Broadband micro-pulse differential absorption laser radar

Abstract

The invention provides a broadband micro-pulse differential absorption laser radar, which comprises a transmitter, a receiver and a telescope primary mirror, and is characterized in that the transmitter comprises an on-line transmitting unit, an off-line transmitting unit, a first collimating lens, a second collimating lens, a first 1 / 2 wave plate, a second 1 / 2 wave plate, a polarization beam combiner, a first 1 / 4 wave plate and a first negative lens; the receiver comprises an on-line detection unit, an off-line detection unit, a 45-degree total reflection mirror with a center hole, a second negative lens, a second 1 / 4 wave plate, a polarization beam splitter, a first optical filter, a second optical filter, a first convergent lens and a second convergent lens. And constructing an objective function by the broadband emission beam spectral function, the water vapor absorption cross section spectral function and the normalized receiving signal, and estimating water vapor molecular density elevation distribution which minimizes the objective function value to form a water vapor vertical profile. The laser radar is low in power consumption, simple and reliable in structure, safe in human eyes and unattended, and can become a node sensor of a new generation of national and foundation meteorological observation networks.

Description

Technical Field

[0001] The present invention relates to the technical field of lidar, and particularly to a broadband micropulse differential absorption lidar. Background Art

[0002] In order to better understand the water and energy cycles, there is an urgent need to build a nationally comprehensive thermodynamic monitoring network in the near-surface boundary layer to fill the key gaps that currently hinder the weather model forecasting ability.

[0003] Radiosondes remain the standard means for globally detecting the true value of water vapor. Due to practical circumstances, most meteorological service stations only launch radiosondes twice a day. These locations are usually hundreds of kilometers apart. The existing radiosonde network cannot provide sufficient temporal resolution and spatial resolution across the surface to fully sample the rapidly changing water vapor distribution, especially in the atmospheric boundary layer (ABL). The key to improving the spatial resolution across the surface is to find a reliable and low-cost device that can be deployed in large quantities.

[0004] Multi-channel microwave radiometers (MWRs) and infrared Fourier transform interferometers (AERls), which are passive remote sensing instruments, have been commercialized. Through inversion using appropriate radiative transfer models, vertically resolved water vapor mixing ratio profiles have been obtained, but the vertical resolution is still relatively sparse. Active remote sensing techniques for measuring water vapor include differential absorption lidar and Raman lidar. Raman lidar infers the water vapor mixing ratio using the weak inelastic scattering from atmospheric water vapor molecules. Raman lidar has been proven to be able to provide good temporal and vertical resolutions and accurate water vapor mixing ratio profiles. Currently, only a few of these instruments are operating globally. These expensive, research-grade instruments require powerful (usually eye-unsafe) laser transmitters and large receiving apertures to detect the weak inelastic backscattering. Raman lidar also requires an independent, co-located radiosonde for calibration. In addition, Raman lidar exhibits a certain degree of residual overlap factor, which may lead to significant deviations in the low-altitude water vapor mixing ratio.

[0005] Currently, there is a need to find a reliable, low-cost lidar that can be deployed in large quantities to provide strong support for building a nationally comprehensive near-surface boundary layer thermodynamic monitoring network. Summary of the Invention

[0006] The problem to be solved by the present invention is to provide a broadband micropulse differential absorption lidar that can reduce the usage cost and facilitate large-scale deployment.

[0007] To solve the above technical problems, a broadband micro-pulse differential absorption lidar provided by the present invention is adopted, which includes a transmitter, a receiver, and a telescope primary mirror. The transmitter includes an online transmitting unit, an offline transmitting unit, a first collimating lens, a second collimating lens, a first half-wave plate, a second half-wave plate, a polarization beam combiner, a first quarter-wave plate, and a first negative lens. The receiver includes an online detecting unit, an offline detecting unit, a 45° total reflection mirror with a central hole, a second negative lens, a second quarter-wave plate, a polarization beam splitter, a first filter, a second filter, a first converging lens, and a second converging lens. The transmitter and the receiver share the telescope primary mirror.

