TDLAS (Tunable Diode Laser Absorption Spectroscopy) detection system and method for eliminating nonlinear influence of laser based on reference light path

Abstract

The invention relates to a TDLAS (Tunable Diode Laser Absorption Spectroscopy) detection system and method for eliminating nonlinear influence of a laser based on a reference light path, and relates to the technical field of environmental gas leakage detection. A laser is connected with the input end of a 1 * 2 coupler; the photoelectric detector is connected with the output end of the 1 * 2 coupler; the air chamber is connected with the output end of the 1 * 2 coupler; the photoelectric detector is connected with the air chamber; the laser generates modulation signal laser, the modulation signal laser is divided into two paths of laser through the 1 * 2 coupler, the laser is received by the photoelectric detector, the laser is received by the photoelectric detector after passing through the gas chamber, and the synchronous acquisition card receives signals received by the photoelectric detector I and the photoelectric detector through the ADC. And processing the detection signal through a second harmonic concentration demodulation method of a background difference value to obtain the gas concentration in the gas chamber. According to the invention, the detection precision of the TDLAS technology can be effectively improved, the required length of the optical path pool is effectively reduced, and the overall cost is effectively reduced.

Description

Technical Field

[0001] The invention relates to the technical field of environmental gas leakage detection, and in particular to a TDLAS detection system and method for eliminating nonlinear effects of lasers based on a reference optical path. Background Art

[0002] Liquefied petroleum gas (LPG) has the advantages of clean and low carbon, abundant sources, high accessibility, convenient use, safety and controllability. It is one of the important energy sources for optimizing my country's energy structure and has played a very important role in promoting the popularization of urban gas in my country. Propane (C3H8) is the highest content component in liquefied petroleum gas, which generally accounts for more than 90%. It is a colorless and odorless gas with a high combustion calorific value and a low boiling point. Therefore, it is widely used in combustion and heating in households and industries. In addition, it is also widely used as a raw material in the petrochemical industry, and its explosion limit in air is 2.2% to 9.5%. In recent years, the proportion of liquefied petroleum gas in users of various gas sources in the country has reached nearly 30%, which is an important supplement to the energy structure other than natural gas. However, under the premise that the number of its users is far lower than that of natural gas, the number of accidents and the number of casualties account for more than natural gas. Therefore, the research and development of sensors for safe and real-time monitoring of gas concentration is crucial.

[0003] At present, the mainstream gas detector solutions are mainly semiconductor, catalytic combustion, infrared absorption and laser. According to the test comparison data of the research, laser gas sensors have shown excellent performance in various complex conditions in household and industrial and commercial scenarios. At the same time, the spectral absorption method also has the advantages of high accuracy, fast response, continuous measurement, and not easily affected by background gas interference.

[0004] Infrared absorption spectroscopy has broad potential in detecting combustible gases such as propane, however, the instability and short life 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, acetylene, etc. In addition, combined with fiber optic components, TDLAS can provide an ideal solution for combustible gas sensing in the above industries, because the transmission of infrared laser for monitoring is very simple when used in actual situations, and there is no need to use main power in the monitored area, making this sensor intrinsically safe and has 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, the traditional TDLAS solution does not show the expected detection effect when facing the detection of trace gases with certain low absorption coefficients. This is because the nonlinear effect of the laser output will superimpose an interfering noise signal on the demodulated waveform, especially for gases with wide spectrum absorption. This noise interference is more obvious, which reduces the detection sensitivity.

[0006] Therefore, how to provide a high-precision trace gas detection system and method, especially a high-precision detection system and method for trace gases with broad spectrum absorption, has become a technical problem that technical personnel in this field urgently need to solve. Summary of the invention

[0007] To solve at least one technical problem in the background technology, the present invention provides a TDLAS detection system and method based on a reference optical path to eliminate the nonlinear effects of the laser, 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 object, the present invention provides a TDLAS detection system based on a reference optical path to eliminate the nonlinear effect of the laser, comprising:

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

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

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

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

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

[0014] The laser generates a modulated signal laser, which is divided into two completely identical lasers through a 1×2 coupler. Laser one is directly received by photodetector one, and laser two is received by photodetector two after passing through the gas chamber. The synchronous acquisition card uses ADC to receive the signal received by photodetector one and the signal received by photodetector two, and processes the detection signal through the second harmonic concentration demodulation method of background difference to obtain the gas concentration in the gas chamber.

[0015] Furthermore, the air chamber is provided with opening one, opening two and a cavity, and opening two is used to connect the cavity with photodetector two; the 1×2 coupler is provided with an input end, output end one and output end two, the input end is connected to the laser, the output end one is connected to the cavity through opening one, and the output end two is connected to photodetector one.

[0016] 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.

[0017] Furthermore, the reflector is a gold-plated reflector.

