Visible light frequency comb generation device and method based on on-chip silicon oxide microcavity

Abstract

The embodiment of the invention discloses a visible light optical frequency comb generation device and method based on an on-chip silicon oxide microcavity. The device comprises a pump light source, an optical fiber polarization controller, an optical fiber and an optical microcavity, wherein the optical fiber is coupled with the optical microcavity through a conical structure; the pump light source provides pump light, and the pump light is coupled into the optical microcavity through the conical structure; the wavelength and the polarization state of the pump light and the coupling state of the conical structure and the optical microcavity are adjusted to generate the optical frequency comb of the visible light wave band. The visible light optical frequency comb generation device provided by the embodiment of the invention has the following advantages: dispersion engineering is simple, abnormal dispersion can be naturally formed only by adjusting a geometric structure, complex dispersion engineering such as mode overlapping and a high-order waveguide mode does not need to be introduced, the structural sensitivity is low, the preparation process tolerance is friendly, and the device can be repeatedly realized; the silicon oxide material is transparent in a visible light window and has relatively low absorption loss, so that the preparation of a high-quality factor micro-disk cavity can be realized, and the power consumption for generating the optical frequency comb is low.

Description

Technical Field

[0001] The present invention relates to the technical field of optical frequency combs, and particularly to a visible light optical frequency comb generation device and method based on an on-chip silica microcavity. Background Art

[0002] Optical frequency combs based on on-chip optical microcavities provide a way to miniaturize traditional large optical frequency combs, and with their advantages of low power consumption, high integration, and compatibility with complementary metal-oxide-semiconductor (CMOS) processes, they have become a research hotspot in the current optical field. Microcavity optical frequency combs have been widely used in multiple fields, including optical frequency synthesis, secondary time standards, and dual-comb spectroscopy, etc.

[0003] In the near-infrared (IR) band, microcavity optical frequency combs based on magnesium fluoride, silica, and silicon nitride are being intensively studied for precision optical applications such as frequency metrology and frequency synthesis. In the mid-infrared band, silicon nitride, crystalline materials, and silicon-based Kerr microcavity optical combs and quantum cascade microcavity optical combs are being explored for applications such as molecular fingerprint recognition. In the visible light band, microcavity optical frequency combs show great application potential in fields such as quantum computing, underwater lidar, biochemical sensing, and cold atom physics. Notably, the miniaturization of optical atomic clocks, for example, in 2016, L. Maleki et al. proposed a microcavity optical frequency comb optical atomic clock based on the D1 transition line (794.8 nm) of rubidium.

[0004] Since the generation of optical frequency combs based on whispering gallery mode optical microcavities is achieved through the Kerr nonlinear effect, and the Kerr nonlinearity requires the condition of anomalous dispersion to be satisfied. However, in the visible light band, materials such as silicon nitride and magnesium fluoride have very large normal dispersion, making it impossible to generate optical frequency combs. Therefore, it is necessary to introduce dispersion engineering to control the dispersion of the microcavity, which makes the generation and application of optical frequency combs in the visible light band difficult. Summary of the Invention

[0005] Embodiments of the present invention provide a visible light optical frequency comb generation device and method based on an on-chip silica microcavity. The visible light optical frequency comb generation device uses silica material, which has smaller material dispersion than other materials in the visible light band. In addition, the thickness of the optical microcavity is adjusted to control the anomalous dispersion of the microcavity mode by reducing the thickness. Through photolithography and dry etching processes, an optical microcavity with a quality factor as high as 3.9×10 7For the silica microdisk cavity, by injecting pump light from a commercial tunable pump light source to a specific cavity mode resonance point to generate a visible light optical frequency comb, it has the following advantages: 1) The dispersion engineering is simple. Only by adjusting the geometric structure can anomalous dispersion be naturally formed without introducing complex dispersion engineering such as mode overlap and high-order waveguide modes. The structure has low sensitivity and is friendly to the preparation process tolerance, and can be realized repeatedly; 2) The silica material is transparent in the visible light window and has low absorption loss, enabling the preparation of a high-quality factor microdisk cavity and low power consumption for generating the optical frequency comb.

[0006] According to one aspect of the present invention, there is provided a visible light optical frequency comb generating device based on an on-chip silica microcavity, including a pump light source in the 780nm band, a fiber polarization controller, an optical fiber, and an optical microcavity. The output end of the pump light source is connected to the input optical fiber of the fiber polarization controller, the output optical fiber of the fiber polarization controller is connected to the first end of the optical fiber, the second end of the optical fiber extends to the optical microcavity, and the optical fiber extending to the optical microcavity includes a tapered structure, and the optical fiber is coupled to the optical microcavity through the tapered structure;

[0007] The pump light source is used to provide pump light. The pump light is transmitted to the optical fiber after the polarization state is adjusted by the fiber polarization controller, and is coupled into the optical microcavity through the tapered structure;

[0008] Wherein, the optical microcavity includes a substrate, a support column located on one side of the substrate, and a silica microdisk cavity;

[0009] Adjust the wavelength of the pump light, the polarization state of the pump light, and the coupling state of the tapered structure and the optical microcavity, so that the Kerr effect occurs in the optical microcavity to generate an optical frequency comb in the visible light band with a repetition frequency below 100GHz.

