CN110098556A - A kind of 828nm atmosphere vapour detection differential absorption lidar transmitter system - Google Patents

Abstract

The invention discloses an 828nm atmospheric water vapor detection differential absorption laser radar transmitter system, which includes an 828nm continuous wave seed laser, a single longitudinal mode narrow line width pulse Nd:YAG laser, a KTP ring cavity optical parametric oscillator, and a multi-channel laser based on water vapor absorption. The PDH seed laser wavelength stabilization unit of the channel absorption cell, and the optical parametric oscillator resonator cavity length locking unit based on scan-and-hold technology. The invention can generate single-frequency laser pulses suitable for atmospheric water vapor detection, and is the core structure of the differential absorption laser radar.

Description

An 828nm Atmospheric Water Vapor Detection Differential Absorption LiDAR Transmitter System

technical field

The invention relates to the field of laser radar remote sensing measurement, and also relates to a ground-based or vehicle-mounted atmospheric water vapor remote sensing instrument, in particular to a differential absorption laser radar transmitter system that detects atmospheric water vapor distribution and works in the 828nm band.

Background technique

Lidar is an optical active remote sensing device that emits laser beams to detect target feature quantities. It has become one of the most effective means to detect atmospheric composition and its vertical distribution. The transmitter emits ultra-short laser pulses of 5 to 200 ns, and after entering the atmosphere, it continuously interacts with the substances to be measured in the atmosphere during its travel, and the back Raman scattering, back Mie scattering and back Rayleigh scattering can all be used as echoes. The wave signal is received by the telescope, detected by the photodetector and converted into a voltage signal, which is sent to the analog-to-digital conversion device for subsequent signal processing and inversion.

Since the 1980s, differential absorption lidars based on various platforms to detect tropospheric atmospheric water vapor profiles have been successively established internationally. Because atmospheric water vapor is mainly concentrated in the atmospheric boundary layer, according to international experience, the differential absorption lidar for airborne and space applications transmits the laser beam from the upper atmosphere to the lower atmosphere, and the water vapor concentration on the downward path of the beam changes from low to high. The absorption ranges from weak to strong. It is generally believed that it is more appropriate for water vapor to absorb a larger cross-section of the emitted laser light at this time; the ground-based and vehicle-mounted differential absorption lidar detects the atmospheric water vapor content from the surface to the tropopause, and generally uses a relatively small absorption cross-section. near 815-820nm or near 720-730nm.

The ground-based differential absorption lidar transmitter systems that have been established internationally to detect tropospheric water vapor are mainly divided into two categories. One is built with low repetition rate and large pulse energy, such as using titanium sapphire laser or alexandrite laser. The Ti:Sapphire laser transmitter adopts seed injection and resonant cavity active frequency stabilization technology. The spectral performance of the transmitter is relatively stable. The wavelength is generally close to the wavelength of 815-820nm with higher Ti:Sapphire gain; the alexandrite laser generally works in the water vapor of 720-730nm However, in the 720-730nm band, the spectral line broadening caused by the backscattering of atmospheric molecules is serious, and the inversion error increases. Another type of transmitter uses a high repetition rate and low pulse energy, with a diode laser and a semiconductor amplifier as the core, and an operating wavelength of 823 and 828nm. This type of lidar is called a micropulse differential absorption lidar.

With the continuous advancement of optical parametric oscillator technology and the continuous maturity of narrow linewidth single longitudinal mode Nd:YAG lasers, the optical parametric oscillator method based on seed injection can generate excellent 828nm-828nm laser beams, covering an absorption spectral region of water vapor , so it can be used as a laser transmitter for ground-based or vehicle-mounted differential absorption lidar to detect the water vapor profile of the atmospheric troposphere.

Contents of the invention

The purpose of this invention is to propose a transmitter system for differential absorption lidar operating in the 820-830nm band. The purpose of doing so is to further enrich the construction means of differential absorption lidar for water vapor detection and enhance the universality of differential absorption lidar applications. sex.