[0018] Furthermore, the air chamber is made of aluminum material, and a collimating mirror with an inclination angle is arranged on a surface of the opening of the air chamber.

[0019] 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.

[0020] Furthermore, the second harmonic concentration demodulation method of the background difference demodulates the detection signal according to the signals of the two-way photodetector 1 and the photodetector 2 collected by the synchronous acquisition card, and then performs linear calibration to obtain the gas concentration in the gas chamber.

[0021] A TDLAS detection method based on a reference optical path to eliminate the nonlinear effect of a laser, comprising:

[0022] The laser generates pulsed laser, which is divided into two identical laser beams through the input end of the 1x2 coupler and transmitted to output end 1 and output end 2. Laser 1 is directly received by photodetector 1 to generate a detection signal. The laser output from output end 1 is reflected back and forth in the cavity and incident on photodetector 2 through opening 2, causing the photodetector to generate a detection signal. The two detection signals are received by the synchronous acquisition card and processed by the PC data to obtain the gas concentration in the gas chamber.

[0023] The beneficial effects of the present invention are:

[0024] First, the gas chamber adopts an open structure, which can facilitate the entry of external gas into the cavity for trace gas concentration detection;

[0025] Second, due to the adoption of the reference optical path solution, compared with the traditional laser detection system, the influence of the laser power jitter and the residual amplitude noise can be eliminated, that is, the TDLAS detection system of the present invention achieves high-precision measurement;

[0026] Third, the method of the present invention effectively eliminates the nonlinear effect in the laser output by comparing the two signals, which not only reduces the high requirements for laser performance, but also further reduces the overall cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is an overall schematic diagram of the detection system of the present invention.

[0028] Figure 2 It is a schematic structural diagram of the air chamber of the present invention.

[0029] Figure 3 It is a schematic diagram of the butterfly package laser of the present invention.

[0030] Figure 4 Schematic diagram of the photoelectric detector of the present invention (photoelectric detector 1 and photoelectric detector 2 are of the same model).

[0031] Figure 5 It is a schematic diagram of a 1×2 coupler of the present invention.

[0032] Figure 6 It is a normalized second harmonic concentration signal diagram detected at multiple concentrations according to the present invention.

[0033] Figure 7 It is a fitted line graph of the detection signal and concentration calibration of the present invention.

[0034] Among them, 1-air chamber, 101-opening one, 102-opening two, 103-cavity, 104-air inlet, 105-air outlet, 2-laser, 3-photodetector one, 4-photodetector two, 5-1x2 coupler, 501-input end, 502-output end one, 503-output end two. DETAILED DESCRIPTION

[0035] The technical solutions in the embodiments of the present invention are described clearly and completely below. 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.

[0036] 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 here. 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 comprising 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.

[0037] In the present application, the terms "upper", "lower", "left", "right", "front", "back", "top", "bottom", "inner", "outer", "middle", "vertical", "horizontal", "lateral", "longitudinal" and the like indicate positions or positional relationships based on the positions or positional relationships shown in the drawings. These terms are mainly used to better describe the present application and its embodiments, and are not used to limit the indicated devices, elements or components to have a specific orientation, or to be constructed and operated in a specific orientation.

[0038] In addition, some of the above terms may be used to express other meanings in addition to indicating orientation or positional relationship. For example, the term "on" may also be used to express a certain dependency or connection relationship in some cases. For those of ordinary skill in the art, the specific meanings of these terms in this application can be understood according to specific circumstances.

[0039] In addition, the terms "installed", "set", "provided with", "connected", "connected", and "socketed" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral structure; it can be a mechanical connection, or an electrical connection; it can be a direct connection, or an indirect connection through an intermediate medium, or it can be an internal connection between two devices, elements, or components. For those of ordinary skill in the art, the specific meanings of the above terms in this application can be understood according to specific circumstances.

[0040] When considering how to obtain a highly sensitive trace gas detector (taking propane molecules as an example), the relative intensity noise (RIN) of the laser is a very important type of noise in the system. In the frequency range of interest, the intensity of RIN is inversely proportional to the frequency, that is, it exhibits the characteristics of 1 / f noise, while other noises such as shot noise and thermal noise exhibit the characteristics of white noise. Therefore, the gas absorption signal can be moved to a high-frequency range with lower noise intensity by modulating the light source, which can greatly improve the signal-to-noise ratio of the system. In order to obtain higher detection accuracy, it is necessary to deeply consider several 2f background signal sources in the experimental measurement. These sources can be roughly divided into three categories: background signals caused by the etalon effect in the optical path; background signals caused by the nonlinear intensity modulation effect in the laser; and background absorption of external gases caused by problems with the sealing of the gas chamber. In recent years, the research on laser propane detection technology has mainly focused on the mid-infrared and near-infrared bands. The high price of lasers and optical fibers in the mid-infrared band limits the practicability of the technology. Therefore, more and more people have begun to pay attention to gas monitoring technology in the near-infrared band. However, the absorption coefficient of gas molecules at this time is often lower than that in the mid-infrared band. This embodiment adopts a reference optical path solution to effectively increase the detection accuracy of laser propane and effectively reduce the required optical path cell length. At the same time, due to the use of near-infrared band detection technology, low laser cost and optical fiber cost can be obtained.

