A Ferro-Resonant Fluorescent Doppler LiDAR System

Abstract

This invention belongs to the field of lidar technology and discloses an iron resonance fluorescence Doppler lidar system. It employs a seed injection method, injecting a single-longitudinal-mode, frequency-stabilized seed light of a certain power into an oscillator stage unit. Through active cavity control technology, precise matching of the seed light frequency and the longitudinal-mode frequency of the oscillator stage is achieved, enabling the oscillator stage to output Q-switched pulses with good transverse-mode and frequency characteristics. Subsequently, the Q-switched pulses are amplified in the amplification stage unit using a MOPA configuration. After two nonlinear frequency transformations, single-longitudinal-mode narrow-linewidth laser output at a specific wavelength is achieved. This system uses iron atoms in the atmosphere as tracers, enabling high-precision detection of atmospheric temperature, wind field, and iron atom number density in the region from the top of the mesosphere to the bottom of the thermosphere.

Description

Technical Field

[0001] This invention belongs to the field of lidar technology, and particularly relates to an iron resonance fluorescence Doppler lidar system. Background Technology

[0002] Temperature and wind field are important environmental parameters characterizing atmospheric conditions. In the region from the top of the mesosphere to the bottom of the thermosphere, at altitudes of approximately 75 km to 115 km, observational data on the vertical distribution of temperature and wind field in this altitude range are relatively scarce due to a lack of effective observation methods. Resonant fluorescence lidar utilizes metal atoms and ions unique to this region as tracers. By exciting their resonant fluorescence signals, temperature and wind field can be detected. Compared with sounding rockets, passive remote sensing, and microwave active remote sensing, resonant fluorescence lidar offers advantages such as continuous observation, high spatiotemporal resolution, and high precision, making it a powerful tool for detecting temperature and wind field profiles in this region. Currently, sodium resonant fluorescence lidar is mainly used internationally to measure temperature and wind field, while iron resonant fluorescence Doppler lidar systems offer advantages such as all-weather detection, making them another effective means of detecting temperature and wind field profiles in this region. Since narrow-band, frequency-stabilized 372nm pulsed lasers are required for wind field measurements, the laser source is one of the most critical technologies in the development of iron resonant fluorescence Doppler lidar systems. Compared with sodium fluorescence resonant fluorescence lidar, 372nm wavelength laser sources have the advantages of solid-state and miniaturization. However, the generation of narrowband laser pulses and the precise stabilization of laser frequency are the key challenges in the development, which greatly limits the development of iron resonant fluorescence Doppler lidar systems.

[0003] Two techniques have been validated for generating narrowband laser pulses. One involves using an alexandrite laser to generate a single-mode 744nm wavelength laser pulse, followed by frequency doubling to obtain a 372nm wavelength pulse. Alexandrite lasers have a wide gain spectrum, and typically employ composite cavity and dispersive cavity techniques to select a specific laser output frequency. The output laser frequency is changed by adjusting the grating used as a back cavity mirror. However, the grating angle is extremely sensitive to shocks and vibrations in the laser's operating environment, easily causing changes in the output laser frequency. The second technique involves using an Nd:YAG laser to generate an 1116nm wavelength laser pulse, followed by sequential frequency doubling and tripling to produce a 372nm wavelength laser pulse. Nd:YAG crystals have a gain linewidth of approximately 1.3nm near the 1116nm wavelength, and specific frequency laser output can be obtained using seed injection techniques. However, the gain of Nd:YAG crystals at the 1116nm wavelength is low, requiring special laser path design. Summary of the Invention

[0004] To enable the iron resonance fluorescence Doppler lidar system to output sufficiently powerful, narrow-linewidth, frequency-stabilized pulsed laser light, achieving high-precision detection of atmospheric temperature, wind field, and iron atom number density in the region from the top of the mesosphere to the bottom of the thermosphere, and to make the lidar miniaturized and all-solid-state, this invention proposes an iron resonance fluorescence Doppler lidar system, the specific scheme of which is as follows:

[0005] The lidar system includes a seed laser, an oscillation stage unit, an amplification stage unit, a frequency multiplication unit, a frequency combining unit, a transmitting unit, a receiving unit, and a data processing unit;

[0006] The seed laser is used to output continuous light with a preset wavelength of λ, and the frequency of the continuous light jumps between f-f0, f, and f+f0, where f is the frequency corresponding to λ and f0 is a preset frequency adjustment amount.

[0007] The oscillation stage unit is connected to the seed laser and is used to output single-longitudinal-mode Q-switched pulse laser.

[0008] The amplification stage unit is connected to the oscillation stage unit and is used to amplify the power of the Q-switched pulse laser.

[0009] The frequency doubling unit is connected to the amplification stage unit and is used to perform a nonlinear frequency transformation on the fundamental frequency light to obtain λ / 2 wavelength frequency-doubled light.

