic actuator drive voltage adjustment, and outputs the corresponding control signal to the high-voltage driver through the signal interface.

[0042] The memory also stores the backup reference record, the calibration relationship between the transmittance and wavelength of the etalon corresponding to each working band, and the position of the longitudinal mode resonance point of the optical cavity obtained in the previous measurement cycle.

[0043] Furthermore, the pattern matching lens group includes a collimator, a first lens, and a second lens arranged sequentially along the emission direction of the laser; the detection component includes a receiving mirror and a photodetector, the receiving mirror being disposed on the emission light path of the second super-reflecting mirror, the photodetector being disposed on the reflection light path of the receiving mirror, and the photodetector being connected to the algorithm control unit.

[0044] The laser, collimator, first lens, second lens, isolator, first reflector, second reflector, first supermirror, second supermirror, and photodetector can all be existing optical devices adapted to the target laser wavelength. These optical devices can be mounted on an optical bracket, adjustable mirror mount, or optical cavity mount, and adjusted by translation, pitch, or deflection to match the laser propagation direction, spot size, and divergence state with the resonant mode of the optical cavity. The substrate material, radius of curvature, coating structure, and reflectivity of the first and second supermirrors can be set according to the optical cavity stability conditions, operating wavelength, and measurement sensitivity.

[0045] Furthermore, the high-voltage driver includes a 16-bit digital-to-analog converter and a voltage amplifier circuit. The input terminal of the 16-bit digital-to-analog converter is connected to the algorithm control unit, and the output terminal of the 16-bit digital-to-analog converter is connected to the piezoelectric ceramic actuator via the voltage amplifier circuit. The maximum driving voltage of the piezoelectric ceramic actuator is 150V, and the maximum axial stroke is 3.9μm. When the system is powered on, the algorithm control unit controls the high-voltage driver to apply an initial driving voltage of 75V to the piezoelectric ceramic actuator.

[0046] This invention utilizes the absorption spectra of carbon dioxide and methane of the gas to be measured within the same optical cavity as candidate references. When neither absorption spectrum meets the reference conditions, a wavelength reference provided by a standard etalon is used. Hysteresis switching is achieved through continuous multi-frame judgment and different exit and recovery thresholds, reducing repeated switching of the reference object near the critical signal-to-noise ratio. Simultaneously, the target reference frequency is matched with the longitudinal mode resonance point of the specific optical cavity. The driving voltage adjustment is determined based on the frequency deviation, the relationship between the longitudinal mode resonance frequency and the cavity length, and the calibration relationship of the piezoelectric ceramic actuator. This causes the piezoelectric ceramic actuator to perform axial extension and directly drive the second super-reflecting mirror to move, thus forming a closed-loop adjustment process that connects reference selection, longitudinal mode positioning, and cavity length compensation.

[0047] This reduces the impact of cavity length drift on the sampling position of the absorption peak and the stability of gas concentration measurement during long-term operation of the optical cavity. In continuous measurement of approximately 397 ppm carbon dioxide standard gas, the deviation of the absorption peak center after locking is mainly maintained within the range of approximately ±2 MHz, and the long-term unidirectional change of the concentration measurement results is correspondingly reduced. Attached Figure Description

[0048] Figure 1 This is a flowchart of the optical cavity length locking method based on gas absorption spectral lines according to the present invention;

[0049] Figure 2 This is a graph showing the shift of the absorption peak center as a function of sampling points when the cavity length is not locked.

[0050] Figure 3 A graph showing the variation of absorption peak center deviation with sampling points after cavity length locking;

[0051] Figure 4 A graph showing the change of carbon dioxide concentration measurement results over time after optical cavity length locking;

[0052] Figure 5 The graph shows the change of carbon dioxide concentration over time without optical cavity length locking.

[0053] Figure 6 This is a schematic diagram of the optical cavity ring-down gas measurement system with cavity length locking function of the present invention;

[0054] Figure 7 This is a schematic diagram of the optical path for wavelength detection of the etalon in this invention.

[0055] In the diagram: 11. Laser; 20. Pattern-matching lens group; 21. Collimator; 22. First lens; 23. Second lens; 31. Isolator; 32. First mirror; 33. Second mirror; 40. Optical cavity; 41. First super-reflecting mirror; 42. Second super-reflecting mirror; 43. Piezoelectric ceramic actuator; 51. Algorithm control unit; 52. High-voltage driver; 60. Detection assembly; 61. Photodetector; 62. Receiving mirror; 71. Enotype collimator; 72. Half-wave plate; 73. First etalon mirror; 74. Second etalon mirror; 75. Enotype; 76. Transmission photodetector; 77. Reflection photodetector. Detailed Implementation

[0056] Example 1

[0057] like Figure 6 As shown, this embodiment provides a cavity ring-down gas measurement system with cavity length locking function. The system includes a laser 11, and a mode-matching lens group 20, an isolator 31, a first reflector 32, and a second reflector 33 are sequentially arranged on the output optical path of the laser 11. An optical cavity 40 is arranged on the reflected optical path of the second reflector 33. A detection component 60 is arranged on the output side of the optical cavity 40, and a piezoelectric ceramic actuator 43 for adjusting the cavity length is arranged on the cavity mirror of the optical cavity 40. The system also includes an etalon 75 for providing wavelength data, a high-voltage driver 52 for driving the piezoelectric ceramic actuator 43, and an algorithm control unit 51 that establishes signal communication with the laser 11, the detection component 60, the etalon 75, and the high-voltage driver 52, respectively.