[0041] Since the cost of an extremely stable output current source is high and the noise influence of the driving circuit is difficult to avoid, in order to eliminate this series of influences, this embodiment uses a 1×2 fiber coupler after the laser output to obtain two completely identical outputs, one passing through the gas chamber, where propane molecules produce a trace absorption, and the other directly received by the PD. The two are received by the synchronous acquisition card and then transmitted to the computer for data processing. The background subtraction method is used to improve its detection lower limit (the influence of background absorption and nonlinear intensity modulation effect can be eliminated), which can be expressed as follows:

[0042]

[0043] in, is the final signal processing result, X 2f and Y 2f It indicates the two-way output signal when the reference signal obtained according to the principle of 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.

[0044] To achieve the above purpose, Figure 1-5As shown, this embodiment provides a TDLAS detection system for eliminating the nonlinear effect of the laser based on a reference optical path, including:

[0045] 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;

[0046] A photodetector 3, wherein the photodetector 3 is connected to an output terminal 502 of the 1×2 coupler;

[0047] Air chamber 1, the air chamber 1 is connected to the second output terminal 503 of the 1×2 coupler;

[0048] A photoelectric detector 2 4, wherein the photoelectric detector 2 4 is connected to the gas chamber 1;

[0049] A synchronous acquisition card, wherein the synchronous acquisition card is connected to the photoelectric detector 1 3 and the photoelectric detector 2 4;

[0050] Laser 2 generates a modulated signal laser, which is divided into two completely identical lasers 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 ADC to receive the signals received by photodetector one 3 and the signals received by photodetector two 4, and processes the detection signal through the second harmonic concentration demodulation method of background difference to obtain the propane concentration in gas chamber 103.

[0051] The 1×2 coupler divides the laser into two completely identical light paths. After being processed, the two laser paths can eliminate the influence of the power fluctuation noise and residual amplitude noise of the laser 2 itself.

[0052] The air chamber 1 is provided with an opening 101, an opening 102 and a cavity 103, wherein the opening 102 is used to connect the cavity 103 with the photodetector 2 4; the 1×2 coupler is provided with an input end 501, an output end 502 and an output end 503, wherein the input end 501 is connected with the laser 2, the output end 502 is connected with the cavity 103 through the opening 101, and the output end 503 is connected with the photodetector 3. The air chamber 1 is made of aluminum material, and a collimating mirror with an inclination is provided on the surface of the opening 101 of the air chamber 1. The laser 2 is connected to the input end 501 of the 1x2 coupler 5 through a flange, and the output end 102 is connected to a fiber collimator and installed in the corresponding opening 101 in the air chamber 1. The position is fixed, so that the generated laser is reflected back and forth along the predetermined optical path in the cavity 103, and emitted from the opening 102 to the photodetector 2 4. The output end 2 503 of the 1×2 coupler is directly connected to the photodetector 1 3. After the overall structure is completed, it has good stability and repeatability. The air chamber 1 is provided with an air inlet 104 and an air outlet 105. The shape, size and setting position of the air inlet 104 and the air outlet 105 are not specifically limited here, as long as it is convenient for external gas to enter the cavity 103. The opening 101 and the opening 2 102 of the air chamber 1 also provide installation positions for other functional components. For example, the photodetector 1 3, the photodetector 2 4 and the 1x2 coupler 5 can be installed on the 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 the service life.

[0053] This embodiment further includes at least one reflector, which is 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.

[0054] This embodiment further includes a temperature control chip for the laser 2 , which is electrically connected to the laser 2 and is used to accurately control the temperature of the laser 2 .

[0055] To further optimize the technical solution, the second harmonic concentration demodulation method of the background difference is to demodulate the detection signal according to 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 103.

[0056] 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 use of a reference optical path solution, compared with the traditional laser propane detection system, the influence of the laser's own power jitter and residual amplitude noise can be eliminated, that is, 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 the mid-infrared detection solution.

