A photothermal spectroscopy gas detection system based on asymmetric micro-nano optical waveguides

Abstract

The present invention discloses a photothermal spectroscopy gas detection system based on an asymmetric micro-nano optical waveguide, which relates to the field of gas measurement. The system includes a detection light source component, a pump laser component, a polarization interference ring of the asymmetric micro-nano optical waveguide, and a data acquisition component. After the detection light is input into the polarization interference ring, the detection light propagating in opposite directions generates a phase difference between two orthogonal polarization modes, and forms polarization interference at the coupler. At the same time, the exterior of the micro-nano optical waveguide is filled with the gas to be measured; the pump laser excites an evanescent wave in the micro-nano optical waveguide, and the gas absorbs the evanescent wave to produce a photothermal effect, thereby changing the refractive index of the gas and the optical waveguide; the data acquisition component detects the additional phase difference generated and obtains gas concentration information. By locking the wavelength of the detection light at an orthogonal point near the inflection point of the group birefringence, the present invention achieves a significant improvement in detection sensitivity, which can meet the detection requirements of high sensitivity and fast response.

Description

Technical Field

[0001] The present invention relates to the field of gas measurement, and in particular to a photothermal spectrum gas detection system based on asymmetric micro-nano optical waveguides. Background Art

[0002] High-precision spectral gas analysis is a research hotspot in the field of fiber optic sensing. Compared with traditional gas chromatography-mass spectrometry, spectroscopy-based gas detection technology, which relies on the characteristic "absorption fingerprints" of different molecules, offers advantages such as high selectivity, simple operation, fast measurement, and maintenance-free operation.

[0003] Fiber optic spectroscopy gas detection technology uses optical fiber as the light transmission medium. Early direct absorption sensors mainly used lens groups as open path gas chambers, with optical fiber serving only as the light transmission medium. In addition, problems such as optical collimation and volume limitations meant that the absorption length could not be too long, limiting its detection sensitivity.

[0004] To improve detection sensitivity, researchers are using an indirect absorption method—photothermal spectroscopy—to detect gas. This technology, combined with microstructured hollow-core optical fibers as optical waveguides and absorption chambers, achieves high-precision gas detection. However, technologies that rely on increasing the length of hollow-core optical fibers to improve measurement accuracy generally suffer from slow response times. For example, if the hollow-core optical fiber length is increased to ten or even a hundred meters, and gas enters the fiber primarily through free diffusion, while sensitivity improves, response speed is significantly slowed. Therefore, existing photothermal spectroscopy detection systems struggle to achieve both rapid and highly sensitive dynamic gas monitoring, failing to meet the comprehensive requirements for gas measurement systems in multiple fields, including environmental protection, security, and healthcare. Summary of the Invention

[0005] Based on this, an embodiment of the present invention provides a photothermal spectroscopy gas detection system based on asymmetric micro-nano optical waveguides, which meets the detection requirements of both high sensitivity and fast response.

[0006] To achieve the above objectives, the present invention provides the following solutions:

[0007] A photothermal spectroscopy gas detection system based on an asymmetric micro-nano optical waveguide includes: a detection laser component, a pump laser component, a polarization interference ring, and a data acquisition component;

[0008] The polarization interference ring includes an asymmetric micro-nano optical waveguide; the asymmetric micro-nano optical waveguide has a polarization interference group birefringence inflection point;

[0009] The detection laser assembly is used to generate a detection laser; the central wavelength of the detection laser is the wavelength at the orthogonal point within a set range; the set range is a linear wavelength range with the polarization interference group birefringence inflection point as the center and the distance at which the light intensity at the polarization interference group birefringence inflection point drops to half the light intensity as the radius;

[0010] The pump laser assembly is used to generate pump laser;

[0011] The asymmetric micro-nano optical waveguide is filled with a gas to be measured; the asymmetric micro-nano optical waveguide is used to receive the probe laser and the pump laser; the pump laser is used to excite an evanescent wave in the asymmetric micro-nano optical waveguide; the gas to be measured absorbs the evanescent wave, generating a photothermal effect accompanied by heat conduction, thereby changing the refractive index of the gas to be measured and the asymmetric micro-nano optical waveguide; the asymmetric micro-nano optical waveguide with a changed refractive index is used to generate an additional phase difference between two orthogonal polarization modes after the probe laser passes through;

[0012] The data acquisition component is used to detect the additional phase difference and determine the concentration of the gas to be measured according to the additional phase difference.

[0013] Optionally, the detection laser assembly includes: a laser feedback controller, a piezoelectric optical fiber stretcher, a detection light source and a second isolator;

[0014] The laser feedback controller is connected to the piezoelectric optical fiber stretcher and the detection light source;

[0015] The piezoelectric optical fiber stretcher is used to drive the laser feedback controller to set the central wavelength of the detection light source to the wavelength at the orthogonal point within a set range.

