Short-optical-path miniaturized laser methane sensor

Abstract

The invention relates to the technical field of sensors, and discloses a short-optical-path miniaturized laser methane sensor, which comprises a working box, the top of the working box is provided with a miniature gas chamber module and is reflected for multiple times through reflected light, so that the effective optical path is improved, and the gas chamber module is provided with a gas inlet and a gas outlet; the micro air chamber module comprises a first plane mirror, a second plane mirror and a concave mirror; the first plane reflecting mirror and the second plane reflecting mirror are installed at an asymmetric angle and used for forming a staggered reflecting light path, and the concave reflecting mirror is located at the bottom of an inner cavity of the micro air chamber module and used for focusing and diverging laser beams. According to the invention, through the multi-reflection design in the micro gas chamber module, especially the focusing and multi-reflection functions of the concave reflector, the effective optical path is obviously improved, so that the miniaturization of the sensor is maintained, and meanwhile, the high-sensitivity detection of methane gas is realized.

Description

Technical Field

[0001] The present invention relates to the technical field of sensors, and particularly to a short optical path miniaturized laser methane sensor. Background Technique

[0002] Natural gas is composed of alkanes such as methane, ethane, propane, and butane. Methane is the main component of natural gas and is a combustible gas. Under standard conditions, methane is a colorless and odorless gas. Methane is basically non-toxic to humans, but when the concentration of methane is too high, methane will reduce the oxygen content in the air, causing people in the same environment to be unable to breathe. Natural gas is usually transported through underground gas pipelines. Some pipelines leak or rupture due to lack of timely maintenance, causing irreparable losses. Therefore, it is usually necessary to set a laser methane sensor in the gas pipeline network to monitor the concentration of methane in real time.

[0003] After retrieval, the patent with the Chinese patent number CN108844895A discloses a laser methane sensor, including a sensor housing. A cooling fan is provided on the rear surface of the sensor housing. A fixing plate is provided at the rear side of the sensor housing. The sensor housing and the fixing plate are fixedly connected through a connecting plate. Rollers are provided on both sides of the fixing plate. The rollers and the fixing plate are rotatably connected through a rotating shaft. An installation plate is provided on one side of the fixing plate. A fixing block is provided at one end of the installation plate. A card slot is provided inside the installation plate. A fixing rod is provided on one side of the card slot; through the cleaning brush designed on the surface of the collection head, when the sensor is used for a long time, a lot of dust will be adsorbed on the surface of the collection head. When the dust accumulates to a certain extent, it will affect the collection of methane information by the collection head. The cleaning motor can be driven to drive the cleaning brush to rotate to clean the dust on the surface of the collection head, preventing the dust from affecting the collection of methane information by the collection head.

[0004] In the above technical solution, laser methane sensors generally rely on tunable diode laser absorption spectroscopy technology to achieve concentration detection through the absorption characteristics of methane molecules for laser with a specific wavelength. Therefore, the optical path length directly affects the detection sensitivity. Usually, a longer optical path is required to enhance the absorption signal, but this will lead to an increase in the volume of the sensor, making it difficult to meet the installation requirements of narrow spaces or portable devices. Moreover, limited by the length of the gas chamber and power consumption, it is difficult to balance the needs of miniaturization and high precision. Based on this, the present invention designs a short optical path miniaturized laser methane sensor to solve the above problems. Summary of the Invention

[0005] The purpose of the present invention is to provide a short optical path miniaturized laser methane sensor, which solves the problem of difficulty in meeting miniaturization and high precision in the background technique.

[0006] To solve the above technical problems, the present invention provides the following technical solutions:

[0007] A short optical path miniaturized laser methane sensor, comprising:

[0008] A working box, on the top of which a micro gas chamber module is installed and multiple reflections are carried out through reflected light, so as to improve the effective optical path. The micro gas chamber module includes a first plane mirror, a second plane mirror and a concave mirror;

[0009] The first plane mirror and the second plane mirror are installed at an asymmetric angle to form an interleaved reflection optical path. The concave mirror is located at the bottom of the inner cavity of the micro gas chamber module and is used for focusing the divergent laser beam. The concave mirror and the first plane mirror and the second plane mirror form an asymmetric reflection optical path to realize spiral optical path folding and improve the effective optical path;

[0010] A laser emission and reception unit for laser emission and reception. The laser emission and reception unit is connected to the micro gas chamber module through optical fiber coupling;

[0011] A signal processing module combines a digital quadrature lock-in amplifier implemented based on FPGA to extract second harmonic signals and combines a second harmonic detection algorithm to achieve a lower limit of methane concentration detection ≤ 10 ppm;

[0012] A temperature compensation unit adjusts the drive current in real time through a proportional-integral-derivative algorithm, stabilizes the gas chamber temperature at 25 ± 0.3 °C, and suppresses the laser wavelength drift within ±0.001 nm.