[0010] Optionally, the thickness of the silica microdisk cavity is greater than or equal to 1.1μm and less than or equal to 1.5μm, and the diameter of the silica microdisk cavity is greater than or equal to 660μm.

[0011] Optionally, the support column is formed by etching, and the etching depth of the support column is greater than or equal to 8μm and less than or equal to 20μm.

[0012] Optionally, it further includes a first coupler, a photodetector, an oscilloscope, and a spectrometer;

[0013] The second end of the optical fiber is connected to the input end of the first coupler. The first output end of the first coupler is connected to the photodetector, the photodetector is connected to the oscilloscope, and the second output end of the first coupler is connected to the spectrometer;

[0014] The oscilloscope is used to output the time-domain waveform detected by the photodetector, and the spectrometer is used to measure the output spectrum of the second output end of the first coupler.

[0015] Optionally, an optical amplifier is further included on the optical path between the pump light source and the fiber polarization controller, and the optical amplifier is used to amplify the pump light.

[0016] Optionally, the optical amplifier includes a semiconductor optical amplifier.

[0017] Optionally, a second coupler and a power meter are further included;

[0018] The input end of the second coupler is connected to the output optical fiber of the fiber polarization controller, the first output end of the second coupler is connected to the power meter, and the second output end of the second coupler is connected to the first end of the optical fiber.

[0019] Optionally, an attenuator and / or an isolator are further included on the optical path between the pump light source and the optical microcavity.

[0020] Optionally, the pump light source includes a wavelength-tunable semiconductor laser, and the materials of the substrate and the support posts include silicon.

[0021] According to another aspect of the present invention, a method for generating a visible light optical frequency comb based on an on-chip silicon oxide microcavity is provided, which is executed by the above-mentioned device for generating a visible light optical frequency comb based on an on-chip silicon oxide microcavity. The method for generating a visible light optical frequency comb includes:

[0022] The pump light source outputs pump light, and the pump light is coupled into the optical microcavity through a tapered structure;

[0023] Adjust the wavelength, polarization state of the pump light, and the coupling strength between the tapered structure and the optical microcavity to make the mode in a just-coupled state;

[0024] Set the wavelength of the pump light at a blue detuning away from the pump mode, and gradually increase the wavelength of the pump light to make the intracavity resonance of the pump light entering the optical microcavity generate the Kerr effect, and generate an optical frequency comb in the visible light band with a repetition frequency below 100 GHz;

[0025] Wherein, in the just-coupled state, the inherent loss rate of the optical microcavity is equal to the external coupling rate, and in the blue detuning, the frequency of the pump light is greater than the frequency of the pump mode, and the pump mode is the frequency of the pump light when generating the optical frequency comb.

[0026] The visible light optical frequency comb generation device based on an on-chip silicon oxide microcavity provided by an embodiment of the present invention includes a pump light source in the 780 nm band, an optical fiber polarization controller, an optical fiber, and an optical microcavity. The output end of the pump light source is connected to the input optical fiber of the optical fiber polarization controller. The output optical fiber of the optical fiber polarization controller is connected to the first end of the optical fiber. The second end of the optical fiber extends to the optical microcavity. The optical fiber extending to the optical microcavity includes a tapered structure, and the optical fiber is coupled to the optical microcavity through the tapered structure. Wherein, the optical microcavity includes a substrate, a support pillar, and a silicon oxide microdisk cavity located on one side of the substrate. The pump light source provides pump light. After the polarization state of the pump light is adjusted by the optical fiber polarization controller, it is transmitted to the optical fiber and coupled into the optical microcavity through the tapered structure. By adjusting the wavelength of the pump light, the polarization state of the pump light, and the coupling state between the tapered structure and the optical microcavity, the optical microcavity is exactly in the anomalous dispersion region for the band of the pump light and has a strong Kerr effect. Under the action of the pump light, four-wave mixing occurs in the optical microcavity, absorbing two photons from the pump light and generating a pair of photons with frequencies symmetric about the pump light, namely the signal light with an increased frequency and the idler light with a decreased frequency. When the resonance mode of the optical microcavity exactly satisfies the phase matching and energy conservation conditions, that is, when the frequencies of the signal light and the idler light are consistent with the resonance mode of the optical microcavity, the optical parametric oscillation will be enhanced, and the pump light power will be transferred to the resonance mode that satisfies the phase matching conditions, thereby generating a pair of sidebands symmetric about the pump light frequency. When the power of the pump light is further increased, the already generated signal light and idler light sidebands will serve as the pump light and continue to perform optical parametric oscillation, occurring degenerate four-wave mixing and cascaded four-wave mixing, generating multiple new sidebands with equal intervals in the frequency domain, thereby generating an optical frequency comb in the visible light band with a repetition frequency below 100 GHz. Moreover, the embodiment of the present invention utilizes an on-chip integrated optical microcavity, which is beneficial to the development of miniaturized and integrated devices.