[0058] The laser 11 in this embodiment includes two butterfly-shaped DFB lasers and a MEMS optical switch. One butterfly-shaped DFB laser operates at a wavelength of 1603 nm and is used to scan the frequency band containing the carbon dioxide absorption line; the other butterfly-shaped DFB laser operates at a wavelength of 1651 nm and is used to scan the frequency band containing the methane absorption line, and can also be used to measure the water absorption signal. The two butterfly-shaped DFB lasers are respectively connected to the MEMS optical switch, and the selection state of the MEMS optical switch is controlled by the algorithm control unit 51, so that the two butterfly-shaped DFB lasers can share the same measurement optical path and the same optical cavity 40 in a time-division multiplexing manner. The center wavelength of the target absorption line for carbon dioxide measurement is 1603.2266 nm, and the center wavelength of the target absorption line for methane measurement is 1650.9606 nm. Since the water absorption signal is relatively weak, and the corresponding application scenario usually involves dehydration treatment of the gas to be measured, this embodiment does not use the water absorption peak as a cavity length locking reference.

[0059] The mode-matching lens group 20 includes a collimator 21, a first lens 22, and a second lens 23 arranged sequentially along the laser propagation direction. The measurement laser selected by the MEMS optical switch is first collimated by the collimator 21, and then the first lens 22 and the second lens 23 are used for beam shaping to match the beam waist position, beam waist radius, and divergence angle of the incident beam with the resonant mode of the optical cavity 40. The collimator 21, the first lens 22, and the second lens 23 can be fiber collimators, plano-convex lenses, biconvex lenses, aspherical lenses, or other lenses with corresponding optical parameters suitable for the corresponding operating wavelength. Their focal length, aperture, coating band, and adjacent spacing can be determined according to the laser beam parameters and the optical cavity mode-matching conditions.

[0060] An isolator 31 is disposed in the output optical path of the mode-matching lens group 20 to suppress reflected light from subsequent optical elements from returning to the laser 11. A first reflector 32 is disposed in the output optical path of the isolator 31, and a second reflector 33 is disposed in the reflected optical path of the first reflector 32. The first reflector 32 and the second reflector 33 together change the propagation direction of the measurement laser, so that the measurement laser after mode matching and isolation is incident along the axis of the optical cavity 40. The first reflector 32 and the second reflector 33 can be dielectric film reflectors suitable for the 1603nm and 1651nm operating wavelength bands, and can be mounted on a reflector frame with pitch and yaw adjustment functions to adjust the coaxiality between the measurement laser and the optical cavity 40.

[0061] The optical cavity 40 includes a first supermirror 41 and a second supermirror 42 arranged opposite each other along the cavity axis, forming a resonant space between them to accommodate the gas to be measured. The first supermirror 41 is located at the incident end of the optical cavity 40, and the second supermirror 42 is located at the exit end of the optical cavity 40. Both can be high-reflectivity cavity mirrors with a reflectivity greater than 99.99%. After the measurement laser is coupled into the optical cavity 40 through the first supermirror 41, it propagates back and forth between the first supermirror 41 and the second supermirror 42 multiple times, thereby forming a relatively long equivalent absorption optical path within the limited physical cavity length.

[0062] The optical cavity 40 and its corresponding measurement optical path are housed in a constant-temperature space. The set temperature of the constant-temperature space is 45℃, and the temperature fluctuation is controlled within ±0.002℃, ensuring that the optical cavity 40 operates under the corresponding constant-temperature conditions. The optical cavity 40 is also mounted on the system base via soft silicone supports or other flexible vibration isolation structures to reduce the transmission of environmental vibrations to the optical cavity 40. While the constant-temperature and vibration isolation measures can reduce environmental disturbances, they cannot completely eliminate the slow drift in cavity length caused by thermal expansion and contraction of the cavity, stress release of the support structure, and long-term changes in the device. Therefore, the remaining cavity length variation is still compensated by a piezoelectric ceramic actuator 43.

[0063] The gas to be tested enters the resonant space through the inlet of the optical cavity 40 and exits through the outlet of the optical cavity 40. A pressure sensor for detecting the pressure of the gas to be tested is located at the outlet of the optical cavity 40, and a proportional valve for pressure regulation is located between the outlet of the optical cavity 40 and the pump. The algorithm control unit 51 adjusts the opening of the proportional valve according to the detection result of the pressure sensor to control the pressure fluctuation of the gas to be tested in the optical cavity 40 within ±5Pa, thereby reducing the influence of changes in the refractive index and spectral parameters of the gas to be tested on the spectral measurement and cavity length compensation.

[0064] A detection component 60 is provided on the output side of the second supermirror 42. The detection component 60 includes a receiving mirror 62 and a photodetector 61. The receiving mirror 62 is disposed in the output optical path of the second supermirror 42, and the photodetector 61 is disposed in the reflected optical path of the receiving mirror 62. After the measurement laser is turned off, the light energy in the optical cavity 40 gradually decays. The ring-down light output by the second supermirror 42 is reflected by the receiving mirror 62 to the photodetector 61. The photodetector 61 converts the ring-down light intensity into an electrical signal and sends it to the algorithm control unit 51.

[0065] The piezoelectric ceramic actuator 43 is disposed on the incident side of the second supermirror 42 along the axial direction of the optical cavity 40. The piezoelectric ceramic actuator 43 is generally annular, with a light-passing hole formed at its center for the measurement laser to pass through. The optical cavity tube, the piezoelectric ceramic actuator 43, and the second supermirror 42 are arranged concentrically and coaxially. One axial end of the piezoelectric ceramic actuator 43 is connected to the optical cavity tube, and the other axial end is bonded to the non-effective light-passing area on the incident side of the second supermirror 42 by optical epoxy resin, thereby avoiding the piezoelectric ceramic actuator 43 and its bonding area from blocking the measurement beam.

[0066] The piezoelectric ceramic actuator 43 utilizes the inverse piezoelectric effect to convert changes in driving voltage into axial extension and contraction displacement. When the driving voltage applied to the piezoelectric ceramic actuator 43 increases, the piezoelectric ceramic actuator 43 extends along the optical cavity axis and drives the second super-reflecting mirror 42 to move away from the optical cavity tube, thereby increasing the physical distance between the first super-reflecting mirror 41 and the second super-reflecting mirror 42; when the driving voltage decreases, the piezoelectric ceramic actuator 43 shortens along the optical cavity axis and drives the second super-reflecting mirror 42 to move closer to the optical cavity tube, thereby reducing the physical cavity length of the optical cavity 40.

