Stable optical clock system based on double microcavity optical comb and two-photon transition and implementation method

Abstract

This invention discloses a stabilized optical clock system and its implementation method based on a dual-microcavity optical comb and two-photon transitions. The system includes a microcavity optical comb section, a locking section, and a photodetector. The microcavity optical comb section contains two SiO2 microchip ring cavity optical comb oscillators, with center wavelengths matched to the two-photon transition lines of rubidium and cesium atoms, respectively. The locking section provides an atomic reference frequency through a two-photon transition generator, and combines a six-way phase-locked loop, an acousto-optic modulator, and a phase modulator to achieve initial frequency locking of the optical comb, interlocking of the repetition frequency, and suppression of thermally induced detuning. The photodetector converts the stabilized optical comb repetition frequency into an RF frequency standard output. This invention solves the problems of complex system design, large integrated size, cumbersome locking structure, and wavelength mismatch in existing ultra-high frequency stabilized atomic clock technologies.

Description

Technical Field

[0001] This invention relates to the field of atomic clock technology, specifically to a stable optical clock system and its implementation method based on a dual-microcavity optical comb and two-photon transition. Background Technology

[0002] Time and frequency standards, as a fundamental support in modern science and technology, are widely used in satellite navigation, quantum communication, precision measurement, and other scenarios. Their frequency stability directly determines the performance ceiling of related systems. Atomic clocks, with their ultra-high stability of atomic energy level transitions, have become the mainstream high-precision time and frequency references. Among them, miniaturized atomic clocks have become a research hotspot in the field of time and frequency technology due to their suitability for portable and integrated applications.

[0003] The frequency stability of miniaturized atomic clocks has now reached 10. -14 While achieving significant speeds, on-chip frequency stabilization and division remain bottlenecks. Optical frequency combs, acting as a bridge between optical and radio frequencies, can transfer the ultra-high stability of optical frequencies to the radio frequency band, providing an effective solution for high-precision frequency division. Microcavity optical combs, with their high quality factor (Q-value), small mode size, and on-chip integration potential, are more suitable for the development needs of miniaturized optical clocks compared to traditional solid-state laser and fiber laser optical combs, becoming a core component for realizing the miniaturization of optical clock systems.

[0004] Currently, optical clock technology solutions based on microcavity optical combs have emerged, such as the solution published by the NIST (National Institute of Standards and Technology) team in 2019. The core idea is to lock the microcavity optical comb onto a high-stability atomic frequency standard, achieving frequency stability through the synergistic effect between the combs. However, this solution has significant technical drawbacks. Using a 1550nm band optical comb, its center wavelength does not match the two-photon transition wavelength of rubidium and cesium atoms, requiring wavelength conversion via a frequency doubling crystal to achieve locking. This not only introduces additional noise and energy loss but also increases the system size. Furthermore, this solution uses an f-2f locking method to stabilize the initial frequency (fceo) of the optical comb, requiring an additional nonlinear crystal, resulting in a cumbersome locking structure. This further reduces system stability and integration, making true miniaturization difficult.

[0005] In China, existing research is mostly focused on theoretical simulation and preliminary experimental verification. A complete design scheme for a microcavity optical comb frequency standard system has not yet been formed, and there is a lack of mature experimental verification data to support it. There are obvious deficiencies in the development of high-stability optical clock systems based on dual microcavity optical combs and dual-photon transitions. Summary of the Invention

[0006] To address this, the present invention provides a stable optical clock system and implementation method based on dual microcavity optical combs and two-photon transitions, solving the problems of complex system design, large integrated volume, cumbersome locking structure, and wavelength mismatch in existing ultra-high frequency stable atomic clock technologies.

[0007] To achieve the above objectives, the present invention provides the following technical solution: a stable optical clock system based on dual microcavity optical combs and two-photon transitions, comprising a microcavity optical comb section for generating optical comb signals matching atomic transition wavelengths, a locking section for achieving stable locking of the optical comb frequency, and a photodetector for converting the stable optical comb signal into a standard radio frequency output.

[0008] The microcavity optical comb section includes two optical comb oscillators using SiO2 micro-core ring cavities, as well as a pump laser and an amplifier that provide pump light for the optical comb oscillators. The center wavelengths of the two optical comb oscillators correspond to the cesium 6S-6D two-photon transition line and the rubidium 5S-5D two-photon transition line, respectively.

[0009] The locking part includes a rubidium two-photon transition generator and a cesium two-photon transition generator that provide atomic reference frequencies, and a phase-locked loop that locks the optical comb frequency through error signal feedback. The phase-locked loop is associated with two optical comb oscillators and the corresponding two-photon transition generators respectively, so as to simultaneously perform frequency interlocking between the two optical combs.

[0010] The photodetector receives the microcavity optical comb repetition frequency signal after it has been partially stabilized by locking, and converts the stabilized microcavity optical comb repetition frequency signal into an RF frequency standard signal for output.

[0011] As a preferred solution for a stable clock system based on dual microcavity optical comb and two-photon transition, the optical comb oscillator corresponding to the cesium 6S-6D two-photon transition line has a center wavelength of 885nm, and the matching pump laser outputs 894nm CW pump light.

[0012] The optical comb oscillator corresponding to the rubidium 5S-5D two-photon transition line has a center wavelength of 778nm, and the matching pump laser outputs 780nm CW pump light.

[0013] As a preferred embodiment of a stable clock system based on dual microcavity optical combs and two-photon transitions, the rubidium two-photon transition generating device includes a 778nm DBR laser and a first photomultiplier tube. The first photomultiplier tube detects the fluorescence of the rubidium atom two-photon transition and extracts a first error signal. The first error signal is used to modulate the current of the DBR laser so that the frequency of the DBR laser is stabilized at the rubidium two-photon transition peak.

[0014] As a preferred embodiment of a stable optical clock system based on dual-microcavity optical combs and two-photon transitions, the cesium two-photon transition generating device includes an 894nm ECDL laser and a second photomultiplier tube. The second photomultiplier tube detects the fluorescence of the two-photon transition of cesium atoms and extracts a second error signal. The second error signal is superimposed on the current modulation signal of the ECDL laser, so that the frequency of the 885nm laser generated by the ECDL laser is stabilized at the cesium two-photon transition peak.

