Water vapor column concentration laser telemetering system and method based on TDLAS-WMS

Abstract

The invention discloses a water vapor column concentration laser telemetering system and method based on TDLAS-WMS. The system comprises an electronics control unit, a laser modulator, a near-infrared laser, a collimator, a dichroscope, a first reflector, a second reflector, an optical convergence telescope, a narrow-band filter, a detector, a ranging radar and a visible light laser. According to the invention, the TDLAS-WMS-2f / 1f technology is used, and high-precision and high-stability water vapor column concentration measurement is realized. The tunable semiconductor laser absorption spectrum is combined with the wavelength modulation spectrum technology, signal processing is carried out according to the ratio of second harmonics to first harmonics, and interference caused by light intensity fluctuation, atmospheric turbulence and non-absorption loss in the light path transmission process can be effectively eliminated.

Description

Technical Field

[0001] This invention relates to the field of water vapor remote sensing, specifically to a laser remote sensing system and method for water vapor column concentration based on TDLAS-WMS. Background Technology

[0002] Water vapor, as the most spatiotemporally varied and thermodynamically active component of the Earth's atmosphere, plays an irreplaceable role in atmospheric physical properties, weather forecasting, and ecological environment monitoring. Its column concentration is a crucial parameter for monitoring weather warnings, predicting agricultural drought disasters, and analyzing regional eco-hydrological cycles. In meteorology, water vapor column concentration is directly related to atmospheric precipitable water, and its high-frequency fluctuations are often precursory signals of severe convective weather systems such as rainstorms and thunderstorms. In optoelectronic engineering and geodesy, it is a key factor determining the attenuation of infrared laser atmospheric transmission, essential for correcting atmospheric transmission models and improving system accuracy.

[0003] In the field of water vapor column concentration remote sensing, existing technologies have limitations in medium- and long-range detection. Traditional differential absorption lidar and Raman radar, while having long detection ranges, are large, complex, expensive, and have low temporal resolution. While traditional Fourier transform infrared spectrometers are relatively simple to use, they rely on ambient background radiation conditions, and weak signals result in bottlenecks in signal-to-noise ratio and range resolution, making it difficult to meet the high-precision quantification requirements at medium- and long-range distances.

[0004] To address this technological gap, an active spectroscopic detection method is needed to overcome atmospheric turbulence and environmental noise interference during long-distance optical transmission. Therefore, this invention proposes a water vapor column concentration remote sensing system based on tunable semiconductor laser absorption spectroscopy combined with wavelength modulation spectroscopy. This system effectively suppresses background noise during medium- and long-distance transmission, improving the system's detection sensitivity and anti-interference capabilities. Summary of the Invention

[0005] The purpose of this invention is to provide a laser remote sensing system and method for water vapor column concentration based on TDLAS-WMS, so as to solve the problems mentioned in the background art.

[0006] To achieve the above objectives, the present invention provides the following technical solution: A water vapor column concentration laser telemetry system based on TDLAS-WMS includes an electronic control unit, a laser modulator, a near-infrared laser, a collimator, a dichroic mirror, a first reflecting mirror, a second reflecting mirror, an optical converging telescope, a narrowband filter, a detector, a ranging radar, and a visible light laser. The water vapor column concentration laser telemetry system is configured as follows: A laser modulator controls a near-infrared laser to emit a continuous wavelength of near-infrared laser light. The laser light passes sequentially through a collimator, a dichroic mirror, a first reflecting mirror, and a second reflecting mirror before being directed toward the atmospheric target area. After selective absorption by water vapor in the atmosphere, the laser light is reflected by the hard target in the target area, forming an echo signal. The echo signal is captured by an optical converging telescope, filtered by a narrow-band filter, and then converged to a detector. The detector converts the signal into an electrical signal, which is transmitted to the electronic control unit. The ranging radar simultaneously measures the straight-line distance between the near-infrared laser and the hard target in the target area while acquiring the optical signal. It is synchronized with the clock of the electronic control unit and transmits the distance data. The electronic control unit first smooths and denoises the echo signal output by the detector through a filter, then separates the first harmonic signal and the second harmonic signal through a quadrature lock-in amplifier and calculates the normalized amplitude. The water vapor concentration is then derived from the normalized amplitude. The water vapor concentration is matched with the straight-line distance measured by the ranging radar, and finally, the water vapor column concentration data of the target area is output.