[0027] It should be understood that the content described in this part is not intended to identify the key or important features of the embodiments of the present invention, nor is it used to limit the scope of the present invention. Other features of the present invention will become easily understood through the following description. Brief Description of the Drawings

[0028] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0029] Figure 1 It is a schematic structural diagram of a visible light optical frequency comb generation device based on an on-chip silicon oxide microcavity provided by an embodiment of the present invention;

[0030] Figures 2 to 4 Schematic diagram of the microscopic structure of an optical microcavity provided by an embodiment of the present invention;

[0031] Figure 5 Dispersion curve diagrams of several materials provided by an embodiment of the present invention;

[0032] Figure 6 Dispersion curve diagram of the TM00 mode of a silica microdisk cavity varying with thickness provided by an embodiment of the present invention;

[0033] Figure 7 Microscopic schematic diagrams of optical microcavities with different etching depths provided by an embodiment of the present invention;

[0034] Figures 8 to 10 Spectral schematic diagrams of an optical frequency comb provided by an embodiment of the present invention;

[0035] Figure 11 Flow schematic diagram of a method for generating a visible light frequency comb based on an on-chip silica microcavity provided by an embodiment of the present invention. Detailed implementation manners

[0036] In order to enable those skilled in the art to better understand the solution of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts shall fall within the protection scope of the present invention.

[0037] It should be noted that the terms "first", "second", etc. in the specification and claims of the present invention and the above-mentioned drawings are used to distinguish similar objects, and do not necessarily need to describe a specific order or sequence. It should be understood that such data can be interchanged under appropriate circumstances so that the embodiments of the present invention described here can be implemented in an order other than those illustrated or described here. In addition, the terms "comprising" and "having" and any variations thereof are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or device that includes a series of steps or units does not necessarily have to be limited to those steps or units clearly listed, but may include other steps or units not clearly listed or inherent to these processes, methods, products, or devices.

[0038] In 2011, the research group of L. Maleki proposed that by appropriately designing the shape of the microcavity to control the group velocity dispersion, a comb-like frequency centered at almost any frequency within the transparent window of the main material of the microresonator can be generated, and the first frequency comb centered at a wavelength of 794 nm was demonstrated through a calcium fluoride microcavity.

[0039] In 2016, the research group of A. M. Weiner demonstrated that the interaction between the fundamental wave and the second harmonic can provide a completely new phase-matching method for four-wave mixing in an optical resonator, thereby generating an optical frequency comb under normal dispersion mechanism under conditions where the generation of comb-shaped waves is usually prohibited. The research results provide a new method to overcome the dispersion limitations of simultaneously forming Kerr combs and second harmonics, making it possible to generate optical frequency combs in the near-visible to visible range, because large normal material dispersion may dominate in this range.

[0040] In 2017, the research group of K. Vahala reduced the pump wavelength of the silica microcavity to the edge of the visible light wavelength, and controlled the mode dispersion through geometric dispersion and mode overlap. At the same time, the microcavity quality factor of 8×10 7 enabled parametric oscillation to be achieved at low pump power, generating a mode-locked Kerr soliton optical frequency comb with a repetition frequency of 20 GHz, which was also the soliton optical frequency comb with the shortest wavelength at that time.

[0041] In 2020, the research group of A. L. Gaeta demonstrated a mode-locked Kerr soliton crystal optical frequency comb with a spectral range from 700 nm to 850 nm in a silicon nitride microresonator by using dispersion engineering of high-order waveguide modes in the visible and near-visible bands to overcome the normal dispersion of materials and allowing small anomalous group velocity dispersion and phase matching of the four-wave mixing process in these bands, and it can be used as a narrowband light source aligned with the rubidium atomic transition line.

[0042] In the above methods, calcium fluoride used in the first method as the cavity material cannot achieve on-chip integration. The second method requires the introduction of a second-order nonlinear process, the mode matching process is complex, and the second-order nonlinear conversion efficiency is not high. The third method generates anomalous dispersion through high-order mode coupling, which has high design requirements and is sensitive to the preparation process accuracy. The fourth method requires the participation of high-order waveguide modes, the cross-sectional structure of the prepared microcavity is large, and the quality factor is only 7.7×10 5 , and a large pump power is required.

