High-precision detection device for dipulse laser energy of laser radar

Abstract

The dual-pulse laser energy high-precision detection device is suitable for an integral path differential absorption (IPDA) laser radar dual-pulse laser energy high-precision detection device, can realize high-precision real-time monitoring of laser energy, and meets the strict precision requirement of the IPDA laser radar in the aspect of energy monitoring. The device comprises a laser emission module, a dual-wavelength pulse laser and a spectroscope; the speckle suppression module comprises an attenuator, a rough diffuse scatterer and a rotating motor capable of adjusting the rotating speed; the receiving module comprises an integrating sphere, a delay optical fiber, a collimating mirror, a receiving telescope and a photoelectric detector; and a data acquisition and processing module. According to the invention, speckle errors are suppressed through the rotating rough diffuse scatterer, the influence of laser pointing change is eliminated in combination with the integrating sphere, and the same detector is used for receiving energy signals of the detection light beam and the monitoring light beam, so that the precision of energy monitoring is improved, and the measurement error of the IPDA laser radar is reduced.

Description

Technical Field

[0001] This invention belongs to the field of lidar energy monitoring technology, specifically relating to a high-precision detection device for dual-pulse laser energy of integral path differential absorption (IPDA) lidar, which can be used for real-time monitoring and correction of laser energy in atmospheric greenhouse gas concentration detection. Background Technology

[0002] Integral path differential absorption (IPDA) lidar is currently the most accurate device for detecting atmospheric greenhouse gas concentrations. Its measurement accuracy depends on the energy ratio of the emitted laser pulse to the echo pulse. Since the emitted pulse enters the atmosphere directly and cannot be measured directly, existing technologies typically employ a spectroscopic monitoring scheme: the emitted pulse is divided into a probe beam (for atmospheric detection) and a monitoring beam (for energy monitoring). The initial energy change of the probe beam is indirectly reflected by the energy fluctuation of the monitoring beam.

[0003] Existing energy monitoring technologies suffer from two major problems: First, the speckle effect. After the monitoring beam is scattered by the integrating sphere, the coherent light interference forms an alternating bright and dark speckle pattern, causing energy signal jitter and reducing monitoring accuracy. Second, the difference between dual detectors. Some schemes use independent detectors to receive the echo signal and the monitoring signal separately. In long-term operation, the different aging levels of the detectors can introduce additional errors. In addition, the roughness and rotation speed of the existing diffuser have not been systematically optimized, resulting in limited speckle suppression effects and making it difficult to meet the high-precision requirements of energy monitoring in high-precision detection.

[0004] In summary, there is an urgent need to design a laser energy detection device that can effectively suppress speckle error, eliminate the influence of laser pointing jitter, avoid the difference between dual detectors, and has high stability, so as to improve the overall measurement accuracy and long-term reliability of IPDA lidar. Summary of the Invention

[0005] The present invention aims to solve the technical problems of large monitoring errors caused by speckle effect in existing IPDA lidar energy detection devices, and to provide a highly stable and accurate energy monitoring device that meets the energy monitoring requirements of IPDA lidar for greenhouse gas concentration detection.

[0006] The technical solution of this invention is as follows: A high-precision detection device for dual-pulse laser energy in lidar, characterized in that it includes: Laser emission module: includes a dual-wavelength pulsed laser and a beam splitter; the beam splitter is set in the output optical path of the dual-wavelength pulsed laser to split the incident laser pulse into two paths, one is a probe beam that is directly emitted into the atmosphere, and the other is a monitoring beam; Speckle suppression module: Along the propagation direction of the monitoring beam, an attenuator, a rough diffuser, and a rotary motor with adjustable speed are arranged in sequence; the rough diffuser is fixedly installed on the rotating shaft of the rotary motor. The monitoring beam first passes through the attenuator to attenuate its energy, and then is incident on the surface of the rough diffuser driven to rotate by the rotary motor. By adjusting the rotation speed of the rotary motor, the rotation speed of the rough diffuser is changed to suppress laser speckle. The receiving module includes an integrating sphere, a time-delay fiber, a collimating lens, a receiving telescope, and a photodetector. The entrance of the integrating sphere faces the exit surface of the rough diffuse scatterer to receive the monitoring beam after speckle suppression. The exit port of the integrating sphere is coupled to the input end of the time-delay fiber, and the output end of the time-delay fiber is connected to the collimating lens. The exit optical path of the collimating lens is aligned with the photosensitive surface of the photodetector. The output end of the receiving telescope is also aligned with the same photosensitive surface of the photodetector to receive the echo signal of the detection beam reflected by the hard target. Data acquisition and processing module: Its input end is electrically connected to the output end of the photodetector, and it is used to acquire and process the monitoring beam signal and the detection beam echo signal.

