TDLAS detection system

Abstract

The utility model relates to a TDLAS (Tunable Diode Laser Absorption Spectroscopy) detection system, and relates to the technical field of environmental gas leakage detection. Comprising a laser which is connected with the input end of a 1 * 2 coupler. The first photoelectric detector is connected with the first output end of the 1 * 2 coupler; the air chamber is connected with the output end II of the 1 * 2 coupler; the second photoelectric detector is connected with the gas chamber; and the synchronous acquisition card is connected with the photoelectric detector I and the photoelectric detector II. According to the utility model, the detection precision of laser propane can be effectively increased, the required length of an optical path pool is effectively reduced, and the cost is reduced.

Description

Technical Field

[0001] The utility model relates to the technical field of environmental gas leakage detection, in particular to a TDLAS detection system based on a reference light path. Background Art

[0002] Liquefied petroleum gas (LPG) boasts the advantages of being clean, low-carbon, abundant, readily available, convenient to use, and safe and controllable. It is a key energy source for optimizing my country's energy mix and has played a crucial role in promoting the widespread use of natural gas in Chinese cities. Propane (C3H8) is the most abundant component in LPG, typically accounting for over 90%. This colorless, odorless gas has a high calorific value and a low boiling point, making it widely used for combustion and heating in homes and industries. It is also widely used as a raw material in the petrochemical industry, with an explosive limit in air of 2.2% to 9.5%. In recent years, LPG has accounted for nearly 30% of all gas users nationwide, making it a crucial supplement to natural gas. However, while its user base far outweighs natural gas, LPG accounts for a higher proportion of accidents and fatalities than natural gas. Therefore, the development of sensors that can safely and real-timely monitor gas concentrations is crucial.

[0003] Currently, mainstream gas detection solutions include semiconductor, catalytic combustion, infrared absorption, and laser. Research and comparative test data show that laser gas sensors demonstrate excellent performance under a variety of complex conditions in both home and commercial environments. Spectral absorption also offers advantages such as high accuracy, fast response, continuous measurement, and immunity to background gas interference.

[0004] Infrared absorption spectroscopy has broad potential for detecting combustible gases such as propane. However, the instability and short lifespan of most traditional infrared light sources (such as lamps and heat sources) make it difficult for sensors based on these sources to achieve long-term, stable, and accurate monitoring. Tunable diode laser absorption spectroscopy (TDLAS) is an infrared absorption spectroscopy technique that uses a tunable diode laser. Due to its advantages such as stable laser source, high spectral resolution, and simple system structure, it has been widely used in the detection of various combustible gases such as methane, ethylene, and acetylene. In addition, combined with fiber optic components, TDLAS can provide an ideal solution for combustible gas sensing in the above-mentioned industries because the transmission of infrared laser for monitoring is very simple when used in actual situations and does not require the use of mains power in the monitored area, making this sensor intrinsically safe and having great potential for small-scale in-situ propane monitoring that is very suitable for industrial and domestic applications.

[0005] In recent years, TDLAS technology has been widely used in various commercial and industrial scenarios, demonstrating excellent long-term stability and safety. However, traditional TDLAS solutions have not performed as expected when detecting trace gases with low absorption coefficients. This is because the nonlinear effect of the laser output will superimpose an interfering noise signal on the demodulated waveform. This noise interference is particularly obvious for gases with broad spectrum absorption, reducing detection sensitivity.

[0006] Therefore, how to provide a high-precision trace detection system has become a technical problem that technicians in this field urgently need to solve. Utility Model Content

[0007] In order to solve at least one technical problem in the background technology, the present invention provides a TDLAS detection system, which can effectively increase the detection accuracy of the TDLAS technology and effectively reduce the required optical path cell length.

[0008] To achieve the above objectives, the present invention provides a TDLAS detection system, comprising:

[0009] a laser connected to an input end of the 1×2 coupler;

[0010] A first photodetector connected to an output terminal 1 of the 1×2 coupler;

[0011] an air chamber connected to the second output terminal of the 1×2 coupler;

[0012] A second photoelectric detector connected to the gas chamber;

[0013] A synchronous acquisition card is connected to the first photoelectric detector and the second photoelectric detector.

[0014] Furthermore, the gas chamber is provided with opening 1, opening 2 and a cavity, and opening 2 is used to connect the cavity with photodetector 2; the 1×2 coupler is provided with an input end, output end 1 and output end 2, the input end is connected to the laser, the output end 1 is connected to the cavity through opening 1, and the output end 2 is connected to photodetector 1.

