A small laser radar for detecting water vapor in the lower troposphere

Abstract

This invention discloses a small lidar for detecting water vapor in the lower troposphere. Using the 935.6845nm strong absorption line as a reference, a 20GHz semiconductor microwave synthesizer is used for phase-locked loop (PLL) and frequency stabilization to maintain the emission wavelength of the front-stage semiconductor laser at a point of 935.7429nm on the slope of the suitable water vapor absorption line, adapting to the physical state where most water vapor is distributed within the boundary layer. The beginning of an optical fiber delay line is connected to the common transmit / receive end of the circulator, with the end of the delay line located at the telescope focal point. All end faces of the delay line are non-perpendicular. A gating time is defined as the period from the start of the emitted pulse until the pulse completely leaves the optical fiber delay line, which puts the photon counter into a dormant state to protect it, thereby shortening the blind zone. The period and duty cycle of the 935nm emitted laser pulse are controlled by another periodic laser pulse with a wavelength of 905nm.

Description

Technical Field

[0001] This invention relates to the field of low-light detection in the optical sensing technology industry, and to a small lidar, particularly a small lidar for detecting the vertical profile of water vapor in the lower troposphere. Background Technology

[0002] While significant progress has been made in space-based remote sensing of atmospheric temperature and water vapor, measured data from the lower troposphere remains indispensable for numerical weather models. The increasing frequency of sudden, localized extreme weather events such as localized torrential rains and short-duration heavy rainfall in recent years has added to the difficulty of accurate weather forecasting. Ground-based measurements of lower tropospheric thermodynamic vertical profile data still have significant gaps in both geographical and temporal distribution. Temporally, atmospheric water vapor vertical profiles change rapidly, and radiosonde balloons can only be released twice a day at fixed times, preventing continuous observation. Geographically, radiosonde balloons can only be deployed from fixed meteorological stations, resulting in a very sparse geographical distribution of measured meteorological data. To improve the accuracy of forecasts for mesoscale (geographical scale) and sudden (temporal scale) meteorological events, establishing a ground-based meteorological observation network, especially for intensive monitoring of rapid geographical and temporal changes in the atmospheric boundary layer water vapor vertical profile, and integrating ground-based observation network data with satellite remote sensing data, is a promising solution. Small lidar is the most promising node sensor for ground-based meteorological observation networks.

[0003] In 2004, a team led by Janet L. Machol at the University of Colorado and the NOAA (National Oceanic and Atmospheric Administration) Environmental Technology Laboratory reported on their development of an unmanned, automated micropulse differential absorption lidar (CODI) capable of detecting the vertical profile of atmospheric water vapor within 2 km of the troposphere. The transmitter is a distributed feedback (DFB) semiconductor laser coupled with a diode-flared amplifier, operating at a wavelength around 823 nm, with a pulse repetition frequency of 6 kHz-10 kHz, a pulse width of 600 ns, and a pulse energy of 0.15 mJ. The receiver is equipped with an avalanche diode Geiger photon counter (EG&G), a receiving telescope diameter of 35 cm, and a filter bandwidth of 160 pm, enabling the detection of atmospheric water vapor concentrations in the range of 0.5 g / m³. 3 -10g / m 3The error is within 15%. In 2010, Australian scientist Alex Dinovitser also established a micropulse differential absorption lidar for water vapor detection in this band. Its transmitter is a Fabry-Perot type diode laser, and it uses a 33m optical path, multi-channel water vapor optical absorption cell as a reference to lock the detection wavelength (online). A microwave oscillator locks the reference wavelength (offline) at a frequency 16 GHz above the detection wavelength. The optical frequency can be stabilized at about 1 MHz, the pulse energy is 300 nJ, the pulse repetition frequency is 3 kHz, and the pulse width is 1 μs. The receiver includes a 356 mm diameter telescope and a photon counter with a photomultiplier tube as the detector. Since 2010, literature has also reported the progress of scientists at NASA Langley Research Center, the University of Montana, and the National Center for Atmospheric Research in developing micropulse differential absorption lidar for atmospheric water vapor detection in the 828-830 nm band, which has now reached the fourth generation. The pulse width is 0.9μs-1μs, the pulse energy is 5μJ-10μJ, the pulse repetition frequency is 9kHz-10kHz, the telescope diameter is 406mm, the absolute accuracy of online and offline wavelength locking is ±0.2pm, and the wavelength repeatability is ±0.03pm. The same telescope is used for beam collimation and reception. The detector is a Geiger photon counting mode avalanche diode, capable of detecting the vertical distribution of water vapor below 5km, setting a record of 50 days of unattended automatic operation.