[0007] Furthermore, the rough diffuse scatterer is frosted glass, a ceramic diffuser, or a metal diffuse reflector, and its surface roughness is optimized according to the wavelength and energy monitoring accuracy requirements of the dual-wavelength pulsed laser. The rotary motor is an automatically rotating platform with continuously adjustable speed or a DC brushless motor, and its speed adjustment range covers the speed range that optimizes the speckle suppression effect, so that the surface linear velocity of the rough diffuse scatterer is greater than the movement correlation length of the speckle pattern within one laser pulse cycle, thereby reducing the standard deviation of energy measurement caused by speckle to below 0.5%.

[0008] Furthermore, the inner wall of the integrating sphere is made of a highly diffuse reflective material, and its entrance is kept at a preset gap or optical path alignment with the exit surface of the rough diffuse scatterer. This is used to receive the monitoring beam after speckle suppression, and to eliminate the influence of the pointing jitter and uneven energy distribution of the monitoring beam on energy measurement through multiple diffuse reflections inside the integrating sphere. The exit light of the integrating sphere is uniformly scattered light from a Lambertian body.

[0009] Furthermore, the delay fiber is a multimode fiber, and its length is set according to the detection distance of the detection beam, so that the echo signals of the monitoring beam and the detection beam generated by the same laser pulse are separated in time, and the monitoring beam signal arrives at the same photosensitive surface of the photodetector before the detection beam echo signal, thereby realizing time-division reception by a single detector.

[0010] Furthermore, the photodetector is an avalanche photodiode (APD), a photomultiplier tube (PMT), or a PIN photodiode, with its photosensitive surface located simultaneously in the output optical path of the collimating lens and the output optical path of the receiving telescope. The echo signals of the monitoring beam and the detection beam do not overlap in time to avoid signal crosstalk.

[0011] Furthermore, the data acquisition and processing module includes a high-speed data acquisition card and an FPGA embedded processing board; the control output terminal of the FPGA embedded processing board is connected to the trigger input terminal of the dual-wavelength pulsed laser, used to control the laser emission timing; the FPGA embedded processing board also synchronously triggers the high-speed data acquisition card to record the voltage waveform output by the photodetector; the FPGA embedded processing board has a peak extraction algorithm and a normalized energy ratio calculation unit embedded in it, used to extract the monitoring pulse peak value (P) from the waveform. on0 P off0 ) and echo pulse peak (P on P off The normalized energy ratio was calculated to correct the gas concentration inversion of the IPDA lidar.

[0012] Furthermore, the two wavelengths output by the dual-wavelength pulsed laser are located on and outside the absorption line of the target gas, respectively, for atmospheric greenhouse gas concentration detection by the integral path differential absorption lidar; the beam splitter has a splitting ratio of 99:1, 98:2 or 95:5, so that most of the laser energy is used as the detection beam and a small portion of the energy is used as the monitoring beam.

[0013] Furthermore, the receiving telescope is a Cassegrain reflecting telescope, whose optical axis is parallel or coaxial with the emission optical axis of the detection beam, and is used to collect the echo signal after diffuse reflection from the hard target and converge it to the photosensitive surface of the photodetector.

[0014] Furthermore, the attenuator is a neutral density attenuator or an adjustable attenuator, and its attenuation factor is set according to the initial energy of the monitoring beam and the linear response range of the photodetector, so as to avoid saturation of the photodetector and ensure that it operates in the linear region.

[0015] Furthermore, the speckle suppression module, the integrating sphere, and the photodetector work together to form a single-detector time-division receiving architecture. The speckle suppression module is used to suppress speckle noise of the monitoring beam, the integrating sphere is used to eliminate pointing jitter and uneven energy distribution of the monitoring beam, and the photodetector receives the monitoring beam signal and the probe beam echo signal in a time-division manner, thereby simultaneously solving the energy measurement error problems caused by speckle effect, pointing jitter, and differences between the two detectors.

[0016] The integrating sphere is used to eliminate the effects of pointing jitter and uneven energy distribution of the monitoring beam.

[0017] The photodetector synchronously receives the detection beam and the monitoring beam, which can eliminate the influence of the difference between the two detectors and lay the foundation for subsequent high-precision data inversion.

