Kerr microcomb dynamics gas sensing system and method based on functionalized microbubble cavities

Abstract

The application discloses a Kerr microcomb dynamics gas sensing system and method based on a functionalized microbubble cavity, which is characterized in that the system combines a graphene inner wall functionalized microbubble cavity and a self-beating frequency response mechanism in the evolution of a Kerr microcomb. The method utilizes the thermal self-locking effect in the blue detuning region, and inverses the gas concentration by monitoring the specific comb tooth beating frequency shift caused by the adsorption of gas molecules. At the same time, the PDH technology is adopted to lock the pump laser frequency to the super-stable cavity, solving the problems of unstable comb tooth frequency and short duration of the sub-comb caused by frequency drift in the existing microcomb system. The to-be-detected gas is introduced into the internal closed circulation of the microbubble cavity, enhancing the interaction between the gas molecules and the optical evanescent field, thereby constructing a trace gas dynamic monitoring system with the advantages of operation safety, long-term stability and ppb-level ultra-high sensitivity.

Description

Technical Field

[0001] This invention belongs to the field of optical sensing and measurement of precision instruments, and particularly relates to a highly sensitive gas detection system and method based on the dynamic evolution of inner wall modified hollow microbubble cavities and Kerr microcomb. Background Technology

[0002] Whispering-gallery mode optical microcavities, due to their extremely high quality factor (Q value) and extremely small mode volume, can greatly enhance the interaction between light and matter, attracting much attention in the field of micro-nano sensing. Currently, most mainstream microcavity gas sensing technologies are based on linear optical response mechanisms, detecting the shift in the microcavity's resonant wavelength by monitoring changes in the microcavity's refractive index caused by ambient gas. The detection resolution of this method is limited by the laser linewidth, ambient thermal noise, and the reading accuracy of the spectrometer. When the concentration of the analyte gas drops to the ppb level, the resonant wavelength shift is often smaller than the fluctuation range caused by the aforementioned factors, leading to a decrease in the signal-to-noise ratio of the extracted signal, making it difficult to achieve high-precision quantitative detection.

[0003] In terms of device structure, traditional microcavity gas sensors are mostly solid structures, typically with sensitive functionalized materials coated on the outer surface of the microcavity to achieve specific sensing. This detection method has the following problems: First, when detecting toxic, harmful, flammable, or explosive gases, the gas is directly exposed to an open environment, posing a risk of leakage; second, the optical coupling of the microcavity usually relies on tapered optical fibers, and externally flowing gases and environmental impurities may contaminate or corrode the fiber coupling area, thereby affecting coupling efficiency and device stability.

[0004] In recent years, the development of optical frequency combs, especially high-repetition-rate Kerr microcomb technology, has provided new ideas for improving sensing performance. Compared with single-wavelength detection, optical frequency combs can provide a large number of phase-locked frequency comb teeth, and their multi-dimensional information (such as repetition frequency, carrier envelope offset frequency, and inter-comb beat frequency) provides more observable parameters for gas detection. However, current microcavity optical frequency comb sensing mainly relies on the translation of the overall comb tooth spectral envelope, and the mining of dynamic information in nonlinear optical processes is still insufficient. In addition, the generation of optical frequency combs requires high frequency stability of the pump source; long-term free operation may cause frequency drift, thereby introducing system measurement errors and affecting its application in practical continuous monitoring.

[0005] In summary, existing microcavity sensing technologies still have room for improvement in terms of detection resolution, long-term system stability, and operational safety. Based on this background, this invention proposes a high-precision gas detection method that combines ultra-stable cavity locking, inner-wall modified hollow microbubble cavities, and Kerr microcomb dynamic evolution. Summary of the Invention

[0006] This invention aims to overcome the problems of limited sensitivity, long-term frequency drift of microcomb systems, and cumbersome system structure and potential for harmful gas leakage caused by external modification relying on additional encapsulated gas chambers in traditional microcavity gas sensing. It proposes a Kerr microcomb dynamic gas sensing system and method based on functionalized microbubble cavities.

[0007] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0008] A Kerr microcomb dynamics gas sensing system based on a functionalized microbubble cavity includes a narrow linewidth continuous wave laser, a polarization-maintaining coupler, a main optical path modulation and amplification unit, a fiber taper-microbubble cavity coupled sensing system, a gas control system, a beat frequency shift detection unit, and an exhaust gas treatment unit, which are sequentially connected by optical fibers in the optical path.

[0009] Furthermore, the narrow-linewidth continuous-wave laser outputs a tunable continuous laser with a center wavelength of 1550 nm, which is used to excite the WGM microcavity to generate a Kerr optical frequency comb. The laser has narrow linewidth characteristics to ensure the coherence required for exciting the optical frequency comb.

[0010] Furthermore, the polarization-maintaining coupler is connected to the output of a narrow-linewidth continuous-wave laser, splitting its output laser into two paths at a power ratio of 1:99. One percent of the weak light power is injected into the ultra-stable cavity, and the laser's emission frequency is locked at the ultra-high precision resonant peak of the ultra-stable cavity using Pound-Drever-Hall (PDH) frequency-locking technology. This effectively suppresses long-term frequency drift of the pump laser, providing a light source with extremely low frequency noise for subsequent high-sensitivity gas sensing. The remaining 99% of the laser power enters the main optical path modulation and amplification unit.