[0016] Optionally, the polarization interference ring further includes: a first optical coupler, a first wavelength division multiplexer, a polarization controller and a second wavelength division multiplexer;

[0017] The output end of the detection laser assembly is connected to the first port of the first optical coupler; the second port of the first optical coupler and the output end of the pump laser assembly are both connected to the input end of the second wavelength division multiplexer; the output end of the second wavelength division multiplexer is connected to the input end of the asymmetric micro-nano optical waveguide; the output end of the asymmetric micro-nano optical waveguide is connected to the third port of the first optical coupler through the first wavelength division multiplexer and the polarization controller in sequence; the fourth port of the first optical coupler is connected to the input end of the data acquisition assembly;

[0018] The first port of the first optical coupler serves as the input end of the polarization interference ring; the fourth port of the first optical coupler serves as the output end of the polarization interference ring;

[0019] The detection laser enters the first optical coupler through the first port, and the detection laser is emitted through the second port and the third port respectively. The two emitted lasers propagate in opposite directions, wherein the first emitted laser passes through the second wavelength division multiplexer, the asymmetric micro-nano optical waveguide, the first wavelength division multiplexer, and the polarization controller in sequence, and then enters the first optical coupler through the third port. The second emitted laser passes through the polarization controller, the first wavelength division multiplexer, the asymmetric micro-nano optical waveguide, and the second wavelength division multiplexer in sequence, and then enters the first optical coupler through the second port. After propagation, the two emitted lasers entering the first optical coupler are light in two orthogonal polarization directions.

[0020] The second wavelength division multiplexer is used to input the first output laser and the pump laser into the asymmetric micro-nano optical waveguide; the first wavelength division multiplexer is used to filter out the pump laser; the polarization controller is used to convert the fast and slow axes of the first output laser after being transmitted through the asymmetric micro-nano optical waveguide;

[0021] After the two lights in orthogonal polarization directions meet each other in the first coupler, polarization interference is generated to form an additional phase difference, and a polarization interference spectrum is outputted at the fourth port of the first coupler.

[0022] Optionally, the pump laser assembly includes: a laser driver, a pump light source, and a first isolator connected in sequence; the output end of the first isolator is connected to the polarization interference ring; the input end of the laser driver serves as the input end of the pump laser assembly;

[0023] The laser driver is used to drive the pump light source to generate wavelength-modulated pump laser.

[0024] Optionally, the data acquisition component includes: a second optical coupler, a first photodetector, a lock-in amplifier, and an acquisition card connected in sequence; the output end of the second optical coupler is also connected to the input end of the detection laser component; the output end of the lock-in amplifier is also connected to the input end of the pump laser component;

[0025] The second optical coupler is used to divide the detection laser light transmitted through the asymmetric micro-nano optical waveguide into a first laser path and a second laser path according to a set optical power ratio;

[0026] The first photodetector is used to receive the first laser beam, convert the first laser beam into a first electrical signal, and transmit the first electrical signal to the lock-in amplifier;

[0027] The lock-in amplifier is used to demodulate the harmonic signal from the electrical signal; the harmonic signal carries an additional phase difference generated by two orthogonal polarization modes after being transmitted through the asymmetric structure micro-nano optical waveguide;

[0028] The acquisition card is used to receive the harmonic signal and determine the peak-to-peak value of the harmonic signal, and determine the concentration of the gas to be measured based on the relationship that the peak-to-peak value is proportional to the concentration of the gas to be measured;

[0029] The detection laser component is further used to receive the second laser beam; after the second laser beam enters the detection laser component, it generates a feedback laser control signal.

[0030] Optionally, the detection laser assembly further comprises: a second photodetector and a low-pass filter connected in sequence;

[0031] The input end of the second photodetector is connected to the output end of the second optical coupler; the output end of the laser feedback controller is connected to the piezoelectric optical fiber stretcher;

[0032] The second photodetector is used to convert the second laser beam into a second electrical signal;

[0033] The low-pass filter is used to filter the second electrical signal to obtain a feedback laser control signal;

[0034] The laser feedback controller is used to drive the piezoelectric fiber stretcher to control the detection light source to lock the central wavelength of the detection laser to the wavelength at the orthogonal point within a set range according to the feedback laser control signal.

[0035] Optionally, the asymmetric micro-nano optical waveguide is an elliptical micro-nano optical waveguide.

[0036] Optionally, the elliptical micro-nano optical waveguide is formed by applying a force moving uniformly in two opposite directions to the elliptical optical fiber using a two-step tapering method, so that the two ends of the elliptical optical fiber gradually decrease and the middle part gradually becomes uniformly thinner;

[0037] The elliptical micro-nano optical waveguide comprises: tapered transition areas at both ends and an elliptical uniform area in the middle.

[0038] Optionally, the ellipticity of the elliptical optical fiber is 0.4-0.6; the major axis size of the elliptical optical fiber is 1.4 μm-1.9 μm; and the length of the elliptical uniform area in the elliptical micro-nano optical waveguide is 3 mm-6 mm.

[0039] Optionally, the polarization interference ring further includes: a gas collecting chamber; the asymmetric micro-nano optical waveguide is arranged in the gas collecting chamber; and the gas collecting chamber is filled with the gas to be measured.

[0040] According to the specific embodiments provided by the present invention, the present invention discloses the following technical effects:

[0041] The embodiment of the present invention achieves a significant improvement in detection sensitivity by locking the central wavelength of the detection laser at an orthogonal point near the birefringence inflection point of the polarization interference group, thereby solving the problem of difficulty in achieving both detection sensitivity and response speed. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0043] Figure 1 A structural diagram of a photothermal spectroscopy gas detection system based on asymmetric micro-nano optical waveguides with enhanced group birefringence inflection points provided by an embodiment of the present invention;

[0044] Figure 2 A schematic diagram of an asymmetric micro-nano optical waveguide provided by an embodiment of the present invention;

[0045] Figure 3 A schematic cross-sectional view of an elliptical optical fiber provided in an embodiment of the present invention;

[0046] Figure 4 A graph showing theoretical calculation of detection sensitivity of a gas sensor according to an embodiment of the present invention;

[0047] Figure 5 This is the output interference spectrum of the polarization interference ring provided in an embodiment of the present invention.

