A graphene functionalized microcavity gas sensor based on secondary comb

Abstract

The present application belongs to the technical field of microcavity optical frequency comb sensing, and particularly relates to a graphene functionalized microcavity gas sensor based on a secondary comb. The present application uses a secondary comb to realize sensing, and only needs blue detuning of a pump at a resonant cavity. Compared with a conventional optical frequency comb sensing mode relying on a soliton state, the present application utilizes a thermal self-locking effect of blue detuning, avoids a complex tuning process and auxiliary means required for entering red detuning, and reduces system complexity. In addition, the combination of a microcavity and graphene solves the inertness problem of a conventional microcavity material, improves gas adsorption efficiency, and greatly improves sensing sensitivity (as low as the ppb order). More importantly, the present application provides an advanced sensing mode suitable for various platforms, breaks the inherent view that a secondary comb is only a transitional state, and provides an example for the wide application of the secondary comb in the future.

Description

Technical Field

[0001] This invention belongs to the field of microcavity optical frequency comb sensing technology, specifically relating to a graphene-functionalized microcavity gas sensor based on a secondary comb. Background Technology

[0002] Microcavity-based sensing technology has a long history of development. Microcavities, as containers for confined light, possess extremely high energy densities in their light fields, leading to rich nonlinear effects and light-matter interactions. Since the 21st century, several highly efficient microcavity sensing mechanisms have emerged, such as microcavity sensors based on effects like mode drift, mode broadening, and mode splitting, exhibiting high sensitivity and selectivity. In recent years, microcavity-based optical frequency combs have brought revolutionary changes to the field of microcavity sensing. The inherent stability of optical frequency combs, combined with their low power consumption, has improved both the detection limit and energy utilization efficiency.

[0003] In the field of optical frequency comb sensing, soliton states are commonly used, leveraging their phase-locking and low-noise stability to achieve accurate detection of various sensing targets. However, the generation of soliton states requires pumping into thermally unstable red detuning, and the cumbersome tuning methods and auxiliary means significantly increase the system's complexity. Furthermore, since microcavities are mostly made of inert materials such as silicon dioxide and silicon nitride, they lack adsorption properties for target molecules, limiting the sensing sensitivity.

[0004] Therefore, finding a microcavity frequency comb sensing mechanism with low system complexity and high detection sensitivity is crucial and has significant scientific and application value. Summary of the Invention

[0005] To address the aforementioned problems and shortcomings, and to solve the issues of high complexity and low detection sensitivity in existing microcavity comb sensor systems, this invention provides a graphene-functionalized microcavity gas sensor based on a secondary comb. By utilizing the merging properties of the secondary comb, a plug-and-play, readily available, and ultrasensitive gas sensor with a detection limit as low as ppb is realized in a microcavity with a single layer of graphene attached to its surface.

[0006] A graphene-functionalized microcavity gas sensor based on a secondary comb includes a tunable narrowband laser, a microcavity, a pump optical coupling kit, a photodetector, a spectrum analyzer, and a temperature control module.

[0007] The tunable narrow-bandwidth laser is used as pump light to generate the secondary comb state.

[0008] The microcavity is covered with a single layer of graphene, and the pump light generated by the tunable narrow bandwidth laser is coupled to the microcavity through a pump light coupling kit.

[0009] The photodetector is used for photoelectric conversion, converting optical signals into electrical signals.

[0010] The spectrum analyzer is used to detect the changing frequency position of the coupled light in the microcavity, calculate the changing frequency drift, and thus calculate the gas concentration.

[0011] The temperature control module is used to provide a stable preset temperature environment for the microcavity.

[0012] Furthermore, the microcavity is a microsphere cavity, a microrod cavity, a microdisc cavity, an FP cavity, or an on-chip microring cavity.

[0013] Furthermore, the material of the microcavity is silicon dioxide (SiO2), silicon nitride (Si3N4), magnesium fluoride (MgF2), or lithium niobate (LiNbO3).

[0014] Furthermore, the secondary comb's mode overlaps with the monolayer graphene, maximizing the influence of the gas adsorption on the mode.

[0015] Furthermore, the microcavity is a silica microsphere cavity, and the corresponding pump optical coupling kit is a tapered optical fiber; the cavity entry end of the tapered optical fiber is connected to an adjustable narrow bandwidth laser, and the transmission end of the tapered optical fiber is connected to a photodetector.

