LiDAR system for measuring atmospheric methane column concentration and water vapor profile

Abstract

A lidar system for simultaneously measuring the column concentration of methane and the profile concentration of water vapor in the atmosphere is disclosed. The device includes a laser emission module: a 1645nm monoline seed laser, a 1645nm offline1 seed laser, a 1645nm offline2 seed laser, a 1064nm pump laser, a laser frequency-locking module, an optical parametric oscillator, a frequency doubling module, and emitting optical components; an echo receiving module: an integrating sphere module, an optical receiving telescope assembly, a beam splitter, filter assembly 1, filter assembly 2, photodetector 1, and photodetector 2; and a data acquisition and processing module. This invention utilizes the frequency doubling relationship between the absorption lines of methane and water vapor to simplify the complex structure of a dual-band, multi-wavelength laser. It employs seed-injected optical parametric oscillator technology to obtain a high-frequency stable 1645nm fundamental frequency light, which is then frequency-doubled to obtain an 822nm laser. The profile concentration of water vapor and the column concentration of methane in the atmosphere can be measured using differential absorption lidar (DIAL) and integral path differential absorption (IPDA) technologies, respectively.

Description

Technical Field

[0001] This invention belongs to the field of lidar technology, specifically a lidar system for measuring atmospheric methane column concentration and water vapor profile concentration. Background Technology

[0002] Atmospheric remote sensing equipment generally includes passive and active remote sensing devices. Passive remote sensing devices can simultaneously retrieve information on multiple gases, but they rely on solar radiation, cannot perform all-day detection, and are easily affected by clouds and aerosols, resulting in lower measurement accuracy. Lidar technology, on the other hand, has significant advantages in measuring gas composition and concentration. Lidar for detecting atmospheric water vapor profiles mainly includes Raman scattering lidar and differential absorption lidar. Raman lidar has an advantage in system complexity compared to differential absorption lidar, but because Raman scattering has a small scattering cross-section, the echo signal is weak, making it unsuitable for long-distance spaceborne applications. Active remote sensing lidar equipment has advantages such as continuous detection, high accuracy, and high spatiotemporal resolution. However, because lidar equipment needs to emit lasers with wavelengths related to the absorption and scattering characteristics of the corresponding gas components, and differential absorption lidar requires narrow linewidth and high spectral purity lasers, traditional differential absorption lidar can only measure one gas. Differential absorption lidar for measuring multiple gases often consists of multiple discrete laser sources, increasing system complexity and cost. Summary of the Invention

[0003] In order to overcome the aforementioned problems and difficulties of active detection lidar, this invention proposes a lidar system for measuring atmospheric methane column concentration and water vapor profile concentration by utilizing the harmonic relationship between the absorption lines of methane and water vapor.

[0004] The technical solution of this invention is as follows:

[0005] A lidar system for measuring atmospheric methane column concentration and water vapor profile concentration is characterized by comprising a laser emitting module, an echo receiving module, and a data acquisition and processing module.

[0006] The laser emission module includes: a 1645nm online seed laser, a first 1645nm offline seed laser, a second 1645nm offline seed laser, a 1064nm pump laser, a laser frequency locking module, an optical parametric oscillator, a frequency doubling module, and emission optical components. The positional relationship of the above components is as follows: the 1645nm online seed laser, the first 1645nm offline seed laser, and the second 1645nm offline seed laser inject the output seed light into the optical parametric oscillator as the starting seed light. The 1064nm pump light output from the 1064nm pump laser is injected into the optical parametric oscillator as the pump source of the optical parametric oscillator. The output terminal of the laser frequency locking module is connected to the second input terminal of the optical parametric oscillator. The required frequency is locked by dynamically adjusting the cavity length. The 1645nm fundamental frequency light generated by the optical parametric oscillator enters the frequency doubling module to generate a frequency-doubled 822nm laser and the remaining 1645nm fundamental frequency light. The laser light then enters the atmosphere through the emission optical components. The above components constitute a laser emission module.

[0007] The echo receiving module includes an integrating sphere module, an optical receiving telescope assembly, a beam splitter, a first filter assembly, a second filter assembly, a first photodetector, and a second photodetector. The emitting optical assembly splits a portion of the light into the integrating sphere module to monitor the energy of the emitted laser. The laser echo signal, after atmospheric absorption, is received by the optical receiving telescope assembly. The beam splitter divides the laser echo signal into two paths: a 1645nm signal and an 822nm signal. The first 1645nm signal passes through the first filter assembly and is received by the first photodetector, while the second 822nm signal passes through the second filter assembly and is received by the second photodetector. These components constitute the echo receiving module.

