A High-Sensitivity Mid-Infrared Single-Photon Gas Detection System and Method Based on Quantum Correlation Detection

Abstract

This invention discloses a highly sensitive mid-infrared single-photon gas detection system and method based on quantum correlation detection, belonging to the technical field of detection equipment. This invention fabricates a nonlinear crystal and uses it as an optical device in the detection system. The system includes a mid-infrared quantum light source fabrication module, a gas sample analysis module, a frequency upconversion module, and a noise suppression and correlation measurement module. This invention achieves the detection of various gases with absorption characteristics in the mid-infrared band through quantum correlation light pair generation, phase-matching tuning, gas absorption analysis, frequency upconversion detection, and coincidence counting noise suppression. This invention provides an effective quantum correlation-enhanced detection architecture, solving the problems of difficult mid-infrared weak light detection, weak absorption signals of trace gases, and susceptibility to background interference, thus improving detection sensitivity.

Description

Technical Field

[0001] This invention relates to the field of detection equipment technology, specifically to a high-sensitivity detection system and method for mid-infrared single-photon gas based on quantum correlation detection. Background Technology

[0002] The mid-infrared band, with wavelengths between 2.5 and 25 μm, corresponds to a large number of molecular vibrational-rotational transition absorptions. It is a crucial band for the selective identification and quantitative analysis of gas molecules, often referred to as the "molecular fingerprint region." Compared to overtone or combination frequency absorption in the near-infrared band, the intrinsic absorption in the mid-infrared band typically exhibits stronger absorption intensity, thus holding significant importance in trace gas detection, industrial process monitoring, environmental pollution monitoring, leak detection, and hazardous gas identification.

[0003] However, existing mid-infrared gas detection technologies still have some limitations. First, mid-infrared weak light, especially mid-infrared single-photon detection, is quite challenging, and direct detectors typically suffer from high dark noise. Second, in trace gas detection scenarios, the signal is weak and the background is strong, resulting in low signal-to-noise ratios for traditional intensity measurement methods in complex environments. Therefore, further improving sensitivity is a key research direction for various detection instrument manufacturers. Summary of the Invention

[0004] To address the aforementioned limitations of existing technologies, the present invention aims to provide a highly sensitive mid-infrared single-photon gas detection system and method based on quantum correlation detection. The core of this invention lies in enhancing the extraction capability of weak absorption signals through quantum correlation measurement. The invention uses a pump light to drive a first nonlinear crystal, generating correlated photon pairs, one of which is a mid-infrared signal photon, and the other is a prediction photon. After the mid-infrared signal photon passes through the gas sample, its event count or correlation count changes due to gas absorption. Subsequently, this mid-infrared signal photon enters a frequency upconversion module and is converted into an upconverted photon detectable by a near-infrared single-photon detector. Simultaneously, the prediction photon is detected by another detection channel. The system extracts the effective absorption signal and suppresses background noise by performing coincidence counting and time correlation analysis on the two detection events.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a high-sensitivity detection system for mid-infrared single-photon gas based on quantum correlation detection, comprising a mid-infrared quantum light source preparation module, a gas chamber sample analysis module, a frequency up-conversion module, and a noise suppression and correlation measurement module; The mid-infrared quantum light source preparation module includes a 795nm continuous laser 1-1 as a pump light source, a first isolator 1-2, a first filter 1-3, a first Glan prism 1-4, a first half-wave plate 1-5, a first high-reflection mirror 1-6, a second high-reflection mirror 1-7, a first lens 1-8, a first nonlinear crystal 1-9 for generating quantum correlated photon pairs, an off-axis parabolic mirror 1-10, and a first dichroic mirror 1-11. The noise suppression and correlation measurement module includes a third filter 4-1, a tenth high-reflectivity mirror 4-2, a sixth lens 4-3, and a second single-photon detector 4-4 arranged sequentially along the optical path; an eighth high-reflectivity mirror 4-6, a ninth high-reflectivity mirror 4-7, a filter group 4-8, a fifth lens 4-9, and a first single-photon detector 4-10 arranged sequentially along the optical path; and a time-correlated single-photon counter 4-5 that receives signals from the first single-photon detector 4-10 and the second single-photon detector 4-4.

[0006] Furthermore, the gas chamber sample analysis module includes a third high-reflection mirror 2-1, a fourth high-reflection mirror 2-2, a gas cell 2-3, a fifth high-reflection mirror 2-4, and a sixth high-reflection mirror 2-5 arranged sequentially along the optical path.

[0007] Furthermore, the frequency upconversion module includes a second half-wave plate 3-8, a second lens 3-9, a second dichroic mirror 3-10, a second nonlinear crystal 3-11, and a fourth lens 3-12 arranged sequentially along the optical path, a 1064nm continuous laser 3-1 serving as the pump source, a second isolator 3-2, a second filter 3-3, a second Glan prism 3-4, a third half-wave plate 3-5, a seventh high-reflection mirror 3-6, and a third lens 3-7 arranged sequentially along the optical path.