[0015] As a preferred embodiment of the optical clock system based on dual microcavity optical combs and two-photon transitions, the locking part further includes an acousto-optic modulator and a phase modulator. The acousto-optic modulator receives the beat frequency error signal between the microcavity optical comb and the corresponding locked laser, and adjusts the pump light frequency according to the beat frequency error signal to lock the initial frequency fceo of the microcavity optical comb.

[0016] The phase modulator, in conjunction with the Pound-Drever-Hall locking method and the single-sideband suppressed carrier SSB-SC modulation mode, adjusts the pump frequency to suppress frequency detuning of the SiO2 micro-core ring cavity.

[0017] As a preferred solution for a stable optical clock system based on dual microcavity optical combs and two-photon transitions, the two optical comb oscillators lock the repetition frequency frep through comb tooth interlocking. The two comb teeth output by each optical comb oscillator participate in the beat frequency, and the two sets of error signals generated by the beat frequency modulate the pump light of the two pump lasers respectively to achieve synchronous locking of the repetition frequency of the dual microcavity optical comb.

[0018] As a preferred embodiment of a stable optical clock system based on dual-microcavity optical comb and two-photon transition, the SiO2 micro-core ring cavity is a whispering-gallery type micro-resonant cavity. The optical trajectory of the SiO2 micro-core ring cavity after processing is located around the SiO2 disk, and the silicon substrate below the SiO2 disk is selectively removed to prevent optical power from leaking to the silicon substrate.

[0019] The Kerr nonlinear effects of the SiO2 microchip ring cavity include self-phase modulation, cross-phase modulation, four-wave mixing, and modulation instability. The generation of broadband equidistant comb teeth of the microcavity optical comb is achieved by cascaded non-degenerate four-wave mixing.

[0020] This invention also provides a method for implementing a stable optical clock system based on a dual-microcavity optical comb and two-photon transitions, applied to the aforementioned stable optical clock system based on a dual-microcavity optical comb and two-photon transitions, comprising the following steps:

[0021] S1. Prepare two optical comb oscillators with SiO2 microchip ring cavity structure, and match the center wavelength of the two optical comb oscillators to the cesium 6S-6D and rubidium 5S-5D two-photon transition lines, respectively.

[0022] S2. Construct a microcavity optical comb generation optical path. Input 894nmCW and 780nmCW pump light to two optical comb oscillators respectively through a pump laser and an amplifier. Utilize the Kerr nonlinear effect of the SiO2 micro-core ring cavity to generate a dual microcavity optical comb.

[0023] S3. Construct a two-photon transition locking optical path for rubidium and cesium. Excite two-photon transitions of rubidium atoms with a 778nm DBR laser and generate 885nm laser to excite two-photon transitions of cesium atoms by modulation with an 894nm ECDL laser. Stabilize the frequencies of the two lasers at the corresponding atomic two-photon transition peaks to form atomic frequency standards.

[0024] S4. The error signal between the microcavity optical comb and the corresponding atomic frequency standard laser is extracted by the beat frequency method. The pump light frequency is adjusted by the phase-locked loop and acousto-optic modulator to achieve independent locking of the initial frequency fceo of the dual microcavity optical comb.

[0025] S5. Beat the teeth of the two microcavity optical combs, extract the beat frequency error signal, and use a phase-locked loop to modulate the pump light phase to achieve interlocking of the repetition frequency frep of the two microcavity optical combs.

[0026] S6. The PDH locking method is combined with the single-sideband suppressed carrier SSB-SC modulation method. The pump light is modulated by the phase modulator, the reflected light signal of the micro-core ring cavity is detected and the detuning error signal is extracted, so as to adjust the pump frequency in real time and suppress the thermal frequency detuning of the SiO2 micro-core ring cavity.

[0027] S7. The repetition frequency of the dual microcavity optical comb is detected by a photodetector, and the radio frequency standard signal of frequency stability is directly output.

[0028] As a preferred method for realizing a stable optical clock system based on dual-microcavity optical combs and two-photon transitions, step S1, the process for fabricating the SiO2 microchip ring cavity includes photolithography, dry etching, isotropic etching of the silicon substrate, and SiO2 reflow molding, specifically including:

[0029] S11. A 2mm thick SiO2 layer is grown on a high-quality silicon substrate by a wet heat oxidation method. Photoresist is coated on the SiO2 layer and photolithography is performed to form a disk-shaped photoresist pad with a diameter of 160mm.

[0030] S12. The photoresist pad is baked and reflowed to smooth the edges. Using the photoresist pad as a mask, the SiO2 layer is selectively etched with buffered hydrofluoric acid solution to form a SiO2 disk. Acetone is used to remove residual photoresist and organic matter.

[0031] S13. Using a SiO2 disk as a mask, an isotropic etching process is performed on the silicon substrate using XeF2 gas under a pressure of 3 Torr to form silicon pillars supporting the SiO2 disk, and the silicon substrate below the outer periphery of the SiO2 disk is removed.

[0032] S14. A CO2 laser is used to heat the surface of the SiO2 disk by normal irradiation. The laser beam intensity distribution is Gaussian, and the diameter of the focused circular spot is approximately 200 mm. The laser power density during reflow soldering is 100 MWm. -2 This allows for selective reflux of the SiO2 disk to form a SiO2 microchip ring cavity structure.

[0033] As a preferred method for implementing a stable optical clock system based on dual-microcavity optical combs and two-photon transitions, step S2, which utilizes the Kerr nonlinear effect of the SiO2 microchip ring cavity to generate the dual-microcavity optical comb, is as follows:

[0034] The pump light first generates secondary sidebands in the SiO2 micro-core ring cavity through modulation instability, then forms pairs of equidistant sidebands through degenerate four-wave mixing, and finally generates a broadband equidistant comb-like spectrum through cascaded non-degenerate four-wave mixing, forming a microcavity optical comb. The modulation instability occurs in the anomalous dispersion region of the SiO2 micro-core ring cavity, and the pump light energy exceeds the modulation instability threshold.

