Random noise modulation laser absorption spectrum system

Abstract

The invention discloses a laser absorption spectrum system for random noise modulation, which relates to the technical field of trace gas detection of absorption spectrum technology, and is characterized in that a signal generation module generates random noise and low-frequency triangular waves with different powers, different bandwidths and different distribution characteristics; laser generated by the light source module is modulated through random noise and low-frequency triangular waves; laser generated by the light source module enters the light flux cell, namely the annular optical multi-pass cell and reciprocates; the signal processing module receives emergent light of the light flux cell and generates a first feedback signal and a second feedback signal according to the emergent light of the light flux cell; the signal generation module adjusts random noise according to the first feedback signal; the light source module adjusts the laser incident angle according to the second feedback signal. According to the invention, random noise modulation is carried out on the light source, interference noise of the system is suppressed, and under the condition that the physical size and the structure of the light flux cell are ensured, the distribution number of mirror light spots is increased, and the effective absorption optical path of gas in the light flux cell is improved, so that high-sensitivity detection of trace gas is realized.

Description

Technical Field

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

[0002] Laser absorption spectroscopy (TDLAS) technology has the advantages of high environmental adaptability, fast response speed, and good selectivity, and has received widespread attention in trace gas detection. In order to analyze the causes and mechanisms of air pollution, it is necessary to conduct rapid field monitoring of these trace gases under various measurement conditions such as on-board and airborne, so there are high requirements for the sensitivity and portability of the system. To improve the sensitivity of the laser absorption spectroscopy system, it is extremely important to increase the length of the effective absorption optical path of the gas. For the same concentration of the gas to be tested, the longer the absorption optical path, the greater the absorption signal and the higher the detection sensitivity of the system. In the laser absorption spectroscopy system, to obtain a longer effective absorption optical path, one method is to increase the number of light spot distributions on the mirror surface, thereby increasing the number of reflections of the laser beam in the light flux pool. However, when the number of light spot distributions on the mirror surface is too large, interference will occur between adjacent light spots, thereby increasing the optical noise and affecting the detection sensitivity of the system. Aspherical mirrors with different structural parameters can also be used as reflectors at both ends of the light flux pool to form a light spot distribution different from that of the traditional Herriot pool, thereby improving the utilization rate of the mirror surface. However, the design of this light flux pool is relatively complex, and it is difficult to achieve a uniform distribution of the light spot position, such as the multi-mirror light flux pool disclosed in Chinese patent CN114062315A. The second method is to increase the length of the light flux pool to increase the absorption optical path under the same number of reflections, but when the length of the light flux pool is too long, the portability of the instrument will be greatly reduced, which is not convenient for real-time measurement in the field. Summary of the invention

[0003] In order to overcome the defects in the above-mentioned prior art, the present invention provides a random noise modulated laser absorption spectroscopy system, which suppresses the interference noise of the system by performing random noise modulation on the light source. While ensuring the physical size and structure of the light flux cell, the number of mirror light spot distributions can be increased, thereby improving the effective absorption optical path of the gas.

[0004] To achieve the above object, the present invention adopts the following technical solutions, including:

[0005] A random noise modulated laser absorption spectroscopy system, characterized by a light source module for generating laser light, a signal generating module for modulating the laser light, a light flux cell for the laser light to be injected and then reciprocated, and a signal processing module;

[0006] The signal generating module is used to generate random noises of different powers, different bandwidths and different distribution characteristics, and to generate low-frequency triangular waves, and to modulate the laser generated by the light source module using the random noises and the low-frequency triangular waves;

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

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

[0009] The signal processing module is used to receive the output light of the light flux pool, and generate a first feedback signal and a second feedback signal according to the output light of the light flux pool; the signal generating module adjusts the power, bandwidth and distribution characteristics of the random noise according to the first feedback signal; the light source module is adjusted according to the second feedback signal to adjust the incident angle of the laser.

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

[0011] The photoelectric detector is used to receive the output light of the light flux 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 and generates a first feedback signal based on the detection signal of the photoelectric detector.

[0012] Preferably, the outgoing light of the light flux cell is split by a beam splitter; the signal processing module further includes a four-quadrant detector;

[0013] A beam of light split by the beam splitter enters the photodetector;

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

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

[0016] The laser is used to generate laser light, and the output light of the laser passes through the aperture, the first reflector and the second reflector in sequence and then enters the light flux pool;

[0017] The laser driving module is used to control the driving current of the laser; the laser temperature control module is used to control the temperature of the laser;

[0018] The signal generating module is connected to the laser driving module, and injects random noise and low-frequency triangle wave into the laser driving module for laser modulation of the laser.

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

[0020] Preferably, a small hole for the laser to enter and emit is provided on the cell body of the light flux cell.

[0021] The advantages of the present invention are:

[0022] (1) While ensuring the physical size and structure of the light flux pool, the present invention modulates the driving current of the laser by random noise, so that the output laser line width is widened and its coherence is weakened. Even if the light incident into the light flux pool is reflected back and forth multiple times and the round-trip light beams overlap each other, no serious interference effect will occur. Therefore, the effective absorption optical path of the gas can be increased by increasing the number of light spot distributions on the mirror.