[0048] Explanation of symbols:

[0049] Detection laser assembly—1, pump laser assembly—2, polarization interference ring—3, data acquisition assembly—4. DETAILED DESCRIPTION

[0050] The technical solutions in the embodiments of the present invention will be described below with reference to the accompanying drawings in the embodiments of the present invention.

[0051] The purpose of the present invention is to provide a photothermal spectroscopic gas detection system based on an asymmetric micro-nano optical waveguide. By locking the central wavelength of the detection laser at an orthogonal point near the birefringence inflection point of the polarization interference group, a significant improvement in detection sensitivity is achieved. The unique polarization interference group birefringence inflection point enhancement characteristics of the asymmetric micro-nano optical waveguide with high birefringence characteristics are utilized to solve the problem of achieving both detection sensitivity and response speed.

[0052] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0053] See also Figure 1 The photothermal spectroscopic gas detection system provided in this embodiment is a photothermal spectroscopic gas detection system based on group birefringence inflection point enhancement of asymmetric micro-nano optical waveguides. The system includes: a detection laser component 1, a pump laser component 2, a polarization interference ring 3 and a data acquisition component 4.

[0054] The polarization interference ring 3 includes an asymmetric micro-nano optical waveguide; the asymmetric micro-nano optical waveguide has a polarization interference group birefringence inflection point.

[0055] The detection laser assembly 1 is used to generate a detection laser; the central wavelength of the detection laser is the wavelength at the orthogonal point within the set range; the set range is a linear wavelength range with the polarization interference group birefringence inflection point as the center and the distance where the light intensity at the polarization interference group birefringence inflection point drops to half the light intensity (for example, the light intensity at the inflection point drops by 3dB) as the radius; the detection laser assembly 1 achieves the purpose of locking the central wavelength of the detection laser at the orthogonal point near the polarization interference group birefringence inflection point, thereby greatly improving the detection sensitivity.

[0056] The pump laser assembly 2 is used to generate pump laser.

[0057] The asymmetric micro-nano optical waveguide is filled with a gas to be measured, which is an absorbing gas, such as acetylene. The asymmetric micro-nano optical waveguide is used to receive the probe laser and the pump laser. The pump laser is used to excite an evanescent wave in the asymmetric micro-nano optical waveguide. The gas to be measured absorbs the evanescent wave, generating a photothermal effect accompanied by heat conduction, thereby changing the refractive index of the gas to be measured and the asymmetric micro-nano optical waveguide. The asymmetric micro-nano optical waveguide with a changed refractive index generates a phase difference between two orthogonal polarization modes after the probe laser passes through. The asymmetric micro-nano optical waveguide with a changed refractive index is used to generate an additional phase difference between the two orthogonal polarization modes after the probe laser passes through. The pump laser plays a role in the asymmetric micro-nano optical waveguide, causing the probe laser to generate an additional phase difference on top of the existing phase difference. The additional phase difference can be used to characterize the parameters of the gas to be measured.

[0058] The data acquisition component 4 is used to detect the additional phase difference and determine the concentration of the gas to be measured based on the additional phase difference. High-precision gas measurement is achieved by utilizing the unique group birefringence inflection point enhancement characteristics of the asymmetric micro-nano optical waveguide and the additional phase difference generated by the photothermal effect between the pump laser and the gas to be measured, which is generated by the probe light.

[0059] In this embodiment, the detection system is implemented as follows: the central wavelength of the pump laser scans through the absorption peak of the gas to be measured, and the central wavelength of the detection laser is aligned with the orthogonal point near the birefringence inflection point of the polarization interference group; when the gas surrounding the micro-nano optical waveguide absorbs the evanescent wave excited by the pump laser in the micro-nano optical waveguide, a photothermal effect is generated, accompanied by heat conduction, which changes the refractive index of the gas medium and the micro-nano optical waveguide; based on the evanescent field effect in different polarization directions, its effective refractive index changes to different degrees, thereby generating an additional phase difference, and the concentration of the gas to be measured is determined based on the additional phase difference.

[0060] The detection system of the above embodiment adopts an asymmetric micro-nano optical waveguide as a key sensing component, and utilizes the special group birefringence inflection point characteristics of the waveguide to enhance the gas detection sensitivity. The actual length of the sensing optical fiber can be shortened, thereby accelerating the air intake time and improving the response speed, while meeting the requirements of high detection sensitivity and fast response. In addition, it is small in size and high in accuracy, which can greatly improve the applicability of the sensor.

[0061] In one example, the wavelength at the birefringence inflection point of the polarization interference group may be 1450 nm, and the setting range of the central wavelength of the above-mentioned detection laser may be 1469 nm-1471 nm.

[0062] In one example, the asymmetric micro-nano optical waveguide is an elliptical micro-nano optical waveguide.

[0063] The elliptical micro-nano optical waveguide is formed by applying a force in two opposite directions to the elliptical optical fiber using a two-step taper method, so that the ends of the elliptical optical fiber gradually decrease and the middle part gradually becomes thinner. The elliptical micro-nano optical waveguide includes: a tapered transition zone at both ends and an elliptical uniform area in the middle. The cross section of the elliptical optical fiber is as follows: Figure 3 As shown, the elliptical fiber has asymmetry.

[0064] In practical applications, the exemplary preparation method of the elliptical micro-nano optical waveguide is as follows:

[0065] (1) An asymmetric micro-nano optical waveguide with a group birefringence inflection point is produced. The two ends of the micro-nano optical waveguide are connected to a broadband light source and an optical fiber spectrum analyzer through standard optical fibers, respectively, so that the entire tapering process can be monitored in real time.

