Device and method for high-speed tuning of soliton micro-comb

Abstract

A device and method for high speed tuning of soliton micro-combs includes an on-chip high Q lithium niobate (LN) micro-resonant cavity as a comb resonant cavity whose dispersion is designed for soliton comb generation in which a strong electro-optic Pockels effect is used to dynamically tune soliton repetition rate, electro-optic tuning and modulation elements are integrated directly into the comb resonant cavity.

Description

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0001] This invention was made with government support under Department of Defense grant HDTRA11810047, Defense Advanced Research Projects Agency grant HR001-20-2-0044, and National Science Foundation grants ECCS-1810169, ECCS-1842691, and OMA-2138174. The government has certain rights in this invention. CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to and the benefit of U.S. Provisional Patent Application Serial No. 63 / 341,632, filed on May 13, 2023, entitled “Device and Method for High-Speed ​​Tuned Soliton Microcomb,” the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present application relates to comb generation, and more particularly to soliton comb generation. Background Art

[0004] Spectrally pure microwaves are essential for many applications ranging from wireless communications, radar, imaging, clocks to high-speed electronics. Highly coherent microwaves can be generated using various photonic techniques such as optoelectronic oscillators, dual-frequency lasers, Brillouin lasers, etc. Among them, phase-locked Kerr frequency combs generated on a monolithic chip soliton combs hold great promise due to their excellent coherence properties. The superior coherence of soliton microcombs has led to various applications such as optical communications, spectral sensing, distance measurement, optical frequency synthesis, neuromorphic computing, etc. Recently, there has been a strong interest in developing soliton microcombs with repetition rates in the radio and microwave frequency range, which have the potential for microwave synthesis on an integrated chip.

[0005] High-speed frequency tuning and modulation of microwaves are crucial for a variety of important applications, such as ranging, communication, imaging, gesture recognition, etc. The repetition rate of the current soliton microcomb is fundamentally determined by the physical size of the monolithic comb resonator, which cannot be changed after the device is fabricated. Therefore, tuning of the microwave frequency must rely on external methods, such as relatively slow external laser modulation or external comb modulation with rather limited efficiency. Summary of the invention

[0006] Coherent microwaves with rapidly tunable frequencies are the basis for many important applications, ranging from sensing, imaging, ranging, timing, wireless communications to high-speed electronics. Soliton microcombs have excellent coherence properties and show great promise for synthesizing microwaves on a chip. However, due to the monolithic nature of the underlying comb resonator, its repetition rate exhibits rather limited tunability.

[0007] In an embodiment, the first microwave frequency soliton microcomb with high-speed data repetition rate tuning is provided. By integrating the electro-optic tuning / modulation elements directly into the lithium niobate comb microresonator, a frequency modulation speed of up to 75 MHz is achieved, which is several orders of magnitude faster than other soliton comb devices reported to date, and a frequency of up to 1.0x10 15 Hz / s frequency modulation rate, which is even faster than the most advanced electronic microwave frequency modulation techniques. The device provides a remarkable bandwidth of up to tens of GHz for locking the repetition rate to an external microwave reference, thereby achieving direct injection locking and feedback locking of the comb resonator itself without involving external modulation. The demonstrated fully integrated electro-optical reconfigurable soliton micro-comb now opens a great path for high-speed dynamic control and processing of microwaves, which is expected to have a profound impact on the wide range of applications of microwave photonics.

[0008] In an exemplary embodiment, a device and method for high-speed tuning of a soliton microcavity includes an on-chip high-Q lithium niobate (LN) microresonator as a comb resonator whose dispersion is designed for soliton comb generation, wherein a strong electro-optic Pockels effect is used to dynamically tune the soliton repetition rate, while the electro-optic tuning and modulation elements are directly integrated into the comb resonator.

[0009] The repetition rate of the microwave soliton microcomb can be tuned at high speed.

