Echo wall functionalized micro-flow cavity hydrogen sensing system based on laser tracking technology

Abstract

The invention belongs to the field of optical sensing, and particularly relates to an echo wall functionalized micro-flow cavity hydrogen sensing system based on a laser tracking technology. An ultrahigh-Q-value WGM optical micro-flow cavity is etched on the outer wall of a silicon dioxide capillary tube, a Pt / WO3 functionalized nano-film is deposited on the inner wall of the silicon dioxide capillary tube, a functionalized material is in indirect contact with the micro-flow cavity, hydrogen reacts with the Pt / WO3 material to release heat, and the optical characteristic change of the WGM optical micro-flow cavity is caused by heat conduction; and then the laser-trigger technology is adopted to ensure that the wavelength of the sensing laser tracks the resonance displacement of the microcavity, and photoelectric heterodyne detection is adopted to accurately measure the position and displacement of the resonance wavelength, so that the detection of the gas concentration is realized. The sensing system provided by the invention has a significant detection limit of 5ppb and large-range detection crossing five orders of magnitude, and realizes integration of high sensitivity, large dynamic range and real-time detection of hydrogen sensing; an advanced detection method is provided for microcavity sensing, and a good basis is provided for safe and large-scale application of hydrogen.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of optical sensing, relates to material science and microcavity sensing technology, and specifically relates to an echo wall functionalized micro-flow cavity hydrogen sensing system based on laser tracking (Laser-trigger) technology. BACKGROUND

[0002] Hydrogen, as a clean and efficient green energy carrier, has a wide range of applications in new energy vehicles and metallurgical processes. In practice, the safe use of hydrogen depends on high-performance hydrogen sensors with selectivity, real-time response, high sensitivity and wide dynamic range. Existing hydrogen sensors mainly include types based on thermal conductivity, electrochemistry, catalysis and optics. However, these methods still have some drawbacks, such as slow response time, poor stability, poor anti-interference ability, etc.

[0003] Whispering Gallery-mode Microcavity (WGM) has high quality factor, strong light-matter interaction, high energy density and small mode volume, and has been widely used in gas sensing, showing excellent performance of anti-electromagnetic interference, stability, high sensitivity and fast response. Therefore, WGM is an ideal platform for realizing ultra-sensitive, fast-response and micro-nano integrated hydrogen sensing.

[0004] However, most common microcavity materials and microcavity surface functionalization materials are not sensitive to hydrogen and have difficulty in responding to hydrogen. Traditional surface functionalization methods directly combine functionalization materials with microcavities, which inevitably reduces the quality factor of the microcavity and thus affects the sensing sensitivity. The limited size of functionalization materials and the saturation of the reaction due to factors such as reaction reaching equilibrium limit the response range of the microcavity to hydrogen. In addition, the transmission spectrum demodulation scheme in the traditional sensing scheme cannot realize real-time demodulation, and the frequency intensity demodulation scheme limits the dynamic range of the sensor, so the traditional sensing scheme often cannot balance real-time performance and large dynamic range. SUMMARY

[0005] In order to solve the problems of the single type of hydrogen sensor, the low sensitivity, the long response time, the small dynamic range and the electromagnetic interference, the echo wall functionalized micro-flow cavity hydrogen sensing system based on laser tracking technology is provided; the Pt / WO3 material is combined with the silica capillary micro-flow cavity and the micro-nano optical fiber to form the hydrogen sensing functionalized sensor device, the sensing laser frequency is locked on the slope edge of the micro-cavity resonance peak through the laser tracking technology, the sensing laser wavelength is tracked with the micro-cavity resonance displacement, the sensing laser and the super-stable reference laser are frequency-mixed, the photoelectric heterodyne detection method is used to accurately measure the resonance wavelength position and displacement, and thus the detection of the gas concentration is realized.

[0006] In order to achieve the above object, the technical scheme adopted by the present application is:

[0007] The echo wall functionalized micro-flow cavity hydrogen sensing system based on laser tracking technology comprises a silica capillary, a micro-nano optical fiber, a polarization controller, an adjustable narrow-bandwidth laser, a coupler, a photoelectric detector, a spectrum analyzer, a laser servo, a super-stable reference laser and a temperature controller.