[0066] (2) Place the wide side of the elliptical fiber upward, and fix both ends of the elliptical fiber on a linear platform through a fiber clamp. Use the fused taper technology to draw the micro-nano optical waveguide through a two-step taper method:

[0067] Step 1: Place the elliptical fiber section at the corresponding high-temperature center of a butane flame jet. Ignite the flame jet and, after preheating the fiber for 20 seconds until it melts, apply a uniform force in two opposite directions at a speed of 5 mm / s. The heated portion of the fiber gradually elongates, forming a tapered region that tapers gradually at both ends and evenly in the middle. When a dense spectrum appears in the spectrometer—that is, a polarization interference spectrum with a relatively small free spectral range (FSR)—the flame jet is turned off, pausing the tapering process.

[0068] The second step is to stop the movement of the linear platform, reduce the output of the flamethrower, aim at the thinnest part of the micro-nano optical waveguide, and slowly scan the flamethrower at a speed of 0.5 mm / s. At this time, the dense spectrum displayed by the spectrometer begins to become sparse, that is, the free spectrum range gradually widens, until the micro-nano optical waveguide shows an obvious group birefringence inflection point, then stop tapering.

[0069] In addition, the above-mentioned elliptical fiber can be a polarization-maintaining elliptical fiber or an integrated polarization-maintaining elliptical fiber.

[0070] In one example, the detection laser assembly 1 includes: a laser feedback controller, a piezoelectric fiber stretcher, a detection light source, and a second isolator. The laser feedback controller connects the piezoelectric fiber stretcher and the detection light source; the piezoelectric fiber stretcher is configured to, through the drive of the laser feedback controller, set the center wavelength of the detection light source to a wavelength at an orthogonal point within a set range.

[0071] In one example, the polarization interference ring 3 further includes: a first optical coupler, a first wavelength division multiplexer, a polarization controller, and a second wavelength division multiplexer. The probe laser passes through the second isolator and is input into the first coupler, i.e., the input end of the polarization interference ring 3 of the asymmetric micro-nano optical waveguide. Light propagating in opposite directions, after passing through the asymmetric micro-nano optical waveguide and the polarization controller, generates a phase difference between the two orthogonal polarization modes, which then meet at the first optical coupler to form polarization interference.

[0072] Specifically, the output end of the detection laser component 1 is connected to the first port a of the first optical coupler; the second port b of the first optical coupler and the output end of the pump laser component 2 are both connected to the input end of the second wavelength division multiplexer; the output end of the second wavelength division multiplexer is connected to the input end of the asymmetric micro-nano optical waveguide; the output end of the asymmetric micro-nano optical waveguide is connected to the third port c of the first optical coupler through the first wavelength division multiplexer and the polarization controller in sequence; the fourth port d of the first optical coupler is connected to the input end of the data acquisition component 4.

[0073] The first port a of the first optical coupler serves as the input end of the polarization interference ring 3 ; the fourth port d of the first optical coupler serves as the output end of the polarization interference ring 3 .

[0074] The detection laser enters the first optical coupler through the first port a, and the detection laser is emitted through the second port b and the third port c respectively. The two emitted lasers propagate in opposite directions, wherein the first emitted laser passes through the second wavelength division multiplexer, the asymmetric micro-nano optical waveguide, the first wavelength division multiplexer and the polarization controller in sequence, and then enters the first optical coupler through the third port c. The second emitted laser passes through the polarization controller, the first wavelength division multiplexer, the asymmetric micro-nano optical waveguide and the second wavelength division multiplexer in sequence, and then enters the first optical coupler through the second port b. After propagation, the two emitted lasers entering the first optical coupler are light in two orthogonal polarization directions.

[0075] The second wavelength division multiplexer is used to input the first output laser and the pump laser into the asymmetric micro-nano optical waveguide. When the detection laser enters the asymmetric micro-nano optical waveguide, the micro-nano optical waveguide with a non-circularly symmetric structure has a large birefringence characteristic. The first wavelength division multiplexer is used to filter out the pump laser; the polarization controller is used to convert the fast and slow axes of the first output laser after being transmitted through the asymmetric micro-nano optical waveguide.

[0076] After two lights with orthogonal polarization directions meet in the first coupler, polarization interference is generated to obtain polarization interference light, and then an additional phase difference is generated due to gas absorption, and a polarization interference spectrum is output at the fourth port d of the first coupler.

[0077] The polarization interference ring 3 may further include: a gas collection chamber; the asymmetric micro-nano optical waveguide is disposed in the gas collection chamber; the gas collection chamber is filled with the gas to be measured, and the gas collection chamber is provided with an air inlet and an air outlet to facilitate the inflow and outflow of the gas to be measured.

[0078] In one example, the pump laser assembly 2 includes: a laser driver, a pump light source and a first isolator connected in sequence; the output end of the first isolator is connected to the polarization interference ring 3; and the input end of the laser driver serves as the input end of the pump laser assembly 2.

[0079] The laser driver is used to drive the pump light source to generate wavelength-modulated pump laser.

[0080] The output of the pump laser assembly 2 is fed through a second wavelength division multiplexer into an asymmetric micro-nano optical waveguide, which is filled with an absorbing gas. The gas absorbs the evanescent wave excited by the pump laser in the micro-nano optical waveguide, generating a photothermal effect accompanied by heat conduction, which changes the refractive index of the gas medium and the micro-nano optical waveguide. The additional phase difference generated by the probe light after passing through the polarization interference ring 3 is detected, and the concentration of the gas to be measured is determined based on this additional phase difference.

