Iron resonance fluorescence Doppler laser radar and laser generation device and method

Abstract

The invention belongs to the technical field of laser radars, and discloses a laser generation device and method of an iron resonance fluorescence Doppler laser radar. The first seed source and the second seed source respectively output continuous light; after the first laser processor carries out power amplification, frequency multiplication, frequency stabilization and frequency beating on the continuous light, the central wavelength of the continuous light is adjusted in real time; the second laser processor performs beam combination and beat frequency on the continuous light adjusted by the first seed source and the continuous light emitted by the second seed source, locks the phase of a difference frequency signal on an external radio frequency signal, and controls the radio frequency signal to circularly jump in a certain interval; and the pulse laser performs amplification and nonlinear frequency conversion on the continuous light emitted by the locked second seed source, outputs single-frequency stable-frequency narrow-band pulse laser, and realizes time-sharing frequency hopping output of the pulse light among three frequencies at the same time. The laser generating device provides a reliable light source for high-precision detection of the atmospheric temperature, the wind field and the iron atom number density of the area from the top of the middle layer to the bottom of the thermal layer.

Description

Technical Field

[0001] The present invention belongs to the technical field of lidar, and particularly relates to an iron resonance fluorescence Doppler lidar, a laser generating device and a method. Background Art

[0002] Temperature and wind field are important environmental parameters characterizing the atmospheric state. In the region from the top of the mesosphere to the bottom of the thermosphere, in the altitude range of about 75 km to 115 km, due to the lack of effective observation means, the observational data of the vertical distributions of temperature and wind field in this altitude range are relatively scarce. Resonance fluorescence lidar uses unique metal atoms, ions, etc. in the region from the top of the mesosphere to the bottom of the thermosphere as tracers, and by exciting their resonance fluorescence signals, the detection of temperature and wind field can be realized. Compared with means such as sounding rockets, passive remote sensing, and microwave active remote sensing, resonance fluorescence lidar has the advantages of continuous observation, high spatio-temporal resolution, high precision, etc., and has become a powerful tool for detecting the region from the top of the mesosphere to the bottom of the thermosphere. Currently, sodium resonance fluorescence lidar is mainly used internationally to measure temperature and wind field, while iron resonance fluorescence Doppler lidar has advantages in all-day detection and other aspects, and is another effective means for detecting the temperature and wind field profiles in the region from the top of the mesosphere to the bottom of the thermosphere. In the iron resonance fluorescence Doppler lidar system, the laser light source is the most important component, which is required to output narrow-linewidth pulsed laser with single frequency and frequency stabilization, and can also realize time-division frequency hopping output of three frequencies. It is precisely due to the difficulties in laser technology that the development of iron resonance fluorescence Doppler lidar is restricted. Currently, only the University of Illinois at Urbana-Champaign, the University of Colorado Boulder in the United States, and the German Aerospace Center have successfully developed this type of lidar. Summary of the Invention

[0003] In order to provide stable narrow-linewidth, single-frequency, frequency-stabilized pulsed laser and realize time-division frequency hopping output of three frequencies, the present invention proposes an iron resonance fluorescence Doppler lidar, a laser generating device and a method, and the specific solutions are as follows:

[0004] The laser generating device includes a first seed source, a first laser processor, a second seed source and a second laser processor;

[0005] The first seed source is used to output continuous seed laser;

[0006] The first laser processor is used to perform power amplification, frequency doubling and frequency stabilization processing on the continuous seed laser, mix the laser after power amplification and frequency doubling and frequency stabilization processing to obtain a difference frequency signal, and adjust the central wavelength of the continuous seed laser according to the difference frequency signal;

[0007] The second seed source is used to output continuous seed laser;

[0008] The second laser processor is used to combine and beat-frequency the continuous seed laser after being adjusted by the first seed source and the continuous seed laser emitted by the second seed source, and phase-lock the difference-frequency signal to an externally provided radio frequency signal, where the signal loaded on the radio frequency signal has a fixed carrier frequency difference w, the central frequency required for temperature and wind measurement, and the hopping amount Δf between the two wings frequencies; control the radio frequency signal to cycle and hop between w - Δf, w, and w + Δf, and after locking, amplify and output the continuous light emitted by the second seed source.

[0009] Further, the first laser processor includes a first fiber amplifier, an active frequency stabilization unit, and a first servo feedback unit;

[0010] The first fiber amplifier is used to amplify the power of the continuous seed laser;

[0011] The active frequency stabilization unit is used to perform frequency doubling on the continuous seed laser after power amplification and then perform frequency stabilization processing;

[0012] The first servo feedback unit is used to mix the laser after frequency stabilization processing with the laser after power amplification to obtain a difference-frequency signal as the input of negative feedback regulation to adjust the central wavelength of the first seed source.