[0008] Preferably, the online transmitting unit and the offline transmitting unit alternately transmit for 5 seconds to eliminate the optical crosstalk between the online transmitting unit and the offline transmitting unit.

[0009] Preferably, both the online transmitting unit and the offline transmitting unit include laser diodes, the emission wavelength range of the laser diodes is 905 nm - 945 nm, and the absorption wavelength difference between the laser diodes of the online transmitting unit and the laser diodes of the offline transmitting unit is within 0.6 nm.

[0010] Preferably, both the online detecting unit and the offline detecting unit include avalanche photodiode detectors.

[0011] The beneficial effects of the present invention are as follows: The present invention provides a broadband micro-pulse differential absorption lidar. After the emission beams of the online transmitting unit and the offline transmitting unit pass through the first half-wave plate and the second half-wave plate respectively, the polarization directions of the online laser beam and the offline laser beam are orthogonal. The two beams are combined by the polarization beam combiner, and then emitted after passing through the first quarter-wave plate. Finally, they are expanded and collimated by the telescope primary mirror and enter the atmosphere. The backscattered light collected by the telescope primary mirror is collimated with an appropriate magnification, and then passes through the second quarter-wave plate and the polarization beam splitter to separate the backscattered light of the online wavelength and the offline wavelength, which are respectively detected by the online detecting unit and the offline detecting unit. By adjusting the postures of the polarization beam combiner and the polarization beam splitter through the servo system, it is ensured that the online laser beam and the offline laser beam are completely coincident. This lidar has low power consumption, a simple structure, is reliable, eye-safe, and does not require personnel on duty. It can become a node sensor for a new generation of national and ground-based meteorological observation networks, can be deployed in large quantities, and provides strong guarantee for building a nationwide near-surface boundary layer thermodynamics monitoring network. Description of the Drawings

[0012] Figure 1 Illustrates the schematic composition of the lidar of the present invention.

[0013] Figure 2 Illustrates the normalized water vapor absorption cross-section, normalized online laser emission spectrum, and normalized offline laser emission spectrum of the present invention at 296K and 1atm.

[0014] Figure 3 Illustrates the physical meaning of some expressions appearing in the objective function cost(z) of the present invention.

[0015] Explanation of the reference numerals in the drawings: transmitter 1, online transmitting unit 10, offline transmitting unit 11, first collimating lens 12, second collimating lens 13, first 1 / 2 wave plate 14, second 1 / 2 wave plate 15, polarization beam combiner 16, first 1 / 4 wave plate 17, first negative lens 18, receiver 2, online detection unit 20, offline detection unit 21, 45° total reflection mirror 22, middle hole 220, second negative lens 23, second 1 / 4 wave plate 24, polarization beam splitter 25, first filter 26, second filter 27, first converging lens 28, second converging lens 29, telescope primary mirror 3. Detailed implementation manners

[0016] To make the objectives, technical solutions, and advantages of the embodiments of the present disclosure clearer, the technical solutions of the embodiments of the present disclosure will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present disclosure. Obviously, the described embodiments are some, but not all, of the embodiments of the present disclosure.

[0017] All other embodiments obtained by those of ordinary skill in the art based on the described embodiments of the present disclosure without creative efforts shall fall within the scope of protection of the present disclosure.

[0018] Refer to Figures 1-3 .

[0019] The present invention provides a broadband micropulse differential absorption lidar, which includes a transmitter 1, a receiver 2, and a telescope primary mirror 3. The transmitter 1 includes an online transmitting unit 10, an offline transmitting unit 11, a first collimating lens 12, a second collimating lens 13, a first 1 / 2 wave plate 14, a second 1 / 2 wave plate 15, a polarization beam combiner 16, a first 1 / 4 wave plate 17, and a first negative lens 18; the receiver 2 includes an online detection unit 20, an offline detection unit 21, a 45° total reflection mirror 22 with a middle hole 220, a second negative lens 23, a second 1 / 4 wave plate 24, a polarization beam splitter 25, a first filter 26, a second filter 27, a first converging lens 28, and a second converging lens 29. The transmitter 1 and the receiver 2 share the telescope primary mirror 3.