[0011] Furthermore, the main optical path modulation and amplification unit sequentially includes an optical attenuator, a polarization controller, an optical isolator, and an erbium-doped fiber amplifier along the optical path. Wherein:

[0012] Optical attenuators are used to coarsely adjust the pump power, keeping it within an adjustable and safe range.

[0013] The polarization controller is used to finely adjust the polarization state of the beam to precisely match the whispering-gallery mode in the microbubble cavity that supports a high quality factor, thereby achieving optimal phase-matching conditions.

[0014] Optical isolators are used to effectively prevent back-reflected light from the end face of the microbubble cavity from returning and damaging the pump laser;

[0015] Erbium-doped fiber amplifiers are used to amplify pump light power to a nonlinear threshold beyond the excitation of Kerr microcomb, ensuring that a stable four-wave mixing process can be established within the cavity.

[0016] Furthermore, the fiber taper-microbubble cavity coupled sensing system comprises a tapered fiber taper and a microbubble cavity with a hollow channel. The diameter of the waist cone of the fiber taper is preferably 1.8~2.2 μm, and it is prepared using a melt-drawing method to ensure a sufficiently strong evanescent field is generated in the waist cone region. The microbubble cavity is formed by locally heating and expanding a quartz capillary, and its inner wall is coated with a graphene functional layer sensitive to the target gas. The fiber taper is in near-field contact with the equatorial plane of the microbubble cavity, and the amplified pump light is coupled into the microbubble cavity through the evanescent field, establishing resonance within the cavity and exciting a Kerr frequency comb.

[0017] Furthermore, the gas control system is connected to a gas source at one end and sealed to the hollow channel inlet of the microbubble cavity at the other end. This provides the microbubble cavity with the precise gas environment required for two operating modes: high-precision constant static pressure during the microbubble cavity preparation stage, and a precisely proportioned target gas during the gas sensing and testing stage. The hollow channel outlet of the microbubble cavity is connected to the waste gas treatment unit, ensuring a safe closed-loop process throughout the entire experiment.

[0018] Furthermore, the beat frequency shift detection unit is connected to the transmission output end of the fiber taper, splitting the transmitted light carrying microcavity dispersion change information into two paths: one path is connected to a high-speed photodetector, which converts the optical signal containing the subcomb beat frequency information into a microwave radio frequency electrical signal and transmits it to a radio frequency spectrum analysis device; the other path passes through an optical attenuator and is then connected to a spectrometer for real-time monitoring of the spectral envelope evolution state of the Kerr subcomb in the optical domain. This dual-domain parallel detection method can comprehensively capture nonlinear evolution characteristics, providing multi-dimensional data support for subsequent concentration inversion.

[0019] The present invention also provides a Kerr microcomb dynamic gas sensing method based on the above system, comprising the following steps:

[0020] S1. System Construction and In-situ Preparation of Microbubble Cavities: The system described in claim 1 is constructed, and a constant static pressure is applied to the inside of the quartz capillary through the preparation path of the gas control system. At the same time, the ablation area is heated again by the arc discharge method to soften its inner wall, so that it expands and takes shape under a specific gas pressure, and finally a hollow microbubble cavity with an outer diameter of 170~190μm is prepared. Subsequently, a graphene functional layer is formed on the inner wall of the microbubble cavity by coating method.

[0021] S2. Pump Laser Locking and Main Optical Path Modulation: The narrow-linewidth continuous-wave laser is turned on, outputting a laser with a center wavelength of 1550 nm and a power of 22 mW. The laser is split into two paths by a polarization-maintaining coupler. One-third of the optical signal is injected into the ultra-stable cavity, and the absolute frequency of the pump laser is locked by the PDH feedback loop to suppress long-term frequency drift of the system. The remaining 99% of the laser passes through an optical attenuator to adjust the power, a polarization controller to match the whispering-gallery mode polarization state of the microbubble cavity, an optical isolator to block the reverse propagation light, and finally is amplified by an erbium-doped fiber amplifier to meet the nonlinear threshold power of Kerr microcomb excitation.

[0022] S3. Microcomb Excitation and Blue Detuning Thermal Lock-in: The amplified pump light is passed into the fiber taper, and the fiber taper and the hollow microbubble cavity with functionalized inner wall are adjusted by a three-dimensional precision displacement stage to achieve near-field evanescent wave coupling; the wavelength of the pump laser is finely tuned to the blue detuning region of a specific resonance peak of the microbubble cavity, that is, the region where the pump wavelength is located on the short-wavelength side of the resonance peak. The photothermal lock-in effect established in the cavity is used to stabilize the pump detuning, thereby exciting a stable secondary comb in the cavity;

[0023] S4. Gas Adsorption and Intracavity Dispersion Disturbances: The gas to be tested is introduced into the hollow channel inside the microbubble cavity through the sensing path of the gas control system. After the test is completed, the exhaust gas is discharged into the waste gas treatment unit. The gas molecules to be tested are adsorbed with the graphene functional layer on the inner wall, which causes the carrier density of the functional layer to change. This in turn causes changes in the effective refractive index and group velocity dispersion of the microbubble cavity. According to the microcavity resonance condition, this will lead to the shift of the resonant wavelength inside the cavity and the dynamic change of the pump light detuning.

[0024] S5. Photoelectric conversion and dual-domain monitoring of beat frequency signals: The transmitted light from the microbubble cavity is split into two paths after being output through the fiber taper. One path is connected to a spectrometer for real-time monitoring of the spectral envelope evolution of the Kerr subcomb in the optical domain. The other path is connected to a high-speed photodetector to convert the optical signal containing the subcomb beat frequency information into a microwave radio frequency electrical signal and transmit it to the radio frequency spectrum analysis equipment.