[0043] Based on this, the embodiments of the present invention provide a visible light optical frequency comb generating device based on an on-chip silica microcavity. The visible light optical frequency comb generating device overcomes the above problems by precisely controlling the geometric structure of the sample, especially the control of the sample thickness, in the silica microdisk cavity, prepares a silica microdisk cavity with a higher quality factor, has good repeatability, and obtains a visible light optical frequency comb with a repetition frequency that can be detected and a spectral range that can cover the D1 and D2 transition lines (794.8 nm, 780.0 nm) of the rubidium cell at a lower power. Through photolithography and dry etching processes, a quality factor as high as 3.9×10 7The silica microdisk cavity generates a visible light optical frequency comb by injecting pump light from a commercial tunable pump light source to a specific cavity mode resonance point, and has the following advantages: 1) The dispersion engineering is simple. Only by adjusting the geometric structure can anomalous dispersion be naturally formed without introducing complex dispersion engineering such as mode overlap and high-order waveguide modes. The structure has low sensitivity and is friendly to the preparation process tolerance, and can be repeatedly realized; 2) The silica material is transparent in the visible light window and has low absorption loss, enabling the preparation of a high-quality factor microdisk cavity and consuming low power for generating the optical frequency comb.

[0044] Figure 1 FIG. is a schematic structural diagram of a visible light optical frequency comb generating device based on an on-chip silica microcavity provided by an embodiment of the present invention. Refer to Figure 1 , the visible light optical frequency comb generating device includes a pump light source 10 in the 780 nm band, a fiber polarization controller 20, a fiber 30, and an optical microcavity 40. The output end of the pump light source 10 is connected to the input fiber of the fiber polarization controller 20. The output fiber of the fiber polarization controller 20 is connected to the first end of the fiber 30. The second end of the fiber 30 extends to the optical microcavity 40. The fiber 30 extending to the optical microcavity 40 includes a tapered structure ( Figure 1 not shown in FIG.), and the fiber 30 is coupled to the optical microcavity 40 through the tapered structure; the pump light source 10 is used to provide pump light. The pump light is transmitted to the fiber 30 after the polarization state is adjusted by the fiber polarization controller 20 and is coupled into the optical microcavity 40 through the tapered structure; wherein, the optical microcavity 40 includes a substrate, a support column and a silica microdisk cavity located on one side of the substrate; by adjusting the wavelength of the pump light, the polarization state of the pump light, and the coupling state between the tapered structure and the optical microcavity 40, the Kerr effect occurs in the optical microcavity 40 to generate an optical frequency comb in the visible light band with a repetition frequency below 100 GHz.

[0045] Among them, the pump light source 10 can output continuously adjustable pump light in the 780 nm band. Optionally, the pump light source 10 may include a wavelength-tunable semiconductor laser, and the embodiment of the present invention does not limit this. The fiber polarization controller 20 can adopt a three-ring type or an embedded polarization controller. By adjusting the state of the fiber polarization controller 20, the coupling efficiency between the pump light and the optical microcavity 40 can be adjusted. The pump light transmitted in the fiber 30 generates an evanescent field in the tapered structure to realize coupling with the optical microcavity 40. The tapered structure can be obtained by fiber fusion tapering. The fiber taper coupling method has the advantages of high coupling efficiency, strong controllability, and convenient adjustment.

[0046] The optical microcavity 40 can be a whispering-gallery-mode optical microcavity, which is an important micro-nano photon device and has wide applications in low-threshold lasers, cavity optomechanics, biosensing, etc. The optical microcavity can be used to generate a new type of optical frequency comb based on the Kerr effect - the microcavity optical frequency comb, thereby making up for the shortcomings of traditional optical frequency combs. In recent years, a series of progress has been made in the practical applications of microcavity optical frequency combs, and it has been experimentally proven that they can be applied to multiple fields including optical communication, optical clocks, detection of exoplanets, lidar ranging, frequency synthesizers, arbitrary waveform generation, optical coherence tomography, etc.

[0047] Optionally, the materials of the substrate and the support posts of the optical microcavity include silicon. The optical microcavity includes a substrate, support posts located on one side of the substrate, and a silica microdisk cavity. In this embodiment, the silica microdisk cavity is frustum-shaped, and the generatrix of the frustum can be 35° with the bottom surface of the frustum. In order to make the optical microcavity in the anomalous dispersion region in the pump light band, in the embodiment of the present invention, by adjusting the size (thickness and diameter) of the silica microdisk cavity, the dispersion and transmission mode of the optical microcavity can be controlled, so as to realize optical frequency combs with different bands and different frequency intervals (from several GHz to hundreds of GHz). Optionally, the thickness of the silica microdisk cavity is greater than or equal to 1.1 μm and less than or equal to 1.5 μm, and the diameter of the silica microdisk cavity is greater than or equal to 660 μm.