[0008] Furthermore, the first nonlinear crystal 1-9 is fixed to an angle-adjustable rotating platform.

[0009] Furthermore, the first nonlinear crystal 1-9 has a replaceable structure.

[0010] In a second aspect, the present invention provides a high-sensitivity detection method for mid-infrared single-photon gas based on quantum correlation detection, which is implemented using the aforementioned high-sensitivity detection system for mid-infrared single-photon gas, and includes the following steps: (1) determining the target working band and the target detection wavelength based on the characteristic absorption peak of the target gas to be measured; (2) Adjust the phase matching angle of the first nonlinear crystal to output mid-infrared signal photons of the corresponding wavelength; (3) Use the mid-infrared quantum light source preparation module to generate mid-infrared signal photons and their corresponding prediction photons; the mid-infrared signal photons pass through the gas sample to be tested; the frequency of the absorbed mid-infrared signal photons is upconverted; (4) Perform single-photon detection on the predicted photon and the upconversion photon respectively; (5) Perform coincidence counting, time gating and correlation statistical analysis on the detection results of step (4); (6) Extract absorption information based on changes in effective related events and output gas detection results.

[0011] The beneficial effects of this invention are: 1. This invention prepares a high-sensitivity mid-infrared single-photon gas detection system based on quantum correlation detection, comprising a mid-infrared quantum light source preparation module, a gas sample analysis module, a frequency up-conversion module, and a noise suppression and correlation measurement module. It fully utilizes the correlation between the prediction light and the signal light to perform coincidence counting and noise suppression, providing an effective quantum correlation-enhanced detection architecture.

[0012] 2. This invention utilizes quantum correlation measurement and coincidence counting to extract weak gas absorption information from complex backgrounds, improving the detection capability of weak absorption signals. It solves the problems of weak gas absorption signals and small signal variations in trace detection, as well as susceptibility to background interference caused by gas absorption, such as environmental thermal radiation, stray light, and detector dark counting. In the detection of carbon dioxide, the system of this invention achieves a detection sensitivity of 1E-10 mol / mol, which is superior to the typical detection sensitivity of existing technologies, which is 1E-9 mol / mol.

[0013] This invention uses a frequency up-conversion method to convert mid-infrared single photons into near-infrared photons for detection, which improves the feasibility of the system and solves the problems of existing mid-infrared direct detectors having high dark noise, high thermal noise, high system complexity, and being unsuitable for high-sensitivity detection under single-photon or extremely weak light conditions.

[0014] This invention allows for switching between different detection wavelengths within the same operating band by adjusting the phase matching angle of the first nonlinear crystal, thus providing intra-band tuning capability. This solves the problem that different target gases correspond to different absorption wavelengths, making it impossible to quickly switch target wavelengths through phase matching adjustment.

[0015] By replacing different nonlinear crystals, this invention enables the system to be applicable to the detection of target gases in multiple mid-infrared sub-bands, possessing cross-band extension capability. This invention has a wide range of applications, not only for the detection of nitrogen dioxide and sulfur dioxide, but also for the detection of carbon dioxide, methane, and other gases with absorption characteristics in the mid-infrared band. Attached Figure Description

[0016] Figure 1This is a high-sensitivity mid-infrared single-photon gas detection system based on quantum correlation detection. In this system, 1-1 is a 795nm continuous-wave laser, 1-2 is the first isolator, 1-3 is the first filter, 1-4 is the first Glan prism, 1-5 is the first half-wave plate, 1-6 is the first high-reflection mirror, 1-7 is the second high-reflection mirror, 1-8 is the first lens, 1-9 is the first nonlinear crystal, 1-10 is an off-axis parabolic mirror, and 1-11 is the first dichroic mirror; 2-1 is the third high-reflection mirror, 2-2 is the fourth high-reflection mirror, 2-3 is the gas cell, 2-4 is the fifth high-reflection mirror, and 2-5 is the sixth high-reflection mirror; 3-1 is a 1064nm continuous-wave laser, and 3-2 is... The second isolator, 3-3 is the second filter, 3-4 is the second Glan prism, 3-5 is the third half-wave plate, 3-6 is the seventh high-reflection mirror, 3-7 is the third lens, 3-8 is the second half-wave plate, 3-9 is the second lens, 3-10 is the second dichroic mirror, 3-11 is the second nonlinear crystal, 3-12 is the fourth lens; 4-1 is the third filter, 4-2 is the tenth high-reflection mirror, 4-3 is the sixth lens, 4-4 is the second single-photon detector, 4-5 is the time-correlated single-photon counter, 4-6 is the eighth high-reflection mirror, 4-7 is the ninth high-reflection mirror, 4-8 is the filter group, 4-9 is the fifth lens, and 4-10 is the first single-photon detector.

