NO2 differential absorption Sham imaging lidar based on wavelength switchable laser

Abstract

The invention discloses a NO2 differential absorption Sham imaging laser radar based on a wavelength switchable laser. It comprises a wavelength switchable external cavity compression blue light laser and a Sham imaging unit. The output beam of a high-power blue light diode laser in the external cavity compression blue light laser passes through a first cylindrical lens, a second cylindrical lens, a blazed grating, a plane reflector and a polarizer respectively. After passing through the polarizer, the beam is divided into two beams of light: one beam is injected into a beam shaper as a main energy beam, and the laser output from the beam shaper is the output laser of the wavelength switchable external cavity compression blue light laser, which is injected into the atmospheric space to be measured; the other beam is injected into a wavelength meter as a test beam, and the wavelength meter is connected to a computer; the invention utilizes the wavelength switching characteristics and high power characteristics of the external cavity compression blue light laser, reduces the cost of the gas differential absorption Sham imaging laser radar, and improves the system stability.

Description

Technical Field

[0001] The present invention relates to the technical field of atmospheric pollution gas detection by laser radar, in particular to a NO2 differential absorption Sham imaging laser radar based on a wavelength-switchable laser. Background Art

[0002] NO2, the most common nitrogen oxide gas in the atmosphere, is one of the most important pollutants and a major component of many NOx emissions. It primarily originates from the combustion of fossil fuels and automobile exhaust emissions. NOx and volatile organic compounds (VOCs) in the atmosphere trigger a chemical reaction with OH radicals under light, generating O3. This process oxidizes NOx, SO2, and VOCs to form secondary particulate matter. High concentrations of O3 and PM2.5 pollution coexist, forming a complex atmospheric pollution system. With increasing demands for environmental quality, the detection of nitrogen oxides, including NO2, in the atmosphere is of paramount importance.

[0003] Differential absorption lidar has gained wide attention and made great progress, improvement and development due to its excellent performance in detecting trace gases in the atmosphere, such as fast and real-time detection, high spatial resolution, and large measurement range. This radar can be used to detect the concentration of NO2 in the atmosphere. Traditional pulsed differential absorption lidar uses two high-energy pulsed lasers with different wavelengths, one of which is λ on Located at the absorption peak of the gas to be measured, another wavelength λ off This method deviates from the gas absorption peak and uses the different absorption rates of gases at different wavelengths to measure the concentration of the gas being measured. This type of lidar requires a high-energy pulsed laser light source and a sophisticated optical transmission and reception system, resulting in extremely high equipment and maintenance costs.

[0004] Differential absorption Sham imaging lidar is based on the Sham imaging principle: in an imaging system, even when the object plane is not parallel to the lens, as long as the image plane, object plane, and lens plane intersect on a straight line, the imaging system can still form a clear image of the object plane with infinite depth of field. Its advantage lies in the use of a large-aperture lens system to enhance echo signal reception while achieving a theoretically infinite depth of field, allowing for clear images of objects over a wide range. This is of great significance for lidar technology that measures weak atmospheric backscatter signals. Compared to traditional pulsed lidar, Sham imaging lidar offers a number of advantages, including a simpler structure, lower system design and maintenance costs, and reduced light source requirements.

[0005] The NO2 differential absorption Sham imaging lidar that has been studied and reported uses two independent laser light sources, one of which has a wavelength of λ on Located at the absorption peak of the gas to be measured, another wavelength λ offDeviating from the gas absorption peak, the two independent laser light sources increase the system cost and weaken the system stability. Summary of the Invention

[0006] The present invention addresses the deficiencies of the prior art and proposes a NO2 differential absorption Sham imaging lidar based on a wavelength-switchable laser.

[0007] A NO2 differential absorption Sham imaging lidar based on a wavelength-switchable laser includes a wavelength-switchable external cavity compressed blue light laser and a Sham imaging unit.