[0010] The frequency combining unit is connected to the frequency doubling unit and is used to perform nonlinear frequency transformation on the frequency-doubled light to obtain λ / 3 wavelength pulsed laser.

[0011] The transmitting unit is used to emit the λ / 3 wavelength pulsed laser output by the frequency combining unit into the atmosphere;

[0012] The receiving unit is used to collect the echo signal after the interaction between the λ / 3 wavelength pulsed laser and the atmosphere;

[0013] The data processing unit is connected to the receiving unit and is used to process the echo signal to obtain the detection results of temperature, wind field and iron atom number density.

[0014] Furthermore, the oscillation stage unit includes, in sequence, a first half-wave plate, a first optical isolator, a second half-wave plate, a first quarter-wave plate, a first high-reflection mirror, an RTP crystal, a second quarter-wave plate, a first polarizer, a third quarter-wave plate, a first crystal rod, a fourth quarter-wave plate, a second high-reflection mirror, a second optical isolator, a first negative lens, and a first total reflection mirror;

[0015] A first piezoelectric ceramic and a second piezoelectric ceramic are respectively attached to the first and second high-reflection mirrors. A resonant detection box, a detector, and a Q-switching drive are arranged sequentially between the first and second piezoelectric ceramics.

[0016] The RTP crystal, the second quarter-wave plate, the first polarizer, and the Q-switching drive constitute an electro-optic Q-switching system, and the detector is used to detect the interference waveform passing through the first polarizer.

[0017] Furthermore, the amplification stage unit includes a second total reflection mirror, a third optical isolator, a second polarizer, a second crystal rod, a second negative lens, a first quartz rotator, a third crystal rod, a fifth quarter-wave plate, a third negative lens, a first filter, and a third total reflection mirror arranged sequentially.

[0018] The fourth total reflection mirror, the fourth optical isolator, the third polarizer, the fourth crystal rod, the fourth negative lens, the second quartz rotator, the fifth crystal rod, the sixth quarter-wave plate, the fifth negative lens, the second filter, and the fifth total reflection mirror are arranged in sequence.

[0019] The fourth total reflection mirror receives the polarized light from the second polarizer.

[0020] Furthermore, the second, third, fourth, and fifth crystal rods are pumped using a semiconductor end-face pumping method for Nd:YAG crystals.

[0021] Furthermore, the frequency doubling unit uses a type I phase-matched LBO crystal to complete the first frequency conversion;

[0022] The frequency combining unit uses a type II phase-matched LBO crystal to complete the second frequency conversion.

[0023] Furthermore, the transmitting unit includes a first beam splitter, a second beam splitter, a beam expander, an adjustment frame, and a laser reflector;

[0024] The laser reflector is mounted on the adjustment frame;

[0025] The first beam splitter is coated with a λ / 3 wavelength reflective film, a λ / 2 wavelength anti-reflective film, and a λ wavelength anti-reflective film;

[0026] The second beam splitter is coated with a λ / 2 wavelength reflective film and a λ wavelength antireflective film;

[0027] The beam expander has an adjustable magnification of 2-5 times.

[0028] Furthermore, the receiving unit includes a receiving telescope and a pinhole aperture, and the receiving telescope adopts a coaxial card system;

[0029] The receiving telescope includes a concave parabolic primary mirror, a hyperboloid secondary mirror, and a corner reflector arranged in sequence.

[0030] The pinhole aperture is located at the rear of the receiving telescope and is used to control the field of view of the receiving telescope.

[0031] Furthermore, the receiving unit also includes a movable lens group, a daytime detection unit, and a nighttime detection unit;

[0032] The receiving unit is used to control the transmission of the echo signal received by the receiving telescope to the daytime detection unit or the nighttime detection unit;

[0033] The night detection unit includes a first narrowband interferometer filter, a first collimating lens, a first narrowband filter, a first converging lens, and a first photodetector arranged in sequence.

[0034] The daytime detection unit includes a second narrowband interferometer filter, a second collimating lens, a second narrowband filter, a second converging lens, and a second photodetector arranged in sequence.

[0035] Furthermore, the data processing unit includes a data acquisition card and a computing unit;

[0036] The data acquisition card is connected to the first photodetector and the second photodetector and is used to convert electrical signals into digital signals;

[0037] The computing unit is connected to the data acquisition card and is used to acquire temperature, wind field and iron atom number density detection results through the digital signal.

[0038] Furthermore, the lidar system also includes an energy monitoring unit and a frequency monitoring unit.

[0039] The energy monitoring unit is used to monitor the λ / 3 wavelength laser and remove the original lidar data within the time period corresponding to abnormal energy fluctuations.

[0040] The frequency monitoring unit is used to combine the remaining λ wavelength of the lidar, the frequency-hopping pulse light and the continuous light to obtain the frequency stability of the pulse light, and to remove the original lidar data within abnormal time periods. The abnormal time period is the time period corresponding to the frequency stability being greater than a set value.