[0010] By exploiting the strong electro-optical Pockels effect of LN and integrating the electro-optic tuning and modulation components directly into the LN comb resonator, we achieve frequency modulation speeds up to 75 MHz, which is several orders of magnitude faster than other soliton comb devices reported to date, and high-frequency modulation of up to 1.0 × 10 15 Hz / s frequency modulation rate, faster than even the most advanced electronic microwave FM technology.

[0011] The device can exhibit a modulation efficiency of about 2 MHz / V, which can be further improved by a factor of six when all three sets of driving electrodes are employed.

[0012] The demonstrated device can provide considerable bandwidth (up to tens of GHz) for feedback locking the repetition rate to an external reference source, thereby achieving direct injection locking and feedback locking of the comb resonator itself without involving external modulation.

[0013] Low frequency phase noise can be directly suppressed to the phase noise of the reference source.

[0014] The demonstrated devices and methods perfectly combine the excellent coherence of soliton microcombs with high-speed dynamic modulation, opening up a great path for electro-optical processing of coherent microwaves, and are expected to be widely used in microwave synthesis, timing, ranging, 5G / 6G communications and other fields.

[0015] Direct electro-optical (EO) tuning and modulation of soliton comb lines is expected to have a profound impact on general photonic applications of soliton microcombs, including frequency metrology, optical frequency synthesis, lidar, and optical communications. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The features of the present application may be better understood with reference to the accompanying drawings and claims described below. The accompanying drawings are not necessarily drawn to scale, and emphasis is generally placed on illustrating the principles described herein. In the accompanying drawings, the same numerals are used to indicate the same components in various views.

[0017] Figure 1 is a diagram of a high-speed tunable microwave soliton device according to one or more exemplary embodiments of the present invention.

[0018] Figure 2 According to one or more exemplary embodiments of the present invention Figure 1 A view of the device’s LN die.

[0019] Figures 3A-3C According to one or more exemplary embodiments of the present invention Figure 1 and Figure 2 View of the device’s drive electrodes and resonant cavity waveguide.

[0020] Figure 4 is a view showing a spectrum of a soliton comb with a repetition rate of 200 GHz for a device according to one or more exemplary embodiments of the present invention.

[0021] Figure 5A and 5B is a view showing a spectrum of a soliton comb with a repetition rate of 200 GHz for a device according to one or more exemplary embodiments of the present invention.

[0022] Figures 6A-6C is a view showing a spectrum of a soliton comb with a repetition rate of 19.8 GHz for a device according to one or more exemplary embodiments of the present invention.

[0023] Figures 7A-7C is a view showing a spectrum of a soliton comb with a repetition rate of 13.5 GHz for a device according to one or more exemplary embodiments of the present invention.

[0024] Fig. 8A is a diagram illustrating the electro-optic tuning performance of a comb resonator of a device according to one or more exemplary embodiments of the present invention.

[0025] Figure 8B is a diagram illustrating an electro-optic tuning modulation response of a comb resonator of a device according to one or more exemplary embodiments of the present invention.

[0026] Figures 9A-9Cis a frequency modulation diagram showing a soliton comb repetition rate at a modulation frequency of 1 MHz according to one or more exemplary embodiments of the present invention.

[0027] Figures 10A-10C Frequency modulation of the soliton comb repetition rate at a modulation frequency of 10 MHz according to one or more exemplary embodiments of the present invention is shown.

[0028] Figures 11A-11C Frequency modulation of the soliton comb repetition rate at a modulation frequency of 50 MHz is shown according to one or more exemplary embodiments of the present invention.

[0029] Figures 12A-12C Frequency modulation of the soliton comb repetition rate at a modulation frequency of 75 MHz is shown according to one or more exemplary embodiments of the present invention.

[0030] Fig.13 A schematic diagram of an experimental test for locking the soliton repetition rate of a device to a reference microwave source according to one or more exemplary embodiments of the present invention is shown.

[0031] 14A is a diagram showing a user interface according to one or more exemplary embodiments of the present invention. Fig.13 Schematic view of the electrical modulation response of the comb resonator of the evaluated device.