[0004] Most water vapor is concentrated in the near-surface boundary layer. Ground-based differential absorption lidar can only operate using wavelengths corresponding to the weak absorption lines of water vapor. Otherwise, using strong absorption lines as the detection wavelength results in backscattered echoes failing to reach the receiving telescope due to the significant absorption and attenuation by water vapor near the ground. Existing small pulse lidars also suffer from the following drawbacks: Since the weak absorption lines of water vapor are used as the detection wavelength, if the seed laser's detection wavelength is referenced to a multi-channel absorption cell for water vapor, the absorption cell requires a very long optical path (36m-100m, 2 Torr). Using a high-precision wavelength meter as the error acquisition component and establishing a negative feedback loop to actively stabilize the seed laser wavelength, the high-precision wavelength meter and the long-path multi-channel water vapor absorption cell constitute the main cost components of small lidars. Furthermore, existing lidars also suffer from an excessively long near-surface blind zone, meaning reliable data cannot be provided within 300m of the ground. These unfavorable factors continue to restrict the widespread deployment and networking of small lidars for detecting lower tropospheric water vapor profiles. Summary of the Invention

[0005] The purpose of this invention is to provide a small lidar for detecting the vertical profile of water vapor in the lower troposphere. This invention can achieve the following three objectives: First, it greatly shortens the optical path length of the multi-channel gas optical absorption cell, which serves as the reference for the active control loop of the detection wavelength; second, it removes the high-precision wavelength meter from the active locking control loop of the seed laser wavelength, further reducing the construction cost of the small lidar; and third, it reduces the near-ground blind zone of the lidar from more than 300 meters to less than 75 meters.

[0006] The present invention relates to a 935nm band miniature lidar comprising: a 935nm laser diode continuous wave master oscillator, a pulsed semiconductor optical amplifier for emitting high repetition frequency, low energy laser pulses; a transceiver circulator, a reflective telescope, and an optical fiber delay line; a combined filter including a quarter-wave plate, a Fabry-Perot etalon, and a narrowband filter; a weak signal detection unit with an enhanced silicon avalanche photodiode and a photon counter as its core; and several other components including system coordination and system management.

[0007] Between 934nm and 936nm wavelengths, water vapor molecules exhibit a strong absorption line at 935.6845nm (all wavelength values ​​mentioned throughout this text are under vacuum conditions), and the absorption cross-section at room temperature and pressure is greater than 2.05 × 10⁻⁶. -21 cm 2 (2.05×10 - 25 m 2 Using this wavelength as the target control wavelength for the water vapor multi-channel optical absorption cell, the optical path length of the water vapor multi-channel optical absorption cell can be selected to be much shorter, such as 80 cm (Wavelength Reference, USA). Using the 935.6845 nm water vapor strong absorption line as the wavelength reference, after a 20 GHz frequency offset lock (equivalent to shifting the wavelength 58.4 pm towards longer wavelengths), the detection wavelength (online) of the laser diode is stabilized near the side slope of the aforementioned strong absorption spectral line at approximately 935.7429 nm. The laser absorption cross-section of water vapor at wavelengths near 935.7429 nm is approximately 5.61 × 10⁻⁶. -23 cm 2 The reference wavelength (offline) was chosen to be around 935.3869 nm, which has a relatively small absorption cross-section for water vapor. Its absorption cross-section at room temperature and pressure is approximately 5.19 × 10⁻⁶. -24 cm 2 .

[0008] In mid-latitude winters, the atmosphere is relatively dry, and wavelengths around 935.7429 nm and 935.3869 nm are selected as the detection and reference wavelengths for lidar. In mid-latitude summers, the atmosphere is relatively humid, and wavelengths around 935.8525 nm and 935.3869 nm are selected as the detection and reference wavelengths for lidar. The continuous wave laser of the online / offline laser diode is alternately selected and connected to the semiconductor optical amplifier by an electro-optic high-speed switch, outputting laser pulses with a time width of 500-900 ns, a repetition frequency of 7 kHz, and an energy of 5 μJ-10 μJ.

