An aerosol measurement device with multi-longitudinal mode optical feedback composite spectrum

Abstract

The invention discloses an aerosol measurement device of multi-longitudinal mode optical feedback composite spectrum, aiming to solve the shortcomings of the prior art such as large measurement error of aerosol optical properties, complex optical structure, and high-cost single-frequency narrow linewidth laser. The invention comprises a laser diode, a composite spectrum measurement cavity, a laser collimator, a λ / 2 wave plate, a polarization beam splitter cube, a mode matching and shaping module, a laser detector, a high-sensitivity microphone, etc.; the high-sensitivity microphone and the laser detector constitute a measurement optical path to realize the composite measurement of photoacoustic spectrum and cavity ring-down spectrum, and the composite spectrum measurement cavity, the λ / 2 wave plate, the polarization beam splitter cube and the laser diode constitute an optical feedback optical path to realize laser mode locking and linewidth narrowing, improve the measurement signal-to-noise ratio, and use a single polarization beam splitter cube to realize optical isolation and optical feedback; the invention has a simple structure, low cost, easy implementation and a wide range of applications.

Description

Technical Field

[0001] The invention relates to the technical field of gas detection, and in particular to a composite spectrum aerosol measuring device based on multi-longitudinal mode light feedback technology. Background Art

[0002] Atmospheric aerosols can affect the energy budget of the Earth's atmosphere and affect climate change by scattering and absorbing solar radiation. The radiative effect of atmospheric aerosols depends on the optical properties of aerosols, including the aerosol scattering coefficient, absorption coefficient and extinction coefficient. The integral of the atmospheric aerosol extinction coefficient in the vertical direction is the aerosol optical depth, and the ratio of the atmospheric aerosol scattering coefficient to the extinction coefficient is the aerosol single scattering albedo. Aerosol optical depth and single scattering albedo are particularly important in the study of aerosol radiative forcing, and their slight changes may lead to large changes in aerosol radiative forcing. Therefore, to accurately assess aerosol radiative forcing, it is necessary to accurately measure the aerosol scattering coefficient, absorption coefficient and extinction coefficient at the same time.

[0003] At present, black carbon meters or absorption spectrophotometers are commonly used to measure aerosol absorption coefficients, and integral turbidity meters are used to measure aerosol scattering coefficients, and then the extinction coefficient is calculated. Since two instruments are used to extract aerosols to measure aerosol absorption coefficients and scattering coefficients respectively, there are inconsistencies in the measurement objects in space and time, and there are certain problems with the comparability of the measurement results. There are also studies reporting that cavity ring-down spectroscopy or photoacoustic spectroscopy and integral turbidity technology are combined to achieve real-time measurement of aerosol extinction coefficients, scattering coefficients, and absorption coefficients. However, there are systematic deviations in measuring aerosol scattering coefficients using the integral turbidity method, and the measurement results are related to the aerosol particle size and complex refractive index.

[0004] The Chinese invention patent application publication number is CN108896449A, the application publication date is November 27, 2018, and the name is "A Synchronous Aerosol Measurement System". It discloses an aerosol synchronous measurement system, which includes an optical path module, an air path module, a cavity module, a photoacoustic spectroscopy signal processing module, a cavity ring-down signal processing module and a control module. The cavity ring-down and photoacoustic spectroscopy technology are used to complete the measurement of the extinction coefficient, absorption coefficient and scattering coefficient of the aerosol in the same body and the same background, and the optical properties of the aerosol are accurately obtained. However, this invention patent does not involve optical feedback technology, but only integrates traditional cavity ring-down technology and photoacoustic spectroscopy technology. The measurement accuracy depends on the performance of the laser, and the optical path part is relatively complex. The Chinese invention patent application publication number is CN102445423A, the application publication date is May 9, 2012, and the name is "An optical feedback continuous wave cavity ring-down spectroscopy technology", which discloses an optical feedback continuous wave cavity ring-down spectroscopy technology, using a V-shaped folded cavity as a ring-down cavity to lock the DFB laser and narrow the spectrum line, thereby achieving high spectral resolution and high sensitivity measurement of the gas. However, the V-shaped folded cavity introduces a folded lens, which increases the loss in the cavity, is not conducive to higher precision measurement, and requires the use of a single-frequency narrow-linewidth DFB laser and cavity length scanning, which is costly and difficult to achieve technically. Summary of the invention