[0008] The inner wall of the silica capillary is attached with Pt / WO3 functionalized material, the outer wall is etched with a WGM optical micro-flow cavity, and the stable preset environment temperature of the silica capillary is provided by the temperature controller; the two ends of the silica capillary are gas input and output channels.

[0009] The adjustable narrow-bandwidth laser emits a laser signal which is split into two paths by the coupler:

[0010] The output light of one path of the coupler is connected to the micro-nano optical fiber after passing through the polarization controller, the optical signal transmitted by the micro-nano optical fiber is coupled into the WGM optical micro-flow cavity, and the WGM optical micro-flow cavity is caused to resonate; the output of the micro-nano optical fiber is connected to the first photoelectric detector, the output optical signal of the micro-nano optical fiber is converted into an electric signal, i.e. a feedback signal, by the first photoelectric detector; and the feedback signal is input into the laser servo, and the laser servo controls the adjustable narrow-bandwidth laser to lock the laser output wavelength on the slope edge of the WGM optical micro-flow cavity resonance peak.

[0011] The laser tracking technology in the present application refers to that the intensity change of the output signal of the micro-nano optical fiber coupled with the WGM optical micro-flow cavity is used as an error signal (i.e. a feedback signal), the laser servo modulates the laser signal according to the error signal feedback, the laser frequency is locked on the slope edge of the WGM optical micro-flow cavity resonance peak, when the resonance peak is frequency-shifted, the laser also moves with the resonance peak, the laser is locked in the micro-cavity resonance mode, and the laser tracking (Laser-trigger) function is realized.

[0012] The other output light of the coupler is coupled with the super-stable reference laser, and a beat signal is formed by photoelectric heterodyne detection as a sensing signal, which is output to a second photoelectric detector and then connected to a frequency spectrum analyzer, so as to directly observe the sensing signal.

[0013] The frequency spectrum analyzer is used to detect the frequency change of the beat signal, calculate the frequency drift amount of the change, and inversely calculate the gas concentration.

[0014] Further, the outer diameter of the silica capillary is 3mm, and the thickness is 400μm, and rubber tubes are sleeved at both ends of the silica capillary for gas input and output.

[0015] Further, the WGM optical microcavity is prepared by polishing, etching and annealing processes on the outer wall of the silica capillary.

[0016] Further, the tunable narrow-bandwidth laser is used for pump light for exciting the resonant mode and locking light for realizing high-precision resonance tracking, the sweep frequency range of the tunable narrow-bandwidth laser is 1550-1551nm, the minimum scanning precision of the laser is less than 100Hz, and the output laser linewidth is less than 1kHz.

[0017] Further, the micro-nano fiber is perpendicular to the silica capillary and is located at the equatorial plane of the WGM optical microcavity, and is used for inputting pump light and outputting sensing signals and optically coupling with the WGM optical microcavity; the micro-nano fiber is prepared by a fusion taper method from a silica single-mode optical fiber, the taper region diameter is 500nm-2μm, the overall length is 2-4cm, and the loss is 0.05-0.2dB, so that the pump light is coupled into the microcavity in the form of an evanescent field.

[0018] Further, the Pt / WO3 functional material is prepared by a sol-gel method and is uniformly deposited on the inner wall of the silica capillary, and has specific adsorption function for hydrogen molecules. In the present application, the functional material does not directly contact the microcavity, so that the high quality factor of the whispering gallery mode optical microcavity is not deteriorated, the Pt / WO3 and hydrogen gas are used to generate a redox reaction to release heat, the temperature change of the inner wall of the silica capillary is conducted to the outer wall of the microcavity etched with the whispering gallery mode optical microcavity through heat conduction, so that the local temperature of the microcavity is increased, the refractive index of the microcavity material is changed, the cavity size is changed, the resonance condition is affected, and the resonance wavelength is red-shifted, so that the purpose of specific sensing of hydrogen gas is achieved.

[0019] Compared with the prior art, the application combines the advanced technologies of optics, metamaterials and micro-nano processing, adopts a brand-new sensing mechanism and an advanced signal processing technology; through the indirect contact between the functionalized material and the micro flow cavity, based on the heat release of hydrogen and Pt / WO3 material reaction, the sensing mechanism of using heat conduction to cause the change of WGM optical micro flow cavity optical characteristics, not only provides a brand-new hydrogen sensor, but also greatly improves the sensitivity of sensing. In addition, the laser-trigger technology used in the application ensures accurate tracking of the movement of the microcavity resonance mode by locking the sensing laser frequency in real time to the microcavity resonance, realizes the real-time and large dynamic range sensing performance. Therefore, the sensing system of the application achieves a significant detection limit of 5ppb and a large range detection of five orders of magnitude.