[0009] like Figure 2 As shown, the spectral characteristic curve of the water vapor absorption cross-section is relatively flat near the wavelength of 935.8525 nm. The change in the absorption cross-section is small with wavelength fluctuations and drift. Therefore, the measurement error caused by these wavelength fluctuations and drifts is relatively small. Thus, a laser diode emitting at a wavelength of 935.8525 nm can be stabilized using a passive frequency stabilization method with constant temperature and constant current. Similarly, a laser diode emitting at a wavelength of 935.3869 nm can also be stabilized using a passive frequency stabilization method with constant temperature and constant current.

[0010] This arrangement eliminates the need for a high-precision wavelength meter to be used in the active-locked loop for the detection wavelength around 935.7429nm, and for a laser diode whose reference wavelength is passively stabilized at 935.3869nm. Because the multi-channel optical absorption cell for water vapor uses the strong absorption line at 935.6845nm as its control target, the optical path of the multi-channel optical absorption cell for water vapor is greatly shortened.

[0011] Using a circulator as a switching component for lidar transmission / reception, with a single telescope shared for both laser transmission and echo reception, maximizes the diameter of the transmitted beam, minimizes its divergence angle, and reduces eye safety risks. However, absolute isolation between the transmitter and receiver ends of the circulator is impossible; a small portion of the transmitted laser pulse energy will always crosstalk into the receiving optical path. This crosstalk pulse is significantly stronger than the echo, and it can irradiate and damage the photosensitive surface of the avalanche photodiode. Therefore, on the one hand, the primary and secondary mirrors of the telescope are off-axis curved mirrors to completely prevent reflected light from returning to the circulator; on the other hand, a multimode fiber is connected between the common transmitter / receive end of the circulator and the focal point of the telescope. The use of a delay line, specifically an optical fiber delay line, adds a time delay between the moment the laser pulse is emitted into the atmosphere and the moment the backscattered echo enters the detector. This time delay becomes the gating time of the photon counting module. During the gating time, the emitted laser pulse is fed into the single-photon counting module, putting the avalanche photodiode into a dormant, non-operating state, thus preventing strong crosstalk light from damaging the avalanche photodiode. This is achieved by placing a 1V forward bias voltage across the PN junction of the avalanche photodiode during the gating time. Once the emitted laser pulse has completely left the optical delay line, the PN junction of the avalanche photodiode is transposed to a voltage slightly higher than the breakdown voltage, entering normal operation. Due to the presence of the optical fiber delay line, there is a time difference between the backscattered light echo and the emitted pulse crosstalk light pulse, making it easier to distinguish between the emitted pulse signal and the echo signal. This timing and structural arrangement ensures that the detector detects the backscattered echo as soon as the emitted light pulse enters the atmospheric boundary layer, thereby shortening the blind zone length of the lidar for near-surface water vapor observation.

[0012] The following section, in conjunction with the accompanying drawings, details a small lidar in the 935nm band and a method for detecting water vapor in the lower troposphere. Attached Figure Description

[0013] Figure 1 This is a diagram of a small lidar system in the 935nm band;

[0014] Figure 2 This is a diagram showing the composition of unit 1-1 (detection wavelength stabilization unit) and unit 1-2 (reference wavelength unit) in a small lidar.

[0015] Figure 3 This is the spectral distribution of the absorption cross section of water vapor in the 935.2nm-936.1nm range;

[0016] Figure 4 This is the optical thickness spectral distribution of water vapor in the 935.2nm-936.1nm range. Detailed Implementation