[0005] In order to solve the shortcomings of the prior art, such as large measurement errors of aerosol optical properties, complex optical structure, and the need for high-cost single-frequency narrow-linewidth lasers, the present invention proposes a composite spectral aerosol measurement device based on multi-longitudinal mode optical feedback technology, which organically integrates cavity ring-down spectroscopy technology and photoacoustic spectroscopy technology, and uses multi-longitudinal mode optical feedback technology to achieve mode locking and linewidth narrowing of the laser, thereby improving the measurement signal-to-noise ratio and making the measurement more accurate; optical isolation and optical feedback are achieved through a single polarization beam splitting cube, the structure is simpler, and a multi-longitudinal mode laser diode is used, without the need for longitudinal mode and phase matching, low cost, easy implementation, and a wide range of applications.

[0006] The technical solution of the present invention is: an aerosol measurement device with multi-longitudinal mode light feedback composite spectrum, comprising an optical path module, the optical path module comprising a laser diode, a laser collimator and a first λ / 2 wave plate are arranged in sequence in the laser emission direction of the laser diode, and an isolated transmission optical path formed by the first λ / 2 wave plate emitting through a multi-stage optical path, and an isolated feedback optical path formed by the cavity module emitting through a multi-stage optical path to reach the laser diode; a cavity module, the cavity module comprising a composite spectrum measurement cavity capable of allowing the emitted laser to carry the optical property information of the aerosol to be measured; a laser beam capable of splitting the laser beam into two is arranged behind the output end of the composite spectrum measurement cavity An optical beam splitter, one path is received by the laser detector, and the other path is fed back to the laser diode through the isolated feedback optical path; a signal processing module, the signal processing module includes a high-sensitivity microphone connected to the composite spectrum measurement cavity and a laser detector arranged on the rear side of the output end of the composite spectrum measurement cavity, and the high-sensitivity microphone is connected to a phase-locked amplifier module; a control module, the control module includes a high-speed acquisition card, a display module and a driving module connected in sequence, the high-speed acquisition card acquires the signals processed by the laser detector and the phase-locked amplifier module, and then the display module receives, processes and displays them, and finally drives the laser diode by controlling the driving module. In the present invention, a laser diode emits a multi-longitudinal mode linear polarized laser, which is collimated by a laser collimator and then emitted. After being rotated and adjusted by a first λ / 2 wave plate, a polarization beam splitter cube performs polarization separation. After the parallel polarized light is transmitted, it is transmitted along the original transmission direction, and the isolated transmission optical path is transmitted to the composite spectrum measurement cavity. The incident laser beam coincides with the optical axis of the composite spectrum measurement cavity. After the laser beam resonates in the measurement cavity, it is emitted through the output high reflector of the measurement cavity. The emitted laser carries the optical property information of the aerosol to be measured. The emitted laser is split into two by a laser beam splitter, one of which is received by a laser detector and the other is transmitted. The light is fed back to the laser diode through the isolated feedback optical path; since the laser in the measuring cavity is absorbed by the aerosol, heat is generated after absorption to form a sound signal, and the high-sensitivity microphone detects the photoacoustic signal in the measuring cavity and outputs it to the phase-locked amplifier module for harmonic detection; the high-speed acquisition card receives the ring-down spectrum signal obtained by the laser detector and the photoacoustic spectrum signal extracted by the phase-locked amplifier module, which are received and processed by the display module, and the extinction coefficient, absorption coefficient and scattering coefficient of the aerosol to be measured are calculated and displayed. At the same time, the output signal controls the driving module, and the driving module drives the laser diode according to the control signal.

[0007] Preferably, the laser diode emits linearly polarized light, including multiple laser longitudinal modes, with a radiation spectrum bandwidth of the order of GHz, a single longitudinal mode line width of the order of MHz, and a laser power of (0-50) mW.

