A random noise modulated laser absorption spectroscopy system

Abstract

This invention discloses a random noise-modulated laser absorption spectroscopy system, relating to the field of trace gas detection technology in absorption spectroscopy. A signal generation module generates random noise with different powers, bandwidths, and distribution characteristics, as well as a low-frequency triangular wave. The random noise and low-frequency triangular wave are used to modulate the laser generated by the light source module. The laser generated by the light source module is incident into an optical multi-pass cell (i.e., a ring-shaped optical multi-pass cell) and oscillates. A signal processing module receives the outgoing light from the optical multi-pass cell and generates a first feedback signal and a second feedback signal based on the outgoing light. The signal generation module adjusts the random noise according to the first feedback signal. The light source module adjusts the laser incident angle according to the second feedback signal. This invention, by modulating the light source with random noise, suppresses the system's interference noise. While maintaining the physical size and structure of the optical multi-pass cell, it increases the number of mirror spots, improves the effective absorption optical path of the gas within the optical multi-pass cell, and thus achieves high-sensitivity detection of trace gases.

Description

Technical Field

[0001] This invention relates to the field of trace gas detection technology using absorption spectroscopy, and in particular to a random noise modulated laser absorption spectroscopy system. Background Technology

[0002] TDLAS (Transient Laser Absorption Spectroscopy) technology has attracted widespread attention in trace gas detection due to its advantages such as high environmental adaptability, fast response speed, and good selectivity. To analyze the causal mechanisms of air pollution, rapid field monitoring of these trace gases is required under various measurement conditions, including vehicle-mounted and airborne operations. Therefore, high demands are placed on the system's sensitivity and portability. Increasing the effective absorption optical path length of the gas is crucial to improving the sensitivity of the TDLAS system. For the same concentration of analyte gas, a longer absorption optical path results in a stronger absorption signal and higher system detection sensitivity. In laser absorption spectroscopy systems, to obtain a longer effective absorption optical path, one approach is to increase the number of light spots distributed on the mirrors, thereby increasing the number of reflections of the laser beam within the optical passage cell. However, when the number of light spots is too large, interference will occur between adjacent spots, increasing optical noise and affecting the system's detection sensitivity. Alternatively, aspherical mirrors with different structural parameters can be used as reflectors at both ends of the optical passage cell, forming a light spot distribution different from the traditional Herriot cell, thus improving mirror utilization. However, this type of optical passage cell design is relatively complex, and it is difficult to achieve a uniform distribution of light spot positions, as exemplified by the multi-mirror optical passage cell disclosed in Chinese patent CN114062315A. A second method is to increase the length of the optical passage cell, thereby increasing the absorption optical path while maintaining the same number of reflections. However, when the length of the optical passage cell is too long, the portability of the instrument is greatly reduced, making real-time measurement in the field inconvenient. Summary of the Invention

[0003] To overcome the shortcomings of the prior art, the present invention provides a laser absorption spectroscopy system with random noise modulation. By randomly modulating the light source, the interference noise of the system is suppressed. While ensuring the physical size and structure of the optical flux cell, the number of mirror spot distributions can be increased, thereby improving the effective absorption optical path of the gas.

[0004] To achieve the above objectives, the present invention adopts the following technical solution, including:

[0005] A random noise modulated laser absorption spectroscopy system, characterized by a light source module for generating laser light, a signal generation module for modulating laser light, an optical cell for the laser light to reciprocate after being injected, and a signal processing module.

[0006] The signal generation module is used to generate random noise with different power, bandwidth and distribution characteristics, as well as to generate low-frequency triangular waves, and to modulate the laser generated by the light source module using random noise and low-frequency triangular waves.

[0007] The laser generated by the light source module is incident on the optical flux cell and reciprocates;

[0008] The optical cell is a ring-shaped optical multi-pass cell;

[0009] The signal processing module is used to receive the emitted light from the optical flux cell and generate a first feedback signal and a second feedback signal based on the emitted light from the optical flux cell; the signal generation module adjusts the power, bandwidth and distribution characteristics of random noise based on the first feedback signal; the light source module adjusts the incident angle of the laser based on the second feedback signal.

[0010] Preferably, the signal processing module includes a photodetector and a computer;

[0011] The photodetector is used to receive the emitted light from the optical cell, convert it into an electrical signal, and send it to the computer for processing. The computer obtains the signal-to-noise ratio of the second harmonic based on the detection signal of the photodetector and generates a first feedback signal.

[0012] Preferably, the emitted light from the optical channel cell is split by a beam splitter; the signal processing module further includes a four-quadrant detector;

[0013] One beam of light, after being split by the beam splitter, enters the photodetector.