[0013] Further, the second laser processor includes a signal generation unit, a signal beat-frequency and frequency locking unit, a second servo feedback unit, a second fiber amplifier, and a pulsed laser;

[0014] The signal generation unit is used to add the frequency difference w between the output frequency and the central frequency of the iron fluorescence spectrum line, the central frequency required for temperature and wind measurement, and the hopping amount Δf between the two wings frequencies to the radio frequency modulation signal;

[0015] The signal beat-frequency and frequency locking unit is used to combine and beat-frequency the continuous seed laser output after being adjusted by the first seed source and the continuous light of the second seed source, and lock the detected difference-frequency signal to the radio frequency signal through an optical phase-locked loop;

[0016] The second servo feedback unit is used to make the radio frequency signal cycle and hop between w + Δf, w, and w + Δf through a PID control circuit;

[0017] The second fiber amplifier is used to amplify the power of the second seed source and then inject it into the pulsed laser.

[0018] Further, the active frequency stabilization unit includes a frequency doubling crystal and a frequency stabilization unit;

[0019] The frequency doubling crystal is used to double the frequency of the continuous seed laser;

[0020] The frequency stabilization unit is used to stabilize the frequency of the continuous seed laser by using the saturated absorption spectroscopy technology and utilizing the iodine molecular absorption spectrum line.

[0021] In another embodiment of the present invention, a lidar is also disclosed. The lidar includes the above-mentioned iron resonance fluorescence Doppler lidar laser generating device, and the laser generating device further includes a transmitting unit. The transmitting unit includes a first beam splitter, a second beam splitter, a beam expander, an adjustment bracket, and a laser mirror;

[0022] The laser mirror is installed on the adjustment bracket;

[0023] The first beam splitter is coated with a λ / 3 wavelength reflection film, a λ / 2 wavelength and a λ wavelength antireflection film;

[0024] The second beam splitter is coated with a λ / 2 wavelength reflection film and a λ wavelength antireflection film, where λ is the central wavelength output by the second seed source;

[0025] The beam expander has an adjustable magnification of 2 - 5 times.

[0026] Further, the lidar further includes a receiving unit. The receiving unit includes a receiving telescope and a pinhole aperture. The receiving telescope adopts a coaxial Cassegrain system;

[0027] The receiving telescope includes a concave parabolic primary mirror, a hyperbolic secondary mirror, and a corner reflector arranged in sequence;

[0028] The pinhole aperture is arranged at the rear side of the receiving telescope and is used to control the field of view angle of the receiving telescope.

[0029] Further, the receiving unit further includes a movable lens group, a daytime detection unit, and a nighttime detection unit;

[0030] The receiving unit is used to control the echo signal received by the receiving telescope to be transmitted to the daytime detection unit or the nighttime detection unit;

[0031] The nighttime detection unit includes a first narrowband interferometric filter, a first collimating lens, a first narrowband filter, a first converging lens, and a first photodetector arranged in sequence;

[0032] The daytime detection unit includes a second narrowband interferometric filter, a second collimating lens, a second narrowband filter, a second converging lens, and a second photodetector arranged in sequence.

[0033] Further, the lidar further includes a data processing unit. The data processing unit includes a data acquisition card and a calculation unit;

[0034] The data acquisition card is connected to the first photodetector and the second photodetector, and is used to convert the electrical signal into a digital signal;

[0035] The calculation unit is connected to the data acquisition card, and is used to obtain the detection results of temperature, wind field and iron atom number density through the digital signal.

[0036] Furthermore, the lidar further includes an energy monitoring unit and a frequency monitoring unit,

[0037] The energy monitoring unit is used to monitor the λ / 3 wavelength laser, and eliminate the lidar raw data corresponding to the time period with abnormal energy fluctuation;

[0038] The frequency monitoring unit is used to perform beam combination and beat frequency on the remaining λ wavelength, frequency hopping pulsed light and the continuous light of the lidar to obtain the frequency stability of the pulsed light, and eliminate the lidar raw data corresponding to the abnormal time period, where the abnormal time is the time period corresponding to the frequency stability greater than the set value.

[0039] In another embodiment of the present invention, a laser generation method for an iron resonance fluorescence Doppler lidar is also disclosed. The laser generation method includes the following steps:

[0040] Amplify the power of the first laser beam with a central wavelength of λ and then perform frequency doubling, and then adopt the saturated absorption spectroscopy technology to realize the frequency stabilization output of the first laser beam by using the iodine molecular transition spectrum line with a wavelength of λ / 2, and calculate the frequency difference w between the locking point of the iodine molecular frequency stabilization and the central frequency after the first laser beam passes through frequency doubling;

[0041] Mix the first beam with frequency stabilization output and the first beam with amplified power to obtain a difference frequency signal, and use a PID control circuit to adjust the central wavelength λ of the first laser beam with the difference frequency signal as the negative feedback input;

[0042] Add the required corrected frequency difference w, the central frequency required for temperature measurement and wind measurement, and the frequency hopping amount between the two wings to the radio frequency modulation signal;

[0043] Perform beam combination and beat frequency on the adjusted first laser beam and the second laser beam, lock the difference frequency signal on the modulation signal through an optical phase-locked loop, and use a PID control circuit to realize the cyclic jump of the modulation signal between w-Δf, w, and w-Δf;

[0044] Phase-lock and power-amplify the modulation signal and then inject it into the pulsed laser to obtain a pulsed laser with a wavelength of λ / 3.