[0007] Furthermore, the optical converging telescope is a Cassegrain telescope.

[0008] Furthermore, the detector is a silicon-based avalanche photodiode.

[0009] This invention also provides a laser telemetry method for water vapor column concentration based on TDLAS-WMS, implemented using the water vapor column concentration laser telemetry system described above, comprising: Step 1: Complete the overall setup of the water vapor column concentration laser remote sensing system; Step 2: Calibrate the optical path of the water vapor column concentration laser telemetry system; Step 3: Set up the calibration device; Step 4: Establish the calibration function, conduct calibration experiments to obtain the normalized amplitude during the calibration stage, and substitute the normalized amplitude during the calibration stage into the calibration function to obtain the calibration function coefficients. Step 5: After removing the calibration device, the water vapor column concentration is measured using a water vapor column concentration laser telemetry system. First, the normalized amplitude of the measurement phase is measured. Then, the normalized amplitude of the measurement phase is substituted into the calibration function to obtain the water vapor concentration before correction. The water vapor concentration before correction is substituted into the calculation formula of the water vapor concentration after correction to invert the water vapor concentration in the area to be measured. Finally, the water vapor column concentration in the target area is calculated by matching the time stamp with the distance data at the same time.

[0010] Further, step 3 includes: During the calibration phase, the Heriot-Lieutenant cell is placed between the dichroic mirror and the first reflecting mirror. Before introducing standard concentration water vapor, the Heriot-Lieutenant cell is purged with high-purity nitrogen for a long time to remove residual gases.

[0011] Furthermore, the calibration function constructed in step 4 is: , In the formula, S 2f / 1f Let η be the normalized amplitude of the first harmonic relative to the second harmonic, η be the calibration function coefficient, C be the water vapor concentration, S(T) be the linear intensity of the characteristic absorption line of water vapor molecules at temperature T, and P be the atmospheric pressure.

[0012] Furthermore, the corrected water vapor concentration C in step 8 meas The calculation formula is: , In the formula, C0 is the water vapor concentration before correction, P0 is the standard atmospheric pressure, and P meas and T meas The pressure and temperature of the environment during measurement are used. γ and β are regression parameters extracted by performing a quadratic polynomial fitting on the nonlinear curve of the theoretical line intensity ratio calculated based on the HITRAN database within the working temperature range.

[0013] Compared with the prior art, the beneficial effects of the present invention are: 1) This invention utilizes TDLAS-WMS-2f / 1f technology to achieve high-precision and high-stability water vapor column concentration measurement. By employing tunable semiconductor laser absorption spectroscopy combined with wavelength modulation spectroscopy, and using the ratio of the second harmonic to the first harmonic for signal processing, this invention effectively eliminates interference caused by light intensity fluctuations, atmospheric turbulence, and non-absorption losses during optical path transmission. This anti-interference measurement method improves the system's detection sensitivity and signal-to-noise ratio, ensuring the accuracy and reliability of water vapor column concentration measurement data.

[0014] 2) The system of this invention possesses advantages such as miniaturization, low cost, ease of deployment, and strong environmental adaptability. Compared to the bulky size and high hardware cost of traditional differential absorption lidar, this invention significantly reduces the system size and lowers the complexity of debugging by using a single near-infrared laser in conjunction with a Cassegrain telescope. Simultaneously, the system simplifies the adaptation requirements for measurement scenarios, eliminating the need for dedicated Lambertian reflectors and enabling effective detection using natural hard targets such as buildings and trees as reflective carriers. This characteristic greatly reduces the difficulty of on-site deployment and expands the system's applicability in complex environments such as mountainous areas, towns, and remote regions. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of a laser telemetry system for water vapor column concentration based on TDLAS-WMS.

[0016] Figure 2 This is a wavelength scanning characteristic curve of the absorbance of water vapor from a laser target in a TDLAS-WMS-based laser telemetry system for water vapor column concentration.

[0017] Figure 3 Figure 1 shows the time-scan characteristic curves of the laser driving signal in a TDLAS-WMS-based laser telemetry system for water vapor column concentration. Figure 2(a) shows the laser driving current versus time, Figure 3(b) shows the laser output wavelength versus time, and Figure 4(c) shows the laser output power versus time.

[0018] Figure 4 This is a flowchart of a laser telemetry method for water vapor column concentration based on TDLAS-WMS.