[0016] Furthermore, the quality factor of the silica microsphere cavity is ≥1×10⁻⁶. 6 .

[0017] Furthermore, the monolayer graphene is attached to the equator of the silica microsphere cavity, and the horizontal height of the tapered optical fiber is on the same plane as the equator of the silica microsphere and does not contact the monolayer graphene to improve the coupling effect. The tapered optical fiber is made of silica single-mode optical fiber by fused taper method, with a taper diameter of 500nm-2μm and an overall length of 2-4cm, and couples the pump light into the microcavity in the form of an evanescent field.

[0018] This invention utilizes the secondary comb state during the generation process of the optical frequency comb, especially the merging effect in the secondary comb, to reflect the distance between two comb teeth growing in the same resonant cavity on the radio frequency using heterodyne beat frequency, and uses its frequency position as a sensing basis to detect gas concentration.

[0019] Compared with the prior art, the superior effects of the present invention are as follows:

[0020] 1. This invention solves the problems of traditional optical frequency comb sensing relying on soliton states, having a cumbersome tuning process, and high system complexity. It utilizes secondary comb states and benefits from the thermal lock-in effect of blue detuning at the resonance peak. No auxiliary means or complex tuning methods are required; the pump detuning amount can be freely adjusted, and once the pump wavelength is determined, a plug-and-play effect can be achieved.

[0021] 2. Microcavity materials are mostly inert materials such as silicon dioxide (SiO2) and silicon nitride (Si3N4), which are insensitive to gases and have poor adsorption capacity. The introduction of graphene effectively solves this problem. Graphene has strong gas adsorption properties, and the adsorption of gas molecules can change the Fermi level of graphene, affecting the refractive index of the microcavity and thus altering its dispersion. The change in dispersion further changes the spacing of the secondary comb teeth, which is reflected in the frequency position of the beat frequency signal, greatly improving the sensitivity to gases.

[0022] This invention organically integrates optical frequency comb dynamics, graphene optoelectronics, and heterodyne beat frequency technology to form a novel, simple, practical, and efficient interdisciplinary gas detection method. More importantly, this invention provides a universal, platform-independent sensing method that can be used on multiple platforms and applied to various sensing objects, such as multi-parameter sensing of gases and on-chip biochemical sensing.

[0023] In summary, this invention has the advantages of low system complexity, plug-and-play functionality, high detection sensitivity, and low cost; it effectively solves the problems of existing microcavity frequency comb sensors, such as complex systems, difficult operation, low sensitivity, and high cost. Attached Figure Description

[0024] Figure 1 This is a conceptual block diagram of an embodiment;

[0025] Figure 2 These are the spectra and radiation spectrum diagrams of each stage of the optical frequency comb pumping modulation process;

[0026] Figure 3 This is a diagram of the sensing system in an embodiment;

[0027] Figure 4 This is a diagram showing the detection peak positions corresponding to different NO2 concentrations in the examples;

[0028] Figure 5 This is an example based on Figure 4 The calculated NO2 concentration. Detailed Implementation

[0029] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments.

[0030] This embodiment is based on a graphene-functionalized microsphere gas sensor using a secondary comb. The system platform consists of an adjustable narrow-bandwidth pump light source (around 1550nm), an optical fiber polarization controller, an erbium-doped optical fiber amplifier (C-band), a tapered optical fiber, a graphene-coated silica microsphere cavity, a photodetector (detection bandwidth greater than or equal to 400MHz), a vacuum chamber, and a temperature controller.

[0031] The pump source, fiber optic erbium-doped amplifier, and fiber optic polarization controller are connected in sequence and then inserted into the cavity entrance of a tapered optical fiber. The tapered optical fiber and the microsphere are in the optimal coupling position, and the transmission end of the tapered optical fiber is connected to a photodetector. The silica microsphere is fixed on a brass thermally conductive base, and a temperature controller is attached to the side of the base. The entire assembly of the base and the microsphere is placed in a vacuum cavity.

[0032] After adjusting the coupling, a tunable narrow linewidth laser is used as the pump to excite the secondary comb. A detection peak is selected on the spectrum and its frequency position is recorded. Then, different amounts of the gas to be tested are gradually introduced. After stabilization, the position of the peak is recorded and the corresponding gas concentration is calculated.

[0033] The tapered optical fiber used is drawn from ordinary single-mode optical fiber using a commercial tapering machine under oxyhydrogen flame heating conditions. The diameter of the tapered region is 500nm-2μm, the length of the tapered region is about 0.5-1cm, and the total length is 4cm.