[0025] S6. Dynamic frequency shift extraction and concentration inversion: The characteristic microwave signal generated by the beat frequency of adjacent Kerr comb teeth is extracted using radio frequency spectrum analysis equipment. Due to the change in detuning caused by gas adsorption in step S4, the center frequency of the characteristic beat frequency signal is shifted. By tracking the frequency shift of the beat frequency signal in real time and substituting it into the pre-calibrated linear mapping relationship of "beat frequency shift - gas concentration", the trace gas concentration introduced into the microbubble cavity is calculated.

[0026] Furthermore, the generation mechanism of the beat frequency shift signal described in step S5 can be explained based on four-wave mixing theory. When the pump light couples into the hollow microbubble cavity, degenerate four-wave mixing occurs under the action of Kerr nonlinearity, generating the first pair of first-order sidebands, located at the pump light frequency. The frequency spacing between the two sides of the pump light is Subsequently, the pump light and the first-order sidebands are excited to generate more frequency components through non-degenerate four-wave mixing. These frequency components are distributed in regular groups on both sides of the pump light frequency, and each group is called a higher-order sideband bundle. Adjacent frequency components within each higher-order sideband bundle have a fixed intrinsic spacing. The intrinsic spacing is equal to the free spectral range of the microbubble cavity, determined by the cavity's geometry and material dispersion characteristics. Each higher-order sideband exhibits a frequency shift relative to the pump light frequency. When adjacent higher-order sideband bundles overlap in the spectrum, the frequency components of different sideband bundles within the overlapping region coexist in the same resonant mode, and the frequency spacing between them... This corresponds to the center frequency of the beat frequency signal output by the photodetector. When gas adsorption causes group velocity dispersion... When changes occur, according to the phase matching condition of four-wave mixing, This will change accordingly, leading to This generates a frequency shift, the amount of which corresponds to the concentration of the gas being measured.

[0027] Compared with the prior art, the beneficial effects of the present invention are:

[0028] First, this invention abandons the linear detection method in traditional microcavity sensing that relies on a single resonant wavelength redshift. Instead, it innovatively utilizes the photothermal self-locking effect in the blue detuning region of the Kerr microcomb dynamics evolution to invert concentration by monitoring the frequency shift of specific comb teeth caused by gas molecule adsorption. Since the frequency measurement accuracy of the beat frequency signal is far higher than that of wavelength drift, this invention can achieve the ppb-level detection limit for trace gases.

[0029] Secondly, by introducing an ultra-stable cavity to perform PDH frequency locking on the pump laser, this invention effectively overcomes the physical bottlenecks of unstable comb frequency and short duration of secondary combs caused by long-term pump frequency drift in micro-comb systems. This significantly improves the long-term stability of the system and the reliability of data acquisition, laying the foundation for practical continuous monitoring applications.

[0030] Third, this invention employs a design that functionalizes the inner wall of the microbubble cavity and creates a dual-mode gas path with sealed connectivity, allowing the gas to be measured to circulate within the microbubble cavity in a closed loop. This innovative structure completely avoids the problems associated with traditional microbubble cavities, such as the cumbersome system structure, susceptibility to contamination of the fiber optic coupling area, and the risk of harmful gas leakage, which are caused by the need for additional gas chamber encapsulation due to the functional layer being coated on the outer surface. At the same time, because the gas and the optical evanescent field interact directly within the extremely thin microbubble cavity wall, the gas-light interaction efficiency is significantly enhanced.

[0031] Fourth, this invention establishes a complete technology chain from microbubble cavity preparation, ultra-stable frequency locking, kinetic evolution monitoring to concentration inversion. The system has a compact structure, is easy to operate, and is safe and reliable. It can be widely used in environmental monitoring, industrial safety, medical diagnosis and other fields for real-time dynamic detection of trace amounts of toxic and harmful gases. Attached Figure Description

[0032] Figure 1 A schematic diagram of the functionalized structure of the inner wall of the microbubble cavity and the cross-section of the gas flow field;

[0033] Figure 2 Optical path diagram of a Kerr microcomb dynamics gas sensing system based on functionalized microbubble cavities;

[0034] Figure 3 Wavelength fluctuation diagram of pump laser locked to ultrastable cavity for 12 hours; Detailed Implementation

[0035] The technical solutions in the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings.

[0036] This embodiment is based on a Kerr microcomb dynamics gas sensing system with functionalized microbubble cavities. The system platform consists of a narrow linewidth continuous wave (CW) laser, a polarization-maintaining coupler, a main optical path modulation and amplification unit, a fiber taper-microbubble cavity coupled sensing system, a gas control system, a beat frequency shift detection unit, and an exhaust gas treatment unit.

[0037] This embodiment provides a method for preparing hollow microbubble cavities and coating the inner wall with a graphene functional layer, the structure of which is as follows: Figure 1 As shown, the solid micrograph is as follows Figure 2 As shown.

[0038] First, the microbubble cavities were prepared. A quartz capillary with an outer diameter of 160 μm and an inner diameter of 100 μm was selected, and a capillary segment approximately 5 cm in length was cut using a ceramic cutter. The cut capillary segment was fixed horizontally, and its middle section was locally ablated using a hydrogen flame for approximately 1-2 seconds. During the ablation process, black ablation residue was observed to gradually form on the surface of the heated area. This was due to the thermal desorption of dopants from the quartz material at high temperatures. The purpose of this step was to ablate the high-temperature resistant coating of the capillary and to initially reduce the wall thickness.