[0019] In the picture: 1 is the electronic control unit; 2 is a laser modulator; 3 is a near-infrared laser; 4 is the collimator; 5 is a dichroic mirror; 6 is the Heriot-Trench pool, installed only during the laboratory calibration phase before actual testing; 7 is the first reflecting mirror; 8 represents the hard target area to be tested; 9 is the second reflecting mirror; 10 is an optical converging telescope; 11 is a narrowband filter; 12 is a detector; 13 is a ranging radar; 14 is a visible light laser; 15 is signal line one; 16 is signal line two; 17 is signal line three; 18 is the fourth signal line; 19 represents optical fiber; 20 represents a coaxial, collinear beam of visible and near-infrared light; 21 represents the echo beam. Detailed Implementation

[0020] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] Please see Figure 1A laser remote sensing system for water vapor column concentration based on TDLAS-WMS is proposed. The system enables large-scale, low-cost measurement of water vapor column concentration. It includes an electronic control unit 1, a laser modulator 2, a near-infrared laser 3, a collimator 4, a dichroic mirror 5, a Heriot-Limiter cell 6, a first reflecting mirror 7, a hard target in the measurement area 8, a second reflecting mirror 9, an optical converging telescope 10, a narrowband filter 11, a detector 12, a ranging radar 13, and a visible light laser 14.

[0022] Among them, the optical converging telescope 10 is preferably a Cassegrain telescope, and the detector 12 is preferably a silicon-based avalanche photodiode.

[0023] The optical path of the water vapor column concentration laser remote sensing system is arranged as follows: a near-infrared laser 3, a collimator 4, a dichroic mirror 5, a Heriot-Limiter cell 6, and a first reflecting mirror 7 are arranged in a straight line. The continuous-wavelength near-infrared laser emitted by the near-infrared laser 3 is collimated by the collimator 4, and after passing through the dichroic mirror 5 and the Heriot-Limiter cell 6, it is transmitted in a straight line to the first reflecting mirror 7. The first reflecting mirror 7 and the second reflecting mirror 9 are arranged parallel to each other. The coaxial and collinear visible and near-infrared light beam 20 is reflected by the two reflecting mirrors, completing the optical path translation and directing it towards the hard target 8 in the measurement area. The echo beam 21 formed by the reflection of the hard target 8 is converged by the optical converging telescope 10, filtered by the narrowband filter 11, and then focused by the detector 12. The beam is converted into an electrical signal by the detector 12 and transmitted to the electronic control unit 1. The ranging radar 13 simultaneously measures the straight-line distance between the near-infrared laser 3 and the hard target in the area to be measured when the optical signal is collected. It is synchronized with the clock of the electronic control unit 1 and transmits the distance data. The electronic control unit 1 first smooths and denoises the electrical signal output by the detector 12 through a filter, and then separates the first harmonic signal and the second harmonic signal through the quadrature lock-in amplifier and calculates the normalized amplitude.

[0024] The electronic control unit 1 is connected to three components via three independent signal lines. Signal line 16 connects it to detector 12, signal line 17 connects it to laser modulator 2, and signal line 18 connects it to ranging radar 13. Laser modulator 2 is connected to near-infrared laser 3 via signal line 15, and laser 3 is connected to collimator 4 via optical fiber 19.

[0025] The target absorption wavelength selected in this example is 815.6 nm, and the absorption intensity is approximately 1.9 × 10⁻⁶ under the condition of 1% water vapor mole fraction at room temperature and standard pressure. -6 cm -2 / atm, such as Figure 2 As shown. Considering the typical concentration range of water vapor in the atmosphere (0.5%-3%) and the maximum target measurement distance of 100m, this wavelength band can avoid excessive absorption of the laser by water vapor during long-distance transmission, and the detector 12 can effectively collect signals.

[0026] The electronic control unit 1 generates a 10Hz low-frequency sawtooth wave superimposed with a 20kHz high-frequency sine wave modulation signal to form the drive signal required by the near-infrared laser, such as... Figure 3 As shown, the signal is transmitted to the laser modulator 2 via signal line 317. The laser modulator 2 is electrically connected to the near-infrared laser 3. Based on the current-wavelength tuning characteristics of the laser, the near-infrared laser 3 is driven to output laser light covering the characteristic absorption peak of 815.6nm, and its wavelength scanning range is set to 815.55nm to 815.63nm.