[0008] The data acquisition and processing module is connected to the echo receiving module and includes a data acquisition card, a computer, etc., and is responsible for subsequent data acquisition and processing of the lidar system.

[0009] The 1645nm online seed laser, the first 1645nm offline seed laser, and the second 1645nm offline seed laser need to output three seed lights with very small wavelength differences, which are respectively located at the 1645nm absorption peak and absorption valley of methane. The seed light has high frequency stability and low power continuous wave.

[0010] The optical parametric oscillator, together with the seed laser and the 1064nm pump laser, constitutes a seed-injected optical parametric oscillator, generating a narrow-linewidth amplified pulsed laser.

[0011] The laser frequency locking module uses the seed laser and the output beat frequency detection of the optical parametric oscillator to control the dynamic adjustment of the piezoelectric ceramic cavity length, thereby precisely locking the output laser.

[0012] In the laser emission module, based on the frequency doubling relationship between the absorption lines of methane (1645nm) and water vapor (822nm), the frequency doubling module is used to replace the seed light, resonant cavity, etc., to generate 822nm laser light.

[0013] The aforementioned emitting optical component is connected to the integrating sphere module to monitor the output laser.

[0014] The beam splitter splits the 1645nm and 822nm signals in the echo into two paths, corresponding to the detection channels for water vapor and methane, respectively.

[0015] The first and second filter components respectively filter out stray light such as solar background light from the signals collected by the water vapor and methane detection channels.

[0016] The advantages of this invention are:

[0017] 1. Using active detection methods, IPDA technology, and DIAL technology, the profile concentrations of methane column and water vapor are obtained respectively. It does not rely on the reflection of sunlight, has high detection accuracy, and can achieve all-day measurement.

[0018] 2. The lidar system of the present invention, according to Figure 2 The absorption line information of the two gases shown and the principle of differential absorption lidar are as follows: the online absorption line of methane corresponds to the offline absorption line of water vapor after frequency doubling. The offline1 and offline2 of the methane absorption line are exactly located online1 and online2 of the water vapor absorption line after frequency doubling. In the laser emission module of this system, only one frequency doubling module is used to directly generate an 822nm water vapor detection laser with high beam quality.

[0019] 3. The lidar system of the present invention sets two offline wavelengths, offline1 and offline2, for methane, and after frequency doubling, they correspond to two online wavelengths for water vapor, taking into account the detection of profile concentrations of different water vapor contents from the middle troposphere to the surface boundary layer.

[0020] 4. The lidar system of the present invention is equipped with filter components in both receiving channels to limit the spectral range of the received echo and filter out background light noise. Attached Figure Description

[0021] Figure 1 This is a block diagram of the overall structure of the lidar system for measuring atmospheric methane column concentration and water vapor profile concentration according to the present invention.

[0022] Figure 2 The methane absorption line at 1645 nm and its overtone absorption line at 822 nm are the basis of this invention. Detailed Implementation

[0023] The present invention will be further described below with reference to examples and accompanying drawings, but this should not be construed as limiting the scope of protection of the present invention.

[0024] Please see Figure 1 , Figure 1 This is a block diagram of the overall structure of the lidar system for measuring atmospheric methane column concentration and water vapor profile according to the present invention. Figure 1 As shown, the lidar system for measuring atmospheric methane column concentration and water vapor profile of the present invention includes a laser emission module: a 1645nm online seed laser 1, a first 1645nm offline seed laser 2, a second 1645nm offline seed laser 3, a 1064nm pump laser 4, a laser frequency locking module 5, an optical parametric oscillator 6, a frequency doubling module 7, and an emission optical assembly 8; an echo receiving module: an integrating sphere module 9, an optical receiving telescope assembly 10, a beam splitter 11, a first filter assembly 12, a second filter assembly 13, a first photodetector 14, and a second photodetector 15; and a data acquisition and processing module 16. The positional relationship of the above components is as follows:

[0025] The 1645nm online seed laser 1, the first 1645nm offline seed laser 2, and the second 1645nm offline seed laser 3 inject their output seed light into the optical parametric oscillator 6 as the oscillation seed light. The 1064nm pump light output from the 1064nm pump laser 4 is injected into the optical parametric oscillator 6 as the pump source of the optical parametric oscillator. The laser frequency locking module 5 is connected to the optical parametric oscillator 6 and locks the required frequency by dynamically adjusting the cavity length. The 1645nm fundamental frequency light generated by the optical parametric oscillator 6 enters the frequency doubling module 7 to generate a frequency-doubled 822nm laser and the remaining 1645nm fundamental frequency light, which then enters the atmosphere through the transmitting optical component 8.

