Laser frequency stabilization and self-calibration device of differential absorption laser radar

Abstract

The invention provides a laser frequency stabilization and self-calibration device of a differential absorption laser radar, which is characterized in that an emergent light beam of a seed laser of the laser radar is divided into two beams, one beam is emitted into the atmosphere through high-power pulse laser output units connected in series, and back scattering light of the atmosphere is collected and detected by a receiving optical unit; and the other beam is input into a series weak-power continuous laser output unit and enters a long-optical-path absorption cell, and transmission light of the beam is detected. And the two paths of detection signals are simultaneously sent to the signal processing board. According to the invention, the output light frequency of the laser is locked on a gas absorption spectral line by utilizing a detection signal of the long-optical-path absorption cell and adopting an active frequency stabilization technology, so that a laser frequency locking function is realized. Meanwhile, the gas concentration in the long-optical-path absorption cell is known, and the real-time absorption section of the gas is measured based on the Lambert-Beer law, so that the self-calibration of the system is realized.

Description

Technical Field

[0001] The present invention belongs to the technical field of laser radar, and in particular relates to a laser frequency stabilization and self-calibration device of a differential absorption laser radar. Background Art

[0002] Water vapor is the largest contributor to the greenhouse effect, and its temporal and spatial distribution has a significant impact on climate change. Measuring the distribution of water vapor is of great significance for studying storm phenomena, atmospheric boundary layer dynamics, and other analyses. Differential absorption water vapor radar has great advantages in detecting water vapor and is an ideal means of achieving high-resolution water vapor detection throughout the day, which can achieve high temporal and spatial resolution.

[0003] Differential absorption lidar uses two laser beams with a small wavelength interval. One beam is at the absorption peak and will be strongly absorbed by the gas to be measured, which is recorded as online. The other beam is at the absorption valley and has very little absorption, which is called offline. The distribution of water vapor is inverted by measuring the change in water vapor absorption with increasing height. During detection, since the water vapor absorption spectrum is very narrow, the laser output light is easily affected by the external environment and causes broadening, reducing its detection accuracy. The drift of the laser center wavelength is not conducive to water vapor detection. Therefore, the laser output wavelength needs to be accurately stabilized at the online wavelength, and the requirements for the offline wavelength are relatively low. Therefore, it is particularly necessary to actively stabilize the frequency of the laser in the online state. Usually, the differential absorption cross section is obtained by searching the HITRAN database to achieve system calibration. However, in actual applications, the system will be affected by factors such as temperature, resulting in inconsistency between the differential absorption cross section and the database. Therefore, real-time acquisition of the differential absorption cross section is very important for achieving high-precision and high-stability operation of the lidar system. Summary of the invention

[0004] In order to solve the problem of laser light source frequency stability and system real-time self-calibration in the current water vapor differential absorption laser radar, the present invention proposes a laser frequency stabilization and self-calibration device for the laser radar system. The specific technical solution is as follows:

[0005] A laser frequency stabilization and self-calibration device for a differential absorption laser radar, comprising a laser, a first laser beam splitter, a high-power pulse laser output unit, a weak-power continuous laser output unit, a receiving optical unit, a long optical path absorption cell, a second laser beam splitter, a first detector, and a signal processing board;

[0006] The first laser beam splitter splits the light generated by the laser into two, one of which is emitted into the atmosphere after passing through a high-power pulse laser output unit connected in series, and the atmospheric backscattered signal is collected and received by a receiving optical unit and then output to a signal processing board; the other laser is input into a low-power continuous laser output unit connected in series and a long optical path absorption cell, and the transmitted light of the long optical path absorption cell is detected by the first detector, and the signals detected by the detector and the receiving optical unit are sent to the signal processing board.