[0020] In summary, the application provides an advanced and stable detection system for hydrogen sensing, which is not easy to be disturbed, and also has the advantages of high sensitivity, large dynamic range and real-time detection integration, which provides a good foundation for the safe large-scale application of hydrogen. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 For the hydrogen response and sensing mechanism in the application;

[0022] Figure 2 For the field distribution simulation result graph in the embodiment;

[0023] Figure 3 For the principle diagram of the Laser-trigger technology in the application;

[0024] Figure 4 For the system block diagram of the application;

[0025] Figure 5 For the frequency spectrum diagram of the beat frequency generated in the embodiment;

[0026] Figure 6 For the test result graph of the embodiment;

[0027] Figure 7 For the WGM microcavity preparation flowchart of the functionalized silica capillary in the embodiment. DETAILED DESCRIPTION

[0028] The application will be further described in detail below in combination with the drawings and embodiments.

[0029] This embodiment provides a whispering-gallery functionalized microfluidic cavity hydrogen sensing system based on laser-trigger technology. It relies on a microfluidic cavity sensor deposited in Pt / WO3 and uses laser-trigger technology for online H2 detection. By utilizing the thermal conduction effect, this invention overcomes the trade-off between high precision and wide range, and exhibits excellent hydrogen sensing performance.

[0030] A whispering-gallery functionalized microfluidic cavity hydrogen sensing system based on laser tracking technology, such as Figure 4 As shown: a silica capillary tube, micro / nano optical fiber, polarization controller, tunable narrow-bandwidth laser, coupler, two photodetectors, spectrum analyzer, laser servo, ultra-stable reference laser, and temperature controller.

[0031] The silica capillary has an outer diameter of 3 mm and a thickness of 400 μm. The inner wall is coated with a Pt / WO3 specific material, and the outer wall is etched with WGM microcavities. Rubber tubes are fitted at both ends of the silica capillary to handle gas input and output. The silica capillary is fixed by a clamp, and the ambient temperature of the silica capillary microcavities is stabilized by a temperature controller.

[0032] Pt / WO3 functionalized materials were prepared using a sol-gel method and uniformly deposited on the inner wall of a silica capillary, exhibiting specific adsorption capabilities for hydrogen molecules. WGM optical microfluidic cavities were etched onto the outer wall of the silica capillary. The fabrication process is as follows: Figure 7 As shown, the Pt / WO3 functionalized material does not directly contact the WGM optical microfluidic cavity to ensure that the high quality factor of the whispering-gallery mode optical microfluidic cavity is not degraded. The exothermic redox reaction between Pt / WO3 and hydrogen allows the temperature change of the inner wall of the silica capillary to be conducted to the outer wall of the etched whispering-gallery mode optical microfluidic cavity, causing a local temperature increase in the microcavity. This leads to changes in the refractive index of the microcavity material and the cavity size, affecting the resonance conditions and causing a redshift of the resonance wavelength, thereby achieving the purpose of specific hydrogen sensing.

[0033] Combination Figure 1 As shown, the sensing scheme in this invention is as follows: WO3 can undergo a reduction reaction with hydrogen under the catalysis of Pt. Therefore, the enhanced response to hydrogen is achieved by bonding Pt / WO3 material to the inner wall of the silica capillary. The hydrogen reaction is exothermic, and through heat transfer, it indirectly affects the outer wall resonant cavity, influencing the cavity's geometry and optical parameters. When the gas concentration changes, the heat release accompanying the reaction also changes accordingly, thus causing a frequency shift in the mode. This exothermic process is relatively linear, and the response does not saturate in a short time, achieving a large dynamic range sensing. Figure 2The simulation results of field distribution show that the sensing structure of the etched microcavity of the outer wall of the silicon dioxide capillary blood vessel and the deposited material of the inner wall do not change the quality factor and mode distribution of the microcavity itself, and the sensing scheme proposed in the application is feasible.