[0017] The transmitter includes an online probe wavelength diode laser 1-1 (TOPTICA Photonics AG, #LD-0935-0080-DFB-1, EYP-DFB-0935-00080-1500-TOC03-0005; Sacher-laser), a current-driven temperature controller, and an active wavelength control loop; an offline reference wavelength diode laser 1-2, a current-driven temperature controller, and a passive wavelength stabilization unit; optical cones 2-2 and 2-1; 1×1 (Agiltron, NanoSpeed) electro-optic switches 3-1 and 3-2; and a 2×1 (Agiltron, NanoSpeed) 2-to-1 electro-optic switch 4. Under the coordination of the microprocessor 30, the online probe wavelength laser and the offline reference wavelength laser are alternately coupled to the semiconductor optical amplifier 8 (Eagleyard Photonics) according to the set program. GmbH, EYP-TPA-0925-01500-3006-CMT03-0000) connects, 5 is an optical isolator to prevent reflected light from returning to diode lasers 1-1 and 1-2, 6 is a dichroic filter to combine the transmitted 935nm laser and the reflected 905nm laser into a single optical path, and lens 7 couples the diode laser into the semiconductor optical amplifier 8. 9 is a constant temperature and constant current driver. The spherical and cylindrical combination lens 10 couples the amplified laser beam out of the semiconductor optical amplifier 8 and shapes the elliptical beam output from it. 11 is an optical beam expander, which reduces the divergence angle of the laser beam after energy amplification. The laser beam collimated by the beam expander 11 enters the optical circulator 12. The optical circulator 12 is a switching device between the receiving and transmitting parts of the lidar. The laser beam output from the optical circulator 12 is coupled by the lens 13 into the multimode fiber delay line 14. The optical output end of the fiber delay line 14 coincides with the focal point of the telescope 16. 15 is the aperture at the focal point. The laser beam emitted from the focal point is expanded by the telescope 16 into the atmospheric troposphere. The backscattered light from the lower troposphere is collected by the telescope 16 and converges at the focal point of the aperture 15. It is then coupled into the fiber delay line 14 and the optical circulator 12. The backscattered echo output from the circulator then enters the detection part of the lidar system.

[0018] The echo light is coupled by lens 17 into multimode fiber 18 (numerical aperture NA = 0.22, core diameter 105 μm). Multimode fiber 18 facilitates structural separation of the detection section from other parts of the system; in other words, the flexibility of the fiber allows for flexible placement of the detection section within the lidar system. The echo light output from multimode fiber 18 is collimated by lens 19 and then passes through a polarization beam splitter 24, a quarter-wave plate, a Fabry-Perot etalon (CVI, laseroptics, USA; Light Machinery), and a narrowband filter (Andover Corporation; Barr.INC.). A composite filter composed of (USA) components effectively suppresses ambient light entering the system from telescope 16. After separation by the composite filter, the echo light enters three detection channels according to wavelength. The first channel consists of a 935.7979nm quarter-wave plate 20-1, a Fabry-Perot etalon 21-1 with a center wavelength of 935.3869nm, a narrowband filter 22-1 with a center wavelength of 935.3869nm, a coupling lens 23-1, a multimode fiber 18, and a single-photon detection module 25, which detects the number of photons at the 935.3869nm wavelength. The second channel consists of a 935.3869nm quarter-wave plate 20-2 (JDSU CASIX, The system comprises: a Fabry-Perot etalon 21-2 with a center wavelength of 935.8525nm, a narrowband filter 22-2 with a center wavelength of 935.8525nm, a coupling lens 23-2, a multimode fiber 18, and a single-photon detection module 26, for detecting the number of photons at a wavelength of 935.8525nm; and a quarter-wave plate 20-3 with a center wavelength of 935.3869nm, a Fabry-Perot etalon 21-3 with a center wavelength of 935.7429nm, a narrowband filter 22-3 with a center wavelength of 935.7429nm, a coupling lens 23-2, a multimode fiber 18, and a single-photon detection module 27 (Excelitas, SPCM-AQRH-13-FC), for detecting the number of photons at a wavelength of 935.7429nm.The other end face of the multimode fiber 18 is closely attached to the photosensitive surface of the avalanche photodiode in the single-photon detection modules 25, 26, and 27 (Perkin Elmer C30955ETC, Laser Component, A-CUBE-S1500-25). The values ​​output by the single-photon detection modules 25, 26, and 27 are sent to the data input ports 28-1, 28-2, and 28-3 of the multi-channel data accumulator (Sigma Space Corporation). 28-4 is the synchronization control port of the multi-channel data accumulator 28, which is connected to the pulse generator 29 and receives the synchronization trigger pulse from the pulse generator 29. 28-5 is the data output terminal of the multi-channel data accumulator 28, which is connected to the microprocessor 30 and sends the accumulated data to the microprocessor 30.