[0017] Figure 2 The results show the detection of methane at a wavelength of approximately 3.2 μm in the 2–5 μm band. In the figure, the vertical axis represents the photon response signal intensity, the black dots represent the detected photons, the red line is the photon signal line, and the black line is the signal fitting line.

[0018] Figure 3 The results show the detection of carbon dioxide at a wavelength of approximately 4.3 μm in the 2–5 μm band. In the figure, the vertical axis represents the photon response signal intensity, the black dots represent the detected photons, the red line is the photon signal line, and the black line is the signal fitting line.

[0019] Figure 4 The results show the detection of nitrogen dioxide at a wavelength of approximately 6.3 μm in the 6–8 μm band. In the figure, the vertical axis represents the photon response signal intensity, the black dots represent the measured photons, the red line is the photon signal line, and the black line is the signal fitting line.

[0020] Figure 5 The results show the detection of sulfur dioxide at a wavelength of approximately 7.3 μm in the 6–8 μm band. In the figure, the vertical axis represents the photon response signal intensity, the black dots represent the detected photons, the red line is the photon signal line, and the black line is the signal fitting line. Detailed Implementation

[0021] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of this application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains.

[0022] To enable those skilled in the art to better understand the technical solution of this application, the technical solution of this application will be described in detail below with reference to specific embodiments.

[0023] The quantum correlation detection technology employed in this invention generates correlated photon pairs through a nonlinear process. One pair serves as a mid-infrared signal light that interacts with the analyte gas, while the other acts as a prediction light for reference detection. The temporal correlation between these two pairs allows for coincidence counting and background filtering, enabling the extraction of effective absorption events even in high-noise environments. Through frequency up-conversion technology, mid-infrared single photons can be converted to the near-infrared band for detection by a mature near-infrared single-photon detector, reducing the difficulty of direct mid-infrared detection and ultimately improving detection sensitivity.

[0024] The test materials used in the embodiments of this invention, unless otherwise specified, are all conventional test materials in the art and can be purchased through commercial channels. The nonlinear crystal used in this invention was purchased from Chengdu Dean Optoelectronics Technology Co., Ltd. (1) The parameters of the first nonlinear crystal are as follows: Crystal type: selenium germanium gallium barium; Crystal characteristics: trigonal crystal system, space group R3; Nonlinear coefficient: d 11 =23.6 pm / V (10.591 µm frequency doubling process); damage threshold: 201 Mw / cm 2 @8.6 ns, 1.053 µm; Size: 7 7 5 mm 3 (2) The parameters of the second nonlinear crystal are as follows: Crystal type: silver sulfide gallium; Crystal characteristics: tetragonal crystal system, space group 42m; Nonlinear coefficient: d 36 =12.6 pm / V (10.591 µm frequency doubling process); damage threshold: 30 Mw / cm 2 @10ns, 1.064 µm; Size: 7 7 10 mm 3 .

[0025] Example 1: A High-Sensitivity Mid-Infrared Single-Photon Gas Detection System Based on Quantum Correlation Detection 1. System Construction The system of this invention includes a mid-infrared quantum light source preparation module, a gas chamber sample analysis module, a frequency up-conversion module, and a noise suppression and correlation measurement module, see [link to relevant documentation]. Figure 1The details are as follows: 1.1 Mid-infrared quantum light source fabrication module The mid-infrared quantum light source preparation module is used to generate mid-infrared signal photons and associated prediction photons. This module includes a 795nm continuous laser 1-1 as the pump source, a first isolator 1-2, a first filter 1-3, a first Glan prism 1-4, a first half-wave plate 1-5, a first high-reflection mirror 1-6, a second high-reflection mirror 1-7, a first lens 1-8, a first nonlinear crystal 1-9 for generating quantum-correlated photon pairs, an off-axis parabolic mirror 1-10, and a first dichroic mirror 1-11.

[0026] This module mainly consists of a narrow-linewidth continuous-pump light source, a first nonlinear crystal, and a crystal mounting and adjustment mechanism. The first nonlinear crystal is mounted on an angle-rotatable platform. By rotating this platform, the phase matching angle of the first nonlinear crystal can be changed, thereby altering the center wavelength of the generated mid-infrared signal photons to match the characteristic absorption peaks of different gases. The first nonlinear crystal is replaceable. By replacing it with different types of nonlinear crystals, the achievable operating wavelength range of the system can be changed, making the system suitable for gas detection in different mid-infrared sub-bands. In other words, this invention has two levels of adjustment capability: Level 1 adjustment: Under the same nonlinear crystal, changing the phase matching angle by rotating the platform achieves target wavelength switching within the same operating wavelength range; Level 2 adjustment: By replacing different nonlinear crystals, the operating wavelength range can be extended from one mid-infrared sub-band to another.