[0008] Preferably, the composite spectrum measurement cavity is a metal hollow structure, with a hollow thin cavity in the middle and hollow thick cavities symmetrically distributed at both ends. The two ends of the composite spectrum measurement cavity are sealed and installed by input window plates and output window plates. An input high-reflection mirror and an output high-reflection mirror are installed inside the composite spectrum measurement cavity, and they are coaxially installed at the two ends of the composite spectrum measurement cavity facing each other via an adjustment device. The composite spectrum measurement cavity is a metal hollow structure, with a hollow thin cavity in the middle, with a cavity diameter of D and a length of L, and hollow thick cavities symmetrically distributed at both ends, with a cavity diameter of d and a length of l, satisfying D≤0.25d, L=2l; two windows are used to seal the two ends of the composite spectrum measurement cavity; two high-reflection mirrors are installed at the two ends of the measurement cavity facing each other via an adjustment device, and the curvature radii of the high-reflection mirrors are R1 and R2 respectively, satisfying The two high-reflection mirrors are both installed and fixed in the measuring cavity by an adjusting device. The high-reflection mirrors can be precisely adjusted in three dimensions through the adjusting device, so that the optical axis of the resonant cavity composed of the high-reflection mirrors coincides with the optical axis of the measuring cavity.

[0009] Preferably, a highly sensitive microphone is sealed and installed in the middle of the composite spectrum measurement cavity, and the sensing part of the microphone is connected to the hollow thin cavity.

[0010] Preferably, the composite spectrum measurement cavity is provided with a protective gas port, an aerosol port to be measured and a mixed gas port, and the protective gas port is located on both sides of the composite spectrum measurement cavity; the aerosol port to be measured and the mixed gas port are both arranged on both sides of the composite spectrum measurement cavity, and are arranged near the middle of the hollow rough cavity.

[0011] Preferably, the input high reflector and the output high reflector are coated with a dielectric film, the front reflectivity of the high reflector is ≥99.9%, and the back of the high reflector can rotate the polarization direction of the incident positive line polarized light by 90°.

[0012] Preferably, the input window and the output window are coated with a high-transmittance film, which can realize low-loss laser incidence and emission, and can be wedge-shaped to suppress laser interference.

[0013] Preferably, the isolated transmission optical path is formed by laser passing through a polarization beam splitter cube, a mode matching and shaping module, a first laser reflector and a second laser reflector, an input window and an input high reflector in sequence. After the parallel polarized light from the laser diode passes through the input high reflector, a part of it is transmitted to the composite spectrum measurement cavity, and the other part is reflected. The polarization direction of the reflected light is rotated 90° to become vertically polarized light. The vertically polarized light returns to the polarization beam splitter cube along the original path, and is emitted from the optical path after reflection, forming optical isolation with the laser diode, thereby avoiding interference from light return.

[0014] Preferably, the isolated feedback optical path is formed by the output high reflector, the output window, the laser beam splitter, the third laser reflector, the second λ / 2 wave plate, the fourth laser reflector, the polarization beam splitter cube, the first λ / 2 wave plate, the laser collimator and the laser diode; the vertically polarized light from the laser diode is reflected by the output high reflector, and the polarization direction is rotated 90° to become horizontally polarized light, and the horizontally polarized light returns to the polarization beam splitter cube along the original path, and is emitted from the optical path after transmission, forming optical isolation with the laser diode, thereby avoiding interference from light return; the output horizontally polarized light from the composite spectrum measurement cavity after resonant frequency selection is reflected by the laser beam splitter and the third laser reflector, and the polarization direction is rotated 90° at the second λ / 2 wave plate to become vertically polarized light, and the vertically polarized light is reflected by the fourth laser reflector and the polarization beam splitter cube and then enters the laser diode, forming optical feedback with the laser diode, thereby achieving laser mode locking and line width narrowing.

[0015] Compared with the prior art, the present invention has the following advantages: light isolation and light feedback are achieved by using a single polarization beam splitter cube, and the structure is simple; multi-longitudinal mode laser diodes are used, and mode and phase matching is not required, and the cost is low and the implementation is easy. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Attached Figure 1 It is a schematic diagram of the structure of the present invention.

[0017] In the figure: 1. laser diode, 2. laser collimator, 3. first λ / 2 wave plate, 4. polarization beam splitter cube, 5. mode matching and shaping module, 6. first laser reflector, 7. second laser reflector, 8. composite spectrum measurement cavity, 9. laser beam splitter, 10. third laser reflector, 11. second λ / 2 wave plate, 12. fourth laser reflector, 13. laser detector, 14. phase-locked amplifier module, 15. high-speed acquisition card, 16. drive module, 17. display module, 18. protective gas port, 19. aerosol port to be measured, 20. mixed gas port, 81. input window, 82. input high reflector, 83. high-sensitivity microphone, 84. output high reflector, 85. output window. DETAILED DESCRIPTION

[0018] The technical solution of the present invention is further specifically described below through embodiments and in conjunction with the accompanying drawings.