[0032] 14B is a diagram showing a user interface according to one or more exemplary embodiments of the present invention. Fig.13 Schematic diagram of the evaluation device with a view of the detected phase noise spectrum of the comb resonator. DETAILED DESCRIPTION

[0033] In an exemplary embodiment, the present invention relates to a microwave soliton microcomb whose repetition rate can be tuned at speeds up to, for example, 75 MHz, which is several orders of magnitude faster than conventional soliton microcomb sources. The device has a repetition rate of about 1×10 15 The device provides a remarkable frequency modulation (FM) rate of Hz / s, greater than even the most advanced electronic microwave FM technology, and significantly higher phase noise performance. In addition, the device provides a greater bandwidth for locking the microwave frequency directly to an external reference source, without the bandwidth limitations of current locking methods that rely on external laser modulation. The high-speed dynamic control and processing capabilities of microwaves provided by the microcomb of the exemplary embodiment provide greater potential for a wide range of applications in microwave photonics.

[0034] Reference now Figure 1, showing an illustrative embodiment of the systems and devices of the present invention. The underlying device 10 is an on-chip high-Q lithium niobate (LN) microresonator 12 whose dispersion structure is used for soliton comb generation. The device platform is capable of self-starting and bidirectional switching of soliton states with the help of photorefractive effects. LN also exhibits a strong electro-optical Pockels effect. Therefore, in an exemplary embodiment, the present invention exploits this effect to dynamically tune the soliton repetition rate by integrating electro-optic tuning and modulation elements directly into the comb resonator. The LN resonator acts as both a soliton comb generator and a high-speed electro-optical (EO) modulator. The fully integrated device not only enables high-speed tuning of the soliton repetition rate, but also provides huge bandwidth for locking the repetition rate to an external microwave reference through direct feedback to the comb resonator itself.

[0035] In one illustrative embodiment, the device is a dispersion engineered high quality microring resonator 12 on a lithium niobate platform 14 for the generation of soliton combs. A drive electrode 16 is directly integrated with the comb generation resonator and is used to modulate the resonator at high speed via the electro-optical Pockels effect. A coupling waveguide 18 couples the pump laser into the comb resonator and generates a soliton comb output from the comb resonator. RF / microwaves from an external microwave source 20 are applied to the drive electrode to electro-optically modulate the comb resonator. This can work in two ways: a) it can produce high speed modulation of the repetition rate of the soliton comb, thereby frequency modulation of the generated microwaves; and / or it can be used to injection lock or feedback lock the soliton repetition rate to an external reference microwave source. In addition, the detected microwaves (from an optical detector) can be applied to the drive electrode to form self-injection locking of the soliton comb, thereby forming a self-sustaining coherent microwave oscillator.

[0036] Figure 2 An example of an LN chip fabricated on a z-cut LN-on-insulator (LNOI) wafer platform is shown. Figures 3A-3C The micro-ring resonator, pulley bus waveguide and driving electrode ( Figure 1 ) is a device composed of a LN microring resonator. In an embodiment, the ring resonator is designed to have a waveguide width of, for example, 2.2 microns, which generates a group velocity dispersion of about -0.035 ps2 / m in the telecommunications band for the fundamental quasi-transverse electric (quasi-TE) mode family suitable for soliton generation. The electrodes are placed along the ring resonator waveguide with an electrode-waveguide spacing of about 4 microns to optimize the electro-optic tuning / modulation efficiency without affecting the optical Q of the resonator. The electrodes are designed to be 525 nm thick and 5 microns wide to support high-speed modulation. The intrinsic optical Q of the LN microring resonator is about 4 million. The optical Q value remains unchanged for resonators of different sizes with radii up to 1.5 mm.