[0013] Preferably, the micro gas chamber module includes a protective seat installed on the top of the working box, an outer shell is installed on the top of the protective seat, the second plane mirror is installed on the top of the concave mirror, the first plane mirror is installed on the second plane mirror, and a rotating mechanism is also installed in the inner cavity of the working box.

[0014] Preferably, the rotating mechanism includes a support plate installed in the inner cavity of the working box, a servo motor is installed on the top of the support plate, the output shaft of the servo motor is fixedly connected with a rotating shaft, a rotating disk is installed on the top of the rotating shaft, the concave mirror is installed on the top of the rotating disk, and a limiting component is installed on one side of the second plane mirror.

[0015] Preferably, the limiting component includes a limiting groove opened on the top of the working box, a limiting piece is installed on one side of the second plane mirror, the limiting piece is slidably connected in the inner cavity of the limiting groove, and a buffer component is also installed in the inner cavity of the protective seat.

[0016] Preferably, the buffer assembly includes a buffer seat installed on the top of the working box. A connecting rod is installed on the inner wall of the buffer seat. One side of the connecting rod is installed with a buffer rod. One side of the buffer rod is installed with a buffer plate. One side of the buffer plate is installed with a buffer wheel.

[0017] Preferably, the laser emission and reception unit includes a support seat. A support rod is installed on the top of the support seat. A DFB laser is installed on the top of the support rod. A collimating lens is installed at the bottom of the DFB laser, and the collimating lens is an aspherical lens. A photodetector is installed on one side of the DFB laser.

[0018] Preferably, for a short optical path miniaturized laser methane sensor, the signal processing module includes the following sub-units:

[0019] A high-frequency modulation unit that generates a composite modulation signal of a triangular wave and a sine wave superposition, drives the injection current of the DFB laser, and realizes wavelength modulation spectroscopy scanning; a second harmonic detection unit that uses a digital quadrature lock-in amplifier to extract the second harmonic signal, and fits the absorption peak by the least squares method, with a detection limit ≤ 10 ppm; an adaptive baseline correction unit that based on a Kalman filter, deducts background noise in real time, with a correction residual ≤ ±1% FS and a cross-sensitivity ≤ ±2%; a dynamic gain control unit that automatically adjusts the gain of the photodetector according to the methane concentration range to ensure that the linearity error of the output signal ≤ ±0.5%;

[0020] The high-frequency modulation unit, the second harmonic detection unit, the adaptive baseline correction unit, and the dynamic gain control unit are all installed on the back of the inner cavity of the working box.

[0021] Preferably, the implementation steps of the digital quadrature lock-in amplifier are as follows:

[0022] Step S1, realizing digital down-conversion through FPGA hardware, with a sampling rate ≥ 10 MSPS and a passband ripple ≤ 0.01 dB; Step S2, using a CIC filter cascaded with an FIR filter, with an order of 128, a cut-off frequency of 1 kHz, and an out-of-band rejection ≥ 80 dB; Step S3, calculating the amplitude and phase of the second harmonic in real time, with a data output rate ≥ 10 Hz and a delay ≤ 50 ms.

[0023] Preferably, the temperature compensation unit includes a multi-region temperature monitoring module, a thermoelectric cooler, and a wavelength locking module. The temperature monitoring module includes multiple groups of platinum resistance temperature sensors, and the monitored temperature gradient ≤ 0.2 °C / cm; the maximum cooling power of the thermoelectric cooler is 5 W, the response time ≤ 30 s, and the temperature is controlled at 25 ± 0.3 °C through an H-bridge drive circuit; the wavelength locking module based on a proportional-integral-derivative algorithm adjusts the laser drive current in real time to suppress the laser wavelength drift within ±0.001 nm.

[0024] Preferably, the laser transmitting and receiving unit further includes an optical system for collimating and focusing the laser beam, and the optical system includes at least one convex lens and concave lens to ensure the accurate transmission and reception of the laser beam.

