A folded optical path optical multipass cell, gas detection system and method of use thereof

Abstract

The present application relates to the technical field of gas detection. In order to solve the problems existing in the existing optical multi-cell, such as difficult to realize mass production and stable control of open-loop regulation, inconvenient debugging of mid-infrared light path, high cost of mirror group parallelism detection, dead volume in gas path and lack of real-time monitoring, etc. A kind of folded light path type optical multi-cell, gas detection system and its use method, including sealed cavity, laser, detector and closed loop angle adjusting system. Two groups of concave mirrors are arranged in the sealed cavity, and a plane mirror is arranged in the middle to fold the light path. After laser incidence, it is emitted after multiple reflections. Each control mirror is matched with a coaxial high-precision rotary encoder to collect the angle. The controller compares the difference between the set and measured angles, relies on the encoder feedback to control the piezoelectric element in a closed loop, can compensate for the piezoelectric hysteresis, eliminates manual calibration, and facilitates batch assembly and long-term stable operation of the equipment.

Description

Technical Field

[0001] This invention relates to the field of gas detection technology, specifically to a folded optical path type optical multi-pass cell, a gas detection system, and a method of using the same. Background Technology

[0002] An optical multipass cell is an optical device based on highly reflective mirrors. After the laser beam enters the sealed cavity through the entrance aperture, it is reflected multiple times between two or more mirrors before finally reaching the photodetector through the exit aperture. This design can achieve an effective optical path of several meters or even hundreds of meters within a limited space, thus significantly improving the sensitivity of gas detection. Due to its relatively simple structure, strong spectral adaptability, and good stability, the optical multipass cell has become a core component of tunable semiconductor laser absorption spectroscopy (TDLAS) gas sensing systems, widely used in atmospheric environmental monitoring, industrial process control, medical diagnostics, and safety monitoring.

[0003] Currently, the mainstream multipass cells mainly include the White type and the Herriott type. The White type multipass cell, proposed by White in 1942, uses three concave mirrors to form an elliptical beam distribution on the mirror surface. This structure allows for repeated reflections of beams with large divergence angles, but due to the involvement of multiple mirrors, the optical path adjustment is relatively complex, the mirror utilization efficiency is relatively low, and the size is large, which is not conducive to miniaturization. The Herriott type multipass cell, proposed by Herriott in 1964, consists of a sealed cavity composed of a pair of coaxial concave mirrors. The beam reflects back and forth between the two, forming an elliptical or circular trajectory. This structure has a relatively easy optical path control and has become the most widely used type of multipass cell on the market. However, the traditional Herriott cell has high requirements for light source collimation, and the beam spot is distributed in a single circular ring on the mirror surface, resulting in low utilization of the central area of ​​the mirror. To achieve a longer optical path, the axial length usually needs to be increased, resulting in a slender cylindrical shape, which is not conducive to the miniaturization and integration of instruments.

[0004] Regarding tunable multi-optical-path systems, patent CN201210371938.5, "An Automatic Tunable Multi-optical-Path System," discloses such a system. This system employs a plane mirror and a concave mirror assembly consisting of upper and lower concave mirror halves. The horizontal rotation angle of the upper concave mirror is adjusted via a piezoelectric crystal, while the length of the sealed cavity is changed by driving a bellows extension and retraction via a stepper motor. Its control system uses a DSP controller to calculate the required voltage based on preset optical path requirements and drive the piezoelectric crystal, operating in an open-loop control mode. This patent also discloses basic inlet and outlet gas lines for introducing standard sample gas or connecting a vacuum pump. Furthermore, regarding coaxial laser calibration, patent CN202211379235.7, "A Method for Coaxial Calibration of Array Lasers," discloses a method for coaxial calibration of array lasers, using an autocollimator and a standard mirror to achieve beam alignment of multiple lasers. However, several technical problems remain unresolved in the existing technology.

[0005] First, existing automatic tunable multi-path optical systems employ open-loop control, meaning they output a preset voltage to drive the piezoelectric crystal based on theoretical calculations, but without any angle or position feedback. In practical applications, piezoelectric crystals exhibit hysteresis, creep, and nonlinearity. Furthermore, factors such as mechanical assembly tolerances, temperature drift, and structural loosening after long-term operation can all cause the actual reflector angle to deviate from the set value. This means that each device requires precise assembly and adjustment by professional optical engineers before leaving the factory, which is time-consuming and costly, failing to meet the demands of modular, rapid assembly and adjustment in mass production. Moreover, during long-term operation, the open-loop system cannot automatically compensate for angle drift, affecting detection stability.

[0006] Secondly, in the field of mid-infrared gas detection, quantum cascade lasers or interband cascade lasers are commonly used. Their operating wavelengths, such as 3.345 micrometers, are within the range invisible to the human eye. Traditional calibration methods rely on infrared observation cards, thermal imagers, or power meters to scan point by point, which is extremely inefficient and makes it difficult to accurately determine whether the optical path is aligned. Although existing patents involve laser coaxial calibration techniques, they are all for array beam combining scenarios of multiple visible or near-infrared lasers. The principles and purposes are different from those of single-wavelength gas detection scenarios, and they cannot be directly applied to the efficient coaxial alignment of invisible mid-infrared lasers with visible light.

[0007] Third, in multi-pass cell structures employing a combination of plane and concave mirrors, the parallelism between the two mirrors directly affects the light spot pattern and effective optical path. Current technologies typically rely on interferometers or high-precision goniometers to determine parallelism; these devices are expensive and complex to operate, making them unsuitable for rapid on-site debugging or rapid inspection in mass production. There is a lack of a simple method that utilizes the inherent characteristics of the optical path for intuitive judgment.