[0019] Embodiment 1:

[0020] Basic composition: Figure 1As shown, an aerosol measurement device of multi-longitudinal mode light feedback composite spectrum mainly includes a laser diode 1, a laser collimator 2, a first λ / 2 wave plate 3, a polarization beam splitter cube 4, a mode matching and shaping module 5, a first laser reflector 6, a second laser reflector 7, a composite spectrum measurement cavity 8, a laser beam splitter 9, a third laser reflector 10, a second λ / 2 wave plate 11, a fourth laser reflector 12, a laser detector 13, a phase-locked amplifier module 14, a high-speed acquisition card 15, a drive module 16 and a display module 17, an input window plate 81, an input high reflector 82, a high-sensitivity microphone 83, an output high reflector 84, and an output window plate 85. An optical path module, the optical path module comprises a laser diode 1, a laser collimator 2 and a first λ / 2 wave plate 3 are sequentially arranged in the laser emission direction of the laser diode, and an isolated transmission optical path formed by the first λ / 2 wave plate 3 emitting through a multi-stage optical path, and an isolated feedback optical path formed by the cavity module emitting through a multi-stage optical path to reach the laser diode 1; a cavity module, the cavity module comprises a composite spectral measurement cavity 8 that enables the emitted laser to carry the optical property information of the aerosol to be measured; a laser beam splitter 9 that can split the laser beam into two is arranged behind the output end of the composite spectral measurement cavity 8, one of which is received by a laser detector 13 and the other is received by the isolation The feedback optical path is fed back to the laser diode 1; the signal processing module includes a high-sensitivity microphone 83 connected to the composite spectrum measurement cavity 8 and a laser detector 13 placed at the rear side of the output end of the composite spectrum measurement cavity 8, and the high-sensitivity microphone 83 is connected to a phase-locked amplifier module 14; the control module includes a high-speed acquisition card 15, a display module 17 and a drive module 16 connected in sequence, and the high-speed acquisition card 15 collects the signals processed by the laser detector 13 and the phase-locked amplifier module 14, and then the display module 17 receives, processes and displays, and finally controls the drive module 16 to drive the laser diode. The laser diode 1 is LDC205C, and when the temperature is controlled at 20°C and the current is controlled at 60mA, the output wavelength is 520nm, the emission power is 3mW, the spectral bandwidth is about 1.5GHz, and the longitudinal mode line width is about 3MHz. The laser collimator 2 is an aspherical lens with a focal length f of about 10mm. The laser reflectors are all gold-plated reflectors with a reflectivity of 95%. The mode matching and shaping module 5 is composed of a cylindrical lens, a plano-convex lens and an aperture, and the spot waist radius is about 400μm, ensuring that the waist radius and position of the outgoing laser match the composite spectrum measurement cavity 8 to achieve transverse mode matching of the laser. The composite spectrum measurement cavity 8 is made of metal copper, with a hollow interior, a hollow thin cavity in the middle, and hollow thick cavities symmetrically distributed at both ends. The surface of the internal cavity is polished, and the corners are rounded transition processed; the input high reflector and the output high reflector are coated with a dielectric film, and the reverse side of the high reflector can rotate the polarization direction of the positive incident line polarized light by 90°.The middle part of the composite spectrum measurement cavity 8 is a hollow thin cavity with a cavity diameter of D and a length of L. Hollow thick cavities are symmetrically distributed at both ends, with a cavity diameter of d and a length of l, satisfying D≤0.25d, L=2l; two windows are used at both ends of the composite spectrum measurement cavity for sealing installation; two high-reflection mirrors are installed at both ends of the measurement cavity facing each other through an adjustment device, and the curvature radii of the high-reflection mirrors are R1 and R2 respectively, satisfying. Two high-reflection mirrors are installed and fixed in the measurement cavity by an adjustment device. The high-reflection mirrors can be precisely adjusted in three dimensions through the adjustment device, so that the optical axis of the resonant cavity composed of the high-reflection mirrors coincides with the optical axis of the measurement cavity. Among them, for the hollow thin cavity, the cavity diameter is D=6mm, and the length is L=200mm. For the hollow thick cavity, the cavity diameter is d=36mm, and the length is l=100mm. Two 520nm anti-reflection film-coated window pieces are used for sealing installation at both ends of the composite spectrum measurement cavity 8. Two 520nm high-reflection mirrors are installed facing each other inside through an adjustment device. Precise three-dimensional adjustment can be achieved through the adjustment device to ensure that the optical axis of the resonant cavity composed of the high-reflection mirrors coincides with the optical axis of the measurement cavity. The curvature radius of the high-reflection mirrors is R1=R2=1000mm, and the reflectivity r reaches 99.99%, satisfying A highly sensitive microphone 83 is sealed and installed in the middle of the composite spectrum measurement cavity 8, and its sensing part is connected to the hollow thin cavity. The microphone is EK23133 with a sensitivity of 22mV / Pa. The high-speed acquisition card 15 is USB-6361, which has dual-channel input and TTL trigger control functions and a bandwidth of 50MHz. The drive module 16 adopts the ITC4001 controller, which has control functions such as current temperature drive and external triggering. The composite spectrum measurement cavity 8 is provided with a protective gas port 18, a test aerosol port 19 and a mixed gas port 20, wherein the protective gas port is located on both sides of the composite spectrum measurement cavity 8, close to the input high reflector 82 and the output high reflector 84, and the test aerosol port and the mixed gas port are both located on both sides of the composite spectrum measurement cavity 8, and are located near the middle of the hollow thick cavity. Before the measurement, nitrogen was used as a protective gas to prevent the high reflector from being contaminated, and the inflow gas flow was 50ml / min; during the measurement, the aerosol to be measured was pumped in at a flow rate of 1L / min from the aerosol port to be measured, and the mixed gas was pumped out from the mixed gas port at a flow rate of 1.1L / min. After the flow rate stabilized, the optical properties of the aerosol were measured. The input window and the output window are coated with a high-transmittance film to achieve low-loss laser incidence and emission, and can be wedge-shaped to suppress laser interference.