[0025] Compared with the prior art, the beneficial effects achieved by the present invention are as follows:

[0026] 1. In the present invention, through the multi-reflection design in the micro gas chamber module, especially the focusing and multiple reflection functions of the concave mirror, the effective optical path is significantly increased. Thus, while maintaining the miniaturization of the sensor, high-sensitivity detection of methane gas is achieved, solving the problem that the detection sensitivity of traditional laser methane sensors is limited due to insufficient optical path, enabling the sensor to be effectively applied in narrow spaces or portable devices.

[0027] 2. In the present invention, a high-precision DFB laser and a collimating lens are adopted to ensure the directivity and collimation of the laser beam, improving the interaction efficiency between the laser and methane gas. At the same time, the signal processing module adopts the high-frequency triangular wave superposed sine wave modulation technology, combined with the second harmonic detection algorithm, effectively distinguishing the methane absorption signal from the background noise, further improving the detection lower limit and accuracy.

[0028] 3. In the present invention, through multi-region temperature monitoring and closed-loop control of the thermoelectric cooler, the temperature stability in the micro gas chamber module is ensured, reducing the drift of the laser wavelength, thereby improving the measurement accuracy. The wavelength locking module adopts the PID control algorithm to monitor and adjust the laser drive current in real time, controlling the laser wavelength drift amount within a very small range, further enhancing the stability of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 is a front view three-dimensional structure schematic diagram of the present invention;

[0030] Figure 2 is a structure schematic diagram of the present invention;

[0031] Figure 3 is of the present invention Figure 2 enlarged view of part A;

[0032] Figure 4 is a side view structure schematic diagram of the present invention;

[0033] Figure 5 is of the present invention Figure 4 enlarged view of part B;

[0034] Figure 6 is a structure schematic diagram of the rotating mechanism of the present invention;

[0035] Figure 7Schematic diagram of the signal processing module and temperature compensation unit of the present invention;

[0036] Figure 8 Schematic structural diagram of the digital quadrature lock-in amplifier of the present invention.

[0037] Wherein: 1. Micro gas chamber module; 2. Laser emission and reception unit; 3. Signal processing module; 4. Temperature compensation unit; 5. Rotating mechanism; 6. Limiting component; 7. Buffer component; 101. Working box; 102. First plane mirror; 103. Second plane mirror; 104. Concave mirror; 105. Protection seat; 106. Outer shell; 201. Support seat; 202. Support rod; 203. DFB laser; 204. Collimating lens; 205. Photoelectric detector; 301. High-frequency modulation unit; 302. Second harmonic detection unit; 303. Adaptive baseline correction unit; 304. Dynamic gain control unit; 401. Multi-region temperature monitoring module; 402. Thermoelectric cooler; 403. Wavelength locking module; 501. Support plate; 502. Servo motor; 503. Rotating shaft; 504. Rotating disk; 601. Limiting groove; 602. Limiting member; 701. Buffer seat; 702. Connecting rod; 703. Buffer rod; 704. Buffer plate; 705. Buffer wheel. Specific embodiments

[0038] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0039] Embodiment 1;

[0040] Please refer to Figures 1 - 8 , in the embodiment of the present invention, a short optical path and miniaturized laser methane sensor includes:

[0041] A working box 101, on the top of the working box 101, a micro gas chamber module 1 is installed and multiple reflections are carried out through the reflected light, so as to improve the effective optical path. The micro gas chamber module 1 includes a first plane mirror 102, a second plane mirror 103 and a concave mirror 104;

[0042] The first planar mirror 102 and the second planar mirror 103 are installed at an asymmetric angle to form an interleaved reflection optical path. The concave mirror 104 is located at the bottom of the inner cavity of the micro gas chamber module 1 and is used to focus the diverging laser beam. The concave mirror 104, the first planar mirror 102, and the second planar mirror 103 form an asymmetric reflection optical path to achieve spiral optical path folding and improve the effective optical path.

[0043] The laser transmitting and receiving unit 2 is used for laser transmission and reception. The laser transmitting and receiving unit 2 is connected to the micro gas chamber module 1 through fiber optic coupling.

[0044] The signal processing module 3 combines a digital quadrature lock-in amplifier implemented based on FPGA to extract the second harmonic signal and combines a second harmonic detection algorithm to achieve a lower limit of methane concentration detection ≤ 10 ppm.

[0045] The temperature compensation unit 4 adjusts the driving current in real time through a proportional-integral-derivative algorithm, stabilizes the gas chamber temperature at 25 ± 0.3 °C, and suppresses the laser wavelength drift within ±0.001 nm.

