Laser frequency stabilization and self-calibration device for differential absorption lidar

Abstract

The application provides a laser frequency stabilizing and self-calibrating device for differential absorption lidar, which divides an outgoing light beam of a seed laser of the lidar into two parts, one part is emitted to the atmosphere by a series of high-power pulsed laser output units, and the backscattered light of the atmosphere is collected and detected by a receiving optical unit; the other part is input into a series of weak-power continuous laser output units and enters a long-path absorption cell, and the transmitted light is detected. The two detection signals are sent to a signal processing board at the same time. The application uses the detection signal of the long-path absorption cell, adopts active frequency stabilizing technology to lock the frequency of the laser output light on the gas absorption spectrum, so that the laser frequency locking function is realized. At the same time, the concentration of the gas in the long-path absorption cell is known, and the real-time absorption cross section of the gas is measured based on the Lambert-Beer law, so that the system self-calibration is realized.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of laser radar, and particularly relates to a laser frequency stabilization and self-calibration device of a differential absorption laser radar. BACKGROUND

[0002] Water vapor is the largest contributor to the greenhouse effect, and the spatial and temporal distribution of water vapor has a significant impact on climate change. Measuring the distribution of water vapor is of great significance for analyzing storm phenomena, atmospheric boundary layer dynamics, etc. Differential absorption water vapor radar has great advantages in detecting water vapor and is an ideal detection means for realizing all-weather high-resolution water vapor detection, which can obtain high temporal and spatial resolution.

[0003] The differential absorption laser radar uses two laser beams with a small wavelength interval, one of which is at the absorption peak and is strongly absorbed by the gas to be measured, and is recorded as online, and the other is at the absorption valley and has little absorption, and is called offline. By measuring the change in water vapor absorption with increasing height, the distribution of water vapor is inverted. During detection, due to the narrow water vapor absorption spectrum, the laser output light is easily affected by the external environment, resulting in broadening, which reduces the detection accuracy. The drift of the center wavelength of the laser is not conducive to water vapor detection. Therefore, the output wavelength of the laser needs to be accurately stabilized at the online wavelength, and the offline wavelength requires 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 realize system calibration. However, in actual application, the system will be affected by factors such as temperature, resulting in a difference between the differential absorption cross section and the database, so real-time acquisition of the differential absorption cross section is very important for realizing high precision and high stability of the laser radar system. SUMMARY

[0004] In order to solve the problem of laser frequency stabilization and real-time self-calibration of the differential absorption water vapor laser radar, the application provides a laser frequency stabilization and self-calibration device for a laser radar system, and the specific technical scheme is as follows:

[0005] The laser frequency stabilization and self-calibration device of the differential absorption laser radar comprises a laser, a first laser beam splitter, a high-power pulsed 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 divides the laser generated light into two, one of which is emitted into the atmosphere after passing through a series of high-power pulsed laser output units, and the atmospheric backscattering signal is collected and received by the receiving optical unit and output to the signal processing board; the other laser is input to a series of weak power continuous laser output units 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 divides the transmitted light of the long optical path absorption cell into another beam and emits it into the detector; the radio frequency source is electrically connected with the phase shifter, the output end of the phase shifter and the output end of the second detector are respectively connected with the mixer, the output end of the mixer is electrically connected with the servo control unit, and the servo control unit is connected with the laser.

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

[0010] The signal obtained after the first signal is filtered in the signal processing board includes signals corresponding to online / offline two laser wavelengths respectively and signal .

[0011] According to the Lambert-Beer law:

[0012]

[0013] In the formula, I represents the transmitted signal intensity through the absorption cell, I 0 represents the light intensity signal when the laser is emitted, represents the absorption coefficient, L represents the optical path length, represents the absorption cross section, N represents the number density of the gas molecules to be measured;

[0014] The differential absorption cross section is

[0015]

[0016] denotes the difference absorption cross section of two wavelengths, and denotes the absorption cross section of online / offline two laser wavelengths, I on and I off denotes the transmission signal of the absorption cell corresponding to online / offline two laser wavelengths;

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

[0018] Meanwhile, 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 respectively, the corresponding two radar echo signals of online / offline wavelengths are respectively obtained, and the water vapor molecular number density distribution in the atmosphere is calculated by combining the obtained difference absorption cross section;

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

[0020]

[0021] wherein R denotes the distance of the echo signal.

[0022] The high-power pulsed laser output unit comprises an amplifier and an acousto-optic modulator, the laser is amplified by the amplifier and then modulated by the acousto-optic modulator to form pulsed light.

[0023] The weak-power continuous laser output unit amplifies the received light beam, and outputs a continuous light signal of the same frequency as the high-power pulsed 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 lines and non-absorption spectral line characteristics, and measures the radar echo signals of online / offline laser wavelengths.

[0025] The long optical path absorption cell is based on the water vapor absorption line selected according to the Lambert-Beer law, and 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 application are:

[0027] 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 laser frequency stabilization function.