[0045] The beneficial effects of the present invention are as follows:

[0046] (1) The present invention proposes to divide the output light of the first seed source and the second seed source into two beams. One beam of continuous light from the first seed source is power-amplified by a first fiber amplifier and undergoes a first non-linear frequency conversion through a frequency doubling crystal, and then enters an iodine absorption cell to achieve frequency stabilization. Another beam of frequency-stabilized light is combined with the output light of the second seed source for beat frequency. The beat frequency signal is locked on a radio frequency signal through an optical phase-locked loop technology, and the radio frequency signal is controlled to cycle and jump within a certain range while ensuring frequency stability. Another beam of continuous light from the second seed source also has the characteristics of frequency stabilization and frequency hopping accordingly. It is power-amplified by a second fiber amplifier and used as the seed injection source for a pulsed laser. The processes of seed injection locking, power amplification, and non-linear frequency conversion are completed in a power laser to achieve the output of the required pulsed laser, which has corresponding narrow linewidth, frequency stabilization, and frequency hopping characteristics.

[0047] (2) The present invention proposes to use an optical phase-locked loop technology to replace the acousto-optic modulator commonly used in a sodium resonance fluorescence Doppler lidar to achieve frequency hopping of different frequencies. Compared with frequency hopping using an acousto-optic modulator, the proposed method has higher optical efficiency and can obtain higher output power of the seed light. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 is a structural diagram of a laser generation device for an iron resonance fluorescence Doppler lidar proposed by the present invention;

[0049] Figure 2 is a working principle diagram of a laser generation device for an iron resonance fluorescence Doppler lidar proposed by the present invention;

[0050] Figure 3 is a schematic diagram showing the relationship between the relative position of the absorption peak and the signal intensity in an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0051] As Figure 1 shown, the present invention discloses a device for generating a Doppler lidar laser. The laser generation device includes a first seed source, a first laser processor, a second seed source, and a second laser processor;

[0052] The first seed source is used to output continuous seed laser;

[0053] The first laser processor is used to perform power amplification, frequency doubling and frequency stabilization processing on the continuous seed laser, mix the laser after power amplification and frequency doubling and frequency stabilization processing to obtain a beat frequency signal, and adjust the central wavelength of the continuous seed laser according to the beat frequency signal;

[0054] The second seed source is used to output continuous seed laser;

[0055] The second laser processor combines and beats the continuous seed laser after being adjusted by the first seed source and the continuous seed laser emitted by the second seed source, and phase-locks the difference frequency signal to an externally provided radio frequency signal, where the signal loaded on the radio frequency signal has a fixed carrier frequency difference w, the central frequency required for temperature and wind measurement, and the frequency hopping amount Δf between the two wings frequencies. Control the radio frequency signal to cycle and hop between w - Δf, w, and w + Δf. After locking, amplify and output the continuous light emitted by the second seed source.

[0056] Exemplarily, the first seed source outputs a continuous seed laser with a narrow linewidth of 1116 nm in the kHz range and a power of 20 mW, and the central wavelength can be adjusted within a certain range. The continuous seed laser output by the first seed source is processed by the first laser processor. First, the first laser processor amplifies the power of the continuous seed laser and outputs the amplified 1116 nm continuous light; then it performs frequency doubling on the amplified 1116 nm continuous light to obtain continuous light with a wavelength of 558 nm; it stabilizes the frequency of the frequency-doubled continuous light; finally, it mixes the power-amplified and frequency-stabilized continuous light, and the difference frequency signal obtained after mixing is used as the input for negative feedback regulation. Finally, the central wavelength of the first seed source is adjusted through a PID control circuit, so that the first seed source outputs continuous light with a stable frequency of 1116 nm wavelength, where the main operation steps of the PID control circuit are proportional-differential-integral. The second seed source also outputs a continuous seed laser with a narrow linewidth of 1116 nm in the kHz range and a power of about 20 mW, and the central wavelength is adjustable within a certain range. The second laser processor combines and beats the continuous seed laser emitted by the first seed source after adjustment and the continuous seed laser emitted by the second seed source, and locks the difference frequency signal generated by the combination and beating to a high-precision radio frequency signal through an optical phase-locked loop. Among them, the radio frequency signal is composed of the frequency difference w generated after frequency doubling of the first seed source, the central frequency required for temperature and wind measurement, and the frequency hopping amount wf between the two wings frequencies added to the radio frequency modulation signal. Finally, a PID control circuit is used to realize the cyclic hopping of the radio frequency signal between w - Δf, w, and w + Δf; after locking, the radio frequency signal adds the frequency difference and the frequency modulation amount to the continuous light emitted by the second seed source, that is, the continuous light emitted by the second seed source also has the characteristics of frequency stabilization and frequency hopping. The second seed source can output continuous light with a stabilized frequency of 1116 nm wavelength, and the laser frequency will hop between 1116 nm - Δf, 1116 nm, and 1116 nm + Δf. After power amplification, stable narrow-linewidth, single-frequency, and frequency-stabilized pulsed laser can be output.

[0057] Further, the first laser processor includes a first fiber amplifier, an active frequency stabilization unit, and a first servo feedback unit;

[0058] The first fiber amplifier is used to amplify the power of the continuous seed laser;

[0059] The active frequency stabilization unit is used to perform frequency doubling on the continuous seed laser after power amplification and then perform frequency stabilization processing;

[0060] The first servo feedback unit is used to mix the laser after frequency stabilization processing with the laser after power amplification to obtain a difference frequency signal as the input for negative feedback regulation to adjust the central wavelength of the first seed source.