[0020] Specifically, the laser beam with the absorption wavelength Online and the laser beam with the comparison wavelength offline are simultaneously emitted through a common aperture, and their atmospheric backscattering is also collected through the common aperture at the same time. The transmitting aperture and the receiving aperture share the same telescope primary mirror 3. The beam from the transmitting optical path exits from the central circular area of the telescope primary mirror 3; the outer peripheral circular area of the telescope primary mirror 3 collects the backscattered light, which is reflected by the 45° total reflection mirror 22 and then enters the echo detection optical path. The 45° total reflection mirror 22 with the central hole 220 is the separation element between the transmitting optical path and the receiving optical path. The transmitting beam leaves through the central hole 220 of the 45° total reflection mirror 22, and the echo beam is deflected from the peripheral circular ring of the 45° total reflection mirror 22 to the receiving optical channel. After the transmitting beam of the online transmitting unit and the transmitting beam of the offline transmitting unit pass through the first 1 / 2 wave plate 14 and the second 1 / 2 wave plate 15 respectively, the polarization directions of the online laser beam and the offline laser beam are orthogonal. The two beams are overlapped by the polarization beam combiner 16, and then emitted after passing through the first 1 / 4 wave plate 17. Finally, they are expanded and collimated by the telescope primary mirror 3 and then enter the atmosphere; the backscattered light collected by the telescope primary mirror 3 is collimated with an appropriate magnification, and then passes through the second 1 / 4 wave plate 24 and the polarization beam splitter 25 to separate the backscattered light of the online wavelength and the offline wavelength, which are respectively detected by the online detection unit 20 and the offline detection unit 21. By adjusting the postures of the polarization beam combiner 16 and the polarization beam splitter 25 through the servo system, it is ensured that the online laser beam and the offline laser beam are completely overlapped. This lidar has low power consumption, a simple structure, is reliable, eye-safe, and does not require personnel on duty. It can become a node sensor for a new generation of national and ground-based meteorological observation networks, can be deployed in large quantities, and provides strong guarantee for building a thermodynamics monitoring network covering the near-surface boundary layer nationwide.

[0021] Based on the above embodiments, the online transmitting unit 10 and the offline transmitting unit 11 alternately emit for 5 seconds to eliminate the optical crosstalk between the online transmitting unit 10 and the offline transmitting unit 11.

[0022] Based on the above embodiments, both the online transmitting unit 10 and the offline transmitting unit 11 include laser diodes. The emission wavelength range of the laser diodes is 905 nm - 945 nm, and the difference in absorption wavelength between the laser diodes of the online transmitting unit 10 and the laser diodes of the offline transmitting unit 11 is within 0.6 nm. The commercial supply of light sources in this wavelength band is relatively easy, meeting the requirement of low cost; secondly, the optical efficiency of the diode lasers in this wavelength band is high, making it easier to meet the need for low power consumption; the diode lasers and their drivers are small in volume and light in weight, which is conducive to the simple structure of the lidar system.

[0023] Based on the above embodiments, both the online detection unit 20 and the offline detection unit 21 include avalanche photodiode detectors to measure the signals at each wavelength in a linear analog mode.

[0024] The broadband micropulse differential absorption lidar consists of a transmitter 1, a receiver 2, a data processing and transmission system for coordination and timing control, a power supply, etc. Data processing and transmission, system coordination and timing control are almost essential software and hardware parts for all lidars. The following presents specific implementation solutions for the implementation of the transmitter 1 and the receiver 2.

[0025] This lidar consists of two measurement units. One is the near-field measurement unit, and the other is the far-field measurement unit. The near-field telescope has a diameter of φ150 ± 20m; the far-field telescope has a diameter of φ300 ± 50m. Each measurement unit uses one transmitter, a common optical path for online and offline signals, and a detection unit. This ensures that the receiver efficiency and overlap performance of the online and offline signals are basically the same. The near-field unit is responsible for detecting the vertical water vapor profile in the range of 50m - 600m above the ground, and the far-field unit is responsible for the vertical water vapor profile above 400m above the ground. The data in the profile transition area is generated by linearly weighted averaging of the two profile values. In this transition area, the weight changes linearly, combining the near-field water vapor profile and the far-field water vapor profile into a continuous water vapor profile. Therefore, the data at the front end of the transition area comes entirely from the near-field unit below 450 meters, and the data at the end of the transition area comes entirely from the far-field unit above 550 meters.