[0041] The beneficial effects of this invention are as follows:

[0042] (1) The iron resonance fluorescence Doppler lidar system proposed in this invention is based on a 372nm wavelength Nd:YAG solid-state laser. The operating wavelength is in the ultraviolet band, where the background light intensity is relatively weak, which can effectively improve the lidar's detection capability during the day. At the same time, the ultraviolet laser beam is invisible to the naked eye, which is more conducive to the confidentiality of the measurement mission of the detection system.

[0043] (2) The iron resonance fluorescence Doppler lidar system proposed in this invention is based on a 372nm wavelength Nd:YAG solid-state laser. It uses the resonance fluorescence signal of iron atoms at 372nm to simultaneously measure atmospheric temperature, wind field, and iron atom number density at a distance of 80-100km. Compared with the commonly used fluorescence spectrum of sodium atoms at 589nm wavelength, the fluorescence spectrum of iron atoms at 372nm wavelength lacks fine structure, and its forward model is simpler, which is beneficial to improving the detection accuracy of atmospheric parameters. In addition, when using iron atoms as tracers to measure atmospheric parameters, the inversion algorithm is different from that of sodium atoms, which can mutually verify the correctness of the lidar system and the inversion algorithm.

[0044] (3) The iron resonance fluorescence Doppler lidar system proposed in this invention is based on a 372nm wavelength Nd:YAG solid-state laser and has real-time status monitoring capabilities. Laser beat frequency technology is used to monitor frequency stability in real time, eliminating raw data corresponding to occasional frequency anomalies to ensure high-precision measurement of the atmospheric wind field;

[0045] (4) The iron resonance fluorescence Doppler lidar system proposed in this invention is based on a 372nm wavelength Nd:YAG solid laser and uses Nd:YAG crystal as the gain medium. Compared with the emerald crystal used abroad, it has higher efficiency and stability at the same output power. At the same time, it can reduce the volume of the transmitting unit and is more suitable for airborne and shipborne environments with limited space and where it is impossible to guarantee relative stability.

[0046] (5) The iron resonance fluorescence Doppler lidar system proposed in this invention is based on a 372nm wavelength Nd:YAG solid-state laser and adopts a semiconductor pumping method for the Nd:YAG crystal. The pump source is an 808nm laser diode. Compared with flash lamp pumping, semiconductor pumping has better stability, more compact structure and higher efficiency. Under the same pumping power, the heat generated is one-third of that of flash lamp pumping, which greatly reduces the thermal effect in the crystal. Attached Figure Description

[0047] Figure 1 This is a schematic diagram of the transmitter of the iron resonance fluorescence Doppler lidar system in an embodiment of the present invention;

[0048] Figure 2 This is a schematic diagram of the oscillation stage unit in an embodiment of the present invention;

[0049] Figure 3 This is a schematic diagram of the amplification stage unit in an embodiment of the present invention;

[0050] Figure 4 This is a schematic diagram of the nonlinear frequency conversion module in an embodiment of the present invention.

[0051] In the diagram: 1. Seed laser; 2. Oscillator unit; 3. Amplifier unit; 4. Frequency doubling unit; 5. Frequency combining unit; 6. Transmitting unit; 201. First half-wave plate; 202. First optical isolator; 203. Second half-wave plate; 204. First quarter-wave plate; 205. First high-reflection mirror; 206. RTP crystal; 207. Second quarter-wave plate; 208. First polarizer; 209. Third quarter-wave plate; 210. First crystal rod; 211. Fourth quarter-wave plate; 212. Second high-reflection mirror; First total reflection mirror; 213. First piezoelectric ceramic; 214. Resonant detector box; 215. Detector; 216. Q-switching driver; 217. Second piezoelectric ceramic; 218. Second optical isolator; 219. First negative lens; 220. First total reflection mirror; 301. Second total reflection mirror; 302. Reflecting mirror; 303. Third optical isolator; 304. Second polarizer; 305. Second crystal rod; 306. Second negative lens; 307. First quartz rotator; 308. Third crystal rod; 309. Fifth quarter-wave plate; 310. Third negative lens; 311. First filter; 312. Third total reflection mirror; 313. Fourth total reflection mirror; 314. Fourth polarizer; 315. Fourth crystal rod; 316. Fourth negative lens; 317. Second quartz rotator; 318. Fifth crystal rod; 319. Sixth quarter-wave plate; 320. Fifth negative lens; 321. Second filter; 322. Fifth total reflection mirror; 401. Sixth total reflection mirror; 402. Seventh quarter-wave plate; 403. Type I phase-matched LBO crystal; 404. Type II phase-matched LBO crystal. Detailed Implementation

[0052] like Figure 1 As shown, the present invention discloses an iron resonance fluorescence Doppler lidar system, the lidar system comprising a seed laser 1, an oscillation stage unit 2, an amplification stage unit 3, a frequency doubling unit 4, a frequency combining unit 5, a transmitting unit 6, a receiving unit, and a data processing unit;

[0053] The seed laser 1 is used to output continuous light with a preset wavelength of λ, and the frequency of the continuous light jumps between f-f0, f, and f+f0, where f is the frequency corresponding to λ and f0 is a preset frequency adjustment amount.