[0061] Exemplarily, a 20 mW, kHz-order narrow linewidth, 1116 nm continuous seed laser emitted by the first seed source is preliminarily amplified in the optical fiber working substance of the first fiber amplifier, and the amplified 1116 nm laser is output. Exemplarily, the first fiber amplifier is a ytterbium-doped fiber amplifier. The active frequency stabilization unit performs frequency doubling on the laser after power amplification by the first seed source and then performs frequency stabilization processing to obtain a laser with a more stable frequency. The first servo feedback unit mixes the laser after frequency stabilization processing with the laser after power amplification to obtain a difference frequency signal, and uses the difference frequency signal as the input of negative feedback to reversely adjust the central wavelength of the laser emitted by the first seed source, so that the first seed source outputs a continuous light with a stable frequency of 1116 nm wavelength.

[0062] Further, the active frequency stabilization unit includes a frequency doubling crystal and a frequency stabilization unit;

[0063] The frequency doubling crystal is used to perform frequency doubling on the continuous seed laser;

[0064] The frequency stabilization unit is used to adopt the saturated absorption spectroscopy technique and utilize the iodine molecular absorption line to realize the stabilization of the frequency of the continuous seed laser.

[0065] Specifically, the 1116 nm wavelength continuous light output by the first fiber amplifier obtains a 558 nm wavelength laser after passing through the frequency doubling crystal. The saturated absorption spectroscopy technique is adopted to utilize the iodine molecular absorption near 558 nm to realize the stabilization of the frequency of the 1116 nm wavelength laser. However, there is a frequency difference w between the locking point of iodine molecular frequency stabilization and the central frequency after frequency doubling of the 1116 nm wavelength.

[0066] Further, the second laser processor includes a signal generation unit, a signal beat frequency locking unit, a second servo feedback unit, a second fiber amplifier, and a pulsed laser;

[0067] The signal generation unit is used to add the frequency difference w between the output frequency and the central frequency of the iron fluorescence spectrum line, the central frequency required for temperature and wind measurement, and the frequency hopping amount Δf between the two wings frequencies to the radio frequency modulation signal;

[0068] The signal beat frequency locking unit is used to combine and beat the continuous seed laser output after adjustment by the first seed source and the continuous light of the second seed source, and lock the detected difference frequency signal on the radio frequency signal through an optical phase-locked loop;

[0069] The second servo feedback unit is used to make the radio frequency signal cyclically jump between w-Δf, w, and w+Δf through a PID control circuit;

[0070] The second fiber optic amplifier is used to amplify the power of the radio frequency signal and then inject it into the pulsed laser.

[0071] Specifically, the signal generation unit is used for frequency difference correction and frequency hopping amount input, and adds the frequency difference w to be corrected, the frequency modulation amount Δf between the central frequency and the two wings frequencies required for temperature and wind measurement to the radio frequency modulation signal. At the same time, the signal beat frequency locking unit is used to combine and beat the continuous seed laser output after adjusting the first seed source and the continuous light of the second seed source, and lock the detected beat frequency signal on the radio frequency signal through an optical phase-locked loop. The second servo feedback unit makes the radio frequency signal cyclically jump between w-Δf, w, and w+Δf through a PID control circuit. After locking, the second seed source can output a frequency-stabilized continuous light with a wavelength of 1116 nm, and the laser frequency jumps between 1116 nm-Δf, 1116 nm, and 1116 nm+Δf. After power amplification by the second fiber optic amplifier, it is injected into a pulsed laser with a wavelength of 372 nm, and finally outputs a frequency-stabilized pulsed light with a wavelength of 372 nm that is consistent with the central wavelength of the iron resonance fluorescence spectrum line, and realizes cyclic frequency hopping between the central wavelength of the iron resonance fluorescence spectrum line -3Δf, 0, and +3Δf.

[0072] Exemplarily, the first seed source outputs a continuous seed laser with a wavelength of 1116 nm, the laser power is 23 mW, the laser line width is 3.3 kHz, and the central wavelength tuning range is from 1115.90 nm to 1116.70 nm; the first fiber optic amplifier amplifies the laser power. The 1116 nm wavelength laser after amplification passes through the frequency doubling crystal to obtain a laser with a wavelength of 558 nm. The frequency stabilization unit uses the modulation saturated absorption spectroscopy technology to lock the wavelength of the frequency-doubled laser at the half waist position of the absorption peak near the 558 nm wavelength. The vacuum central wavelength of the iron atom fluorescence spectrum line is 372.0993 nm, and the precise laser wavelength corresponding to the required seed light is 1116.2979 nm. There is a frequency difference of about 1.163 GHz between the locking point stabilized by iodine molecules near 558 nm and the central frequency after frequency doubling of the 1116.2979 nm wavelength. The first servo feedback unit mixes the 1116 nm wavelength continuous light output from the frequency stabilization unit with the 1116 nm wavelength continuous light directly output from the first fiber optic amplifier, and uses the obtained frequency difference as the input of negative feedback regulation. After PID system control, the central wavelength of the first seed source is adjusted so that the first seed source outputs a frequency-stabilized continuous light with a wavelength of 1116 nm.