[0008] Fourth, most existing multi-pass gas meters only have simple inlets and outlets, without considering the gas flow distribution within the sealed cavity. In actual use, the gas may flow along the shortest path, resulting in dead volumes in certain areas of the sealed cavity, long gas replacement times, and delayed concentration measurement response. Furthermore, the lack of real-time monitoring of gas pressure and flow rate makes it impossible to determine whether the gas path is blocked or leaking, affecting measurement reliability.

[0009] In summary, existing technologies have not yet effectively solved the following core problems: the problem of rapid and consistent assembly and adjustment of multi-pass cell optical paths in mass production; the problem of efficient and low-cost alignment of invisible mid-infrared lasers; the problem of rapid and intuitive judgment of the parallelism of plane mirrors; and the problem of real-time monitoring of the uniformity of gas exchange and the status of gas paths within multi-pass cells. Summary of the Invention

[0010] To address the aforementioned problems, this invention proposes a gas detection system with a folded optical path multi-pass cell. By introducing closed-loop control of a rotary encoder, coaxial adjustment of dichroic mirrors, parallelism criterion of equally spaced light spots, and optimized gas path design, this invention effectively solves the shortcomings of existing technologies.

[0011] To achieve the aforementioned objective, the present invention relates to a gas detection system with a folded optical path optical multi-pass cell. In a first aspect, the invention comprises: a sealed cavity containing a first concave reflector, a second concave reflector, and at least two planar reflectors, the first and second concave reflectors being disposed opposite each other, and the first concave reflector having a 3mm × 3mm rectangular off-axis aperture located on the side where the first concave reflector and the planar reflectors meet; the planar reflectors being disposed in the optical path between the first and second concave reflectors for folding the optical path so that the incident laser beam is reflected multiple times within the sealed cavity before exiting; a laser for generating a detection beam with a mid-infrared wavelength of 3.345μm, the detection beam entering the sealed cavity through an entrance aperture; a photodetector for receiving the beam emitted from the sealed cavity and converting it into an electrical signal; and a closed-loop angle adjustment system comprising: at least one piezoelectric drive element for adjusting the angle of the planar reflector or the first and second concave reflectors. The tilt angle is as follows: A two-axis piezoelectric tilting platform is mounted on the back of each of the two concave and planar reflectors adjacent to the second concave reflector. Each two-axis piezoelectric tilting platform is directly connected to the rigid back plate of the reflector via its built-in flexible hinge, achieving nanometer-level precision angle driving. At least one rotary encoder is used to detect the angle information of the adjusted first concave reflector, second concave reflector, and planar reflector in real time. Each adjusted first concave reflector, second concave reflector, and planar reflector is equipped with a corresponding high-precision rotary encoder. The rotor of this rotary encoder is coaxially connected to the rotating shaft of the first concave reflector, second concave reflector, and planar reflector or the output shaft of the two-axis piezoelectric tilting platform via a coupling. The stator is fixed on the sealed cavity base. A controller receives the feedback signal from the rotary encoder and controls the action of the piezoelectric drive element according to the deviation between the preset angle value and the actual angle value, forming a closed-loop control. Through closed-loop feedback, nanometer-level precision angle adjustment and automatic compensation are achieved, overcoming piezoelectric hysteresis, creep and temperature drift. No manual calibration by professional engineers is required, enabling rapid batch assembly and adjustment and long-term self-stabilization.

[0012] Furthermore, it also includes a coaxial adjustment optical path, comprising a visible laser for generating a visible adjustment beam, and a dichroic mirror positioned in the optical path of the detection beam to ensure coaxial propagation of the visible adjustment beam and the detection beam. The dichroic mirror has a high transmittance of greater than 95% in the 3.345μm wavelength band and a high reflectance of greater than 99.5% in the 400-700nm visible light band. Its substrate material is ZnSe, coated with a dielectric film. This method of using a dichroic mirror to achieve coaxiality between the visible light and the invisible mid-infrared detection beam reduces adjustment time from hours to minutes, eliminates the need for an infrared camera, and significantly reduces adjustment costs and difficulty.

[0013] Furthermore, the refraction of the probe beam by the dichroic mirror results in a minute parallel gap between the transmitted beam and the incident beam. This minute parallel gap is compensated by a one-dimensional precision translation stage mounted under the dichroic mirror frame. A rotating differential head drives the frame to perform a micrometer-level translation in a direction perpendicular to the optical axis. By compensating for the parallel gap caused by the dichroic mirror refraction, the visible adjustment optical path and the actual measurement optical path are ensured to be truly coaxial, avoiding beam deviation that could lead to emission failure or reduced receiving efficiency, thus improving alignment accuracy.

[0014] Furthermore, the parallelism adjustment of the plane mirrors is judged using the following method: an off-axis laser source is introduced, and the reflected light spots are observed to be evenly distributed on the receiving screen. The spot diameter is 1 mm, and the deviation of the distance between adjacent spots is controlled within 0.1 mm-0.2 mm. If the deviation of the distance between adjacent spots is not within 0.1 mm-0.2 mm, it is determined that the parallelism of the two plane mirrors does not meet the requirements and needs to be readjusted. This invention provides a low-cost, rapid parallelism judgment method that does not require an interferometer or a high-precision goniometer. It is suitable for on-site debugging and mass production, and the quantified numerical range eliminates subjective errors, ensuring consistent adjustment.

[0015] Furthermore, the sealed cavity is rectangular in shape, with dimensions of 100mm in length, 150mm in width, and 80mm in height. By introducing two plane mirrors to fold the optical path, an effective absorption optical path of 14 meters is achieved within the internal space of the rectangular shape. The volume compression ratio, i.e., the ratio of the equivalent cylindrical Herriott cell volume to the internal space volume of the rectangular shape, is 7:1. This compresses the traditional cylindrical cell into a compact rectangular shape, facilitating internal installation, improving space utilization, achieving a long optical path within a smaller volume, and making portable devices possible.