[0034] Combining Figure 3 As shown, the laser-trigger technology is to realize the precise locking of the laser frequency by introducing the reference laser and the sensing laser and combining the feedback loop. The high stability of the reference laser guarantees the reference precision of the system, and the sensing laser captures the small frequency change caused by the gas concentration through the interaction with the gas in the sensing cavity. Through beat frequency detection of the two lasers, the mode frequency shift in the sensing cavity can be accurately converted into a measurable frequency signal, and high-precision tracking of the gas concentration is realized.

[0035] Combining Figure 4 The working process of the embodiment is described as follows:

[0036] After the system is built in sequence, the silicon dioxide capillary is placed on the clamp, the tunable narrow linewidth laser is turned on to emit the pump light signal, the position of the micro-nano fiber is adjusted to be coupled with the microcavity equator, and whether the pump light enters the cavity is judged by observing the change of the power. After the pump light enters the cavity, the polarization controller is adjusted to the best polarization state; the optical signal is connected to the coupler after passing through the polarization controller.

[0037] One output of the coupler enters the micro-nano fiber, and the optical signal transmitted by the micro-nano fiber is coupled into the WGM optical mode micro-flow cavity. The pump wavelength is changed to meet the resonance condition, the pump laser is locked with the resonance peak by using the laser servo, and the feedback function of the servo makes the pump laser and the resonance peak move synchronously.

[0038] The other output of the coupler is coupled with the super-stable laser source, a beat frequency signal is formed by photoelectric heterodyne detection as a sensing signal, and is output to the photoelectric detector and the spectrum analyzer. The photoelectric detector is connected to the spectrum analyzer. After the beat frequency signal is obtained, hydrogen is introduced through the rubber tubes at both ends of the silicon dioxide capillary. The hydrogen and the functionalized material on the inner wall of the capillary have an oxidation-reduction reaction to release heat. The heat is indirectly transferred to the microcavity etched on the outer wall of the capillary through the wall of the capillary. The microcavity is heated, the refractive index and the geometric size are changed, and then the resonance condition is changed, the resonance peak is red-shifted, and the signal generated by the beat frequency of the pump light and the super-stable laser locked on the resonance peak is also red-shifted. Figure 5 As shown, after being stabilized, the new frequency position is recorded, the frequency drift amount is calculated, and the gas concentration is calculated inversely.

[0039] The embodiment is compared with the actual concentration to prove the accuracy of the sensing of the application. As shown in Figure 6As shown, when the hydrogen concentration increases from 5 ppm to 30,000 ppm, the beat frequency offset increases from 8 MHz to 600 MHz. At low hydrogen concentration, the device does not respond well due to low heat release. Due to the feedback voltage limit of the laser servo system, the maximum detection range is about 0%-3%, and the sensitivity is approximately 21 MHz / p(H ppm). The detection limit (LOD) of H2 sensing reaches 5 ppb.

[0040] In conjunction Figure 7 As shown, the preparation process of the functionalized silica capillary WGM microcavity is as follows:

[0041] The processing of the microcavity requires the use of a high-power CO2 laser as a heat source, which is focused on a rotating silica capillary. Because the surface tension will form a disc-like area at the focal position, the CO2 laser passes through a mirror, enters a beam expander, and then passes through a focusing mirror to form a light spot with a diameter of about 10 μm on the silica rod. The mirror, beam expander, and focusing mirror are all fixed on a two-dimensional precision motorized displacement platform, which can be controlled by a program to adjust the focal length and laser fusion etching position appropriately.

[0042] Step 1, set the power of the CO2 laser, move the focusing mirror away from the silica capillary to make the laser spot larger and cover a larger area. Under high-speed rotation, use the displacement table to move repeatedly to polish the silica capillary and remove impurities on the surface.

[0043] Step 2, reduce the laser power and rotation speed to remove the milky white coating layer that appears during the polishing process, and make the silica capillary uniform in shape. At this time, move the focusing mirror close to the quartz rod, and move the displacement table at a certain distance and repeat the fusion etching. Two fusion etching points will appear concave, and the microcavity shape will be formed by surface tension.

[0044] Step 3, through annealing treatment, remove the dust in the etching, smooth the microcavity surface, change the radius of curvature, and finally get a microcavity with high Q value and controllable mode number.

[0045] Step 4, after etching the cavity on the outer wall of the silica capillary, configure the Pt / WO3 solution, stir uniformly, inject the solution into the inner wall of the capillary, rotate the capillary to ensure that the inner wall is evenly covered, then place the capillary coated with sol in the oven, dry for 1-2 hours to remove water and solvent, and get the functionalized microcavity.