[0073] The second seed source is used to output a 1116 nm continuous seed laser with a laser power of 23 mW, a laser linewidth of 4.2 kHz, and a central wavelength tuning range of 1115.95 nm to 1116.65 nm; the signal generation unit applies the required frequencies of 1.163 GHz - 250 MHz, 1.163 GHz, and 1.163 GHz + 250 MHz to the radio frequency modulator signal; the signal beat frequency and frequency locking unit combines and beats the 1116 nm wavelength frequency-stabilized continuous light after adjusting the first seed source with the 1116 nm wavelength continuous light emitted by the second seed source, and the detected beat frequency signal is locked on a high-precision radio frequency signal through an optical phase-locked loop. The second servo feedback unit uses a PID control circuit to achieve cyclic hopping of the radio frequency signal among 1.163 GHz - 250 MHz, 1.163 GHz, and 1.163 GHz + 250 MHz, and the hopping relocking time is on the order of ms. After locking, the second seed source can output a 1116 nm wavelength frequency-stabilized continuous light, and the laser frequency will hop among 1116.2979 nm - 250 MHz, 1116.2979 nm, and 1116.2979 nm + 250 MHz; the laser output by the second seed source after locking is power-amplified by the second fiber amplifier and then injected into a 372 nm wavelength pulsed laser, and finally a 372.0993 nm wavelength frequency-stabilized pulsed light consistent with the central wavelength of the iron resonance fluorescence spectrum is output, and cyclic hopping among the frequencies of the central wavelength of the iron resonance fluorescence spectrum - 750 MHz, 0 MHz, and + 750 MHz is achieved.

[0074] Exemplarily, due to the laser frequency jitter, the deviation of the effective scattering cross-section between the central frequency and the two-wing frequencies is caused, resulting in systematic errors in the temperature and radial wind speed obtained by the three-frequency ratio Doppler technique. The systematic errors in temperature and wind speed caused by 1 MHz laser frequency jitter are 0.25 K and 0.32 m / s respectively. Therefore, it is necessary to lock the absolute frequency of the emitted laser.

[0075] Affected by factors such as frequency chirping, the frequency stability of the seed source should be controlled within hundreds of kHz. Since there is no suitable atomic or molecular spectral line at the 1116 nm wavelength as a frequency reference point, while there is a strong iodine molecular transition spectral line near the 558 nm wavelength, the saturated absorption spectrum of iodine molecules near 558 nm after frequency doubling is selected as the reference. The full width at half maximum of the 558 nm iodine molecular spectral line is about hundreds of kHz. If the frequency stabilization system and the subsequent feedback circuit can lock the frequency at the zero point of the saturated absorption spectrum error signal, the frequency stability of the continuous light output by the seed source is also hundreds of kHz.

[0076] Due to the frequency difference between the locking point of iodine molecular frequency stabilization and the 1116.2979 nm wavelength, the triple frequency is 372.0993 nm, and the central frequency after frequency doublingFigure 3 At the position of absorption peak 4, w is 1.163 GHz, which makes the wavelength of the output seed light after frequency stabilization unable to precisely correspond to the 1116.2979 nm wavelength. It is necessary to introduce another second seed source with similar performance and use optical beat frequency locking technology to complete the correction of the frequency difference w, so that the laser frequency of the 2 channels output by the second seed source is the required frequency. In addition, through optical beat frequency locking technology, the commonly used acousto-optic modulator in sodium resonance fluorescence Doppler lidar can be replaced to achieve the jump switching between three frequencies.

[0077] Among them, optical beat frequency locking technology is a technology that uses the method of electrical feedback control to lock the frequency and phase of optical signals, aiming to achieve the synchronization of frequencies and phases between two or more different optical carriers. The frequency difference between two laser beams is locked on the externally provided radio frequency signal w0, so that the frequency difference between the two laser beams is changed to a quantity set according to the magnitude of w.

[0078] Assume that the laser frequency of the first seed source after frequency stabilization is f1, and the frequency difference between the central frequency f0 corresponding to 1116.2979 nm and f1 is w, that is, f0 = f1 + w. The laser frequency of the second seed source is f2, and the two laser beams are combined for beat frequency. According to the laser beat frequency principle, the combined optical signal will contain the frequency difference component f2 - f1 and the frequency sum component f2 + f1 of the two lasers. Due to the limitation of the bandwidth of the photodetector, the sum frequency component is filtered out, and the detector can only detect the signal of the difference frequency part. The detected difference frequency signal is locked on the high-precision radio frequency signal w0, that is, after locking, f2 - f1 = w0. Using a circuit to make the radio frequency signal w0 change between w - 250 MHz, w and w + 250 MHz, then after locking, the main output of the second seed source, that is, the laser frequency f2 = f1 + w0 output by the second seed source will jump between three frequencies of f0 - 250 MHz, f0, and f0 + 250 MHz.