[0019] The microprocessor 30 receives and processes data from the data accumulator 28, sends trigger commands to the pulse generator 29, sends switching commands to the digital switch driver 31, and sends error signals of the negative feedback control loop to the diode laser driver 32.

[0020] On one hand, pulse generator 29 sends switching pulses to the current driver 33 of 905nm laser diode 34. Under the control of the periodic pulses of pulse generator 29, 905nm laser diode 34 emits 905nm periodic pulsed laser light. On the other hand, pulse generator 29 sends gate pulses to single-photon detection modules 25, 26, and 27 to cut off the detected photon signal, and also sends synchronization signal pulses to multi-channel digital accumulator 28 to coordinate the timing between 935nm laser pulse emission and multi-channel digital accumulator receiving photon signal. Digital switch driver 31 provides high and low levels for switches 3-1, 3-2, and 4. 32 represents the current driver and temperature controller of the laser diodes in units 1-1 and 1-2 of the lidar.

[0021] A 905nm pulsed laser and a 935nm continuous wave, single-frequency low-power seed laser are simultaneously incident on the open end face of the semiconductor optical amplifier 8, entering the active region of the semiconductor optical amplifier 8. The 905nm high-power light pulse will block the active region and will not amplify the 935nm laser. When the 905nm high-power light pulse is interrupted, the semiconductor optical amplifier 8 will release the 935nm pulsed laser and amplify the peak power of the 935nm laser. Until the next 905nm high-power light pulse arrives, the semiconductor optical amplifier 8 will stop releasing the 935nm laser again. This cycle repeats, realizing the use of the 905nm laser pulse to control the output of the 935nm laser pulse of the semiconductor optical amplifier, including the width and duty cycle of the 935nm laser output pulse. The constant temperature and constant current driver 9 provides a stable continuous current and constant temperature for the semiconductor optical amplifier 8.

[0022] All end faces of the optical elements through which the laser beam passes are coated with anti-reflection films. The end faces of the optical fibers are never perpendicular to the optical axis; instead, the incident angle of the beam entering the end face of the optical fiber is less than 90°. The primary and secondary mirrors of telescope 16 are off-axis curved mirrors to avoid back-transmission of light caused by mirror reflection.

[0023] The pulse generator 29 indirectly controls the semiconductor optical amplifier 8 to cut off or emit a 935nm laser pulse by controlling the emission and cutoff of the 905nm laser pulse. The 935nm light echo, after passing through the combined filter, is coupled into the multimode fiber 18 by the lens 23. Changing the distance between the coupling lens 23 and the end face of the multimode fiber 18 adjusts the receiving field of view. Under the coordination of the pulse generator 29, the single-photon detection modules 25, 26, and 27 enter a cutoff, dormant, and non-operating state before the 935nm laser pulse is emitted. A 1V forward voltage is applied across the PN junction of their avalanche photodiodes, with the timing advance determined by the pulse generator 29. From the emission of the 935nm laser pulse until it completely leaves the fiber delay line, the single-photon detection modules 25, 26, and 27 are inactive and in a dormant protection state to prevent 935nm emission. The optical pulse enters the avalanche photodiode through crosstalk of the circulator, causing a large current that damages the single-photon counter. After the 935nm optical pulse is emitted, a fiber delay line 14 is immediately following the transmit / receive common terminal of the optical circulator 12, which provides a gate time for the avalanche photodiode to enter its cutoff and sleep state. The gate time is the width of the emitted optical pulse plus the time delay caused by the fiber delay line 14. Once the 935nm optical pulse completely leaves the fiber delay line 14, the avalanche photodiode immediately ends its sleep state and enters normal operation, starting to detect and record photons. The PN junction of the avalanche photodiode is subjected to a reverse voltage, slightly higher than the reverse breakdown voltage, which means it is in Geiger mode.