[0035] As a preferred scheme for implementing a stable optical clock system based on dual microcavity optical combs and two-photon transitions, in step S5, the pump light phase is modulated using a phase-locked loop to achieve the interlocking process of the repetition frequency frep of the dual microcavity optical comb:

[0036] By changing the output power of the pump laser, the effective path length of the SiO2 micro-core ring cavity is altered using thermal effects and Kerr nonlinearity, thereby adjusting the mode spacing of the microcavity optical comb and achieving frequency locking of the repetition frequency (frep).

[0037] The present invention has the following advantages:

[0038] The microcavity optical comb of this invention has a center wavelength that covers the two-photon transition frequency band of rubidium and cesium atoms, eliminating the need for a frequency doubling crystal; it also eliminates the f-2f locking structure, simplifying system design and facilitating on-chip integration and true miniaturization.

[0039] This invention uses dual optical comb interlocking and dual atomic frequency standard locking technology to suppress thermal detuning and additional noise, achieving high frequency stability and significantly optimizing system reliability.

[0040] This invention employs SiO2 microchip ring cavity technology, combined with phase-locked loop and optical path design, to reduce redundant components, improve system integration, and meet the needs of portability and practicality.

[0041] This invention achieves high-precision direct frequency division from optical frequency to radio frequency by utilizing dual optical comb collaboration and atomic frequency standard locking, eliminating the need for additional conversion modules and reducing energy loss and signal distortion. Attached Figure Description

[0042] To more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings in the following description are merely exemplary, and those skilled in the art can derive other embodiments based on the provided drawings without creative effort.

[0043] The structures, proportions, sizes, etc. illustrated in this specification are only for the purpose of assisting those skilled in the art in understanding and reading the content disclosed herein, and are not intended to limit the conditions under which the present invention can be implemented. Therefore, they have no substantial technical significance. Any modifications to the structure, changes in the proportions, or adjustments to the size, without affecting the effects and objectives that the present invention can produce, should still fall within the scope of the technical content disclosed in the present invention.

[0044] Figure 1 This is a schematic diagram of the stable optical clock system architecture based on dual microcavity optical comb and two-photon transition provided in an embodiment of the present invention;

[0045] Figure 2 This is a schematic diagram of the modulation instability of a stable optical clock system based on dual microcavity optical combs and two-photon transitions provided in an embodiment of the present invention.

[0046] Figure 3 This is a schematic diagram of four-wave mixing in a nonlinear medium in a stable optical clock system based on dual microcavity optical combs and two-photon transitions, provided in an embodiment of the present invention.

[0047] Figure 4 This is a schematic diagram of the process for implementing a stable optical clock based on dual microcavity optical combs and two-photon transitions, as provided in an embodiment of the present invention.

[0048] Figure 5 This is the SiO2 microchip ring cavity fabrication process in the method for realizing a stable optical clock based on dual microcavity optical comb and two-photon transition provided in the embodiments of the present invention. Detailed Implementation

[0049] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0050] See Figure 1This invention provides a stable optical clock system based on dual microcavity optical combs and two-photon transitions, including a microcavity optical comb section that generates optical comb signals matching atomic transition wavelengths, a locking section that achieves stable locking of the optical comb frequency, and a photodetector that converts the stable optical comb signal into a standard radio frequency output.

[0051] The microcavity optical comb section includes two optical comb oscillators using SiO2 micro-core ring cavities, a pump laser, and an amplifier to provide pump light to the optical comb oscillators. The center wavelengths of the two optical comb oscillators correspond to the cesium 6S-6D and rubidium 5S-5D two-photon transition lines, respectively. The SiO2 micro-core ring cavity, with its high Q value and small mode volume, is the carrier for generating the microcavity optical comb, and its Kerr nonlinear effect enables multi-frequency expansion of the optical frequency. The pump laser provides the initial energy, which, after amplification, meets the energy threshold for optical comb generation. The two-photon transitions of rubidium and cesium atoms have fixed and stable frequencies, corresponding to the center wavelength of the optical comb, allowing direct establishment of a correlation between the optical comb and the atomic frequency standard, avoiding errors introduced by wavelength conversion.

[0052] The locking mechanism includes a rubidium two-photon transition generator and a cesium two-photon transition generator to provide atomic reference frequencies, and a phase-locked loop (PLL) that locks the optical comb frequency through error signal feedback. The PLL is associated with two optical comb oscillators and their corresponding two-photon transition generators to simultaneously interlock the frequencies between the two optical combs. The atomic transition frequencies generated by the rubidium and cesium two-photon transition generators are naturally highly stable reference standards, providing a calibration basis for the optical comb frequency. The PLL detects the deviation between the optical comb frequency and the atomic reference frequency and adjusts the optical comb parameters accordingly to achieve frequency locking. The interlocking between the two optical combs further offsets the frequency drift of a single optical comb, improving overall frequency stability through the synergistic effect of the two optical combs.

[0053] The photodetector receives the microcavity optical comb repetition frequency signal after it has been partially stabilized by locking, and converts the stabilized microcavity optical comb repetition frequency signal into an RF frequency standard signal for output. The repetition frequency of the microcavity optical comb belongs to the RF band, and its stability has been achieved through atomic frequency standard locking, ensuring high stability of the optical frequency. The photodetector can directly convert the optical signal of the optical comb into an electrical signal without the need for additional frequency division or multiplication modules, reducing signal loss and distortion, and directly outputting a standard RF frequency standard that meets application requirements.

[0054] In this embodiment, the center wavelength of the optical comb oscillator corresponding to the cesium 6S-6D two-photon transition line is 885nm, and the matching pump laser outputs 894nm CW pump light.

[0055] Specifically, the characteristic wavelength of the 6S-6D two-photon transition of cesium atoms is 885nm, and the center wavelength of the optical comb can be matched with it to achieve frequency coupling directly. The pump light is 894nm CW (continuous wave). The laser of this wavelength can efficiently excite the optical comb with a center wavelength of 885nm in the SiO2 micro-core ring cavity through the Kerr nonlinear effect. Continuous wave pumping can ensure the stability of the optical comb output and avoid the fluctuations introduced by pulse pumping.