[0081] In one example, the data acquisition component 4 includes: a second optical coupler, a first photodetector, a phase-locked amplifier and an acquisition card connected in sequence; the output end of the second optical coupler is also connected to the input end of the detection laser component 1; the output end of the phase-locked amplifier is also connected to the input end of the pump laser component 2.

[0082] The second optical coupler is used to divide the detection laser (ie, polarized interference light) transmitted through the asymmetric micro-nano optical waveguide into a first laser path and a second laser path according to a set optical power ratio.

[0083] The first photodetector is used to receive a first laser beam (with a phase change), convert the first laser beam into a first electrical signal, and transmit the first electrical signal to the lock-in amplifier.

[0084] The lock-in amplifier is used to demodulate the harmonic signal from the electrical signal; the harmonic signal carries an additional phase difference generated by two orthogonal polarization modes after being transmitted through the asymmetric structure micro-nano optical waveguide.

[0085] The acquisition card is used to receive the harmonic signal and determine the peak-to-peak value of the harmonic signal, and determine the concentration of the gas to be measured based on the relationship that the peak-to-peak value is proportional to the concentration of the gas to be measured.

[0086] The detection laser component is further used to receive the second laser beam; after the second laser beam enters the detection laser component, it generates a feedback laser control signal.

[0087] In one example, the detection laser assembly 1 further includes: a second photodetector and a low-pass filter connected in sequence. The input end of the second photodetector is connected to the output end of the second optical coupler; the output end of the laser feedback controller is connected to the piezoelectric fiber stretcher. The second photodetector is used to convert the second laser into a second electrical signal. The low-pass filter is used to filter the second electrical signal to obtain a feedback laser control signal. The laser feedback controller is used to drive the piezoelectric fiber stretcher to control the detection light source to lock the central wavelength of the detection laser to the wavelength at the orthogonal point within a set range according to the feedback laser control signal, that is, the piezoelectric fiber stretcher locks the central wavelength of the detection light source at the orthogonal point near the birefringence inflection point of the polarization interference group through the drive of the feedback controller.

[0088] The wavelength tuning port of the piezoelectric fiber stretcher adjusts the central wavelength of the detection light so that its wavelength is always stable at the orthogonal point near the group birefringence inflection point of the polarization interference spectrum, thereby improving the sensitivity of phase detection and enhancing the accuracy of gas detection, while avoiding operating point drift and signal disturbance caused by environmental changes.

[0089] In one example, the ellipticity of the elliptical optical fiber is e=b / a, where a and b are the major axis and minor axis dimensions of the elliptical optical fiber, respectively, and the value of the ellipticity e is 0.4-0.6; the major axis dimension of the elliptical optical fiber is 1.4μm-1.9μm; the length of the elliptical uniform area in the elliptical micro-nano optical waveguide is 3mm-6mm.

[0090] The detection system of the above embodiment is implemented as follows: After the probe laser is input into the polarization interference ring 3 based on the asymmetric micro-nano optical waveguide through a first optical coupler, the probe light propagating in opposite directions generates a phase difference between two orthogonal polarization modes after passing through the asymmetric micro-nano optical waveguide and the polarization controller. These two orthogonal polarization modes meet at the optical coupler, forming polarization interference. Simultaneously, the pump laser is input into the asymmetric micro-nano optical waveguide through a second wavelength division multiplexer. The micro-nano optical waveguide is filled with an absorbing gas. The gas absorbs the evanescent wave in the micro-nano optical waveguide, generating a photothermal effect accompanied by heat conduction, which changes the refractive index of the gas medium and the micro-nano optical waveguide. The additional phase difference generated by the probe light after passing through the polarization interference ring 3 is detected to obtain gas concentration information. The detection system of this embodiment achieves a significant improvement in detection sensitivity by locking the probe laser wavelength at an orthogonal point near the birefringence inflection point of the polarization interference group, thus resolving the difficulty in achieving both detection sensitivity and response speed. Furthermore, the micro-nano optical waveguide of this embodiment features a simple fabrication process and high photothermal efficiency, enabling sensor miniaturization and significantly improving sensor applicability.

[0091] The following is a further detailed introduction to the above detection system based on the specific principles of detection:

[0092] Due to the birefringence effect of the asymmetric micro-nano optical waveguide, when the probe light enters the fiber ring through the input end of the first coupler and propagates in opposite directions, the probe light generates a phase difference between the two orthogonal polarization modes through the asymmetric micro-nano optical waveguide and the polarization controller, and then meets at the first optical coupler to form polarization interference. The gas detection sensitivity S of this photothermal spectroscopic gas sensor can be expressed as:

[0093]

[0094] Where λ is the wavelength of the probe light, G is the group birefringence of the asymmetric micro-nano optical waveguide, and its magnitude is:

[0095]

[0096] The mode birefringence B of the above asymmetric micro-nano optical waveguide is x -n y , n x and n y The effective refractive index group birefringence in the two principal axis directions is respectively. Formula (1) shows that the gas detection sensitivity S depends on the following factors: the detection light wavelength λ, the group birefringence G and the birefringence change caused by the external refractive index. When the group birefringence G approaches zero, the second term This plays a dominant role in gas detection sensitivity S, significantly enhancing it. The condition where the group birefringence G approaches zero is defined as the group birefringence inflection point. The gas absorbs the evanescent wave excited by the pump light in the micro-nano optical waveguide, generating a photothermal effect accompanied by heat conduction, which changes the refractive index of the gas medium and the micro-nano optical waveguide. This is then demodulated using the polarization interference loop 3 to obtain gas concentration information.