[0054] The oscillation stage unit 2 is connected to the seed laser 1 and is used to output single longitudinal mode Q-switched pulse laser. Preferably, the oscillation stage unit 2 is connected to the seed laser 1 through a single-mode optical fiber.

[0055] The amplification stage unit 3 is connected to the oscillation stage unit 2 and is used to amplify the power of the Q-switched pulse laser.

[0056] The frequency doubling unit 4 is connected to the amplification stage unit 3 and is used to perform a nonlinear frequency transformation on the fundamental frequency light to obtain λ / 2 wavelength frequency-doubled light.

[0057] The frequency combining unit 5 is connected to the frequency doubling unit 4 and is used to perform nonlinear frequency transformation on the frequency-doubled light to obtain λ / 3 wavelength pulsed laser.

[0058] The transmitting unit 6 is used to emit the λ / 3 wavelength pulsed laser output by the frequency combining unit into the atmosphere;

[0059] The receiving unit is used to collect the echo signal after the interaction between the λ / 3 wavelength pulsed laser and the atmosphere;

[0060] The data processing unit is connected to the receiving unit and is used to process the echo signal to obtain the detection results of temperature, wind field and iron atom number density.

[0061] Specifically, after the seed laser 1 outputs three-frequency continuous light, the oscillation stage unit 2 uses active cavity length control technology to match the seed light frequency with the longitudinal diaphragm frequency of the oscillation stage unit 2, enabling the oscillation stage unit 2 to output a single-longitudinal-diaphragm Q-switched pulsed laser at a certain power level. The amplification stage unit 3 amplifies the power of the Q-switched pulsed laser from the oscillation stage unit 2 to obtain a fundamental frequency light with a preset wavelength of λ and a laser power of 3.6W or higher. The frequency doubling unit 4 performs a nonlinear frequency transformation on the fundamental frequency light to obtain a frequency-doubled light with a wavelength of λ / 2 that meets the power index. The frequency combining unit 5 performs another nonlinear frequency transformation on the frequency-doubled light to obtain a pulsed laser with a wavelength of λ / 3 and a power of 0.6W or higher. The transmitting unit 6 is used to transmit the pulsed laser output by the frequency combining unit 5 into the atmosphere. The receiving unit receives the backscattered echo signal after the pulsed laser interacts with the atmosphere. The backscattered echo signal mainly includes Rayleigh scattering signal and resonant fluorescence scattering signals at three frequencies. After receiving the backscattered echo signal, the data processing unit obtains the atmospheric temperature, wind field, and iron atom number density detection results.

[0062] For example, the seed laser 1 is a 1116nm wavelength tri-frequency seed laser, capable of outputting continuous light with a center wavelength of 1116.2979nm, and its frequency stability root mean square is less than 200kHz. Furthermore, the laser frequency can jump between f1-250MHz, f1, and f1+250MHz, where f1 is the center frequency corresponding to the 1116.2979nm center wavelength, specifically 268.559529THz. In this embodiment, a 372nm wavelength Nd:YAG solid-state laser is used as the laser source, a laser diode as the pump source, and an Nd:YAG crystal as the gain medium to output 1116nm fundamental frequency light. The fundamental frequency light is then amplified, and finally, after two nonlinear frequency transformations, a 372nm wavelength pulsed laser is output. The iron resonance fluorescence Doppler lidar system based on a 372nm wavelength Nd:YAG solid-state laser emits a narrowband, frequency-stabilized laser. It can not only measure atmospheric temperature and iron atom number density like a broadband iron Boltzmann lidar, but also simultaneously measure atmospheric wind field.

[0063] Furthermore, such as Figure 2 As shown, the oscillation stage unit 2 includes, in sequence, a first half-wave plate 201, a first optical isolator 202, a second half-wave plate 203, a first quarter-wave plate 204, a first high-reflection mirror 205, an RTP crystal 206, a second quarter-wave plate 207, a first polarizer 208, a third quarter-wave plate 209, a first crystal rod 210, a fourth quarter-wave plate 211, a second high-reflection mirror 212, a second optical isolator 218, a first negative lens 219, and a first total reflection mirror 220;

[0064] A first piezoelectric ceramic 213 and a second piezoelectric ceramic 217 are respectively disposed on the first high-reflection mirror 205 and the second high-reflection mirror 211. A resonant detection box 214, a detector 215, and a Q-switching drive 216 are sequentially disposed between the first piezoelectric ceramic 213 and the second piezoelectric ceramic 217.

[0065] The RTP crystal 206, the second quarter-wave plate 207, the first polarizer 208, and the Q-switching driver 216 constitute an electro-optic Q-switching system, and the detector 215 is used to detect the interference waveform passing through the first polarizer 208.