[0015] Furthermore, it also includes at least one reflector, which is installed on the inner wall of the cavity, and the modulated signal laser emitted by the laser is reflected by the reflector to the second photodetector.

[0016] Furthermore, the reflector is a gold-plated reflector.

[0017] Furthermore, the air chamber is made of aluminum material, and a collimating mirror with an inclination angle is provided on one surface of the opening of the air chamber.

[0018] Furthermore, it also includes a laser temperature control chip, which is electrically connected to the laser and is used to accurately control the temperature of the laser.

[0019] The beneficial effects of the present invention are:

[0020] First, the gas chamber adopts an open structure, which can facilitate the entry of external gas into the cavity for trace gas concentration detection;

[0021] Second, due to the use of a reference optical path, the influence of the laser's own power jitter and residual amplitude noise can be eliminated compared to traditional laser detection systems. In other words, the TDLAS detection system of the utility model achieves high-precision measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is an overall schematic diagram of the TDLAS detection system of the present utility model.

[0023] Figure 2 This is a schematic structural diagram of the air chamber of the present utility model.

[0024] Figure 3 This is a schematic diagram of the butterfly packaged laser of the present invention.

[0025] Figure 4 This is a schematic diagram of the photoelectric detector of the present invention (photoelectric detector 1 and photoelectric detector 2 are of the same model).

[0026] Figure 5 This is a schematic diagram of a 1×2 coupler of the present invention.

[0027] Figure 6 This is a normalized second harmonic concentration signal diagram detected at multiple concentrations according to the present invention.

[0028] Figure 7 This is a fitted line graph of the detection signal and concentration after calibration of the utility model.

[0029] Among them, 1-gas chamber, 101-opening 1, 102-opening 2, 103-cavity, 104-air inlet, 105-air outlet, 2-laser, 3-photodetector 1, 4-photodetector 2, 5-1x2 coupler, 501-input end, 502-output end 1, 503-output end 2. DETAILED DESCRIPTION

[0030] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0031] It should be noted that the terms "first", "second", etc. in the specification and claims of the present application and the above-mentioned drawings are used to distinguish similar objects and are not necessarily used to describe a specific order or sequential order. It should be understood that the data used in this way can be interchanged where appropriate, so that the embodiments of the present application described herein. In addition, the terms "including" and "having" and any of their variations are intended to cover non-exclusive inclusions. For example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0032] In this application, terms such as "upper," "lower," "left," "right," "front," "back," "top," "bottom," "inner," "outer," "center," "vertical," "horizontal," "transverse," and "longitudinal" indicate positions or locations based on the positions or locations shown in the accompanying drawings. These terms are primarily intended to better describe this application and its embodiments and are not intended to limit the devices, elements, or components indicated to having a specific orientation, or to being constructed or operated in a specific orientation.

[0033] Furthermore, some of the above terms may be used to express other meanings besides indicating a position or location. For example, the term "on" may also be used to indicate a dependency or connection in certain circumstances. Those skilled in the art will understand the specific meanings of these terms in this application based on the specific circumstances.

[0034] Furthermore, the terms "installed," "disposed," "provided with," "connected," "connected," and "socketed" should be interpreted broadly. For example, they can refer to fixed connections, removable connections, or integral structures; mechanical connections or electrical connections; direct connections, indirect connections through an intermediary, or internal communication between two devices, elements, or components. Those skilled in the art will understand the specific meanings of these terms in this application based on the specific circumstances.

[0035] When considering how to obtain a highly sensitive detector for trace gases (for example, propane molecules), the laser's relative intensity noise (RIN) is a crucial component of the system. Within the frequency range of interest, the intensity of RIN is inversely proportional to the frequency, exhibiting the characteristics of 1 / f noise, while other noises, such as shot noise and thermal noise, exhibit white noise characteristics. Therefore, by modulating the light source, the gas absorption signal can be shifted to a higher-frequency range with lower noise intensity, significantly improving the system's signal-to-noise ratio. To achieve higher detection accuracy, it's necessary to thoroughly consider several 2f background signal sources in experimental measurements. These sources can be broadly categorized into three types: background signals caused by etalon effects in the optical path; background signals caused by nonlinear intensity modulation in the laser; and background absorption from external gases due to chamber sealing issues. In recent years, research on laser propane detection technology has primarily focused on the mid-infrared and near-infrared bands. The high cost of mid-infrared lasers and optical fibers has limited the practicality of this technology. Consequently, increasing interest has been placed on gas monitoring technology in the near-infrared band. However, the absorption coefficient of gas molecules in this band is often lower than that in the mid-infrared band. This embodiment, by employing a reference optical path, can effectively increase the accuracy of laser propane detection and reduce the required optical path length. Furthermore, the use of near-infrared detection technology can reduce laser and optical fiber costs.

