CN112285674B - Micro-pulse differential absorption laser radar transmitter for detecting atmospheric water vapor - Google Patents
Micro-pulse differential absorption laser radar transmitter for detecting atmospheric water vapor Download PDFInfo
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
- G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
- G01S17/88—Lidar systems specially adapted for specific applications
- G01S17/95—Lidar systems specially adapted for specific applications for meteorological use
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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- Y02A—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE
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Abstract
The invention discloses a transmitter of a micro-pulse differential absorption laser radar for detecting atmospheric water vapor, wherein a DFB semiconductor continuous wave laser reference unit forms a stable wavelength reference of continuous wave laser, a DFB semiconductor continuous wave laser on unit and a DFB semiconductor continuous wave laser off unit are chopped by an acousto-optic modulator respectively through a 2 x 1 gating switch to form pulse laser with the repetition frequency of 10kHz magnitude and the pulse width within 1 mu s, and the pulse laser with small energy passes through a Raman fiber amplifier to improve the single pulse energy; the energy of the pulse laser is improved by the Raman fiber amplifier and then converted into 825.5 +/-0.25 nm pulse laser by the frequency doubler, and finally the energy of the pulse laser reaches 10 mu J magnitude. Wherein, the pumping source of the Raman fiber amplifier is served by a high-power semiconductor laser near 1540 nm. The invention has the advantages that: the laser wavelength of the system is stable, and the system cost is low; the peak power of the transmitted pulse is small, and the requirement of human eye safety is met.
Description
Technical Field
The invention relates to an optical remote sensor for atmospheric water vapor vertical profile, in particular to a differential absorption laser radar for atmospheric water vapor vertical profile detection, and particularly relates to a transmitter of a micro-pulse differential absorption laser radar.
Background
The most representative differential absorption laser radar with the wave band of 720nm-730nm belongs to an airborne differential absorption laser radar LEANDRE-II developed by French scientists Didier Bruneau and the like, can detect the vertical profile of the water-gas mixing ratio at the lower half part of a troposphere, a transmitter is a flash lamp pumped Alexandrite laser, the dual-pulse laser works, and the dual wavelength is positioned in the spectral range of 727 nm-736 nm. Laser pulse energy of 50mJ, pulse repetition frequency of 10Hz, line width of 2.4X 10 -2 cm -1 The spectral purity is greater than 99.99%, and the time interval between double pulse emission is 50 mus. One feature of the LEANDRE-II differential absorption lidar is that the emission wavelength is monitored in real time by a high precision wavemeter calibrated by a stable 632.991372nm He-Ne laser (model Hewlett-Packard 5517B) providing a reference to the wavemeter to an absolute accuracy of 5 x 10 -3 cm -1 Within.
Atmospheric water vapor also has a proper absorption line in a 817nm-820nm wave band, and the titanium gem has a high gain coefficient in the wave band, so that scientists such as gard Wagner and the like at Hohenheim university of Germany develop a vehicle-mounted, three-dimensional scanning differential absorption laser radar based on a titanium gem laser as a transmitter in the wave band to detect the atmospheric water vapor distribution of a lower convection layer. A dynamically stable, ring resonator comprising a Brewster's cut angle titanium sapphire (Ti: sapphire) crystal, pumped on both sides at twice the frequency (532 nm) of a diode pumped Nd: YAG laser. Pulse repetition frequency 250Hz, pulse energy 27mJ. The resonant cavity adopts the active stabilization technology of seed injection and phase sensitive detection. This model (UHOH DIAL) differential absorption lidar is characterized by the use of a high-precision wavemeter (model WS7, high finesse/Angstrom) as the error measuring device of the wavelength control loop, and a frequency stabilized helium-neon laser (SL 03, sio Messtechnik GmbH) as the reference for the wavemeter.