[0014] The other beam of light, after being split by the beam splitter, enters the four-quadrant detector to monitor the position of the light spot in real time and send the real-time position data of the light spot to the computer for processing. The computer senses the position change of the light spot based on the real-time position data of the light spot and generates a second feedback signal when the position of the light spot shifts.

[0015] Preferably, the light source module includes: a laser, an aperture, a first reflector, a second reflector, a laser driver module, and a laser temperature control module;

[0016] The laser is used to generate laser light, and the laser light emitted from the laser enters the optical cell after passing through the aperture, the first mirror and the second mirror in sequence.

[0017] The laser driver module is used to control the drive current of the laser; the laser temperature control module is used to control the temperature of the laser.

[0018] The signal generation module is connected to the laser driver module and injects random noise and low-frequency triangular waves into the laser driver module for laser modulation of the laser.

[0019] Preferably, the first and second reflectors are adjusted according to the second feedback signal to adjust the incident angle of the laser.

[0020] Preferably, the optical transmission cell has small holes on its body for the laser to enter and exit.

[0021] The advantages of this invention are:

[0022] (1) Under the premise of ensuring the physical size and structure of the optical flux cell, the present invention modulates the driving current of the laser by random noise, thereby widening the linewidth of the output laser and weakening its coherence. Even if the light incident into the optical flux cell undergoes multiple back-and-forth reflections and the back-and-forth beams overlap, there will be no serious interference effect. Therefore, the effective absorption optical path of the gas can be improved by increasing the number of light spots distributed on the mirror.

[0023] (2) By randomly modulating the light source, the present invention suppresses the interference noise of the system. While ensuring the physical size and structure of the optical cell, it can increase the number of mirror spot distributions. Specifically, by using a ring-shaped optical multi-pass cell, multiple spot distribution patterns are realized. That is, the number of spots can be changed by adjusting the laser incident angle, thereby increasing the effective absorption optical path of the gas in the optical cell and realizing high-sensitivity detection of trace gases.

[0024] (3) The present invention receives the emitted light from the optical cell through the signal processing module, can detect the signal-to-noise ratio of the second harmonic, and generate a first feedback signal. The signal generation module adjusts the power, bandwidth and distribution characteristics of random noise according to the first feedback signal, providing closed-loop feedback for the adjustment of random noise.

[0025] (4) The present invention adds a beam splitter and a four-quadrant detector at the position of the emitted light to sense the position change of the light spot. When the light spot shifts due to changes in external factors, a feedback signal can be provided to facilitate the adjustment of the first and second reflectors. The position change of the light spot can be achieved by adjusting the laser incident angle, thereby improving the stability of the system.

[0026] (5) The optical cell adopts a ring-shaped optical multi-pass cell, which is an integrated design. There is no need to adjust the internal structure of the optical cell. Only the external incident optical path needs to be adjusted to adjust the laser incident angle to achieve the change of the spot position, which simplifies the system structure. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of a laser absorption spectroscopy system with random noise modulation.

[0028] Figure 2 Schematic diagram of a ring-shaped optical multipass cell

[0029] Figure 3This is a schematic diagram of the half-spot distribution of a ring-shaped optical multipass cell. Detailed Implementation

[0030] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0031] Depend on Figure 1 As shown, a random noise modulated laser absorption spectroscopy system includes: a light source module 2 for generating laser light, a signal generation module 1 for modulating laser light, an optical flux cell 6 for reciprocating laser light after it enters the cell, and a signal processing module 10.

[0032] The light source module includes: laser 2, aperture 3, first reflector 4, second reflector 5, laser driver module and laser temperature control module.

[0033] In this embodiment, the emitted light from laser 2 passes sequentially through aperture 3, first reflector 4, and second reflector 5 before entering optical flux cell 6. Laser driver module 19 controls the drive current of laser 2. Laser temperature control module 1 controls the temperature of laser 2. The temperature and drive current of laser 2 directly affect the output performance of the light source; therefore, highly stable driver and temperature control modules are required, specifically including current control circuitry and temperature control circuitry. In this embodiment, laser 2 is an infrared quantum cascade laser (QCL).

[0034] The signal generation module 1 is used to generate random noise and low-frequency triangular waves with different powers, bandwidths, and distribution characteristics, and to modulate the laser 2 using the random noise and low-frequency triangular waves. Specifically, the random noise and low-frequency triangular waves are injected into the laser driver module 19 for laser modulation of the laser 2.