[0056] The optical comb oscillator corresponding to the rubidium 5S-5D two-photon transition line has a center wavelength of 778nm, and the matching pump laser outputs 780nm CW pump light.

[0057] Specifically, the characteristic wavelength of the 5S-5D two-photon transition of rubidium atoms is 778nm. The center wavelength of the optical comb precisely matches this transition line, and it can be locked without wavelength conversion. The 780nm CW pump light is the optimal choice for exciting the SiO2 micro-core ring cavity to generate an optical comb with a center wavelength of 778nm. Its energy is easily absorbed by the micro-cavity, and its continuous wave characteristics can ensure the uniformity and stability of the optical comb teeth.

[0058] In this embodiment, the rubidium two-photon transition generating device includes a 778nm DBR laser and a first photomultiplier tube. The first photomultiplier tube detects the fluorescence of the rubidium atom two-photon transition and extracts a first error signal. The first error signal is used to modulate the current of the DBR laser so that the frequency of the DBR laser is stabilized at the rubidium two-photon transition peak.

[0059] Specifically, the 778nm DBR laser has the characteristics of narrow linewidth and high stability, making it suitable as a light source for exciting two-photon transitions of rubidium atoms. When the laser frequency deviates from the two-photon transition peak of rubidium atoms, the fluorescence intensity generated by the atomic transition will change. The first photomultiplier tube converts the fluorescence signal into an electrical signal, and after processing, the first error signal reflecting the frequency deviation is extracted. By current-modulating the laser, its output frequency can be precisely adjusted so that the frequency is always stable at the transition peak, forming a highly stable rubidium atom frequency standard.

[0060] In this embodiment, the cesium two-photon transition generating device includes an 894nm ECDL laser and a second photomultiplier tube. The second photomultiplier tube detects the fluorescence of the cesium atom two-photon transition and extracts a second error signal. The second error signal is superimposed on the current modulation signal of the ECDL laser, so that the frequency of the 885nm laser generated by the ECDL laser is stabilized at the cesium two-photon transition peak.

[0061] Specifically, the 6S-6D two-photon transition of cesium atoms requires excitation by an 885nm laser, but continuous-wave lasers of this wavelength are difficult to obtain directly. However, the 894nm ECDL laser has a large tunable range and can generate 885nm laser through current modulation. A second photomultiplier tube detects the fluorescence of cesium atom transitions and extracts a second error signal reflecting the deviation between the laser frequency and the transition peak. This error signal is superimposed on the current modulation signal of the laser to correct the laser frequency in real time, stabilizing it at the two-photon transition peak of cesium atoms, thus forming a cesium atom frequency standard.

[0062] In this embodiment, the locking part further includes an acousto-optic modulator and a phase modulator. The acousto-optic modulator receives the beat frequency error signal between the microcavity optical comb and the corresponding locked laser, and adjusts the pump light frequency according to the beat frequency error signal to lock the initial frequency fceo of the microcavity optical comb.

[0063] Specifically, the initial frequency fceo (Carrier-Envelope Offset Frequency) of the microcavity optical comb is a parameter that affects the stability of the comb and needs to be locked with the atomic frequency standard. When the comb teeth of the microcavity optical comb are beat with the stabilized atomic frequency standard laser, a beat error signal will be generated when fceo deviates from the target value. The acousto-optic modulator can adjust the frequency of the pump light by changing the ultrasonic frequency, thereby directly changing the fceo of the microcavity optical comb. Through feedback, fceo is stabilized at the target value, thus completing the initial frequency locking.

[0064] Specifically, the phase modulator, in conjunction with the Pound-Drever-Hall (PDH) locking method and single-sideband suppressed carrier (SSB-SC) modulation, adjusts the pump frequency to suppress frequency detuning of the SiO2 micro-core ring cavity. The SiO2 micro-core ring cavity is temperature-sensitive; thermal effects can cause changes in cavity length, leading to frequency detuning. The phase modulator performs SSB-SC modulation on the pump light, retaining only the single-sideband signal and suppressing the carrier, thus reducing modulation noise. The PDH locking method extracts the detuning error signal by detecting the phase information carried in the reflected light from the microcavity. Combining these two methods allows for precise adjustment of the pump frequency, counteracting the frequency shift caused by changes in cavity length, and achieving real-time suppression of microcavity frequency detuning.

[0065] In this embodiment, the two optical comb oscillators lock the repetition frequency frep through comb tooth interlocking. The two comb teeth output by each optical comb oscillator participate in the beat frequency. The two sets of error signals generated by the beat frequency modulate the pump light of the two pump lasers respectively to achieve synchronous locking of the repetition frequency of the dual microcavity optical comb.

[0066] Specifically, the repetition frequency (frep) is another core parameter of the optical comb. Dual optical comb interlocking can improve its stability. The beat frequency of the two comb teeth of each optical comb can reflect the fluctuation of its own frep, and the beat frequency of the comb teeth between the two optical combs can reflect the deviation of their frep. The two sets of error signals are fed back to the phase modulators of the corresponding pump lasers. By adjusting the pump light phase, the effective path length of the microcavity is changed, thereby adjusting the frep, so that the frep of the two optical combs are kept synchronized, and high-precision interlocking of the repetition frequency is achieved.

[0067] In this embodiment, the SiO2 microchip ring cavity is a whispering-gallery type microresonant cavity. The optical trajectory of the SiO2 microchip ring cavity after processing is located around the SiO2 disk, and the silicon substrate below the SiO2 disk is selectively removed to prevent optical power from leaking to the silicon substrate.

[0068] Specifically, the whispering-gallery microresonator allows light to propagate by total internal reflection along the inner wall of the cavity, greatly extending the interaction time between light and the medium, enhancing the Kerr nonlinear effect, and facilitating the generation of the optical comb; the optical trajectory located on the periphery of the SiO2 disk can maximize the utilization of the cavity's high Q value; the refractive index of the silicon substrate is higher than that of SiO2, and selective removal of the silicon substrate below the periphery can prevent optical power leakage to the substrate, reduce energy loss, and improve the strength and stability of the optical comb.