[0021] Working principle: In the present invention, the laser diode 1 emits a multi-longitudinal mode linear polarized laser, which is collimated by the laser collimator 2 and then emitted. After being rotated and adjusted by the first λ / 2 wave plate 3, it is polarized and separated by the polarization beam splitter cube 4, wherein the parallel polarized light is transmitted along the original transmission direction, and after being formed into a matching circular spot by the mode matching and shaping module 5, it is reflected by the first laser reflector 6 and the second laser reflector 7 in turn and transmitted to the composite spectrum measurement cavity 8. The laser beam injected into the cavity coincides with the optical axis of the composite spectrum measurement cavity 8, and the laser beam goes back and forth in the measurement cavity 8 and resonates, and is emitted through the output high reflector 84 of the measurement cavity. The emitted laser carries the optical property information of the aerosol to be measured, and is split into two by the laser beam splitter 9. One path is received by the laser detector 13, and the other path is reflected by the high-transmittance film 10 and then passes through the second λ / 2 wave plate 11. The parallel polarized light becomes vertically polarized light. The vertically polarized light is reflected by the fourth laser reflector 12, then reflected by the polarization beam splitter cube 4 and sequentially passes through the first λ / 2 wave plate 3 and the laser collimator 2 before being fed back to the laser diode 1. First, the laser in the measurement cavity 8 is absorbed and scattered by the aerosol, and the comprehensive benefit is manifested as an extinction characteristic. The laser carrying the extinction information is output through the output high-reflection mirror 84 to form an output ring-down spectrum signal, and the signal size is related to the extinction coefficient of the aerosol; second, the absorption effect of the aerosol causes heat to be generated in the cavity, thereby forming a sound signal, that is, a photoacoustic spectrum signal, and the signal size is related to the absorption coefficient of the aerosol. The high-sensitivity microphone 83 detects the photoacoustic spectrum signal in the measuring cavity and outputs it to the phase-locked amplifier module 14 for harmonic detection; the high-speed acquisition card 15 receives the ring-down spectrum signal obtained by the laser detector and the photoacoustic spectrum signal extracted by the phase-locked amplifier module 14, which are received and processed by the display module 17, and the extinction coefficient, absorption coefficient and scattering coefficient of the aerosol to be measured are calculated and displayed. At the same time, the output signal controls the driving module 16, and the driving module 16 drives and controls the laser diode 1 according to the control signal.