[0048] Exemplarily, Figures 2 to 4 are respectively the schematic microstructural diagrams of an optical microcavity provided by the embodiment of the present invention, wherein the thickness of the silica microdisk cavity is 1.3 μm, Figure 2 the diameter of the silica microdisk cavity in [diagram] is 1000 μm, Figure 3 the diameter of the silica microdisk cavity in [diagram] is 2000 μm, Figure 4 the diameter of the silica microdisk cavity in [diagram] is 3000 μm.

[0049] Figure 5 are the dispersion curve diagrams of several materials provided by the embodiment of the present invention, Figure 6 is the dispersion curve diagram of the TM00 mode of the silica microdisk cavity provided by the embodiment of the present invention changing with thickness. Referring to Figure 5 , compared with several other common microcavity materials, silica shows smaller material dispersion characteristics. This characteristic makes silica have significant advantages in microcavity design, especially in application scenarios that require anomalous dispersion. From Figure 6In the simulation results, the variation trend of the modal dispersion curve with the sample thickness can be further observed: as the sample thickness decreases, it is easier for the mode to achieve anomalous dispersion. However, as the sample thickness decreases, the stress problem generated in the sample during the etching process becomes more significant, which may affect the structural stability and optical performance of the microcavity. Based on these considerations, the embodiments of the present invention finally selected a thickness of 1.3 ± 0.2 μm as the design parameter of the silica microdisk cavity. This thickness achieves a good balance between achieving anomalous dispersion and avoiding etching stress. The silica microdisk cavity prepared in the embodiments of the present invention is realized based on a silica wafer through photolithography and etching processes. The thickness accuracy of the commercially available silica wafer used reaches the nanometer level, fully meeting the requirements of the experimental design. In addition, the diameter of the microdisk cavity is precisely controlled through the photolithography process, and the inclination angle is achieved through inductively coupled plasma (ICP) dry etching technology, ensuring the high precision and consistency of the overall structure.

[0050] Optionally, the support pillars are formed by etching, and the etching depth of the support pillars is greater than or equal to 8 μm and less than or equal to 20 μm.

[0051] Among them, the silica wafer used in the embodiments of the present invention is prepared by a thermal growth process, and silica is grown at a high temperature close to 1000 °C. When the temperature cools to room temperature, due to the significant difference in the thermal expansion coefficients of silica and silicon, well-known compressive stress will be generated in the oxide layer. This stress may cause damage to the microdisk cavity structure in extreme cases and also have a significant impact on the quality factor of the microdisk cavity. In a certain embodiment of the present invention, the designed microdisk cavity has a relatively large diameter (3200 μm) and a relatively thin thickness (1.3 μm), and these structural characteristics make it more susceptible to stress. Therefore, when etching the silicon substrate with xenon difluoride ( ), special attention needs to be paid to controlling the etching rate and precisely adjusting the etching depth to minimize the impact of stress on the microdisk cavity structure. Figure 7 It is a microscopic schematic diagram of the optical microcavities with different etching depths provided by the embodiments of the present invention. Among them, the etching depth of Figure a is 20.73 μm, the etching depth of Figure b is 25.50 μm, and the etching depth of Figure c is 43.87 μm. Refer to Figure 7, through multiple experiments, it is found that when the etching depth exceeds 20 μm, obvious stress distribution begins to appear in the microdisk cavity. In particular, the microdisk cavity with an etching depth of 43.87 μm is damaged after being placed for a period of time. This indicates that excessive etching will lead to stress concentration, thus affecting the structural stability of the microdisk cavity. On the other hand, when the etching depth is too small, due to insufficient etching, annular broken silicon will remain at the bottom edge of the microdisk cavity, which will significantly reduce the quality factor of the microdisk cavity. The mode field distribution of the TM00 mode designed in the embodiment of the present invention is relatively close to the edge of the microdisk cavity. Therefore, the etching depth has an important impact on the optical characteristics of the mode. In order to further study the influence of the etching depth on the quality factor of the TM00 mode, the embodiment of the present invention systematically experiments on the change of the quality factor of the TM00 mode under different etching depths. The experimental results show that when the etching depth is about 8 μm, the TM00 mode can be observed for the first time, but the quality factor at this time is relatively low. As the etching depth increases, the quality factor of the mode gradually increases. When the etching depth reaches 16 μm - 20 μm, the quality factor of the TM00 mode tends to be stable and no longer increases significantly with the increase of the etching depth. Based on the above experimental results, the embodiment of the present invention sets the target etching depth at 16 μm - 20 μm, preferably 18 μm.