[0046] The micro gas chamber module 1 includes a protective seat 105 installed on the top of the working box 101. The top of the protective seat 105 is installed with a housing 106. The second planar mirror 103 is installed on the top of the concave mirror 104. The first planar mirror 102 is installed on the second planar mirror 103. A rotating mechanism 5 is also installed in the inner cavity of the working box 101.

[0047] The rotating mechanism 5 includes a support plate 501 installed in the inner cavity of the working box 101. The top of the support plate 501 is installed with a servo motor 502. The output shaft of the servo motor 502 is fixedly connected with a rotating shaft 503. The top of the rotating shaft 503 is installed with a rotating disk 504. The concave mirror 104 is installed on the top of the rotating disk 504. A limiting component 6 is installed on one side of the second planar mirror 103.

[0048] The limiting component 6 includes a limiting groove 601 opened on the top of the working box 101. A limiting piece 602 is installed on one side of the second planar mirror 103. The limiting piece 602 is slidably connected in the inner cavity of the limiting groove 601. A buffer component 7 is also installed in the inner cavity of the protective seat 105.

[0049] The buffer component 7 includes a buffer seat 701 installed on the top of the working box 101. A connecting rod 702 is installed on the inner wall of the buffer seat 701. A buffer rod 703 is installed on one side of the connecting rod 702. A buffer plate 704 is installed on one side of the buffer rod 703. A buffer wheel 705 is installed on one side of the buffer plate 704.

[0050] The working principle of the embodiment of the present invention is as follows: In the working process of the short optical path miniaturized laser methane sensor, first, the laser emission and reception unit 2 is connected to the micro gas chamber module 1 through fiber optic coupling, and is responsible for emitting a laser beam into the micro gas chamber module 1. The laser beam first encounters the first planar mirror 102, and after the initial reflection, the laser beam continues to propagate to the second planar mirror 103 and is reflected again. At this time, the laser beam has completed two reflections, and the effective optical path has been initially increased; subsequently, the laser beam is directed to the concave mirror 104 located at the bottom of the inner cavity of the micro gas chamber module 1. The design of the concave mirror 104 is not only used to focus the divergent laser beam, but also further guides the laser beam to perform multiple reflections. Through its unique curved surface design, the laser beam forms multiple reflection paths inside the micro gas chamber module 1, thereby increasing the effective optical path.

[0051] While the laser beam is undergoing multiple reflections, the signal processing module 3 starts to work. This module uses the high-frequency triangular wave superposed sine wave modulation technology to modulate the laser beam, and combines the second harmonic detection algorithm to achieve accurate measurement of the methane concentration, and can effectively distinguish the methane absorption signal from the background noise, improve the detection limit, and ensure the accuracy of the measurement.

[0052] In order to maintain the stability of the laser wavelength, the temperature compensation unit 4 precisely adjusts the temperature inside the micro gas chamber module 1 through a closed-loop control algorithm. The temperature compensation unit 4 can real-time monitor the temperature inside the gas chamber and adjust the heating or cooling device as needed to ensure that the temperature inside the gas chamber is stable within a preset range.

[0053] In addition, the present invention also designs a rotating mechanism 5 to further optimize the reflection path of the laser beam inside the micro gas chamber module 1. The servo motor 502 drives the rotating shaft 503 and the rotating disk 504 to rotate, thereby driving the concave mirror 104 and the second planar mirror 103 to make small-angle adjustments. This adjustment can change the reflection angle of the laser beam, enabling it to more evenly cover the internal space of the micro gas chamber module 1 and improve the detection efficiency of methane gas.

[0054] In order to ensure the stability of the second planar mirror 103 during rotation, the present invention designs a limit component 6. The cooperation between the limit groove 601 and the limit member 602 limits the movement range of the second planar mirror 103 to prevent it from undergoing excessive offset or damage during rotation; at the same time, the introduction of the buffer component 7 provides additional protection for the entire system. When the rotating mechanism 5 works, the buffer wheel 705 slides along the connecting rod 702 and the buffer rod 703 under the guidance of the buffer plate 704, effectively absorbing and dispersing the vibration and impact force generated during rotation, and protecting the micro gas chamber module 1 and other sensitive components from damage.

[0055] Embodiment 2;

[0056] Please refer to Figures 1 - 8 In the embodiment of the present invention, the laser transmitting and receiving unit 2 includes a support base 201. A support rod 202 is installed on the top of the support base 201. A DFB laser 203 is installed on the top of the support rod 202. A collimating lens 204 is installed at the bottom of the DFB laser 203, and the collimating lens 204 is an aspherical lens. A photodetector 205 is installed on one side of the DFB laser 203.