[0022] Implementation of optical feedback: output reflector 84, output window 85, laser beam splitter 9, third laser reflector 10, second λ / 2 wave plate 11, fourth laser reflector 12, polarization beam splitter cube 4, first λ / 2 wave plate 12, laser collimator 2 and laser diode 1 form an optical feedback optical path. The output horizontal polarized light after the composite spectrum measurement cavity 8 resonates and selects the mode is reflected by laser beam splitter 9 and third laser reflector 10. The polarization direction is rotated 90° at the second λ / 2 wave plate 12 to become vertical polarized light. The vertical polarized light is reflected by polarization beam splitter cube 4 and then enters laser diode 1, forming optical feedback with laser diode 1 to achieve laser mode locking and line width narrowing. The laser used is a multi-longitudinal mode laser diode with many longitudinal mode components, and there is no need to match the phase and longitudinal mode, which saves the scanning of cavity length and phase. It is low in cost, easy to implement and simple in structure.

[0023] Implementation of optical isolation: First, the polarization beam splitter cube 4, the mode matching and shaping module 5, the first laser reflector 6 and the second laser reflector 7, the input window 81 and the input high reflector 82 form an optical isolation optical path. After the parallel polarized light from the laser diode 1 is reflected by the first high reflector 82, the polarization direction is rotated 90° to become vertically polarized light. The vertically polarized light returns to the polarization beam splitter cube 4 along the original path, and emits the optical path after reflection, forming optical isolation with the laser diode 1, thereby avoiding light return interference; the polarization beam splitter cube 4, the fourth laser reflector 12, the second λ / 2 wave plate 11, the third laser reflector 10, the laser beam splitter 9, the second window 85 and the output high reflector 84 form an optical isolation optical path. After the vertical polarized light from the laser diode 2 is reflected by the second high reflector 84, the polarization direction is rotated 90° to become horizontally polarized light. The horizontally polarized light returns to the polarization beam splitter cube 4 along the original path, and emits the optical path after transmission, forming optical isolation with the laser diode 1, thereby avoiding light return interference. A single polarization beam splitter cube is used to realize an optical isolator, thereby avoiding the influence of return light on the laser diode, effectively improving the signal-to-noise ratio, and making the structure more compact and simple.

[0024] Measurement of composite spectrum: Since the laser in the measurement cavity 8 is absorbed and scattered (extinction) by the aerosol, the laser loss in the cavity increases, and the energy and decay time of the laser in the cavity will decrease. After quickly cutting off the current of the laser diode 1, the laser detector 13 is used to obtain the measurement cavity ring-down curve. The ring-down time has a certain relationship with the aerosol extinction coefficient. After data processing, the aerosol extinction coefficient is measured. Since the laser in the measurement cavity is absorbed by the aerosol, heat is generated after absorption to form a sound signal. The larger the aerosol absorption coefficient, the more heat is absorbed, and the stronger the sound signal is. The photoacoustic signal in the cavity has a certain relationship with the aerosol absorption coefficient. The high-sensitivity microphone 83 detects the photoacoustic signal in the measurement cavity and outputs it to the phase-locked amplifier module 14 for harmonic detection. After data processing, the aerosol absorption coefficient is measured. Using the obtained extinction coefficient and absorption coefficient, the scattering coefficient of the aerosol can be measured.

Claims

1. An aerosol measurement device with multi-longitudinal mode optical feedback composite spectrum, characterized in that: include An optical path module, the optical path module comprising a laser diode, a laser collimator and a first λ / 2 wave plate are sequentially arranged in the laser emission direction of the laser diode, an isolated transmission optical path formed by the first λ / 2 wave plate passing through a multi-stage optical path, and an isolated feedback optical path formed by the cavity module passing through a multi-stage optical path to reach the laser diode; A cavity module, the cavity module comprising a composite spectral measurement cavity capable of causing the emitted laser to carry the optical property information of the aerosol to be measured; a laser beam splitter capable of splitting the laser beam into two is arranged behind the output end of the composite spectral measurement cavity, one of which is received by the laser detector and the other is fed back to the laser diode through the isolated feedback optical path; A signal processing module, the signal processing module comprising a high-sensitivity microphone connected to the composite spectrum measurement cavity and a laser detector arranged at the rear side of the output end of the composite spectrum measurement cavity, the high-sensitivity microphone being connected to a phase-locked amplifier module; The control module includes a high-speed acquisition card, a display module and a driving module connected in sequence. The high-speed acquisition card collects the signals processed by the laser detector and the phase-locked amplifier module, and then the display module receives, processes and displays the signals. Finally, the driving module is controlled to realize the driving of the laser diode.