[0027] The detailed structure and specific functions of this module are as follows: A narrow-linewidth continuous-wave laser with a center wavelength of 795 nm, an output power greater than 1 W, a linewidth less than 1 MHz, and a power stability better than 1% is used as the pump source, outputting spatially collimated linearly polarized light. An optical isolator corresponding to the wavelength band is configured at the laser's output end to effectively block potential interference from back-reflected light to the laser body, ensuring stable system operation. The pump light first passes through a bandpass filter to remove stray light, retaining only the 795 nm center wavelength laser. Then, the polarization purity is further improved by a Glan laser polarizing prism (Thorlabs GL10-B), followed by fine polarization angle adjustment by an achromatic half-wave plate (Lbtek AHWP20-SNIR) to achieve strict matching with the phase-matching conditions in the nonlinear crystal. After polarization adjustment, the pump light passes through two dielectric high-reflection mirrors (Lbtek BDM1-B) to change the beam propagation direction and collimate. The pump light is focused onto a nonlinear crystal using a plano-convex lens (Lbtek MCX10613-B) selected for the target wavelength, and the down-conversion efficiency is improved by adjusting the appropriate focal length. In the nonlinear crystal, the pump light generates a pair of quantum-correlated photons through a type-I angle-phase-matched spontaneous parametric down-conversion (SPDC) process: a mid-infrared signal photon with a wavelength in the 6-8 μm band and a near-infrared prediction photon with a wavelength range of 882.7-916.4 nm. This process satisfies energy conservation and angle-phase-matching conditions, resulting in a two-photon state that is anti-correlated in frequency and synchronized in time, thus forming a strongly quantum-correlated two-photon state. The output end of the nonlinear crystal uses a gold-coated 90° off-axis parabolic mirror (Lbtek MPM10101-90-AU), which combines high reflectivity and achromatic properties to eliminate chromatic aberration between the mid-infrared signal photon and the near-infrared prediction photon, and can collimate and shape the beam into parallel light. Subsequently, the two photons are spatially separated using a dichroic mirror (Edmund). The coating must ensure that the mid-infrared signal photons propagate along the transmission direction, while the near-infrared prediction photons and residual pump light are reflected to another optical path, providing good input light conditions for subsequent detection and analysis.

[0028] The nonlinear crystal is fixed to an angle-adjustable rotating platform. The rotation angle of the platform is pre-set according to the phase matching angle corresponding to the target signal wavelength. When changing the wavelength, a light barrier must be used to block the pump laser from incident on the system to prevent damage to the crystal. The nonlinear crystal mounted on the angle-adjustable rotating platform is adjusted to the corresponding phase matching angle by an electrically controlled rotating platform, and the pitch and deflection angles are fine-tuned to optimize the maximum signal strength. To achieve the desired angle phase matching condition by rotating the nonlinear crystal when switching between different test light sources, an electric rotary stage is configured in the system for precise adjustment of the crystal's rotation angle. A Kohzu RA04A-W01 electric rotary stage is selected, with a rotation range of ±177°, which is much larger than the actual rotation angle range required during wavelength switching. Simultaneously, this rotary stage has an angular resolution of up to 0.0002°, providing extremely high rotation control accuracy and effectively meeting the requirements for precise phase matching and repeatable adjustment of the nonlinear crystal.

[0029] 1.2 Gas Sample Analysis Module The gas sample analysis module is used to allow mid-infrared signal photons to pass through and be absorbed by the gas sample being tested. This module is positioned along the transmission path of the mid-infrared signal photons. The gas sample analysis module can take the form of a closed gas chamber, a multi-pass cell, an open sampling channel, or a replaceable sample cell. The gas chamber sample analysis module includes a third high-reflectivity mirror 2-1, a fourth high-reflectivity mirror 2-2, a gas cell 2-3, a fifth high-reflectivity mirror 2-4, and a sixth high-reflectivity mirror 2-5 arranged sequentially along the optical path. This module improves detection selectivity and sensitivity by setting the wavelength of the mid-infrared signal photons to match the characteristic absorption peaks of the gas being tested within the corresponding wavelength band.

[0030] The detailed structure and specific functions of this module are as follows: Before entering the subsequent frequency upconversion module, the mid-infrared signal photons generated by the mid-infrared quantum light source preparation module are first collimated by two mid-infrared enhanced gold film mirrors (Thorlabs PF10-03-M02) and then incident into the mid-infrared gas cell (Thorlabs HC10L / M-M02). The mid-infrared gas cell enables multiple transmission interactions with the target molecular sample. By adjusting parameters such as sample concentration and designed optical path length, the absorption information of the analyte gas on the mid-infrared signal photons can be obtained. After exiting the gas cell, the signal photons are again collimated by two mid-infrared enhanced gold film mirrors (Thorlabs PF10-03-M02). The mid-infrared gas cell has a 10.4 m optical path and features a concave mirror coated with a protective gold film to enhance performance in the mid-infrared band. Each gas cell has two gas connection ports, which can be used for both static pressure configuration and flow cell applications.