[0024] For the seed unit with a reference wavelength of 935.3869nm (1-2), it consists of two parts: a laser diode 1-2-1 and a thermoelectric cooler 1-2-2. The laser diode 1-2-1 integrates a thermistor and a thermoelectric cooler (TEC). The constant current source circuit injects a constant current (RMS wideband noise and ripple, 5 Hz..1MHz: typ.10μA, Noise Density at 1kHz: typ.60nA / sqrt (Hz)) into the active region of the laser diode 1-2-1. The laser diode 1-2-1 can emit continuous wave laser light at a relatively stable wavelength. In addition, the thermistor senses the temperature of the laser diode 1-2-1. The proportional-integral-derivative (PID) circuit actively stabilizes the temperature of the laser diode 1-2-1 by changing the voltage width of the pulse width modulator (PWM) supplied to the cooler (TEC).

[0025] The detection wavelength unit 1-1 includes two sub-units 1-1-1 and 1-1-2. Sub-unit 1-1-1 controls the laser diode 1-1-1-1 to operate on a strong absorption line of 935.6845 nm in water vapor. The single-frequency, narrow-linewidth laser emitted by the laser diode 1-1-1-1 is phase-modulated by the electro-optic phase modulator 1-1-1-3 (Photline / Ixblue, NIR-MPX950-LN-0.1, France) with a waveform of Ecos2πft and a modulation frequency f = 240 MHz. The modulated continuous... The wave laser passes through the multi-channel water vapor optical absorption cell (1-1-1-4). After power attenuation due to water vapor absorption, the remaining modulated laser is coherently detected by the silicon detector (1-1-1-5). The signal after coherent detection is further amplified by the transimpedance amplifier (1-1-1-6). The power divider (SBTC-2-10-7550+) (1-1-1-14) splits the 240MHz RF oscillation signal Ecos2πft generated by the RF oscillator (1-1-1-13) into two parts. One part is sent to the RF input port of the electro-optic phase modulator (1-1-1-3), becoming the signal of the electro-optic phase modulator (1-1-1-1). The RF drive signal of 3, and another part of the 240MHz RF oscillation signal, after being phase-shifted by phase shifter 1-1-1-15, are sent to the x input port of multiplier 1-1-1-7 (AD834). At the same time, the signal amplified by transimpedance amplifier 1-1-1-6 is sent to the y input port of multiplier 1-1-1-7. The two signals meet and are mixed. The mixed electrical signal is output from the out output port of mixer 1-1-1-7, and then filtered by low-pass filter 1-1-1-8 to remove the RF components of frequencies f, 2f, and 3f. If the laser diode 1-1-1 If the wavelength of the continuous wave emitted by the laser diode 1-1-1-1 is equal to the center wavelength of the absorption line of the multi-channel water vapor optical absorption cell (935.6845nm), then the output signal of the 1-1-1-8 low-pass filter will be 0. If the wavelength of the continuous wave emitted by the laser diode 1-1-1-1 is not equal to the center wavelength of the absorption line of the multi-channel water vapor optical absorption cell (935.6845nm), the 1-1-1-8 low-pass filter will output a DC level. The magnitude and sign of this DC level correspond to the difference between the wavelength of the continuous wave emitted by the laser diode 1-1-1-1 and the center wavelength of the water vapor absorption line (935.6845nm).Therefore, this DC level is transmitted to the microprocessor 1-1-1-10 via the A / D converter 1-1-1-9. Based on the input digital signal and the set servo control program, the microprocessor 1-1-1-10 outputs a suitable error digital signal, which is then converted into an analog error signal by the D / A converter 1-1-1-11. This changes the additional injection current of the driver 1-1-1-12 of the laser diode 1-1-1-1 and the additional voltage controlled by thermoelectricity, so that the continuous wave wavelength emitted by the laser diode 1-1-1-1 approaches the reference wavelength of 935.6845nm.

[0026] The function of the secondary probe wavelength seed laser unit 1-1-2 is to precisely lock the difference between the frequency of the continuous wave light emitted by the laser diode 1-1-2-1 and the frequency of the continuous wave light emitted by the laser diode 1-1-1-1 to a bias frequency of 20 GHz. On one hand, after passing through optical couplers 1-1-1-2 and 1-1-2-3, the low-power continuous wave laser emitted by laser diode 1-1-1-1 mixes with the low-power continuous wave laser emitted by laser diode 1-1-2-1. The optical frequency difference between the two is detected by high-speed optical receiver 1-1-2-4 (NewFocus Model1414-50, Band Width 25GHz). This difference frequency signal belongs to the microwave band and is amplified by limiting amplifier 1-1-2-5. On the other hand, microwave synthesizer module 1-1-2-7 (LowPhase Noise, 0.2 to 20GHz, SOT-02220313200-SF-B6, SAGE Millimeter, Inc) also accurately outputs a 20GHz microwave radio frequency signal.