[0057] The synchronous acquisition card is electrically connected to the photodetector 1 and the photodetector 2 to ensure the synchronization of the two collected signals for the subsequent signal processing and demodulation process. Since there is a phase difference between the two signals if they are not synchronized, an error caused by this will be introduced during the reference. The use of a synchronous acquisition card can eliminate this error and improve the detection accuracy. The traditional TDLAS system superimposes a high-frequency modulated sinusoidal signal on the sawtooth wave to move the noise to the high-frequency range, which is beneficial to improve the signal-to-noise ratio of the system. A phase-locked amplifier is added to the signal processing part for harmonic extraction and calculation of the concentration of the gas to be measured. However, there are still several 2f background noise sources at this time: one is the background signal caused by the standard instrument effect in the optical path; the second is the background signal caused by the nonlinear intensity modulation effect in the laser; and the last is the influence of the residual amplitude modulation introduced due to the asynchronous wavelength modulation and intensity modulation of the laser. By adopting the second harmonic concentration demodulation method of the reference gas chamber, the nonlinear modulation of the laser itself and the noise influence caused by the residual amplitude modulation can be eliminated; on the other hand, since the first harmonic and the second harmonic are both proportional to the light intensity, the division between the two can eliminate the influence of light intensity fluctuations during long-term measurements, and the accuracy of the propane concentration in the gas chamber obtained in this way is higher, that is, the TDLAS detection system and method of this embodiment based on the reference light path to eliminate the nonlinear influence of the laser 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, the linearity is good, and the Figure 6 It can be seen that there is a significant difference between the detection coefficient of zero-concentration propane gas and that of propane gas, that is, it has a good signal-to-noise ratio.

[0058] The present invention also provides a TDLAS detection method for eliminating the nonlinear influence of the laser based on a reference optical path, comprising:

[0059] The laser generates pulsed laser, which is divided into two identical laser beams through the input end of the 1x2 coupler and transmitted to output end 1 and output end 2. Laser 1 is directly received by photodetector 1 to generate a detection signal. The laser output from output end 1 is reflected back and forth in the cavity and incident on photodetector 2 through opening 2, causing the photodetector to generate a detection signal. The two detection signals are received by the synchronous acquisition card and processed by the PC data to obtain the gas concentration in the gas chamber.

[0060] The above description is only a preferred embodiment of the present invention and does not limit the technical scope of the present invention. Therefore, any slight 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 based on a reference optical path to eliminate the nonlinear effects of lasers, characterized in that: include: A laser connected to an input end of the 1×2 coupler; A photodetector 1, wherein the photodetector 1 is connected to an output terminal 1 of the 1×2 coupler; An air chamber, the air chamber being connected to the second output terminal of the 1×2 coupler; A second photoelectric detector, wherein the second photoelectric detector is connected to the gas chamber; A synchronous acquisition card, wherein the synchronous acquisition card is connected to the first photoelectric detector and the second photoelectric detector; The laser generates a modulated signal laser, which is divided into two completely identical lasers through a 1×2 coupler. Laser one is directly received by photodetector one, and laser two is received by photodetector two after passing through the gas chamber. The synchronous acquisition card uses ADC to receive the signal received by photodetector one and the signal received by photodetector two, and processes the detection signal through the second harmonic concentration demodulation method of background difference to obtain the gas concentration in the gas chamber.

2. A TDLAS detection system based on a reference optical path to eliminate the nonlinear effect of laser as claimed in claim 1, characterized in that: The air 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 with the laser, output end 1 is connected with the cavity through opening 1, and output end 2 is connected with photodetector 1.

3. A TDLAS detection system based on reference optical path to eliminate laser nonlinear effects as claimed in claim 2, characterized in that: It also includes at least one reflector, which is installed on the inner wall of the cavity. The modulated signal laser emitted by the laser is reflected by the reflector to the second photodetector.

4. A TDLAS detection system based on reference optical path to eliminate laser nonlinear effects as claimed in claim 3, characterized in that: The reflecting mirror is a gold-plated reflecting mirror.

5. A TDLAS detection system based on reference optical path to eliminate laser nonlinear effects as claimed in claim 4, characterized in that: The air chamber is made of aluminum material, and a collimating mirror with an inclination angle is arranged on a surface of an opening of the air chamber.

6. A TDLAS detection system based on a reference optical path to eliminate the nonlinear effect of laser as claimed in 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.

7. A TDLAS detection system based on a reference optical path to eliminate the nonlinear effect of laser as claimed in claim 1 or 6, characterized in that: The second harmonic concentration demodulation method of the background difference is to demodulate the detection signal according to the signals of the two-way photodetector 1 and the photodetector 2 collected by the synchronous acquisition card, and then perform linear calibration to obtain the gas concentration in the gas chamber.

8. A TDLAS detection method based on a reference optical path to eliminate the nonlinear effect of the laser, characterized in that: include: The laser generates pulsed laser, which is divided into two identical laser beams through the input end of the 1x2 coupler and transmitted to output end 1 and output end 2. Laser 1 is directly received by photodetector 1 to generate a detection signal. The laser output from output end 1 is reflected back and forth in the cavity and incident on photodetector 2 through opening 2, causing the photodetector to generate a detection signal. The two detection signals are received by the synchronous acquisition card and processed by the PC data to obtain the gas concentration in the gas chamber.

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