[0018] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. A coarse diffuse scatterer is mounted on a rotary motor with adjustable rotation speed. By actively controlling the rotation speed to change the time averaging effect of the speckle pattern, speckle noise caused by laser coherence is suppressed. Compared with static or fixed-rotation diffuse scatterers, this invention allows for flexible adjustment of the rotation speed according to the laser repetition frequency and energy monitoring accuracy requirements, achieving optimal speckle suppression effect.

[0019] 2. The monitoring beam after speckle suppression is introduced into the integrating sphere, and the beam is homogenized by the Lambertian scattering characteristics of the integrating sphere. This effectively eliminates the changes in coupling efficiency caused by laser pointing jitter, beam splitter angle drift, or uneven energy distribution of the monitoring beam, thereby improving the stability and repeatability of energy measurement.

[0020] 3. By using the same photodetector to receive the monitoring beam signal and the echo signal of the probe beam respectively, the system errors such as inconsistent aging rate, temperature response drift, and gain difference introduced by using two independent detectors in the traditional solution are avoided. It is especially suitable for IPDA lidar that requires long-term stable operation. Attached Figure Description

[0021] Figure 1. Schematic diagram of the overall structure of the high-precision detection device for dual-pulse laser energy of the lidar according to the present invention. Detailed Implementation

[0022] The present invention will be further described below with reference to examples and accompanying drawings, but this should not be construed as limiting the scope of protection of the present invention. Any modifications, equivalent substitutions, etc., made based on the concept of the present invention shall fall within the scope of protection of the present invention.

[0023] Please see Figure 1 , Figure 1 This is a schematic diagram of the overall structure of the energy monitoring device of the present invention, as shown below. Figure 1 As shown, this embodiment provides a high-precision detection device for dual-pulse laser energy of lidar, including a laser emission module, a speckle suppression module, a receiving module, and a data acquisition and processing module 11.

[0024] Laser emission module: dual-wavelength pulsed laser 1, beam splitter 2.

[0025] Speckle suppression module: attenuator 3, rough diffuser 4, and adjustable speed rotary motor 5.

[0026] Receiver module: Integrating sphere 6, Delay fiber 7, Collimating lens 8, Receiving telescope 9, Photodetector 10.

[0027] Data acquisition and processing module 11.

[0028] In this embodiment, the dual-wavelength pulsed laser 1, under the control of the FPGA, first outputs an on-line laser pulse (1572nm), which is split into a probe beam and a monitoring beam by the beam splitter 2. After a 1 ms interval, it outputs an off-line laser pulse (1571nm), which is also split into a probe beam and a monitoring beam. This forms a dual-pulse pair with a repetition frequency of 250 Hz (i.e., each pair of pulses has a 2 ms period).

[0029] The monitoring beam's energy is attenuated after passing through attenuator 3, and then it passes through a coarse diffuser 4 mounted on an adjustable-speed rotary motor 5. By controlling the rotational speed of the rotary motor 5, the rotational speed of the coarse diffuser is changed, thereby altering the suppression effect of laser speckle. The monitoring beam (alternating on / off) passes sequentially through attenuator 3, the rotating coarse diffuser 4, and then enters integrating sphere 6. The uniformly diffused light output from integrating sphere 6 is coupled to delay fiber 7, and after being delayed by fiber, it is collimated by collimating lens 8 and illuminates photodetector 10.

[0030] The probe beam (on / off) is directed at a hard target (e.g., the ground or a hard target), and after reflection from the hard target, it is collected by the receiving telescope 9. Assuming the target distance is 5 km, the round-trip time of the light is 33.3 μs. The echo signal arrives at the photodetector 10 approximately 33.3 μs after laser emission. Since the arrival time of the monitoring signal is 0.5 μs and the echo signal is 33.3 μs, the time difference between the two is much greater than the pulse width (10 ns), so they can be received by the same detector in a time-division manner without crosstalk.

[0031] The data acquisition and processing module records the voltage waveform output by the detector and calculates the CO2 column concentration based on the IPDA differential absorption principle.

[0032] Wherein, DAOD is the differential optical thickness, obtained from lidar measurements; IWF is the weighting function for the target gas, related to meteorological parameters such as temperature, humidity, and pressure. The target gas concentration error can be expressed as: The normalized energy ratio of the four pulses of the probe beam to that of the monitoring beam is defined as follows: Then the relationship between the normalized energy ratio error and the target gas column concentration error is:

[0033] Under a given spectrophotometer ratio, neglecting target gas absorption, the normalized energy ratio should be 1. The deviation of the normalized energy ratio from 1 represents the bias of the energy monitoring system, and the fluctuation of the normalized energy ratio represents the accuracy of the energy monitoring system.