[0067] The high-voltage driver 52 includes a 16-bit digital-to-analog converter (DAC) and a voltage amplifier circuit. The input of the DAC is connected to the algorithm control unit 51, and the output of the DAC is connected to the piezoelectric ceramic actuator 43 via the voltage amplifier circuit. The piezoelectric ceramic actuator 43 has a maximum driving voltage of 150V and a maximum axial travel of 3.9μm. With the DAC, its theoretical digital subdivision step is approximately 0.059nm.

[0068] In addition to the main measurement optical path used to generate the cavity ring-down signal, the system also includes an etalon wavelength detection module. For example... Figure 7 As shown, the etalon wavelength detection module includes an etalon collimator 71, a half-wave plate 72, a first etalon reflector 73, a second etalon reflector 74, an etalon 75, a transmission photodetector 76, and a reflection photodetector 77.

[0069] The monitoring light output from the laser selected by the MEMS optical switch enters the etalon wavelength detection module and propagates sequentially through the etalon collimator 71, the half-wave plate 72, the first etalon reflector 73, and the second etalon reflector 74 to the etalon 75.

[0070] After the laser beam is incident on the etalon 75, a portion of the laser beam passes through the etalon 75 and is received by the transmission photodetector 76 located on the transmission side of the etalon 75; the other portion of the laser beam is reflected by the incident surface of the etalon 75 and travels along... Figure 7 The reflected light path shown propagates to the reflective photodetector 77. The transmission photodetector 76 and the reflective photodetector 77 are respectively connected to the algorithm control unit 51, and output the transmission detection voltage and the reflection detection voltage respectively.

[0071] Algorithm control unit 51 calculates the transmittance of the standard etalon according to the following formula:

[0072] ;

[0073] In the formula, Standard transmittance; The transmission detection voltage output by the transmission photodetector 76; This represents the reflection detection voltage output by the reflection photodetector 77. The transmittance of the etalon is characterized by the ratio of the transmission detection voltage to the sum of the transmission and reflection detection voltages, which can reduce the influence of changes in the total input laser power on the wavelength determination result.

[0074] After the etalon wavelength detection module is initially assembled and adjusted, an external wavelength meter is used to perform complete laser scans in the 1603nm and 1651nm working bands, respectively. The transmittance and wavelength of the etalon at each sampling position are recorded simultaneously during the scan, and calibration relationships for the corresponding working bands are established according to the scanning sequence. During gas measurement, the algorithm control unit 51 determines the wavelength data corresponding to the current ringing-down signal from the corresponding calibration relationships based on the current working band of the selected laser, the current scanning position, and the wavelength continuity between adjacent measurement points.

[0075] The standard fixture 75 is subjected to high-precision temperature control during operation, with temperature fluctuations controlled within ±1mK.

[0076] During the initial calibration, the algorithm control unit 51 establishes a backup reference record, which includes a reference working band identifier and a target reference frequency. In this embodiment, the initial backup reference record is established with a predetermined reference frequency of the 1603nm working band.

[0077] When the carbon dioxide absorption peak or the methane absorption peak meets the reference conditions, the algorithm control unit 51 corrects the etalon calibration relationship of the current working band according to the known center wavelength of the corresponding absorption peak, and updates the backup reference record with the center frequency of the corresponding absorption peak. When neither of the two gas absorption peaks can provide a stable reference, the etalon wavelength detection module continues to determine the frequency coordinates of the current optical cavity longitudinal mode resonance point, and the algorithm control unit 51 calls the target reference frequency stored in the backup reference record and calculates the frequency deviation within the reference working band indicated by the backup reference record.

[0078] The connection between the algorithm control unit 51 and the etalon 75 in this embodiment refers to the establishment of a signal connection between the algorithm control unit 51 and the transmission photodetector 76 and the reflection photodetector 77, rather than the direct electrical connection between the algorithm control unit 51 and the passive etalon 75 body.

[0079] The algorithm control unit 51 can be composed of an industrial computer, an embedded processor, a microcontroller, a digital signal processor, a field-programmable gate array, or a combination of these devices. The algorithm control unit 51 receives the detection signals output by the photodetector 61, the transmission photodetector 76, and the reflection photodetector 77, and sends control signals to the laser 11, the MEMS optical switch, and the high-voltage driver 52 to perform laser scanning, ring decay signal acquisition, etalon wavelength calculation, absorption spectrum construction, reference object determination, longitudinal mode matching, frequency deviation calculation, and cavity length compensation.

[0080] Example 2

[0081] like Figure 1 As shown, this embodiment provides a method for locking the cavity length of an optical cavity based on gas absorption spectral lines. The method can be executed by the optical cavity ring-down gas measurement system described in Embodiment 1.

[0082] After system startup, the algorithm control unit 51 controls the high-voltage driver 52 to apply an initial drive voltage of 75V to the piezoelectric ceramic actuator 43. Since the maximum drive voltage of the piezoelectric ceramic actuator 43 is 150V, setting the initial drive voltage to 75V allows the piezoelectric ceramic actuator 43 to retain adjustable stroke in both the extension and contraction directions. Simultaneously, the temperature of the optical cavity 40 and the constant-temperature space containing the measurement optical path is controlled at 45℃, with temperature fluctuations controlled within ±0.002℃. Furthermore, pressure fluctuations of the gas to be measured within the optical cavity 40 are controlled within ±5Pa using a pressure sensor and a proportional valve.

[0083] The algorithm control unit 51 also loads the etalon calibration relationship, backup reference record, and calibration coefficient of the piezoelectric ceramic actuator 43 corresponding to the 1603nm and 1651nm working bands during the initialization process.