[0034] The silica microspheres used are made from ordinary single-mode optical fiber, with a diameter of 600 micrometers and a quality factor of 2×10⁻⁶. 6 .

[0035] The monolayer graphene used was obtained from graphite by mechanical exfoliation and transferred to the equator of silica microspheres by dry transfer, so that the mode of the secondary comb overlapped with the graphene, maximizing the influence of gas adsorption on the mode.

[0036] A silica microsphere coated with a single layer of graphene is placed in a fixture, and a tapered optical fiber is placed on a three-axis precision displacement stage. The horizontal height of the tapered optical fiber is precisely adjusted by rotating the knob of the displacement stage so that it is in the same plane as the equator of the microsphere. Then, the relative distance between the fiber and the microsphere is finely adjusted to achieve a coupling effect close to critical coupling.

[0037] The pump power used depends on whether it can generate a secondary comb with a merging effect, as well as the bandwidth, signal-to-noise ratio (SNR), and frequency stability of the probe peak in the RF spectrum. Typically, a pump power that can excite the merging effect, has a narrow probe peak bandwidth, high SNR, and good frequency stability is selected. Testing in this embodiment showed that the optimal pump power is approximately 260mW, at which point the probe peak bandwidth is approximately 2kHz, the SNR is ≥25dB, and the drift over 5 minutes is approximately ±37kHz.

[0038] The resulting secondary comb has a spectral width of approximately 80 nm, clearly showing each primary comb line and the secondary comb lines growing around them. A noticeable overlap is visible between two adjacent primary comb lines. In the RF spectrum, the number of beat frequencies (manifested as sharp spectral components) from the merging comb teeth varies between 1 and 10, depending on the magnitude of the detuning. The optimal detuning is typically chosen based on the narrowest bandwidth, highest signal-to-noise ratio, and best frequency stability of the beat frequency signal.

[0039] The working process of this embodiment is as follows:

[0040] Set up the system in sequence, turn on the pump, adjust the fiber polarization controller to the optimal polarization state for frequency comb excitation, turn on the erbium-doped fiber amplifier, amplify the pump to the preset power, and gradually increase the pump wavelength to allow it to enter the cavity gradually from blue detuning. During this process, the gain due to modulation instability increases with the increase of cavity power. When the gain exceeds the cavity loss, a pair of primary comb lines can be seen growing symmetrically on both sides of the pump in the spectrum. Continue to increase the pump wavelength, and under the action of non-degenerate four-wave mixing, several primary comb lines grow out on both sides with the same spacing to form a primary comb.

[0041] As the detuning increases further, multiple secondary comb lines grow at equal intervals around each primary comb line. These secondary comb lines extend outwards until they overlap with another cluster of secondary comb lines. At this point, several beat frequency signals gradually emerge from the originally flat radio frequency domain, and the number of signals increases with the increase of the detuning. Select one signal as the observation target and record its frequency position at this time.

[0042] A preset amount of the gas to be measured is introduced into the vacuum chamber using a syringe. After stabilization, the new frequency position is recorded, the frequency drift is calculated, and the gas concentration is calculated in reverse. This embodiment is compared with the actual concentration to confirm the accuracy of the sensing of the present invention.

[0043] In this embodiment: the silica microsphere cavity is fabricated from ordinary single-mode optical fiber under discharge conditions. It is inexpensive, possesses a considerable quality factor, ultra-long photon lifetime, and a low Kerr nonlinear threshold, making it an ideal platform for optical frequency comb excitation. Furthermore, monolayer graphene obtained through mechanical exfoliation is readily available and can be dry-spun and adhered to the microsphere surface. It exhibits strong adsorption of gases, and its Fermi level changes with the amount of adsorbed gas, making it an ideal material for gas sensing. The tapered optical fiber, made from single-mode optical fiber through a tapering mechanism, has a taper diameter on the same order of magnitude as the pump wavelength, facilitating light coupling with the microsphere via an evanescent field.

[0044] like Figure 1As shown in the conceptual diagram, the silica microspheres have a diameter of approximately 600 micrometers, with a single layer of graphene adhered to the equator, and a quality factor of approximately 2 × 10⁻⁶. 6 The tapered optical fiber has a total length of 4 cm and a taper diameter of approximately 1 micrometer. Microspheres are fixed to a fixture, and the tapered optical fiber is fixed to a precision three-dimensional displacement stage. The stage is used to adjust their relative positions to achieve optimal coupling. The output of the tapered optical fiber is connected to a photodetector. When the secondary comb is excited, sharp spectral components appear in the radio frequency domain. After the graphene adsorbs gas, the frequency components shift, and the corresponding gas concentration can be calculated from the amount of this shift.