[0007] Specifically, it also includes a half wave plate, an electro-optic modulator, a second laser beam splitter, a second detector, a radio frequency source, a phase shifter, a mixer, and a servo control unit;

[0008] The half-wave plate and the electro-optic modulator are arranged between the weak-power continuous laser output unit and the long optical path absorption cell. The second laser beam splitter separates another beam of transmitted light from the long optical path absorption cell and transmits it to the detector. The radio frequency source is electrically connected to the phase shifter, the output end of the phase shifter and the output end of the second detector are respectively connected to the mixer, the output end of the mixer is electrically connected to the servo control unit, and the servo control unit is connected to the laser.

[0009] The laser is a fiber seed laser that alternately generates online / offline laser wavelengths, and the online / offline laser wavelengths are switched by switching the applied voltage of the PZT. Specifically, the transmission signal detected by the detector from the long optical path absorption cell is set as the first signal, and the radar echo signal received by the receiving optical unit is set as the second signal. The specific steps are as follows:

[0010] The first signal is obtained after filtering in the signal processing board, including the signals I corresponding to the two laser wavelengths online / offline on and signal I off ;

[0011] According to the Lambert-Beer law:

[0012] I=I 0 exp[-α(v)L]=I 0 exp[-σ(v)NL]

[0013] Where, I represents the intensity of the transmission signal through the absorption cell, I 0 represents the light intensity signal when the laser is emitted, α(v) represents the absorption coefficient, L represents the optical path length, σ(v) represents the absorption cross section, and N represents the number density of gas molecules to be measured;

[0014] It can be seen that the differential absorption cross section is

[0015]

[0016] Δσ represents the differential absorption cross section at two wavelengths, σ on and σ off It represents the absorption cross section corresponding to the two laser wavelengths online / offline, I on and I off Represents the transmission signal of the absorption cell corresponding to the two laser wavelengths online / offline;

[0017] The gas concentration in the long optical path absorption cell is known, and the real-time absorption cross-section value is obtained;

[0018] At the same time, the second signal is filtered in the signal processing board (11) to obtain a signal P on and P off , P on and P off They represent the two corresponding radar echo signals at wavelengths of online / offline, respectively, and the number density distribution of water vapor molecules in the atmosphere is calculated by combining the obtained differential absorption cross section;

[0019] In differential absorption water vapor radar, according to the radar equation, the number density of water vapor molecules is

[0020]

[0021] Where R represents the distance of the echo signal.

[0022] The high-power pulse laser output unit comprises an amplifier and an acousto-optic modulator. The amplifier amplifies the laser light and then modulates and chops the laser light into pulse light through the acousto-optic modulator.

[0023] The low-power continuous laser output unit amplifies the received light beam and outputs a continuous light signal with the same frequency as that of the high-power pulse laser output unit.

[0024] The receiving optical unit is a telescope system, which collects and receives aerosol backscattering echo signals, obtains radar signals containing water vapor absorption spectral line and non-absorption spectral line characteristics, and measures radar echo signals of online / offline laser wavelengths.

[0025] The long optical path absorption cell selects a water vapor absorption line based on the Lambert-Beer law, and the water vapor absorption line is the same as the water vapor absorption line of the differential absorption radar for measuring water vapor in the atmosphere.

[0026] The advantages of the present invention are:

[0027] (1) The frequency stabilization technology uses a long optical path absorption cell filled with water vapor of known concentration and uses active frequency stabilization technology to lock the laser output light frequency to the gas absorption spectrum, thereby achieving the laser frequency stabilization function.

[0028] (2) The system self-calibration uses the same external absorption cell to measure its real-time absorption cross section based on the Lambert-Beer law, thereby achieving real-time self-calibration of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 Schematic diagram of the laser frequency stabilization and self-calibration device of the lidar system.

[0030] Figure 2 Flowchart of processing the first signal and the second signal by the signal processing board for self-calibration of the radar system.