[0084] After the initialization is completed, the algorithm control unit 51 controls the MEMS optical switch to select two butterfly DFB lasers in a time-division multiplexing manner. During the spectral measurement, the injection current of the currently selected butterfly DFB laser is changed to scan the laser frequency, while the piezoelectric ceramic actuator 43 maintains its current driving voltage. The absorption spectrum is obtained by scanning the laser frequency, rather than by scanning the piezoelectric ceramic actuator 43 to change the cavity length. The piezoelectric ceramic actuator 43 is only adjusted after the absorption spectrum of the current period, the target reference frequency is determined, and the frequency deviation is calculated.

[0085] The algorithm control unit 51 first controls the 1603nm butterfly DFB laser to scan the frequency band where the carbon dioxide absorption spectral line with a center wavelength of 1603.2266nm is located, and then controls the 1651nm butterfly DFB laser to scan the frequency band where the methane absorption spectral line with a center wavelength of 1650.9606nm is located.

[0086] The laser has a frequency tuning range of 7.5 GHz near each target absorption line, and the free spectral range of the optical cavity 40 is 500 MHz. The algorithm control unit 51 selects 15 complete longitudinal mode resonance points near the target absorption peak, capable of ring-down fitting, from the cavity mode signals acquired within the frequency tuning range as spectral measurement points. If there are incompletely acquired resonance signals at the edge of the scanning range, they are not included in the 15 longitudinal mode resonance points.

[0087] When the current laser frequency resonates with a longitudinal mode of the optical cavity 40, laser energy is coupled into the optical cavity 40 via the first super-reflecting mirror 41. After the ring-down signal acquisition condition is met, the algorithm control unit 51 shuts off the currently selected butterfly DFB laser, and the photodetector 61 acquires the ring-down signal output from the optical cavity 40. Simultaneously with acquiring the ring-down signal, the algorithm control unit 51 reads the transmission detection voltage output by the transmission photodetector 76. and the reflection detection voltage output by the reflection photodetector 77 And calculate the transmittance of the standard etalon. .

[0088] The algorithm control unit 51 determines the wavelength data corresponding to the current ring-down signal in the calibration relationship of the corresponding working band based on the current working band of the selected laser, the current laser scanning position, and the wavelength continuity between adjacent measurement points.

[0089] The longitudinal mode resonant frequency of the optical cavity satisfies:

[0090] ;

[0091] In the formula, For the first The resonant frequency of the longitudinal mode of the optical cavity; It is the vertical module number, and it is an integer; The speed of light in a vacuum; The refractive index of the gas to be measured inside the optical cavity 40; Let be the physical cavity length between the first super-reflecting mirror 41 and the second super-reflecting mirror 42. From the above relationship, it can be seen that when the longitudinal mode number and the refractive index of the gas being measured remain constant, the longitudinal mode resonant frequency of the optical cavity decreases with increasing physical cavity length and increases with decreasing physical cavity length.

[0092] For each longitudinal mode resonance point of the optical cavity, the algorithm control unit 51 performs exponential fitting on the corresponding ring-down signal. The ring-down intensity can be expressed as:

[0093] ;

[0094] In the formula, The moment after the laser is turned off The intensity of the decaying light; The initial light intensity at the onset of decay; The obtained ring-down time is used for fitting. The frequency coordinates are determined based on the wavelength data corresponding to the longitudinal mode resonance points of each optical cavity, and the absorption signal at the corresponding frequency coordinates is determined based on the fitted ring-down time. The carbon dioxide absorption spectrum and the methane absorption spectrum are formed by multiple frequency coordinates and their corresponding absorption signals, respectively.

[0095] In this embodiment, a carbon dioxide absorption spectrum refers to a carbon dioxide absorption spectrum constructed from multiple ring-down signals and wavelength data obtained by a 1603nm butterfly DFB laser completing a target frequency band scan; a methane absorption spectrum refers to a methane absorption spectrum constructed from multiple ring-down signals and wavelength data obtained by a 1651nm butterfly DFB laser completing a target frequency band scan.

[0096] During continuous measurement, the algorithm control unit 51 uses the frequency positions of the 15 effective optical cavity longitudinal mode resonance signals obtained in the previous measurement cycle as the center, and searches for the corresponding optical cavity longitudinal mode resonance signal in the current cycle within a range of ±10MHz at each frequency position. Since the search range of ±10MHz is significantly smaller than the 500MHz interval between adjacent longitudinal modes, it can continuously track the corresponding longitudinal mode resonance signal, reducing the possibility of misidentifying adjacent longitudinal modes as the current longitudinal mode.

[0097] The algorithm control unit 51 selects multiple spectral measurement points at the baseline positions of the carbon dioxide absorption spectrum and the methane absorption spectrum, respectively. Based on the absorption signal fluctuations at the selected spectral measurement points, it determines the noise amplitude of the corresponding absorption spectrum. Then, it determines the signal-to-noise ratio of the corresponding absorption spectrum based on the ratio of the absorption peak signal amplitude to the noise amplitude. The baseline positions are spectral regions that avoid the main absorption areas of the target absorption peak.

[0098] After determining the signal-to-noise ratio of the carbon dioxide absorption spectrum and the methane absorption spectrum, the algorithm control unit 51 selects the target reference object according to the priority order of carbon dioxide absorption peak, methane absorption peak, and etalon wavelength reference. The carbon dioxide target absorption line is a relatively independent single absorption line, and its absorption peak center fitting accuracy is relatively high; there are other weak methane absorption lines near the methane target absorption peak, and the influence of multiple peaks may increase the center fitting error; the etalon 75 may be affected by its own aging and deformation when it is removed from the gas absorption peak correction for a long time, so the etalon wavelength reference is set as the lowest priority backup reference.

[0099] When the signal-to-noise ratio of both the carbon dioxide absorption spectrum and the methane absorption spectrum is less than 5, the absorption peaks of the two gases are insufficient to provide a stable reference. The algorithm control unit 51 determines the etalon wavelength reference that has been most recently corrected as the target reference.

[0100] When using a standard etalon wavelength reference, the algorithm control unit 51 calls the backup reference record, determines the target reference frequency stored therein as the current target reference frequency, and selects the target optical cavity longitudinal mode resonance point from the 15 optical cavity longitudinal mode resonance points in the corresponding operating band according to the reference operating band identifier. The wavelength data currently output by the standard etalon wavelength detection module is used to determine the current longitudinal mode frequency, and is not directly used as the target reference frequency.