[0057] The signal processing module 3 includes the following sub-units:

[0058] A high-frequency modulation unit 301 generates a composite modulation signal that is a superposition of a triangular wave and a sine wave, drives the injection current of the DFB laser 203, and realizes wavelength modulation spectroscopy scanning; a second harmonic detection unit 302 uses a digital quadrature lock-in amplifier to extract the second harmonic signal, and fits the absorption peak by the least squares method, with a detection limit ≤ 10 ppm; an adaptive baseline correction unit 303 based on a Kalman filter deducts background noise in real time, with a correction residual ≤ ±1% FS and a cross-sensitivity ≤ ±2%; a dynamic gain control unit 304 automatically adjusts the gain of the photodetector 205 according to the methane concentration range to ensure that the linearity error of the output signal ≤ ±0.5%;

[0059] The high-frequency modulation unit 301, the second harmonic detection unit 302, the adaptive baseline correction unit 303, and the dynamic gain control unit 304 are all installed on the back of the inner cavity of the work box 101.

[0060] The implementation steps of the digital quadrature lock-in amplifier are as follows:

[0061] Step S1, digital down-conversion is realized through FPGA hardware, with a sampling rate ≥ 10 MSPS and a passband ripple ≤ 0.01 dB;

[0062] Step S2, a CIC filter is cascaded with an FIR filter, with an order of 128, a cut-off frequency of 1 kHz, and an out-of-band rejection ≥ 80 dB;

[0063] Step S3, the amplitude and phase of the second harmonic are calculated in real time, with a data output rate ≥ 10 Hz and a delay ≤ 50 ms.

[0064] The temperature compensation unit 4 includes a multi-region temperature monitoring module 401, a thermoelectric cooler 402, and a wavelength locking module 403. The temperature monitoring module 401 includes multiple groups of platinum resistance temperature sensors, with a monitored temperature gradient ≤ 0.2 °C / cm; the maximum refrigeration power of the thermoelectric cooler 402 is 5 W, with a response time ≤ 30 s, and the temperature is controlled at 25 ± 0.3 °C through an H-bridge drive circuit; the wavelength locking module 403 based on a proportional-integral-derivative algorithm adjusts the laser drive current in real time to suppress the laser wavelength drift within ±0.001 nm.

[0065] The laser transmitting and receiving unit 2 further includes an optical system for collimating and focusing the laser beam. The optical system includes at least one convex lens and concave lens to ensure the precise transmission and reception of the laser beam.

[0066] The working principle of the embodiment of the present invention is as follows: In the laser transmitting and receiving unit 2, the DFB laser 203 is installed at the top of the support rod 202 as a light source, and the support rod is firmly connected to the support base 201. The DFB laser 203 generates a laser beam with a specific wavelength due to its characteristics of narrow linewidth and high stability. To improve the directivity and collimation of the laser beam, a collimating lens 204 of an aspherical lens is installed at the bottom of the DFB laser 203. The design of the collimating lens 204 ensures that the laser beam enters the micro gas chamber module 1 with the minimum divergence angle, thereby improving the interaction efficiency between the laser and methane gas.

[0067] After the laser beam is reflected multiple times inside the micro gas chamber module 1, part of the laser is absorbed by the methane gas, and the remaining part continues to propagate and is finally received by the photodetector 205. The photodetector 205 converts the received optical signal into an electrical signal, providing a basis for subsequent signal processing.

[0068] The high-frequency modulation unit 301 generates a composite modulation signal superimposed by a triangular wave and a sine wave. This signal is used to drive the injection current of the DFB laser 203, thereby realizing wavelength modulation spectroscopy scanning. This process scans the absorption spectrum line of methane by changing the output wavelength of the laser, so as to capture the absorption characteristics of methane.

[0069] Then, the second harmonic detection unit 302 uses a digital quadrature lock-in amplifier to extract the second harmonic signal. The digital quadrature lock-in amplifier first realizes digital down-conversion through FPGA hardware, samples the received signal at a sampling rate ≥ 10MSPS, and then filters the signal using a CIC filter cascaded with an FIR filter to remove high-frequency noise and interference. The adaptive baseline correction unit 303 deducts the background noise in real time based on the Kalman filter. The Kalman filter can dynamically estimate and update the state of the system, thereby effectively removing the background noise caused by factors such as environmental changes and instrument drift. This process ensures the accuracy and stability of the measurement results, with a calibration residual ≤ ±1% FS and a cross-sensitivity ≤ ±2%.