[0037] The repetition rate fr of the soliton microcomb is mainly determined by the resonator size. By changing the radius of the ring resonator from 100 μm to 450 μm, fr is reduced from 200 GHz to 44.84 GHz, as shown in Figure 1 and Figure 2 As shown. However, further increase of cavity size is not trivial due to interference from stimulated Raman scattering. LN exhibits rich Raman scattering properties and has recently been shown to introduce interesting self-frequency shifts (SFS) on Kerr solitons. However, it can also stimulate Raman lasing when the Stokes frequency matches the cavity resonance. For cavities with radius ≥ 1 mm, the free spectral range is small enough (≤ 20 GHz) that Raman lasing is inevitable, which interferes with the generation of solitons. For the z-cut cavity used in this paper, the main interference comes from the A 1 (TO 4 ) phonon mode-related Raman frequency shift is a Stokes wave of 19 THz. To address this issue, the pulley bus waveguide is designed to be close to critical coupling at a pump wavelength near 1550 nm and overcoupled at the Raman Stokes wavelength near 1720 nm. As a result, Raman lasing can be suppressed and soliton combs can be easily generated.

[0038] Figure 4 -7 shows exemplary microcombs with different repetition rates. More specifically, microcombs with 200 GHz (Fig. 5), 44.84 GHz ( Figure 5A and 5B )、19.8GHz( Figures 6A-6C ) and 13.5GHz( Figures 7A-7C ) repetition rate of the soliton comb, they are respectively in the 100μm ( Figure 4 ), 450 μm (Table 5A and Figure 5B )、1020μm(see Figures 6A-6C ) and 1500μm (with Figures 7A-7C ) radius, and the on-chip powers are 33mW, 396mW, 282mW and 400mW respectively. Figure 5B Shown is the detected microwave spectrum from a 44.84 GHz soliton comb. Figure 6B and 6C Depicted by Fig. 6A Shown is the detected 19.82 GHz microwave spectrum and phase noise generated by the soliton comb. Fig. 7A and 7B Depicted by Fig. 7A The 13.5 GHz microwave spectrum and phase noise of the soliton comb generated and detected are shown. Figure 6B and 7BThe resolution bandwidth (RBW) of the RF spectrum is 200 Hz. In all embodiments, the device is operated without active feedback and the pump laser detuning is stabilized by the photorefractive effect.

[0039] For example, Fig. 6A and 7A Two examples of soliton microcavities with fr of 19.82 GHz and 13.5 GHz are shown, respectively. The slight spectral distortion near 1580 nm is caused by the side effect of insufficient coupling of the designed bus waveguide at this wavelength. However, this side effect does not affect the integrity of the generated Kerr solitons, and the coherence of the soliton comb is determined by Figure 6B and 7B The detected microwaves shown in the figure clearly show that both the 19.82 GHz and 13.5 GHz microwaves exhibit a signal-to-noise ratio greater than 70 dB. Figure 6C and 7C As shown in Figure 1, the microwave phase noise is about -40dBc / Hz at 1kHz, reaches -110dBc / Hz at 10kHz, and finally drops to below -130dBc / Hz at 3MHz. This phase noise level is comparable to that recently demonstrated on other on-chip platforms. The spectral bump around 4kHz is likely due to the frequency noise of the pump laser.

[0040] In order to dynamically tune and modulate microwave solitons, such as Figure 1 As shown, the EQ tuning / modulation components are directly integrated on the comb resonator. For the z-cut resonator, LN's R22 electro-optic tensor element is used to tune the fundamental quasi-TE mode. In particular, for the circular ring resonator, a detailed analysis shows that the EQ tuning efficiency can be maximized by three sets of driving electrodes, each containing two pairs of signal-ground electrodes with a span of 60 degrees. Figures 3A-3C The detailed device layout is shown in , where only two sets of electrodes are fabricated to avoid interference with the coupled bus waveguides (see also Figure 1 ). To simplify the experimental tests, only one set of electrodes for EQ tuning and modulation was used, but the integrated EQ tuning can still achieve a fairly good tuning efficiency of 0.34pm / V for the cavity resonance, Fig. 8A and 8B An example is shown in . Fig. 8A DC tuning of the cavity resonance is shown. Figure 8B The electro-optic modulation response S of the comb resonator is shown. 21 The same figure also shows the EQ modulation response of the comb device, which indicates a 3dB bandwidth of about 61MHz corresponding to the photon lifetime limit of the comb resonator.