[0016] Furthermore, the system includes a gas path optimization system comprising multiple vent holes on the sealed cavity structure to achieve uniform gas exchange within the sealed cavity, and a flow meter and a pressure gauge for real-time monitoring of gas flow rate and pressure data within the sealed cavity, respectively. The flow meter is a mass flow controller with a range of 0-1000 sccm, and the pressure gauge is an absolute pressure or gauge pressure sensor with a range of 0-150 kPa, with its output connected to the controller or an external data acquisition device. This system eliminates dead volume through multiple vent holes, shortens the gas replacement response time, and allows for timely detection of gas path blockages or leaks by real-time monitoring of flow and pressure data, improving measurement reliability and maintainability.

[0017] Furthermore, the controller is also connected to a data storage module and a communication module for recording historical angle adjustment data and real-time gas detection data, and supporting remote monitoring or parameter adjustment. The communication module includes one or more of RS485, CAN, Ethernet, or Wi-Fi, supporting the Modbus RTU / TCP industrial protocol. The data storage module records timestamps, the angles of the first concave reflector, the second concave reflector, and the planar reflector, as well as gas concentration, flow rate, and pressure data in CSV or binary format. This technical solution enables local data storage and remote communication, supports historical data tracing, fault analysis, and remote parameter adjustment, and facilitates the construction of distributed sensor networks or unattended monitoring systems.

[0018] Secondly, the present invention also includes a gas concentration detection method, comprising the following steps: The angles of the first concave reflector, the second concave reflector, and the plane reflector within the system are pre-adjusted to a set value using the closed-loop angle adjustment system, and the angle is confirmed to be in place using the rotary encoder; a laser is activated, allowing the detection beam to enter the sealed cavity through the entrance hole and exit to the photodetector after multiple reflections; the photodetector converts the optical signal into an electrical signal, and the signal processing unit calculates the concentration of the gas to be measured; during gas measurement, the flow meter and pressure gauge of the gas path optimization system monitor the gas path status in real time, and dynamically compensate for angle drift when necessary using the closed-loop angle adjustment system; wherein, the trigger condition for dynamic compensation is: the absolute value of the deviation between the actual angle value fed back by the rotary encoder and the preset angle value continuously exceeds the preset value range for more than 1 second; once triggered, the controller immediately drives the piezoelectric drive element to perform reverse compensation until the angle deviation returns to within the value range. This provides a standardized detection process, clarifies the dynamic compensation trigger condition, avoids miscompensation caused by instantaneous noise, achieves automatic angle maintenance in long-term continuous monitoring, and ensures measurement accuracy.

[0019] Furthermore, during the laser debugging phase, the coaxial debugging optical path is employed: the visible light laser is turned on, and the visible beam and the probe beam propagate coaxially. The alignment of the probe beam's optical path is determined by observing the position of the visible light spot, and compensation is made for the spacing introduced by the dichroic mirror. The standard for successful alignment is that, after multiple reflections, the offset between the center of the final emitted visible light spot and the center mark point of the photodetector target surface is less than 5% of the target surface size. After alignment is completed, the visible light laser is turned off, and formal measurements are performed. By quantifying the alignment completion standard, subjective differences among operators are eliminated, and the abstract alignment is transformed into a measurable engineering indicator, ensuring that the probe beam falls completely within the effective area of ​​the detector and maximizing the signal-to-noise ratio.

[0020] Furthermore, the signal processing unit of this invention employs wavelength modulation spectroscopy technology. It superimposes a high-frequency sinusoidal modulation signal onto the laser injection current and demodulates the second harmonic signal using a lock-in amplifier to invert the gas concentration. The modulation frequency is 10 kHz, the modulation depth is 2.0-2.5 times the half-width of the target gas absorption line, the time constant of the lock-in amplifier is 100 ms, and a 12 dB / oct low-pass filter is used. This technical solution optimizes the modulation parameters, effectively suppressing 1 / f noise and background interference, significantly improving the signal-to-noise ratio, achieving ppb-level trace gas detection sensitivity, and striking a good balance between response speed and detection accuracy.

[0021] By adopting the aforementioned technical solution, this invention achieves the following technical effects: The technical problem solved is that existing automatic tunable multi-path systems employ open-loop piezoelectric control with no angle feedback, failing to compensate for lens angle deviations caused by assembly tolerances, temperature drift, and piezoelectric hysteresis. This results in difficulties in mass production assembly and adjustment, poor long-term stability, and the need for manual calibration by specialized engineers for each device. The technical effect is that by constructing a closed-loop feedback system using piezoelectric drive elements, a rotary encoder, and a controller, the actual angles of the first concave reflector, the second concave reflector, and the plane reflector are detected in real time and compared with preset values. Deviations are automatically corrected, achieving high-precision angle adjustment, modular and rapid assembly and adjustment in mass production, and self-stabilization during long-term operation, thus reducing mass production costs and maintenance difficulty. Attached Figure Description

[0022] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used in conjunction with embodiments of the invention to explain the invention and do not constitute a limitation thereof. In the drawings: Figure 1 This is a schematic diagram of the physical structure of a gas detection system with a folded optical path type optical multi-pass cell according to the present invention.

[0023] Figure 2 This is a schematic diagram of the beam spacing of a gas detection system with a folded optical path type optical multi-pass cell according to the present invention.

[0024] Figure 3 This is an overall workflow diagram of a gas detection system with a folded optical path type optical multi-pass cell according to the present invention.

[0025] Figure 4 This is a schematic diagram of the system initialization and angle pre-adjustment process in the overall workflow of a gas detection system with a folded optical path type optical multi-pass cell according to the present invention.

[0026] Figure 5 This is a schematic diagram of the coaxial adjustment and alignment process in the overall workflow of a gas detection system with a folded optical path type optical multi-pass cell according to the present invention.

[0027] Figure 6 This is a schematic diagram of the formal measurement and concentration measurement process in the overall workflow of a gas detection system with a folded optical path type optical multi-pass cell according to the present invention.