[0101] When the current target reference is the carbon dioxide absorption peak, if the signal-to-noise ratio of the carbon dioxide absorption spectrum for 120 consecutive frames is less than 5, while the signal-to-noise ratio of the methane absorption spectrum is not less than 5, then the target reference will be switched from the carbon dioxide absorption peak to the methane absorption peak; if the signal-to-noise ratio of both the carbon dioxide absorption spectrum and the methane absorption spectrum is less than 5, then the reference will be switched to the etalon wavelength.

[0102] When the current target reference is the methane absorption peak, if the methane absorption spectrum still meets the reference condition of a signal-to-noise ratio (SNR) of not less than 5, then when the SNR of the carbon dioxide absorption spectrum is greater than 10 for 120 consecutive frames, the target reference will be restored to the carbon dioxide absorption peak. When the SNR of the methane absorption spectrum drops below 5, while the SNR of the carbon dioxide absorption spectrum is greater than 5, since the current methane absorption peak can no longer provide a valid reference, the target reference will be directly switched to the carbon dioxide absorption peak, and the requirement that the carbon dioxide absorption spectrum be greater than 10 for 120 consecutive frames will no longer apply. When the SNR of both absorption spectra is less than 5, the reference will be switched to the etalon wavelength.

[0103] When the current target reference is an etalon wavelength reference, the system exits the etalon reference state as long as the signal-to-noise ratio (SNR) of at least one of the absorption spectra of carbon dioxide and methane is greater than 5. If the SNR of the carbon dioxide absorption spectrum is greater than 5, the carbon dioxide absorption peak is preferentially selected; if the SNR of the carbon dioxide absorption spectrum is not greater than 5 but the SNR of the methane absorption spectrum is greater than 5, the methane absorption peak is selected. Simultaneously, the known center wavelength of the selected gas absorption peak is used to correct the correspondence between the etalon transmittance and wavelength.

[0104] The 120 consecutive frames, SNR threshold of 5, and SNR threshold of 10 are all empirical parameters determined based on equipment operation. When the SNR is greater than 5, the gas absorption signal can be clearly identified; when the SNR is greater than 10, the accuracy of the absorption peak center fitting can be further guaranteed. If only one switching threshold is used, the target reference object may frequently switch back and forth when the gas absorption signal fluctuates around that threshold. By setting different exit and recovery thresholds, and combining this with the 120 consecutive frames for judgment, a hysteresis switching process can be formed, reducing the impact of frequent changes in the reference object on cavity length stability.

[0105] When the target reference is a carbon dioxide absorption peak or a methane absorption peak, the Voigt line shape function is used to fit the corresponding absorption spectrum. The Voigt line shape is formed by the convolution of a Gaussian line shape and a Lorentz line shape.

[0106] ;

[0107] The Gaussian line type is as follows:

[0108] ;

[0109] The Lorentz line type is:

[0110] ;

[0111] In the formula, For frequency variables; The center frequency of the absorption peak; The full width at half maximum (FWHM) is Gaussian. The full width is half the height of the Lorentz curve. It is a Gaussian linear function; It is a Lorentz linear function; This is the Voigt linear function.

[0112] The algorithm control unit 51 determines the center frequency of the fitted carbon dioxide absorption peak or methane absorption peak as the target reference frequency. At the same time, the algorithm control unit 51 corrects the etalon calibration relationship of the current working band according to the known center wavelength of the corresponding gas absorption peak, and updates the backup reference record using the current working band identifier and the absorption peak center frequency.

[0113] After determining the target reference frequency, the algorithm control unit 51 calculates the frequency distance between the 15 longitudinal mode resonance points of the optical cavity and the target reference frequency within the current measurement cycle, and determines the longitudinal mode resonance point of the optical cavity with the smallest frequency distance as the target longitudinal mode resonance point. The frequency deviation between the target longitudinal mode resonance point and the target reference frequency is:

[0114] ;

[0115] In the formula, The frequency deviation between the longitudinal mode resonance point of the target optical cavity and the target reference frequency; Target reference frequency; The current frequency of the longitudinal mode resonance point of the target optical cavity.

[0116] During a single cavity length adjustment, the longitudinal mode number of the optical cavity remains unchanged, and under isothermal and isobaric conditions, the change in the refractive index of the gas under test within one adjustment cycle is negligible. Based on the relationship between the longitudinal mode resonant frequency and the physical cavity length, the required cavity length change can be expressed as:

[0117] ;

[0118] In the formula, The amount of cavity length change required to shift the longitudinal mode resonance point of the target optical cavity toward the target reference frequency; This is the current physical cavity length of optical cavity 40.

[0119] when When the value is greater than 0, the target reference frequency is higher than the current frequency of the target optical cavity's longitudinal mode resonance point, thus requiring an increase in the optical cavity's longitudinal mode resonance frequency. <0, the algorithm control unit 51 controls the piezoelectric ceramic actuator 43 to shorten, causing the second super-reflecting mirror 42 to move closer to the optical cavity tube. When When the value is less than 0, the target reference frequency is lower than the current frequency of the target optical cavity's longitudinal mode resonance point, thus requiring a reduction in the optical cavity's longitudinal mode resonance frequency. >0, the algorithm control unit 51 controls the piezoelectric ceramic actuator 43 to extend, so that the second super-reflecting mirror 42 moves away from the optical cavity tube.

[0120] In another calibration method, a sawtooth wave scanning voltage can be applied to the piezoelectric ceramic actuator 43, and calibration can be completed using the linear scanning segment of the sawtooth wave.

[0121] The algorithm control unit 51 synchronously acquires the scanning voltage signal of the piezoelectric ceramic actuator 43 and the cavity resonance peak signal output by the photodetector 61. As the second super-reflecting mirror 42 moves, the longitudinal mode resonance frequency of the cavity changes. When the longitudinal mode resonance frequency of the cavity passes the fixed output frequency of the laser 11, the photodetector 61 detects a cavity resonance peak.

