Long-term laser frequency stabilization system based on on-chip optical reference microcavity

Abstract

The present application relates to laser frequency stabilization technology, in order to solve the contradiction between chip and long-term frequency stability of on-chip frequency stabilization technology, provide a kind of long time laser frequency stabilization system based on on-chip optical reference microcavity, including: two lasers, on-chip optical reference microcavity;Two frequency locking modules are used for locking two lasers to two different modes of on-chip optical reference microcavity respectively;Dual-mode beat frequency measurement and control module is used for real-time measurement of the dual-mode frequency difference between the two different modes locked to on-chip optical reference microcavity;In-cavity temperature slow feedback control loop is used for generating temperature fine control feedback signal based on dual-mode frequency difference, and the fine temperature of on-chip optical reference microcavity is adjusted according to temperature fine control feedback signal, to compensate the optical cavity length change caused by thermo-optic effect and thermal expansion effect, so as to stabilize the resonant frequency of on-chip optical reference microcavity, reduce the long-term drift of on-chip laser light frequency, improve the long time frequency stability of laser single frequency output.

Description

Technical Field

[0001] This invention relates to the field of laser frequency stabilization technology, and in particular to a long-term laser frequency stabilization system based on an on-chip optical reference microcavity. Background Technology

[0002] High-stability laser sources, as core components for precision spectral measurements, have been widely applied in fields such as optical metrology, spectroscopy, optical communication, and quantum information. With the development of integrated photonics technology, the miniaturization and chip-based application of laser frequency stabilization technology have become the foundation and prerequisite for solving the needs of portable, low-cost, and low-power applications.

[0003] High-stability frequency references are crucial for laser frequency stabilization technology. Traditional frequency references typically utilize atomic or molecular transition lines and optical resonant cavities. Current miniaturization and micro-miniaturization of frequency references include fiber optic cavities, micromachined rubidium vapor cells, vacuum gap micro-FP cavities, whispering-gallery resonators, and waveguide-integrated resonators. However, only waveguide-integrated resonators possess CMOS compatibility and can be considered chip-level optical references. Nevertheless, the practical application of waveguide-integrated resonators is limited by their large thermo-optic coefficient, making them exceptionally sensitive to environmental disturbances, especially temperature drift, resulting in poor long-term frequency stability.

[0004] To address the resonant frequency drift caused by environmental thermal fluctuations, several active control, material compensation, and temperature measurement methods have been proposed. However, these methods currently fail to adequately solve the problem, resulting in a long-term frequency stability level of only 10 over periods exceeding 10,000 seconds. -8 The precision level is insufficient for applications such as atomic cooling and lidar.

[0005] Low integration and chip-level integration: Existing mainstream technologies are based on miniaturized reference cavities such as fiber optic cavities, micromachined rubidium vapor batteries, vacuum gap micro FP cavities, and whispering-gallery resonators, which are large in size and currently cannot be CMOS compatible or integrated on-chip.

[0006] Poor long-term frequency stability: Existing frequency stabilization techniques based on on-chip resonators only achieve a long-term frequency stability level of 10% over a period exceeding 10,000 seconds. -8 Its limited size restricts its application in the field of precision measurement.

[0007] The frequency reference has poor flexibility: existing frequency references with good long-term stability can only rely on the transition frequencies of specific atoms or molecules, and the frequency values ​​cannot be adjusted flexibly. Summary of the Invention

[0008] To address the problems existing in the prior art and to resolve the contradiction between "chip-based" and "long-term frequency stability" in on-chip frequency stabilization technology, this invention provides a long-term laser frequency stabilization system based on an on-chip optical reference microcavity. By using dual-mode beat frequency locking, the thermal instability problem of the on-chip microcavity is solved, thereby achieving long-term on-chip frequency stability.

[0009] To achieve the above objectives, the present invention provides the following technical solution:

[0010] This invention provides a long-term laser frequency stabilization system based on an on-chip optical reference microcavity, comprising:

[0011] Two lasers, namely the first laser and the second laser;

[0012] On-chip optical reference microcavity, used to provide frequency reference;

[0013] Two frequency-locking modules, namely the first frequency-locking module and the second frequency-locking module, are used to lock the two lasers to two different modes of the on-chip optical reference microcavity, respectively.

[0014] The dual-mode beat frequency measurement and control module is used to measure the dual-mode frequency difference between two different modes locked to the on-chip optical reference microcavity in real time.

[0015] The intracavity temperature slow feedback control loop is used to generate a temperature fine control feedback signal based on the dual-mode frequency difference. The temperature inside the on-chip optical reference microcavity is finely adjusted according to the temperature fine control feedback signal to compensate for the changes in optical cavity length caused by thermo-optical effect and thermal expansion effect, thereby stabilizing the resonant frequency of the on-chip optical reference microcavity and enabling the laser locked by the two frequency locking modules to obtain long-term frequency stability.

[0016] Furthermore, the first laser and the second laser in the above system are not limited to on-chip lasers; that is, the first laser and the second laser can both be on-chip lasers, or they can be off-chip lasers.

[0017] Furthermore, the selection of the two different modes is based on choosing a pair of different modes within a free spectral range, and the frequency difference between the two different modes is less than 10 GHz. The two different modes differ in at least one parameter: mode type, order, or polarization. For example, the mode types are the same but the orders are different, or the mode types are different but the polarizations are the same, or the mode types are different and the polarizations are different, etc.

[0018] Furthermore, the on-chip optical reference microcavity also includes a microcavity temperature coarse adjustment control module, used to regulate the operating environment temperature of the on-chip optical reference microcavity to a target range, thereby achieving the effect of coarsely adjusting the resonant frequency of the on-chip optical reference microcavity. The type of the microcavity temperature coarse adjustment control module is not limited; it can be a cooling or heating module commonly used in the laser field, such as a microcavity temperature coarse adjustment control module based on a thin-film resistance heater or a semiconductor cooler.

[0019] Furthermore, the present invention also includes an arbitrary laser frequency locking module, which is used to lock an arbitrary output wavelength laser to an on-chip optical reference microcavity after the frequency has been stabilized for a long time, thereby realizing multi-wavelength frequency-stabilized laser output.