[0028] Figure 7 This is a schematic diagram illustrating the real-time monitoring and dynamic compensation process in the overall workflow of a gas detection system with a folded optical path type optical multi-pass cell according to the present invention.

[0029] Figure 8 This is a schematic diagram of the structure of the first concave reflector in a gas detection system with a folded optical path type optical multi-pass cell according to the present invention.

[0030] In the figure: 1. Sealed cavity; 2. First concave mirror; 3. Second concave mirror; 4. Plane mirror; 5. Laser; 6. Photodetector; 7. Rectangular off-axis aperture; 8. Folding mirror. Detailed Implementation

[0031] The preferred embodiments of the present invention will be described below with reference to the accompanying drawings. It should be understood that the preferred embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0032] Example 1 like Figure 1 , Figure 3 As shown, this embodiment provides a gas detection system with a folded optical path type optical multi-pass cell. Its specific structure includes a sealed cavity 1 in the shape of a cuboid, with dimensions of 100mm in length, 150mm in width, and 80mm in height. Inside the sealed cavity are a first concave reflector 2, a second concave reflector 3, and two planar reflectors 4. (See figure) Figure 8As shown, the first concave reflector 2 has a 3mm × 3mm rectangular off-axis aperture. The rectangular off-axis aperture is located on the side where the first concave reflector 2 and the plane reflector 4 are aligned. The collimated incident light spot diameter is ≤1mm. It enters through the off-axis aperture via the two plane reflectors 4, undergoes multiple reflections in the first concave reflector 2, the second concave reflector 3, and the two plane reflectors 4, and exits through the same off-axis aperture. The exiting light is then received by the photodetector 6 via the folding reflector 8. It should be noted that the rectangular off-axis aperture is not a common structure in the field of this invention; currently, over 90% of commercial and patented multi-pass filters use conventionally designed circular apertures for both the entrance and exit apertures. Compared to a circular aperture, a rectangular aperture offers better optical path adaptability. It accommodates linearly arranged light spots, extending along the spot arrangement direction and narrowing vertically. In scenarios where the incident and exit light spots share the same aperture, this design ensures that the linearly distributed light spots do not obstruct each other. Compared to a circular aperture of the same area, it reduces the ineffective mirror opening area by more than 20% and retains more mirror reflection area to increase the number of reflections. A circular aperture, on the other hand, accommodates circularly distributed light spots. If used in this design, the aperture diameter needs to be increased to accommodate the linear light spots, which would occupy additional mirror reflection area, reduce the effective number of reflections, and decrease the optical path. Furthermore, regarding off-axis adaptability, a rectangular aperture can extend elongated along the mirror edge, occupying only the narrow edge area and not encroaching on the central core reflection area of ​​the mirror, maximizing mirror utilization. A circular aperture, when opened at the off-axis edge, occupies a larger radial area, more easily encroaching on the effective reflection area of ​​the mirror and reducing the usable reflection area. Rectangular shapes have inherent directionality, allowing direct alignment with the incident light direction during assembly without requiring additional angle calibration and preventing reverse installation. Circular holes, on the other hand, lack directionality, necessitating additional calibration of the incident light spot angle during assembly, making positioning more challenging.

[0033] In terms of final performance, specifically optical diffraction, the diffraction difference between the two is minimal for small apertures of around 3mm. Circular apertures produce symmetrical Airy disks with uniform energy distribution, while rectangular apertures produce cross-shaped or rectangular spots with less diffraction diffusion along the optical path, making them more suitable for transmission in linear folded optical paths.

[0034] Meanwhile, the folding mirror 8 is only used to fold incident and outgoing light, and its purpose is different from that of the plane mirror 4.

[0035] In the design of multi-pass cells, one of the more important parameters is the optical path volume ratio (RLV).

[0036] , where n is the number of reflected light spots on the two concave surfaces, d is the distance between the two plane mirrors 4, and S is the area of ​​a single plane mirror 4 and a single concave mirror.

[0037] Through the folding effect of two plane mirrors 4, an effective absorption optical path of 14 meters is achieved within this compact volume, and the volume compression ratio, that is, the ratio of the equivalent cylindrical Herriott cell volume to the volume of this cuboid cell, reaches 7:1.

[0038] like Figure 2 As shown, the incident angle of the folded plane mirror 4 is θ, the distance between the two plane mirrors 4 is d, and the plane mirror 4 is a rectangle of 25.4mm*25.4mm.

[0039] Based on geometric relationships, the expression for the distance between the two light spots on a single plane mirror 4, M1 and M2, can be obtained: ; The M1 and M2 light spots are arranged in an alternating pattern with a half-step interval. The vertical spacing between the alternating mirrors is: ; The current off-axis hole size is 3mm*3mm, so the theoretical incident angle θ should satisfy: 6Δx≤25.4+1.5, i.e., θ≤5.14°; With a small offset angle, θ≈tanθ, so when the lens is offset by 0.1°, the spot spacing is offset by 0.1mm.