[0039] After the ablation of the quartz capillary has cooled to room temperature, gently wipe the middle surface of the capillary with a lint-free cloth soaked in anhydrous ethanol to remove the ablation residue and expose a clean quartz surface.

[0040] After cleaning, the quartz capillary is placed in the fiber optic fusion splicer, and its position is adjusted so that the ablation area is aligned with the center of the electrode. One end of the capillary is sealed to a micro-injection pump via a Teflon tubing to apply controllable gas pressure to the inside of the capillary during the discharge process.

[0041] Using the manual discharge mode of a fiber optic fusion splicer, with the discharge intensity set to the standard level and the discharge time set to 2000 ms, a secondary arc heating was applied to the ablation area. Simultaneously, a constant positive pressure was applied to the inside of the capillary via a micro-injection pump. The softened quartz tube wall, under the combined action of the outward thrust of the internal gas pressure and surface tension, uniformly expanded outward, gradually forming spherical or near-spherical hollow microbubble cavities. By adjusting the discharge parameters and the internal gas pressure, the final size of the microbubble cavities could be precisely controlled.

[0042] After the discharge ends, maintain the internal pressure and continue venting for 5-10 seconds to allow the microbubble cavity to fully solidify under pressure. Then, slowly release the internal pressure. After the microbubble cavity cools naturally to room temperature, remove it from the welding machine. The prepared microbubble cavity retains the original capillary tube at one end as the air inlet and a capillary tube of approximately 2 cm length at the other end as the air outlet. The overall outer diameter is approximately 180 μm, and the wall thickness is approximately 5-8 μm. Its microstructure is as follows: Figure 3 As shown.

[0043] Next, the inner wall of the microbubble cavity was functionalized. First, an aqueous dispersion of graphene oxide was prepared: graphene oxide powder with a diameter of 0.5~5 μm and a thickness of 0.8~1.2 nm was weighed and dissolved in deionized water to prepare a dispersion with a concentration of 1 mg / mL. Then, the dispersion was placed in an ultrasonic cleaner and ultrasonically treated for 30 minutes to fully exfoliate and uniformly disperse the graphene oxide nanosheets.

[0044] One end of the prepared hollow microbubble cavity was connected to a microinjection pump via a silicone tubing, and the other end was immersed in the graphene oxide aqueous dispersion. The microinjection pump was set to operate at a positive pressure injection rate of 1 μL / min to slowly inject the dispersion into the hollow channel inside the microbubble cavity. When the dispersion was observed to continuously flow out of the outlet end of the microbubble cavity, the injection was stopped, at which point the microbubble cavity was filled with the dispersion.

[0045] The microbubble cavities filled with the dispersion were transferred to an oven and subjected to constant-temperature drying at 60°C for 12 hours. This temperature was chosen slightly above the boiling point of water to accelerate solvent evaporation while avoiding excessive heat that could lead to thermal decomposition of the graphene oxide; the longer drying time ensured complete evaporation of the solvent from the elongated capillary channels. As the solvent gradually evaporated, graphene oxide nanosheets were uniformly deposited and adhered to the inner walls of the microbubble cavities under the combined action of surface tension and van der Waals forces, forming a graphene oxide layer.

[0046] Finally, a reduction process was performed. A vitamin C aqueous solution with a concentration of 10 mg / mL was prepared. The vitamin C aqueous solution was injected into the microbubble cavity with the deposited graphene oxide layer using a microinjection pump. After filling, the reduction reaction was maintained at room temperature for 2.5 hours to reduce the graphene oxide to reduced graphene oxide, i.e., the graphene layer. Then, the cavity was slowly rinsed three times with deionized water to remove residual vitamin C and reaction byproducts.

[0047] The cleaned microbubble cavities were transferred back to an oven and dried at 50°C for 2 hours. This drying temperature was slightly lower than the initial film-forming drying because the graphene layer was already firmly attached to the inner wall at this point. Excessive temperature was unnecessary to accelerate evaporation; gentle drying was sufficient to remove residual moisture while avoiding potential thermal stress that could damage the formed graphene layer. The final result was a graphene-microbubble cavity composite structure with a functionalized inner wall.

[0048] This embodiment provides a specific method for building an optical system and the coupling adjustment process between the fiber taper and the microbubble cavity.

[0049] The fiber taper is fabricated as follows: A section of single-mode fiber is taken, and the coating layer of about 3 cm in the middle is stripped off. A fiber taper with a waist diameter of 2 μm and diameters of 125 μm at both ends is fabricated using the fused taper method. The transmitted light power is monitored in real time during the tapering process to ensure that the tapered fiber taper has low loss.

[0050] The prepared fiber taper was moved to the vicinity of the microbubble cavity using a three-dimensional precision displacement stage. Under electron microscopy, the fiber taper was slowly moved so that its waist-cone region gently contacted the equatorial plane of the microbubble cavity, forming near-field evanescent wave coupling. The coupling state was evaluated by monitoring the resonance peak depth of the transmission spectrum: the resonance peak depth in the transmission spectrum was maximized when the coupling reached the critical coupling state. The three propeller angles of the polarization controller were adjusted while monitoring the transmitted light power until the transmitted power reached its minimum value, indicating that the beam polarization state and the whispering-gallery mode of the microbubble cavity achieved optimal matching.