[0028] The system self-calibration uses the same external absorption cell and measures 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 picture:

[0032] 1. Laser; 2. First laser beam splitter; 3. High-power pulsed laser output unit; 4. Low-power continuous laser output unit; 5. Receiving optical unit; 6. Half-wave plate; 7. Electro-optical 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 lidar 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 pulsed laser output unit 3. 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-path absorption cell 8. The transmitted light of the long-path absorption cell 8 is detected by the first detector 10. 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, alternately generating laser of online / offline two wavelengths, and switching the applied voltage of the PZT to switch the online / offline laser wavelength. The high-power pulsed laser output unit 3 includes an amplifier and an acousto-optic modulator. The amplifier amplifies the laser, and the acousto-optic modulator modulates the chopped pulse light to emit into the atmosphere. The receiving optical unit 5 adopts a telescope receiving system to collect and receive aerosol backscattering echoes, obtain radar signals containing water vapor absorption spectrum and non-absorption spectrum characteristics, and measure the online / offline laser wavelength radar echo signals. The two wavelength echo signals are different in the signal processing board 11, which can remove the influence of other factors in the system except water vapor absorption, and realize the function of continuous detection in the daytime.

[0036] In another way, the weak power continuous laser output unit 4 amplifies the received light beam, and outputs a continuous light signal of the same frequency as the high-power pulsed laser output unit 3. The laser passes through the half-wave plate 6 to adjust the polarization state of the continuous light signal and coincide 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 the water vapor absorption line of the differential absorption radar for measuring water vapor in the atmosphere is the same. Therefore, it can be used as a frequency stabilization unit and used to obtain the absorption cross section to realize system calibration, saving volume and cost.

[0037] The long optical path absorption cell 8 is an active frequency stabilization unit. The active laser frequency stabilization technology selects a stable standard reference frequency. When the laser is affected by the outside world, the laser frequency deviates 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 the stable standard reference frequency, realizing laser frequency stabilization.

[0038] In order to achieve laser frequency stabilization, the Pound-Drever-Hall (PDH) active frequency stabilization technology is adopted 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 transmits 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 phase-modulates the laser light through the electro-optical modulator 7. Furthermore, the RF source 13 outputs a co-frequency signal with the modulated signal of the electro-optical modulator 7. This co-frequency signal is phase-matched by the phase shifter 14, then mixed with the signal detected by the second detector 12 through the mixer 15 to generate a discrimination signal. The phase shifter 14 is finely adjusted to obtain the maximum discrimination signal for the servo control unit 16. The discrimination signal undergoes proportional-integral conversion by the servo control unit 16 and is then 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 PZT frequency tuning element of the laser 1. Laser frequency stabilization is achieved by controlling the voltage applied to the PZT. This frequency stabilization function is only enabled when the laser wavelength is online. Furthermore, to achieve range resolution, the laser radar detection light source must use a pulsed laser. However, the active control loop for the laser wavelength of a pulsed laser has a hysteresis effect, making real-time servoing difficult. This application implements real-time servoing by feeding an active feedback signal back to the laser 1 before amplification and chopping.

[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 the signal obtained after filtering in the signal processing board, including the signals corresponding to the two laser wavelengths online / offline and signal ;

[0042] According to the Lambert-Beer law:

[0043]

[0044] Where, Irepresents the transmission signal intensity through the absorption cell, I 0 represents the light intensity signal when the laser exits, represents the absorption coefficient, L represents the optical path length, represents the absorption cross section, N represents the number density of the gas molecules to be measured;

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

[0046]

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

[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 respectively represent the corresponding two radar echo signals at the 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 the differential absorption water vapor radar, according to the radar equation, the number density of water vapor molecules is

[0051]

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

[0053] The above is only a preferred embodiment of the present application, and is not used to limit the present application, any modification, equivalent replacement and improvement made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. 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 high-power pulse laser output unit (3) connected in series and is released 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 low-power continuous laser output unit (4) and a long optical path absorption cell (8) connected in series, 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); It also includes a half-wave plate (6), an electro-optical 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-optical modulator (7) are arranged between the weak-power continuous laser output unit (4) and the long-path absorption cell (8); the second laser beam splitter (9) splits another beam of transmitted light from the long-path absorption cell (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).

2. 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) that alternately generates lasers of two wavelengths, online and offline, and switches the online / offline laser wavelengths by switching the applied voltage of a piezoelectric ceramic (PZT).

3. The laser frequency stabilization and self-calibration device for differential absorption laser radar according to claim 2, characterized in that: The transmission signal of the long optical path absorption cell detected by the first detector (10) 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 the signals corresponding to the two laser wavelengths of online / offline and signal ; According to the Lambert-Beer law: , 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, represents the absorption coefficient, L Indicates the optical path length, represents the absorption cross section, N Indicates the number density of gas molecules to be measured; It can be seen that the differential absorption cross section is , represents the differential absorption cross section at two wavelengths, and Indicates the absorption cross section corresponding to the two laser wavelengths online / offline, I on and I off Indicates 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 Respectively represent the two corresponding radar echo signals at wavelengths of online / offline, 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 water vapor molecule number density is , in R Indicates the distance of the echo signal.

4. 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.

5. 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).

6. 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 and non-absorption spectrum line characteristics, and measures radar echo signals of online / offline laser wavelengths.

7. 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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