[0069] The Kerr nonlinear effect of the SiO2 micro-core ring cavity includes self-phase modulation, cross-phase modulation, four-wave mixing, and modulation instability. The generation of broadband equidistant comb teeth of the microcavity optical comb is achieved by cascaded non-degenerate four-wave mixing.

[0070] Traditional optical frequency combs, including those in solid-state lasers and fiber lasers, are generated through stimulated oscillations. This involves pump light acting on a gain medium, causing energy level inversion in the gain medium particles and producing stimulated emission. Microcavity optical combs, however, lack a gain medium and are generated through Kerr nonlinear parametric oscillations. The optical Kerr effect refers to the linear change in refractive index with optical field intensity; media capable of exhibiting this effect are called Kerr media. The Kerr effect in microring resonators mainly includes self-phase modulation, cross-phase modulation, four-wave mixing, and modulation instabilities. Parametric oscillations are primarily caused by modulation instabilities and four-wave mixing.

[0071] Modulation instability refers to a nonlinear process in which continuous or quasi-continuous waves in an optical nonlinear dispersive medium generate self-modulation in amplitude and frequency, thereby enhancing the perturbation superimposed on the continuous or quasi-continuous wave. This process can disrupt the periodicity of light. Kerr nonlinearity amplifies this deviation, generating spectral sidebands and ultimately forming light pulses.

[0072] See Figure 2 The process of modulation instability is described. Typically, modulation instability occurs only under certain conditions, usually under anomalous dispersion, i.e., shorter wavelengths exhibit larger group velocity dispersion. This is actually a type of Kerr nonlinearity, as its refractive index also changes with the optical field density. Furthermore, modulation instability can only be observed when the energy exceeds a certain threshold. Since the optical field density is highly dependent on frequency perturbations, the frequency perturbation is very weak at a specific frequency, but it grows exponentially at other frequencies. Random perturbations generate frequencies containing a large number of frequency components, leading to the generation of frequency sidebands.

[0073] In a nonlinear medium, if light with frequencies ω1, ω2, and ω3 exists, and these three frequencies satisfy the wave vector matching condition, light with frequency ω4 will be generated, satisfying ω1 + ω2 = ω3 + ω4. This process is called four-wave mixing. If the pump light satisfies ω1 = ω2, this four-wave mixing process is called degenerate four-wave mixing; otherwise, it is called non-degenerate four-wave mixing. (See attached diagram.) Figure 3 If the medium originally contains light of frequency ω3, it will generate light of frequency ω4 and amplify the light of frequency ω3, which is called parametric amplification (OPA). Conversely, if there is no light of frequency ω3 originally, it will generate light of both frequency ω3 and frequency ω4, which is called parametric oscillation (OPO).

[0074] The purpose of four-wave mixing is to generate multiple frequencies within a microcavity, thus broadening the spectral range. In a whispering-gallery optical microcavity, parametric oscillations can occur when the scattering rates of the signal light and idler light are higher than the attenuation rates of their respective cavity modes. The microcavity optical comb generation process is as follows:

[0075] First, due to modulation instability, the pump laser can convert pump photons into secondary sidebands with spacing across multiple free spectral ranges (FSRs). The degenerate FWM process also results in pairs of equidistant sidebands. However, the resulting sideband pairs are not necessarily equidistant; that is, they do not need to have the same frequency spacing as when forming a comb. Then, when the generated signal and idler light are used as pump light for the next step of parametric generation, comb-like equidistant sidebands are produced. This process is called cascaded FWM, a type of non-degenerate FWM. Cascaded FWM is the primary process for generating new sidebands when the power of the signal and idler sidebands is comparable to the pump power within the cavity. Through this process, a broadband equidistant comb is generated within the microcavity.

[0076] Since the cavity length of the microcavity comb is fixed and cannot be changed, frequency locking can only be achieved by adjusting the pump light. Locking the initial frequency fceo is simply a matter of directly changing the pump laser frequency. Locking the repetition frequency frep is achieved by changing the power of the pump laser. Due to thermal effects and Kerr nonlinearity, changes in laser power translate into changes in the effective path length of the microresonator, thus altering the laser's mode spacing.

[0077] See Figure 4 and Figure 5 This invention also provides a method for implementing a stable optical clock system based on a dual-microcavity optical comb and two-photon transitions, applied to the stable optical clock system based on a dual-microcavity optical comb and two-photon transitions described in the above embodiments, comprising the following steps:

[0078] S1. Two SiO2 microchip ring cavity optical comb oscillators were fabricated, with their center wavelengths matched to the two-photon transition lines of cesium 6S-6D and rubidium 5S-5D, respectively. The core of the optical comb oscillator is the SiO2 microchip ring cavity, whose structural parameters determine the center wavelength of the optical comb. By controlling the microcavity fabrication process, the center wavelengths of the two optical combs were matched to the two-photon transition lines of rubidium and cesium atoms, respectively, laying the structural foundation for locking without frequency doubling.

[0079] S2. Construct a microcavity optical comb generation path. Pump lasers (894nm CW and 780nm CW) are input to two optical comb oscillators via a pump laser and an amplifier, respectively. The Kerr nonlinear effect of the SiO2 micro-core ring cavity is used to generate a dual microcavity optical comb. The laser output from the pump laser is amplified to the threshold of the microcavity Kerr nonlinear effect. The 894nm and 780nm CW pump lights are coupled to their respective SiO2 micro-core ring cavities. Within the cavity, Kerr nonlinear processes such as modulation instability and four-wave mixing convert the single-frequency pump light into an optical comb signal containing multiple equidistant frequency components, i.e., a dual microcavity optical comb, providing a carrier for frequency locking.