[0008] The wavelength-switchable external cavity compression blue laser comprises a high-power blue diode laser, a first cylindrical lens, a second cylindrical lens, a blazed grating, a dihedral mirror structure, a plane reflector, a wavelength meter and a beam shaper;

[0009] The output beam of the high-power blue light diode laser passes through a first cylindrical lens, a second cylindrical lens, a blazed grating, a plane reflector, and a polarizer, respectively. After passing through the polarizer, the beam is split into two beams: one beam is injected into a beam shaper as a main energy beam. The laser output from the beam shaper is the output laser of the wavelength-switchable external cavity compression blue light laser, which is injected into the atmospheric space to be measured; and the other beam is injected into a wavelength meter as a test beam. The wavelength meter is connected to a computer and is used to monitor the output wavelength of the wavelength-switchable external cavity compression blue light laser in real time, and to lock the output wavelength of the wavelength-switchable external cavity compression blue light laser to a set wavelength through the computer.

[0010] The light beam output by the high-power blue diode laser passes through the first and second cylindrical lenses and becomes a parallel light beam with a rectangular cross section, which is then injected into the blazed grating. The blazed grating selects laser light of a set wavelength and reflects it into the high-power blue diode laser.

[0011] The blazed grating is fixed on one arm of an L-shaped rotatable dihedral mirror structure, and the plane reflector is fixed on the other arm of the L-shaped rotatable dihedral mirror structure, and the plane where the blazed grating is located and the plane where the plane reflector is located form an angle of 90 degrees;

[0012] The Sham imaging unit converts light scattered from the atmospheric space into an electrical signal and inputs the signal into a computer, which then calculates the NO2 concentration information.

[0013] The present invention utilizes the wavelength switching characteristics and high power characteristics of the external cavity compression blue light laser, reduces the cost of the gas differential absorption Sham imaging lidar, improves the system stability, and enhances the sensitivity and detection limit of the atmospheric NO2 concentration detection lidar. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 It is a structural schematic diagram of the present invention.

[0015] Figure 2 This is a partial diagram of the heat dissipation of the present invention. DETAILED DESCRIPTION

[0016] like Figure 1 and Figure 2 As shown, this embodiment includes a metal heat sink 1a, a TEC cooling sheet 1b, a heat dissipation fan 1c, a temperature-controlled heat sink 1d, a thermistor 2, a high-power blue light diode laser 3, a first cylindrical lens 4a, a second cylindrical lens 4b, a piezoelectric ceramic PZT 5, an integrated circuit module 6, a blazed grating 7, an L-shaped rotatable dihedral mirror structure 8, a plane reflector 9, a beam shaper 10, a polarizer 11, a wavelength meter 12, a laser base plate 13, a computer 14, a lens 15, a filter 16, and an image detector 17.

[0017] Referring to the absorption spectrum of NO2 near 448.1nm, the following optical devices and mechanical structures operating in the 440-450nm band are selected to form a wavelength-switchable external cavity compression blue light laser: metal heat sink 1a, TEC cooling plate 1b, cooling fan 1c, temperature-controlled heat sink 1d, thermistor 2, high-power blue light diode laser 3, first cylindrical lens 4a, second cylindrical lens 4b, PZT 5, integrated circuit module 6, blazed grating 7, L-shaped rotatable dihedral mirror structure 8, plane mirror 9, beam shaper 10, polarizer 11, wavelength meter 12, laser base plate 13, computer 14.

[0018] The components of the wavelength-switchable external cavity compression blue laser are connected as follows: a high-power blue diode laser 3 is fixed to a metal heat sink 1a, a thermistor 2 is placed inside the metal heat sink 1a, the lower end surface of the metal heat sink 1a is bonded to the cold surface of a TEC (electrical control) cooling plate 1b using thermal silicone grease, and the hot surface of the TEC (electrical control) cooling plate 1b is bonded to a temperature-controlled heat sink 1d using thermal silicone grease. A cooling fan 1c is fixed to one side of the temperature-controlled heat sink. The output beam of the high-power blue diode laser 3 passes through a first cylindrical lens 4a, a second cylindrical lens 4b, a blazed grating 7, a plane reflector 9, and a polarizer 11. After passing through the polarizer 11, the beam is split into two beams: one beam, serving as the main energy beam, is injected into a beam shaper 10. The laser output from the beam shaper 10 is the output laser of the wavelength-switchable external cavity compression blue laser; the other beam, serving as a test beam, is injected into a wavelength meter 12, which is connected to a computer 14.