[0051] This embodiment details the specific implementation process of Pound-Drever-Hall (PDH) frequency locking for the pump laser using an ultrastable cavity. This locking scheme is a key technology for suppressing long-term frequency drift of the pump laser and ensuring stable excitation of the microcomb system.

[0052] Turn on the narrow linewidth continuous wave laser, set the output wavelength to 1550 nm, and the output power to 22 mW. After the laser output, it first enters the polarization-maintaining fiber beam splitter, which splits the laser into two paths: one path serves as the main pump light for subsequent sensing experiments, and the other path serves as the frequency-locked probe light entering the PDH frequency-locked branch.

[0053] In the PDH frequency-locked branch, the probe light first enters the electro-optic modulator (EOM). A radio frequency modulation signal is applied to the EOM via a signal generator; in this embodiment, the modulation frequency is 20 MHz, modulating the phase of the probe light and thus generating symmetrical positive and negative first-order modulation sidebands on both sides of the carrier frequency. The expression for the modulated optical field can be written as:

[0054]

[0055] in, For the laser carrier frequency, For modulation frequency, For modulation depth, This is a Bessel function.

[0056] The probe light carrying the modulation sideband is coupled into the ultrastable cavity through an optical fiber collimator. The ultrastable cavity has extremely high precision and a stable resonant frequency, with a very narrow transmission peak linewidth, approximately 10 kHz in this embodiment. When the laser frequency scan passes through the resonant peak of the ultrastable cavity, only the light component with a frequency matching the resonant peak can be efficiently coupled into the cavity, while the remaining frequency components are reflected.

[0057] The reflected light from the ultrastable cavity is captured by a high-speed photodetector, converting the optical signal into a radio frequency electrical signal. This reflected light signal contains amplitude and phase information of the carrier wave and the sidebands after their interaction with the ultrastable cavity. When the laser carrier frequency deviates from the resonant frequency of the ultrastable cavity, a specific phase relationship is generated between the carrier component and the sideband components in the reflected light.

[0058] The reflected light signal and the original RF modulation signal (local oscillator signal) are input together to a mixer for demodulation. The mixer multiplies the two signals, outputting a mixed signal containing sum and difference frequency components. After filtering out high-frequency components with a low-pass filter, the error signal ERROR is obtained. The amplitude of this error signal is linearly related to the detuning of the laser frequency relative to the resonant frequency of the ultrastable cavity, and the zero-crossing point corresponds to the laser frequency being precisely locked at the center of the resonant peak. The expression for the error signal is:

[0059]

[0060] in, This is the frequency detuning. and These represent the optical power of the carrier and the sideband, respectively. This is the reflection coefficient function of the ultrastable cavity.

[0061] The demodulated error signal is input to a preset PID controller. The PID controller performs proportional, integral, and derivative operations on the error signal to generate a feedback control signal. This embodiment uses a fast and slow dual-channel feedback mechanism to achieve wide dynamic range and high-precision frequency locking.

[0062] Fast feedback channel: connects the high-frequency feedback signal to the current modulation port of the laser to quickly suppress the frequency jitter and noise of the laser;

[0063] Slow feedback channel: The low-frequency feedback signal is connected to the piezoelectric ceramic module inside the laser to finely adjust the laser frequency to compensate for long-term drift caused by factors such as changes in ambient temperature.

[0064] Through fast and slow dual-channel coordinated feedback, the laser's output frequency is precisely locked to the center of the resonant peak of the ultra-stable cavity. In this embodiment, the long-term drift (within 12 hours) of the locked laser frequency is less than 1 MHz, and the short-term linewidth is compressed to the Hz level, significantly better than the drift of hundreds of MHz in free operation, providing a light source foundation with extremely low frequency noise for subsequent high-sensitivity gas sensing.

[0065] After the pump laser frequency is locked, the other laser beam (approximately 21.78 mW) output from the polarization-maintaining fiber beam splitter enters the main optical path modulation unit. First, the power is fine-tuned to 20 mW by an optical attenuator to ensure it is within a safe range suitable for nonlinear excitation. Then, an optical isolator ensures unidirectional beam transmission, preventing back-reflected light from damaging the laser and optical amplifier.

[0066] The pump light, with its polarization state and power adjusted, is input into an erbium-doped fiber amplifier (EDFA) and amplified to 100 mW to cross the nonlinear threshold of Kerr microcomb excitation. The amplified pump light is then passed into a fiber taper that has been coupled and tuned to the microbubble cavity.

[0067] At this point, the wavelength of the pump laser is fine-tuned. Since the laser frequency is locked by the ultrastable cavity, the offset of the laser frequency relative to the resonant peak of the ultrastable cavity can be precisely controlled by adjusting the bias voltage at the frequency lock point. The laser frequency is continuously increased in steps of 0.1 MHz (i.e., scanning towards longer wavelengths), while the outputs of the spectrometer and the RF spectrum analyzer are monitored in real time.

[0068] As the pump wavelength gradually moves from the short-wavelength side of the resonance peak (the blue detuning region) towards the center of the resonance peak, a decrease in transmitted power is observed. Continuing to increase the wavelength to a specific position within the blue detuning region, symmetrical first-order sidebands begin to appear on the spectrometer, marking the initial excitation of the Kerr microcomb. In the blue detuning region, a photothermal effect exists within the cavity: some pump light is absorbed by the microcavity material, generating heat and causing the microcavity temperature to rise, which in turn redshifts the resonance wavelength. This redshift effect reduces the detuning between the pump wavelength and the resonance peak, forming a negative feedback mechanism—the photothermal self-locking effect. Utilizing this effect, the pump detuning can be stabilized within a certain range without complex active feedback control.