After 1998, G.Ehret et al scientists in DLR (Deutsches Zentrumfur Luft-und Raumfahrt) reported that they developed optical parametric oscillators consisting of a double-frequency 532nm pulsed KTP nonlinear crystal of seed-injected Nd: YAG lasers. The single longitudinal mode and narrow line width pulse laser is output near 935nm, the differential absorption laser radar is used for detecting atmosphere water vapor profile, the Nd is pumped, the YAG laser is used for seed injection, the resonant cavity of the optical parametric oscillator is also used for seed injection of continuous wave semiconductor laser, and the absorption line of the band water vapor is stronger, so the method is more suitable for airborne flight test detection and is more beneficial to atmosphere water vapor detection of the upper half part of the troposphere and the bottom part of the troposphere in relatively dry regions. The differential absorption laser radar (waves) in the wave band also adopts a High-precision wavemeter (model High Finesse WS 7) as an error measuring device of a seed laser wavelength control loop, and the reference standard of the wavemeter is a strong absorption spectral line (around 935.607 nm) of a water vapor self multi-channel absorption cell (model New Focus 5612, the effective length is 100m, and the purified water vapor pressure is 1200 Pa). Pulse energy 45mJ, pulse repetition frequency 100Hz.
For a differential absorption lidar transmitter for detecting atmospheric moisture, a stabilizing unit for the detection wavelength is a key component. The differential absorption lidar for water vapor which is being developed and operated at present has a common characteristic that a wavelength stabilizing unit is used for a negative feedback control loop, and an error measuring device of the negative feedback active control loop is used for an expensive high-precision wavelength meter with a wavelength reference. This arrangement adds significant cost to the composition of the lidar for differential absorption of water vapor. The differential laser radar transmitter adopts a laser with low repetition frequency (within 50 Hz) and high pulse energy (more than tens of milli-joules), the high peak power of the laser pulse has the risk of injuring human eyes, the volume (carrying a water cooling system) and the power consumption of the laser are high, and the high cost of the differential absorption laser radar is also one of the reasons. Just because of the high cost, the differential absorption laser radar for detecting atmospheric water vapor has not been popularized and used yet.
Disclosure of Invention
The invention aims to provide a transmitter of a micro-pulse differential absorption laser radar for detecting atmospheric water vapor, which solves two problems existing in the existing transmitter: the expensive high-precision wavemeter with the wavelength reference is adopted in the wavelength stabilizing unit, so that the system cost is obviously increased; the peak power of the transmitted pulses is so high that the eye safety risk and power consumption are high.
The transmitter comprises a DFB semiconductor continuous wave laser reference unit 1, a DFB semiconductor continuous wave laser on unit 2, a DFB semiconductor continuous wave laser off unit 3, an on/off gating switch 4, an acoustic-optical modulator 5, a Raman optical fiber amplifier 6, a frequency doubler 7 and a pumping light source 8 of the Raman optical fiber amplifier.
The DFB semiconductor continuous wave laser reference cell 1 forms a wavelength reference for the stabilization of the continuous wave laser, the difference between the on wavelength of the DFB semiconductor continuous wave laser in the DFB semiconductor continuous wave laser on cell 2 and the reference wavelength in the DFB semiconductor continuous wave laser reference cell 1 is maintained at a long-term stable wavelength of 0.04nm, and the difference between the off wavelength of the DFB semiconductor continuous wave laser in the DFB semiconductor continuous wave laser off cell 3 and the stable reference wavelength in the DFB semiconductor continuous wave laser reference cell 1 is maintained at a long-term stable wavelength of 0.140nm. The DFB semiconductor continuous wave laser on unit 2 and the DFB semiconductor continuous wave laser off unit 3 are gated by 2 multiplied by 1 of an on/off switch 4, chopped by an acousto-optic modulator 5 respectively to become pulse laser with repetition frequency of 10kHz magnitude and pulse width within 1 mu s, and the pulse laser with small energy passes through a Raman optical fiber amplifier 6 to improve single pulse energy; the pulse laser with the energy improved by the Raman fiber amplifier 6 is converted into 825.5 +/-0.25 nm pulse laser by the frequency doubler 7, and finally the pulse laser energy reaches 10 mu J magnitude. The pump source 8 of the Raman fiber amplifier is a high-power semiconductor laser around 1540 nm.
The water vapor has a proper absorption line between 825 nm and 826nm, for example, the detection wavelength (On-line) of the differential absorption water vapor is selected to be 825.499nm, the reference wavelength (Off-line) is 825.599nm, and an active stable control ring of 825 nm to 826nm laser wavelength is directly established, so that great difficulty is caused; instead, they indirectly establish their active stable control loops for double the lasing wavelength of 1650-1652 nm.