[0041] The broadband EO response of this device enables high-speed control of the soliton microcomb. To characterize this feature, a sinusoidal electrical signal was applied to the 19.81 GHz comb resonator and the frequency of the probe wave was monitored. Figures 9A-9C The frequency modulation of the soliton comb repetition rate at a modulation frequency of 1 MHz is shown. Figures 10A-10C The frequency modulation of the soliton comb repetition rate at a modulation frequency of 10 MHz is shown. Figures 11A-11C The frequency modulation of the soliton comb repetition rate at a modulation frequency of 50 MHz is shown. Figures 12A-12C The frequency modulation of the soliton comb repetition rate at a modulation frequency of 75 MHz is shown. Fig. 9A , 10A , 11A and 12A, the detected microwave spectra are depicted. Fig. 9B , 10B , 11B and 12B, the frequency spectrum relationship between frequency and time is depicted. Fig. 9C , 10C , 11C and 12C, the frequency curves varying with time, i.e., the average trajectory of the frequency vs. time spectrum, are depicted. In all embodiments, the device operates freely without active feedback, and the pump laser cavity detuning is self-stabilized by the photorefractive effect. More specifically, as shown, the sinusoidal EO drive produces direct sinusoidal frequency modulation (FM) of the microwaves. At a modulation frequency of 1 MHz ( Figures 9A-9C ), the peak drive voltage VP = 0.76V produces an FM amplitude of 41.8kHz, which corresponds to an FM efficiency of 55kHz / V. The FM efficiency increases significantly with the increase of modulation frequency. At 10MHz ( Figures 10A-10C ) and 50MHz( Figures 11A-11C ) reaches 463kHz / V and 824kHz / V. Figures 12A-12C As shown, microwaves are modulated at frequencies up to 75 MHz, where a drive voltage VP = 3.0 V produces an FM amplitude of 3.45 MHz, corresponding to an FM efficiency of 1.15 MHz / V. The blurring of the time-spectrum is simply due to the limited bandwidth (160 MHz) of the spectrum analyzer (Tektronics RSA5126B) for time-correlated frequency characterization. For the same reason, time-correlated frequency analysis may underestimate the FM amplitude because it only captures the first-order modulation sidebands as the modulation frequency is so high. Figures 10A-10C The microwave spectra shown in each infer an FM amplitude of about 6.0 MHz, which means an FM efficiency of about 2.0 MHZ / V.

[0042] However, with further increase in modulation frequency, the FM efficiency drops, simply due to the photon lifetime limitation of the resonant cavity. On the other hand, the frequency modulation rate increases with modulation frequency, reaching a value of about 1.0×1015 Hz / s at a modulation frequency of 75 MHz. This FM rate is several orders of magnitude faster than other soliton comb devices. It is even faster than electronic microwave frequency modulation techniques developed so far, while the soliton combs here offer a phase noise that is more than 20 dB lower than their electronic counterparts.

[0043] One mechanism responsible for the observed microwave FM is the Raman-induced SFS of solitons, whose amplitude depends on the laser cavity detuning. The EO modulation of the comb resonator modulates the laser cavity detuning of the pump wave, which in turn changes the amplitude of the SFS and thus the carrier frequency of the Kerr solitons. This shift in the soliton carrier frequency will translate into a change in the free spectral range due to the group velocity dispersion of the resonator, leading to a modulation of the repetition rate. As shown in the SI, this mechanism explains the FM efficiency of ~(100-200) kHz / V, which well explains the phenomenon observed at low modulation frequencies. However, the FM efficiency observed at high modulation frequencies of 50 and 75 MHz is much larger than this value. The underlying reason may be related to the speed of the EO modulation, which is comparable to the photon lifetime in the resonator and the cavity resonance can no longer follow the EO modulation adiabatically.