[0069] This embodiment describes in detail the specific operation procedure for gas sensing testing using the above system, as well as the detection and analysis method for beat frequency shift signals.

[0070] After stable excitation by the Kerr microcomb, the gas to be tested is introduced into the hollow channel inside the microbubble cavity via a gas control system. First, high-purity nitrogen is introduced as the baseline gas, and the frequency value of the characteristic beat frequency signal on the RF spectrum analyzer at this point is recorded as the reference frequency. .

[0071] Subsequently, different concentrations of the target gas were precisely prepared using a mass flow controller. Taking ammonia as an example, the N2 carrier gas flow rate was set to 100 sccm, and the ammonia target gas flow rates were set to 0.01 sccm, 0.05 sccm, 0.1 sccm, 0.5 sccm, 1 sccm, 5 sccm, 10 sccm, 50 sccm, and 100 sccm, respectively, corresponding to diluted concentrations of 0.1 ppm, 0.5 ppm, 1 ppm, 5 ppm, 10 ppm, 50 ppm, 100 ppm, 500 ppm, and 1000 ppm. After thorough mixing in a gas mixer, the mixed gas was introduced into the microbubble chamber. Aeration was maintained for 10 minutes at each concentration point to ensure adsorption equilibrium, and high-purity nitrogen was introduced for 5 minutes between concentration increases to restore the system to baseline.

[0072] After the gas was introduced, the frequency changes of the characteristic beat frequency signal on the radio frequency spectrum analyzer were monitored in real time. It was observed that as the ammonia concentration increased, the center frequency of the beat frequency signal gradually shifted towards higher frequencies (blue shift). The frequency shift after stabilization at each concentration was recorded. Each concentration point was measured three times, the average value was taken, and the standard deviation was calculated as an error bar.

[0073] The concentration was taken as the common logarithm and then linearly fitted with the frequency shift to verify whether the beat frequency shift and the logarithm of the gas concentration showed a good linear relationship over a wide concentration range, and the detection sensitivity of the system was calibrated accordingly.

[0074] To verify the system's selectivity and response characteristics to different gases, this embodiment tested the frequency shift response of various gases (such as ammonia, carbon dioxide, oxygen, etc.) within the same concentration range, compared the sensitivity differences of different gases, analyzed the relationship between the sensitivity and the interaction strength of the functional layer, and verified the system's selective recognition capability for specific gases.

[0075] This embodiment also tests the response time and recoverability of the sensing system, records the time required for the frequency shift to stabilize after different gases are introduced, and the time required for the system to recover to near the baseline after the gas supply is stopped and the system is purged, and analyzes the influence of adsorption-desorption kinetics on the response characteristics.

[0076] To evaluate the long-term stability of the sensor, repeated tests were conducted on a fixed concentration of target gas over several consecutive days. The diurnal fluctuations and relative standard deviations of the frequency shift were analyzed to verify the long-term stability of the system.

[0077] The selectivity of the system is tested in a mixed gas environment. The target gas is mixed with common coexisting gases (such as carbon dioxide and oxygen) and then introduced into the system. The frequency shift is compared with that in a pure target gas environment to verify whether the system is significantly affected by the coexisting gases.

[0078] The above experiments verify that the system has high sensitivity, fast response and recovery characteristics, good repeatability and excellent selectivity for the target gas, meeting the needs of real-time detection of trace gases and having the potential for multi-component gas analysis.

[0079] To verify the generation mechanism of the beat frequency shift signal and its correspondence with gas concentration, this embodiment conducts an in-depth study combining theoretical analysis and numerical simulation.

[0080] According to the four-wave mixing theory, the sideband frequencies of the optical frequency comb excited by the pump light within the microbubble cavity satisfy the phase-matching condition. The frequency spacing between the first-order sideband and the pump light... Determined by the following formula:

[0081]

[0082] In the formula, For group velocity dispersion, These are nonlinear coefficients. For group refractive index, This represents the pumping power within the cavity. When gas molecules are adsorbed onto the graphene functional layer, the carrier density of the graphene changes, leading to a change in its dielectric constant, which in turn affects the overall effective refractive index and group velocity dispersion of the microbubble cavity. According to the above formula, The changes directly led to Things have changed.

[0083] Meanwhile, the beat frequency signal frequency between higher-order sideband bundles and Satisfying Relationship:

[0084]

[0085] in This represents the free spectral range of the microbubble cavity. Therefore, The changes further caused The frequency shift. This theoretical relationship explains why gas adsorption can be detected by beat frequency shift.

[0086] To verify the above theory, this embodiment conducted a numerical simulation based on the Lugiato-Lefever equation, calculating the beat frequency shift corresponding to the dispersion change caused by gas adsorption for microbubble cavities of different sizes. The following parameters were fixed in the simulation: Q value 5 × 10⁻⁶. 8The nonlinear coefficient γ = 1.79 × 10⁻² W⁻¹m⁻¹, pump power 20mW, and group velocity dispersion variation Δβ² = 0.1 ps² / km (simulating dispersion perturbation caused by gas adsorption) are given. The geometric parameters and simulation results for microbubble cavities of different diameters are shown in the table below.