[0122] Under calibration conditions, the refractive index of the medium inside the optical cavity Approximately 1, the cavity length variation corresponding to two adjacent cavity resonance peaks is:

[0123] ;

[0124] In the formula, This indicates that the incident laser wavelength remains constant during the calibration of the piezoelectric ceramic actuator; in this embodiment, it is taken as... .

[0125] In this embodiment, the voltage difference between the piezoelectric ceramic actuators corresponding to the resonance peaks of two adjacent optical cavities is approximately 26V. Therefore, the axial displacement per unit voltage of the piezoelectric ceramic actuator 43 is:

[0126] ;

[0127] In the formula, This represents the driving voltage difference corresponding to the resonance peaks of two adjacent optical cavities.

[0128] Based on the change in cavity length and the axial displacement per unit voltage, the adjustment amount of the driving voltage for the piezoelectric ceramic actuator 43 is as follows:

[0129] .

[0130] The algorithm control unit 51 performs cavity length adjustment whenever it detects a non-zero frequency deviation between the longitudinal mode resonance point of the target optical cavity and the target reference frequency. When the frequency deviation is zero or the driving voltage adjustment is zero after digital-to-analog conversion resolution quantization, the current driving voltage of the piezoelectric ceramic actuator 43 is maintained.

[0131] To avoid excessively large adjustments in a single measurement cycle, the absolute value of the frequency adjustment step size is limited to within 5MHz in each measurement cycle. When the absolute value of the frequency deviation is greater than 5MHz, the cavity length change and drive voltage adjustment are determined only according to 5MHz in this cycle. After completing the adjustment for this cycle, the absorption spectrum is rescanned, the target longitudinal mode is matched, and the frequency deviation is calculated. The frequency is then approached to the target reference frequency through multiple cycles.

[0132] The electromechanical response time of the piezoelectric ceramic actuator 43 from a change in driving voltage to the generation of axial displacement can reach the microsecond level. However, a complete cavity length locking cycle also includes two absorption spectral scans, ring-down signal acquisition, etalon wavelength calculation, absorption spectrum fitting, reference object determination, and driving voltage calculation. Therefore, the actual cavity length adjustment cycle of the system is approximately 1–2 seconds. Since the cavity length change of the optical cavity 40 is mainly manifested as a slow drift, the above adjustment cycle can meet the cavity length compensation requirements.

[0133] After adjusting the drive voltage in each measurement cycle, the algorithm control unit 51 determines whether the updated drive voltage has reached the travel boundary of 0V or 150V. If the travel boundary has not been reached, the positions of the 15 optical cavity longitudinal mode resonance points are retained, and the next measurement cycle is started directly.

[0134] When the driving voltage of the piezoelectric ceramic actuator 43 reaches the travel boundary of 0V or 150V, the algorithm control unit 51 controls the high-voltage driver 52 to restore the driving voltage to 75V and clear the positions of the 15 optical cavity longitudinal mode resonance points saved in the previous measurement cycle. After the driving voltage is restored to 75V, the cavity mode signal is rescanned within the complete 7.5GHz frequency tuning range, the 15 complete optical cavity longitudinal mode resonance points are reselected, and a new initial state for longitudinal mode tracking is established; after the re-initialization is completed, the next measurement cycle begins.

[0135] Because the total stroke of the piezoelectric ceramic actuator 43 is limited, a cavity length change will occur when the drive voltage returns to 75V. This process is used to restore the subsequent bidirectional adjustment capability, but cannot compensate for permanent cavity length changes that exceed the total stroke of the piezoelectric ceramic actuator 43.

[0136] The algorithm control unit 51 performs laser scanning, spectrum construction, reference object determination, target longitudinal mode matching, frequency deviation calculation and cavity length compensation in a cyclical manner according to the above process, so that the longitudinal mode resonance point of the target optical cavity is continuously kept near the target reference frequency.

[0137] Example 3

[0138] This embodiment illustrates the measurement results before and after locking the optical cavity length.

[0139] Continuous measurements were performed using a standard gas of carbon dioxide with a concentration of approximately 397 ppm. The positions of the absorption peak centers and the carbon dioxide concentration measurements were recorded for both cases with and without optical cavity length locking.

[0140] Both sets of comparative experiments used the same optical cavity ring-down gas measurement system and the same carbon dioxide standard gas, and employed the same isothermal conditions, optical cavity pressure control conditions, and spectral processing methods. The difference between the two sets lay in whether or not cavity length compensation for the piezoelectric ceramic actuator 43 was performed. The experimental data were not filtered, averaged, or outlier removed. Figure 2 and Figure 5 Originating from the same measurement in which cavity length locking was not performed, Figure 3 and Figure 4 The measurements were taken during the same cavity length locking test; the difference in the number of sampling points between the two sets of data is due to the different measurement durations in the two experiments. Figure 2 and Figure 3 Each sampling point in the diagram represents the center position of the absorption peak or the deviation of the absorption peak center obtained by fitting after a complete absorption spectrum measurement.

[0141] like Figure 2 As shown, without cavity length locking, the absorption peak center shift gradually increases from approximately 135MHz to approximately 195MHz with increasing sampling points, with a cumulative change of approximately 60MHz. The curve shows a continuous upward trend, indicating that vibration isolation, temperature control, and pressure control alone cannot completely eliminate the slow cavity length drift generated during long-term operation of the optical cavity 40.

[0142] like Figure 3 As shown, after cavity length locking, the absorption peak center deviation is mainly distributed between −2MHz and 2MHz, fluctuating around 0MHz. Figure 2 Compared to the cumulative offset of about 60MHz, the center deviation of the locked absorption peak was limited to the peak-to-peak range of about 4MHz, and there was no longer a continuous increase in unidirectional drift.