[0079] In another embodiment of the present invention, a method for generating laser of an iron resonance fluorescence Doppler lidar is also disclosed. The method for generating laser includes the following steps:

[0080] Amplify the power of the first laser beam with a central wavelength of λ and then perform frequency doubling. Then, adopt saturated absorption spectroscopy technology and use the iodine molecule transition spectrum line with a wavelength of λ / 2 to achieve the frequency-stabilized output of the first laser beam, and calculate the frequency difference w between the locking point of iodine molecule frequency stabilization and the central frequency after the first laser beam undergoes frequency doubling;

[0081] Mix the first light beam with frequency-stabilized output and the first light beam with amplified power to obtain a difference frequency signal, and use a PID control circuit to adjust the central wavelength λ of the first laser beam with the difference frequency signal as the negative feedback input;

[0082] The frequency difference w to be corrected, the center frequency required for temperature and wind measurement, and the frequency hopping amount between the two side frequencies are externally applied to the radio frequency modulation signal;

[0083] The adjusted first laser beam and the second laser beam are combined for beat frequency, and the difference frequency signal is locked on the modulation signal through an optical phase-locked loop, and the modulation signal is cyclically hopped between w - Δf, w, and w + Δf by using a PID control circuit;

[0084] The modulation signal is injected into a pulsed laser based on a Nd:YAG solid laser crystal after frequency locking and power amplification. After seed injection locking, power amplification, and two nonlinear frequency conversion processes, laser pulses with the required wavelength are obtained, and the specific value is 372.0993 nm.

[0085] In another embodiment of the present invention, a lidar is also disclosed. The lidar includes the laser generation device of the ferromagnetic resonance fluorescence Doppler lidar according to any one of the above embodiments. The lidar includes a transmitting unit, and the transmitting unit includes a first beam splitter, a second beam splitter, an expander, an adjustment bracket, and a laser mirror;

[0086] The laser mirror is installed on the adjustment bracket;

[0087] The first beam splitter is coated with a λ / 3 wavelength reflection film, a λ / 2 wavelength and a λ wavelength antireflection film;

[0088] The second beam splitter is coated with a λ / 2 wavelength reflection film and a λ wavelength antireflection film, where λ is the center wavelength output by the second seed source;

[0089] The expander has an adjustable magnification of 2 - 5 times.

[0090] Exemplarily, the first beam splitter is coated with a 372 nm wavelength reflection film, a 558 nm wavelength and a 1116 nm wavelength antireflection film; the second beam splitter is coated with a 558 nm wavelength reflection film and a 1116 nm wavelength antireflection film. The expander has an adjustable magnification of 2 - 5 times, and the laser divergence angle is controlled between 0.1 mrad and 0.2 mrad. The emitted 372 nm wavelength pulsed laser is reflected by a high reflector and enters a Galilean expander to change the laser divergence angle of the lidar so that the laser divergence angle meets the requirements of the receiving telescope field of view angle. At the same time, the attitude of the laser mirror is adjusted through the adjustment bracket to control the direction of the laser beam emitted into the atmosphere, making the transmitting and receiving optical axes of the lidar parallel, thereby ensuring the receiving efficiency.

[0091] Furthermore, the lidar further includes a receiving unit, and the receiving unit includes a receiving telescope and a small aperture stop. The receiving telescope adopts a coaxial Cassegrain system;

[0092] The receiving telescope includes a concave parabolic primary mirror, a hyperbolic secondary mirror, and a corner reflector arranged in sequence;

[0093] The small aperture stop is arranged at the rear side of the receiving telescope and is used to control the field of view angle and divergence angle of the receiving telescope.

[0094] Exemplarily, the receiving telescope adopts a coaxial Cassegrain system, which consists of a concave parabolic primary mirror with a diameter of 1000 mm, a hyperbolic secondary mirror with a diameter of 190 mm, and a corner reflector with a diameter of 25.4 mm, and is used to receive the echo signals of three frequencies of the system; the zenith angle is 30 degrees. If the orientation is due east, it can be used to measure the zonal wind; if the laser beam and the telescope pointing are changed to due north, the meridional wind can be measured.

[0095] The small aperture stop is used to control the field of view angle of the receiving telescope; considering the problem of daytime detection, the field of view angle of the receiving telescope is controlled within 0.4 mrad; at the same time, affected by the iron atom saturation effect, the laser divergence angle cannot be less than 0.1 mard. To ensure the receiving efficiency of the system, the field of view angle of the receiving telescope needs to be greater than the laser divergence angle.

[0096] Further, the receiving unit further includes a movable lens group, a daytime detection unit, and a nighttime detection unit;

[0097] The receiving unit is used to control the echo signal received by the receiving telescope to be transmitted to the daytime detection unit or the nighttime detection unit;

[0098] The nighttime 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;

[0099] 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.

[0100] Specifically, since the background noise of sunlight is extremely strong during daytime detection, in order to more effectively obtain the effective echo signal, different optical structures are used to obtain the echo signal during daytime and nighttime in this embodiment. During the day, through the movable lens group, the echo signal is sent to the daytime detection unit for collection and processing of the echo signal. At night, the position of the movable lens group is adjusted to send the echo signal to the nighttime detection unit for collection and processing of the echo signal. Exemplarily, the bandwidth of the first narrowband filter is 0.15 nm, and the second narrowband interferometer filter is composed of two different FP etalons cascaded, and the comprehensive bandwidth is less than 5 pm.