[0045] like Figure 2 As shown, the evolution of the optical frequency comb in the spectrum and frequency domain can be observed as the pump wavelength gradually redshifts. Initially, before the pump enters the cavity, only one frequency component is visible in the spectrum, and the spectrum is flat. Slowly moving the pump into the cavity, the increased intracavity power excites a stronger gain, resulting in the growth of primary combs with spacing across multiple free spectral ranges on both sides of the pump. Due to the stability of the primary combs, the spectrum exhibits the same low-noise characteristics as before. Gradually increasing the pump wavelength, secondary combs begin to grow around the primary combs and extend to both sides. When the two clusters of secondary combs overlap, obvious beat frequency signals gradually appear in the spectrum, and their number increases with increasing pump wavelength. The peaks with high signal-to-noise ratio, narrow linewidth, and good frequency stability are selected as the detection peaks.

[0046] The system structure of gas sensing is as follows Figure 3 As shown, a microcavity sensor is placed inside a vacuum chamber and connected to the pump using a fiber polarization controller to optimize mode excitation. The output is connected to a photodetector and a frequency counter.

[0047] like Figure 4 As shown, different amounts of nitrogen dioxide (NO2) gas were injected into the vacuum chamber, and the detection peak positions were recorded at different concentrations. As the NO2 concentration increased from 10 ppb to 10000 ppb, the detection peak position shifted from 204.436 MHz to 204.623 MHz. Figure 5 As shown, by plotting the relationship between gas concentration and detection peak shift, the gas detection sensitivity in this embodiment can reach approximately 4 ppb.

[0048] As can be seen from the above embodiments, this invention uses a secondary comb for sensing, requiring only the pumping of the blue detuned state within the resonant cavity. Compared to traditional optical frequency comb sensing methods relying on soliton states, it utilizes the thermal locking effect of blue detuning, avoiding the complex tuning process and auxiliary methods required to achieve red detuning, thus reducing system complexity. Furthermore, the combination of microcavity and graphene solves the inertia problem of traditional microcavity materials, improves gas adsorption efficiency, and significantly enhances sensing sensitivity (down to the ppb level). More importantly, it provides a universal, advanced sensing method suitable for various platforms, breaking the previously held view that secondary combs are merely a transitional state, and providing a paradigm for its widespread future application.

Claims

1. A graphene-functionalized microcavity gas sensor based on a secondary comb, characterized in that: This includes a tunable narrow-bandwidth laser, a microcavity, a pump optical coupling kit, a photodetector, a spectrum analyzer, and a temperature control module. The tunable narrow-bandwidth laser is used as pump light to generate the secondary comb state; The microcavity is a silica microsphere cavity, and the corresponding pump optical coupling kit is a tapered optical fiber; the cavity entrance end of the tapered optical fiber is connected to an adjustable narrow bandwidth laser, and the transmission end of the tapered optical fiber is connected to a photodetector. The microcavity is covered with a single layer of graphene, and the pump light generated by the tunable narrow bandwidth laser is coupled to the microcavity through a pump light coupling kit. The monolayer graphene is attached to the equator of the silica microsphere cavity, and the horizontal height of the tapered optical fiber is on the same plane as the equator of the silica microsphere, and does not contact the monolayer graphene. The tapered optical fiber is made of silica single-mode optical fiber by fused taper method, with a taper diameter of 500nm-2μm and an overall length of 2-4cm. It couples pump light into the microcavity in the form of an evanescent field. The photodetector is used for photoelectric conversion, converting optical signals into electrical signals; The spectrum analyzer is used to detect the changing frequency position of the coupled light in the microcavity, calculate the changing frequency drift, and thus calculate the gas concentration. The temperature control module is used to provide a stable preset temperature environment for the microcavity.

2. The graphene-functionalized microcavity gas sensor based on a secondary comb as described in claim 1, characterized in that: The secondary comb pattern overlaps with the monolayer graphene.

3. The graphene-functionalized microcavity gas sensor based on a secondary comb as described in claim 1, characterized in that: The quality factor of the silica microsphere cavity is ≥1×10⁻⁶. 6 .

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