[0040] To achieve precise adjustment and long-term stability of the reflector angle, this embodiment employs a closed-loop angle adjustment system. Existing automatic tunable multi-optical-path systems, such as the invention patent CN102879898B mentioned in the background section, employ open-loop piezoelectric control. This means that a preset voltage is output based on theoretical calculations to drive the piezoelectric crystal, but without any angle or position feedback. This open-loop approach cannot compensate for the inherent hysteresis, creep, and nonlinearity of piezoelectric ceramics, nor can it eliminate lens angle deviations caused by mechanical assembly tolerances and temperature drift. This results in each device requiring time-consuming and expensive precision assembly and adjustment by professional optical engineers before leaving the factory, and the optical path is prone to deviation after long-term operation, leading to poor detection stability. To address these technical problems, this embodiment adopts the following structure: a two-axis piezoelectric tilting platform is mounted on the back of each of the second planar reflector 4 and the second concave reflector 3. This platform uses Physik Instrumente (PI) S-330 series products. Each platform is directly connected to the rigid backplate of the reflector via its built-in flexible hinge, achieving nanometer-level precision angle driving. Each of the adjusted first concave reflector, second concave reflector, and plane reflector is equipped with a high-precision rotary encoder, using the Netzerprecision VLZ series. The rotor of this rotary encoder is coaxially connected to the rotating shaft of the first concave reflector, second concave reflector, and plane reflector, or the output shaft of the two-axis piezoelectric tilting platform, via a coupling. The stator is fixed on a sealed cavity base, used for direct, real-time measurement of the absolute tilt angle of the reflector. A controller receives the feedback signal from the rotary encoder and controls the action of the piezoelectric drive element according to the deviation between the preset angle value and the actual angle value, forming a closed-loop control. Through this closed-loop feedback structure, the system can detect the actual angle of the reflector in real time and compare it with the preset value, automatically correcting the deviation, thereby accurately compensating for angle deviations caused by assembly tolerances, temperature drift, and piezoelectric hysteresis. This design enables modular rapid assembly and adjustment in mass production without the need for manual calibration by professional engineers, while ensuring self-stability during long-term operation, significantly reducing mass production costs and maintenance difficulty.

[0041] This embodiment also includes a coaxial tuning optical path. This path includes a visible laser 5 for generating a visible tuning beam, and a dichroic mirror positioned within the probe beam's optical path. The dichroic mirror's substrate material is ZnSe, coated with a dielectric film system. It exhibits a high transmittance of greater than 95% in the 3.345μm wavelength band and a high reflectance of greater than 99.5% in the 400-700nm visible light band, thus ensuring the visible tuning beam and probe beam propagate coaxially. The refraction of the probe beam by the dichroic mirror results in a small parallel gap between the transmitted beam and the incident beam. To compensate for this parallel gap, a one-dimensional precision translation stage is installed under the dichroic mirror's frame. A rotating differential head drives the frame to perform micrometer-level translation in a direction perpendicular to the optical axis to compensate for this parallel gap.

[0042] In this embodiment, the one-dimensional precision translation stage is the ThorlabsMT1 platform. It is a high-precision, modular one-dimensional manual translation stage whose core function is to provide a stable and precise uniaxial linear displacement for precise positioning of components in optical or optoelectronic experiments.

[0043] The gas path optimization system of this embodiment is as follows: Four air inlets with a diameter of 1.0 mm are evenly opened at the four corners of the bottom of the cuboid sealed cavity; two air outlets with a diameter of 1.2 mm are opened at the center of the top cover of the sealed cavity; to eliminate local flow stagnation, four auxiliary ventilation holes with a diameter of 0.8 mm are added to the inner wall of the sealed cavity near the first concave reflector 2. The system also includes a flow meter and a pressure gauge. The flow meter is a mass flow controller with a range of 0-1000 sccm. In this embodiment, the Alicat M series product is used. The pressure gauge is an absolute pressure / gauge pressure sensor with a range of 0-150 kPa. In this embodiment, the Omega PX409 series product is used. The output terminals of the mass flow controller and the absolute pressure / gauge pressure sensor are both connected to a controller or an external data acquisition device for real-time detection of gas flow and pressure data in the sealed cavity.

[0044] The controller is also connected to a data storage module and a communication module. The data storage module records data such as timestamps, reflector angles, gas concentrations, flow rates, and pressures in CSV or binary format. The communication module includes one or more of RS485, CAN, Ethernet, or Wi-Fi, supporting the Modbus RTU / TCP industrial protocol for remote monitoring or parameter adjustment.

[0045] During use, in the debugging phase, the parallelism of the plane mirror 4 should be adjusted first. An off-axis laser source should be introduced to observe whether the reflected light spots are evenly distributed on the receiving screen.

[0046] In this embodiment, a 3345nm distributed feedback mid-infrared continuous wave laser 5 from Nanoplus is used, targeting TDLAS or gas sensing applications. It features a TO-66 metal package, an integrated thermistor, and outputs a free-space beam. The beam diameter is approximately 1mm, and the deviation between adjacent beams is controlled within 0.1mm-0.2mm. If the deviation is not within 0.1mm-0.2mm, the parallelism of the two plane mirrors is deemed insufficient and readjustment is required. Then, a coaxial adjustment optical path is enabled. The visible light laser 5 is turned on, allowing the visible beam and the probe beam to propagate coaxially. The alignment of the probe beam's optical path is determined by observing the position of the visible beam. The standard for successful alignment is that, after multiple reflections, the offset between the center of the final emitted visible beam and the center mark point on the target surface of the photodetector 6 is less than 5% of the target surface size.

[0047] Photodetector 6 uses a Thorlabs PDAV-J8 mercury cadmium telluride photodetector with a response wavelength range of 2.0-8.0 micrometers and a photosensitive area of ​​1 mm². 2 The output bandwidth is DC-100 MHz, and it has adjustable gain amplification. The spacing introduced by the dichroic mirror is compensated by a one-dimensional precision translation stage. After alignment is completed, the visible light laser 5 is turned off, and the debugging is now complete.

[0048] like Figures 4-6 As shown, during the formal measurement phase, the angles of all reflectors in the system are pre-adjusted to the set values ​​using a closed-loop angle adjustment system, and the angles are confirmed to be in place using a rotary encoder. Laser 5 is activated, allowing the probe beam to enter the sealed cavity through the entrance aperture and exit to photodetector 6 after multiple reflections. Photodetector 6 converts the optical signal into an electrical signal, which is then used by the signal processing unit to calculate the concentration of the gas to be measured. The signal processing unit employs wavelength modulation spectroscopy technology, injecting a current into laser 5 and superimposing a high-frequency sinusoidal modulation signal. The modulation frequency is 10kHz, and the modulation depth is 2.0-2.5 times the half-width of the target gas absorption line. A lock-in amplifier is used to demodulate the second harmonic signal. The time constant of the lock-in amplifier is 100ms, and a 12dB / oct low-pass filter is used to invert the gas concentration.