[0066] Specifically, the first half-wave plate 201 is used to adjust the intensity of the injected seed light; the first optical isolator 202 and the second optical isolator 218 are used to prevent return light; the second half-wave plate 203 and the first quarter-wave plate 204 are used to adjust the laser polarization state; the first high-reflection mirror 205 and the second high-reflection mirror 212 are two high-reflection mirrors that make up the resonant cavity mirror, and both are coated with anti-reflection films of 946nm, 1064nm and 1319nm, which are used to prevent the high-gain spectral lines in the first crystal rod 210 from oscillating. The first crystal rod 210 is an Nd:YAG crystal rod with a gain medium, and the pumping method of semiconductor end-face pumping of Nd:YAG crystal is adopted, with an 808nm laser diode as the pump source; the setting of the third quarter-wave plate 209 and the fourth quarter-wave plate 211 can avoid the standing wave in the first crystal rod 210 from causing oscillation. A spatial hole-burning effect occurs. The RTP crystal 206, the second quarter-wave plate 207, the first polarizer 208, and the Q-switching driver 216 constitute an electro-optic Q-switching system, wherein the first polarizer 208 is a polarizer incident at Brewster angle. The first piezoelectric ceramic 216 and the second piezoelectric ceramic 217 are used to adjust the cavity length of the resonant cavity. The resonant detector 214 is used to apply a ramp scanning voltage to the first piezoelectric ceramic 216 and the second piezoelectric ceramic 217 to fine-tune the cavity length of the resonant cavity. The detector 215 is used to detect the value of the interference waveform. When a maximum value is detected, the seed light frequency and the longitudinal diaphragm frequency of the oscillation stage unit are precisely matched. At this time, the Q-switching driver triggers the RTP crystal and then outputs a single longitudinal Q-switched pulse with the same frequency as the seed light. The first negative lens 219 is used to compensate for the thermal lensing effect of the Nd:YAG crystal. The first total reflection mirror 220 reflects the Q-switched laser back to the amplification stage unit 3. In this embodiment, the output laser power is greater than 0.3W and the frequency stability is less than 1MHz.

[0067] Furthermore, such as Figure 3 As shown, the amplification stage unit 3 includes

[0068] The following components are arranged in sequence: a second total reflection mirror 301, a third optical isolator 302, a second polarizer 303, a second crystal rod 304, a second negative lens 305, a first quartz rotator 306, a third crystal rod 307, a fifth quarter-wave plate 308, a third negative lens 309, a first filter 310, and a third total reflection mirror 311.

[0069] The fourth total reflection mirror 312, the fourth optical isolator 313, the third polarizer 314, the fourth crystal rod 315, the fourth negative lens 316, the second quartz rotator 317, the fifth crystal rod 318, the sixth quarter-wave plate 319, the fifth negative lens 320, the second filter 321, and the fifth total reflection mirror 322 are arranged in sequence.

[0070] The fourth total reflection mirror 312 receives the polarized light from the second polarizer 303.

[0071] Among them, the third optical isolator 302 and the fourth optical isolator 313 are used to isolate the fundamental frequency light from the oscillating unit from the radiation light reflected back by multiple levels; the second polarizer 303 and the third polarizer 314 separate the laser beam according to the difference in polarization state; the second crystal rod 304, the third crystal rod 307, the fourth crystal rod 315 and the fifth crystal rod 318 are Nd:YAG crystal rods that serve as gain media, and they also adopt the pumping method of semiconductor end face pumping of Nd:YAG crystals; the second negative lens 305, the third negative lens 309, the fourth negative lens 316 and the fifth negative lens 320 are used to compensate for the thermal lensing effect of Nd:YAG crystal rods; the first quartz rotator 306 and the second quartz rotator 317 are 90-degree quartz rotators that compensate for optical distortion caused by thermal birefringence; the first filter 310 and the second filter 321 are 1064nm wavelength filters used to suppress spontaneous emission amplification of the 1064nm wavelength high-gain spectral line.

[0072] Specifically, the Q-switched laser, after being reflected by the second total reflection mirror 301, sequentially passes through the third optical isolator 302, the second polarizer 303, the second crystal rod 304, the second negative lens 305, the first quartz rotator 306, the third crystal rod 307, the fifth quarter-wave plate 308, the third negative lens 309, and the first filter 310 before reaching the third total reflection mirror 311. At this point, the Q-switched laser undergoes two amplifications. The third total reflection mirror 311 reflects the twice-amplified Q-switched laser back, which then sequentially passes through the first filter 310, the third negative lens 309, the fifth quarter-wave plate 308, the third crystal rod 307, the first quartz rotator 306, the second negative lens 305, the second crystal rod 304, and the second polarizer 303. Due to the effect of the third optical isolator 302, the second polarizer 303 can only refract the reflected light to the fourth total reflection mirror 312. At this point, the Q-switched laser has undergone four amplifications.