[0101] Further, the lidar further includes a data processing unit, and the data processing unit includes a data acquisition card and a calculation unit;

[0102] The data acquisition card is connected to the first photodetector and the second photodetector, and is configured to convert an electrical signal into a digital signal;

[0103] The calculation unit is connected to the data acquisition card, and is configured to obtain detection results of temperature, wind field, and iron atom density (iron atom number density) through the digital signal.

[0104] Specifically, after the first photodetector or the 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 calculation unit obtains the detection results of temperature, wind field, and iron density through the digital signal.

[0105] Further, the lidar further includes a high-speed chopper, and the high-speed chopper is configured to suppress strong backscattered echo signals at low altitudes and prevent saturation of the photodetector and the photon counting card; the rotational speed of the high-speed chopper drive motor is set at 7200 revolutions per minute, the frequency of the electrical pulse signal generated by the conversion circuit is 480 Hz, and an 8-frequency division is performed by the digital delay pulse generator DG645 to achieve an output of a 60 Hz pulse signal.

[0106] Further, the lidar further includes a timing control unit, which uses the 60 Hz pulse sequence from the high-speed chopper as the main pulse signal for the timing control of the entire iron resonance fluorescence Doppler lidar, and realizes synchronous triggering of the 372 nm wavelength pulsed laser, the frequency monitoring unit, and the acquisition unit.

[0107] Further, the lidar further includes an energy monitoring unit and a frequency monitoring unit;

[0108] The energy monitoring unit is configured to monitor the λ / 3 wavelength laser and eliminate the original lidar data during the time period corresponding to abnormal energy fluctuations;

[0109] The frequency monitoring unit is configured to perform beam combination and beat frequency on the remaining λ wavelength, frequency hopping pulsed light, and the continuous light of the lidar to obtain the frequency stability of the pulsed light, and eliminate the original lidar data during the abnormal time period, where the abnormal time is the time period corresponding to the frequency stability being greater than the set value.

[0110] Specifically, for the energy monitoring unit, the pulsed laser from the 372-nm wavelength Nd:YAG solid-state laser passes through the first beam splitter. 95% of the pulsed light is reflected to the beam expander, and 5% of the pulsed light passes through the beam splitter and then enters the energy monitoring unit and the frequency monitoring unit respectively through the second beam splitter. Among them, a laser energy meter is used to measure the pulsed light of 372-nm wavelength for a long time. The measured laser pulse energy data is displayed and stored in real time, and the original lidar data in the time period corresponding to the abnormal energy fluctuation is excluded subsequently;

[0111] For the frequency monitoring unit, the remaining 1116-nm wavelength frequency-stabilized and frequency-hopping pulsed light from the 372-nm wavelength Nd:YAG solid-state laser is coupled into the optical fiber through a five-dimensional adjustment frame, and is combined and beat with the 1116-nm wavelength continuous light from the 1116-nm wavelength triple-frequency frequency-stabilized seed laser. The obtained frequency difference can indirectly reflect the frequency stability of the pulsed light; if the frequency stability is greater than the set value of 1 MHz, the original lidar data in the corresponding time period needs to be excluded.

[0112] As mentioned above, the above is only the preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and inventive concept of the present invention, makes equivalent substitutions or changes, and all should be covered by the protection scope of the present invention.

Claims

1. A laser generation device for an iron resonance fluorescence Doppler lidar, characterized in that, The laser generating device includes a first seed source, a first laser processor, a second seed source, and a second laser processor; The first seed source is configured to output continuous seed laser; The first laser processor is configured to perform power amplification, frequency doubling and frequency stabilization processing on the continuous seed laser, mix the laser after power amplification and frequency doubling and frequency stabilization processing to obtain a difference frequency signal, and adjust the central wavelength of the continuous seed laser according to the difference frequency signal; The second seed source is configured to output continuous seed laser; The second laser processor is configured to perform beam combining and beat frequency on the continuous seed laser adjusted by the first seed source and the continuous seed laser emitted by the second seed source, and phase-lock the difference frequency signal to an externally provided radio frequency signal, where the signal loaded on the radio frequency signal has a fixed carrier frequency difference w, the central frequency required for temperature and wind measurement, and the frequency hopping amount Δf between the two wings frequencies; control the radio frequency signal to cycle and jump between w - Δf, w, and w + Δf. After locking, amplify and output the continuous light emitted by the second seed source.

2. The laser generation device for an iron resonance fluorescence Doppler lidar according to claim 1, characterized in that, The first laser processor includes a first fiber amplifier, an active frequency stabilization unit, and a first servo feedback unit; The first fiber amplifier is configured to perform power amplification on the continuous seed laser; The active frequency stabilization unit is configured to perform frequency doubling on the continuous seed laser after power amplification and then perform frequency stabilization processing; The first servo feedback unit is configured to mix the laser after frequency stabilization processing with the laser after power amplification to obtain a difference frequency signal as the input for negative feedback adjustment to adjust the central wavelength of the first seed source.