[0020] Furthermore, the purpose of the frequency locking module described in this invention is to lock the corresponding laser to the on-chip optical reference microcavity. The mode locking technology used in each mode locking module is not limited, and can be one of the following: laser locking module based on PDH frequency stabilization technology, laser locking module based on tilt locking, laser locking module based on edge locking, phase detection locking module based on lock-in amplifier, laser locking module based on self-injection locking, etc.

[0021] Furthermore, the dual-mode beat frequency measurement and control module includes a beam combiner, a photodetector, and a frequency meter;

[0022] The beam combiner is used to combine two different modes of laser beams that have been locked to the on-chip optical reference microcavity;

[0023] The photodetector is used to convert the combined optical signal into an electrical signal.

[0024] The frequency meter is used to measure the dual-mode frequency difference between two different modes locked to the on-chip optical reference microcavity.

[0025] Furthermore, the slow feedback control loop employs low-frequency feedback control with a bandwidth on the order of Hz.

[0026] Furthermore, the intracavity temperature slow feedback control loop includes a Kalman filter, a slow feedback controller, and an intracavity temperature actuator;

[0027] The Kalman filter is used to filter the real-time measured dual-mode frequency difference to generate a real-time temperature deviation signal that characterizes the actual temperature fluctuation of the on-chip optical reference microcavity.

[0028] The slow feedback controller generates a control signal for the cavity temperature actuator based on the real-time temperature deviation signal. The cavity temperature actuator adjusts the temperature of the on-chip optical reference microcavity to compensate for the ambient temperature disturbance of the on-chip optical reference microcavity, so that the dual-mode frequency difference is locked at a preset value, thereby achieving precise closed-loop control of the temperature of the on-chip optical reference microcavity.

[0029] The intracavity temperature actuator is a voltage-controlled adjustable attenuator, which is set in the optical path from the first on-chip laser or the second on-chip laser to the on-chip optical reference microcavity, and is used to adjust the laser intensity input to the on-chip optical reference microcavity from the first on-chip laser or the second on-chip laser.

[0030] The beneficial effects that the present invention can produce through the above technical solution are:

[0031] This invention provides a long-term laser frequency stabilization system based on an on-chip optical reference microcavity. It utilizes the frequency difference between two different modes as a high-precision "sensor" of the resonant frequency to monitor real-time fluctuations in the laser resonant frequency with ambient temperature. Specifically, it includes: two on-chip lasers; an on-chip optical reference microcavity for providing a frequency reference; two frequency-locking modules for locking the two on-chip lasers to two different modes of the on-chip optical reference microcavity, respectively; a dual-mode beat frequency measurement and control module for real-time measurement of the dual-mode frequency difference between the two different modes locked to the on-chip optical reference microcavity; and an intracavity temperature slow feedback control loop for generating a temperature fine-control feedback signal based on the dual-mode frequency difference. This feedback signal is used to finely adjust the temperature of the on-chip optical reference microcavity to compensate for changes in the optical cavity length caused by thermo-optical effects and thermal expansion, thereby stabilizing the resonant frequency of the on-chip optical reference microcavity. This allows the on-chip lasers locked by the two frequency-locking modules to achieve long-term frequency stability. Furthermore, this invention allows for flexible adjustment of the reference frequency value simply by adjusting the temperature of the on-chip optical reference microcavity. Furthermore, this invention can also lock any laser to an on-chip optical reference microcavity after dual-mode beat frequency measurement and control, realizing multi-wavelength frequency-stabilized laser output. By employing the aforementioned on-chip laser frequency stabilization technology, this invention significantly improves the long-term frequency stability of the output laser, building upon the traditional PDH frequency locking effect. It achieves a breakthrough by realizing a sustained stable frequency output for 100,000 seconds, meeting the long-term requirements for laser frequency stability in precision measurement fields such as optical precision measurement, optical communication, optical clocks, spectral metrology, and quantum information. Attached Figure Description

[0032] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0033] Figure 1 This is a block diagram of a long-term laser frequency stabilization system based on an on-chip optical reference microcavity in one embodiment;

[0034] Figure 2This is a schematic diagram of the system structure for achieving dual-wavelength output in a long-term laser frequency stabilization system based on an on-chip optical reference microcavity in one embodiment;

[0035] Figure 3 This is a schematic diagram of the system structure for a long-term laser frequency stabilization system based on an on-chip optical reference microcavity to achieve multi-wavelength output.

[0036] Figure 4 This is a set of measured data and fitting results showing the effect of temperature on laser frequency and dual-mode frequency difference in one embodiment. Figure 4 (a) is a schematic diagram of the linear fitting of the laser frequency to the temperature response. Figure 4 (b) is a schematic diagram of the linear fitting of the dual-mode frequency difference to the temperature response. Figure 4 (c) is a schematic diagram showing the ratio of laser frequency to dual-mode frequency difference;

[0037] Figure 5 This is a comparison chart of on-chip laser frequency stabilization results in one embodiment, wherein... Figure 5 (a) is a time-domain result diagram of laser frequency free operation for 10h, PDH locked for 10h, and dual-mode locked for 100000s; Figure 5 (b) is a graph showing the results of Allen's variance calculation;

[0038] Explanation of the labels in the diagram:

[0039] 1. First on-chip laser; 2. First electro-optic phase modulator; 3. First polarization controller; 4. First circulator; 5. On-chip optical reference microcavity; 6. Second on-chip laser; 7. Voltage-controlled adjustable attenuator; 8. Second electro-optic phase modulator; 9. Second polarization controller; 10. Second circulator; 11. First photodetector; 12. First servo controller; 13. Second photodetector; 14. Second servo controller; 15. Beam combiner; 16. Third photodetector; 17. Frequency counter; 18. Kalman filter; 19. Slow feedback controller; 20. Arbitrary laser; 21. Third electro-optic phase modulator; 22. Optical filter; 23. Fourth photodetector; 24. Third servo controller. Detailed Implementation

[0040] The technical solution of the present invention will now be clearly and completely described through specific embodiments. Obviously, the described embodiments are merely some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0041] In one embodiment, reference is made to Figure 1 A long-term laser frequency stabilization system based on an on-chip optical reference microcavity is provided, comprising:

[0042] Two lasers, namely the first laser and the second laser;

[0043] An on-chip optical reference microcavity, including a microcavity temperature coarse adjustment control module, is used to provide a frequency reference;

[0044] Two frequency-locking modules, namely the first frequency-locking module and the second frequency-locking module, are used to lock the first laser and the second laser to two different modes of the on-chip optical reference microcavity, respectively.