[0087] Microbubble cavity diameter (µm) Perimeter L (mm) Free spectral range (GHz) Frequency shift (kHz) caused by the change 180 0.565 333 28.7 200 0.628 300 24.3 300 0.942 200 16.2

[0088] As shown in the table, when the microbubble cavity diameter decreases from 300 μm to 180 μm, the beat frequency shift caused by the same dispersion change increases from 16.2 kHz to 28.7 kHz. This trend indicates that reducing the microbubble cavity size can enhance the influence of dispersion perturbations caused by gas adsorption on the beat frequency signal, thereby improving sensing sensitivity.

[0089] The above description is merely a preferred embodiment of this application and is not intended to limit the scope of 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 Kerr microcomb dynamics gas sensing system based on functionalized microbubble cavities, characterized in that, The system includes a narrow-linewidth continuous-wave laser (1), a polarization-maintaining coupler (2), an ultra-stable cavity frequency-locking system (3), an optical attenuator (4), a polarization controller (5), an optical isolator (6), an erbium-doped fiber amplifier (7), a fiber taper-microcavity coupled sensing system (8), a photodetector (9), an optical attenuator (10), a spectrometer (11), a gas control system (12), and an exhaust gas treatment unit (13), all connected sequentially via optical fibers. The narrow linewidth continuous wave laser (1) is used to output a 1550 nm band tunable continuous laser to excite the microbubble cavity to generate an optical frequency comb. The polarization-maintaining coupler (2) is connected to the narrow linewidth continuous wave laser (1) and its output laser is divided into two paths with a power ratio of 1:

99. 1% of the optical power is injected into the ultra-stable cavity (3) and the laser frequency is locked by Pound-Drever-Hall frequency locking technology to suppress pump frequency drift. The remaining 99% of the optical power enters the main optical path modulation unit. The main optical path modulation unit consists of an optical attenuator (4), a polarization controller (5), an optical isolator (6), and an erbium-doped fiber amplifier (7) connected in sequence. The optical attenuator (4) is used to coarsely adjust the pump power, the polarization controller (5) is used to adjust the beam polarization state to match the whispering wall mode of the microbubble cavity, the optical isolator (6) is used to block the reverse propagating light, and the erbium-doped fiber amplifier (7) is used to amplify the pump light to above the nonlinear threshold of Kerr microcomb excitation. The fiber taper-microbubble cavity coupled sensing system (8) includes a tapered fiber taper (14) and a hollow microbubble cavity (15). The inner wall of the microbubble cavity is coated with a graphene functional layer. The fiber taper is in near-field contact with the equatorial surface of the microbubble cavity. The amplified pump light is coupled into the microbubble cavity through the evanescent field to establish resonance and excite the optical frequency comb. After the secondary comb is excited, it is input into the beat frequency shift detection unit. The beat frequency shift detection unit is connected to the transmission output end of the fiber taper, and the transmitted light carrying the microcavity dispersion change information is connected to the photodetector (9), and then the light power is reduced by the optical attenuator (10) before being connected to the spectrometer (11) to monitor the spectral frequency shift. The gas control system (12) is connected to a gas source at one end and sealed to the hollow channel inlet of the microbubble cavity at the other end, which is used to provide the microbubble cavity with the target gas required for preparation at a constant static pressure or for testing at a precise concentration ratio; the hollow channel outlet of the microbubble cavity is connected to the waste gas treatment unit (13).

2. The Kerr microcomb dynamics gas sensing system based on functionalized microbubble cavities according to claim 1, characterized in that, The hollow microbubble cavity is obtained by a preparation method comprising the following steps: Step 2.1, Capillary cutting: Select a quartz capillary with an outer diameter of 160μm and an inner diameter of 100μm, and use a ceramic cutter to cut a quartz capillary segment with a length of 4~7cm. Step 2.2, Hydrogen flame ablation treatment: Use a hydrogen flame to locally ablate the middle part of the quartz capillary segment for a duration of 1 to 2 seconds until the polyimide coating on the surface of the heated area is completely carbonized and forms a black pyrolysis residue, then stop the ablation to remove the coating in that area. Step 2.3, Cleaning: After the ablation of the quartz capillary has cooled to room temperature, use a lint-free cloth soaked in anhydrous ethanol to wipe away the ablation residue on the surface of the middle part of the capillary. Step 2.4, Discharge Expansion Molding: The cleaned quartz capillary is placed in the fiber optic fusion splicer. A flat-headed needle tube with an inner diameter of 0.2 mm is inserted into one end of a silicone tube, and the other end of the silicone tube is tightly fitted into one end of the quartz capillary. Air is introduced into the capillary through the flat-headed needle tube. At the same time, the ablation area is heated a second time using an arc discharge method to soften its inner wall, so that it expands and shapes under a specific air pressure, and finally a hollow microbubble cavity with an outer diameter of 170~190 μm is prepared.

3. The Kerr microcomb dynamics gas sensing system based on functionalized microbubble cavities according to claim 2, characterized in that, The functional modification of the inner wall of the microbubble cavity is achieved through a coating method including the following steps: Step 3.1, Dispersion preparation: Prepare an aqueous dispersion of graphene oxide and perform ultrasonic treatment to ensure that the graphene oxide nanosheets are uniformly dispersed in the solvent. Step 3.2, Dispersion injection: Connect one end of the hollow microbubble cavity to a micro-injection pump through a tubing, immerse the other end in the graphene oxide aqueous dispersion, and inject the dispersion into the hollow channel inside the microbubble cavity using a positive pressure injection method through the micro-injection pump. Step 3.3, Drying and film formation: Transfer the microbubble cavity filled with dispersion to an oven for constant temperature drying to evaporate the solvent and allow graphene oxide to be uniformly deposited and attached to the inner wall of the microbubble cavity, forming a graphene oxide layer. Step 3.4, reduction treatment: Inject a vitamin C aqueous solution with a concentration of 5~20 mg / mL into the microbubble cavity after the treatment in step 3.3, maintain the reduction reaction for 2~3 hours, and reduce the graphene oxide layer to a graphene layer; then clean the cavity with deionized water, and transfer the microbubble cavity to an oven for drying treatment again.