The DFB semiconductor continuous wave laser reference unit 1 locks the wavelength of the first single longitudinal mode continuous wave DFB semiconductor laser on the 1650.958nm spectral absorption line of methane. The unit comprises a first DFB semiconductor laser 1-1, a first coupler 1-2, a second coupler 1-3, a phase modulator 1-4, a methane gas molecular absorption cell 1-5, a detector 1-6, a first mixer 1-7, a first low-pass filter 1-8, an analog-to-digital converter 1-9, a first microprocessor 1-10, a first digital-to-analog converter 1-11, and an injection current and working temperature controller 1-12 of the first DFB semiconductor laser. The first DFB semiconductor laser 1-1 is modulated by the phase modulator 1-4, and when the wavelength of the first DFB semiconductor laser 1-1 is equal to the central wavelength of the absorption line of methane gas molecules, the error signal output by the first digital/analog converter 1-11 is 0.0; when the wavelength of the first DFB semiconductor laser 1-1 deviates from the center wavelength of the methane gas molecular absorption line, the first low-pass filter 1-8 outputs a DC signal, which passes through the A/D converter 1-9, the first microprocessor 1-10, and the first D/A converter 1-11, and then outputs a compensation signal, and adjusts the injection current and the operating temperature of the controller 1-12 of the first DFB semiconductor laser so that the wavelength returns to the center wavelength of the methane gas molecular absorption line.
The DFB semiconductor continuous wave laser on unit 2 comprises a second DFB semiconductor laser 2-1, a third coupler 2-2, a fourth coupler 2-3, a first balance detector 2-4, a second mixer 2-5, a first high-frequency generator 2-6 (4.4 GHz), a second low-pass filter 2-7, a first frequency discriminator 2-8, a second microprocessor 2-9, a second digital-to-analog converter 2-10, an injection current and working temperature controller 2-11 of the second DFB semiconductor laser (2-1) and the like. The wavelength of the second DFB semiconductor laser 2-1 is locked at 1650.998nm (which is twice as long as the detection wavelength 825.499nm of the water vapor absorption line online) by offset frequency control based on the locked wavelength of the first DFB semiconductor laser 1-1; the fourth coupler 2-3 and the second coupler 1-3 sample a small portion of laser light from the second DFB semiconductor laser 2-1 and the first DFB semiconductor laser 1-1 respectively and detect a frequency difference signal of the two laser light by heterodyne of the first balanced detector 2-4, the first high frequency generator 2-6 generates a stable 4.4GHz high frequency signal, the two rf signals pass through the second mixer 2-5, the second low pass filter 2-7 and the first frequency discriminator 2-8 to obtain the frequency difference of the two aforementioned high frequency signals, and then the second microprocessor 2-9 and the second digital/analog converter 2-10 generate a correction signal to the injection current and operating temperature controller 2-11 according to the magnitude and the sign of the frequency difference, so that the frequency difference between the second DFB semiconductor laser 3-1 and the first DFB semiconductor laser 1-1 continuous wave laser is stabilized at 4.4GHz and the wavelength difference is stabilized at 0.04nm.
The DFB semiconductor continuous wave laser off unit 3 comprises a third DFB semiconductor laser 3-1, a fifth coupler 3-2, a sixth coupler 3-3, a second balanced detector 3-4, a third mixer 3-5, a second high-frequency generator 3-6 (15.4 GHz), a third low-pass filter 3-7, a second frequency discriminator 3-8, a third microprocessor 3-9, a third digital/analog converter 3-10 and an injection current and working temperature controller 3-11 of the third DFB semiconductor laser 3-1; the wavelength of the third DFB semiconductor laser 3-1 is locked at 1651.098nm (which is twice the reference wavelength of the differential absorption lidar ofline 825.599 nm) by offset frequency control. So that the difference in frequency between the continuous wave laser light of the third DFB semiconductor laser 3-1 and the continuous wave laser light of the first DFB semiconductor laser 1-1 is stabilized at 15.4GHz, that is, the difference in wavelength is stabilized at 0.14nm.
Compared with the differential absorption laser radar for detecting water vapor which is developed and operated at the present time, the invention has the advantages that:
the invention adopts a methane gas absorption cell (0.1 m), takes the absorption line of methane as the control reference of the working wavelength of the system, abandons an error measurement component which takes a high-precision wavemeter as a feedback control loop, and has the total cost which is obviously lower than that of the high-precision wavemeter and a vapor multi-channel gas absorption cell (optical path 36 m) (or other attached reference lasers such as a frequency stabilized helium neon laser).