[0045] The dual-mode beat frequency measurement and control module is used to measure the dual-mode frequency difference between two different modes locked to the on-chip optical reference microcavity in real time.

[0046] The intracavity temperature slow feedback control loop is used to generate a temperature fine control feedback signal based on the dual-mode frequency difference. The temperature of the on-chip optical reference microcavity is finely adjusted according to the temperature fine control feedback signal to compensate for the changes in optical cavity length caused by thermo-optical effect and thermal expansion effect, thereby stabilizing the resonant frequency of the on-chip optical reference microcavity and enabling the on-chip laser locked by the two frequency locking modules to obtain long-term frequency stability.

[0047] In this invention, the first and second lasers are locked to two different modes of the on-chip optical reference microcavity. The selection of these two different modes is based on choosing a pair of different modes within a free spectral range, and the frequency difference between the two different modes is less than 10 GHz. At least one parameter of the mode type, order, or polarization differs between the two different modes. For example, the mode types are the same but the orders are different, or the mode types are different but the polarizations are the same, or the mode types are different and the polarizations are different, etc. Specifically, selecting either the TE mode or the TM mode, or any combination of two modes of different orders within the TE mode or the TM mode, all fall within the dual-mode selection range of this invention. This also includes cases where the two different modes are higher-order transverse electric modes or higher-order transverse magnetic modes with the same polarization state but different orders.

[0048] For a long time, researchers have tended to use a pair of modes with different polarizations to form a dual-mode system because the difference in thermo-optic coefficients between TE and TM modes naturally exists in birefringent crystal materials, easily generating significant beat frequency signals. Furthermore, higher-order modes with the same polarization often have larger quality factors and losses. Therefore, existing technologies often prioritize TE / TM combinations. This invention can actively select a combination of a fundamental mode and a higher-order mode with the same polarization state. It utilizes the advantage that the characteristic parameters between modes with the same polarization are closer than those of orthogonal polarization modes. The frequency difference between modes with the same polarization can better suppress common-mode noise, obtaining a more accurate frequency difference signal reflecting changes in cavity temperature. This is beneficial for improving the accuracy of cavity temperature control and the signal-to-noise ratio, resulting in a high-precision stable frequency signal. In addition, since existing technologies often prioritize TE / TM combinations, this invention is the first to apply a combination of higher-order modes with the same polarization to dual-mode beat frequency stabilization, breaking the long-standing technical prejudice in the field that "higher-order modes are unusable." Moreover, the selection of the same polarization mode overcomes the limitation that it is only applicable to materials with birefringence effects; this invention has universal applicability in any optical resonant cavity material.

[0049] In another embodiment, a long-term laser frequency stabilization system based on an on-chip optical reference microcavity is provided, comprising:

[0050] Two lasers, namely the first laser and the second laser;

[0051] An on-chip optical reference microcavity, including a microcavity temperature coarse adjustment control module, is used to provide a frequency reference;

[0052] The microcavity temperature coarse adjustment control module is used to regulate the operating environment temperature of the on-chip optical reference microcavity to the target range, thereby achieving coarse adjustment of the resonant frequency of the on-chip optical reference microcavity.

[0053] Two frequency-locking modules, namely the first frequency-locking module and the second frequency-locking module, are used to lock the first laser and the second laser to two different modes of the on-chip optical reference microcavity, respectively.

[0054] The dual-mode beat frequency measurement and control module is used to measure the dual-mode frequency difference between two different modes locked to the on-chip optical reference microcavity in real time.

[0055] The intracavity temperature slow feedback control loop is used to generate a temperature fine control feedback signal based on the dual-mode frequency difference. The temperature of the on-chip optical reference microcavity is adjusted according to the temperature fine control feedback signal to compensate for the optical cavity length change caused by thermo-optical effect and thermal expansion effect, thereby stabilizing the resonant frequency of the on-chip optical reference microcavity and enabling the on-chip laser after being locked by the two frequency locking modules to obtain long-term frequency stability.

[0056] The type of microcavity temperature coarse adjustment control module described in the above embodiments is not limited. It can be a cooling or heating module commonly used in the laser field, such as a microcavity temperature coarse adjustment control module based on a thin-film resistance heater or a semiconductor cooler. The microcavity temperature coarse adjustment control module can be integrated with the on-chip optical reference microcavity. The module includes a temperature sensor (using a thermistor or thermocouple), a controller, and a temperature adjustment execution unit (heating wire, thin-film resistance heater, semiconductor cooler, etc.). The temperature sensor is mounted on the on-chip optical reference microcavity to detect the actual temperature of the microcavity in real time and transmit it to the controller. The temperature adjustment execution unit can be arranged on the substrate or packaging base of the on-chip optical reference microcavity. The controller generates a control signal for the temperature adjustment execution unit based on the difference between the set temperature and the actual temperature. The temperature adjustment execution unit coarsely adjusts the temperature of the on-chip optical reference microcavity to the set temperature (which can be a temperature point or a temperature range), ensuring that the resonant frequency of the on-chip optical reference microcavity falls within the range covered by subsequent fine feedback control, avoiding excessive temperature deviation that would prevent fine feedback control from starting or locking. When the long-term laser frequency stabilization system based on an on-chip optical reference microcavity is in operation, the resonant frequency of the on-chip optical reference microcavity is first adjusted to a set range using a coarse temperature control module. Then, a dual-mode beat frequency measurement and control module and an intracavity temperature slow feedback control loop are used to achieve fine adjustment of the resonant frequency of the on-chip optical reference microcavity. The coarse temperature control module first pulls the operating environment temperature of the on-chip optical reference microcavity into the target range, ensuring that the microcavity resonant frequency is close to the target frequency, overcoming the limitation of the capture range of a single fine adjustment module. Subsequently, a high-sensitivity beat frequency signal is generated by the dual-mode beat frequency measurement and control module, and fine adjustment is performed by the intracavity temperature slow feedback control loop to accurately lock the resonant frequency to the target value, thereby achieving high-precision frequency stabilization over a wide temperature range. The microcavity temperature coarse adjustment control module compensates for the slow drift of the ambient temperature (such as day-night temperature difference, laboratory air conditioning fluctuations, etc.), so that the microcavity always works in a suitable temperature range; the dual-mode beat frequency measurement and control module and the cavity temperature slow feedback control loop precisely lock the resonant frequency. The two work together to enable the system to maintain the frequency lock state for a long time, which is suitable for scenarios that require continuous and stable operation.