[0031] The mid-infrared gas cell used in this module is based on the Herriott gas cell (Thorlabs HC10L / M-M02). The Herriott gas cell is an optical component used for gas absorption spectroscopy analysis, particularly suitable for applications requiring long optical paths to improve gas detection sensitivity. A Herriott gas cell typically consists of two highly reflective concave mirrors. This module uses a gold-coated concave mirror covering the 600 nm-10 μm band, including the target's mid-infrared wavelength. The two concave mirrors are placed opposite each other. One mirror has a small hole through which mid-infrared photons enter the gas cell and exit through the other small hole after multiple reflections. After entering through the entrance hole, the mid-infrared photons reflect back and forth between the two concave mirrors, forming a path of multiple reflections. After each reflection, the beam travels a certain distance within the cell, significantly increasing the optical path. The Herriott cell causes the beam to reflect along a specific trajectory, preventing the beam from repeatedly passing through the same area and ensuring uniform interaction between the gas and the light. The optical path length is determined by the number of reflections and the distance between the mirrors. By adjusting the incident angle and the distance between the mirrors, the optical path length for the selected model is 10.439 ± 0.015 m. The Herriott gas cell is characterized by high sensitivity. Because mid-infrared photons extend the optical path within the gas cell, they enhance the interaction between light and gas, significantly improving the sensitivity of gas detection and facilitating the analysis of gas concentration and composition. Compared to other long-path gas cells, the Herriott gas cell is compact and small in size, allowing for adjustment of the optical path length to meet specific needs, thus offering high flexibility.

[0032] 1.3 Frequency Up-Conversion Module The frequency upconversion module is used to convert mid-infrared signal photons, after gas absorption, into a more easily detectable wavelength range. This module includes, sequentially arranged along the optical path, a second half-wave plate 3-8, a second lens 3-9, a second dichroic mirror 3-10, a second nonlinear crystal 3-11, and a fourth lens 3-12; a 1064nm continuous laser 3-1 serving as the pump source; and, sequentially arranged along the optical path, a second isolator 3-2, a second filter 3-3, a second Glan prism 3-4, a third half-wave plate 3-5, a seventh high-reflection mirror 3-6, and a third lens 3-7. This module mainly comprises the second nonlinear crystal and the upconversion pump source. Mid-infrared signal photons and upconverted pump light undergo a sum-frequency interaction in the second nonlinear crystal to generate near-infrared upconverted photons.

[0033] The detailed structure and specific functions of this module are as follows: After passing through the gas chamber analysis module, the mid-infrared photons enter the frequency up-conversion module. To ensure complete polarization alignment with the pump light and meet the first-type phase-matching condition in the nonlinear crystal, a custom-designed wide-band mid-infrared half-wave plate (Edmund) is used for fine-tuning the polarization direction, covering a band of 6-8 μm. After adjustment by the half-wave plate, the mid-infrared signal light is focused onto the center of the nonlinear crystal by a zinc selenide (ZnSe) plano-convex lens (Thorlabs LA7261-E4). This lens selection was optimized through simulation calculations to match the focusing radius with the waist radius of the pump beam, thereby maximizing conversion efficiency. The focused signal light and pump light are then spatially combined using a custom-designed dichroic mirror (Edmund). This dichroic mirror has high reflectivity for the pump light (1064 nm) and high transmittance for mid-infrared light in the 6-8 μm range, ensuring geometrically that the two beams propagate collinearly and are collimated into the nonlinear crystal. In the overall signal optical path, necessary optical components are mounted on adjustable brackets and displacement stages, supporting translation and angle fine-tuning in the XYZ directions to ensure consistent optical axis height, precise alignment of the beam waist position with the crystal center, and improved mode field coincidence and system stability.

[0034] The pump light is provided by a continuous-wave laser with a center wavelength of 1064 nm and an average power of 10 W, outputting a linearly polarized, single-mode, spatially collimated Gaussian beam. Similar to the mid-infrared quantum light source preparation module, to prevent damage to the laser from reflected light generated by subsequent optical components (such as crystals and mirrors), an optical isolator (Thorlabs IO-5-1064-HP) suitable for high laser power is placed at the laser output end of the system. After passing through the optical isolator, the pump light is first filtered by a filter (Thorlabs FLH1064-3) to remove any noise that may be present in the laser, and then purified by a Glan laser crystal polarizing prism (Thorlabs GL10-C) with a high extinction ratio to ensure consistent linear polarization direction. Subsequently, a quartz true zero-order half-wave plate (Lbtek HWP25-1064A-M) is used to fine-tune the polarization direction, aligning it with the polarization direction of the mid-infrared signal photons, thus meeting the first-type phase-matching requirement of a nonlinear crystal angle-phase-matched crystal. To achieve optimal mode field matching, the pump light is focused onto the center of the crystal using a lens with a suitable focal length. This lens is designed for a 1064 nm pump light wavelength, providing high-quality focusing and a stable spot size. Its mounting distance is adjusted in conjunction with beam waist coincidence conditions to ensure optimal spatial overlap with the mid-infrared beam within the crystal.