[0073] The fourth total reflection mirror 312, the fourth optical isolator 313, the third polarizer 314, the fourth crystal rod 315, the fourth negative lens 316, the second quartz rotator 317, the fifth crystal rod 318, the sixth quarter-wave plate 319, the fifth negative lens 320, the second filter 321, and the fifth total reflection mirror 322 are arranged in the same way as the second total reflection mirror 301, the third optical isolator 302, the second polarizer 303, the second crystal rod 304, the second negative lens 305, the first quartz rotator 306, the third crystal rod 307, the fifth quarter-wave plate 308, the third negative lens 309, the first filter 310, and the third total reflection mirror 311, and the principle is the same. Finally, when the Q-switched laser input from the second total reflection mirror 301 is output from the third polarizer 314, eight stages of amplification are completed.

[0074] Furthermore, the frequency multiplication unit 4 uses a type I phase-matched LBO crystal 403 to complete the first frequency conversion;

[0075] The frequency combining unit 5 uses a type II phase-matched LBO crystal 404 to complete the second frequency conversion.

[0076] Specifically, the sixth total reflection mirror (401) receives the polarized light from the third polarizer (303), and the seventh quarter-wave plate (402) adjusts the polarization state of the laser incident on the frequency doubling unit 4. After the frequency doubling unit 4 performs the first frequency conversion on the laser, it sends the converted laser to the frequency combining unit to complete the second frequency conversion, finally obtaining a laser with a wavelength of 372nm.

[0077] Furthermore, the transmitting unit 6 includes a first beam splitter, a second beam splitter, a beam expander, an adjustment frame, and a laser reflector;

[0078] The laser reflector is mounted on the adjustment frame;

[0079] The first beam splitter is coated with a λ / 3 wavelength reflective film, a λ / 2 wavelength anti-reflective film, and a λ wavelength anti-reflective film;

[0080] The second beam splitter is coated with a λ / 2 wavelength reflective film and a λ wavelength antireflective film;

[0081] The beam expander has an adjustable magnification of 2-5 times.

[0082] For example, the first beam splitter is coated with a 372nm wavelength reflective film and anti-reflection films for 558nm and 1116nm wavelengths; the second beam splitter is coated with a 558nm wavelength reflective film and an 1116nm wavelength anti-reflection film. The beam expander uses an adjustable magnification of 2-5 times to control the laser divergence angle between 0.1mrad and 0.2mrad. The emitted 372nm wavelength pulsed laser is reflected by a high-reflectivity mirror and enters a Galilean beam expander to change the laser divergence angle of the lidar, ensuring that the laser divergence angle meets the field of view requirements of the receiving telescope. Simultaneously, the attitude of the laser reflector is adjusted by an adjustment frame to control the direction of the laser beam emitted into the atmosphere, ensuring that the receiving and receiving axes of the lidar are parallel, thereby guaranteeing receiving efficiency.

[0083] Furthermore, the receiving unit includes a receiving telescope and a pinhole aperture, and the receiving telescope adopts a coaxial card system;

[0084] The receiving telescope includes a concave parabolic primary mirror, a hyperboloid secondary mirror, and a corner reflector arranged in sequence.

[0085] The pinhole aperture is located at the rear of the receiving telescope and is used to control the field of view of the receiving telescope.

[0086] For example, the receiving telescope adopts a coaxial cassette system, consisting of a 1000mm diameter concave parabolic primary mirror, a 190mm diameter hyperboloid secondary mirror, and a 25.4mm diameter corner reflector, used to receive echo signals at three frequencies of the system; the zenith angle is 30 degrees, and if the orientation is due east, it can be used to measure zonal winds; if the laser beam and telescope are pointed due north, it can measure meridional winds.

[0087] The pinhole aperture is used to control the field of view of the receiving telescope. Considering the problem of daytime detection, the field of view of the receiving telescope is controlled within 0.4 mrad. At the same time, due to the influence of the iron atom saturation effect, the laser divergence angle cannot be less than 0.1 mrad. In order to ensure the receiving efficiency of the system, the field of view of the receiving telescope needs to be greater than the laser divergence angle.

[0088] Furthermore, the receiving unit also includes a movable lens group, a daytime detection unit, and a nighttime detection unit;

[0089] The receiving unit is used to control the transmission of the echo signal received by the receiving telescope to the daytime detection unit or the nighttime detection unit;

[0090] The night detection unit includes a first narrowband interferometer filter, a first collimating lens, a first narrowband filter, a first converging lens, and a first photodetector arranged in sequence.

[0091] The daytime detection unit includes a second narrowband interferometer filter, a second collimating lens, a second narrowband filter, a second converging lens, and a second photodetector arranged in sequence.

[0092] Specifically, due to the extremely strong background noise from sunlight during daytime detection, different optical structures are used in this embodiment to obtain effective echo signals for daytime and nighttime. During the day, the echo signal is guided to the daytime detection unit via a movable lens group, where it is collected and processed. At night, the position of the movable lens group is adjusted to guide the echo signal to the nighttime detection unit, where it is collected and processed. For example, the bandwidth of the first narrowband filter is 0.15 nm, and the second narrowband interference filter is composed of two cascaded FP etalons, with a combined bandwidth of less than 5 pm.