[0080] S3. Construct a rubidium-cesium two-photon transition locking optical path. Rubidium atoms undergo two-photon transitions using a 778nm DBR laser, while cesium atoms undergo two-photon transitions using an 894nm ECDL laser modulated to generate an 885nm laser. The frequencies of both lasers are stabilized at the corresponding atomic two-photon transition peaks, forming atomic frequency standards. The frequencies of atomic two-photon transitions possess inherent stability, making them ideal references for calibrating the optical comb frequency. Constructing a locking optical path ensures full interaction between the laser and atoms. Error signals are extracted using photomultiplier tubes and fed back to adjust the laser frequency, precisely locking the laser output frequency to the atomic transition peaks, forming highly stable rubidium-cesium atomic frequency standards, providing a reference for optical comb locking.

[0081] S4. The error signal between the microcavity optical comb and the corresponding atomic frequency standard laser is extracted by beat frequency extraction. The pump light frequency is adjusted using a phase-locked loop and an acousto-optic modulator to achieve independent locking of the initial frequency fceo of the two microcavity optical combs. Beat frequency extraction is an effective method for measuring the deviation between two frequency signals. Beating the microcavity optical comb teeth with the atomic frequency standard laser signal will result in a frequency deviation in the form of a beat frequency signal, which is then processed to obtain the error signal. The phase-locked loop filters and amplifies the error signal, and outputs a control signal to the acousto-optic modulator. The acousto-optic modulator directly adjusts the fceo of the microcavity optical comb by changing the pump light frequency, stabilizing fceo at the target value corresponding to the atomic frequency standard, thus completing the independent locking of the fceo of the two optical combs.

[0082] S5. Beat the teeth of the two microcavity optical combs, extract the beat frequency error signal, and use a phase-locked loop to modulate the pump light phase to achieve interlocking of the repetition frequency (FRP) of the two microcavity optical combs. Dual-comb interlocking can cancel the drift of the FRP of a single comb, improving overall stability. When the teeth of the two combs beat each other, a beat frequency error signal is generated if there is a deviation in their FRP. After processing this error signal, the phase-locked loop outputs a phase modulation command to the pump laser. By changing the pump light phase, the effective path length of the microcavity is affected, thereby adjusting the FRP of the optical combs to keep them consistent, achieving interlocking of the repetition frequency.

[0083] S6. A PDH (Pound-Drever-Hall) locking method combined with single-sideband suppressed carrier (SSB-SC) modulation is employed. The pump light is modulated using a phase modulator to detect the reflected light signal from the microcavity ring and extract the detuning error signal. This allows for real-time adjustment of the pump frequency, suppressing thermally induced frequency detuning of the SiO2 microcavity ring. The cavity length of the SiO2 microcavity ring is susceptible to slight temperature variations, leading to frequency detuning. SSB-SC modulation of the pump light by the phase modulator reduces modulation noise interference. The PDH method analyzes the phase change of the reflected light from the microcavity to accurately extract the detuning error signal reflecting changes in cavity length. Based on this error signal, the pump frequency is adjusted in real-time to compensate for the frequency shift caused by changes in cavity length, ensuring long-term stability of the microcavity comb frequency.

[0084] S7. The repetition frequency of the dual microcavity optical comb is detected by a photodetector, and the radio frequency standard signal with frequency stability is directly output. After fceo locking, frep interlocking, and detuning suppression, the repetition frequency of the microcavity optical comb has high stability comparable to that of an atomic frequency standard. The photodetector can directly convert the optical signal of the optical comb into an electrical signal, and the frequency of this electrical signal is the repetition frequency of the optical comb. It belongs to the radio frequency band and can be used as a standard radio frequency standard output without additional signal processing, meeting the application requirements of satellite navigation, precision measurement, and other scenarios.

[0085] In this embodiment, step S1, the process for preparing the SiO2 microchip ring cavity includes photolithography, dry etching, isotropic etching of the silicon substrate, and SiO2 reflow molding, specifically including:

[0086] S11. A 2mm thick SiO2 layer is grown on a high-quality silicon substrate using a hydrothermal oxidation method. Photoresist is then coated onto the SiO2 layer and photolithography is performed to form a 160mm diameter disc-shaped photoresist pad. High-quality silicon substrates possess excellent flatness and stability, making them ideal substrates for fabricating microcavities. The hydrothermal oxidation method can grow a uniform and dense SiO2 layer on the silicon substrate surface; the 2mm thickness ensures the structural strength and optical performance of the microcavity. The photoresist has photosensitive properties, and the pre-defined disc-shaped pattern can be transferred to the photoresist layer through photolithography to form a photoresist pad, providing a mask for etching.

[0087] S12. The photoresist pad is baked and reflowed to smooth the edges. Using the photoresist pad as a mask, the SiO2 layer is selectively etched with a buffered hydrofluoric acid solution to form a SiO2 disk. Acetone is used to remove residual photoresist and organic matter. Baking and reflowing smooth the edges of the photoresist pad, preventing burrs on the edges of the SiO2 disk formed by subsequent etching, which would affect optical performance. The buffered hydrofluoric acid solution selectively etches SiO2, only etching the SiO2 areas not covered by the photoresist pad, preserving the SiO2 beneath the photoresist pad to form a disk-shaped structure. Acetone dissolves residual photoresist and organic matter, ensuring a clean SiO2 disk surface and preventing impurities from affecting microcavity performance.

[0088] S13. Using a SiO2 disk as a mask, isotropic etching of the silicon substrate is performed with XeF2 gas under a 3 Torr pressure to form silicon pillars supporting the SiO2 disk, and the silicon substrate below the periphery of the SiO2 disk is removed. SiO2 is chemically inert to XeF2 gas and can be used as an etching mask; XeF2 gas can react with silicon, and the etching process is isotropic, which can uniformly remove the silicon substrate below the SiO2 disk; the 3 Torr pressure is the optimal pressure for XeF2 gas etching, which can ensure the etching rate and avoid over-etching; retaining the central silicon pillars to support the SiO2 disk and removing the silicon substrate below the periphery can prevent optical power from leaking to the silicon substrate and improve the Q value of the microcavity.