[0035] Depend on Figure 2 and Figure 3 As shown, the optical passage cell 6 is a ring-shaped optical multi-pass cell, with small holes on its body for the laser to enter and exit. The ring-shaped optical multi-pass cell is mainly composed of an integrated ring-shaped reflective mirror, with a reflective film coated on its inner wall and small holes for laser entry and exit. When the laser beam forms a stable distribution within the ring-shaped optical passage cell, the light path typically forms a "polygonal star" shape, with the light spot distribution being a uniform circular distribution around the inner wall of the ring-shaped reflective mirror. The ring-shaped optical multi-pass cell has a stable structure, and the absorption optical path can be changed by altering the incident angle, making it easy to adjust. Figure 3 This is a schematic diagram of the half-spot distribution of a ring-shaped optical multipass cell.

[0036] The light emitted from the optical cell 6 is split into beams by the beam splitter 7.

[0037] The signal processing module includes a photodetector 8, a four-quadrant detector 9, and a computer 10.

[0038] In this system, one beam of light from the beam splitter 7 enters the photodetector 8, where it is converted into an electrical signal and sent to the computer 10 for processing. The computer 10 obtains the signal-to-noise ratio of the second harmonic based on the detection signal from the photodetector 8 and generates a first feedback signal. The signal generation module 1 can adjust the power, bandwidth, and distribution characteristics of the random noise based on the first feedback signal.

[0039] The other beam of light from the beam splitter 7 enters the four-quadrant detector 9 to monitor the spot position in real time and send the real-time spot position data to the computer 10 for processing. The computer 10 senses changes in the spot position based on the real-time spot position data. When the spot position shifts due to changes in external factors, the computer 10 generates a second feedback signal. The first reflector 4 and the second reflector 5 can be adjusted according to the second feedback signal to adjust the incident angle of the laser, thereby changing the spot position and improving the stability of the system.

[0040] Computer 10 includes a processor and a display.

[0041] The above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A laser absorption spectroscopy system modulated by random noise, characterized in that, include: The system includes a light source module (2) for generating laser light, a signal generation module (1) for modulating laser light, a light transmission cell (6) for allowing laser light to reciprocate after being injected, and a signal processing module (10). The signal generation module (1) is used to generate random noise with different power, different bandwidth and different distribution characteristics, and to generate low-frequency triangular waves, and to modulate the laser generated by the light source module (2) using random noise and low-frequency triangular waves. The laser generated by the light source module (2) is incident into the optical cell (6) and reciprocates; The optical cell (6) is a ring-shaped optical multi-pass cell; The signal processing module (10) is used to receive the emitted light from the optical cell (6) and generate a first feedback signal and a second feedback signal based on the emitted light from the optical cell (6); the signal generation module (1) adjusts the power, bandwidth and distribution characteristics of random noise based on the first feedback signal; the light source module adjusts the incident angle of the laser based on the second feedback signal.

2. The random noise modulated laser absorption spectroscopy system according to claim 1, characterized in that, The signal processing module includes a photodetector (8) and a computer (10); The photodetector (8) is used to receive the emitted light from the light-passing cell (6), convert it into an electrical signal and send it to the computer (10) for processing; the computer (10) obtains the signal-to-noise ratio of the second harmonic based on the detection signal of the photodetector (8) and generates a first feedback signal.

3. The random noise modulated laser absorption spectroscopy system according to claim 2, characterized in that, The light emitted from the optical cell (6) is split by a beam splitter (7); the signal processing module also includes a four-quadrant detector (9); One beam of light after being split by the beam splitter (7) enters the photodetector (8); The other beam of light after being split by the beam splitter (7) enters the four-quadrant detector (9) to monitor the position of the light spot in real time and send the real-time position data of the light spot to the computer (10) for processing. The computer (10) senses the position change of the light spot according to the real-time position data of the light spot and generates a second feedback signal when the position of the light spot shifts.

4. The random noise modulated laser absorption spectroscopy system according to claim 1, characterized in that, The light source module includes: a laser (2), an aperture (3), a first reflector (4), a second reflector (5), a laser driving module, and a laser temperature control module; The laser (2) is used to generate laser light, and the emitted light from the laser (2) enters the optical cell (6) after passing through the aperture (3), the first reflector (4), and the second reflector (5) in sequence. The laser driving module (19) is used to control the driving current of the laser (2); the laser temperature control module (1) is used to control the temperature of the laser (2); The signal generation module (1) is connected to the laser driving module (19) and injects random noise and low-frequency triangular waves into the laser driving module (19) for laser modulation of the laser (2).

5. A random noise modulated laser absorption spectroscopy system according to claim 4, characterized in that, The first reflector (4) and the second reflector (5) are adjusted according to the second feedback signal to adjust the incident angle of the laser.

6. The random noise modulated laser absorption spectroscopy system according to claim 1, characterized in that, The optical cell (6) has small holes on its body for the laser to enter and exit.

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