[0093] Furthermore, the data processing unit includes a data acquisition card and a computing unit;

[0094] The data acquisition card is connected to the first photodetector and the second photodetector and is used to convert electrical signals into digital signals;

[0095] The computing unit is connected to the data acquisition card and is used to acquire temperature, wind field and iron atom number density detection results through the digital signal.

[0096] Specifically, after the first or second photodetector converts the optical signal of the echo signal into an electrical signal, the data acquisition card quantizes the electrical signal into a digital signal, and the computing unit obtains the detection results of temperature, wind field and iron density through the digital signal.

[0097] Furthermore, the lidar also includes a high-speed chopper, which is used to suppress strong backscattered echo signals at low altitudes and prevent saturation of the photodetector and photon counter card. The high-speed chopper drive motor speed is set at 7200 rpm, and the frequency of the electrical pulse signal generated by the conversion circuit is 480 Hz. After being divided by 8 by a digital delay pulse generator, a 60 Hz pulse signal output is achieved.

[0098] Furthermore, the lidar also includes a timing control unit, which uses the 60Hz pulse sequence from the high-speed chopper as the main pulse signal for timing control of the entire iron resonance fluorescence Doppler lidar system, thereby achieving synchronous triggering of the 372nm wavelength pulsed laser, the frequency monitoring unit, and the acquisition unit.

[0099] Furthermore, the lidar system also includes an energy monitoring unit and a frequency monitoring unit;

[0100] The energy monitoring unit is used to monitor the λ / 3 wavelength laser and remove the original lidar data within the time period corresponding to abnormal energy fluctuations.

[0101] The frequency monitoring unit is used to combine the remaining λ wavelength of the lidar, the frequency-hopping pulse light and the continuous light to obtain the frequency stability of the pulse light, and to remove the original lidar data within abnormal time periods. The abnormal time period is the time period corresponding to the frequency stability being greater than a set value.

[0102] Specifically, in the energy monitoring unit, the pulsed laser light from the 372nm wavelength Nd:YAG solid-state laser passes through a first beam splitter, where 95% of the pulsed light is reflected to a beam expander, and 5% of the pulsed light passes through the beam splitter before entering the energy monitoring unit and frequency monitoring unit respectively via a second beam splitter. A laser energy meter is used to measure the 372nm wavelength pulsed light over a long period. The measured laser pulse energy data is displayed in real time and stored on a hard drive. Subsequently, the raw lidar data for the time period corresponding to abnormal energy fluctuations is removed.

[0103] The frequency monitoring unit couples the remaining 1116nm wavelength frequency-stabilized and frequency-hopping pulsed light from the 372nm wavelength Nd:YAG solid-state laser into an optical fiber via a five-dimensional adjustment frame, and performs beam combining and frequency matching with the 1116nm wavelength continuous light from the 1116nm wavelength three-frequency stabilized seed laser. The resulting frequency difference can indirectly reflect the frequency stability of the pulsed light. If the frequency stability is greater than the set value of 1MHz, the original lidar data within the corresponding time period must be discarded.

[0104] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A ferrore resonance fluorescence Doppler lidar system, characterized in that, The lidar system includes a seed laser (1), an oscillation stage unit (2), an amplification stage unit (3), a frequency doubling unit (4), a frequency combining unit (5), a transmitting unit (6), a receiving unit, and a data processing unit; The seed laser (1) is used to output continuous light with a preset wavelength of λ, and the frequency of the continuous light jumps between f-f0, f, and f+f0, where f is the frequency corresponding to λ and f0 is the preset frequency adjustment amount; The oscillation stage unit (2) is connected to the seed laser (1) and is used to output a single longitudinal mode Q-switched pulse laser. The amplification stage unit (3) is connected to the oscillation stage unit (2) and is used to amplify the power of the Q-switched pulse laser. The frequency doubling unit (4) is connected to the amplification stage unit (3) and is used to perform a nonlinear frequency transformation on the fundamental frequency light to obtain λ / 2 wavelength frequency-doubled light; The frequency combining unit (5) is connected to the frequency doubling unit (4) and is used to perform nonlinear frequency transformation on the frequency doubling light to obtain λ / 3 wavelength pulsed laser. The transmitting unit (6) is used to transmit the λ / 3 wavelength pulsed laser output by the frequency combining unit into the atmosphere; The receiving unit is used to collect the echo signal after the interaction between the λ / 3 wavelength pulsed laser and the atmosphere; The data processing unit is connected to the receiving unit and is used to process the echo signal and obtain the detection results of temperature, wind field and iron atom number density. The amplification stage unit (3) includes, in sequence, a second total reflection mirror (301), a third optical isolator (302), a second polarizer (303), a second crystal rod (304), a second negative lens (305), a first quartz rotator (306), a third crystal rod (307), a fifth quarter-wave plate (308), a third negative lens (309), a first filter (310), and a third total reflection mirror (311). The fourth total reflection mirror (312), the fourth optical isolator (313), the third polarizer (314), the fourth crystal rod (315), the fourth negative lens (316), the second quartz rotator (317), the fifth crystal rod (318), the sixth quarter-wave plate (319), the fifth negative lens (320), the second filter (321), and the fifth total reflection mirror (322) are arranged in sequence. The fourth total reflection mirror (312) receives the polarized light from the second polarizer (303).