[0089] S14. A CO2 laser is used to heat the surface of the SiO2 disk by normal irradiation. The laser beam intensity distribution is Gaussian, and the diameter of the focused circular spot is approximately 200 mm. The laser power density during reflow soldering is 100 MW / m². -2This allows for selective reflow of the SiO2 disk to form a SiO2 microchip ring cavity structure. The wavelength of the CO2 laser matches the absorption peak of SiO2, enabling efficient heating of the SiO2 disk; the Gaussian distributed laser beam ensures more uniform heating at the disk edge, and the focused spot size allows for precise control of the heating range; 100MW / m -2 The power density allows the edge of the SiO2 disk to reach a molten state, and the reflow through surface tension forms a ring structure (micro-core ring cavity); selective reflow ensures that the central support part is not affected, and finally forms a SiO2 micro-core ring cavity with a regular structure and excellent optical performance.

[0090] In this embodiment, in step S2, the process of generating a dual-microcavity optical comb using the Kerr nonlinear effect of the SiO2 micro-core ring cavity is as follows: the pump light first generates secondary sidebands in the SiO2 micro-core ring cavity through modulation instability, then forms pairs of equidistant sidebands through degenerate four-wave mixing, and finally generates a broadband equidistant comb-shaped spectrum through cascaded non-degenerate four-wave mixing to form a microcavity optical comb; wherein, the modulation instability occurs in the anomalous dispersion region of the SiO2 micro-core ring cavity, and the pump light energy exceeds the modulation instability threshold. Modulation instability is the initial step in optical comb generation. Within the anomalous dispersion region of the microcavity, the energy perturbation of the pump light is amplified, generating secondary sidebands. Degenerate four-wave mixing (pump light frequency ω1=ω2) generates paired equidistant sidebands, expanding the spectral range. In cascaded non-degenerate four-wave mixing, the signal light and idler light generated in the previous step serve as new pump light, continuously generating new sidebands, ultimately forming a broadband, equidistant comb-like spectrum (microcavity optical comb). The pump light energy must exceed the modulation instability threshold to trigger the above nonlinear process and ensure effective optical comb generation.

[0091] In this embodiment, in step S5, during the process of interlocking the repetition frequency frep of the dual microcavity optical comb by modulating the pump light phase using a phase-locked loop: by changing the output power of the pump laser, the effective path length of the SiO2 micro-core ring cavity is changed by utilizing thermal effects and Kerr nonlinearity, thereby adjusting the mode spacing of the microcavity optical comb and achieving the locking of the repetition frequency frep.

[0092] Specifically, changes in the pump laser's output power affect the microcavity through two effects: first, a thermal effect, where power changes lead to changes in the microcavity temperature, which in turn changes the SiO2 refractive index; and second, a Kerr nonlinear effect, where changes in the optical field intensity directly alter the microcavity's refractive index. These two effects combined cause a change in the effective path length of the microcavity. The repetition frequency (frep) of the optical comb is proportional to the microcavity mode spacing, which adjusts as the effective path length changes. By precisely controlling the pump power, the frep can be adjusted to keep the frep of the two optical combs consistent, achieving interlocking of the repetition frequencies.

[0093] Although the present invention has been described in detail above with general descriptions and specific embodiments, modifications or improvements can be made to it, which will be obvious to those skilled in the art. Therefore, all such modifications or improvements made without departing from the spirit of the present invention fall within the scope of protection claimed by the present invention.

Claims

1. A stable optical clock system based on a dual-microcavity optical comb and two-photon transitions, characterized in that, It includes a microcavity optical comb section that generates optical comb signals matching atomic transition wavelengths, a locking section that achieves stable locking of the optical comb frequency, and a photodetector that converts the stable optical comb signal into a standard radio frequency output. The microcavity optical comb section includes two optical comb oscillators using SiO2 micro-core ring cavities, as well as a pump laser and an amplifier that provide pump light for the optical comb oscillators. The center wavelengths of the two optical comb oscillators correspond to the cesium 6S-6D two-photon transition line and the rubidium 5S-5D two-photon transition line, respectively. The locking part includes a rubidium two-photon transition generator and a cesium two-photon transition generator that provide atomic reference frequencies, and a phase-locked loop that locks the optical comb frequency through error signal feedback. The phase-locked loop is associated with two optical comb oscillators and the corresponding two-photon transition generators respectively, so as to simultaneously perform frequency interlocking between the two optical combs. The photodetector receives the microcavity optical comb repetition frequency signal after it has been partially stabilized by locking, and converts the stabilized microcavity optical comb repetition frequency signal into an RF frequency standard signal for output.

2. The stable optical clock system based on dual-microcavity optical comb and two-photon transition as described in claim 1, characterized in that, The optical comb oscillator corresponding to the cesium 6S-6D two-photon transition line has a center wavelength of 885nm, and the matching pump laser outputs 894nm CW pump light; The optical comb oscillator corresponding to the rubidium 5S-5D two-photon transition line has a center wavelength of 778nm, and the matching pump laser outputs 780nm CW pump light.

3. The stable optical clock system based on dual-microcavity optical comb and two-photon transition as described in claim 2, characterized in that, The rubidium two-photon transition generating device includes a 778nm DBR laser and a first photomultiplier tube. The first photomultiplier tube detects the fluorescence of the rubidium atom two-photon transition and extracts a first error signal. The first error signal is used to modulate the current of the DBR laser so that the frequency of the DBR laser is stabilized at the rubidium two-photon transition peak.

4. The stable optical clock system based on dual-microcavity optical comb and two-photon transition as described in claim 2, characterized in that, The cesium two-photon transition generating device includes an 894nm ECDL laser and a second photomultiplier tube. The second photomultiplier tube detects the fluorescence of the cesium atom two-photon transition and extracts a second error signal. The second error signal is superimposed on the current modulation signal of the ECDL laser, so that the frequency of the 885nm laser generated by the ECDL laser is stabilized at the cesium two-photon transition peak.