[0027] The microwave limiting amplifier 1-1-2-5 outputs microwave RF signals to the mixer 1-1-2-6 (GaAs-Double-Balanced Mixer, 4.5-24GHz, MM1-0424S, Marki Microwave, Inc.) RF terminal; the microwave synthesizer module 1-1-2-7 outputs RF signals to the mixer 1-1-2-6 (GaAs-Double-Balanced Mixer, 4.5-24GHz, MM1-0424S, Marki Microwave, Inc.). The LO port of Inc.) The RF signal IF after mixing is called the down-converted signal. Its power is split into two by the power divider 1-1-2-8. One part is sent to the low-pass filter 1-1-2-9 for filtering and then sent to the power detector 1-1-2-10 to detect its power. The other part of the RF signal power after mixing does not pass through the low-pass filter 1-1-2-9 and is directly detected by the detector 1-1-2-10. The power values ​​of both parts are sent to the A / D converter 1-1-2-11 to be converted into digital signals. The microprocessor 1-1-2-12 (30) calculates the transmittance of the down-converted RF signal after mixing relative to the low-pass filter 1-1-2-9 based on the two power values. The microprocessor 1-1-2-12 is based on the transmittance of the down-converted signal relative to the low-pass filter 1-1-2-9. The frequency response curve of the low-pass filter (the frequency range of the low-pass filter is DC-4GHz, and the gain of the low-pass filter signal is in one-to-one correspondence with the frequency of this signal) stored in the microprocessor 1-1-2-12 can be used to determine the frequency of the down-converted signal. This frequency is the difference between the optical difference frequency signal of the two diode lasers detected by the high-speed optical receiver 1-1-2-4 and the frequency of the 20GHz microwave synthesized signal. Therefore, the microprocessor 1-1-2-12 outputs a digital error signal. This digital error signal is converted into an analog error signal by the DA converter 1-1-2-13. This analog error signal changes the additional injection current and additional thermoelectric control voltage of the driver 1-1-2-14 of the laser diode 1-1-2-1, so that the optical frequency difference of the two diode lasers approaches 20GHz. When the optical frequency difference between laser diode 1-1-2-1 and laser diode 1-1-1-1 is stabilized at 20GHz, and the optical wavelength of laser diode 1-1-1-1 is locked at 935.6845nm, then the emission wavelength of laser diode 1-1-2-1 is stabilized at around 935.7429nm.

[0028] The online / offline switching frequency is 100Hz (j=1-30), and the laser emission pulse frequency is 7kHz. This means that for every 70 pulses (i=1-70) emitted by the optical amplifier, switches 3 and 4 switch the emission wavelength once. We assume the system's time resolution is 1 minute, with 210 pulse pairs as a group. A set of vertical profile data is calculated and output, and the cumulative number of photons from the 70 pulse echoes is N. j,on Subtract the background count B on One wavelength switch produces 70 pulse echo photons, accumulating to N. j,off Subtract the background count B j,off The energy of the 70 pulses monitored for transmission is E j,on and E j,off The range resolution ΔR, and the differential absorption cross section of the lidar for water vapor Δσ = σ on -σ off The distance resolution is 75 meters inside the boundary layer and 150-300 meters outside the boundary layer.

[0029] ;

[0030] ;

[0031] ;

[0032] ;

[0033] ;

[0034] ;

[0035] Water vapor concentration: Unit: m -3 .

[0036] In mid-latitude regions during winter, the atmosphere is relatively dry. The wavelengths around 935.7429 nm and 935.3869 nm were chosen as the detection and reference wavelengths for lidar. The absorption cross-section at room temperature and pressure is approximately 5.61 × 10⁻⁶. -23 cm 2 / 5.19×10 -24 cm 2 .