The invention adopts 1651 + -0.5 nm DFB semiconductor laser (and driver) and Raman optical fiber amplifier (and pumping source), to reduce the peak light power of system, to meet the eye safety, and improve the signal-to-noise ratio to realize echo signal accumulation and compensation. The optical paths from the DFB semiconductor laser to the Raman optical fiber amplifier are all connected by optical fibers, so that the reliability of the system can be improved; the DFB semiconductor laser, the acousto-optic modulator, the phase modulator and other core devices used by the system can be purchased commercially, and the cost of the system is limited.
The present invention will be described in further detail with reference to the accompanying drawings and specific embodiments.
Drawings
FIG. 1 shows the components of a 1650.5-1651.5nm wavelength-stabilized differential absorption lidar transmitter. A 1-DFB semiconductor laser wavelength reference unit; a 2-DFB semiconductor laser wavelength on unit; a 3-DFB semiconductor laser wavelength off cell; 4-on/off 2X 1 gating switch; 5-an acousto-optic modulator; 6-Raman fiber amplifier; 7-pump source of Raman fiber amplifier (high power semiconductor laser).
FIG. 2 is a three-unit combination roadmap for wavelength stabilization of DFB semiconductor lasers;
FIG. 3 is a graph of a transmittance spectrum of methane gas;
fig. 4 is a spectral plot of the water vapor absorption cross section.
Detailed Description
Referring to fig. 1, the differential absorption lidar transmitter for detecting moisture comprises several parts, such as a DFB semiconductor continuous wave light source wavelength reference unit 1, a DFB semiconductor continuous wave light source on unit 2, a DFB semiconductor continuous wave light source off unit 3, a 2 × 1 gating switch 4 of the on unit/off unit, an acousto-optic modulator (and its acoustic wave generator) 5, a Raman optical fiber amplifier 6, a nonlinear optical quasi-phase matching crystal frequency doubler 7, and a pumping light source high-power semiconductor Laser 8 (with a wavelength of about 1.54 μm, provided by Laser Components USA inc. Of Raman optical fiber amplifier 6).
The wavelength of the R6 spectral line of methane gas molecules is 1650.958nm, which is very close to twice the detection wavelength of laser 825.499nm which needs to be controlled, so that the wavelength of the absorption line of the methane R6 spectrum is used as the reference standard of an active control link, and the first single longitudinal mode DFB (distributed feedback) semiconductor laser 1-1 (Ideal photonics corporation, TL-DFB-20-A82-W1650-SP1; or nanoplus Nanosystems and Technologies GmbH, the same applies hereinafter) has its emission wavelength actively stabilized on the methane R6 line of 1650.958nm, this single longitudinal mode DFB (distributed feedback) semiconductor laser 1-1 becomes the reference laser (reference cell 1 in fig. 2), and heterodyne detection and phase lock loop technology is used to lock the wavelength difference Δ λ (0.040 nm, optical frequency difference-4.4 GHz) between the second single longitudinal mode DFB (distributed feedback) semiconductor laser 2-1 and the first reference single longitudinal mode DFB (distributed feedback) semiconductor laser 1-1, which is equal to the difference between the R6 absorption line of methane and twice the water vapor absorption line, so that the wavelength of the second single longitudinal mode DFB (distributed feedback) semiconductor laser 2-1 is actively stabilized at a value twice the water vapor absorption line, and the second single longitudinal mode DFB (distributed feedback) semiconductor laser 2-1 becomes the connine seed of the system.