[0070] The dynamic gain control unit 304 automatically adjusts the gain of the photodetector 205 according to the methane concentration range. When the methane concentration is high, the gain is reduced to avoid signal saturation; when the methane concentration is low, the gain is increased to improve signal sensitivity. This process ensures that the linearity error of the output signal ≤ ±0.5%, thereby improving the dynamic range of the measurement.

[0071] In the temperature compensation unit 4, the multi-region temperature monitoring module 401 plays a crucial role. This module is built-in with multiple groups of high-precision platinum resistance temperature sensors, which are evenly distributed at key positions in the micro gas chamber module 1 and its surrounding environment to achieve fine monitoring of the temperature gradient. Through continuous monitoring, it ensures that the temperature gradient is strictly controlled within the range of ≤0.2 °C / cm, providing a stable thermal environment basis for the accurate measurement of methane gas.

[0072] The thermoelectric cooler 402, as the core component for temperature regulation, has a maximum refrigeration power of 5W and a response time of no more than 30 seconds, and can quickly respond to the feedback signal of the temperature monitoring module 401. Through the intelligent control of the H-bridge drive circuit, the thermoelectric cooler 402 precisely maintains the temperature of the micro gas chamber module 1 and its internal components within the narrow range of 25 ± 0.3 °C. This precise temperature control not only reduces the drift of the laser wavelength but also ensures the stability of the methane absorption characteristics, thereby improving the measurement accuracy.

[0073] The wavelength locking module 403 further enhances the stability of the system. This module adopts an advanced proportional-integral-derivative (PID) control algorithm to continuously monitor and adjust the drive current of the DFB laser 203. Through this mechanism, even in the face of minor temperature changes or external environmental disturbances, the wavelength locking module 403 can quickly respond and suppress the drift of the laser wavelength within ±0.001 nm.

[0074] Embodiment 3;

[0075] Provide a specific embodiment, providing a short optical path laser methane sensor based on tunable diode laser absorption spectroscopy (TDLAS). The overall dimensions of the sensor are 120×80×50 mm, the weight is 300 g, the operating voltage is 5V, the communication interface uses TTL level, supports the Modbus RTU protocol, the baud rate is 115200, the data bits are 8, the stop bit is 1, and there is no parity check. The sensor is built-in with a micro gas chamber module, adopting an asymmetric three-mirror system. The installation angles of the first plane mirror and the second plane mirror are 15° and 10° respectively, and the radius of curvature of the concave mirror is 20 mm, forming a spiral folded optical path, achieving an effective optical path of 5 m within a physical cavity length of 8 cm. The lower limit of methane detection reaches 10 ppm, the response time is 0.1 s, and the cross-sensitivity is ≤±2%.

[0076] The laser emission unit uses a distributed feedback (DFB) laser with a central wavelength of 1653.7 nm, an output power of 10 mW, a linewidth ≤ 2 MHz, and is paired with an aspherical collimating lens with a numerical aperture of 0.25 and a divergence angle ≤ 0.5 mrad. The photodetector selects an InGaAs photodiode with a responsivity of 1.2 A / W and a bandwidth of 10 MHz. The gain dynamic adjustment range of the preamplifier circuit is 60 - 100 dB. The signal processing module integrates a high-frequency modulation unit to generate a composite modulation signal of a 100 Hz triangular wave superimposed on a 10 kHz sine wave, driving the injection current of the laser to achieve wavelength scanning. The FPGA digital quadrature lock-in amplifier extracts the second harmonic signal at a sampling rate of 10 million times per second, and a 128-stage CIC-FIR filter cascade is used to achieve a cut-off frequency of 1 kHz with out-of-band rejection ≥ 80 dB.

[0077] The temperature compensation unit is configured with 4 groups of platinum resistance sensors to monitor the temperature gradient of the gas chamber ≤ 0.2 °C / cm. The maximum power of the thermoelectric cooler (TEC) is 5 W. The temperature of the gas chamber is stabilized at 25 ± 0.3 °C through the PID algorithm, and the laser wavelength drift is suppressed within ±0.001 nm.

[0078] The working process includes: the gas enters the gas chamber at a flow rate of 0.5 L / min. The servo motor dynamically adjusts the angle of the concave mirror to optimize the optical path. After the photodetector receives the attenuated optical signal, it undergoes adaptive baseline correction and dynamic gain control, and the output concentration data is transmitted through the RS485 interface. When the standard is exceeded, an audible and visual alarm is triggered with a delay ≤ 50 ms. The protection level of the sensor is IP65, the working temperature range is -20 °C to 55 °C, the humidity tolerance is ≤ 95% RH, it is applicable to coal mines, gas pipelines, and industrial sites, and the stability exceeds 180 days.