[0037] In mid-latitude regions during summer, the atmosphere is relatively humid. The wavelengths around 935.8529 nm and 935.3869 nm were chosen as the detection and reference wavelengths for lidar. The absorption cross-section at room temperature and pressure is approximately 1.21 × 10⁻⁶. -23 cm 2 / 5.19×10 -24 cm2 .

[0038] From the water vapor absorption spectrum curve, the absorption cross section does not change much with wavelength fluctuations and drift around 935.8529 nm. Therefore, the emission wavelength of the laser diode can be stabilized at 935.8529 nm through passive stabilization measures.

Claims

1. A small lidar for detecting water vapor in the lower troposphere, comprising a 935nm laser diode continuous wave master oscillator, a pulsed semiconductor optical amplifier emitting high repetition frequency, low energy laser pulses; a transceiver circulator, a reflective telescope and fiber optic delay line, a quarter-wave plate, a combined filter of Fabry-Perot etalon and narrowband filter, a weak signal detection unit with an enhanced silicon avalanche photodiode and photon counter as its core, and system coordination and management; characterized in that: Water vapor molecules have a strong absorption line at 935.6845 nm in the 934 nm-936 nm range, and the absorption cross-section at room temperature and pressure is approximately 2.05 × 10⁻⁶. -21 cm 2 Using this wavelength as the target control wavelength for the water vapor multi-channel optical absorption cell (1-1-1-4), the optical path length of the water vapor multi-channel optical absorption cell (1-1-1-4) can be selected to be a relatively short value of 80 cm or less. Based on the 935.6845 nm water vapor intensity absorption line, and after 20 GHz frequency polarization locking, the emission wavelength of another laser diode (1-1-2-1) is stabilized at a specific position around 935.7429 nm. The laser absorption cross-section of water vapor at this wavelength is approximately 5.61 × 10⁻⁶. -23 cm 2 The emission wavelength is less than the 935.6845nm water vapor absorption cross-section, and this wavelength is chosen as the detection wavelength for the lidar. The reference wavelength is chosen to be near 935.3869nm, which has a relatively small water vapor absorption cross-section, and its absorption cross-section at room temperature and pressure is approximately 5.19 × 10⁻⁶. -24 cm 2 ; In mid-latitude regions during winter, the atmosphere is relatively dry. The wavelength of 935.7429 nm was selected as the detection wavelength for lidar, and 935.3869 nm was selected as the reference wavelength for lidar. At the wavelength of 935.3869 nm, the spectral characteristic curve of the water vapor absorption cross-section is relatively flat, and the change in the absorption cross-section is not significant with wavelength fluctuations and drifts. The measurement error of water vapor caused by such wavelength fluctuations and drifts is small. Therefore, the laser diode emitting the wavelength of 935.3869 nm adopts passive frequency stabilization measures with constant temperature and constant current. In mid-latitude regions during summer, the atmosphere is relatively hot and humid. The wavelength around 935.8529 nm was chosen as the detection wavelength for lidar, and the wavelength around 935.3869 nm was chosen as the reference wavelength for lidar. At the wavelength around 935.8529 nm, the spectral characteristic curve of the water vapor absorption cross-section is also relatively flat, and the change in the absorption cross-section with wavelength fluctuations and drifts is not significant. The measurement error of water vapor caused by such wavelength fluctuations and drifts is also small. Therefore, the laser diode emitting a wavelength around 935.8529 nm also adopts passive frequency stabilization measures with constant temperature and constant current. The laser emission and echo reception share a single telescope (16), thereby maximizing the diameter of the emitted beam, reducing the divergence angle of the emitted laser beam, and reducing the safety risk to the human eye. One end of the fiber delay line (14) is connected to the common end of the circulator (12) for both emission and reception. The laser pulse from the leading edge of the 935nm emission reaches the fiber delay line (14), and the trailing edge of the laser pulse leaves the other end of the fiber delay line (14). This period is used as the gate time of the photon counting modules (25, 26, 27). During the gate time, the avalanche photodiode is in a dormant, non-working state, preventing a small portion of the power of the 935nm emission laser pulse from interfering with the optical path of the single-photon detector and damaging the avalanche photodiode. Due to the presence of the fiber delay line (14), there is a time difference between the backscattered light echo and the emitted pulse crosstalk light pulse, making it easy to distinguish the emitted pulse signal from the echo signal.

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