[0097] To this end, simulation calculations were performed to obtain specific parameters and study its sensing characteristics. The longest side dimension of the cross section of the asymmetric micro-nano optical fiber is no more than 10 μm. According to formula (1), Figure 4 The gas detection sensitivity S of an asymmetric micro-nano optical waveguide with e = 0.5 and a = 1.56 μm is presented. When the probe wavelength approaches the group birefringence inflection wavelength, the detection sensitivity S significantly increases and approaches ∞. The theoretical curve shows that when the probe wavelength is within ±30 nm of the group birefringence inflection wavelength, the detection sensitivity S increases rapidly; when the probe wavelength is far from the group birefringence inflection wavelength, the detection sensitivity S decreases rapidly.

[0098] Since environmental noise can easily cause the interference spectrum output of this system to drift slightly, it is necessary to lock the working point of the detection light wavelength so that it remains at an orthogonal point near the inflection point of the group birefringence. The locking method specifically relies on the laser feedback controller and the piezoelectric fiber stretcher. Figure 1As shown, after obtaining the polarization interference spectrum, the detection laser light, split by the second coupler at a 10% power ratio, enters the input of the detection laser assembly 1, i.e., the input of the second photodetector. The output of the second photodetector generates a control signal through a low-pass filter and a laser feedback controller. The laser feedback controller connects the detection light source and the piezoelectric fiber stretcher. Driven by the laser feedback controller, the piezoelectric fiber stretcher locks the wavelength of the detection laser light at a quadrature point near the inflection point of the group birefringence of the polarization interference spectrum. Specifically, the second photodetector inputs the detected signal containing operating point drift into the laser feedback controller. The laser feedback controller extracts the noise signal and generates an electrical signal corresponding to the noise signal using a proportional-integral algorithm. This signal is then input into the piezoelectric fiber stretcher. The piezoelectric fiber stretcher changes the cavity length of the optical fiber within the detection light source based on the input electrical signal, thereby adjusting the output wavelength of the detection laser light in real time, keeping it stable at a quadrature point near the inflection point of the group birefringence of the polarization interference spectrum. This improves phase detection sensitivity and enhances gas detection accuracy, while also preventing operating point drift and signal perturbations caused by environmental changes.

[0099] The pump laser assembly 2 generates pump excitation light. This pump laser interacts with the absorbing gas in the evanescent field of the asymmetric micro-nano optical waveguide, producing a photothermal effect. This causes the absorbing gas and the micro-nano optical waveguide to increase in temperature. This temperature change, through the photothermal effect, changes the refractive index of the gas and the micro-nano optical waveguide. This creates an additional phase difference between the two orthogonal polarization modes of the probe light after passing through the polarization interference ring 3, and polarization interference fringes appear at the output of the first coupler. When the phase difference between the two orthogonal polarization modes of the probe light in the optical fiber changes, the interference fringes shift. The data acquisition assembly 4 receives this information about the changes in the interference fringes and determines the concentration of the gas to be measured based on the additional phase difference.

[0100] The pump light source is used to output pump light, and the laser wavelength generated by the pump light source corresponds to any absorption line of the absorbing gas to be measured (such as acetylene gas). In this embodiment, the pump light source can generate single-frequency pump light with a wavelength of 1530.371nm, which corresponds to the P9 absorption line of acetylene gas. The laser driver is arranged at the input end of the pump light source and is used to modulate the wavelength of the pump light. The laser driver in this embodiment can adjust the pump light by sinusoidal wave modulation, triangular wave modulation, etc. The wavelength-modulated pump light and the unmodulated detection light are input into the gas collection chamber through the second wavelength division multiplexer, and the phase of the detection light is modulated in the gas collection chamber by the photothermal effect of the absorbing gas.

[0101] like Figure 2As shown, the asymmetric micro-nano optical waveguide in this embodiment specifically includes two tapered transition zones and an elliptical uniform zone in the middle. The elliptical uniform zone serves as the main functional zone of the asymmetric micro-nano optical waveguide. Its ellipticity e is 0.4-0.6, its major axis a is 1.4μm-1.9μm, and its length is 3mm-6mm. Due to the small diameter of the micro-nano optical waveguide, its evanescent field is strong, which facilitates the generation of a photothermal effect with the absorbing gas. The elliptical uniform zone is located in the middle of the gas collection chamber, which enables the evanescent field to be evenly distributed in the elliptical uniform zone and interact uniformly with the absorbing gas. The acetylene gas in the gas collection chamber can efficiently absorb the energy of the evanescent field and generate heat. The heated micro-nano optical fiber produces additional phase modulation on the detection light through the photothermal effect.

[0102] After the data acquisition assembly 4 in this embodiment obtains the polarization interference spectrum, the detection laser light, which accounts for 90% of the optical power split by the second coupler, enters the input of the first photodetector and is fed into a lock-in amplifier to extract the second harmonic signal. The acquisition card then performs data acquisition and display, thereby obtaining the measurement results. Simultaneously, the output of the lock-in amplifier is connected to the input of the pump laser assembly 2, outputting a sinusoidal signal to sinusoidally modulate the wavelength of the pump laser assembly 2.

[0103] Specifically, the gas collection chamber is first filled with acetylene gas at a concentration of 0.1%. A 1530.371 nm distributed feedback laser can be used as the pump light source. A lock-in amplifier inputs a 15 kHz sinusoidal modulation signal to the laser driver, which simultaneously drives the pump light source at a 0.01 Hz triangular wave sweep frequency, outputting modulated pump laser light. This modulated pump laser light then passes through an isolator and a second wavelength division multiplexer into an asymmetric micro-nano optical waveguide. The interaction between the pump laser light and the acetylene gas produces a photothermal effect and accompanying heat conduction, resulting in periodic changes in the refractive index of the gas under test and the micro-nano optical fiber.