[0026] Transmitting alternately for 5 seconds in each unit eliminates the echo crosstalk between the two units. During each 5-second measurement cycle, the online and offline echo signals are time-averaged. Vertically, a quasi-Gaussian weighting function is also used to smooth the echo signals, where the full width at half maximum (FWHM) of the Gaussian function increases with height, ranging from 10m at the ground surface to 200m at 1000m AGL and 500m at 3000m AGL. This vertical averaging scheme helps to maintain consistent data quality when the signal-to-noise ratio decreases with height.

[0027] The transmitter 1 includes a laser diode and its driving and constant-temperature unit, an emission monitoring and main wave generation module, an emission optical path, etc. The emission wavelength of the laser diode can be selected within the range of 905 nm - 945 nm. The interval between the absorption wavelength online and the comparison wavelength offline does not exceed 0.6 nm. A smaller interval means smaller errors caused by the wavelength dependence of aerosol Mie scattering and molecular Rayleigh scattering. Although it is a broadband lidar, it is still not desirable for the width of the emission spectrum to exceed 200 pm (0.2 nm). It should be said that the wider the emission spectrum, the more likely systematic errors will occur. Of course, the greater the emission power of the laser diode, the better the signal-to-noise ratio of the received signal. The average emission power = single-pulse energy × pulse repetition frequency, which is limited by the heat resistance of the laser diode. There is a restrictive relationship between the emission power and the spectral width of the laser diode. When the spectral width is below 0.2 nm, its emission power cannot be increased. Conversely, the emission spectral width of laser diodes with a large emission power is above several nm. A single-mode output FBG (distributed Bragg grating) laser diode can achieve an emission spectral width below 0.2 nm. Diode lasers can be customized from companies such as IPG photonics. The single-pulse energy is mainly determined by the pulse current of the diode, the pulse repetition frequency is determined by the switching frequency of the diode current driver, and the laser pulse width is limited by the duty cycle of the pulse generator.

[0028] In another scheme, a DBR (distributed Bragg reflector) diode laser + a traveling-wave optical amplifier + a power amplifier, where the traveling-wave optical amplifier is driven by a switching current. The DBR diode laser emits narrow-spectrum low-power continuous laser. The traveling-wave optical amplifier chops the continuous-wave laser into pulsed laser, and then, through the final-stage semiconductor power optical amplifier, finally increases the output power.

[0029] As Figure 1 shown, the online emission unit 10 and the offline emission unit 11 respectively emit an online laser beam and an offline laser beam. The online laser beam and the offline laser beam are respectively collimated by the first collimating lens 12 and the second collimating lens 13. After collimation, the online laser beam and the offline laser beam are respectively adjusted in their polarization directions by the first 1 / 2 wave plate 14 and the second 1 / 2 wave plate 15, and the posture of the polarization beam combiner 16 is adjusted by the servo system to match the polarization directions of the two collimated beams, so that the two beams completely coincide and jointly pass through the first 1 / 4 wave plate 17, and then are collimated again by the first negative lens 18 to reduce the divergence angle, and are vertically emitted into the atmosphere through the main mirror 3 of the telescope. The emission beam only occupies the central circular area of the main mirror 3 of the telescope. The first negative lens 18 also acts as the secondary mirror of a refractive telescope.