3. The laser generation device for an iron resonance fluorescence Doppler lidar according to claim 2, characterized in that, The second laser processor includes a signal generating unit, a signal beat frequency and frequency locking unit, a second servo feedback unit, a second fiber amplifier, and a pulsed laser; The signal generating unit is configured to add the frequency difference w between the output frequency and the central frequency of the iron fluorescence spectrum line, the central frequency required for temperature and wind measurement, and the frequency hopping amount Δf between the two wings frequencies to the radio frequency modulation signal; The signal beat frequency and frequency locking unit is configured to perform beam combining and beat frequency on the continuous seed laser output after adjustment by the first seed source and the continuous light of the second seed source, and lock the detected difference frequency signal to the radio frequency signal through an optical phase-locked loop; The second servo feedback unit is configured to make the radio frequency signal cycle and jump between w - Δf, w, and w + Δf through a PID control circuit; The second fiber amplifier is configured to perform power amplification on the continuous light of the second seed source and then inject it into the pulsed laser.

4. The laser generation device for an iron resonance fluorescence Doppler lidar according to claim 2, characterized in that: The active frequency stabilization unit includes a frequency doubling crystal and a frequency stabilization unit; The frequency doubling crystal is configured to perform frequency doubling on the continuous seed laser; The frequency stabilization unit is configured to use the saturated absorption spectroscopy technique to stabilize the frequency of the continuous seed laser by using the iodine molecular absorption spectrum line.

5. A lidar, the lidar includes the laser generation device for an iron resonance fluorescence Doppler lidar according to any one of claims 1-4, characterized in that, The lidar further includes a transmitting unit, and the transmitting unit includes a first beam splitter, a second beam splitter, a beam expander, an adjustment bracket, and a laser reflector; The laser reflector is installed on the adjustment bracket; The first beam splitter is coated with a λ / 3 wavelength reflection film, a λ / 2 wavelength and a λ wavelength antireflection film; The second beam splitter is coated with a λ / 2 wavelength reflection film and a λ wavelength antireflection film, where λ is the central wavelength output by the second seed source; The beam expander has an adjustable magnification of 2 - 5 times.

6. The lidar according to claim 5, characterized in that, The lidar further includes a receiving unit, which includes a receiving telescope and a pinhole aperture. The receiving telescope adopts a coaxial Cassegrain system; The receiving telescope includes a concave parabolic primary mirror, a hyperbolic secondary mirror, and a corner reflector arranged in sequence; The pinhole aperture is arranged at the rear side of the receiving telescope for controlling the field of view angle of the receiving telescope.

7. The lidar according to claim 6, characterized in that, The receiving unit further includes a movable lens group, a daytime detection unit, and a nighttime detection unit; The receiving unit is used to control the echo signal received by the receiving telescope to be transmitted to the daytime detection unit or the nighttime detection unit; The nighttime detection unit includes a first narrow - band interferometer filter, a first collimating lens, a first narrow - band filter, a first converging lens, and a first photodetector arranged in sequence; The daytime detection unit includes a second narrow - band interferometer filter, a second collimating lens, a second narrow - band filter, a second converging lens, and a second photodetector arranged in sequence.

8. The lidar according to claim 7, characterized in that, The lidar further includes a data processing unit, which includes a data acquisition card and a calculation unit; The data acquisition card is connected to the first photodetector and the second photodetector for converting the electrical signal into a digital signal; The calculation unit is connected to the data acquisition card for obtaining the detection results of temperature, wind field, and iron atom number density through the digital signal.

9. The lidar according to claim 5, characterized in that, The lidar further includes an energy monitoring unit and a frequency monitoring unit, The energy monitoring unit is used to monitor the λ / 3 wavelength laser and eliminate the original lidar data corresponding to the time period with abnormal energy fluctuations; The frequency monitoring unit is used to perform beam combination and beat - frequency on the remaining λ wavelength, frequency - hopping pulsed light, and continuous light of the lidar to obtain the frequency stability of the pulsed light, and eliminate the original lidar data during the abnormal time period. Here, the abnormal time is the time period corresponding to the frequency stability being greater than the set value.

10. A method for generating laser of an iron resonance fluorescence Doppler lidar, characterized in that, The laser generation method includes the following steps: Amplify the power of the first laser beam with a central wavelength of λ and then perform frequency doubling. Then, adopt the saturated absorption spectroscopy technique to achieve the frequency - stabilized output of the first laser beam by using the iodine molecular transition spectrum line of λ / 2 wavelength, and calculate the frequency difference w between the locking point of iodine molecular frequency stabilization and the central frequency after the frequency doubling of the first laser beam; Mix the frequency - stabilized output first beam and the power - amplified first beam to obtain a difference - frequency signal, and use a PID control circuit to adjust the central wavelength λ of the first laser beam with the difference - frequency signal as the negative - feedback input; Add the required corrected frequency difference w, the central frequency required for temperature and wind measurement, and the frequency - hopping amount between the two wings to the radio - frequency modulation signal; Perform beam combination and beat - frequency on the adjusted first laser beam and the second laser beam, lock the difference - frequency signal on the modulation signal through an optical phase - locked loop, and use a PID control circuit to achieve the cyclic jump of the modulation signal between w - Δf, w, and w + Δf; Phase - lock and power - amplify the modulation signal and then inject it into the pulsed laser to obtain a λ / 3 wavelength pulsed laser.

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