The lidar transmitter system includes a single-frequency pump laser 1, an active frequency-stabilized 828nm seed laser 2, a ring cavity optical parametric oscillator 3, and necessary optical lenses 4-7. The overall structure is shown in FIG. 1 . Among them, the seed-injected Nd:YAG laser is used as the pump source, including 1064nm Nd:YAG optical amplifier 1-1, double frequency crystal 1-2, 1064nm Nd:YAG main oscillator 1-3, 1064nm seed source 1-4 . The single-frequency pump laser 1 generates pump laser light, and the pump laser light when it reaches the R1064T532 dielectric film dichroic mirror 4 contains both 1064nm components and 532nm components. R1064T532 dielectric film dichroic mirror 4 is coated with 532nm antireflection film and 1064nm high reflection film, so that 532nm laser light passes through 4 and enters ring cavity optical parametric oscillator 3, 1064nm is reflected to 1064nm mirror 5, and is reflected by 5 again to R532T1064 dielectric film bichromatic Mirror 6. The R532T1064 dielectric film dichroic mirror 6 is coated with 1064nm antireflection coating and 532nm high reflection coating, so that 1064nm can be released through 6. The laser emitted by the ring cavity optical parametric oscillator 3 contains both 828nm and 532nm components. R532T828 dielectric film dichroic mirror 7 is coated with 828nm anti-reflection film and 532nm high-reflection film, so that 828nm laser is emitted through 7, and 532nm laser is reflected by 7 to the R532T1064 dielectric film dichroic mirror 6, and is reflected by 6 again for release.

The laser radar transmitter system uses an active frequency-stabilized 828nm seed laser 2 as an optical parametric oscillator to inject the seed source, as shown in Figure 3, including 828nm DFB laser 2-1, 95:5 fiber splitter 2-2, water vapor absorption Cell 2-3, gas cell photodetector 2-4, seed light control electronics unit 2-5. The wavelength of the seed light is based on the minimum value frequency ω ₀ in the center of the light absorptivity curve of the water vapor standard absorption cell as a reference standard, as shown in Figure 2, using the PDH (Pound–Drever–Hall) frequency stabilization technology to stabilize the seed source wavelength.

Electro-optic phase modulation is performed on the continuous wave seed laser frequency ω, and the phase-modulated electric field before entering the water vapor multi-channel absorption cell becomes

E _inc ＝E ₀ e ^{i(ωt+msinδt) ≈E} ₀ [J ₀ (m)e ^iωt +J ₁ (m)e ^i(ω+δ)t -J ₁ (m)e ^{i(ω-δ) t} ] (1)

Among them, m is the depth of phase modulation, the frequency of δ modulation wave, and msinδt is the phase modulation term. The modulated CW laser is split into three frequency spectra, namely the center frequency component carrier J ₀ (m)e ^iωt , and two sidebands distributed on both sides of the center frequency with equal amplitude and opposite phase. When the seed light frequency ω is aligned with the center frequency ω ₀ of the reference water vapor multi-channel absorption cell, the upper and lower sidebands after water vapor absorption are equal in size and opposite to the carrier beat frequency current, and the detector output P _coherent is zero; otherwise, the seed light When the frequency ω is not equal to the center frequency ω ₀ of the water vapor multi-channel absorption cell, the antisymmetry of the sidebands on both sides is destroyed after being absorbed by the water vapor multi-channel absorption cell, and the upper and lower sidebands and the carrier beat frequency current no longer cancel each other out. When the detector output P _coherent is mixed with the original radio frequency modulation signal sinδt, and after low-pass filtering, the center frequency deviation information containing the seed laser frequency and the water vapor multi-channel absorption cell is obtained. The post-stage PID servo control system feeds back and controls the injection current and piezoelectric voltage of the seed laser according to the deviation information, so that the frequency ω of the seed laser approaches the center frequency ω ₀ of the water vapor multi-channel absorption pool.

The working optical path used by the lidar transmitter system to generate a single-frequency 828nm wavelength laser is shown in FIG. 4 . The 532nm pump laser light produced by the single-frequency pump laser 1 passes through the pump light beam shrinker group 17 and the half-wave-2 16, is reflected by the 532nm mirror 14, and enters the resonant cavity through the cavity mirror M2 3-2; The seed laser generated by the frequency-stabilized 828nm seed laser 2 passes through the shaping prism 8 and the optical isolator 9, enters the single-mode polarization-maintaining fiber 10-2 through the fiber coupling-1 10-1, and is output by the fiber coupler-2 10-3 , after passing through the collimating mirror 11 and the half-wave-1 12, it is reflected by the 828nm mirror 13, and enters the resonant cavity through the cavity mirror M33-3. The ring cavity optical parametric oscillator 3 adopts a four-mirror ring cavity structure, wherein the cavity mirror M4 3-4 is closely attached to the piezoelectric ceramic 3-5. The laser emitted by the ring cavity optical parametric oscillator 3 contains both 828nm and 532nm components, of which 532nm is transmitted and leaked by the R828T532 dielectric film dichroic mirror 18, and 828nm is reflected by 18 and emitted by the 828nm mirror 21. At the same time, a small part of the emitted laser light is sampled by the beam sampling mirror 20 to the resonant cavity photodetector 23 to obtain a cavity length modulation signal, and then the cavity-locked signal is obtained through the resonant cavity control electronic unit 24 and LV amplifier 25 and fed back to the piezoelectric ceramics 3-5 for further processing. The resonant cavity is long locked.