[0049] like Figure 7 As shown, during gas measurement, the flow meter and pressure gauge of the gas path optimization system monitor the gas path status in real time. Simultaneously, the controller continuously reads the feedback from the rotary encoder and dynamically compensates for angle drift. The trigger condition for dynamic compensation is: the absolute value of the deviation between the actual angle value fed back by the rotary encoder and the preset angle value continuously exceeds the preset value range, i.e., 5-10 microradians, for more than 1 second. Once triggered, the controller immediately drives the piezoelectric drive element to perform reverse compensation until the angle deviation returns to within the value range.

[0050] In Embodiment 1 of this invention, a closed-loop feedback system consisting of a two-axis piezoelectric tilt platform and a rotary encoder achieves nanometer-level precision adjustment and real-time automatic compensation of the reflector angle. This effectively overcomes piezoelectric hysteresis, creep, and temperature drift, allowing for batch assembly and adjustment without manual calibration by professional optical engineers. Furthermore, it maintains optical path self-stability during long-term continuous operation, significantly reducing mass production costs and maintenance difficulty. Utilizing a dichroic mirror and a visible laser 5, the invisible mid-infrared detection beam is strictly coaxial with the visible light beam. Combined with a one-dimensional precision translation stage to compensate for the beam parallelism, the optical path alignment time is reduced from hours to minutes, eliminating the need for an infrared camera. The rectangular cavity is folded by two planar reflectors 4, achieving an effective absorption optical path of 14 meters and a volume compression ratio of 7:1, providing a structural basis for portable devices. Multiple vent holes eliminate dead volume, enabling rapid and uniform gas replacement. An integrated mass flow controller and pressure sensor allow for real-time diagnosis of gas path anomalies. A data storage module records all parameters, and a multi-protocol communication module supports remote monitoring and parameter adjustment, enabling the system to connect to the Industrial Internet of Things (IIoT). By employing optimized wavelength modulation spectral parameters, ppb-level trace gas detection sensitivity was achieved. Dynamic compensation triggering conditions avoided false compensation, ensuring that the optical path remained in optimal alignment throughout the measurement process. In summary, Example 1 achieved full-chain automation and high precision from optical path adjustment, alignment, gas path control to signal processing, providing a stable, efficient, and mass-producible system solution for gas detection.

[0051] Example 2 This embodiment further improves the gas path optimization system based on Embodiment 1, and describes its usage method in detail.

[0052] In this embodiment, the sealed cavity 1 is also a cuboid with dimensions of 100mm in length, 150mm in width, and 80mm in height. It contains a first concave reflector 2, a second concave reflector 3, and two planar reflectors 4. Addressing the technical problems of existing open-loop piezoelectric control systems failing to compensate for assembly tolerances, temperature drift, and piezoelectric hysteresis leading to lens angle deviations, resulting in difficulties in mass production assembly and adjustment, poor long-term stability, and the need for manual calibration by professional engineers for each device, this embodiment employs the same closed-loop angle adjustment structure as in Embodiment 1: a PIS-330 series two-axis piezoelectric tilting platform is mounted on the back of each of the second planar reflector 4 and the second concave reflector 3, connected to the rigid back plate of the reflector via flexible hinges; each reflector corresponds to a Netzer VLZ series rotary encoder, with the rotor coaxially connected to the reflector's rotating shaft, and the stator fixed to the sealed cavity base; the controller receives encoder feedback and drives the two-axis piezoelectric tilting platforms to form a closed loop. This structure achieves high-precision angle adjustment, modular rapid assembly and adjustment in mass production, and self-stabilization in long-term operation by detecting the actual angle in real time and comparing it with the preset value, and automatically correcting the deviation. This reduces mass production costs and maintenance difficulty. The structure of the coaxial adjustment optical path is also consistent with that of Example 1: the ZnSe substrate dichroic mirror has a transmittance of greater than 95% in the 3.345μm band and a reflectance of greater than 99.5% in the 400-700nm band. A ThorlabsMT1 one-dimensional precision translation stage is installed under the mirror frame to compensate for the parallel spacing of the beam.

[0053] In this embodiment, the arrangement of the vent holes was optimized. An air inlet with a diameter of 1.2 mm is opened at each of the four bottom corners of the cuboid sealed cavity; an air outlet with a diameter of 1.2 mm is opened at the corresponding positions at the four corners of the top cover; and an auxiliary vent hole with a diameter of 0.6 mm is added to the center of each of the four side walls of the sealed cavity. All vent holes are processed using laser drilling to ensure smooth, burr-free hole walls. Furthermore, miniature one-way valves are installed on the internal pipes of the air inlets and outlets to prevent backflow of gas. The flow meter and pressure gauge are selected the same as in Embodiment 1: an Alicat M series mass flow controller and an Omega PX409 series pressure sensor. The data storage and communication module also integrates RS485, CAN, Ethernet, Wi-Fi interfaces, and Modbus RTU / TCP protocol support.

[0054] Before use, the system should be initialized. The sealed cavity 1 should be evacuated to a pressure below 10 Pa, then flushed with high-purity nitrogen, repeated three times to remove any residual air. Next, the angles of each reflector should be adjusted to the theoretically calculated values ​​using the closed-loop angle adjustment system, and the rotary encoder should be used to confirm proper positioning. Then, optical path alignment should be performed. The visible light laser 5 should be turned on, and the wavelength adjusted to 650 nm. Its beam, after reflection by the dichroic mirror, should be coaxial with the 3.345 μm mid-infrared detection beam. The operator should observe the position of the visible light spot after multiple reflections within the sealed cavity, exiting onto the target surface of the photodetector 6. By adjusting the ThorlabsMT1 translation stage, the offset between the center of the visible light spot and the center mark on the target surface should be less than 5% of the target surface size; that is, less than 50 μm when the target surface diameter is 1 mm. After alignment, the visible light laser 5 should be turned off.