[0046] It can be seen from the above examples that the application ensures the WGM optical microcavity with ultra-high Q value by indirectly contacting the Pt / WO3 functional material with the microcavity; the change of the optical characteristics of the WGM optical microcavity is caused by heat conduction based on the heat release of the reaction of hydrogen and the Pt / WO3 material; the laser-trigger technology is used to ensure that the sensing laser wavelength tracks the microcavity resonance displacement, and the photoelectric heterodyne detection is used to accurately measure the resonance wavelength position and displacement, so that the detection of the gas concentration is realized. The sensing system of the application has a significant detection limit of 5ppb and a large range detection of five orders of magnitude, realizes the integration of high sensitivity, large dynamic range and real-time detection of hydrogen sensing, provides an advanced detection method for microcavity sensing, and provides a good foundation for the safe large-scale application of hydrogen.

Claims

1. An echo-wall functionalized microfluidic cavity hydrogen sensor system based on laser tracking technology, characterized in that: The silica capillary, micro-nano fiber, polarization controller, adjustable narrow-bandwidth laser, coupler, photodetector, spectrum analyzer, laser servo, ultra-stable reference laser and temperature controller are included. The inner wall of the silica capillary is attached with Pt / WO3 functional material, and the outer wall is etched with WGM optical micro-cavity, and the stable preset environment temperature of the silica capillary is provided by the temperature controller; the two ends of the silica capillary are gas input and output channels. The laser signal emitted by the adjustable narrow-bandwidth laser is split into two paths by the coupler. The output light of one path split by the coupler is connected to the micro-nano fiber after passing through the polarization controller, and the optical signal transmitted by the micro-nano fiber is coupled into the WGM optical micro-cavity, causing the WGM optical micro-cavity to resonate; the output of the micro-nano fiber is connected to the first photodetector to convert the optical signal into an electrical signal, i.e. feedback signal, which is then input into the laser servo, and the laser servo controls the adjustable narrow-bandwidth laser to lock the laser output wavelength on the slope of the WGM optical micro-cavity resonance peak. The other path of the output light split by the coupler is coupled with the ultra-stable reference laser, and the beat frequency signal formed by the photodetector is output to the second photodetector and then connected to the spectrum analyzer. The spectrum analyzer is used to detect the frequency change of the beat frequency signal, calculate the frequency drift amount, and then calculate the gas concentration.

2. The hydrogen sensor system based on the laser tracking echo-wall functionalized microfluidic cavity according to claim 1, wherein: The outer diameter of the silica capillary is 3 mm, and the thickness is 400 μm; rubber tubes are provided at the two ends of the silica capillary to input and output gas.

3. The hydrogen sensor system based on the laser tracking echo-wall functionalized microfluidic cavity according to claim 1, wherein: The WGM optical micro-cavity is prepared by polishing, etching and annealing process on the outer wall of the silica capillary using CO2 laser.

4. The hydrogen sensor system based on the laser tracking echo-wall functionalized microfluidic cavity according to claim 1, wherein: The adjustable narrow-bandwidth laser is used to excite the pump light of the resonance mode and realize the high-precision resonance tracking of the locking light; the sweep frequency range of the adjustable narrow-bandwidth laser is 1550-1551 nm, the minimum scanning precision of the laser is less than 100 Hz, and the output laser line width is less than 1 kHz.

5. The hydrogen sensor system based on the laser tracking echo-wall functionalized microcavity according to claim 1, wherein: The micro-nano fiber is perpendicular to the silica capillary and located on the equatorial plane of the WGM optical micro-cavity, and is used to input pump light and output sensing signal and couple with the WGM optical micro-cavity; the micro-nano fiber is prepared by fusing and tapering the silica single-mode fiber, the tapering zone diameter is 500 nm-2 μm, the overall length is 2-4 cm, and the loss is 0.05-0.2 dB; the pump light is coupled into the micro-cavity in the form of evanescent field.

6. The hydrogen sensor system based on the laser tracking echo-wall functionalized microcavity according to claim 1, wherein: The Pt / WO3 functional material is prepared by sol-gel method and is uniformly deposited on the inner wall of the silica capillary, and has specific adsorption function for hydrogen molecules.

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