4. A Kerr microcomb dynamics gas sensing method based on functionalized microbubble cavities using the system described in claim 1, characterized in that, Includes the following steps: Step 4.1, Pump Laser Locking and Main Optical Path Modulation: The narrow linewidth continuous wave laser is turned on, outputting a laser with a center wavelength of 1550nm and a power of 22mW. The laser is split into two paths by a polarization-maintaining coupler; 1% of the optical signal is injected into the ultra-stable cavity, and the absolute frequency of the pump laser is locked by a PID feedback loop to suppress long-term frequency drift of the system; the remaining 99% of the laser passes through an optical attenuator to adjust the power, a polarization controller to match the whispering-gallery mode polarization state of the microbubble cavity, an optical isolator to block the reverse propagation light, and finally amplified by an erbium-doped fiber amplifier to meet the nonlinear threshold power of Kerr microcomb excitation; Step 4.2, Microcomb Excitation and Blue Detuning Thermal Lock-in: The amplified pump light is passed into the fiber taper, and the fiber taper is adjusted to perform near-field evanescent wave coupling with the hollow microbubble cavity with functionalized inner wall; the wavelength of the pump laser is finely tuned to the blue detuning region of a specific resonance peak in the microbubble cavity, and the pump detuning is stabilized by the photothermal lock-in effect established in the cavity, thereby exciting a stable secondary comb in the cavity; Step 4.3, Gas Adsorption and Intracavity Dispersion Disturbance: The gas to be tested is introduced into the hollow channel inside the microbubble cavity, and the exhaust gas after the test is completed is discharged into the waste gas treatment module; the gas molecules to be tested are adsorbed with the graphene functional layer on the inner wall, which causes changes in the effective refractive index and group velocity dispersion of the microbubble cavity, thereby causing frequency shift of the beat frequency. Step 4.4, photoelectric conversion and dual-domain monitoring of beat frequency signal: The transmitted light from the microbubble cavity is output through the fiber taper and then connected to a high-speed photodetector to convert the optical signal containing the subcomb beat frequency information into a microwave radio frequency electrical signal, which is then transmitted to a spectrum analyzer to analyze the frequency shift. Step 4.5, Dynamic Frequency Shift Extraction and Concentration Inversion: The characteristic microwave signal generated by the beat frequency of adjacent Kerr comb teeth is extracted using an RF spectrum analysis device; due to the change in detuning caused by gas adsorption in step 4.3, the center frequency of this characteristic beat frequency signal undergoes a frequency shift; by tracking the frequency shift of this beat frequency signal in real time and substituting it into the pre-calibrated linear mapping relationship of "beat frequency shift - gas concentration", the trace gas concentration introduced into the microbubble cavity is calculated.

5. The Kerr microcomb dynamics gas sensing method based on functionalized microbubble cavities according to claim 4, characterized in that, The generation and detection of the beat frequency shift signal in step 4.4 are based on the following principle: When the pump light couples into the hollow microbubble cavity, a four-wave mixing process occurs under the Kerr nonlinear effect; firstly, degenerate four-wave mixing generates the first pair of first-order sidebands, located at the pump light frequency. The frequency spacing between the two sides of the pump light is ; Subsequently, the pump light and the first-order sidebands are excited to generate more frequency components through non-degenerate four-wave mixing. These frequency components are distributed in regular groups on both sides of the pump light frequency, and each group is called a higher-order sideband bundle. Each higher-order sideband bundle consists of a set of frequency components, and there is a fixed intrinsic spacing between adjacent frequency components within the bundle. The intrinsic spacing is equal to the free spectral range of the microbubble cavity, that is: （1） In the formula, The speed of light in a vacuum. For group refractive index, The perimeter of the microbubble cavity; Each higher-order sideband bundle exhibits a frequency shift relative to the pump light frequency. The shift amount of the nth higher-order sideband bundle is... satisfy: （2） Where n is the ordinal number of the higher-order sideband bundle. The spectral spacing between the pump light and the first-order sideband is determined by the following formula: (3) In the formula, The refractive index of the microbubble cavity material is... These are nonlinear coefficients. To couple the pump power into the microbubble cavity, For group velocity dispersion, For vertical pattern ordinal numbers, The pump light frequency; When adjacent higher-order sideband bundles overlap in the frequency spectrum, the frequency components of different sideband bundles within the overlapping region coexist in the same resonant mode, and the frequency spacing between them... Determined by the difference in offset between adjacent bundles: （4） This frequency interval This corresponds to the center frequency of the beat frequency signal output by the photodetector described in step 4.4; When the gas molecules to be measured are adsorbed onto the functional layer of the microbubble cavity, the carrier density of the functional layer changes, leading to changes in its dielectric constant and effective refractive index, which in turn causes group velocity dispersion in the microbubble cavity. An offset has occurred; based on the above relationship, Changes led to Changes, which in turn cause Frequency shift The frequency shift It corresponds to the concentration of the gas being measured.

Images