[0055] Next, gas measurement is performed. The gas to be measured is adjusted to 500 sccm by a mass flow controller and simultaneously enters the sealed cavity through four inlets at the bottom. The gas forms a uniform diffusion front within the cavity, propelling the existing gas to flow smoothly out through the four outlets on the top cover. Auxiliary small holes ensure that there is no dead volume in the corner areas of the sealed cavity. A pressure sensor monitors the pressure inside the cavity in real time, and data acquisition begins when the pressure stabilizes at 101.3 kPa.

[0056] The injected current is superimposed with a 10kHz sinusoidal modulated signal, with a modulation depth of 2.2 times the half-width of the absorption line. The output signal of photodetector 6 is demodulated with the second harmonic by a lock-in amplifier, with a time constant of 100ms and a filter of 12dB / oct. The controller inverts the gas concentration based on the second harmonic amplitude and stores the data in CSV format on a local SD card, while simultaneously uploading it to the host computer via an Ethernet interface using the Modbus TCP protocol.

[0057] During continuous measurement, the rotary encoder continuously monitors the reflector angle. When the absolute value of the deviation between the actual angle and the preset value exceeds 8 microradians and lasts for 1.5 seconds, the controller determines it as a valid drift and immediately drives the two-axis piezoelectric tilting platform to compensate in the reverse direction until the deviation returns to within the numerical range. At the same time, the output signals of the flow meter and pressure gauge are compared in real time: if the flow rate drops by more than 10% or the pressure fluctuates abnormally by more than ±5 kPa, the system automatically issues an alarm indicating an abnormal gas path.

[0058] After the measurement is completed, the system automatically records all parameters of this operation, including angle adjustment history, gas concentration curve, flow and pressure data, and timestamps, for subsequent analysis. Maintenance personnel can view historical data or adjust system parameters remotely via the Wi-Fi module without needing to go to the site.

[0059] Compared to Example 1, Example 2 features a modified gas path design with four bottom corner air inlets and four top corner air outlets. It also adds auxiliary small holes with a diameter of 0.6mm in the center of the four side walls and installs miniature one-way valves to prevent backflow. Furthermore, it incorporates a pre-treatment step involving three vacuuming and high-purity nitrogen rinsing cycles. The dynamic compensation trigger condition is adjusted to a deviation >8μrad for 1.5 seconds, employing a more conservative strategy to reduce unnecessary compensation actions. The automatic alarm value range for flow rate drops exceeding 10% or pressure fluctuations exceeding ±5kPa is clearly defined. In terms of data communication, it emphasizes uploading data via Ethernet using the Modbus TCP protocol and utilizing Wi-Fi for remote parameter adjustment. Based on the above differences, the technical effects achieved in Example 2 are as follows: a more symmetrical and uniform flow field reduces the gas replacement time by 20%-30% compared to Example 1, making it particularly suitable for rapid continuous monitoring; vacuuming and nitrogen rinsing effectively remove residual air and moisture from the sealed cavity, avoiding background gas interference and meeting the requirements for high-purity or trace detection; more conservative dynamic compensation triggering conditions extend the service life of piezoelectric elements while ensuring stability; a clearly defined fault alarm value range enhances the system's self-diagnostic capabilities, facilitating timely maintenance notifications when unattended; and through deep integration of Ethernet and Wi-Fi, the system can more easily connect to the Industrial Internet of Things, allowing maintenance personnel to view historical data and adjust parameters without going to the site. Therefore, based on the comprehensive performance verification of Example 1, Example 2 specifically strengthens gas path uniformity, system cleanliness, and intelligent maintenance.

[0060] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

[0061] While the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the scope of the invention. Any person skilled in the art can make modifications without departing from the scope of the invention; all equivalent modifications made in accordance with the invention should be covered by the scope of the invention. In the description of this specification, references to terms such as "one embodiment / mode," "some embodiments / modes," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment / mode or example is included in at least one embodiment / mode or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment / mode or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments / modes or examples. Moreover, without contradiction, those skilled in the art can combine and integrate different embodiments / modes or examples and features of different embodiments / modes or examples described in this specification.

[0062] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this application, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0063] Those skilled in the art should understand that the above embodiments are merely for illustrating the present disclosure and are not intended to limit the scope of the disclosure. Those skilled in the art can make other changes or modifications based on the above disclosure, and these changes or modifications still fall within the scope of the present disclosure.

Claims

1. A gas detection system with a folded optical path type optical multi-pass cell, characterized in that, include: A sealed cavity is provided with a first concave reflector, a second concave reflector, and at least two planar reflectors. The first and second concave reflectors are arranged opposite to each other, and the first concave reflector has a 3mm×3mm rectangular off-axis hole located on the side where the first concave reflector and the planar reflector meet. The planar reflectors are arranged in the optical path between the first and second concave reflectors to fold the optical path so that the incident laser beam is reflected multiple times in the sealed cavity before exiting. A laser is used to generate a detection beam with a mid-infrared wavelength of 3.345 μm, which enters the sealed cavity through an entrance aperture; A photodetector is used to receive a light beam emitted from the sealed cavity and convert it into an electrical signal; A closed-loop angle adjustment system includes: At least one piezoelectric drive element is used to adjust the tilt angle of the plane mirror or the first concave mirror and the second concave mirror; wherein, a biaxial piezoelectric tilting platform is mounted on the back of each of the plane mirror and the second concave mirror adjacent to the second concave mirror. At least one rotary encoder is used to detect the angle information of the first concave reflector, the second concave reflector, and the plane reflector being adjusted in real time; each of the first concave reflector, the second concave reflector, and the plane reflector being adjusted is provided with a high-precision rotary encoder, the rotor of which is coaxially connected to the rotating shaft of the first concave reflector, the second concave reflector, and the plane reflector or the output shaft of the two-axis piezoelectric tilting platform via a coupling, and the stator is fixed on the sealed cavity base; The system also includes a controller that receives feedback signals from the rotary encoder and controls the operation of the piezoelectric drive element based on the deviation between a preset angle value and the actual angle value, thus forming a closed-loop control.