[0026] The transmitting optical component 8 splits a portion of the light into the integrating sphere module 9 to monitor the energy of the emitted laser. The laser echo signal, absorbed by the atmosphere, enters the optical receiving telescope component 10 and is received by the lidar device. The optical signal is then split into two signal beams by the beam splitter 11: a 1645nm signal and an 822nm signal. The first 1645nm signal beam passes through the first filter component 12 and is received by the first photodetector 14, while the second 822nm signal beam passes through the second filter component 13 and is received by the second photodetector 15. These components constitute the echo receiving module.

[0027] The data acquisition and processing module 16 is connected to the echo receiving module and includes a data acquisition card, a computer, etc., and is responsible for subsequent data acquisition and processing of the lidar system.

[0028] Please see Figure 2 , Figure 2 This diagram illustrates the methane absorption line at 1645nm and its 822nm water vapor absorption line in the harmonic band, upon which this invention is based. In the diagram: the dashed line represents the 1645nm absorption line used by the methane IPDA lidar, with the horizontal axis (wavelength) at the top and the vertical axis (optical thickness) at the left. The solid line represents the 822nm absorption line used by the water vapor DIAL lidar, with the horizontal axis (wavelength) at the bottom and the vertical axis (optical thickness) at the right. The upper and lower axes represent harmonic relationships. The vertical line corresponding to the dotted-dashed line represents the methane online position (harmonic absorption corresponds to the water vapor offline position), and the two vertical lines corresponding to the double-dashed line represent the two methane offline positions (harmonic absorption corresponds to the two water vapor online positions).

[0029] The following is a specific device used in an embodiment:

[0030] The optical parametric oscillator (OPO) 6 uses a KTA or KTP crystal, and an optical parametric amplifier (OPA) module can be added after the OPO module as needed to meet high pulse energy requirements. The frequency doubling module 7 uses an LBO frequency doubling crystal. The transmitting optical assembly 8 consists of a collimating lens, a beam expander, etc. The integrating sphere module 9 collects the emitted light, transmits it through optical fiber, collimates it through the collimating lens, and then inputs it into the beam splitter 11. The optical receiving telescope assembly 10 uses a Cassegrain telescope; the first photodetector 14 and the second photodetector 15 are APD detectors.

[0031] The specific process of the lidar system for measuring atmospheric methane column concentration and water vapor profile implemented in this invention is as follows:

[0032] A 1645nm online seed laser, a first 1645nm offline seed laser, a second 1645nm offline seed laser, and a 1064nm pump laser generate seed light and pump light, which are injected into an optical parametric oscillator 6 to obtain amplified 1645nm pulsed light (no problem). This pulsed light is then passed through a frequency doubling module 7 to obtain 822nm frequency-doubled light and the remaining 1645nm fundamental frequency light. The emitted light then enters the atmosphere through an emitting optical component 8, and the emitted signal (1645nm online) is monitored by an integrating sphere.

[0033] E1 on0 Offline is E1 off0 .

[0034] The atmospheric echo signal enters the lidar via the optical receiving telescope assembly 10. For the 1645nm methane channel, only the echo signal E1 reflected from the target needs to be received. on E1 off For an 822nm water vapor channel, it is necessary to receive atmospheric backscattered signals P at different locations along the laser path. on P off .

[0035] This allows us to obtain the differential optical thickness of methane. The methane column concentration along the laser path, expressed by the IPDA method, is: Where IWF is the integral of the methane weighting function along the path. According to the DIAL method, the concentration of water vapor at different positions along the laser path is... Where σ on σ off Let R be the absorption cross section of water vapor at the online and offline locations (the absorption cross section is calculated based on the temperature, humidity, and pressure profiles at the measured locations and the line type parameters in the HITRAN database), ΔR = R2 - R1. R1 and R2 are the distances between any two points on the laser path and the radar.