[0027] The near-infrared laser 3 uses a distributed Bragg reflection tunable laser with TO8 packaging and spatial light output design. It has an embedded Bragg grating, an average output power of 100mW, a spectral linewidth of less than 3MHz, a wavelength tuning range of more than 2nm, and is equipped with a thermoelectric cooler to control temperature stability.

[0028] Near-infrared laser 3 is connected to collimator 4 via optical fiber 19. Collimator 4 uses an FC / APC interface, has a wavelength of 850nm, an effective focal length of 18.4mm, a divergence angle of 4rad, and can handle the 100mW power of near-infrared laser 3.

[0029] Dichroic mirror 5 is placed after collimator 4. The passband of dichroic mirror 5 covers 790-1770nm, with an average transmittance of 93%, enabling efficient transmission of a target wavelength of 815.6nm. The cutoff wavelength is 786nm, with an average reflectance of 90%, enabling efficient reflection of visible light.

[0030] The Heriot-Limiter cell 6 is used only in the calibration stage before actual measurement and is placed in the optical path between the dichroic mirror 5 and the first reflecting mirror 7. The Heriot-Limiter cell 6 adopts a double spherical reflecting mirror structure design. After the laser beam enters the cell through the entrance hole, it forms a closed optical path through multiple round trip reflections between the two spherical reflecting mirrors to achieve a distance of 100 meters.

[0031] An optical converging telescope 10, used to receive the echo beam 21, is arranged on the same side as the near-infrared laser 3. This telescope uses a double-reflection focusing mechanism and consists of two curved mirrors: a primary mirror and a secondary mirror. The primary mirror, a parabolic mirror, is located at the bottom of the telescope and reflects and converges the incident parallel light rays to its focal point. The secondary mirror, a hyperboloidal convex mirror, is located in front of the focal point of the primary mirror and reflects the converged light rays again, allowing them to pass through a central aperture in the primary mirror and achieve focusing at the focal point behind the primary mirror.

[0032] A Cassegrain telescope with an effective aperture of 200mm was selected, whose large focal length effectively reduces the size and weight of the entire system. The parabolic structure of the primary mirror can focus the echo beam 21, eliminating spherical aberration, while the secondary mirror cancels the remaining aberrations and expands the field of view. At the same time, the Cassegrain telescope does not rely on light refraction, avoiding the dispersion problem of lens structures and improving the optical purity of the beam.

[0033] A narrowband filter 11 is placed inside the optical converging telescope 10 to filter near-infrared light of the target band in the echo beam. Its center wavelength is 815 nm, its half-width at half-maximum is 10 nm, and its peak transmittance reaches 90%, which can provide a high-purity target band light signal for photoelectric conversion by the subsequent detector 12.

[0034] The echo beam 21, after being focused by the optical converging telescope 10 and filtered by the narrowband filter 11, is incident on the detector 12 for photoelectric conversion. The detector 12 achieves photoelectric conversion based on the avalanche effect, with a wavelength range covering 400-1000 nm. Its response gain reaches 0.8 A / W near the target detection wavelength of 815.6 nm, exhibiting excellent signal response characteristics. The gain of the detector 12 is precisely controlled by adjusting the reverse bias voltage at both ends, with an adjustment range of 10-100 times. Its 3dB bandwidth can reach 1MHz, and its response time is in the nanosecond range.

[0035] The ranging radar 13 is electrically connected to the electronic control unit 1. Based on the Time-of-Flight (TOF) principle, the ranging radar 13 measures the absolute height of hard targets. The distance data from the ranging radar includes a timestamp, which is matched with the synchronously measured water vapor concentration to calculate the water vapor column concentration value of the area to be measured.

[0036] A visible light laser 14 is placed beside the measurement optical path. Its emitted visible laser light is reflected by a dichroic mirror 5 and then overlaps with the near-infrared laser output from the near-infrared laser, forming a coaxial and collinear visible-near-infrared beam 20. The visible light laser 14 has a built-in collimation function, an output power of approximately 5mW, a divergence angle ≤1.5mrad, and outputs a circular spot. Due to the invisible nature of near-infrared light, it is used for auxiliary calibration of the measurement optical path.