When the lidar transmitter system is working on the differential absorption lidar, it needs to switch between the narrowband λ _on laser and the broadband λ _off laser near 828nm at a speed of 5-25Hz. To this end, the semiconductor laser output from the fiber is used as the injection seed, and injected into the resonant cavity of the OPO through a fast optical switch gating method. While the seed is injected, the piezoelectric ceramic 3-5 scans the length of the OPO cavity, and uses the resonant cavity photodetector 23 to monitor the seed light signal leaked from the OPO output. When they coincide exactly, the seed signal will be at the position of the minimum value, and record this position. After the entire scanning period is completed, the resonant cavity control electronic unit 24 generates a control voltage, which is amplified by the LV amplifier 25 and drives the piezoelectric ceramics 3-5 to adjust the length of the OPO cavity to the recorded minimum value to obtain a point and keep it at this position. That is to say, when the pump pulse comes, the OPO can oscillate and output a single-frequency pulse laser that is consistent with the frequency of the injected seed light. The timing sequence of the whole process is shown in Fig. 5 . In order to improve the long-term stability and repeatability of the system, each pulse undergoes a scan-hold process before being output.

Description of drawings

Fig.1 Block diagram of 828nm water vapor detection differential absorption lidar transmitter. Labels in the figure: 1-Single-frequency pump laser light source, 1-1 is 1064nm Nd:YAG optical amplifier, 1-2 is frequency doubler, 1-3 is 1064nm Nd:YAG main oscillator, 1-4 is 1064nm seed source; 2 is 828nm seed laser with active frequency stabilization; 3 is ring cavity optical parametric oscillator; 4 is R1064T532 dielectric film dichromatic mirror; 5 is 1064nm mirror; 6 is R532T1064 dielectric film dichromatic mirror; 7 is R532T828 dielectric film dichroic mirror.

Figure 2 Schematic diagram of the absorption spectrum characteristics and modulation frequency of the water vapor standard absorption cell.

Figure 3 is a block diagram of the active frequency-stabilized 828nm seed laser. Numbers in the figure: 2-1 is 828nm DFB laser, 2-2 is 95:5 fiber splitter, 2-3 is water vapor absorption pool, 2-4 is gas pool photodetector, 2-5 is seed light control electronics unit.

Figure 4. Schematic diagram of the seed-injected optical parametric oscillator. Numbers in the figure: 2 is the 828nm seed laser with active frequency stabilization, 3 is the ring cavity optical parametric oscillator, 3-1 is the cavity mirror M1, 3-2 is the cavity mirror M2, 3-3 is the cavity mirror M3, 3-4 M4 is cavity mirror, 3-5 is piezoelectric ceramic, 3-6 is KTP crystal-1, 3-7 is KTP crystal-2, 8 is shaping prism, 9 is optical isolator, 10-1 is fiber coupling-1 , 10-2 is optical fiber, 10-3 is fiber coupler-2, 11 is collimating mirror, 12 is half-wave-1, 13 is 828nm reflector, 14 is 532nm reflector, 15 is light barrier-1, 16 It is half-wave-2, 17 is the pump light reducer group, 18 is the R828T532 dielectric film dichroic mirror, 19 is the light barrier-2, 20 is the beam sampling mirror, 21 is the 828nm mirror, 22 is the diaphragm, 23 is The resonant cavity photodetector, 24 is the resonant cavity control electronic unit, and 25 is the LV amplifier.

Fig. 5 The timing diagram of the cavity lock in the scan and hold method.

Detailed ways

1. The laser radar transmitter system uses the double frequency 532nm laser pulse of the seed injected Nd:YAG laser as the pump source, such as the French Quantel company YG980 series product collocation 1064nm continuous wave seed laser (SI-2000), the U.S. Continuum The company's Powerlite ^TM series is equipped with a 1064nm continuous wave seed laser (SI-2000), the German InnoLas Laser GmbH company's SpitLight series is equipped with a 1064nm continuous wave seed laser, and the American Spectra-Physics company's Quanta-Ray series is equipped with a 1064nm continuous wave seed laser (Model 6350). And the single longitudinal mode Nd:YAG laser of China Leibao Optoelectronics Technology Co., Ltd.; the 1064nm seed source can be a continuous wave ytterbium-doped distributed feedback semiconductor laser, or a continuous wave laser of a single Nd:YVO ₄ crystal.