[0030] The backward scattered light of the atmosphere is collected by the peripheral ring area of the main mirror 3 of the telescope, and after being deflected by the 45° total reflection mirror 22, it enters the receiving optical channel. The receiving optical channel successively includes a second negative lens 23, a second quarter-wave plate 24, a polarization beam splitter 25, a first filter 26 or a second filter 27. Since then, the backward scattering of the online / offline dual wavelengths is separated respectively, and is detected by the online detection unit 20 and the offline detection unit 21 through the first converging lens 28 and the second converging lens 29 respectively. An avalanche photodiode detector is used to measure the signals of each wavelength in a linear analog mode. Both detectors are infrared-enhanced avalanche diodes, which detect the signal current together with a transimpedance amplifier. The bandwidth of the detection and amplification module 12 is: 10 MHz (D0 - 10 MHz) for the near-field unit, 3 MHz (DC - 3 MHz) for the far-field unit. Immediately following, the acquisition rate of the data acquisition card is not higher than 25 MSPS (near-field unit) or 10 MSPS (far-field unit).

[0031] As Figure 2 shown, the emission spectra of the 0nline beam and the Offline beam both have a certain width (generally less than 200 pm), and their normalized emission spectral functions are S on (v) and S off (v). The emission spectrum of the 0nline beam covers the spectral band with a relatively large water vapor absorption cross-section value, and the emission spectrum of the Offline beam covers the spectral band with a very small water vapor absorption cross-section value; N(z) represents the vertical profile of the water vapor molecular number density, γ(v, z) represents the function of the water vapor molecular absorption cross-section with respect to the optical frequency and height, v represents the optical frequency (wave number), z represents the height from the ground, and the one-way atmospheric transmittance function of the laser beam is (Beer's theorem):

[0032]

[0033] The signal P on (z) output by the online detection unit 20, the signal P on of the online detection unit; the signal P off (z) output by the offline detection unit 21, the signal P off (z) of the offline emission unit 11; the signals of the two channels are processed by subtracting the background noise and normalizing as follows:

[0034] P on ′(z) = [P(z) - P B，on / P on , P off ′(z) = [P(z) - P B，o，f / P off

[0035] From the emission spectral function of the laser beam, the spectral function of the water vapor absorption cross-section (as Figure 3 shown), and the data of the online detection unit 20, the offline detection unit 21, the online emission unit 10, and the offline emission unit 11, the objective function is constructed together.

[0036]

[0037] Find the optimal value of N(z) at each height z such that the value of the objective function cost(z) is minimized; in this way, N(z) is obtained, that is, the vertical distribution profile of water vapor in the z direction is generated.

[0038] The above embodiments are only descriptions of the preferred embodiments of the present invention, and do not limit the scope of the present invention. Without departing from the design spirit of the present invention, various deformations and improvements made by those of ordinary skill in the art to the technical solutions of the present invention shall fall within the protection scope determined by the claims of the present invention.

Claims

1. A broadband micro-pulse differential absorption laser radar, comprising a transmitter, a receiver, and a telescope primary mirror, characterized in that: The transmitter includes an online transmitting unit, an offline transmitting unit, a first collimating lens, a second collimating lens, a first 1 / 2 wave plate, a second 1 / 2 wave plate, a polarization beam combiner, a first 1 / 4 wave plate, and a first negative lens; the receiver includes an online detection unit, an offline detection unit, a 45° total reflection mirror with a central hole, a second negative lens, a second 1 / 4 wave plate, a polarization beam splitter, a first filter, a second filter, a first converging lens, and a second converging lens. The transmitter and receiver share the telescope primary mirror.

2. A broadband micro-pulse differential absorption laser radar according to claim 1, characterized in that: The online transmitting unit and the offline transmitting unit transmit alternately for 5 seconds to eliminate optical crosstalk between the online transmitting unit and the offline transmitting unit.

3. A broadband micro-pulse differential absorption laser radar according to claim 2, characterized in that: The online transmitting unit and the offline transmitting unit both include a laser diode, the emission wavelength range of the laser diode is 905nm-945nm, and the absorption wavelength difference between the laser diode of the online transmitting unit and the laser diode of the offline transmitting unit is within 0.6nm.

4. A broadband micro-pulse differential absorption laser radar according to claim 3, characterized in that: The online detection unit and the offline detection unit both include an avalanche photodiode detector.

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