2. The gas detection system with a folded optical path type optical multi-pass cell according to claim 1, characterized in that, It also includes a coaxial tuning optical path, which includes a visible laser for generating a visible tuning beam and a dichroic mirror disposed in the optical path of the probe beam to make the visible tuning beam and the probe beam propagate coaxially; the dichroic mirror has a high transmittance of more than 95% in the 3.345μm band and a high reflectance of more than 99.5% in the 400-700nm visible light band, and its substrate material is ZnSe, coated with a dielectric film system.

3. The gas detection system with a folded optical path type optical multi-pass cell according to claim 2, characterized in that, The refraction of the probe beam by the dichroic mirror results in a tiny parallel gap between the transmitted beam and the incident beam. This tiny parallel gap is compensated by a one-dimensional precision translation stage installed under the dichroic mirror frame. The frame is driven to perform a micrometer-level translation in a direction perpendicular to the optical axis by rotating a differential head.

4. The gas detection system with a folded optical path type optical multi-pass cell according to claim 1, characterized in that, The parallelism of the plane mirrors is judged by the following method: an off-axis laser source is introduced, and the reflected light spots are observed to be evenly distributed on the receiving screen. The diameter of the light spots is 1 mm, and the deviation of the distance between adjacent light spots is controlled within 0.1 mm-0.2 mm. If the deviation of the distance between adjacent light spots is not within 0.1 mm-0.2 mm, it is determined that the parallelism of the two plane mirrors does not meet the requirements and needs to be readjusted.

5. The gas detection system with a folded optical path type optical multi-pass cell according to claim 1, characterized in that, The sealed cavity is rectangular in shape, with dimensions of 100mm in length, 150mm in width, and 80mm in height. By introducing two plane mirrors to fold the optical path, an effective absorption optical path of 14 meters is achieved within the internal space of the rectangular shape. The volume compression ratio, i.e., the ratio of the equivalent cylindrical Herriott pool volume to the internal space volume of the rectangular shape, is 7:

1.

6. The gas detection system with a folded optical path type optical multi-pass cell according to claim 2, characterized in that, It also includes a gas path optimization system, which includes multiple small ventilation holes set on the sealed cavity structure to achieve uniform gas exchange within the sealed cavity, as well as a flow meter and a pressure gauge, used to detect gas flow rate and pressure data within the sealed cavity in real time, respectively; the flow meter is a mass flow controller with a range of 0-1000 sccm, and the pressure gauge is an absolute pressure or gauge pressure sensor with a range of 0-150 kPa, and its output terminal is connected to the controller or an external data acquisition device.

7. The gas detection system with a folded optical path type optical multi-pass cell according to claim 1, characterized in that, The controller is also connected to a data storage module and a communication module for recording historical angle adjustment data and real-time gas detection data, and supporting remote monitoring or parameter adjustment; the communication module includes one or more of RS485, CAN, Ethernet or Wi-Fi, and supports the Modbus RTU / TCP industrial protocol; the data storage module records timestamps, angles of the first concave reflector, the second concave reflector and the plane reflector, gas concentration, flow rate and pressure data in CSV or binary format.

8. A gas concentration detection method, employing the gas detection system with a folded optical path type optical multi-pass cell as described in claim 6; characterized in that, Includes the following steps: The closed-loop angle adjustment system pre-adjusts the angles of the first concave mirror, the second concave mirror, and the plane mirror to the set values, and uses the rotary encoder to confirm that the angles are in place. The laser is activated, and the detection beam enters the sealed cavity through the entrance hole, and after multiple reflections, it exits to the photodetector. The photodetector converts light signals into electrical signals, and the signal processing unit calculates the concentration of the gas to be measured. During the gas measurement process, the flow meter and pressure gauge of the gas path optimization system monitor the gas path status in real time, and the closed-loop angle adjustment system dynamically compensates for angle drift when needed. The trigger condition for dynamic compensation is as follows: the absolute value of the deviation between the actual angle value fed back by the rotary encoder and the preset angle value continues to exceed the preset value range for more than 1 second. Once triggered, the controller immediately drives the piezoelectric drive element to perform reverse compensation until the angle deviation returns to within the preset value range.

9. The gas concentration detection method according to claim 8, characterized in that, During the laser debugging phase, the coaxial debugging optical path is adopted: the visible light laser is turned on, and the visible beam and the probe beam are propagated coaxially. The position of the visible light spot is observed to determine whether the optical path of the probe beam is aligned, and the spacing introduced by the dichroic mirror is compensated. The standard for alignment is that after multiple reflections, the offset between the center of the final emitted visible light spot and the center mark point of the photodetector target surface is less than 5% of the target surface size. After alignment is completed, the visible light laser is turned off, and formal measurement is performed.

10. The gas concentration detection method according to claim 8, characterized in that, The signal processing unit employs wavelength modulation spectroscopy technology. It superimposes a high-frequency sinusoidal modulation signal onto the laser injection current and demodulates the second harmonic signal using a lock-in amplifier to invert the gas concentration. The modulation frequency is 10kHz, the modulation depth is 2.0-2.5 times the half-width of the target gas absorption line, the time constant of the lock-in amplifier is 100ms, and a low-pass filter of 12dB / oct is used.

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