[0034] In this embodiment, the rough diffuse scatterer 4 is made of 220-mesh frosted glass. The roughness of the frosted glass can be changed according to the energy monitoring accuracy requirements. The adjustable-speed rotary motor 5 is an OSMS series automatic rotary platform, which can change the speckle suppression effect by adjusting the rotation speed of the frosted glass. In this embodiment, the surface roughness of the frosted glass Ra≈1.5 μm, and the scattering angle of the 1572 nm laser is approximately 30° (full angle). The frosted glass is fixed to the shaft of the rotary motor 5 by a clamp. The rotary motor 5 is an OSMS series automatic rotary platform (Sigkokki, Japan), and the speed can be continuously adjusted within the range of 0~3000 rpm. In this embodiment, based on the laser repetition frequency of 500 Hz, the speed is set to 1800 rpm, so that the movement distance of the frosted glass surface in each laser pulse interval is greater than one speckle correlation length, effectively achieving speckle averaging.

[0035] Integrating sphere 6 is a Thorlabs IS200 integrating sphere with an inner diameter of 50 mm. The inner wall is made of Spectralon® high diffuse reflectance material with a reflectivity >98%. The inlet aperture of the integrating sphere is 10 mm, and the outlet aperture is 5 mm. The monitoring beam enters the integrating sphere through the inlet after passing through the scatterer, and after multiple diffuse reflections, a uniform Lambertian spot is output from the outlet.

[0036] The receiving telescope 9 is a Cassegrain-type reflecting telescope with an aperture of 200 mm, a focal length of 800 mm, and a field of view of 0.5 mrad. The probe beam, after being reflected by a hard target (ground or cloud), is collected by the telescope and focused onto the photosensitive surface of the photodetector 10. The photodetector 10 is an InGaAs avalanche photodiode (APD), model Thorlabs APD410C, with a response wavelength of 900~1700 nm, adjustable gain, bandwidth of 10 MHz, and a photosensitive surface diameter of 0.5 mm. The detector output signal is connected to the data acquisition and processing module 11 after 50 Ω impedance matching.

[0037] This invention features a compact structure, mature components, and easy integration, making it suitable for ground-based, airborne, and spaceborne IPDA lidar systems for high-precision column concentration detection of atmospheric greenhouse gases (CO2, CH4, etc.). It can also be extended to other fields requiring high-precision pulsed laser energy monitoring, such as laser ranging and laser remote sensing.

[0038] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of the patent. All equivalent variations and modifications made within the scope of the present invention should fall within the scope of the present invention.

Claims

1. A high-precision detection device for dual-pulse laser energy in lidar, characterized in that, include: Laser emission module: includes a dual-wavelength pulsed laser (1) and a beam splitter (2); the beam splitter (2) is set on the output optical path of the dual-wavelength pulsed laser (1) to split the incident laser pulse into two paths, one of which is a probe beam that is directly emitted into the atmosphere, and the other is a monitoring beam; Speckle suppression module: Along the propagation direction of the monitoring beam, an attenuator (3), a rough diffuser (4), and a rotary motor (5) with adjustable speed are arranged in sequence; the rough diffuser (4) is fixedly installed on the rotating shaft of the rotary motor (5). The monitoring beam first passes through the attenuator (3) to attenuate its energy, and then is incident on the surface of the rough diffuser (4) driven to rotate by the rotary motor (5). By adjusting the rotation speed of the rotary motor (5), the rotation speed of the rough diffuser (4) is changed to suppress laser speckle; The receiving module includes an integrating sphere (6), a time-delay fiber (7), a collimating lens (8), a receiving telescope (9), and a photodetector (10). The entrance of the integrating sphere (6) faces the exit surface of the rough diffuse scatterer (4) and is used to receive the monitoring beam after speckle suppression. The exit of the integrating sphere (6) is coupled to the input end of the time-delay fiber (7), and the output end of the time-delay fiber (7) is connected to the collimating lens (8). The exit optical path of the collimating lens (8) is aligned with the photosensitive surface of the photodetector (10). The output end of the receiving telescope (9) is also aligned with the same photosensitive surface of the photodetector (10) and is used to receive the echo signal of the detection beam after reflection from the hard target. Data acquisition and processing module (11): Its input end is electrically connected to the output end of the photodetector (10) and is used to acquire and process the monitoring beam signal and the detection beam echo signal.