[0037] During calibration or measurement of the water vapor column concentration laser telemetry system, the electronic control unit generates a specific driving signal to drive the near-infrared laser to emit continuous wavelengths of near-infrared laser light. The relationship between gas concentration and harmonic amplitude is determined based on the Lambert-Beer law and Fourier expansion. Specifically, the electronic control unit generates a driving signal consisting of a low-frequency sawtooth wave and a high-frequency sine wave superimposed on each other, and transmits this signal to the laser modulator. The laser modulator, based on the near-infrared laser's current-wavelength mapping relationship, drives the near-infrared laser to emit continuous wavelengths of near-infrared laser light. The driving signal is a modulation signal formed by superimposing a low-frequency sawtooth wave with a high-frequency sine wave of frequency f. The frequency v(t) of the superimposed near-infrared laser can be written as: , in, Here, 'a' is the center frequency of the laser frequency scanning range, 'a' is the modulation amplitude, 't' is time, and the laser intensity I(t) can be written as: , in, Let be the center light intensity of the laser scanning target band. According to the Lambert-Beer law, the transmittance τ(v) after gas absorption can be written as: , Where α is the absorption coefficient of the water vapor to be measured at v(t), and L is the optical path length in the region to be measured.

[0038] Furthermore, considering the weak absorption of atmospheric water vapor, The above formula can be further simplified to: , Where C is the water vapor concentration, S(T) is the line intensity of the characteristic absorption line of water vapor molecules at temperature T, P is the atmospheric pressure, X is the mole fraction of water vapor, and Φ is the line shape function. Simultaneously, a Fourier series expansion of the transmittance τ(v) yields: , Let θ = 2πft, then the nth order Fourier expansion coefficients It can be written as: , The expression for X can be obtained by rearranging the above formula: , It can be seen that the gas concentration X is related to the harmonic amplitude. Proportional.

[0039] Define two dimensionless parameters: normalized frequency x and modulation depth m, each with the following expression: , , Where Δv is the half-width and half-height of the target absorption line. It can be seen that the odd harmonics of the absorbance signal have a value of 0 at x=0, and the waveform is symmetrical about the origin. The even harmonic waveform is symmetrical about the x=0 axis, reaching its maximum value at x=0. As the harmonic order increases, the amplitude of the harmonic waveform gradually decreases. Furthermore, the measured signal amplitude is not the absorbance; the intensity of the echo signal and the detector gain must also be considered. The nth harmonic intensity signal S actually measured by the silicon-based avalanche photodiode can be written as: , Where G is the electro-optic gain of the detector, I0 is the intensity of the laser echo beam, and H... n This represents the amplitude of the nth harmonic signal. Detector gain and system noise, among other factors, can affect the measured value. Therefore, the second harmonic is normalized using the first harmonic, and the normalized amplitude S is... 2f / 1f It can be written as: , Where f is the frequency, S 1f S is the signal amplitude of the first harmonic. 2f S represents the signal amplitude of the second harmonic. 1f and S 2f i1 represents the amplitude of the first and second harmonics, respectively, and i1 is the normalized linear modulation coefficient of the near-infrared laser intensity.

[0040] The electronic control unit uses an FIR filter to smooth and preprocess the electrical signal output by the detector to eliminate noise. The preprocessed electrical signal is then separated into first and second harmonic signals by a quadrature lock-in amplifier, as detailed below: Assume the frequency of the input signal s(t) to be measured is The amplitude is A, and the phase is If the signal is accompanied by noise N(t), then the signal can be expressed as: , The quadrature lock-in amplifier internally generates reference signals with the same frequency but orthogonal phases, which are respectively in-phase reference signals. Signals and quadrature reference signals Phase is : , , The input signal s(t) is compared with the in-phase reference signal. Orthogonal reference signal Perform multiplication to obtain the mixed signal of the in-phase branch. Mixed signals from orthogonal branches : , , The above results include DC components, harmonic components, and noise terms. The mixed signal is fed into a low-pass filter for integration, filtering out high-frequency components and noise unrelated to the reference frequency, retaining only the DC component I of the in-phase branch and the DC component Q of the quadrature branch. , , in, Let I be the phase difference between the input signal and the reference signal. Based on the values ​​of I and Q, the amplitude A of the measured signal, independent of the phase difference, can be calculated using the sum of squares and the square root of the square roots. , Based on the above-mentioned orthogonal demodulation principle, using The amplitude calculation model shown extracts the fundamental frequency component and harmonic components of the signal by adjusting the frequencies of the in-phase and quadrature reference signals. The reference signal frequency is set to ω, and the DC component obtained after low-pass filtering is denoted as I. 1ω With Q 1ω Substitute The first harmonic amplitude A is calculated. 1ω That is, S 1f The reference signal frequency is set to 2ω, and the DC component obtained after low-pass filtering is denoted as I. 2ω With Q 2ω Substitute The first harmonic amplitude A is calculated. 2ω That is, S 2f To eliminate the influence of laser intensity fluctuations and optical path transmission losses on the measurement results, both are normalized, i.e.: .