[0035] The upconversion pump lens is Thorlabs' ACA254-150-B air-spacing achromatic doublet. This lens offers excellent spherical and chromatic aberration correction performance, with broadband antireflective coatings of 650-1050 nm on both sides, achieving a transmittance greater than 97% near 1 μm. This ensures efficient pump light transmission while maintaining low lateral and longitudinal aberrations. Its effective focal length of 100 mm maintains a compact focusing path while forming a suitable focused spot size and divergence angle within the crystal, balancing spatial overlap and phase matching. In engineering design, the air-spacing achromatic doublet is manufactured without any adhesives, exhibiting high resistance to laser damage. Compared to cemented doublets, the two surfaces within the air-spacing lens do not need to have identical curvature; this variable characteristic allows it to outperform cemented doublets in some cases.

[0036] A dielectric high-reflectivity mirror (Lbtek BDM1-B) is also used for guidance and collimation in the pump light path, and spatially combined with the signal light by the aforementioned dichroic mirror (Edmund) after the focusing lens. The two beams are then collinearly incident on the nonlinear crystal to generate sum-frequency photons with wavelengths of 904-939 nm, and collimated and shaped into parallel light output by a cemented doublet achromatic lens (Thorlabs AC254-100-B). The lenses before and after the frequency up-conversion crystal are fixed on a lens frame with XY dual-axis translation and a Z-axis mechanical displacement platform. A one-inch translation adjustment frame (Thorlabs POLARIS-1XY) keeps the lens fixed and provides high-resolution adjustment (0.2 mm / rev) to ensure the center is aligned with the optical axis. Test data shows that the deviation of this adjustment frame is less than 5 µrad after a 10°C temperature cycle, achieving long-term alignment stability.

[0037] 1.4 Noise Suppression and Correlation Measurement Module The noise suppression and correlation measurement module includes a third filter 4-1, a tenth high-reflectivity mirror 4-2, a sixth lens 4-3, and a second single-photon detector 4-4 arranged sequentially along the optical path; an eighth high-reflectivity mirror 4-6, a ninth high-reflectivity mirror 4-7, a filter group 4-8, a fifth lens 4-9, and a first single-photon detector 4-10 arranged sequentially along the optical path; and a time-correlated single-photon counter 4-5 that receives signals from the first single-photon detector 4-10 and the second single-photon detector 4-4.

[0038] The noise suppression and correlation measurement module performs functions including forecast light detection, upconversion signal detection, and correlation measurement and signal processing. Forecast light detection: This module performs single-photon detection on forecast photons and outputs the detection event time and count information. The forecast photons serve as a reference channel for mid-infrared signal photons. Upconversion signal detection: This module performs single-photon detection on upconverted signal photons and outputs the detection event time and count information. Correlation measurement and signal processing: This module receives the detection event data output from the forecast light detection module and the upconversion signal detection module and performs time correlation analysis, coincidence counting, gating filtering, background subtraction, and correlation statistics. By counting only events that temporally meet the correlation conditions, the impact of background thermal noise, stray light, and dark counts on the results can be significantly reduced.

[0039] The detailed structure and specific functions of this module are as follows: To ensure the effective detection of the spectrum-shifted sum-frequency photons under high signal-to-noise ratio conditions, the noise suppression and correlation measurement module further spatially guides, suppresses noise, and precisely focuses the aligned sum-frequency photons before finally connecting them to the single-photon detector. The sum-frequency photons from nonlinear crystal 2 are guided by reflection from two dielectric film mirrors (Lbtek BDM1-B) and then enter the filter array. This filter array plays a crucial role in filtering out pump residual light, background scattered light, and non-target wavelength signals from the system. The filter array consists of multiple filter groups that can be flexibly combined and adjusted according to the test wavelength. These include a notch filter (Thorlabs NF1064-44) designed for the 1064nm pump laser wavelength to suppress possible residual pump signals. Additionally, it includes short-pass (Thorlabs FESH0950) and long-pass (Thorlabs FELH0875) filters with steep edges to block long-wavelength background infrared light and the 795nm pump light component that may leak from the mid-infrared quantum light source preparation module, further reducing the detection noise floor. To eliminate the strong scattering noise and laser safety hazards that may be caused by the 1μm band pump light reflected by the notch filter, an optical trap (BT600 / M, Thorlabs) is placed in the direction of the reflected light path of the notch filter. This optical trap can withstand continuous light with an average power of up to 80 W, covering a wavelength range of 200 nm to 3 µm, and can meet the requirement of absorbing continuous light energy on the order of 10 W. The optical trap uses a high-damage-threshold absorbing material inside, which can effectively dissipate incident energy and minimize back reflection.

[0040] For different upconversion center wavelengths, a variety of high-transmittance bandpass filters are provided for precise selection of the pass and pass signals. Considering that different test wavelengths require different filter types, the system is equipped with a 4-position sliding filter mounting bracket. The corresponding bandpass filter is installed for the test wavelength, and the filter can be quickly switched through modular insert control to adapt to test tasks of different long infrared target wavelengths.