5. The stable optical clock system based on dual-microcavity optical comb and two-photon transition as described in claim 2, characterized in that, The locking part also includes an acousto-optic modulator and a phase modulator. The acousto-optic modulator receives the beat frequency error signal between the microcavity optical comb and the corresponding locked laser, and adjusts the pump light frequency according to the beat frequency error signal to lock the initial frequency fceo of the microcavity optical comb. The phase modulator, in conjunction with the Pound-Drever-Hall locking method and the single-sideband suppressed carrier SSB-SC modulation mode, adjusts the pump frequency to suppress frequency detuning of the SiO2 micro-core ring cavity.

6. The stable optical clock system based on dual-microcavity optical comb and two-photon transition as described in claim 1, characterized in that, Two optical comb oscillators lock the repetition frequency (frep) through comb tooth interlocking. The two comb teeth output by each optical comb oscillator participate in the beat frequency. The two sets of error signals generated by the beat frequency modulate the pump light of the two pump lasers respectively to achieve synchronous locking of the repetition frequency of the dual microcavity optical comb.

7. The stable optical clock system based on dual-microcavity optical comb and two-photon transition as described in claim 1, characterized in that, The SiO2 microchip ring cavity is a whispering-gallery type microresonant cavity. The optical trajectory of the SiO2 microchip ring cavity after processing is located around the SiO2 disk, and the silicon substrate below the SiO2 disk is selectively removed to prevent optical power from leaking to the silicon substrate. The Kerr nonlinear effects of the SiO2 microchip ring cavity include self-phase modulation, cross-phase modulation, four-wave mixing, and modulation instability. The generation of broadband equidistant comb teeth of the microcavity optical comb is achieved by cascaded non-degenerate four-wave mixing.

8. A method for implementing a stable optical clock system based on a dual-microcavity optical comb and two-photon transitions, characterized in that, The optical clock system based on dual microcavity optical combs and two-photon transitions, as described in any one of claims 1-7, comprises the following steps: S1. Prepare two optical comb oscillators with SiO2 microchip ring cavity structure, and match the center wavelength of the two optical comb oscillators to the cesium 6S-6D and rubidium 5S-5D two-photon transition lines, respectively. S2. Construct a microcavity optical comb generation optical path. Input 894nmCW and 780nmCW pump light to two optical comb oscillators respectively through a pump laser and an amplifier. Utilize the Kerr nonlinear effect of the SiO2 micro-core ring cavity to generate a dual microcavity optical comb. S3. Construct a two-photon transition locking optical path for rubidium and cesium. Excite two-photon transitions of rubidium atoms with a 778nm DBR laser and generate 885nm laser to excite two-photon transitions of cesium atoms by modulation with an 894nm ECDL laser. Stabilize the frequencies of the two lasers at the corresponding atomic two-photon transition peaks to form atomic frequency standards. S4. The error signal between the microcavity optical comb and the corresponding atomic frequency standard laser is extracted by the beat frequency method. The pump light frequency is adjusted by the phase-locked loop and acousto-optic modulator to achieve independent locking of the initial frequency fceo of the dual microcavity optical comb. S5. Beat the teeth of the two microcavity optical combs, extract the beat frequency error signal, and use a phase-locked loop to modulate the pump light phase to achieve interlocking of the repetition frequency frep of the two microcavity optical combs. S6. The PDH locking method is combined with the single-sideband suppressed carrier SSB-SC modulation method. The pump light is modulated by the phase modulator, the reflected light signal of the micro-core ring cavity is detected and the detuning error signal is extracted, so as to adjust the pump frequency in real time and suppress the thermal frequency detuning of the SiO2 micro-core ring cavity. S7. The repetition frequency of the dual microcavity optical comb is detected by a photodetector, and the radio frequency standard signal of frequency stability is directly output.

9. The method for implementing a stable optical clock system based on a dual-microcavity optical comb and two-photon transitions according to claim 8, characterized in that, In step S1, the process for fabricating the SiO2 microchip ring cavity includes photolithography, dry etching, isotropic etching of the silicon substrate, and SiO2 reflow molding, specifically including: S11. A 2mm thick SiO2 layer is grown on a high-quality silicon substrate by a wet heat oxidation method. Photoresist is coated on the SiO2 layer and photolithography is performed to form a disk-shaped photoresist pad with a diameter of 160mm. S12. The photoresist pad is baked and reflowed to smooth the edges. Using the photoresist pad as a mask, the SiO2 layer is selectively etched with buffered hydrofluoric acid solution to form a SiO2 disk. Acetone is used to remove residual photoresist and organic matter. S13. Using a SiO2 disk as a mask, an isotropic etching process is performed on the silicon substrate using XeF2 gas under a pressure of 3 Torr to form silicon pillars supporting the SiO2 disk, and the silicon substrate below the outer periphery of the SiO2 disk is removed. S14. A CO2 laser is used to heat the surface of the SiO2 disk by normal irradiation. The laser beam intensity distribution is Gaussian, and the diameter of the focused circular spot is approximately 200 mm. The laser power density during reflow soldering is 100 MWm. -2 This allows for the selective reflow of SiO2 disks to form a SiO2 microcore annular cavity structure. In step S2, the process of generating a dual-microcavity optical comb using the Kerr nonlinear effect of the SiO2 microchip ring cavity is as follows: The pump light first generates secondary sidebands in the SiO2 micro-core ring cavity through modulation instability, then forms pairs of equidistant sidebands through degenerate four-wave mixing, and finally generates a broadband equidistant comb-like spectrum through cascaded non-degenerate four-wave mixing, forming a microcavity optical comb. The modulation instability occurs in the anomalous dispersion region of the SiO2 micro-core ring cavity, and the pump light energy exceeds the modulation instability threshold.

10. The method for implementing a stable optical clock system based on a dual-microcavity optical comb and two-photon transitions according to claim 8, characterized in that, In step S5, the pump light phase is modulated using a phase-locked loop to achieve interlocking of the repetition frequency frep of the dual microcavity optical comb: By changing the output power of the pump laser, the effective path length of the SiO2 micro-core ring cavity is altered using thermal effects and Kerr nonlinearity, thereby adjusting the mode spacing of the microcavity optical comb and achieving frequency locking of the repetition frequency (frep).

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