2. The high-precision detection device for dual-pulse laser energy of lidar according to claim 1, characterized in that, The rough diffuse scatterer (4) is made of frosted glass, ceramic diffuser, or metal diffuse reflector, and its surface roughness is optimized according to the wavelength and energy monitoring accuracy requirements of the dual-wavelength pulsed laser (1). The rotary motor (5) is an automatic rotating platform or a DC brushless motor with continuously adjustable speed. Its speed adjustment range covers the speed range that makes the speckle suppression effect optimal, so that the surface linear velocity of the rough diffuse scatterer (4) is greater than the movement correlation length of the speckle pattern within one laser pulse cycle, thereby reducing the standard deviation of energy measurement caused by speckle to below 0.5%.

3. The high-precision detection device for dual-pulse laser energy of lidar according to claim 1, characterized in that, The inner wall of the integrating sphere (6) is made of a highly diffuse reflective material. Its entrance is aligned with the exit surface of the rough diffuse scatterer (4) by a preset gap or optical path alignment. It is used to receive the monitoring beam after speckle suppression and eliminate the influence of the pointing jitter and uneven energy distribution of the monitoring beam on energy measurement through multiple diffuse reflections inside the integrating sphere. The outgoing light of the integrating sphere (6) is uniformly scattered light from a Lambertian body.

4. The high-precision detection device for dual-pulse laser energy of lidar according to claim 1, characterized in that, The delay fiber (7) is a multimode fiber, and its length is set according to the detection distance of the detection beam, so that the monitoring beam and the detection beam echo signal generated by the same laser pulse are separated in time, and the monitoring beam signal arrives at the same photosensitive surface of the photodetector (10) before the detection beam echo signal, thereby realizing time-division reception of a single detector.

5. The high-precision detection device for dual-pulse laser energy of lidar according to claim 1, characterized in that, The photodetector (10) is an avalanche photodiode (APD), a photomultiplier tube (PMT) or a PIN photodiode. Its photosensitive surface is located simultaneously on the outgoing optical path of the collimating lens (8) and the output optical path of the receiving telescope (9). The echo signals of the monitoring beam and the detection beam do not overlap in time to avoid signal crosstalk.

6. The high-precision detection device for dual-pulse laser energy of lidar according to claim 1, characterized in that, The data acquisition and processing module (11) includes a high-speed data acquisition card and an FPGA embedded processing board; the control output terminal of the FPGA embedded processing board is connected to the trigger input terminal of the dual-wavelength pulsed laser (1) to control the laser emission timing. The FPGA embedded processing board also synchronously triggers the high-speed data acquisition card to record the voltage waveform output by the photodetector (10); the FPGA embedded processing board has a peak extraction algorithm and a normalized energy ratio calculation unit embedded in it, which are used to extract the monitoring pulse peak value (P) from the waveform. on0 P off0 ) and echo pulse peak (P on P off The normalized energy ratio was calculated to correct the gas concentration inversion of the IPDA lidar.

7. The high-precision detection device for dual-pulse laser energy of lidar according to claim 1, characterized in that, The two wavelengths output by the dual-wavelength pulsed laser (1) are located on the absorption line and outside the absorption line of the target gas, respectively, and are used for atmospheric greenhouse gas concentration detection by the integral path differential absorption lidar; the beam splitter (2) has a splitting ratio of 99:1, 98:2 or 95:5, so that most of the laser energy is used as the detection beam and a small part of the energy is used as the monitoring beam.

8. The high-precision detection device for dual-pulse laser energy of lidar according to claim 1, characterized in that: The receiving telescope (9) is a Cassegrain reflecting telescope, whose optical axis is parallel or coaxial with the emission optical axis of the detection beam, and is used to collect the echo signal after diffuse reflection from the hard target and converge it to the photosensitive surface of the photodetector (10).

9. The high-precision detection device for dual-pulse laser energy of lidar according to claim 1, characterized in that: The attenuator (3) is a neutral density attenuator or an adjustable attenuator. Its attenuation factor is set according to the initial energy of the monitoring beam and the linear response range of the photodetector (10) to avoid saturation of the photodetector and ensure that it works in the linear region.

10. The high-precision detection device for dual-pulse laser energy of lidar according to claim 1, characterized in that: The speckle suppression module, the integrating sphere (6), and the photodetector (10) work together to form a single detector time-division receiving architecture. The speckle suppression module is used to suppress speckle noise of the monitoring beam, the integrating sphere (6) is used to eliminate pointing jitter and uneven energy distribution of the monitoring beam, and the photodetector (10) receives the monitoring beam signal and the probe beam echo signal in a time-division manner, thereby simultaneously solving the energy measurement error problems caused by speckle effect, pointing jitter and the difference between the two detectors.

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