[0041] Please see Figure 4 A laser telemetry method for water vapor column concentration based on TDLAS-WMS includes: Step 1: Based on the principle of tunable semiconductor laser absorption spectroscopy, complete the overall construction of the water vapor column concentration laser remote sensing system, including hardware wiring and software communication debugging between modules, specifically including: A distributed Bragg reflector tunable laser is connected to a collimator via optical fiber. After collimation, the laser forms a low-divergence-angle near-infrared continuous-wave beam. This beam serves as the measurement light and is incident on a dichroic mirror along the optical path. A visible light laser is used to assist the near-infrared measurement light. The visible light is incident on the reflecting surface of the dichroic mirror. Based on the difference in transmission and reflection of different wavelengths of light by the dichroic mirror, the reflected visible beam and the transmitted near-infrared beam form a completely overlapping optical path on the exit side. This coaxial and collinear visible-near-infrared beam is sequentially incident on the first and second reflecting mirrors, and the optical path is laterally translated through two consecutive 90° orthogonal turns, eventually reaching the atmospheric measurement area. Absorption by water vapor molecules in the atmosphere at the characteristic absorption peak causes the laser intensity to attenuate, becoming a weak signal. This weak signal is reflected by hard targets in the atmosphere, forming an echo beam, which is captured and received by a Cassegrain telescope placed on the same side as the laser. The light beam is reflected by the parabolic primary mirror within the Cassegrain telescope and focused to the focal region of the mirror assembly. It is then reflected a second time by a hyperboloidal convex secondary mirror located in front of the primary mirror's focal point, causing the beam to be redirected and pass through the central aperture of the primary mirror, forming a converging beam at the focal plane behind the primary mirror. A narrow-band filter is placed at the focal plane, filtering the coaxial, collinear visible and near-infrared light beam before it converges to a silicon-based avalanche photodiode, where the optical signal is converted into an electrical signal.

[0042] Step 2, calibrate the optical path of the water vapor column concentration laser telemetry system, specifically including: This process involves three steps to calibrate the visible and near-infrared coaxial collinear beams. First, the laser alignment disk is positioned on the exit side of the dichroic mirror. By adjusting the angle of the dichroic mirror and the emission angle of the visible laser, the near-infrared and visible beams are made to completely overlap on the alignment disk, ensuring an initial coaxial and collinear optical path at the exit side of the dichroic mirror. Second, the laser alignment disk is moved to the exit side of the second reflector. The pitch angle and vertical height of the first and second reflectors are fine-tuned, adjusting the offset of the optical path after two 90° orthogonal turns, ensuring that the near-infrared and visible beams remain overlapped after exiting the second reflector. Third, the laser alignment disk is placed at a certain distance from the exit side of the second reflector. Fine-tuning the reflectors eliminates optical path deviations during long-distance propagation, ensuring the two beams are overlapped for the third time. Through three progressive optical path adjustments and calibrations, the near-infrared measurement beam and the visible light auxiliary beam are made completely coaxial and collinear, which greatly controls the alignment of the echo beam formed by reflection from hard atmospheric targets with the receiving field of view of the Cassegrain telescope, thereby improving the acquisition efficiency of the echo signal.

[0043] Step 3, set up the calibration device, specifically including: A Heriot-Lieutenant multipass cell was positioned between the dichroic mirror and the first reflecting mirror, and the cell was adjusted so that the incident and exit beams of the laser did not deviate within the cell. Before introducing standard concentration water vapor, the Heriot-Lieutenant cell was purged with high-purity nitrogen for an extended period to remove residual gases.