[0019] The L-shaped rotatable dihedral mirror structure 8 is fixed to the laser base plate 13 via a knob structure at the intersection of its two arms. The blazed grating 7 is fixed to one arm of the L-shaped rotatable dihedral mirror structure 8, and the plane reflector 9 is fixed to the other arm of the L-shaped rotatable dihedral mirror structure 8. The plane of the blazed grating 7 and the plane reflector 9 form a 90-degree angle. The PZT 5 is in perpendicular contact with one arm of the L-shaped rotatable dihedral mirror structure 8 and is fixed to the laser base plate 13. The PZT 5 is connected to the integrated circuit module 6, which is respectively connected to the high-power blue light diode laser 3, the TEC cooling plate 1b, the thermistor 2, and the computer 14.

[0020] The working principle of the wavelength-switchable external cavity compression blue laser is as follows: (1) The light beam output by the high-power blue diode laser 3 passes through the first cylindrical lens 4a and the second cylindrical lens 4b and becomes a parallel light beam with a rectangular cross section and is injected into the blazed grating 7. The blazed grating 7 selects a laser beam of a specific wavelength and reflects it into the high-power blue diode laser 3. Based on the working principle of the optical feedback semiconductor laser, the output wavelength of the high-power blue diode laser at this time is determined by the light reflected by the blazed grating 7, and the laser line width becomes narrower. (2) When the computer 14 controls the integrated circuit module 6 to drive the PZT 5 so that the L-shaped rotatable dihedral mirror structure 8 rotates slightly, the wavelength of the light reflected by the blazed grating 7 and injected into the high-power blue diode laser 3 changes, thereby causing the wavelength-switchable external cavity compression blue laser to switch to another wavelength. (3) When the PZT 5 drives the L-shaped rotatable dihedral mirror structure 8 to rotate slightly, the blazed grating 7 and the plane reflector 9 remain relatively stationary, which can ensure that the output laser direction of the wavelength-switchable external cavity compression blue laser remains unchanged and that the beam does not shift in the lateral direction. (4) The wavelength meter 12 is used to monitor the output wavelength of the wavelength-switchable external cavity compression blue light laser in real time, and can lock the output wavelength of the wavelength-switchable external cavity compression blue light laser at a specific wavelength through the computer 14. The output wavelength of the wavelength-switchable external cavity compression blue light laser is locked at a specific wavelength. The wavelength meter 12 measures the difference between the output wavelength and the locked wavelength, and the computer 14 provides negative feedback based on the PZT 5 to achieve output wavelength tuning of the external cavity compression blue light laser to lock the output wavelength.

[0021] (2) On the basis of realizing a wavelength-switchable external cavity compressed blue laser, the following optical devices with a working band covering 440-450nm are further selected to form the Sham imaging lidar: lens 15, filter 16 and image detector 17.

[0022] The components of the Sham imaging lidar are connected as follows: a wavelength-switchable external-cavity compressed blue laser output beam is injected into the atmospheric space to be measured; light scattered from the atmospheric space is injected into lens 15, then passed through filter 16 and injected into image detector 17. The plane of lens 15, the plane of image detector 17, and the wavelength-switchable external-cavity compressed blue laser output beam are collinear at a specific plane in the atmospheric space at the measured location. These three planes intersect on a straight line, satisfying the Sham imaging principle.

[0023] The plane where the atmospheric scattering cross section is located, the plane where the lens 15 is located, and the plane where the image detector 17 is located intersect in a straight line, satisfying the Sham imaging principle. After the scattered light passes through the lens, it is filtered out by the filter 16 to remove stray light and finally injected into the image detector 17. The image detector 17 converts the optical signal into an electrical signal and transmits it to the computer 14 for processing. Finally, the computer 14 calculates the final NO2 concentration information.