[0036] Because extremely stable output current sources are expensive and the influence of driver circuit noise is difficult to avoid, to eliminate these effects, this embodiment uses a 1×2 fiber coupler after the laser output to obtain two identical outputs. One output passes through the gas chamber, where propane molecules absorb it slightly, and the other is directly received by the PD. Both are received by a synchronous acquisition card and transmitted to a computer for data processing. Background subtraction is used to improve the detection limit (to eliminate the influence of background absorption and nonlinear intensity modulation effects), which can be expressed as follows:

[0037]

[0038] in, is the final signal processing result, X 2f and Y 2f It represents the two-way output signal when the reference signal obtained according to the principle of the phase-locked amplifier is twice the sinusoidal frequency of the original signal, R 1f It represents the final output signal when the sinusoidal frequency of the reference signal is equal to that of the original signal. The subscript raw represents the detection optical path, and the subscript bg represents the background reference optical path.

[0039] To achieve the above purpose, Figure 1-5As shown, this embodiment provides a near-infrared laser propane detection system based on a reference optical path, including:

[0040] Laser 2, the laser 2 is connected to the input end 501 of the 1×2 coupler, and the lasing wavelength of the laser 2 is 1686 nm;

[0041] A photodetector 3 connected to an output terminal 502 of the 1×2 coupler;

[0042] Air chamber 1, said air chamber 1 being connected to the second output terminal 503 of said 1×2 coupler;

[0043] A second photoelectric detector 4, wherein the second photoelectric detector 4 is connected to the gas chamber 1;

[0044] A synchronous acquisition card connected to the photoelectric detector 1 3 and the photoelectric detector 2 4;

[0045] Laser 2 generates a modulated signal laser, which is divided into two identical laser beams through a 1×2 coupler. Laser one is directly received by photodetector one 3, and laser two is received by photodetector two 4 after passing through gas chamber 1. The synchronous acquisition card uses an ADC to receive the signals received by photodetector one 3 and the signals received by photodetector two 4. The detection signals are processed by the second harmonic concentration demodulation method of background difference to obtain the propane concentration in cavity 103.

[0046] The 1×2 coupler divides the laser light into two completely identical light paths. After processing, the two laser paths can eliminate the influence of the power fluctuation noise and residual amplitude noise of the laser 2 itself.

[0047] The gas chamber 1 has an opening 101, a second opening 102, and a cavity 103. The second opening 102 is used to connect the cavity 103 to the second photodetector 4. The 1×2 coupler has an input end 501, an output end 502, and a second output end 503. The input end 501 is connected to the laser 2, the first output end 502 is connected to the cavity 103 through the first opening 101, and the second output end 503 is connected to the first photodetector 3. The gas chamber 1 is made of aluminum, and the surface of the first opening 101 of the gas chamber 1 is provided with an angled collimating mirror. Laser 2 is connected to the input end 501 of the 1x2 coupler 5 via a flange. Output end 102 is connected to a fiber collimator and mounted in the corresponding opening 101 in the air chamber 1. This fixed position facilitates the generated laser light to reflect back and forth along a predetermined optical path within the cavity 103 and exit through opening 2 102 onto photodetector 2 4. Output end 2 503 of the 1x2 coupler is directly connected to photodetector 1 3. Once the overall structure is complete, it exhibits excellent stability and repeatability. Air chamber 1 is provided with an air inlet 104 and an air outlet 105. The shape, size, and location of air inlet 104 and air outlet 105 are not specifically limited herein, as long as they facilitate the entry of external gas into the cavity 103. Openings 101 and 2 102 of air chamber 1 also provide mounting locations for other functional components. For example, photodetector 1 3, photodetector 2 4, and 1x2 coupler 5 can be mounted on air chamber 1. The surface of the air chamber 1 is electroplated with a corrosion-resistant and oxidation-resistant layer using electrophoresis technology to increase its service life.

[0048] This embodiment further includes at least one reflector mounted on the inner wall of the cavity 103. The modulated signal laser emitted by the laser 2 is reflected by the reflector to the photodetector 2 4. Preferably, the reflector is a gold-plated reflector.