[0057] Furthermore, such as Figure 1As shown, this invention can also lock any laser to an on-chip optical reference microcavity after dual-mode beat frequency measurement and control, achieving multi-wavelength frequency-stabilized laser λi output. Specifically, the system also includes an arbitrary laser frequency locking module, used to lock an arbitrary output wavelength laser to the on-chip optical reference microcavity after long-term frequency stabilization, achieving multi-wavelength frequency-stabilized laser λi output. The arbitrary laser frequency locking module includes at least one arbitrary laser, each arbitrary laser corresponding to a third frequency locking module. Each arbitrary laser is locked to the on-chip optical reference microcavity through its corresponding third frequency locking module to obtain the desired frequency-stabilized laser λi output. The number and wavelength of the arbitrary lasers are unlimited, and the output wavelengths of the arbitrary lasers can be the same or different.

[0058] The purpose of each frequency-locking module is to lock the corresponding laser onto the on-chip optical reference microcavity. This invention does not limit the specific implementation of the frequency-locking module; those skilled in the art can choose a suitable implementation method from existing frequency-locking technologies. For example, the frequency-locking module can use any one of the following: a laser locking module based on PDH frequency stabilization technology, a laser locking module based on tilt locking, a laser locking module based on edge locking, a phase-detection locking module based on a lock-in amplifier, or a laser locking module based on self-injection locking, to achieve the purpose of locking the laser onto the on-chip optical reference microcavity.

[0059] As is generally the case, one embodiment employs a laser locking module based on PDH frequency stabilization technology, which includes an electro-optic phase modulator, a polarization controller, a circulator, a photodetector, and a servo controller.

[0060] The electro-optic phase modulator is used to generate modulation sidebands;

[0061] The polarization controller is used to control the laser polarization state;

[0062] The circulator is used to guide the laser into and out of the on-chip optical reference microcavity;

[0063] A photodetector is used to monitor the output light field of the on-chip optical reference microcavity and convert it into an electrical signal output to the controller.

[0064] The servo controller is used to demodulate the electrical signal to obtain a DC error signal proportional to the deviation between the laser and the reference microcavity resonant frequency, and to provide real-time feedback control to the on-chip laser to achieve laser frequency locking.

[0065] This invention achieves long-term frequency stability of on-chip lasers through dual-mode locking of an on-chip optical reference microcavity. The first and second lasers can both be on-chip lasers, or they can be non-on-chip lasers.

[0066] In one embodiment, the first laser and the second laser are respectively a first on-chip laser and a second on-chip laser. A first frequency-locking module and a second frequency-locking module are used to lock the first on-chip laser and the second on-chip laser onto the on-chip optical reference microcavity, respectively, so that the first on-chip laser and the second on-chip laser produce frequency-stabilized laser output, achieving frequency-stabilized laser λ1 output and frequency-stabilized laser λ2 output, respectively. Simply locking the laser frequency to the high-Q on-chip optical reference microcavity through the frequency-locking module is insufficient to achieve high frequency stability, because the optical cavity length of the resonant cavity is inevitably affected by environmental fluctuations, especially drift caused by temperature changes. This invention ensures short-term frequency stability while simultaneously generating a precise temperature control feedback signal based on the dual-mode beat frequency measurement and control module and an intracavity temperature slow feedback control loop. This signal is used to finely adjust the temperature of the on-chip optical reference microcavity, compensating for changes in optical cavity length caused by thermo-optical effects and thermal expansion. This suppresses long-term frequency drift caused by ambient temperature fluctuations and stabilizes the on-chip optical reference microcavity's own resonant frequency, thus achieving long-term frequency stability for the laser after the frequency-locking module is engaged. Furthermore, this invention can lock any wavelength of laser output from any laser to the on-chip optical reference microcavity after dual-mode beat frequency measurement and control, achieving multi-wavelength frequency-stabilized laser output.

[0067] The on-chip optical reference microcavity described in this invention is an optical resonant cavity integrated on a semiconductor substrate, with dimensions ranging from micrometers to submicrometers. It utilizes microfabrication techniques (such as photolithography and etching) to construct tiny spatial structures (such as microdisks, microrings, and microstrips) on silicon-based or III-V group semiconductor materials to achieve extreme localization of the optical field (typically at the micrometer or submicrometer scale).

[0068] Reference Figure 2 One embodiment provides a long-term laser frequency stabilization system based on an on-chip optical reference microcavity, including a first on-chip laser 1, a first electro-optic phase modulator 2, a first polarization controller 3, a first circulator 4, an on-chip optical reference microcavity 5, a second on-chip laser 6, a voltage-controlled adjustable attenuator 7, a second electro-optic phase modulator 8, a second polarization controller 9, a second circulator 10, a first photodetector 11, a first servo controller 12, a second photodetector 13, a second servo controller 14, a beam combiner 15, a third photodetector 16, a frequency counter 17, a Kalman filter 18, and a slow feedback controller 19.

[0069] Both the first and second frequency-locking modules are laser locking modules based on PDH frequency stabilization technology. The first frequency-locking module includes: a first electro-optic phase modulator 2, a first polarization controller 3, a first circulator 4, a first photodetector 11, and a first servo controller 12. The second frequency-locking module includes a second electro-optic phase modulator 8, a second polarization controller 9, a second circulator 10, a second photodetector 13, and a second servo controller 14.