[0143] Figure 2 and Figure 3 The comparison shows that the algorithm control unit 51 does not only calibrate the optical cavity 40 once when the system starts, but periodically obtains the frequency deviation between the longitudinal mode resonance point of the target optical cavity and the target reference frequency during continuous measurement, and continuously corrects the position of the second super mirror 42 through the piezoelectric ceramic actuator 43, so that the longitudinal mode resonance point of the target optical cavity is kept near the target reference frequency.

[0144] like Figure 4 As shown, during the measurement of the optical cavity length locking, the carbon dioxide concentration mainly fluctuated around 397.68 ppm, and the curve did not show a significant continuous upward or downward trend. The change in this concentration curve is consistent with... Figure 3 The state corresponding to the center deviation of the absorption peak fluctuating around 0MHz corresponds to this.

[0145] like Figure 5 As shown, during the measurement process without optical cavity length locking, the carbon dioxide concentration gradually increased from approximately 397.65 ppm to nearly 397.90 ppm, with a cumulative change of approximately 0.25 ppm, exhibiting a trend of continuous change with increasing measurement time. The direction of this concentration change is consistent with... Figure 2 The direction of the shift in the center of the absorption peak is consistent.

[0146] Figures 2 to 5 This indicates that the long-term shift of the longitudinal mode resonance point of the optical cavity relative to the center of the gas absorption peak will be transmitted to the concentration inversion results. By determining the target reference frequency based on the absorption peak of carbon dioxide, the absorption peak of methane, or the wavelength reference of the etalon, continuously tracking the longitudinal mode resonance point of the optical cavity adjacent to the target reference frequency, and using the piezoelectric ceramic actuator 43 to drive the second super-reflecting mirror 42 to move along the axis of the optical cavity 40, the influence of the change in the cavity length on the absorption peak sampling position and the long-term measurement stability of the gas concentration can be reduced.

Claims

1. A cavity length locking method based on gas absorption spectral lines, applied to a cavity ring-down gas measurement system, characterized in that, include: S1. Control the laser (11) to scan the frequency bands where the carbon dioxide absorption spectrum and the methane absorption spectrum are located. When the laser frequency of the laser (11) resonates with the longitudinal mode of the optical cavity (40), the laser is turned off to collect the ring-down signal and obtain the wavelength data corresponding to the ring-down signal. S2. Based on the ring-down signal and wavelength data, obtain the carbon dioxide absorption spectrum and methane absorption spectrum of the gas to be measured from the optical cavity (40), respectively; S3. Based on the signal-to-noise ratio of the carbon dioxide absorption spectrum and the methane absorption spectrum, the current target reference object, and the preset hysteresis switching conditions, determine the target reference object from the carbon dioxide absorption peak, the methane absorption peak, and the wavelength reference provided by the pre-calibrated standard, and determine the target reference frequency based on the target reference object. The preset hysteresis switching conditions include a preset number of consecutive frames, a first threshold, and a second threshold that is higher than the first threshold; S4. From the multiple longitudinal mode resonance points of the optical cavity (40), determine the target optical cavity longitudinal mode resonance point whose frequency is closest to the target reference frequency, and determine the frequency deviation between the target optical cavity longitudinal mode resonance point and the target reference frequency. S5. Based on the frequency deviation, the relationship between the longitudinal mode resonance frequency of the optical cavity and the cavity length, and the pre-calibrated correspondence between the driving voltage of the piezoelectric ceramic actuator and the cavity length change, determine the driving voltage adjustment amount of the piezoelectric ceramic actuator (43), control the high voltage driver (52) to adjust the piezoelectric ceramic actuator (43) according to the driving voltage adjustment amount, so that the piezoelectric ceramic actuator (43) extends and retracts along the axial direction of the optical cavity (40), and drives the second super-reflecting mirror (42) bonded to it to move along the axial direction of the optical cavity (40), and repeat steps S1 to S5.

2. The method for locking the cavity length of an optical cavity based on gas absorption spectral lines according to claim 1, characterized in that, Before performing step S1, the driving voltage of the piezoelectric ceramic actuator (43) is set to 75V, so that the piezoelectric ceramic actuator (43) retains the adjustment stroke in the extension direction and the shortening direction respectively; The temperature of the optical cavity (40) is controlled at 45±0.002℃, and the pressure fluctuation inside the optical cavity (40) is controlled within ±5Pa.

3. The method for locking the cavity length of an optical cavity based on gas absorption spectral lines according to claim 1, characterized in that, Steps S1 and S2 include: By changing the injection current of the laser (11), the frequency bands of the carbon dioxide absorption spectrum and the methane absorption spectrum are scanned; Using the free spectral range of the optical cavity (40) as the frequency interval between the longitudinal mode resonance points of adjacent optical cavities, the absorption spectra of carbon dioxide and methane are measured point by point. Record the wavelength data fed back by the etalon when the laser (11) is turned off each time; Based on the wavelength data fed back by the etalon and the free spectral range of the optical cavity (40), the frequency coordinates of the longitudinal mode resonance points of each optical cavity are determined.

4. The method for locking the cavity length of an optical cavity based on gas absorption spectral lines according to claim 3, characterized in that, Step S2 includes: Exponential fitting is performed on each decay signal to obtain the decay time corresponding to the longitudinal mode resonance point of each optical cavity; In step S3, when the target reference is a carbon dioxide absorption peak or a methane absorption peak, the Voigt line-shape function, formed by the convolution of Gaussian and Lorentz line shapes, is used to fit the absorption spectrum corresponding to the target reference to obtain the center frequency of the corresponding absorption peak, and the center frequency of the absorption peak is determined as the target reference frequency.

5. The method for locking the cavity length of an optical cavity based on gas absorption spectral lines according to claim 4, characterized in that, Step S3 includes: Multiple spectral measurement points were selected at the baseline positions of the carbon dioxide absorption spectrum and the methane absorption spectrum, respectively. The noise amplitude of the corresponding absorption spectrum is determined based on the absorption signals from multiple selected spectral measurement points; The signal-to-noise ratio of carbon dioxide absorption spectrum and methane absorption spectrum is determined based on the ratio of the absorption signal amplitude to the noise amplitude of the corresponding absorption spectrum.