[0044] Step 4: Establish the calibration function, conduct calibration experiments to obtain the normalized amplitude during the calibration phase, and substitute the normalized amplitude during the calibration phase into the calibration function to obtain the calibration function coefficients. Specifically, this includes: The constructed calibration function is as follows: , In the formula, S 2f / 1f Let η be the normalized amplitude of the first harmonic relative to the second harmonic, η be the calibration function coefficient, C be the water vapor concentration, S(T) be the linear intensity of the characteristic absorption line of water vapor molecules at temperature T, and P be the atmospheric pressure.

[0045] In the calibration experiment, the electronic control unit generates a drive signal and loads it onto the laser modulator. The laser modulator controls the near-infrared laser to output a near-infrared modulated laser with a center wavelength covering the target water vapor absorption line. This near-infrared laser propagates along the optical path of the water vapor column concentration laser telemetry system. After atmospheric transmission and reflection from the hard target in the measurement area, it converges to the detector, where it is converted from an optical signal into an electrical signal. A Heriot-Trench multi-pass cell is used to introduce standard water vapor at different concentrations, collecting 600 sets of raw signals. An FIR filter is used to smooth and reduce noise in the raw signals. Demodulation of the first and second harmonics is achieved using a quadrature lock-in amplifier, and the normalized amplitude ratio of the second and first harmonics is calculated. The average value of the 600 sets of normalized amplitude ratios of the second and first harmonics corresponding to each standard water vapor concentration point is calculated. Then, the normalized amplitude from the calibration stage is substituted into the calibration function to obtain the calibration function coefficient η.

[0046] The water vapor concentration C0 before correction and the normalized amplitude S of the first harmonic to the second harmonic. 2f / 1f The temperature T and pressure P are linearly correlated when they are constant.

[0047] Since the atmospheric water vapor concentration is relatively large, and its measurement accuracy is limited by the changes in saturated vapor pressure caused by large temperature differences and fluctuations in ambient air pressure, this invention introduces a calibration formula that includes temperature and pressure parameters to correct the measurement results: The absorption line intensity S(T) is a function of temperature T, which is a well-known function, expressed as: , Q(T) is the molecular partition function, expressed using a polynomial fitting: , Where A, B, D, and E are the constant coefficients of the polynomial fitting, and c2 is the second radiation constant. For low-state transition energy, Let T0 be the center wavenumber of the spectral line, T0 be the ambient temperature, and exp be the natural exponential function.

[0048] Substitute the ambient temperature during the measurement into Dynamic corrections are applied to the water vapor absorption line intensity to calculate the actual water vapor absorption line intensity under the current flight environment. , and Combining these, we obtain the corrected water vapor concentration C. meas The calculation formula is as follows: , Where C0 is the water vapor concentration before correction, P0 is the standard atmospheric pressure, and P meas and T meas The pressure and temperature of the environment during measurement are taken. An engineering approximation is used to convert the linear intensity ratio term... This can be simplified to a quadratic polynomial in terms of temperature difference, which can then be fitted and replaced. , Among them, the coefficients of γ and β are regression parameters extracted by performing a quadratic polynomial fitting on the nonlinear curve of the theoretical line intensity ratio calculated based on the HITRAN database within the working temperature range.

[0049] The final formula for calculating water vapor concentration is obtained as follows: , This method significantly reduces the dependence on laser intensity changes caused by beam steering and scattering, and suppresses common-mode noise introduced by laser drive current fluctuations, detector gain fluctuations, and non-resonant transmission losses.

[0050] By verifying the goodness-of-fit coefficient R... 2 After confirming the linear correlation of the calibration function with a value >0.99, the calibration function is stored in the electronic control unit to provide a basis for the subsequent water vapor concentration inversion in the experimental stage.

[0051] Step 5: After removing the calibration device, the water vapor column concentration is measured using a laser telemetry system. First, the normalized amplitude of the measured phase is obtained. This normalized amplitude is then substituted into the calibration function to calculate the water vapor concentration before correction. The water vapor concentration before correction is then substituted into the formula for calculating the water vapor concentration after correction to retrieve the water vapor concentration in the area to be measured. Finally, the water vapor column concentration in the target area is calculated by matching the time stamp with the distance data from the same period. Specifically, this includes: Substituting the normalized amplitude into the established formula for calculating water vapor concentration The water vapor concentration in the area to be measured is obtained; the electronic control unit matches the water vapor concentration obtained by inversion with the distance data received at the same time according to the timestamp, and calculates the water vapor column concentration in the target area.