[0070] The first on-chip laser 1 is connected to the first electro-optic phase modulator 2, the first electro-optic phase modulator 2 is connected to the first polarization controller 3, the first polarization controller 3 is connected to the first circulator 4, the first on-chip laser 1 is modulated by the first electro-optic phase modulator 2 to obtain a sideband, and then coupled into the on-chip optical reference microcavity 5 through the first polarization controller 3 and the first circulator 4.

[0071] The second on-chip laser 6 is connected to a voltage-controlled adjustable attenuator 7, which is connected to a second electro-optic phase modulator 8. The second electro-optic phase modulator 8 is connected to a second polarization controller 9, which is connected to a second circulator 10. After passing through the voltage-controlled adjustable attenuator 7, the second on-chip laser 6 is modulated by the second electro-optic phase modulator 8 to obtain a sideband, which is then coupled into the on-chip optical reference microcavity 5 after passing through the second polarization controller 9 and the second circulator 10.

[0072] When the scanning frequency of the first on-chip laser 1 is near the resonant frequency of the on-chip optical reference microcavity 5, a resonance peak can be observed in the output light field at the other end of the on-chip optical reference microcavity 5, which is far from the first on-chip laser 1. The output light field, after passing through the second circulator 10, is converted into an electrical signal by the first photodetector 11. Finally, it is frequency demodulated by the first servo controller 12 to obtain a DC error signal proportional to the deviation between the output laser frequency of the first on-chip laser 1 and the resonant frequency of the reference cavity. This error signal is then fed back to control the first on-chip laser 1 in real time, achieving laser locking of the first on-chip laser 1 and realizing the output of a frequency-stabilized laser λ1. Similarly, when the scanning frequency of the second on-chip laser 6 is near the resonant frequency of the on-chip optical reference microcavity 5, a resonance peak can be observed in the output light field at the other end of the on-chip optical reference microcavity 5, which is far from the second on-chip laser 6. The output light field from the first circulator 4 is converted into an electrical signal by the second photodetector 13. Finally, the signal is demodulated in the second servo controller 14 to obtain a DC error signal proportional to the deviation between the output laser frequency of the second on-chip laser 6 and the resonant frequency of the reference cavity. This error signal is then fed back in real-time to control the second on-chip laser 6, achieving laser locking and frequency-stabilized laser λ2 output. The key components of the frequency locking module are the on-chip laser and the on-chip reference cavity. Short-term frequency stability is mainly affected by the quality factor (Q) of the on-chip reference cavity, requiring a load quality factor of 10 for the on-chip reference cavity. 7 above.

[0073] During the frequency locking process described above, the DC error signal ε Proportional to the deviation between the laser frequency and the reference cavity resonant frequency δν It can transmit DC error signals ε Represented as:

[0074]

[0075] in P c and P s For the power of the modulated carrier and sideband, Δ ν Let be the linewidth of the resonant peak of the reference cavity. In an on-chip optical reference microcavity, the linewidth of the resonant peak can be expressed as Δ. ν = ν / Q, where ν Q is the resonant frequency, and Q is the quality factor of the on-chip optical reference microcavity 5. Therefore, the higher the quality factor, the narrower the resonant peak linewidth, the greater the slope of the error signal, and the stronger the frequency discrimination capability of the on-chip optical reference microcavity.

[0076] Simply locking the laser frequency to a high-Q on-chip optical reference microcavity via a frequency-locking module is insufficient to achieve high frequency stability. This is because the optical cavity length of the resonant cavity is inevitably affected by environmental fluctuations, especially temperature-induced drift. According to fluctuation dissipation theory, thermal noise determines the stability limit achievable by the reference cavity. Thermal noise mainly includes thermal refractive noise, thermal expansion noise, and thermoelastic noise. For on-chip optical reference microcavities, light propagation within the cavity is confined to a dielectric material. The thermal refractive index and thermal expansion coefficient of this material are much larger than those of traditional vacuum gap cavities, with thermal refractive indices typically around 10⁻⁶. -5 K -1 The order of magnitude, the coefficient of thermal expansion is typically in the range of 10. -6 K -1 The magnitude of the change is significant. Unstable ambient temperature leads to variations in refractive index and cavity length, which in turn affects the instability of the resonant frequency. Therefore, high-precision temperature control of the on-chip optical reference microcavity is crucial for on-chip laser frequency stability.

[0077] The resonant frequency is caused by the change in temperature T. ν The change can be represented as:

[0078] d ν / dT= - ν ( α n + α l )

[0079] in α n and αl These are the thermal refractive index and thermal expansion coefficient of the dielectric material, respectively. For different resonant modes, the transverse field distribution differs, with energy concentrated in the core layer (refractive index...). n 1) and cladding (refractive index) n 2) The distribution ratios differ. For example, the energy of the fundamental mode is highly concentrated in the core layer, while the energy of higher-order modes leaks into the cladding. Therefore, the effective refractive index of higher-order modes decreases with increasing order. Let Γ (called the confinement factor) be the proportion of optical field energy in the core layer, and the effective refractive index... n eff It can be viewed as a weighted average of the spatial refractive index sensed by the mode electromagnetic field energy:

[0080]

[0081] Accordingly, due to the differences in the constraint factor Γ among the different modes, the effective thermal refractive index of the different modes also differs:

[0082]

[0083] Compared to the thermal refractive index, the thermal expansion coefficient depends on the optical path length of light within the resonant cavity. For an on-chip optical reference microcavity, the optical path lengths of different modes are almost the same, so the thermal expansion coefficients are essentially equal.

[0084] The selection of dual modes is based on choosing a pair of different modes within a free spectral range. In the specific implementation of this invention, TE0 and TE2 modes are selected as the dual modes. In fact, any other mode is included within this invention. For example, selecting one of the TE mode and one of the TM mode, or any combination of two modes of different orders within the TE mode or TM mode, are all within the dual-mode selection range of this invention. When the ambient temperature changes, the resonant frequency of the TE0 mode... ν TE0 Resonant frequency of TE2 mode ν TE2 Dual-mode frequency difference δ = ν TE0 - ν TE2 The change of temperature T can be expressed as:

[0085] d δ / dT≈ - ν ( α n TE0 - α n TE2 )

[0086] in α n TE0 and αn TE2 These are the thermal deflection coefficients for TE0 mode and TE2 mode, respectively.