As shown in fig. 2, "reference unit 1", a first DFB semiconductor laser 1-1 emits continuous wave single frequency (optical frequency ω) laser light around 1650.958nm driven by a temperature control/current driver 1-12 (light wave company), in which a part of the laser light is fed to (electro-optical) phase modulator 1-4 (Thorlabs, LN65S-C under a constant temperature condition), the phase of the single frequency laser light is modulated by a radio frequency (frequency 200 MHz) signal generated by a radio frequency oscillator 1-13, the modulated continuous wave laser light can be decomposed into three single frequency components, and besides the original carrier laser light (optical frequency ω), there are two side frequency lights, one of which has an optical frequency of carrier optical frequency (ω) plus a 200MHz modulated radio frequency (frequency ω +200 MHz) output by the radio frequency oscillator 1-13, and the other has an optical frequency of carrier frequency (ω) minus the modulated radio frequency (optical frequency ω -200 MHz); the continuous wave laser after phase modulation is absorbed by a methane gas molecular optical absorption cell 1-5 (wavelet Reference U.S. company), and a detector 1-6 detects the results of two-coherent and two-coherent three light wave components after passing through the methane gas cell 1-5; the amplitude and the positive and negative of the radio frequency (200 MHz) signal finally output by the detector 1-6 reflect the difference between the laser wavelength (optical frequency) of the first DFB semiconductor laser 1-1 and the absorption peak wavelength of methane gas, the radio frequency (frequency is 200 MHz) output by the detector 1-6 and the radio frequency (frequency is also 200 MHz) of the original phase modulation wave are subjected to frequency mixing by a first multiplier 1-7 and are filtered by a first low-pass filter 1-8, the remaining direct current signal is an error signal of a feedback loop, the error signal is subjected to an analog-to-digital converter 1-9, a first microprocessor 1-10 and a first digital-to-analog converter 1-11 and finally is negatively fed back to a temperature control/current driver 1-12 of the first DFB semiconductor laser 1-1, and the current and the working temperature injected into the first DFB semiconductor laser 1-1 are adjusted, so that the laser wavelength output by the first DFB semiconductor laser 1-1 approaches the absorption peak wavelength of methane gas. The first DFB semiconductor laser 1-1 was stabilized at 1650.958nm in the methane absorption line by such active closed loop control. The power divider 1-14 in fig. 2 functions to divide the radio frequency (frequency 200 MHz) signal generated by the oscillator 1-13 to the phase modulator 1-4 and the first mixer 1-7.
As shown in fig. 2, "on unit 2", the second DFB semiconductor laser 2-1 emits a continuous wave single frequency laser beam near 1650.998nm under the driving of the temperature control/current driver 2-11, and through the second fiber coupler 1-3 and the fourth fiber coupler 2-3, the second DFB semiconductor laser 2-1 samples a part of the laser beam and a part of the reference laser beam emitted by the first DFB semiconductor laser 1-1 to be mixed and then heterodyne-detects a radio frequency signal by the first balanced detector 2-4 (New Focus1437M,25GHz Bandwidth, the same applies below), and the operating frequency of the radio frequency signal is equal to the difference between the laser frequencies of the first DFB semiconductor laser 1-1 and the second DFB semiconductor laser 2-1. The first offset frequency oscillator 2-6 has a frequency equal to the reference frequency of 4.4GHz and the amplitude remains stable. The radio frequency signal output by the first offset frequency oscillator 2-6 and the radio frequency signal detected by the heterodyne of the two lasers are mixed by a second multiplier 2-5, and after the radio frequency signal is subjected to second low pass filtering 2-7, a difference frequency signal is output, the difference frequency signal is subjected to a first frequency discriminator 2-8 to output an error signal, the error signal is processed by a second microprocessor 2-9 and a second digital/analog converter 2-10, and according to the processing result, the driving current and the working temperature of the second DFB semiconductor laser 2-1 are adjusted in a negative feedback mode (through a driver 2-11), so that the output wavelength of the second DFB semiconductor laser 2-1 is adjusted until the optical frequency difference of the two DFB semiconductor lasers is equal to the working frequency (-4.4 GHz) of the 2-6 reference radio frequency signal, namely, the wavelength difference is +0.040nm.