6. The method for locking the cavity length of an optical cavity based on gas absorption spectral lines according to claim 5, characterized in that, Step S3 includes: When the signal-to-noise ratio of both the carbon dioxide absorption spectrum and the methane absorption spectrum is less than 5, the target reference object is switched to the wavelength reference provided by the pre-calibrated etalon, and the frequency corresponding to the wavelength reference is determined as the target reference frequency. If the aforementioned conditions are not met, the current target reference is the carbon dioxide absorption peak, and the signal-to-noise ratio of 120 consecutive frames of carbon dioxide absorption spectra is less than 5 and the signal-to-noise ratio of the methane absorption spectrum is not less than 5, the target reference will be switched to the methane absorption peak. When the current target reference is the methane absorption peak, the target reference will be switched to the carbon dioxide absorption peak when the signal-to-noise ratio of 120 consecutive frames of carbon dioxide absorption spectra is greater than 10.

7. The method for locking the cavity length of an optical cavity based on gas absorption spectral lines according to claim 1, characterized in that, The relationship between the driving voltage of the piezoelectric ceramic actuator and the change in cavity length is pre-calibrated through the following steps: Keeping the output frequency of the laser (11) constant, a triangular wave scanning voltage is applied to the piezoelectric ceramic actuator (43) to cause the piezoelectric ceramic actuator (43) to stretch and deform along the axis of the optical cavity (40), and drive the second super mirror (42) bonded to it to move along the axis of the optical cavity (40). Multiple optical cavity resonance peaks formed during the movement of the second super-reflecting mirror (42) are collected by photodetector (61), and the driving voltage of the piezoelectric ceramic actuator corresponding to each optical cavity resonance peak is determined. Based on the frequency interval between two adjacent optical cavity resonance peaks being the free spectral range of the optical cavity (40), a correspondence is established between the driving voltage of the piezoelectric ceramic actuator and the change in the longitudinal mode resonance frequency of the optical cavity. Based on the relationship between the longitudinal mode resonant frequency of the optical cavity and the physical cavity length of the optical cavity (40), the correspondence between the driving voltage of the piezoelectric ceramic actuator and the change in the longitudinal mode resonant frequency of the optical cavity is converted into the correspondence between the driving voltage of the piezoelectric ceramic actuator and the change in the cavity length.

8. A cavity ring-down gas measurement system with cavity length locking function, characterized in that, Includes a laser (11), and the laser (11) has a pattern matching lens group (20), an isolator (31), a first reflector (32) and a second reflector (33) arranged sequentially on the output optical path; The second reflector (33) has a light cavity (40) in its reflected light path. The light cavity (40) includes a first super-reflecting mirror (41) and a second super-reflecting mirror (42) arranged opposite to each other along the optical cavity axis. A resonant space for the gas to be tested is formed between the first super-reflecting mirror (41) and the second super-reflecting mirror (42). The second super mirror (42) is provided with a detection component (60) on the exit side. The piezoelectric ceramic actuator (43) is arranged along the optical cavity axis and bonded to the incident side of the second super mirror (42). The piezoelectric ceramic actuator (43) can extend and retract along the optical cavity axis. The piezoelectric ceramic actuator (43) is connected to a high voltage driver (52). The system also includes a etalon and an algorithm control unit (51), which is connected to the laser (11), the detection assembly (60), the etalon and the high-voltage driver (52), respectively. The algorithm control unit (51) is configured to obtain the carbon dioxide absorption spectrum and methane absorption spectrum of the gas to be tested in the resonant space based on the ring-down signal collected by the detection component (60) and the wavelength data obtained by the etalon, and determine the target reference frequency from the carbon dioxide absorption peak, the methane absorption peak and the wavelength reference provided by the pre-calibrated etalon based on the signal-to-noise ratio of the carbon dioxide absorption spectrum and the methane absorption spectrum, the current target reference object and the hysteresis switching conditions including a preset number of consecutive frames, a first threshold and a second threshold higher than the first threshold; The algorithm control unit (51) is also configured to determine the target optical cavity longitudinal mode resonance point that is closest to the target reference frequency. Based on the frequency deviation between the target optical cavity longitudinal mode resonance point and the target reference frequency, the relationship between the optical cavity longitudinal mode resonance frequency and the optical cavity length, and the correspondence between the piezoelectric ceramic actuator driving voltage and the cavity length change, the high voltage driver (52) is controlled to adjust the piezoelectric ceramic actuator (43) so that the piezoelectric ceramic actuator (43) extends and retracts along the optical cavity axis under the action of the high voltage driving voltage, and the axial deformation is transmitted to the second super-reflecting mirror (42) bonded to it.

9. The optical cavity ring-down gas measurement system with cavity length locking function according to claim 8, characterized in that, The pattern matching lens group (20) includes a collimator (21), a first lens (22), and a second lens (23) arranged sequentially along the emission direction of the laser (11); The detection component (60) includes a receiving reflector (62) and a photodetector (61). The receiving reflector (62) is disposed on the outgoing light path of the second super-reflecting mirror (42), and the photodetector (61) is disposed on the reflected light path of the receiving reflector (62). The photodetector (61) is connected to the algorithm control unit (51).

10. The optical cavity ring-down gas measurement system with cavity length locking function according to claim 8, characterized in that, The high-voltage driver (52) includes a 16-bit digital-to-analog converter and a voltage amplifier circuit. The input terminal of the 16-bit digital-to-analog converter is connected to the algorithm control unit (51), and the output terminal of the 16-bit digital-to-analog converter is connected to the piezoelectric ceramic actuator (43) via the voltage amplifier circuit. The piezoelectric ceramic actuator (43) has a maximum driving voltage of 150V and a maximum axial stroke of 3.9μm; The algorithm control unit (51) is configured to control the high voltage driver (52) to apply an initial drive voltage of 75V to the piezoelectric ceramic actuator (43) when the system is powered on.

Images