[0052] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A laser remote sensing system for water vapor column concentration based on TDLAS-WMS, characterized in that, This includes electronic control units, laser modulators, near-infrared lasers, collimators, dichroic mirrors, first reflecting mirrors, second reflecting mirrors, optical converging telescopes, narrowband filters, detectors, ranging radars, and visible light lasers. The water vapor column concentration laser telemetry system is configured as follows: A laser modulator controls a near-infrared laser to emit a continuous wavelength of near-infrared laser light. The laser light passes sequentially through a collimator, a dichroic mirror, a first reflecting mirror, and a second reflecting mirror before being directed toward the atmospheric target area. After selective absorption by water vapor in the atmosphere, the laser light is reflected by the hard target in the target area, forming an echo signal. The echo signal is captured by an optical converging telescope, filtered by a narrow-band filter, and then converged to a detector. The detector converts the signal into an electrical signal, which is transmitted to the electronic control unit. The ranging radar simultaneously measures the straight-line distance between the near-infrared laser and the hard target in the target area while acquiring the optical signal. It is synchronized with the clock of the electronic control unit and transmits the distance data. The electronic control unit first smooths and denoises the echo signal output by the detector through a filter, then separates the first harmonic signal and the second harmonic signal through a quadrature lock-in amplifier and calculates the normalized amplitude. The water vapor concentration is then derived from the normalized amplitude. The water vapor concentration is matched with the straight-line distance measured by the ranging radar, and finally, the water vapor column concentration data of the target area is output.

2. The water vapor column concentration laser telemetry system based on TDLAS-WMS according to claim 1, characterized in that, The optical converging telescope is a Cassegrain telescope.

3. The water vapor column concentration laser remote sensing system based on TDLAS-WMS according to claim 1, characterized in that, The detector is a silicon-based avalanche photodiode.

4. A laser telemetry method for water vapor column concentration based on TDLAS-WMS, implemented using a laser telemetry system for water vapor column concentration as described in any one of claims 1-3, characterized in that... include: Step 1: Complete the overall setup of the water vapor column concentration laser remote sensing system; Step 2: Calibrate the optical path of the water vapor column concentration laser telemetry system; Step 3: Set up the calibration device; Step 4: Establish the calibration function, conduct calibration experiments to obtain the normalized amplitude during the calibration stage, and substitute the normalized amplitude during the calibration stage into the calibration function to obtain the calibration function coefficients. Step 5: After removing the calibration device, the water vapor column concentration is measured using a water vapor column concentration laser telemetry system. First, the normalized amplitude of the measurement phase is measured. Then, the normalized amplitude of the measurement phase is substituted into the calibration function to obtain the water vapor concentration before correction. The water vapor concentration before correction is substituted into the calculation formula of the water vapor concentration after correction to invert the water vapor concentration in the area to be measured. Finally, the water vapor column concentration in the target area is calculated by matching the time stamp with the distance data at the same time.

5. The laser remote sensing method for water vapor column concentration based on TDLAS-WMS according to claim 4, characterized in that, Step 3 includes: During the calibration phase, the Heriot-Lieutenant cell is placed between the dichroic mirror and the first reflecting mirror. Before introducing standard concentration water vapor, the Heriot-Lieutenant cell is purged with high-purity nitrogen for a long time to remove residual gases.

6. The laser remote sensing method for water vapor column concentration based on TDLAS-WMS according to claim 4, characterized in that, The calibration function constructed in step 4 is: ， In the formula, S 2f / 1f Let η be the normalized amplitude of the first harmonic relative to the second harmonic, η be the calibration function coefficient, C be the water vapor concentration, S(T) be the linear intensity of the characteristic absorption line of water vapor molecules at temperature T, and P be the atmospheric pressure.

7. The laser remote sensing method for water vapor column concentration based on TDLAS-WMS according to claim 6, characterized in that, The corrected water vapor concentration C in step 8 meas The calculation formula is: ， In the formula, C0 is the water vapor concentration before correction, P0 is the standard atmospheric pressure, and P meas and T meas The pressure and temperature of the environment during measurement are used. γ and β are regression parameters extracted by performing a quadratic polynomial fitting on the nonlinear curve of the theoretical line intensity ratio calculated based on the HITRAN database within the working temperature range.

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