[0041] In the near-infrared forecast photon channel, spectral selection is performed using a bandpass filter (Thorlabs FBH880-40) to retain only near-infrared forecast photon information within the target band. The filtered sum-frequency photons and near-infrared forecast photons are then focused onto the end face of a multimode fiber by coupling lenses (C220TMD-B, Thorlabs), and efficient photon coupling is achieved using a fiber optic coupling system (MBT613D / M, Thorlabs) with an FC connector fiber support. This coupling stage features three-dimensional flexible adjustment capabilities, enabling precise alignment between the lens and the fiber, thereby maximizing coupling efficiency.

[0042] The photon detection section employs two high-performance silicon-based single-photon counters (SPCM-900-14), operating in the 400-1000 nm wavelength range. They achieve a detection efficiency of up to 58% at 850 nm, with a dark count as low as 100 cps and a timing jitter of 350 ps. The detector operates in free-running mode with a dead time of 40 ns, a typical saturation count rate of 37 Mc / s, and a post-pulse probability of only 1%. Their high detection efficiency and low dark count characteristics ensure high signal-to-noise ratio and accuracy for coincidence measurements, featuring low background counting, high efficiency, and outputting a standard TTL level pulse signal. The two coincidence counts are measured using a time-correlated single-photon counter (Xingmiao Optoelectronics MT16). By improving the ratio of coincidence events to non-coincidence events, the system's anti-interference capability and effective event extraction capability under different noise levels can be accurately evaluated.

[0043] 1.2 Application of the Mid-Infrared Single-Photon Gas High-Sensitivity Detection System This invention relates to a highly sensitive mid-infrared single-photon gas detection method based on quantum correlation detection, comprising the following steps: (1) Determine the target working band and target detection wavelength based on the characteristic absorption peaks of the target gas to be measured; (2) Adjust the phase matching angle of the first nonlinear crystal to output mid-infrared signal photons of the corresponding wavelength; (3) Use the mid-infrared quantum light source preparation module to generate mid-infrared signal photons and their corresponding prediction photons; the mid-infrared signal photons pass through the gas sample to be tested; the frequency of the absorbed mid-infrared signal photons is upconverted; (4) Perform single-photon detection on the predicted photon and the upconversion photon respectively; (5) Perform coincidence counting, time gating and correlation statistical analysis on the detection results of step 4; (6) Extract absorption information based on changes in effective related events and output gas detection results.

[0044] Example 2: Gas Detection 1. Detection of methane at a wavelength of approximately 3.2 μm in the 2–5 μm band. A methane gas standard in helium with a concentration of approximately 1E-10 mol / mol was generated using an ultra-trace gas standard preparation system (National Defense Applied Chemistry Professional Metrology Station, critical sharp-orifice method, GB / T5275.6-2023) at a concentration of 0.1 MPa, 293 K, and 1 L / min. The system was used for testing, and the results are shown in the figure. Figure 2 In the figure, the vertical axis represents the intensity of the photon response signal, the black dots represent the measured photons, the red lines are the photon signal lines, and the black lines are the signal fitting lines.

[0045] according to Figure 2 As a result, a photon response signal was measured near 3230 nm, which was basically consistent with the theoretical gas absorption spectrum, indicating that the detection sensitivity of the system of the present invention for methane gas reached 1E-10 mol / mol.

[0046] 2. Detection of carbon dioxide at a wavelength of approximately 4.3 μm in the 2–5 μm band. Using an ultra-trace gas standard material preparation system (National Defense Applied Chemistry Professional Metrology Station), a carbon dioxide gas standard material in helium with a concentration of approximately 1E-10 mol / mol was generated (0.1 MPa, 293 K, 1 L / min). This standard gas material was introduced into the system of this invention for testing, and the results are shown in […]. Figure 3 In the figure, the vertical axis represents the intensity of the photon response signal, the black dots represent the measured photons, the red lines are the photon signal lines, and the black lines are the signal fitting lines.

[0047] according to Figure 3 As a result, a photon response signal was measured near 4250 nm, which was basically consistent with the theoretical gas absorption spectrum, indicating that the detection sensitivity of the system for carbon dioxide gas reached 1E-10 mol / mol.

[0048] 3. Detection of nitrogen dioxide at a wavelength of approximately 6.3 μm in the 6–8 μm band. A nitrogen dioxide gas standard in helium with a concentration of approximately 1E-10 mol / mol was generated using an ultra-trace gas standard preparation system (National Defense Applied Chemistry Professional Metrology Station, critical sharp orifice method, GB / T5275.6-2023) at 0.1 MPa, 293 K, and 1 L / min. The system was used for testing, and the results are shown in the figure. Figure 4In the figure, the vertical axis represents the intensity of the photon response signal, the black dots represent the measured photons, the red lines are the photon signal lines, and the black lines are the signal fitting lines.