[0031] In the figure:

[0032] 1. Laser; 2. First laser beam splitter; 3. High-power pulse laser output unit; 4. Low-power continuous laser output unit; 5. Receiving optical unit; 6. Half-wave plate; 7. Electro-optic modulator; 8. Long optical path absorption cell; 9. Second laser beam splitter; 10. First detector; 11. Signal processing board; 12. Second detector; 13. RF source; 14. Phase shifter; 15. Mixer; 16. Servo control unit. DETAILED DESCRIPTION

[0033] like Figure 1 As shown, the laser frequency stabilization and self-calibration device of the differential absorption laser radar includes a laser 1, a first laser beam splitter 2, a high-power pulse laser output unit 3, a low-power continuous laser output unit 4, a receiving optical unit 5, a half-wave plate 6, an electro-optical modulator 7, a long optical path absorption cell 8, a second laser beam splitter 9, a first detector 10, a signal processing board 11, a second detector 12, a radio frequency source 13, a phase shifter 14, a mixer 15, and a servo control unit 16.

[0034] The first laser beam splitter 2 splits the light generated by the laser 1 into two, one laser beam is emitted into the atmosphere through the high-power pulse laser output unit 3, and the atmospheric backscattered light is collected and detected by the receiving optical unit 5 and then output to the signal processing board 11; the other laser beam is input into the series-connected weak-power continuous laser output unit 4 and the long optical path absorption cell 8, and the transmitted light of the long optical path absorption cell 8 is detected by the first detector 10, and the signals detected by the detector 10 and the receiving optical unit 5 are sent to the signal processing board 11. The signals detected by the detector 10 and the receiving optical unit 5 are sent to the signal processing board 11 for system self-calibration.

[0035] Specifically, the laser 1 is a fiber seed laser, which alternately generates online / offline laser wavelengths, and the purpose of switching the online / offline laser wavelength is achieved by switching the applied voltage of the PZT. The high-power pulse laser output unit 3 includes an amplifier and an acousto-optic modulator. The amplifier amplifies the laser, modulates and chops it into pulse light through the acousto-optic modulator, and then emits it into the atmosphere; the receiving optical unit 5 uses a telescope receiving system to collect and receive aerosol backscattering echoes, and obtain radar signals containing water vapor absorption spectral lines and non-absorption spectral line characteristics; and measures the radar echo signals of the online / offline laser wavelengths. The two wavelength echo signals are differentiated in the signal processing board 11, which can remove the influence of other factors in the system besides water vapor absorption, and realize the continuous detection function during the day.

[0036] In the other path, the weak-power continuous laser output unit 4 amplifies the received light beam and outputs a continuous light signal with the same frequency as the high-power pulse laser output unit 3. The laser adjusts the polarization state of the continuous light signal through the half-wave plate 6 and coincides with the optical axis of the electro-optic modulator 7 to obtain the maximum modulation depth. The long optical path absorption cell 8 selects the water vapor absorption line based on the Lambert-Beer law, and is the same as the water vapor absorption line of the water vapor differential absorption radar in the atmosphere. In this way, it can be used as a frequency stabilization unit and can also be used to obtain the absorption cross section to realize system calibration, saving volume cost.

[0037] The long optical path absorption cell 8 serves as an active frequency stabilization unit. The active laser frequency stabilization technology is to select a stable standard reference frequency. When the laser is affected by external factors, the laser frequency will deviate from the standard frequency. The deviation is measured, and the cavity length of the laser is adjusted through an electrical feedback system to restore the laser frequency to a stable standard reference frequency, thereby achieving laser frequency stabilization.