[0104] In one example, when the probe laser enters a micro-nano optical waveguide with birefringence properties, orthogonal polarization modes are excited.

[0105] After receiving the majority of the detection laser light from the polarization interference fiber ring, the first photodetector transmits the converted electrical signal to a lock-in amplifier. The lock-in amplifier demodulates the electrical signal to extract the harmonic signal, an intermediate signal carrying additional phase difference information generated by two orthogonal polarization modes after transmission through the asymmetric micro-nano optical waveguide. After receiving the harmonic signal, the acquisition card processes it to obtain the peak-to-peak value of the harmonic signal, which is proportional to the concentration of the gas being measured, thereby obtaining gas concentration information.

[0106] See Figure 5 , Figure 5The transmission spectrum of the polarization interference ring based on the asymmetric micro-nano optical waveguide is shown. The transmission spectrum is used to determine the wavelength range of the detection light used. The long axis a of the micro-nano optical waveguide is 1.56μm, and the length of the uniform area is 5.2mm. Figure 4 The interference spectrum caused by the coupling of two orthogonal polarization modes can be observed. The loss and extinction ratio of the interference ring are 2dB and 23dB respectively. In addition, a clear group birefringence inflection point can be observed, with the inflection point wavelength located near 1450nm.

[0107] Specifically, the detection system of the above embodiment has the following advantages:

[0108] 1. Using a doubly symmetrical micro-nano optical fiber with birefringence, the unique polarization interference group birefringence of the optical fiber itself is used to enhance its external sensitivity, eliminating the need for additional resonant cavity enhancement. The micro-nano optical waveguide structure is also simple and can be fabricated using a fused taper method, reducing manufacturing difficulty and cost.

[0109] 2. Asymmetric micro-nano optical waveguides have extremely high thermo-optical coefficients and extremely low transmission losses. Combined with photothermal spectroscopy technology, they produce larger phase modulations, achieving ultra-high sensitivity gas sensing.

[0110] 3. The gas medium to be measured is wrapped in a micro-nano optical waveguide with a length of centimeters. The photothermal signal response is instantaneous. Therefore, while improving the detection sensitivity, it can also shorten the air intake time and reduce gas consumption, significantly improving the response speed and achieving rapid measurement.

[0111] 4. Birefringent micro-nano optical waveguides can achieve zero background signal during gas detection. In traditional direct absorption systems, gas absorption in the optical path introduces background signal, interfering with the results. In a photothermal fiber system, only the photothermal changes caused by gas absorption near the asymmetric micro-nano optical waveguide are detected by the detection module, resulting in zero background signal.

[0112] 5. Compared with the traditional free-space optical resonator, the present invention does not require a complex alignment optical system and has the advantages of small size, light weight, compatibility with optical fiber systems, and remote monitoring, which greatly improves the applicability of the sensor.

[0113] The various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referenced to each other.

[0114] This document uses specific examples to illustrate the principles and implementation methods of the present invention. The above examples are only intended to help understand the method and core concept of the present invention. At the same time, those skilled in the art will find that the specific implementation methods and application scopes may vary based on the concept of the present invention. In summary, the contents of this specification should not be construed as limiting the present invention.

Claims

1. A photothermal spectroscopy gas detection system based on asymmetric micro-nano optical waveguides, characterized in that: include: Detection laser assembly, pump laser assembly, polarization interference ring and data acquisition assembly; The polarization interference ring includes an asymmetric micro-nano optical waveguide; the asymmetric micro-nano optical waveguide has a polarization interference group birefringence inflection point; the wavelength at the polarization interference group birefringence inflection point is 1450nm; the polarization interference ring also includes: a first optical coupler, a first wavelength division multiplexer, a polarization controller and a second wavelength division multiplexer; the output end of the detection laser assembly is connected to the first port of the first optical coupler; the second port of the first optical coupler and the output end of the pump laser assembly are both connected to the input end of the second wavelength division multiplexer; the output end of the second wavelength division multiplexer is connected to the input end of the asymmetric micro-nano optical waveguide; the output end of the asymmetric micro-nano optical waveguide is connected to the third port of the first optical coupler through the first wavelength division multiplexer and the polarization controller in sequence; the fourth port of the first optical coupler is connected to the input end of the data acquisition assembly; the detection laser enters the first optical coupler through the first port, and the detection laser is emitted through the second port and the third port respectively, and the two emitted lasers propagate in opposite directions; The detection laser assembly is used to generate a detection laser; the central wavelength of the detection laser is the wavelength at the orthogonal point within a set range; the set range is a linear wavelength range with the polarization interference group birefringence inflection point as the center and the distance where the light intensity at the polarization interference group birefringence inflection point drops to half the light intensity as the radius; when the detection laser enters the micro-nano optical waveguide with birefringence characteristics, it excites an orthogonal polarization mode; The pump laser assembly is used to generate a pump laser; the pump laser assembly includes: a laser driver, a pump light source, and a first isolator connected in sequence; the output end of the first isolator is connected to the polarization interference ring; the input end of the laser driver serves as the input end of the pump laser assembly; the laser driver is used to drive the pump light source to generate a wavelength-modulated pump laser; the laser wavelength generated by the pump light source corresponds to any absorption spectrum line of the absorbing gas to be measured; the pump light source generates a single-frequency pump light with a wavelength of 1530.371 nm; The asymmetric micro-nano optical waveguide is filled with a gas to be measured; the asymmetric micro-nano optical waveguide is used to receive the probe laser and the pump laser; the pump laser is used to excite an evanescent wave in the asymmetric micro-nano optical waveguide; the gas to be measured absorbs the evanescent wave, generating a photothermal effect accompanied by heat conduction, thereby changing the refractive index of the gas to be measured and the asymmetric micro-nano optical waveguide; the asymmetric micro-nano optical waveguide with a changed refractive index is used to generate an additional phase difference between two orthogonal polarization modes after the probe laser passes through; The data acquisition component is used to detect the additional phase difference and determine the concentration of the gas to be measured according to the additional phase difference.