[0052] In the technical solution of the embodiment of the present invention, pump light is provided by a pump light source. After the polarization state of the pump light is adjusted by a fiber polarization controller, it is transmitted to an optical fiber and coupled into an optical microcavity through a tapered structure; by adjusting the wavelength of the pump light, the polarization state of the pump light, and the coupling state between the tapered structure and the optical microcavity, the optical microcavity is exactly in the anomalous dispersion region for the wavelength band of the pump light and has a strong Kerr effect. Under the action of the pump light, four-wave mixing will occur in the optical microcavity, absorbing two photons from the pump light and generating a pair of photons with frequencies symmetric about the pump light, namely the signal light with an increased frequency and the idler light with a decreased frequency; when the resonance mode of the optical microcavity exactly satisfies the phase matching and energy conservation conditions, that is, when the frequencies of the signal light and the idler light are consistent with the resonance mode of the optical microcavity, the optical parametric oscillation will be enhanced, and the pump light power will be transferred to the resonance mode that satisfies the phase matching condition, thereby generating a pair of sidebands symmetric about the pump light frequency. When the power of the pump light is further increased, the already generated signal light and idler light sidebands will serve as pump light and continue to perform optical parametric oscillation, resulting in degenerate four-wave mixing and cascaded four-wave mixing, generating multiple new sidebands with equal intervals in the frequency domain, thereby generating an optical frequency comb in the visible light band with a repetition frequency below 100 GHz. Moreover, the embodiment of the present invention utilizes an on-chip integrated optical microcavity, which is beneficial to the development of miniaturized and integrated devices.

[0053] Continue to refer to Figure 1, Optionally, the visible light optical frequency comb generating device further includes a first coupler 50, a photodetector 51, an oscilloscope 52, and a spectrometer 53; the second end of the optical fiber 30 is connected to the input end of the first coupler 50, the first output end of the first coupler 50 is connected to the photodetector 51, the photodetector 51 is connected to the oscilloscope 52, and the second output end of the first coupler 50 is connected to the spectrometer 53; the oscilloscope 52 is used to output the time-domain waveform detected by the photodetector 51, and the spectrometer 53 is used to measure the output spectrum of the second output end of the first coupler 50.

[0054] It can be understood that in order to verify whether the visible light optical frequency comb generating device provided by the embodiments of the present invention generates an optical frequency comb, tests need to be carried out. By observing the time-domain waveform of the oscilloscope 52 and the spectrum measured by the spectrometer 53, it can be determined whether an optical frequency comb is generated. In implementation, the first coupler 50 can be selected as an optical fiber coupler with a splitting ratio of 50:50 between the first output end and the second output end.

[0055] Continue to refer to Figure 1 , Optionally, the visible light optical frequency comb generating device further includes an optical amplifier 60 disposed on the optical path between the pump light source 10 and the optical fiber polarization controller 20, and the optical amplifier 60 is used to amplify the pump light.

[0056] In specific implementation, the power of the pump light output by the pump light source 10 may be small and unable to reach the threshold power for generating an optical frequency comb. Therefore, an optical amplifier 60 can be disposed on the optical path between the wavelength tunable light source 10 and the optical fiber polarization controller 20 to amplify the power of the pump light to above the threshold power for exciting the optical frequency comb. Optionally, the optical amplifier 60 includes a semiconductor optical amplifier SOA. In other embodiments, the optical amplifier 60 can also select other types of amplifiers, which can be designed according to actual situations in specific implementation.

[0057] Continue to refer to Figure 1 , Optionally, the visible light optical frequency comb generating device further includes a second coupler 70 and a power meter 71; the input end of the second coupler 70 is connected to the output optical fiber of the optical fiber polarization controller 20, the first output end of the second coupler 70 is connected to the power meter 71, and the second output end of the second coupler 70 is connected to the first end of the optical fiber 30.

[0058] Among them, the second coupler 70 has a preset splitting ratio (for example, the splitting ratio between the first output end and the second output end is 1:99). By setting the power meter 71, the optical power of the pump light can be monitored in real time. Combining with the oscilloscope and the spectrometer, the threshold power for generating an optical frequency comb can also be measured.

[0059] In other embodiments, optionally, the visible light optical frequency comb generating device may further include an attenuator and / or an isolator disposed on the optical path between the pump light source and the optical microcavity. The attenuator is used to adjust the attenuation power of the pump light, and the isolator ensures the unidirectional transmission of the pump light, avoiding the influence of the reverse beam on the stability of the pump light. Its specific installation position can be designed according to the actual situation.