2. The aerosol measurement device of multi-longitudinal mode optical feedback composite spectrum according to claim 1 is characterized in that: The laser diode emits linearly polarized laser light, which contains multiple laser longitudinal mode components, has a radiation spectrum bandwidth of the GHz order of magnitude, and a single longitudinal mode line width of the MHz order of magnitude.

3. The aerosol measurement device of multi-longitudinal mode optical feedback composite spectrum according to claim 1, characterized in that: The composite spectrum measurement cavity is a metal hollow structure, with a hollow thin cavity in the middle and hollow thick cavities symmetrically distributed at both ends. The two ends of the composite spectrum measurement cavity are sealed and installed by an input window piece and an output window piece. An input high-reflection mirror and an output high-reflection mirror are installed inside the composite spectrum measurement cavity, and are coaxially installed at the two ends of the composite spectrum measurement cavity facing each other through an adjustment device.

4. The aerosol measurement device of multi-longitudinal mode optical feedback composite spectrum according to claim 3 is characterized in that: A high-sensitivity microphone is sealed and installed in the middle of the composite spectrum measurement cavity, and the sensing part of the high-sensitivity microphone is connected with the hollow thin cavity.

5. The aerosol measurement device of multi-longitudinal mode optical feedback composite spectrum according to claim 3 is characterized in that: The composite spectrum measurement cavity is provided with a protective gas port, a test aerosol port and a mixed gas port, wherein the protective gas port is located on both sides of the composite spectrum measurement cavity; the test aerosol port and the mixed gas port are both located on both sides of the composite spectrum measurement cavity, and are arranged near the middle of the hollow rough cavity.

6. The aerosol measurement device of multi-longitudinal mode optical feedback composite spectrum according to claim 3, characterized in that: The input high reflector and the output high reflector are plated with dielectric films, and the reverse side of the high reflector can rotate the polarization direction of the positive incident line polarized light by 90 degrees.

7. The aerosol measurement device of multi-longitudinal mode optical feedback composite spectrum according to claim 3 is characterized in that: The input window piece and the output window piece are plated with a high-transmittance film.

8. An aerosol measurement device for multi-longitudinal mode optical feedback composite spectrum according to any one of claims 1 or 2 or 3 or 4 or 5 or 6 or 7, characterized in that: The isolated transmission optical path is formed by laser passing through a polarization beam splitter cube, a mode matching and shaping module, a first laser reflector and a second laser reflector, an input window and an input high reflector in sequence. After the parallel polarized light from the laser diode passes through the input high reflector, a part of it is transmitted to the composite spectrum measurement cavity, and the other part is reflected. The polarization direction of the reflected light is rotated 90° to become vertically polarized light. The vertically polarized light returns to the polarization beam splitter cube along the original path, and is emitted from the optical path after reflection, forming optical isolation with the laser diode, thereby avoiding interference from light return.

9. An aerosol measurement device of multi-longitudinal mode optical feedback composite spectrum according to any one of claims 3, 4, 5, 6 or 7, characterized in that: The isolated feedback optical path is formed by the output high reflector, the output window plate, the laser beam splitter, the third laser reflector, the second λ / 2 wave plate, the fourth laser reflector, the polarization beam splitter cube, the first λ / 2 wave plate, the laser collimator and the laser diode; the vertical polarized light from the laser diode is reflected by the output high reflector, and the polarization direction is rotated 90° to become horizontal polarized light, and the horizontal polarized light returns to the polarization beam splitter cube along the original path, and is emitted from the optical path after transmission, and forms optical isolation with the laser diode, thereby avoiding interference of optical return; the output horizontal polarized light from the composite spectrum measurement cavity after resonant frequency selection is reflected by the laser beam splitter and the third laser reflector, and the polarization direction is rotated 90° at the second λ / 2 wave plate to become vertical polarized light, and the vertical polarized light is reflected by the fourth laser reflector and the polarization beam splitter cube and then enters the laser diode, and forms optical feedback with the laser diode, thereby realizing laser mode locking and line width narrowing.

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