[0049] This embodiment further includes a temperature control chip for the laser 2 , which is electrically connected to the laser 2 and is used to precisely control the temperature of the laser 2 .

[0050] To further optimize the technical solution, the second harmonic concentration demodulation method of the background difference is to demodulate the detection signal based on the signals of the two-way photodetector 1 3 and the photodetector 2 4 collected by the synchronous acquisition card and then perform linear calibration to obtain the propane concentration in the gas chamber 1.

[0051] The gas chamber of this embodiment adopts an open structure, which can facilitate the entry of external gas into the cavity for propane concentration detection; due to the adoption of a reference optical path solution, compared with traditional laser propane detection systems, the influence of the laser's own power jitter and residual amplitude noise can be eliminated. In other words, the laser propane detection system of this embodiment achieves high-precision measurement; this embodiment uses near-infrared band lasers and related optical fiber equipment, which greatly reduces the cost compared with mid-infrared detection solutions.

[0052] The synchronous acquisition card is electrically connected to photodetector 1 3 and photodetector 2 4 to ensure the synchronization of the two collected signals for subsequent signal processing and demodulation. A phase difference exists between the two signals if they are not synchronized, introducing an error during reference comparison. Using a synchronous acquisition card can eliminate this error, improving detection accuracy. Traditional TDLAS systems superimpose a high-frequency modulated sinusoidal signal on a sawtooth wave, shifting the noise to the high-frequency range. This improves the system's signal-to-noise ratio (SNR). A phase-locked amplifier is added to the signal processing section for harmonic extraction and calculation of the gas concentration to be measured. However, several sources of 2f background noise still exist: the background signal caused by the etalon effect in the optical path; the background signal caused by the nonlinear intensity modulation effect in laser 2; and the residual amplitude modulation introduced by the asynchrony between the wavelength modulation and intensity modulation of the laser. By adopting the second harmonic concentration demodulation method of the reference gas chamber 1, the noise effects caused by the nonlinear modulation of the laser 2 itself and the residual amplitude modulation can be eliminated. On the other hand, since both the first harmonic and the second harmonic are proportional to the light intensity, the division between the two can eliminate the influence of light intensity fluctuations during long-term measurement. The resulting propane concentration in the cavity 103 is more accurate, which means that the near-infrared laser propane detection system based on the reference optical path of this embodiment has high detection accuracy. Figure 7 The figure shows the calibration data and fitting line diagram of the detection module concentration and the second harmonic concentration demodulation method based on the reference optical path. The linear correlation is greater than 0.99, and the linearity is good. Figure 6 It can be seen that there is a significant difference between the detection coefficients of zero-concentration propane gas and those with propane gas, which means that the signal-to-noise ratio is good.

[0053] The above description is merely a preferred embodiment of the present invention and does not limit the technical scope of the present invention. Therefore, any minor modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention are still within the scope of the technical solution of the present invention.

Claims

1. A TDLAS detection system, characterized in that: include: a laser connected to an input end of the 1×2 coupler; A first photodetector connected to an output terminal 1 of the 1×2 coupler; an air chamber connected to the second output terminal of the 1×2 coupler; A second photoelectric detector connected to the gas chamber; A synchronous acquisition card is connected to the first photoelectric detector and the second photoelectric detector.

2. A TDLAS detection system according to claim 1, characterized in that: The gas chamber is provided with opening 1, opening 2 and a cavity, and opening 2 is used to connect the cavity with photodetector 2; the 1×2 coupler is provided with an input end, output end 1 and output end 2, the input end is connected to the laser, the output end 1 is connected to the cavity through opening 1, and the output end 2 is connected to photodetector 1.

3. A TDLAS detection system according to claim 2, characterized in that: It also includes at least one reflecting mirror, which is installed on the inner wall of the cavity. The modulated signal laser emitted by the laser is reflected by the reflecting mirror to the second photodetector.

4. A TDLAS detection system according to claim 3, characterized in that: The reflecting mirror is a gold-plated reflecting mirror.

5. A TDLAS detection system according to claim 4, characterized in that: The air chamber is made of aluminum material, and a collimating mirror with an inclination angle is arranged on one surface of the opening of the air chamber.

6. A TDLAS detection system according to claim 5, characterized in that: It also includes a laser temperature control chip, which is electrically connected to the laser and is used to accurately control the temperature of the laser.

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