[0060] Exemplarily, Figure 1 it is shown in that the visible light optical frequency comb generating device further includes an attenuator 61 and an isolator 62. For the visible light optical frequency comb generating device provided in this embodiment, the pump light source 10 outputs pump light in the 780 nm band, and a visible light optical frequency comb covering the rubidium cell D1D2 transition lines (794.8 nm, 780.0 nm) can be realized. After the pump light passes through the optical amplifier 60, the attenuator 61, and the fiber polarization controller 20, a part of the pump light is received by the power meter 71 from the first output end of the second coupler 70 for detecting the power of the pump light, and another part of the pump light is output from the second output end of the second coupler 70, enters the first port 1 of the optical microcavity 40 after passing through the isolator 62, and the pump light is continuously injected into the optical microcavity 40. An optical frequency comb is generated in the optical microcavity 40 through the degenerate four-wave mixing and cascaded four-wave mixing effects. The generated optical frequency comb is output through the second port 2 of the optical microcavity. After passing through the first coupler 50, a part of the light enters the photodetector 51 from the first output end of the first coupler 50, and the light intensity signal is converted into a voltage signal. The photodetector 51 is connected to the oscilloscope 52 through a cable, and the voltage signal is displayed on the oscilloscope 52. Another part of the light enters the spectrometer 53 from the second output end of the first coupler 50. In Figure 1 it, the solid line represents the optical path connected by single-mode fiber, and the dashed line represents the circuit connected by cable.

[0061] Figures 8 to 10 are respectively the spectral schematic diagrams of an optical frequency comb provided by the embodiments of the present invention, where Figure 8 is the spectral diagram of a silica microdisk cavity with a diameter D = 1000 μm and a repetition frequency of 66 GHz, Figure 9 is the spectral diagram of a silica microdisk cavity with a diameter D = 2000 μm and a repetition frequency of 33 GHz, Figure 10 is the spectral diagram of a silica microdisk cavity with a diameter D = 3200 μm and a repetition frequency of 20 GHz. Compared with the repetition frequency of the optical frequency comb generated by the prior art, which is at least 800 GHz, the repetition frequency of the optical frequency comb generated by the embodiments of the present invention can be directly detected by the photodetector, which is beneficial to reducing the cost during measurement.

[0062] Figure 11Schematic flowchart of a method for generating a visible light optical frequency comb based on an on-chip silica microcavity provided by an embodiment of the present invention. This method for generating a visible light optical frequency comb is executed by any one of the on-chip silica microcavity-based visible light optical frequency comb generating devices provided by the above embodiments. Refer to Figure 11 , the method for generating a visible light optical frequency comb includes:

[0063] S110. The pump light source outputs pump light, and the pump light is coupled into the optical microcavity through a tapered structure.

[0064] Among them, the pump light source can be a semiconductor laser in the 780nm band. In the embodiment of the present invention, a triangular wave piezoelectric signal can be input into the semiconductor laser as a scanning trigger signal. This signal is used to control the frequency scanning range of the semiconductor laser. Then, the silica microcavity is placed on a three-dimensional adjustable fine displacement stage. By precisely adjusting the relative position between the microcavity and the tapered structure, the evanescent field coupling between the two reaches the best state, so as to ensure that the pump light can effectively enter the optical microcavity.

[0065] S120. Adjust the wavelength, polarization state of the pump light, and the coupling strength between the tapered structure and the optical microcavity to make the mode in the critical coupling state.

[0066] In specific implementation, within a free spectral range, find the TM00 mode designed in the embodiment of the present invention. By adjusting the wavelength, polarization state of the pump light, and the coupling strength between the tapered structure and the optical microcavity, make the mode in the critical coupling state. Among them, in the critical coupling state, the intrinsic loss rate of the optical microcavity is equal to the external coupling rate. Specifically, microcavity critical coupling refers to a coupling state in which the coupling between the optical microcavity and the external waveguide or free space reaches the best matching state, so that the incident light energy can be maximally absorbed or utilized by the optical microcavity. In the critical coupling state: the intrinsic loss rate (γ 0 ) of the microcavity is equal to the external coupling rate (γ e ), at this time the incident light energy is completely absorbed by the microcavity, and the reflected or transmitted signal can theoretically be reduced to zero, and the system reaches the maximum energy conversion efficiency.

[0067] S130. Set the wavelength of the pump light at a blue detuning away from the pump mode, and gradually increase the wavelength of the pump light to make the intracavity resonance of the pump light entering the optical microcavity generate the Kerr effect, and generate an optical frequency comb in the visible light band with a repetition frequency below 100GHz.

[0068] Among them, at blue detuning, the frequency of the pump light is greater than the frequency of the pump mode, and the pump mode is the frequency of the pump light when generating the optical frequency comb.

[0069] In specific implementation, after adjusting to the exact coupling state, gradually increase the power of the pump light. Subsequently, turn off the triangular wave signal to stop the frequency scanning of the semiconductor laser. Set the wavelength of the laser at a blue detuning away from the pump mode. Specifically, the frequency of the pump light can be set to be more than 20 MHz greater than the frequency of the pump mode. Then gradually increase the wavelength of the semiconductor laser (wavelength redshift) to make the pump light resonate inside the cavity mode, thereby generating a visible Kerr optical comb. Continuously adjust the wavelength until the spectrum observed on the spectrometer tends to be stable and no longer undergoes significant evolution. Refer to Figures 8 to 10 , which shows visible optical frequency combs with three different repetition frequencies and a pump power of approximately 60 mW. The stable generation of these optical combs verifies the effectiveness of the experimental design and operation steps of the embodiments of the present invention.