[0079] Working principle: In the working process of the short optical path miniaturized laser methane sensor, the laser emission and reception unit 2 is connected to the micro gas chamber module 1 through fiber optic coupling. A laser beam is emitted into the micro gas chamber module 1. After the laser beam is initially reflected by the first plane mirror 102 and the second plane mirror 103, it is directed to the concave mirror 104, which focuses and diverges the laser beam and guides it for multiple reflections, significantly increasing the effective optical path and enhancing the methane detection sensitivity.

[0080] Meanwhile, the signal processing module 3 uses the high-frequency triangular wave superimposed sine wave modulation technology to modulate the laser beam, and combines the second harmonic detection algorithm to effectively distinguish the methane absorption signal from the background noise, achieving accurate measurement of the methane concentration. The temperature compensation unit 4 precisely adjusts the temperature in the micro gas chamber module 1 through a closed-loop control algorithm, reducing the laser wavelength drift and improving the detection accuracy.

[0081] In addition, the servo motor 502 of the rotating mechanism 5 drives the rotating shaft 503 and the rotating disk 504 to rotate, driving the concave mirror 104 and the second plane mirror 103 to make small-angle adjustments, optimizing the laser beam reflection path, improving the detection efficiency. The limiting component 6 restricts the movement range of the second plane mirror 103 to prevent excessive deviation or damage. The buffer component 7 absorbs and disperses the vibration and impact force generated during rotation to protect sensitive components.

[0082] In the laser emission and reception unit 2, the DFB laser 203 generates a laser beam with a specific wavelength. The collimating lens 204 improves the collimation of the laser beam. After the laser beam is reflected multiple times inside the micro gas chamber module 1, the remaining part is received by the photodetector 205 and converted into an electrical signal. The high-frequency modulation unit 301 generates a composite modulation signal to drive the DFB laser 203 to achieve wavelength modulation spectroscopy scanning. The second harmonic detection unit 302 extracts the second harmonic signal. The adaptive baseline correction unit 303 deducts the background noise in real time. The dynamic gain control unit 304 automatically adjusts the gain of the photodetector 205 according to the methane concentration range to ensure the linearity of the output signal. The multi-region temperature monitoring module 401 of the temperature compensation unit 4 finely monitors the temperature gradient. The thermoelectric cooler 402 quickly responds to the feedback signal to maintain temperature stability. The wavelength locking module 403 adjusts the laser drive current in real time based on the PID algorithm to suppress the laser wavelength drift and further enhance the system stability.

[0083] Although the embodiments of the present invention have been shown and described, for those of ordinary skill in the art, it can be understood that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A short optical path miniaturized laser methane sensor, characterized in that Including: A working box (101), on the top of which a micro gas chamber module (1) is installed and multiple reflections are carried out through reflected light, so as to improve the effective optical path. The micro gas chamber module (1) includes a first plane mirror (102), a second plane mirror (103) and a concave mirror (104); The first plane mirror (102) and the second plane mirror (103) are installed at an asymmetric angle to form a staggered reflection optical path. The concave mirror (104) is located at the bottom of the inner cavity of the micro gas chamber module (1) and is used to focus the divergent laser beam. The concave mirror (104) and the first plane mirror (102) and the second plane mirror (103) form an asymmetric reflection optical path to realize spiral optical path folding and improve the effective optical path; A laser emission and reception unit (2), which is used for laser emission and reception. The laser emission and reception unit (2) is connected to the micro gas chamber module (1) through optical fiber coupling; A signal processing module (3), which combines a digital quadrature lock-in amplifier based on FPGA to extract second harmonic signals and combines a second harmonic detection algorithm to achieve a lower limit of methane concentration detection ≤ 10 ppm; A temperature compensation unit (4), which adjusts the drive current in real time through a proportional-integral-derivative algorithm, stabilizes the gas chamber temperature at 25 ± 0.3 °C, and suppresses the laser wavelength drift within ± 0.001 nm.

2. The short optical path miniaturized laser methane sensor according to claim 1, characterized in that: The micro gas chamber module (1) includes a protective seat (105) installed on the top of the working box (101). The top of the protective seat (105) is installed with a housing (106). The second plane mirror (103) is installed on the top of the concave mirror (104). The first plane mirror (102) is installed on the second plane mirror (103). A rotating mechanism (5) is also installed in the inner cavity of the working box (101).