[0087] When the resonant frequency is within a small fluctuation range (wavelength variation is negligible), the dual-mode frequency difference in the radio frequency band... δ The fluctuations can reflect temperature changes in real time, and are also related to the resonant frequency. ν Presenting proportional relationships:

[0088]

[0089] When the temperature controller on the surface of the on-chip optical reference microcavity is set to different temperatures, the frequency and dual-mode frequency difference of the on-chip laser locked to this pair of modes are measured. The experimental measurement results are as follows: Figure 4 As shown, Figure 4 The measured data and fitting results for the effect of temperature on laser frequency and dual-mode frequency difference are presented, among which... Figure 4 (a) is a schematic diagram of the linear fitting of the laser frequency to the temperature response. Figure 4 (b) is a schematic diagram of the linear fitting of the dual-mode frequency difference to the temperature response. Figure 4 (c) is a schematic diagram of the ratio of laser frequency to dual-mode frequency difference. When the ambient temperature increases from 24℃ to 24.09℃, the laser frequency and dual-mode frequency difference change by 140.92MHz and 0.97MHz, respectively. Linear fitting of the results yields proportionality coefficients of 1.575GHz / K and 10.76MHz / K for the laser frequency and dual-mode frequency difference as a function of temperature, respectively. For an on-chip optical reference microcavity with a free spectral range of FSR, the temperature adjustment value of the microcavity temperature controller reaches... The resonant frequency can then cover the entire frequency band without dead zones, enabling arbitrary reference frequency output. For the 30GHz free spectral range in this embodiment, the corresponding microcavity temperature needs to be adjusted by 19°C, which is easily achieved by the temperature controller.

[0090] Fitting analysis of these two sets of data showed that the change in laser frequency with temperature was 146.28 times the dual-mode frequency difference, and the coefficient of determination R for linear fitting was [missing value]. 2 =0.99982. The good linear fit confirms the feasibility of using the dual-mode frequency difference as a resonant frequency and a sensor for intracavity temperature changes.

[0091] This invention utilizes the difference in effective refractive index temperature dependence of different modes of light field within an on-chip microcavity, and uses the difference in resonant frequency change between modes as a high-precision sensor of the cavity temperature, with a temperature measurement accuracy far exceeding that of external temperature sensors.

[0092] Based on the dual-mode beat frequency locking principle of on-chip microcavities, such as Figure 2As shown, the dual-mode beat frequency measurement and control module includes a beam combiner 15, a third photodetector 16, and a frequency meter 17.

[0093] The beam combiner 15 is used to combine two different modes of laser beams that have been locked to the on-chip optical reference microcavity 5.

[0094] The third photodetector 16 is used to convert the combined optical signal into an electrical signal.

[0095] The frequency meter 17 is used to measure the dual-mode frequency difference between two different modes locked to the on-chip optical reference microcavity 5. According to the dual-mode locking principle, the dual-mode frequency difference can be used as a sensing unit for the reference cavity temperature and the laser resonant frequency. Real-time measurement of the fluctuation of the dual-mode frequency difference can reflect the frequency change of the laser.

[0096] To address the long-term resonant frequency drift caused by thermal noise in the reference cavity, fine-grained slow-feedback control of the cavity temperature is necessary. This is because, due to temperature relaxation, excessively fast feedback can actually worsen short-term stability. The cavity temperature slow-feedback control loop described in this invention includes a Kalman filter 18, a slow-feedback controller 19, and a cavity temperature actuator.

[0097] The Kalman filter 18 is used to filter the real-time measured dual-mode frequency difference to generate a real-time temperature deviation signal that characterizes the real temperature fluctuation of the on-chip optical reference microcavity.

[0098] The slow feedback controller 19 generates a temperature precision control feedback signal for the cavity temperature actuator based on the real-time temperature deviation signal. The cavity temperature actuator finely adjusts the temperature of the on-chip optical reference microcavity, compensates for the environmental temperature disturbance of the on-chip optical reference microcavity, locks the dual-mode frequency difference at a preset value, and realizes precise closed-loop control of the temperature of the on-chip optical reference microcavity.

[0099] In the specific implementation case, the selected actuators are the voltage-controlled adjustable attenuator 7 and the Kalman filter 18. Other actuators and feedback schemes, such as semiconductor coolers and modulators, are all alternatives to this dual-mode beat frequency measurement and control module.

[0100] This invention further utilizes the dual-mode frequency difference sensing principle, considering the hysteresis characteristics of temperature feedback, and designs a slow feedback control loop with a feedback bandwidth of 2Hz. It selects a voltage-controlled adjustable attenuator capable of adjusting the intensity of the laser light entering the reference cavity, and incorporates a Kalman filter to enhance the stability of the module. Specifically, refer to... Figure 3The cavity temperature slow feedback control loop includes a Kalman filter 18, a slow feedback controller 19, and a voltage-controlled adjustable attenuator 7. The voltage-controlled adjustable attenuator 7 is disposed in the optical path from the first on-chip laser or the second on-chip laser to the on-chip optical reference microcavity, and is used to adjust the laser intensity input to the on-chip optical reference microcavity from the first on-chip laser or the second on-chip laser. Figure 2 In the embodiment shown, the voltage-controlled adjustable attenuator 7 is disposed between the second on-chip laser 6 and the second electro-optic phase modulator 8.

[0101] The Kalman filter 18 is used to filter the dual-mode frequency difference measured in real time by the frequency meter 17, generating a real-time temperature deviation signal characterizing the actual temperature fluctuation of the on-chip optical reference microcavity. The slow feedback controller 19 generates a control voltage signal (i.e., a temperature precision control feedback signal) for the voltage-controlled adjustable attenuator 7 based on the real-time temperature deviation signal, finely adjusts the attenuation of the voltage-controlled adjustable attenuator 7, changes the laser intensity input from the second on-chip laser 6 to the on-chip optical reference microcavity 5, and compensates for the ambient temperature disturbance of the on-chip optical reference microcavity 5 through photothermal effect, so that the dual-mode frequency difference is locked at a preset value, realizing precise closed-loop control of the temperature of the on-chip optical reference microcavity 5.