2. The photothermal spectroscopic gas detection system based on asymmetric micro-nano optical waveguide according to claim 1 is characterized in that: The detection laser assembly includes: a laser feedback controller, a piezoelectric optical fiber stretcher, a detection light source and a second isolator; The laser feedback controller is connected to the piezoelectric optical fiber stretcher and the detection light source; The piezoelectric optical fiber stretcher is used to drive the laser feedback controller to set the central wavelength of the detection light source to the wavelength at the orthogonal point within a set range.

3. The photothermal spectroscopy gas detection system based on asymmetric micro-nano optical waveguide according to claim 1 is characterized in that: The first port of the first optical coupler serves as the input end of the polarization interference ring; the fourth port of the first optical coupler serves as the output end of the polarization interference ring; The first output laser sequentially passes through the second wavelength division multiplexer, the asymmetric micro-nano optical waveguide, the first wavelength division multiplexer, and the polarization controller, and then enters the first optical coupler through the third port. The second output laser sequentially passes through the polarization controller, the first wavelength division multiplexer, the asymmetric micro-nano optical waveguide, and the second wavelength division multiplexer, and then enters the first optical coupler through the second port. After propagation, the two output lasers entering the first optical coupler are light in two orthogonal polarization directions. The second wavelength division multiplexer is used to input the first output laser and the pump laser into the asymmetric micro-nano optical waveguide; the first wavelength division multiplexer is used to filter out the pump laser; the polarization controller is used to convert the fast and slow axes of the first output laser after being transmitted through the asymmetric micro-nano optical waveguide; After the two lights with orthogonal polarization directions meet each other in the first optical coupler, polarization interference is generated, and a polarization interference spectrum is outputted at the fourth port of the first optical coupler.

4. The photothermal spectroscopy gas detection system based on asymmetric micro-nano optical waveguide according to claim 2 is characterized in that: The data acquisition assembly includes: a second optical coupler, a first photodetector, a lock-in amplifier, and an acquisition card connected in sequence; the output end of the second optical coupler is also connected to the input end of the detection laser assembly; the output end of the lock-in amplifier is also connected to the input end of the pump laser assembly; The second optical coupler is used to divide the detection laser light transmitted through the asymmetric micro-nano optical waveguide into a first laser path and a second laser path according to a set optical power ratio; The first photodetector is used to receive the first laser beam, convert the first laser beam into a first electrical signal, and transmit the first electrical signal to the lock-in amplifier; The lock-in amplifier is used to demodulate the harmonic signal from the electrical signal; the harmonic signal carries an additional phase difference generated by two orthogonal polarization modes after being transmitted through the asymmetric structure micro-nano optical waveguide; The acquisition card is used to receive the harmonic signal and determine the peak-to-peak value of the harmonic signal, and determine the concentration of the gas to be measured based on the relationship that the peak-to-peak value is proportional to the concentration of the gas to be measured; The detection laser component is further used to receive the second laser beam; after the second laser beam enters the detection laser component, it generates a feedback laser control signal.

5. The photothermal spectroscopic gas detection system based on asymmetric micro-nano optical waveguide according to claim 4 is characterized in that: The detection laser assembly further comprises: a second photodetector and a low-pass filter connected in sequence; The input end of the second photodetector is connected to the output end of the second optical coupler; the output end of the laser feedback controller is connected to the piezoelectric optical fiber stretcher; The second photodetector is used to convert the second laser beam into a second electrical signal; The low-pass filter is used to filter the second electrical signal to obtain a feedback laser control signal; The laser feedback controller is used to drive the piezoelectric fiber stretcher to control the detection light source to lock the central wavelength of the detection laser to the wavelength at the orthogonal point within a set range according to the feedback laser control signal.

6. The photothermal spectroscopy gas detection system based on asymmetric micro-nano optical waveguide according to claim 1 is characterized in that: The asymmetric micro-nano optical waveguide is an elliptical micro-nano optical waveguide.

7. The photothermal spectroscopic gas detection system based on asymmetric micro-nano optical waveguide according to claim 6 is characterized in that: The elliptical micro-nano optical waveguide is formed by applying a force moving uniformly in two opposite directions to the elliptical optical fiber using a two-step tapering method, so that the two ends of the elliptical optical fiber gradually decrease and the middle part gradually becomes thinner. The elliptical micro-nano optical waveguide comprises: tapered transition areas at both ends and an elliptical uniform area in the middle.

8. The photothermal spectroscopic gas detection system based on asymmetric micro-nano optical waveguide according to claim 7 is characterized in that: The ellipticity of the elliptical optical fiber is 0.4-0.6; the major axis size of the elliptical optical fiber is 1.4 μm-1.9 μm; and the length of the elliptical uniform area in the elliptical micro-nano optical waveguide is 3 mm-6 mm.

9. The photothermal spectroscopic gas detection system based on asymmetric micro-nano optical waveguide according to claim 1 is characterized in that: The polarization interference ring further includes: a gas collection chamber; the asymmetric micro-nano optical waveguide is arranged in the gas collection chamber; and the gas collection chamber is filled with the gas to be measured.

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