2. The laser radar transmitter system uses KTP crystal as the nonlinear working substance. KTP is cut into 8mm×8mm×12mm, the surface of the crystal is coated with broadband anti-reflection coating, and the crystal is cut along the x-axis in the xy plane at θ=62°, which meets the critical phase matching of type II. The crystal is precisely positioned so that the pump beam passes through the center of the crystal. The pump beam shrinker group 17 is used to adjust the diameter of the pump beam entering the resonant cavity.

3. The resonant cavity 3 of the laser radar transmitter system includes four planar mirrors, the cavity mirror M2 3-2 has a high reflectivity to the signal light and a high transmittance to the pump light, and the reflection angle is 45 °; the cavity mirror M3 3-3 has 70% reflectivity for signal light, high transmittance for pump light, and an incident angle of 45°; cavity mirror M4 3-4 has high reflectivity for 828nm signal light and 532nm pump light; The cavity mirror M1 3-1 has a high transmittance for the pump light and a high reflectivity for the signal light. All the cavity mirrors have high transmittance for the idle light, and the idle light leaks out from each cavity mirror of the resonator, and only the signal light is positively fed back to oscillate in the resonator.

4. The lidar transmitter system introduces the 828nm seed light produced by the DFB laser or the ECDL laser or the DBR laser as an optical parametric oscillator resonator injection seed; use a multi-channel absorption pool filled with water vapor and an external cavity tuned semiconductor laser to form a PDH (Pound–Drever–Hall) feedback servo system, so that the wavelength of the seed laser is located at the center wavelength of the water vapor absorption line.

Claims (3)

1. a kind of 828nm atmosphere vapour detects differential absorption lidar transmitter system, including single-frequency pump laser (1), The 828nm seed laser (2) and annular cavity optical parametric oscillator (3) of active frequency stabilization, it is characterised in that:

Annular cavity optical parametric oscillator (3) are passed through by the 532nm pumping laser pulse that the single-frequency pump laser (1) emits The exploring laser light pulse of 828nm is converted to, conversion process needs the kind for generating the 828nm seed laser (2) of active frequency stabilization Sub- laser injects annular cavity optical parametric oscillator (3).

2. 828nm atmosphere vapour as described in claim 1 detects differential absorption lidar transmitter system, feature exists In the 828nm seed laser (2) of the active frequency stabilization module includes 828nm Distributed Feedback Laser (2-1), 95:5 optical fiber point Road device (2-2), water vapor absorption pond (2-3), gas cell photodetector (2-4), seed photocontrol electronic unit (2-5).By The seed laser that 828nm Distributed Feedback Laser (2-1) generates is divided into two-way by 95:5 optical fiber splitter (2-2), and 95% is sent to ring Shape chamber optical parametric oscillator (3), 5% is sent to water vapor absorption pond (2-3)；By gas cell photodetector (2-4) after water vapor absorption Absorption signal is obtained, seed photocontrol electronic unit (2-5) carries out 828nm Distributed Feedback Laser (2-1) according to absorption signal anti- Feedback control.

3. 828nm atmosphere vapour as described in claim 1 detects differential absorption lidar transmitter system, it is characterised in that The annular cavity optical parametric oscillator (3) includes hysteroscope M1 (3-1), hysteroscope M2 (3-2), hysteroscope M3 (3-3), hysteroscope M4 (3- 4), piezoelectric ceramics (3-5), ktp crystal 1 (3-6), ktp crystal 2 (3-7)；By the 532nm pump of single-frequency pump laser (1) transmitting Pu laser pulse enters ring resonator, the 828nm laser arteries and veins that annular cavity optical parametric oscillator (3) generates by hysteroscope M2 (3-2) Punching leaves resonant cavity via hysteroscope M3 (3-3), and the annular cavity optical parametric oscillator (3) has used resonant cavity stable mode simultaneously Block, including Resonant Cavity Photodetectors (23), resonant cavity control electronic unit (24) and LV amplifier (25), and resonant cavity photoelectricity is visited It surveys the chamber long modulated signal that device (23) obtain and the calculating of electronic unit (24) and putting for LV amplifier (25) is controlled by resonant cavity Piezoelectric ceramics (3-5) is fed back to after big.