[0102] One of the key aspects of the above embodiments lies in the design of the slow feedback actuator. To control the cavity temperature with the highest possible sensitivity, this invention uses a voltage-controlled adjustable attenuator 7 to control the light intensity input from a single on-chip laser to the on-chip optical reference microcavity 5. The control voltage of the voltage-controlled adjustable attenuator 7 serves as the actuator for the reference cavity temperature feedback control, achieving the effect of regulating the cavity heat. Furthermore, to minimize the increase in cavity thermal noise caused by light intensity changes, the slow feedback control loop employs a 2Hz bandwidth feedback control, sets the feedback control time interval to 0.5s, and performs Kalman filtering on the measured dual-mode frequency difference fluctuation to reduce dynamic measurement errors and minimize overshoot of the feedback system due to temperature relaxation.

[0103] In summary, the entire long-term laser frequency stabilization system based on an on-chip optical reference microcavity comprises two PDH fast feedback loops locking the on-chip laser to the on-chip optical reference microcavity, a slow feedback loop using dual-mode frequency difference as high-precision temperature sensing data to control the temperature within the reference cavity, and an arbitrary laser frequency locking loop. A comparison of the results after free laser frequency operation, locking to the on-chip reference cavity via the PDH feedback loop, and locking based on the high-precision dual-mode temperature feedback loop is shown below. Figure 5 As shown, where Figure 5 (a) is a time-domain result diagram of laser frequency free operation for 10h, PDH locked for 10h, and dual-mode locked for 100000s; Figure 5(b) is a graph showing the Allen variance calculation results. This invention can achieve an Allen variance better than 4×10 over an average time of more than 10,000 s. -10 .

[0104] This invention further enables the locking of an arbitrary output wavelength laser to an on-chip optical reference microcavity after long-term frequency stabilization, achieving multi-wavelength frequency-stabilized laser output. (Refer to...) Figure 3 This is a schematic diagram of a long-term laser frequency stabilization system based on an on-chip optical reference microcavity to achieve multi-wavelength output. The system includes a first on-chip laser 1, a first electro-optic phase modulator 2, a first polarization controller 3, a first circulator 4, an on-chip optical reference microcavity 5, a second on-chip laser 6, a voltage-controlled adjustable attenuator 7, a second electro-optic phase modulator 8, a second polarization controller 9, a second circulator 10, a first photodetector 11, a first servo controller 12, a second photodetector 13, a second servo controller 14, a beam combiner 15, a third photodetector 16, a frequency counter 17, a Kalman filter 18, a slow feedback controller 19, an arbitrary laser 20, a third electro-optic phase modulator 21, an optical filter 22, a fourth photodetector 23, and a third servo controller 24. That is, based on... Figure 2 The system of the embodiment shown achieves long-term frequency stability. By using an arbitrary laser frequency locking module, an arbitrary output wavelength laser is locked to an on-chip optical reference microcavity after long-term frequency stability, thereby achieving frequency-stabilized laser λi output and thus realizing multi-wavelength frequency-stabilized laser output. Figure 3 The system shown is in Figure 2 Based on the existing structure, an arbitrary laser 20, a third electro-optic phase modulator 21, an optical filter 22, a fourth photodetector 23, and a third servo controller 24 are added. The arbitrary laser 20 is connected to the third electro-optic phase modulator 21, which is connected to the first polarization controller 3. The first polarization controller 3 is connected to the first circulator 4. The arbitrary laser 20 is modulated by the third electro-optic phase modulator 21 to obtain a sideband, which is then coupled into the on-chip optical reference microcavity 5 through the first polarization controller 3 and the first circulator 4. The light output from the on-chip optical reference microcavity 5 is filtered by the optical filter 22. One output port of the optical filter 22 is connected to the first photodetector 11, and the other output port is connected to the fourth photodetector 23. The fourth photodetector 23 is connected to the third servo controller 24, which controls the arbitrary laser 20. The arbitrary frequency-stabilized laser output is controlled by adjusting the temperature of the on-chip optical reference microcavity 5 so that the laser frequency falls near the cavity resonant frequency. The laser output from the arbitrary laser 20 is combined with the optical path of the first on-chip laser 1 after passing through the third electro-optic phase modulator 21. The output light from the on-chip optical reference microcavity 5 is filtered at the corresponding frequency by the optical filter 22, and the frequency of the arbitrary laser 20 is locked by the fourth photodetector 23 and the third servo controller 24 to obtain the desired frequency-stabilized laser λi output.

[0105] The TE mode described in this invention is one of the two basic guided modes (TE and TM) in dielectric waveguides, and is one of the most commonly used polarization modes in integrated optics, on-chip microcavities, and laser frequency stabilization. The TE mode, also called the transverse electric mode, has an electric field vector that is completely perpendicular to the transmission direction in the waveguide.

[0106] The dual-mode described in this invention refers to two different optical modes within a free spectral range (FSR) in an optical waveguide. In a specific embodiment of this invention, these modes are classified into different orders based on the number of nodes in the cross-section of the optical field under the same polarization (TE mode). Specifically, they are the fundamental mode TE0 (the mode with the simplest field distribution and no nodes, whose electric field has a single peak distribution in the core layer) and the higher-order mode TE2 (with two nodes in the cross-section, whose electric field has a three-peak distribution in the core layer).

[0107] The on-chip laser frequency stabilization of this invention utilizes a monolithic / heterogeneous integrated micro-nano photonic structure to replace the traditional bulk optical reference cavity and frequency discriminator, thereby achieving laser frequency locking, calibration, and long-term stable output. The goal is to compress narrow-linewidth, low-frequency drift coherent light sources from desktop systems to the chip scale, adapting them to applications such as on-chip optical atomic clocks, coherent optical communication, and precision sensing.

[0108] The Allen bias described in this invention is a statistic used to evaluate the stability of a frequency source. It reflects the frequency fluctuation over an average time period, and the smaller the value, the higher the stability.