[0024] The principle of NO2 differential absorption Sham imaging lidar using wavelength-switchable laser is as follows: when the wavelength-switchable external cavity compressed blue laser emits two center wavelengths alternately at a certain time period, the two center wavelengths are located at the NO2 absorption peak wavelength λ on (e.g. 448.1 nm) and the NO2 absorption valley wavelength λ near the NO2 absorption peak off Atmospheric NO2 absorbs light of two different wavelengths (for example, 449 nm). Therefore, the scattered light echoes have different intensities. Based on this differential absorption, a computer compares the measured data with the NO2 absorption spectra in a known database to infer the NO2 concentration in the gas being measured. Using the Sham imaging principle, the spatial distribution of atmospheric NO2 concentration can be obtained. In this example, the detected atmospheric nitrogen dioxide concentration was 11 ppbv, with a sensitivity of 0.6 ppbv (10 min).

[0025] In summary, this invention is applicable to the field of atmospheric detection lidar technology. It utilizes the wavelength switching and high-power characteristics of an external-cavity compression blue laser to reduce the cost of gas differential absorption Sham imaging lidar, improve system stability, and enhance the sensitivity and detection limit of atmospheric NO₂ concentration detection lidar. This invention can be applied to atmospheric NO₂ concentration detection, offering advantages such as high sensitivity and system stability.

Claims

1. NO2 differential absorption Sham imaging lidar based on wavelength switchable laser, including wavelength switchable external cavity compression blue laser and Sham imaging unit, characterized in that: The wavelength-switchable external cavity compressed blue laser comprises a high-power blue diode laser, a first cylindrical lens, a second cylindrical lens, a blazed grating, a dihedral mirror structure, a plane reflector, a wavelength meter and a beam shaper; The output beam of the high-power blue light diode laser passes through the first cylindrical lens, the second cylindrical lens, the blazed grating, the plane reflector and the polarizer respectively. After passing through the polarizer, the beam is divided into two beams of light: one beam is injected into the beam shaper as the main energy beam, and the laser output from the beam shaper is the output laser of the wavelength-switchable external cavity compression blue light laser, which is injected into the atmospheric space to be tested; the other beam is injected into the wavelength meter as the test beam, and the wavelength meter is connected to the computer. The wavelength meter is used to monitor the output wavelength of the wavelength-switchable external cavity compression blue light laser in real time, and the output wavelength of the wavelength-switchable external cavity compression blue light laser is locked at a set wavelength through the computer; The light beam output by the high-power blue diode laser passes through the first cylindrical lens and the second cylindrical lens, and becomes a parallel light beam with a rectangular cross section, which is injected into the blazed grating. The blazed grating selects the laser with a set wavelength to reflect and inject into the high-power blue diode laser. The blazed grating is fixed on one arm of the L-shaped rotatable two-sided mirror structure, and the plane reflector is fixed on the other arm of the L-shaped rotatable two-sided mirror structure, and the plane where the blazed grating is located and the plane where the plane reflector is located form an angle of 90 degrees; The Sham imaging unit converts the light scattered back from the atmospheric space into an electrical signal and inputs it into a computer, and the computer calculates the NO2 concentration information; The Sham imaging unit includes a lens, a filter and an image detector with a working band covering 440-450nm. The light scattered back from the atmospheric space is injected into the lens, and then injected into the image detector through the filter. The image detector converts the light signal into an electrical signal and transmits it to a computer for processing. The plane where the atmospheric scattering cross section is located, the plane where the lens is located and the plane where the image detector is located intersect in a straight line, satisfying the Sham imaging principle.

2. The NO2 differential absorption Sham imaging laser radar based on a wavelength switchable laser according to claim 1, characterized in that: The rotation of the dihedral mirror structure is driven by piezoelectric ceramics.

3. The NO2 differential absorption Sham imaging laser radar based on a wavelength switchable laser according to claim 1, characterized in that: The high-power blue light diode laser is fixed on a metal heat sink base, a thermistor is placed inside the metal heat sink base, the lower end surface of the metal heat sink base is bonded to the cold surface of the TEC cooling plate through thermal grease, the hot surface of the TEC cooling plate is bonded to the temperature-controlled heat sink base with thermal grease, and a heat dissipation fan is fixed on one side of the temperature-controlled heat sink base.

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