[0044] The broadband EO response of the device will provide huge bandwidth for locking the soliton repetition rate to an external microwave reference. Fig.13 The locking of the soliton repetition rate to a reference microwave source is shown. Schematic diagram of the experimental test setup including an EDFA (erbium-doped fiber amplifier), a FBG (fiber Bragg grating filter) and an amplifier (electrical amplifier). The soliton repetition rate is locked to an external reference microwave source by two different methods. In the first "direct locking" method, the external microwave directly drives the comb resonator. In the second method, called "RF feedback locking", the detected microwaves are compared with an external reference microwave and the error signal is used to feedback lock the comb resonator.

[0045] The drive electrodes are designed to support modulation speeds significantly beyond the lifetime limit of the photons in the resonant cavity. Fig.14A and 14B , where the resonantly enhanced EO response is clearly visible at modulation frequencies of 19.81 and 39.62 GHz, which are equal to one and two times the free spectral range, and comb sidebands can be generated by driving the comb resonator at a frequency of 19.81 GHz. The device provides an EO bandwidth of up to tens of gigahertz for locking the soliton repetition rate, which is several orders of magnitude larger than other methods such as piezoelectric actuation. Through this broadband modulation response of the comb resonator, the soliton repetition rate is locked in two ways. On the one hand, in Fig.13During the soliton generation process, a 19.81 GHz reference microwave is directly "directly locked" to the comb resonator. The generated EO comb can trigger the generation of solitons and potentially stabilize its repetition rate. This approach is similar to the injection locking method, but the reference microwave is now fed directly to the comb resonator itself, rather than through external modulation on the pump laser. Fig. 14B As shown by the yellow curve in , this direct locking method is able to suppress the phase noise by about 40 dB in the frequency range of 1 Hz-1 kHz. In an alternative, a feedback loop approach can be adopted by comparing the detected microwaves with reference microwaves and applying the error signal to feedback lock the pump laser cavity detuning. Here, this feedback locking can be applied directly to the comb resonator instead of tuning the pump laser frequency externally. Fig. 14B As shown in the red curve, this feedback locking method can almost completely suppress the phase noise to the phase noise of the reference microwave in the frequency range of 1Hz-3kHz. The residual peak around 10kHz is mainly due to the bandwidth limitation of the servo unit.

[0046] Considering the huge bandwidth provided by feedback-locked devices (up to tens of GHz, see Fig.14A and 14B ), the microwave phase noise performance can be further improved. The current comb resonator chip is unpackaged, where the LN chip and the lens fiber ( Fig.13 ) are placed on a separate stage, whose mechanical vibrations will be converted into fluctuations in the pump power launched into the comb resonator. When the device is free running and direct RF locking is implemented, this disturbance mainly causes phase noise of low-frequency microwaves ( Fig. 14B , blue and yellow curves). In order to mitigate its impact on feedback locking, a proportional-integral servo controller was implemented, but this controller limits the operating bandwidth of the feedback lock. By properly packaging the chip [21,23], the servo unit can be removed and the full bandwidth of the device can be used for feedback locking, which is expected to significantly improve the phase noise performance over a wide bandwidth. On the other hand, the soliton microcomb exhibits a certain "quiet point", near which the phase noise of the microwave can be significantly suppressed due to the recoil effect between the soliton SFS and the dispersive waves generated by mode crossing. This approach can be implemented in the current design to further improve the phase noise performance of the LN soliton microcomb.

[0047] In an exemplary embodiment, the device targets microwave soliton microcombs whose repetition rate can be tuned at high speed. By exploiting the strong electro-optical Pockels effect of LN and integrating the electro-optical tuning and modulation components directly into the LN comb resonator, frequency modulation speeds of up to 75 MHz are achieved, which is several orders of magnitude faster than other soliton comb devices reported to date, with frequency modulation rates up to 1.0×1015 Hz / s, even faster than the state-of-the-art electronic microwave FM technology. The modulation efficiency of the device is about 2 MHz / V, which can be further improved by a factor of six by using three sets of driving electrodes. The demonstrated device provides a significant bandwidth (up to tens of GHz) for feedback locking of the repetition rate to an external reference source, achieving direct injection locking and feedback locking to the comb resonator itself without involving external modulation. In this way, low-frequency phase noise is suppressed directly to the phase noise of the reference source. The demonstrated devices and methods perfectly combine the excellent coherence of soliton microcombs with high-speed dynamic modulation, opening up a great path for electro-optical processing of coherent microwaves, which is expected to be widely used in microwave synthesis, timing, ranging, 5G / 6G communications, etc. In addition, in addition to microwave processing, the direct electro-optical tuning and modulation of soliton comb lines is expected to have a profound impact on general photonic applications of soliton microcombs, including frequency metrology, optical frequency synthesis, lidar, and optical communications.