[0038] In order to achieve laser frequency stabilization, the Pound-Drever-Hall (PDH) active frequency stabilization technology is used here. The structure of the PDH active frequency stabilization technology is specifically as follows: the half wave plate 6 and the electro-optical modulator 7 are arranged between the weak-power continuous laser output unit 4 and the long optical path absorption pool 8, and the second laser beam splitter 9 separates another beam of transmitted light from the long optical path absorption pool 8 and emits it to the detector 12; the radio frequency source 13 is electrically connected to the phase shifter 14, and the output end of the phase shifter 14 and the output end of the second detector 12 are respectively connected to the mixer 15, and the output end of the mixer 15 is electrically connected to the servo control unit 16, and the servo control unit 16 is connected to the laser 1. Specifically, the RF source 13 performs phase modulation on the laser through the electro-optical modulator 7; in addition, the RF source 13 outputs a modulation signal of the electro-optical modulator 7 as a co-frequency signal, and the co-frequency signal is phase-matched by the phase shifter 14 and mixed with the signal detected by the second detector 12 by the mixer 15 to obtain a discrimination signal, and the phase shifter 14 is finely adjusted to obtain the maximum discrimination signal for the servo control unit 16; the discrimination signal is proportional-integral transformed by the servo control unit 16 and output to the tuning actuator of the laser 1 for frequency stabilization. Specifically, the signal generated by the servo control unit 16 is applied to the drive of the frequency tuning element PZT of the laser 1, and the laser frequency stabilization function is realized by controlling the voltage applied to the PZT, wherein the frequency stabilization function is only turned on when the laser wavelength is online. In addition, in order to have the distance resolution capability, the light source in the laser radar detection needs to use a pulsed laser, and the active control loop of the laser wavelength of the pulsed laser has a hysteresis effect, which makes it difficult to realize real-time servo. In this application, the active feedback signal is fed back to the laser 1 before the amplification chopping to realize real-time servo.

[0039] The long optical path absorption cell 8 is used as a real-time acquisition unit for the differential absorption cross section. Figure 2 As shown, the first signal is Figure 1 The middle detector 10 detects the transmission signal of the long optical path absorption cell 8 , and the second signal is the radar echo signal received by the receiving optical unit 5 .

[0040] The specific steps are as follows:

[0041] The first signal is obtained after filtering in the signal processing board, including the signals I corresponding to the two laser wavelengths online / offline on and signal I off ;

[0042] According to the Lambert-Beer law:

[0043] I=I 0 exp[-α(v)L]=I 0 exp[-σ(v)NL]

[0044] Where, I represents the intensity of the transmission signal through the absorption cell, I 0 represents the light intensity signal when the laser is emitted, α(v) represents the absorption coefficient, L represents the optical path length, σ(v) represents the absorption cross section, and N represents the number density of gas molecules to be measured;

[0045] It can be seen that the differential absorption cross section is

[0046]

[0047] Δσ represents the differential absorption cross section at two wavelengths, σ on and σ off It represents the absorption cross section corresponding to the two laser wavelengths online / offline, I on and I off Represents the transmission signal of the absorption cell corresponding to the two laser wavelengths online / offline;

[0048] The gas concentration in the long optical path absorption cell is known, and the real-time absorption cross-section value is obtained;

[0049] At the same time, the second signal is filtered in the signal processing board 11 to obtain a signal P on and P off , P on and P off They represent the two corresponding radar echo signals at wavelengths of online / offline, respectively, and the number density distribution of water vapor molecules in the atmosphere is calculated by combining the obtained differential absorption cross section;

[0050] In differential absorption water vapor radar, according to the radar equation, the number density of water vapor molecules is

[0051]

[0052] Where R represents the distance of the echo signal.

[0053] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A laser frequency stabilization and self-calibration device for differential absorption laser radar, characterized in that: It comprises a laser (1), a first laser beam splitter (2), a high-power pulse laser output unit (3), a low-power continuous laser output unit (4), a receiving optical unit (5), a long optical path absorption cell (8), a second laser beam splitter (9), a first detector (10), and a signal processing board (11); The first laser beam splitter (2) splits the light generated by the laser (1) into two, wherein one laser beam passes through a series-connected high-power pulse laser output unit (3) into the atmosphere, and the atmospheric backscattered light is received by a receiving optical unit (5) and then output to a signal processing board (11); the other laser beam is input into a series-connected weak-power continuous laser output unit (4) and a long optical path absorption cell (8), and the transmitted light of the long optical path absorption cell (8) is detected by a first detector (10), and the signals detected by the detector and the receiving optical unit (5) are sent to the signal processing board (11).