As shown in FIG. 2, the "off unit 3", the third DFB semiconductor laser 3-1 emits a continuous wave monochromatic laser beam around 1651.098nm under the driving of the temperature/current controller 3-11, the third DFB semiconductor laser 3-1 samples a part of the laser beam and a part of the laser beam emitted by the first DFB semiconductor laser 1-1 to be aliased by the first fiber coupler 1-2 and the sixth fiber coupler 3-3, and a radio frequency signal is heterodyne-detected by the second balanced detector 3-4, the operating frequency of the radio frequency signal is equal to the difference between the laser frequencies of the first DFB semiconductor laser 1-1 and the third DFB semiconductor laser 3-1. The second offset oscillator (Mini-Circuits, USA) 3-6 has a frequency equal to the reference frequency of 15.4GHz, and the amplitude is kept stable. The reference radio frequency signal output by the second offset frequency oscillator 3-6 and the radio frequency signal detected by heterodyne of the two lasers are mixed by a third multiplier 3-5, and after being filtered by a third low pass filter 3-7, a difference frequency signal is output, the difference frequency signal outputs an error signal through a second frequency discriminator 3-8, the error signal is processed by a third microprocessor 3-9 and a third digital/analog converter 3-10, and according to the processing result, the driving current or the control temperature of the third DFB semiconductor laser 3-1 is adjusted by negative feedback (through a controller 3-11), so that the output wavelength of the third DFB semiconductor laser 3-1 is adjusted until the optical frequency difference of the two DFB semiconductor lasers is equal to the working frequency (-15.4 GHz) of the reference radio frequency signal of the second offset frequency oscillator 3-6, namely the wavelength difference is 0.140nm.
The second and third DFB semiconductor lasers 2-1 and 3-1, which are wavelength-stabilized, continuous wave lasers are connected to an on/off switch 4 (agitron. Inc) through third and fifth couplers 2-2 and 3-2, respectively, and are gated by 2 × 1, and are chopped into pulse lasers with a pulse repetition frequency of 10kHz within a pulse width of 1 μ s by an acousto-optic modulator 5 (including a 40-60 MHz rf driver, brimrose Corporation of America, models AMM-27-2, the same below), which amplifies pulse energy by a Raman fiber amplifier 6 (ipgpotonics, usa; sichuan ultra optical communication limited), which can reach a pulse energy of 50 μ J magnitude, and which passes through a frequency converter 7 (MgO: PPLN, or PPKTP, or PPLN, PPKTA, etc.), the doubled wavelength of which is equal to a certain on wavelength of a moisture absorption line (825 nm), or the wavelength of the pulse lasers is equal to a radar with a double absorption wavelength of water vapor (reference of 599 nm).
The working substance of the frequency doubler 7 is a periodically polarized quasi-phase matching frequency crystal, and the quasi-phase matching crystal (MgO: PPLN, PPKTP, PPLN, PPKTA) is arranged in a box which is dry, constant in temperature and transparent at two ends. The conversion efficiency of the frequency doubler can be generally 20% -40% and is not less than 10%.
Claims (3)
1. A transmitter of a micropulse differential absorption laser radar for detecting atmospheric water vapor comprises a DFB semiconductor continuous wave laser reference unit (1), a DFB semiconductor continuous wave laser on unit (2), a DFB semiconductor continuous wave laser off unit (3), an on/off gating switch (4), an acousto-optic modulator (5), a Raman optical fiber amplifier (6), a frequency doubler (7) and a pumping light source (8) of the Raman optical fiber amplifier; the method is characterized in that:
the DFB semiconductor continuous wave laser reference unit (1) forms a wavelength reference for stabilizing the continuous wave laser, the difference between the on wavelength of the DFB semiconductor continuous wave laser in the DFB semiconductor continuous wave laser on unit (2) and the wavelength of the DFB semiconductor continuous wave laser reference unit (1) which is stable for a long time is 0.04nm, the off wavelength of the DFB semiconductor continuous wave laser in the DFB semiconductor continuous wave laser off unit (3) and the wavelength of the DFB semiconductor continuous wave laser reference unit (1) which is stable for a long time are 0.140nm; the DFB semiconductor continuous wave laser on unit (2) and the DFB semiconductor continuous wave laser off unit (3) are chopped into pulse laser with repetition frequency of 10kHz magnitude and pulse width within 1 mu s respectively by an acousto-optic modulator (5) through an on/off gated 2 multiplied by 1 switch (4), and the pulse laser with small energy is subjected to single pulse energy improvement through a Raman fiber amplifier (6); the energy of the pulse laser is improved by the Raman fiber amplifier (6), and then the pulse laser is converted into 825.5 +/-0.25 nm pulse laser by the frequency doubler (7), and finally the energy of the pulse laser reaches 10 mu J magnitude; wherein, the pumping source (8) of the Raman fiber amplifier is acted by a high-power semiconductor laser near 1540 nm;