[0109] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0110] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this application should be determined by the appended claims.

[0111] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A long-term laser frequency stabilization system based on an on-chip optical reference microcavity, characterized in that, include: Two lasers, namely the first laser and the second laser; On-chip optical reference microcavity, used to provide frequency reference; Two frequency-locking modules, namely the first frequency-locking module and the second frequency-locking module, are used to lock the two lasers to two different modes of the on-chip optical reference microcavity, respectively. The dual-mode beat frequency measurement and control module is used to measure the dual-mode frequency difference between two different modes locked to the on-chip optical reference microcavity in real time. An intracavity temperature slow feedback control loop is used to generate a temperature fine-control feedback signal based on the dual-mode frequency difference. This signal is used to finely adjust the intracavity temperature of the on-chip optical reference microcavity to compensate for changes in the optical cavity length caused by thermo-optical effects and thermal expansion, thereby stabilizing the resonant frequency of the on-chip optical reference microcavity and ensuring long-term frequency stability of the laser after the two frequency-locking modules are locked. The intracavity temperature slow feedback control loop includes a Kalman filter, a slow feedback controller, and an intracavity temperature actuator. The Kalman filter filters the real-time measured dual-mode frequency difference to generate a real-time temperature deviation signal characterizing the actual temperature fluctuation of the on-chip optical reference microcavity. The slow feedback controller generates a control signal for the intracavity temperature actuator based on the real-time temperature deviation signal. The actuator adjusts the temperature of the on-chip optical reference microcavity to compensate for environmental temperature disturbances, locking the dual-mode frequency difference at a preset value, thus achieving high-precision closed-loop control of the on-chip optical reference microcavity temperature.

2. The long-term laser frequency stabilization system based on an on-chip optical reference microcavity according to claim 1, characterized in that, The on-chip optical reference microcavity also includes a microcavity temperature coarse adjustment control module, which is used to regulate the operating environment temperature of the on-chip optical reference microcavity to the target range, thereby achieving coarse adjustment of the resonant frequency of the on-chip optical reference microcavity.

3. The long-term laser frequency stabilization system based on an on-chip optical reference microcavity according to claim 1 or 2, characterized in that, It also includes an arbitrary laser frequency locking module, which is used to lock an arbitrary output wavelength laser to an on-chip optical reference microcavity after the frequency has been stabilized for a long time, so as to realize multi-wavelength frequency-stabilized laser output.

4. The long-term laser frequency stabilization system based on an on-chip optical reference microcavity according to claim 3, characterized in that, The arbitrary laser frequency locking module includes at least one arbitrary laser, each arbitrary laser corresponds to a third frequency locking module, and each arbitrary laser is locked to the on-chip optical reference microcavity through the corresponding third frequency locking module to output a frequency-stabilized laser.

5. The long-term laser frequency stabilization system based on an on-chip optical reference microcavity according to claim 1, 2, or 4, characterized in that, The selection of the two different modes is based on selecting a pair of different modes within a free spectral range, and the frequency difference between the two different modes is less than 10 GHz.

6. The long-term laser frequency stabilization system based on an on-chip optical reference microcavity according to claim 5, characterized in that, The two different modes differ in at least one parameter: mode type, order, or polarization.

7. The long-term laser frequency stabilization system based on an on-chip optical reference microcavity according to claim 1, 2, 4, or 6, characterized in that, Both the first laser and the second laser are on-chip lasers, and the frequency locking module is one of the following: a laser locking module based on PDH frequency stabilization technology, a laser locking module based on tilt locking, a laser locking module based on edge locking, a phase detection locking module based on a lock-in amplifier, and a laser locking module based on self-injection locking.

8. The long-term laser frequency stabilization system based on an on-chip optical reference microcavity according to claim 7, characterized in that, The frequency locking module is a laser locking module based on PDH frequency stabilization technology, which includes an electro-optic phase modulator, a polarization controller, a circulator, a photodetector, and a servo controller. The electro-optic phase modulator is used to generate modulation sidebands; The polarization controller is used to control the laser polarization state; The circulator is used to guide the laser into and out of the on-chip optical reference microcavity; A photodetector is used to monitor the output light field of the on-chip optical reference microcavity and convert it into an electrical signal output to the controller. The servo controller is used to demodulate the electrical signal to obtain a DC error signal proportional to the deviation between the laser and the reference microcavity resonant frequency, and to provide real-time feedback control to the on-chip laser to achieve laser frequency locking.

9. The long-term laser frequency stabilization system based on an on-chip optical reference microcavity according to claim 1, 2, 4, 6, or 8, characterized in that, The slow feedback control loop employs low-frequency feedback control with a bandwidth on the order of Hz.

10. The long-term laser frequency stabilization system based on an on-chip optical reference microcavity according to claim 1, characterized in that, The intracavity temperature actuator is a voltage-controlled adjustable attenuator, which is set in the optical path from the first on-chip laser or the second on-chip laser to the on-chip optical reference microcavity, and is used to adjust the laser intensity input to the on-chip optical reference microcavity from the first on-chip laser or the second on-chip laser. The slow feedback controller generates a control voltage signal for the voltage-controlled adjustable attenuator based on the real-time temperature deviation signal, adjusts the attenuation of the voltage-controlled adjustable attenuator, changes the laser intensity input from the first on-chip laser or the second on-chip laser to the on-chip optical reference microcavity, and compensates for the ambient temperature disturbance of the on-chip optical reference microcavity through photothermal effect, so that the dual-mode frequency difference is locked at a preset value, thereby realizing precise closed-loop control of the temperature of the on-chip optical reference microcavity.

11. The long-term laser frequency stabilization system based on an on-chip optical reference microcavity according to claim 1, 2, 4, 6, 8, or 10, characterized in that, The dual-mode beat frequency measurement and control module includes a beam combiner, a photodetector, and a frequency meter; The beam combiner is used to combine two different modes of laser beams that have been locked to the on-chip optical reference microcavity; The photodetector is used to convert the combined optical signal into an electrical signal. The frequency meter is used to measure the dual-mode frequency difference between two different modes locked to the on-chip optical reference microcavity.

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