2. The laser frequency stabilization and self-calibration device for differential absorption laser radar according to claim 1, characterized in that: It also includes a half-wave plate (6), an electro-optic modulator (7), a second laser beam splitter (9), a second detector (12), a radio frequency source (13), a phase shifter (14), a mixer (15), and a servo control unit (16); The half-wave plate (6) and the electro-optic modulator (7) are arranged between the weak-power continuous laser output unit (4) and the long optical path absorption pool (8); the second laser beam splitter (9) splits another beam of transmitted light from the long optical path absorption pool (8) and emits it into the detector (12); the radio frequency source (13) is electrically connected to the phase shifter (14); the output end of the phase shifter (14) and the output end of the second detector (12) are respectively connected to the mixer (15); the output end of the mixer (15) is electrically connected to the servo control unit (16); and the servo control unit (16) is connected to the laser (1).

3. The laser frequency stabilization and self-calibration device for differential absorption laser radar according to claim 1, characterized in that: The laser (1) is a fiber seed laser (1) which alternately generates lasers of two wavelengths, online and offline, and switches the online / offline laser wavelengths by switching the applied voltage of piezoelectric ceramics (PZT).

4. The laser frequency stabilization and self-calibration device for differential absorption laser radar according to claim 3, characterized in that: The transmission signal of the long optical path absorption cell detected by the detector is set as the first signal, and the radar echo signal received by the receiving optical unit is set as the second signal. The specific steps are as follows: The first signal is obtained after filtering in the signal processing board (11), including signals I corresponding to the two laser wavelengths online / offline respectively. on and signal I off ; According to the Lambert-Beer law: I=I0exp[-α(v)L]=I0exp[-σ(v)NL] Where, I represents the intensity of the transmission signal through the absorption cell, I0 represents the light intensity signal when the laser is emitted, α(v) represents the absorption coefficient, L represents the optical path length, σ(v) represents the absorption cross section, and N represents the number density of gas molecules to be measured; It can be seen that the differential absorption cross section is Δσ represents the differential absorption cross section at two wavelengths, σ on and σ off It represents the absorption cross section corresponding to the two laser wavelengths online / offline, I on and I off Represents the transmission signal of the absorption cell corresponding to the two laser wavelengths online / offline; The gas concentration in the long optical path absorption cell (8) is known, and the real-time absorption cross-section value is obtained; At the same time, the second signal is filtered in the signal processing board (11) to obtain a signal P on and P off , P on and P off They represent the two corresponding radar echo signals at wavelengths of online / offline, respectively, and the number density distribution of water vapor molecules in the atmosphere is calculated by combining the obtained differential absorption cross section; In differential absorption water vapor radar, according to the radar equation, the number density of water vapor molecules is Where R represents the distance of the echo signal.

5. The laser frequency stabilization and self-calibration device for differential absorption laser radar according to claim 1, characterized in that: The high-power pulse laser output unit (3) comprises an amplifier and an acousto-optic modulator. The amplifier amplifies the laser light and then modulates and chops the laser light into pulse light through the acousto-optic modulator.

6. The laser frequency stabilization and self-calibration device for differential absorption laser radar according to claim 1, characterized in that: The low-power continuous laser output unit (4) amplifies the received light beam and outputs a continuous light signal with the same frequency as that of the high-power pulse laser output unit (3).

7. The laser frequency stabilization and self-calibration device for differential absorption laser radar according to claim 1, characterized in that: The receiving optical unit (5) is a laser radar, which emits laser light and simultaneously receives aerosol backscatter echoes, obtains radar signals containing water vapor absorption spectral line and non-absorption spectral line characteristics, and measures radar echo signals of online / offline laser wavelengths.

8. The laser frequency stabilization and self-calibration device for differential absorption laser radar according to claim 1, characterized in that: The long optical path absorption cell (8) selects a water vapor absorption line based on the Lambert-Beer law, and the water vapor absorption line is the same as the water vapor absorption line of the differential absorption radar for measuring water vapor in the atmosphere.

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