[0023] (2) The present invention suppresses the interference noise of the system by performing random noise modulation on the light source, thereby increasing the number of mirror light spot distributions while ensuring the physical size and structure of the light pass cell. Specifically, a ring-shaped optical multi-pass cell is used to achieve multiple light spot distribution patterns, that is, the number of light spots can be changed by adjusting the laser incident angle, thereby increasing the effective absorption optical path of the gas in the light pass cell, thereby achieving high-sensitivity detection of trace gases.

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

[0025] (4) The present invention adds a beam splitter and a four-quadrant detector at the position of the outgoing 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 reflector and the second reflector. The change of the light spot position is achieved by adjusting the laser incident angle, thereby improving the stability of the system.

[0026] (5) The luminous flux cell adopts a ring-shaped optical multi-pass cell with an integrated design. There is no need to adjust the internal structure of the luminous flux cell. It only needs to adjust the external incident light path, that is, adjust the laser incident angle to achieve the change of the spot position, which simplifies the system structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 Schematic diagram of a random noise modulated laser absorption spectroscopy system.

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

[0029] Figure 3Schematic diagram of the half-edge spot distribution of the annular optical multi-pass cell. DETAILED DESCRIPTION

[0030] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work 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 generating module 1 for modulating the laser light, a light flux pool 6 for the laser light to be injected and then reciprocated, and a signal processing module 10.

[0032] The light source module comprises: a laser 2, an aperture 3, a first reflector 4, a second reflector 5, a laser driving module and a laser temperature control module.

[0033] Among them, the output light of the laser 2 enters the light flux pool 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 temperature and driving current of the laser 2 directly affect the output performance of the light source, so a high-stability driving module and temperature control module are required, specifically including a current control circuit and a temperature control circuit. In this embodiment, the laser 2 adopts an infrared quantum cascade laser (QCL).

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

[0035] Depend on Figure 2 and Figure 3 As shown, the light-pass pool 6 is an annular optical multi-pass pool, and a small hole for the laser to enter and exit is arranged on the pool body of the light-pass pool 6. The annular optical multi-pass pool is mainly composed of an integrated annular reflective lens, the inner wall of which is coated with a reflective film layer and has small holes for the laser to enter and exit. When the laser beam forms a stable distribution in the annular light-pass pool, the light path usually forms a "polygonal star" shape, and the light spot distribution is a uniform circular distribution around the inner wall of the annular reflective lens. The annular optical multi-pass pool has a stable structure, and the absorption optical path can be changed by changing the incident angle, which is easy to adjust. Among them, Figure 3 Schematic diagram of the half-edge spot distribution of the annular optical multi-pass cell.

[0036] The outgoing light of the light flux cell 6 is split by a beam splitter 7 .

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

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

[0039] Another beam of light from the beam splitter 7 enters the four-quadrant detector 9, which is used to monitor the spot position in real time and send the spot real-time position data to the computer 10 for processing. The computer 10 senses the spot position change according to the spot real-time 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 to achieve the change of the spot position and improve the stability of the system.

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

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

Claims

1. A random noise modulated laser absorption spectroscopy system, characterized in that: include: A light source module (2) for generating laser light, a signal generation module (1) for modulating the laser light, a light flux pool (6) for the laser light to be injected and then reciprocated, and a signal processing module (10); The signal generating module (1) is used to generate random noises of different powers, different bandwidths and different distribution characteristics, and to generate low-frequency triangular waves, and to modulate the laser light generated by the light source module (2) using the random noise and the low-frequency triangular waves; The laser light generated by the light source module (2) is incident on the light flux pool (6) and reciprocates; The light-pass cell (6) is a ring-shaped optical multi-pass cell; The signal processing module (10) is used to receive the output light of the light flux pool (6), and to generate a first feedback signal and a second feedback signal according to the output light of the light flux pool (6); the signal generating module (1) adjusts the power, bandwidth and distribution characteristics of the random noise according to the first feedback signal; and the light source module is adjusted according to the second feedback signal to adjust the incident angle of the laser.

2. A random noise modulated laser absorption spectroscopy system according to claim 1, characterized in that: The signal processing module comprises a photoelectric detector (8) and a computer (10); The photoelectric detector (8) is used to receive the output light of the light flux cell (6), convert it into an electrical signal, and then 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 photoelectric detector (8) and generates a first feedback signal.

3. The random noise modulated laser absorption spectroscopy system according to claim 2, characterized in that: The outgoing light of the light flux pool (6) is split by a beam splitter (7); the signal processing module also includes a four-quadrant detector (9); A beam of light split by the beam splitter (7) enters a photoelectric detector (8); Another beam of light split by the beam splitter (7) enters a four-quadrant detector (9) for real-time monitoring of the light spot position and sending the light spot real-time position data to a computer (10) for processing. The computer (10) senses the position change of the light spot based on the light spot real-time position data and generates a second feedback signal when the light spot position shifts.

4. The random noise modulated laser absorption spectroscopy system according to claim 1, characterized in that: The light source module comprises: a laser (2), an aperture (3), a first reflector (4), a second reflector (5), a laser drive module and a laser temperature control module; The laser (2) is used to generate laser light, and the output light of the laser (2) passes through the aperture (3), the first reflector (4) and the second reflector (5) in sequence and then enters the light flux pool (6); 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 generating module (1) is connected to a laser driving module (19) and injects random noise and a low-frequency triangle wave into the laser driving module (19) for laser modulation of the laser (2).

5. The 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 cell body of the light flux cell (6) is provided with a small hole for the laser to enter and emit.

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