3. The short optical path miniaturized laser methane sensor according to claim 2, characterized in that: The rotating mechanism (5) includes a support plate (501) installed in the inner cavity of the working box (101). The top of the support plate (501) is installed with a servo motor (502). The output shaft of the servo motor (502) is fixedly connected with a rotating shaft (503). The top of the rotating shaft (503) is installed with a rotating disc (504). The concave mirror (104) is installed on the top of the rotating disc (504). A limiting component (6) is installed on one side of the second plane mirror (103).

4. A short optical path miniaturized laser methane sensor according to claim 3, characterized in that: The limiting component (6) includes a limiting groove (601) opened on the top of the working box (101). A limiting part (602) is installed on one side of the second plane mirror (103). The limiting part (602) is slidably connected in the inner cavity of the limiting groove (601). A buffer component (7) is also installed in the inner cavity of the protective seat (105).

5. The short optical path miniaturized laser methane sensor according to claim 4, characterized in that: The buffer assembly (7) includes a buffer seat (701) installed on the top of the working box (101). A connecting rod (702) is installed on the inner wall of the buffer seat (701). A buffer rod (703) is installed on one side of the connecting rod (702). A buffer plate (704) is installed on one side of the buffer rod (703). A buffer wheel (705) is installed on one side of the buffer plate (704).

6. The short optical path miniaturized laser methane sensor according to claim 1, characterized in that: The laser emission and reception unit (2) includes a support seat (201). A support rod (202) is installed on the top of the support seat (201). A DFB laser (203) is installed on the top of the support rod (202). A collimating lens (204) is installed at the bottom of the DFB laser (203), and the collimating lens (204) is an aspherical lens. A photodetector (205) is installed on one side of the DFB laser (203).

7. The miniaturized laser methane sensor with a short optical path according to claim 6, characterized in that, The signal processing module (3) includes the following sub-units: A high-frequency modulation unit (301) that generates a composite modulation signal by superimposing a triangular wave and a sine wave, drives the injection current of the DFB laser (203), and realizes wavelength modulation spectroscopy scanning; a second harmonic detection unit (302) that uses a digital quadrature lock-in amplifier to extract the second harmonic signal and fits the absorption peak by the least squares method, with a detection limit ≤ 10 ppm; an adaptive baseline correction unit (303) that based on a Kalman filter, deducts background noise in real time, with a correction residual ≤ ±1% FS and a cross-sensitivity ≤ ±2%; a dynamic gain control unit (304) that automatically adjusts the gain of the photodetector (205) according to the methane concentration range to ensure that the linearity error of the output signal ≤ ±0.5%; The high-frequency modulation unit (301), the second harmonic detection unit (302), the adaptive baseline correction unit (303) and the dynamic gain control unit (304) are all installed on the back of the inner cavity of the working box (101).

8. A short optical path miniaturized laser methane sensor according to claim 7, characterized in that, The implementation steps of the digital quadrature lock-in amplifier are as follows: Step S1, realizing digital down-conversion through FPGA hardware, with a sampling rate ≥ 10 MSPS and a passband ripple ≤ 0.01 dB; Step S2, using a CIC filter cascaded with an FIR filter, with an order of 128, a cut-off frequency of 1 kHz, and an out-of-band rejection ≥ 80 dB; Step S3, calculating the amplitude and phase of the second harmonic in real time, with a data output rate ≥ 10 Hz and a delay ≤ 50 ms.

9. A short optical path miniaturized laser methane sensor according to claim 1, characterized in that, The temperature compensation unit (4) includes a multi-region temperature monitoring module (401), a thermoelectric cooler (402) and a wavelength locking module (403). The temperature monitoring module (401) contains multiple groups of platinum resistance temperature sensors, with a monitored temperature gradient ≤ 0.2 °C / cm; the maximum refrigeration power of the thermoelectric cooler (402) is 5 W, with a response time ≤ 30 s, and the temperature is controlled at 25 ± 0.3 °C through an H-bridge drive circuit; the wavelength locking module (403) based on a proportional-integral-derivative algorithm adjusts the laser drive current in real time to suppress the laser wavelength drift within ±0.001 nm.

10. A short optical path miniaturized laser methane sensor according to claim 1, characterized in that: The laser transmitting and receiving unit (2) further includes an optical system for collimating and focusing the laser beam, and the optical system includes at least one convex lens and concave lens to ensure the accurate transmission and reception of the laser beam.

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