[0049] according to Figure 4 As a result, a photon response signal was detected near 6250 nm, which was basically consistent with the theoretical gas absorption spectrum, indicating that the detection sensitivity of the system for nitrogen dioxide gas reached 1E-10 mol / mol. 4. Detection of sulfur dioxide at a wavelength of approximately 7.3 μm in the 6–8 μm band. A sulfur dioxide gas standard in helium with a concentration of approximately 1E-10 mol / mol was generated using an ultra-trace gas standard preparation system (National Defense Applied Chemistry Professional Metrology Station, critical sharp-orifice method, GB / T5275.6-2023) at 0.1 MPa, 293 K, and 1 L / min. The system was used for testing, and the results are shown in the figure. Figure 5 In the figure, the vertical axis represents the intensity of the photon response signal, the black dots represent the measured photons, the red lines are the photon signal lines, and the black lines are the signal fitting lines.

[0050] according to Figure 5 As a result, photon response signals were detected near 7270 nm and 7350 nm, which were basically consistent with the theoretical gas absorption spectrum, indicating that the detection sensitivity of the system of the present invention for sulfur dioxide gas reached 1E-10 mol / mol.

[0051] It is evident that the system of this invention achieves a detection sensitivity of 1E-10 mol / mol for methane, carbon dioxide, nitrogen dioxide, and sulfur dioxide, demonstrating a high level of detection sensitivity.

[0052] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A high-sensitivity mid-infrared single-photon gas detection system based on quantum correlation detection, characterized in that, It includes a mid-infrared quantum light source preparation module, a gas chamber sample analysis module, a frequency upconversion module, and a noise suppression and correlation measurement module; The mid-infrared quantum light source preparation module includes a 795nm continuous laser (1-1) as a pump light source, a first isolator (1-2), a first filter (1-3), a first Glan prism (1-4), a first half-wave plate (1-5), a first high-reflection mirror (1-6), a second high-reflection mirror (1-7), a first lens (1-8), a first nonlinear crystal (1-9) for generating quantum correlated photon pairs, an off-axis parabolic mirror (1-10), and a first dichroic mirror (1-11). The noise suppression and correlation measurement module includes a third filter (4-1), a tenth high-reflectivity mirror (4-2), a sixth lens (4-3), and a second single-photon detector (4-4) arranged sequentially along the optical path; an eighth high-reflectivity mirror (4-6), a ninth high-reflectivity mirror (4-7), a filter group (4-8), a fifth lens (4-9), and a first single-photon detector (4-10) arranged sequentially along the optical path; and a time-correlated single-photon counter (4-5) that receives signals from the first single-photon detector (4-10) and the second single-photon detector (4-4).

2. The mid-infrared single-photon gas high-sensitivity detection system based on quantum correlation detection according to claim 1, characterized in that, The gas chamber sample analysis module includes a third high-reflection mirror (2-1), a fourth high-reflection mirror (2-2), a gas cell (2-3), a fifth high-reflection mirror (2-4), and a sixth high-reflection mirror (2-5) arranged sequentially along the optical path.

3. The mid-infrared single-photon gas high-sensitivity detection system based on quantum correlation detection according to claim 1, characterized in that, The frequency upconversion module includes a second half-wave plate (3-8), a second lens (3-9), a second dichroic mirror (3-10), a second nonlinear crystal (3-11), and a fourth lens (3-12) arranged sequentially along the optical path, a 1064nm continuous laser (3-1) serving as the pump source, a second isolator (3-2), a second filter (3-3), a second Glan prism (3-4), a third half-wave plate (3-5), a seventh high-reflectivity mirror (3-6), and a third lens (3-7) arranged sequentially along the optical path.

4. The mid-infrared single-photon gas high-sensitivity detection system based on quantum correlation detection according to claim 1, characterized in that, The first nonlinear crystal (1-9) is fixed to an angle-adjustable rotating platform.

5. The mid-infrared single-photon gas high-sensitivity detection system based on quantum correlation detection according to claim 1, characterized in that, The first nonlinear crystal (1-9) has a replaceable structure.

6. A highly sensitive detection method for mid-infrared single-photon gas based on quantum correlation detection, characterized in that, The method of implementing the mid-infrared single-photon gas high-sensitivity detection system as described in any one of claims 1-5 includes the following steps: (1) determining the target working band and the target detection wavelength based on the characteristic absorption peak of the target gas to be measured; (2) Adjust the phase matching angle of the first nonlinear crystal to output mid-infrared signal photons of the corresponding wavelength; (3) Use the mid-infrared quantum light source preparation module to generate mid-infrared signal photons and their corresponding prediction photons; the mid-infrared signal photons pass through the gas sample to be tested; the frequency of the absorbed mid-infrared signal photons is upconverted; (4) Perform single-photon detection on the predicted photon and the upconversion photon respectively; (5) Perform coincidence counting, time gating and correlation statistical analysis on the detection results of step 4; (6) Extract absorption information based on changes in effective related events and output gas detection results.

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