the DFB semiconductor continuous wave laser reference unit (1) locks the wavelength of a first single longitudinal mode continuous wave DFB semiconductor laser on a 1650.958nm spectral absorption line of methane; the unit comprises a first DFB semiconductor laser (1-1), a first coupler (1-2), a second coupler (1-3), a phase modulator (1-4), a methane gas molecular absorption pool (1-5), a detector (1-6), a first mixer (1-7), a first low-pass filter (1-8), an analog-digital converter (1-9), a first microprocessor (1-10), a first digital-analog converter (1-11), an injection current and working temperature controller (1-12) part of the first DFB semiconductor laser, wherein the first DFB semiconductor laser (1-1) is modulated by the phase modulator (1-4), and when the wavelength of the first DFB semiconductor laser (1-1) is equal to the central wavelength of a methane gas molecular absorption line, an error signal output by the first digital-analog converter (1-11) is 0.0; when the wavelength of the first DFB semiconductor laser (1-1) deviates from the central wavelength of the methane gas molecular absorption line, the first low-pass filter (1-8) outputs a direct current signal, the direct current signal passes through the analog-to-digital converter (1-9), the first microprocessor (1-10) and the first digital-to-analog converter (1-11), and then outputs a compensation signal, and the injection current and the working temperature of the controller (1-12) of the first DFB semiconductor laser are adjusted to enable the wavelength to return to the central wavelength of the methane gas molecular absorption line.
2. The transmitter of the micro-pulse differential absorption lidar for detecting atmospheric moisture according to claim 1, wherein:
the DFB semiconductor continuous wave laser on unit (2) comprises a second DFB semiconductor laser (2-1), a third coupler (2-2), a fourth coupler (2-3), a first balanced detector (2-4), a second mixer (2-5), a first high-frequency generator (2-6) of 4.4GHz, a second low-pass filter (2-7), a first frequency discriminator (2-8), a second microprocessor (2-9), a second digital-to-analog converter (2-10) and an injection current and working temperature controller (2-11) part of the second DFB semiconductor laser, wherein the wavelength of the second DFB semiconductor laser (2-1) is locked at a wavelength of 1650.998nm (which is twice the detection wavelength of an online of moisture absorption line) through offset frequency control of 4.4GHz based on the wavelength of the first DFB semiconductor laser (1-1) as a reference; the fourth coupler (2-3) and the second coupler (1-3) respectively sample a small part of laser light from the second DFB semiconductor laser (2-1) and the first DFB semiconductor laser (1-1) and heterodyne and detect frequency difference signals of the two laser light by the first balance detector (2-4), the first high-frequency generator (2-6) generates stable 4.4GHz high-frequency signals, the two radio-frequency signals obtain the frequency difference of the two high-frequency signals through the second mixer (2-5), the second low-pass filter (2-7) and the first frequency discriminator (2-8), and then the second microprocessor (2-9) and the second digital-to-analog converter (2-10) generate correction signals to the injection current and the working temperature controller (2-11) according to the magnitude and the positive and negative of the frequency difference, so that the stable wave difference between the continuous waves of the second DFB semiconductor laser (2-1) and the first DFB semiconductor laser (1-1) is stable at 4.04 GHz wavelength.
3. The transmitter of the micro-pulse differential absorption lidar for detecting atmospheric moisture according to claim 1, wherein:
the DFB semiconductor continuous wave laser off unit (3) comprises a third DFB semiconductor laser (3-1), a fifth coupler (3-2), a sixth coupler (3-3), a second balanced detector (3-4), a third mixer (3-5), a 15.4GHz second high-frequency generator (3-6), a third low-pass filter (3-7), a second frequency discriminator (3-8), a third microprocessor (3-9), a third digital/analog converter (3-10) and an injection current and working temperature controller (3-11) of the third DFB semiconductor laser; the wavelength of the third DFB semiconductor laser (3-1) is locked at 1651.098nm through offset frequency control of 15.4GHz, so that the difference of the frequency between the third DFB semiconductor laser (3-1) and the continuous wave laser of the first DFB semiconductor laser (1-1) is stabilized at 15.4GHz, namely, the difference of the wavelength is stabilized at 0.14nm.
Priority Applications (1)
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