2. The iron resonance fluorescence Doppler lidar system according to claim 1, characterized in that, The oscillation stage unit (2) includes, in sequence, a first half-wave plate (201), a first optical isolator (202), a second half-wave plate (203), a first quarter-wave plate (204), a first high-reflection mirror (205), an RTP crystal (206), a second quarter-wave plate (207), a first polarizer (208), a third quarter-wave plate (209), a first crystal rod (210), a fourth quarter-wave plate (211), a second high-reflection mirror (212), a second optical isolator (218), a first negative lens (219), and a first total reflection mirror (220). A first piezoelectric ceramic (213) and a second piezoelectric ceramic (217) are respectively attached to the first high-reflection mirror (205) and the second high-reflection mirror (212). A resonant detection box (214), a detector (215), and a Q-switching drive (216) are arranged sequentially between the first piezoelectric ceramic (213) and the second piezoelectric ceramic (217). The RTP crystal (206), the second quarter-wave plate (207), the first polarizer (208) and the Q-switching driver (216) constitute an electro-optic Q-switching system, and the detector (215) is used to detect the interference waveform passing through the first polarizer (208).

3. The iron resonance fluorescence Doppler lidar system according to claim 1, characterized in that, The second crystal rod (304), the third crystal rod (307), the fourth crystal rod (315) and the fifth crystal rod (318) adopt the pumping method of semiconductor end face pumping Nd:YAG crystal.

4. The iron resonance fluorescence Doppler lidar system according to claim 1, characterized in that, The frequency multiplication unit (4) uses a type I phase-matched LBO crystal (403) to complete the first frequency conversion; The frequency combining unit (5) uses a type II phase-matched LBO crystal (404) to complete the second frequency conversion.

5. The iron resonance fluorescence Doppler lidar system according to claim 1, characterized in that, The transmitting unit (6) includes a first beam splitter, a second beam splitter, a beam expander, an adjustment frame, and a laser reflector; The laser reflector is mounted on the adjustment frame; The first beam splitter is coated with a λ / 3 wavelength reflective film, a λ / 2 wavelength anti-reflective film, and a λ wavelength anti-reflective film; The second beam splitter is coated with a λ / 2 wavelength reflective film and a λ wavelength antireflective film; The beam expander has an adjustable magnification of 2-5 times.

6. The iron resonance fluorescence Doppler lidar system according to claim 1, characterized in that, The receiving unit (7) includes a receiving telescope and a pinhole aperture, and the receiving telescope adopts a coaxial card system; The receiving telescope includes a concave parabolic primary mirror, a hyperboloid secondary mirror, and a corner reflector arranged in sequence. The pinhole aperture is located at the rear of the receiving telescope and is used to control the field of view of the receiving telescope.

7. The iron resonance fluorescence Doppler lidar system according to claim 6, characterized in that, The receiving unit also includes a movable lens group, a daytime detection unit, and a nighttime detection unit; The receiving unit is used to control the transmission of the echo signal received by the receiving telescope to the daytime detection unit or the nighttime detection unit; The night detection unit includes a first narrowband interferometer filter, a first collimating lens, a first narrowband filter, a first converging lens, and a first photodetector arranged in sequence. The daytime detection unit includes a second narrowband interferometer filter, a second collimating lens, a second narrowband filter, a second converging lens, and a second photodetector arranged in sequence.

8. The iron resonance fluorescence Doppler lidar system according to claim 7, characterized in that, The data processing unit includes a data acquisition card and a computing unit; The data acquisition card is connected to the first photodetector and the second photodetector and is used to convert electrical signals into digital signals; The computing unit is connected to the data acquisition card and is used to acquire temperature, wind field and iron atom number density detection results through the digital signal.

9. The iron resonance fluorescence Doppler lidar system according to claim 1, characterized in that, The lidar system also includes an energy monitoring unit and a frequency monitoring unit; The energy monitoring unit is used to monitor the λ / 3 wavelength laser and remove the original lidar data within the time period corresponding to abnormal energy fluctuations. The frequency monitoring unit is used to combine the remaining λ wavelength of the lidar, the frequency-hopping pulse light and the continuous light to obtain the frequency stability of the pulse light, and to remove the original lidar data within abnormal time periods. The abnormal time period is the time period corresponding to the frequency stability being greater than a set value.

Images