Claims

1. A lidar system for measuring atmospheric methane column concentration and water vapor profile concentration, comprising a laser emitting module, an echo receiving module, and a data acquisition and processing module, characterized in that, The laser emitting module includes: a 1645nm online seed laser (1), a first 1645nm offline seed laser (2), a second 1645nm offline seed laser (3), a 1064nm pump laser (4), a laser frequency locking module (5), an optical parametric oscillator (6), a frequency doubling module (7), and an emitting optical component (8); the 1645nm online seed laser (1), the first 1645nm offline seed laser (2), and the second 1645nm offline seed laser (3) output seed light, which is injected into the optical parametric oscillator (6) as the starting seed light. The 64nm pump laser (4) outputs 1064nm pump light, which is injected into the optical parametric oscillator (6) as a pump source. The output of the laser frequency locking module (5) is connected to the second input of the optical parametric oscillator (6), and the required frequency is locked by dynamically adjusting the cavity length. The 1645nm pulse light generated by the optical parametric oscillator (6) enters the frequency doubling module (7) to generate 822nm frequency-doubled laser and the remaining 1645nm fundamental frequency light. Most of the light enters the atmosphere through the emission optical component (8), and a small portion enters the integrating sphere module (9) to monitor the remaining 1645nm fundamental frequency pulse energy E1. on0 E1 off0 ; The echo receiving module includes an optical receiving telescope assembly (10), a beam splitter (11), a first filter assembly (12), a second filter assembly (13), a first photodetector (14), and a second photodetector (15). The laser echo signal, after atmospheric absorption, is received by the optical receiving telescope assembly (10). The beam splitter (11) splits the laser echo signal into two paths: a 1645nm signal light and an 822nm signal light. The 1645nm signal light is received by the first photodetector (14) after passing through the first filter assembly (12). on E1 off The 822nm signal light is received by the second photodetector (15) after passing through the second filter assembly (13). on P off ; The data acquisition and processing module (16) is connected to the echo receiving module and is responsible for subsequent data acquisition and processing of the lidar system. methane column concentration Water vapor concentration at different locations along the laser path In the formula: Differential optical thickness of methane IWF is the integral of the weighting function of methane along the path; σ on σ off Let R be the absorption cross section of water vapor at the online and offline points, and ΔR = R2 - R1. R1 and R2 are the distances between any two points on the laser path and the radar.

2. The lidar system for measuring atmospheric methane column concentration and water vapor profile concentration according to claim 1, characterized in that, The 1645nm online seed laser (1), the first 1645nm offline seed laser (2), and the second 1645nm offline seed laser (3) are required to output three seed lights with very small wavelength differences, which are located at the 1645nm absorption peak and absorption valley of methane, respectively. The seed light has high frequency stability and low power continuous wave.

3. The lidar system for measuring atmospheric methane column concentration and water vapor profile concentration according to claim 1, characterized in that, The optical parametric oscillator (6), together with the seed laser and the 1064nm pump laser (4), constitutes a seed-injected optical parametric oscillator, generating a narrow-linewidth amplified pulsed laser.

4. The lidar system for measuring atmospheric methane column concentration and water vapor profile concentration according to claim 1, characterized in that, The laser frequency locking module (5) controls the piezoelectric ceramic to dynamically adjust the cavity length by detecting the beat frequency of the seed laser and the output of the optical parametric oscillator (6), thereby precisely locking the output laser.

5. The lidar system for measuring atmospheric methane column concentration and water vapor profile concentration according to claim 1, characterized in that, In the laser emission module, based on the frequency doubling relationship between the absorption lines of methane at 1645nm and water vapor at 822nm, the frequency doubling module (7) is used to replace the seed light and the resonant cavity to generate an 822nm laser.

6. The lidar system for measuring atmospheric methane column concentration and water vapor profile concentration according to claim 1, characterized in that, The beam splitter (11) splits the 1645nm band signal and the 822nm band signal light in the echo into two paths, which correspond to the detection channels of water vapor and methane, respectively.

7. The lidar system for measuring atmospheric methane column concentration and water vapor profile concentration according to claim 1, characterized in that, The first filter assembly (12) and the second filter assembly (13) respectively filter out the solar background light in the signals collected by the water vapor and methane detection channels.

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