[0070] The above specific implementation manners do not constitute a limitation to the protection scope of the present invention. Those skilled in the art should understand that various modifications, combinations, sub - combinations, and substitutions can be made according to design requirements and other factors. Any modifications, equivalent substitutions, and improvements made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A visible light frequency comb generator based on an on-chip silicon oxide microcavity, characterized in that: The invention comprises a pump light source of 780 nm band, a fiber polarization controller, an optical fiber and an optical microcavity, wherein the output end of the pump light source is connected to the input optical fiber of the fiber polarization controller, the output optical fiber of the fiber polarization controller is connected to the first end of the optical fiber, the second end of the optical fiber extends to the optical microcavity, the optical fiber extending to the optical microcavity comprises a tapered structure, and the optical fiber is coupled to the optical microcavity through the tapered structure; The pump light source is used to provide pump light, and the pump light is transmitted to the optical fiber after the polarization state is adjusted by the optical fiber polarization controller, and is coupled into the optical microcavity through the tapered structure; Wherein, the optical microcavity comprises a substrate and a support column and a silicon oxide microdisk cavity located on one side of the substrate; The wavelength of the pump light, the polarization state of the pump light, and the coupling state of the tapered structure and the optical microcavity are adjusted to cause a Kerr effect in the optical microcavity to generate an optical frequency comb in the visible light band with a repetition frequency below 100 GHz.

2. The visible light frequency comb generator based on on-chip silicon oxide microcavity according to claim 1, characterized in that: The thickness of the silicon oxide micro-disk cavity is greater than or equal to 1.1 μm and less than or equal to 1.5 μm, and the diameter of the silicon oxide micro-disk cavity is greater than or equal to 660 μm.

3. The visible light frequency comb generator based on on-chip silicon oxide microcavity according to claim 2, characterized in that: The support column is formed by etching, and the etching depth of the support column is greater than or equal to 8 μm and less than or equal to 20 μm.

4. The visible light frequency comb generator based on on-chip silicon oxide microcavity according to claim 1, characterized in that: Also includes a first coupler, a photodetector, an oscilloscope and a spectrometer; The second end of the optical fiber is connected to the input end of the first coupler, the first output end of the first coupler is connected to the photodetector, the photodetector is connected to the oscilloscope, and the second output end of the first coupler is connected to the spectrometer; The oscilloscope is used to output the time domain waveform detected by the photodetector, and the spectrometer is used to measure the output spectrum of the second output end of the first coupler.

5. The visible light frequency comb generator based on on-chip silicon oxide microcavity according to claim 1, characterized in that: It also includes an optical amplifier arranged on the optical path between the pump light source and the optical fiber polarization controller, and the optical amplifier is used to amplify the pump light.

6. The visible light frequency comb generator based on on-chip silicon oxide microcavity according to claim 5, characterized in that: The optical amplifier comprises a semiconductor optical amplifier.

7. The visible light frequency comb generator based on on-chip silicon oxide microcavity according to claim 1, characterized in that: Also included is a second coupler and a power meter; The input end of the second coupler is connected to the output optical fiber of the optical fiber polarization controller, the first output end of the second coupler is connected to the power meter, and the second output end of the second coupler is connected to the first end of the optical fiber.

8. The visible light frequency comb generator based on on-chip silicon oxide microcavity according to claim 1, characterized in that: It also includes an attenuator and / or an isolator arranged on the optical path between the pump light source and the optical microcavity.

9. The visible light frequency comb generator based on on-chip silicon oxide microcavity according to claim 1, characterized in that: The pump light source comprises a wavelength-tunable semiconductor laser, and the material of the substrate and the support column comprises silicon.

10. A method for generating a visible light frequency comb based on an on-chip silicon oxide microcavity, characterized in that: The method is performed by the visible light frequency comb generation device based on an on-chip silicon oxide microcavity according to any one of claims 1 to 9, wherein the visible light frequency comb generation method comprises: The pump light source outputs pump light, and the pump light is coupled into the optical microcavity through the cone structure; Adjusting the wavelength and polarization state of the pump light and the coupling strength between the tapered structure and the optical microcavity to put the mode in a just-coupled state; The wavelength of the pump light is set at a blue detuned position far from the pump mode, and the wavelength of the pump light is gradually increased, so that the pump light enters the cavity mode of the optical microcavity and resonates to produce a Kerr effect, thereby generating an optical frequency comb in the visible light band with a repetition frequency below 100 GHz; Wherein, in the just coupled state, the intrinsic loss rate of the optical microcavity is equal to the external coupling rate, and in the blue detuned state, the frequency of the pump light is greater than the frequency of the pump mode, and the pump mode is the pump light frequency when the optical frequency comb is generated.

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