[0048] In an exemplary embodiment, the device can be fabricated on a 610nm z-cut LN-on-insulator (LNOI) wafer. The ring resonator and waveguide structures are defined by etching away about 410nm with the help of Ar ion milling using ZEP520a as a photoresist via a first electron beam lithography. After removing the ZEP520a residue, 525nm Au electrodes are patterned using PMMA as a photoresist via a second exposure and deposited by an electron beam evaporator. An overnight stripping process is used to remove PMMA and unwanted Au.

[0049] Design of pulley coupling waveguide

[0050] In other exemplary embodiments, a pulley-coupled waveguide is provided whereby the resonant cavity is close to critical coupling near a pump wavelength of 1550 nm, but is strongly overcoupled at a Raman Stokes wavelength of around 1720 nm. The effective refractive index of the waveguide is modeled using the finite element method using COMSOL, and the coupling conditions of the bus waveguide can be simulated using coupled mode theory. Detailed modeling shows that the desired coupling conditions can be obtained with a bus waveguide width of 1.765 μm, a pulley angle of 10 degrees, and a constant gap of 300 nm between the bus waveguide and the ring resonant cavity in the pulley coupling region. Figure 5 shows the simulated ratio of the external coupling Q (Qex) to the intrinsic optical Q (Q0). It shows that this pulley waveguide design is able to achieve near critical coupling at a pump wavelength of 1550 nm, but is strongly overcoupled at a Raman Stokes wavelength of around 1720 nm. A side effect of this design is the appearance of a coupled resonance near a wavelength of 1580 nm, where the resonant cavity is deeply undercoupled. This side effect is Fig. 7A and 7C The cause of the spectral distortion of the soliton comb shown. The side effect can be eliminated by further optimizing the pulley waveguide design.

[0051] It should be understood that the above disclosed variants and other features and functions, or alternatives thereof, may be combined into many other different systems or applications. Various currently unforeseeable or unanticipated alternatives, modifications, changes or improvements therein may be subsequently modified by those skilled in the art, and these alternatives, modifications, changes or improvements are also intended to be covered by the appended claims.

Claims

1. A microwave soliton microcomb for high-speed tuning of soliton microcombs, comprising an on-chip high-Q lithium niobate (LN) microresonator as a comb resonator, whose dispersion is designed for soliton comb generation, wherein the soliton repetition rate is dynamically tuned using a strong electro-optical Pockels effect by directly integrating electro-optic tuning and modulation elements into the comb resonator.

2. The microwave soliton micro-comb according to claim 1, wherein the repetition rate can be adjusted at high speed.

3. The microwave soliton microcomb according to claim 1, in, By exploiting the strong electro-optical Pockels effect of LN and by directly integrating the electro-optic tuning and modulation components into the LN comb resonator, a frequency modulation speed of up to 75 MHz and a frequency modulation rate of up to 1.0 × 1015 Hz / s are achieved.

4. The microwave soliton microcomb according to claim 1, in, The device exhibits a modulation efficiency of about 2 MHz / V.

5. The microwave soliton microcomb according to claim 1, in, The device shown provides significant bandwidth (up to tens of GHz) for feedback locking the repetition rate to an external reference source, enabling direct injection locking and feedback locking of the comb resonator itself without involving external modulation.

6. The microwave soliton microcomb according to claim 1, in, The low frequency phase noise is suppressed directly to the low frequency phase noise of the reference source.

7. The microwave soliton microcomb according to claim 1, in, The devices and methods described include soliton microcombs with high-speed dynamic modulation.

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