Abstract
Since its invention, optical frequency comb has revolutionized a broad range of subjects from metrology to spectroscopy. The recent development of microresonatorbased frequency combs (microcombs) provides a unique pathway to create frequency comb systems on a chip. Indeed, microcombbased spectroscopy, ranging, optical synthesizer, telecommunications and astronomical calibrations have been reported recently. Critical to many of the integrated comb systems is the broad coverage of comb spectra. Here, microcombs of more than twooctave span (450 nm to 2,008 nm) is demonstrated through χ^{(2)} and χ^{(3)} nonlinearities in a deformed silica microcavity. The deformation lifts the circular symmetry and creates chaotic tunneling channels that enable broadband collection of intracavity emission with a single waveguide. Our demonstration introduces a new degree of freedom, cavity deformation, to the microcomb studies, and our microcomb spectral range is useful for applications in optical clock, astronomical calibration and biological imaging.
Introduction
Microresonatorbased frequency comb (microcomb) uses Kerr nonlinearity to create parametric gain and offset cavity loss^{1}. It has been demonstrated in various material platforms including silica^{2}, CaF_{2} (ref. ^{3}), MgF_{2} (ref. ^{4}), highindex silica^{5}, silicon nitride^{6}, AlN^{7}, diamond^{8}, and LiNbO_{3}^{9}. The recent development of dissipated Kerr soliton microcombs^{10,11,12,13,14,15,16,17} has enabled applications ranging from spectroscopy to astronomy calibration^{18,19,20,21,22,23,24,25}. The microcomb also provides a novel platform for nonlinear physics studies and has led to observations of Stokes solitons^{26}, soliton crystals^{27}, and soliton interactions^{28}. In these demonstrations, the microcombs are generated in either circularsymmetric whisperinggallerymode (WGM) cavities or waveguidemode cavities, and are coupled out by phasematched, evanescent couplers. The efficiency of the coupler drops significantly at wavelengths far from that of the pump laser. The same challenge is also imposed on microcavity harmonic generation, but it is often overcome by adding additional coupler^{29}. However, this resolution does not directly applied to microcombs with broad spectral coverage. Complicated coupler has recently been designed and tested for octavespanning microcombs^{30,31}.
Asymmetric (deformed) cavity, first proposed by Nöckel and Stone^{32}, can output emissions from whisperinggallery modes to free space without using evanescent couplers. Its applications in directional laser^{33}, suppression of laser spatiotemporal instabilities^{34}, light storage^{35}, as well as single nanoparticle biosensing^{36} have been demonstrated. Generally, ray dynamics in a billiard together with the dynamic tunneling in the momentum space provides a versatile tool to understand the interesting phenomena in asymmetric microcavities^{37,38}. Recently, it was revealed that the chaotic channels in the deformed cavity can assist broadband momentum transformation^{39}, opening up new possibilities to resolve the challenge in outputting broadband nonlinear emissions from microcavities.
In this Article, we report a microcomb spanning from 450 to 2008 nm in a deformed cavity (Fig. 1a). The microcomb is pumped near 1550 nm, and its frequency range is extended by nonlinearities^{40,41,42,43,44} of intracavity χ^{(3)} and symmetrybreaking induced χ^{(2)} (ref. ^{45}) (Fig. 1c). Importantly, to output the broadband emission, the microcavity is deformed slightly to create a chaotic tunneling channel. Because of the chaotic motion, the angular momentum of light is not conserved, and it changes with time, covering a broad range of values (Fig. 1b). Once the angular momenta of the comb reach the corresponding critical lines of total internal reflections, the comb can be effectively collected by a nanofiber through nearly wavelengthindependent refraction process^{39}.
Results
Twooctavespanning microcomb
The silica microtoroid resonator^{46} used in this experiment has principal (minor) diameter of 75 μm (4.2 μm) and its Qfactor is 1.6 × 10^{7} at 1550 nm. The microcavity is weakly deformed to create chaotic channels and the minimum diameter is 5.6% smaller than the maximum diameter. The details of the deformed cavity boundary shape are included in the “Methods” section. A tunable laser at 1550 nm is used as the pump. Its power is amplified to 1.6 W and is coupled into the cavity through a 720nmdiameter nanofiber.
The optical spectrum of the microcomb is presented in Fig. 2a. The frequency span of the microcomb ranges from 150 to 670 THz. An optical spectrum analyzer (OSA, Yokogawa) is used to measure the spectrum above 1200 nm (shown in red), while the shorter wavelength part (shown in blue) is measured by a spectrograph (Andor). The power of the OSAs are calibrated at 1550 and 635 nm. The selection of the calibration wavelength is constrained by the collection of laser sources in our lab. The power mismatch at 1200 nm is likely to come from the variation of OSA’s efficiency versus wavelength. The noise floor of the spectrograph is also well characterized. The violet and green emission from the microtoroid can be observed directly by a CCD camera set on the side of the microtoroid (Fig. 2a, inset). The zoomin spectra of the microcombs at 378.3–389.4, 480–491.1, 549.3–560.4, and 647.4–658.5 THz are shown in Fig. 2b–e.
The formation process of broadband microcomb in deformed cavity is investigated both experimentally and numerically (Fig. 3). In this measurement, the frequency detuning of laser cavity is slowly decreasing while the optical spectrum is recorded. Four typical phases of microcomb are presented in Fig. 3. First, the thresholdless harmonic generation is observed at 386.9 and 580.5 THz as the frequency of pump light is 193.4 THz. With the decrease of detuning, the intracavity pump power reaches the parametric threshold and the primary comb lines are generated near the pump wavelength as well as the harmonic wavelengths. We refer the comb lines near the pump wavelength as seed comb, and the rest of the comb as signal comb. The signal comb is generated from sumfrequency generation (SFG) of the seed comb, and the fourwave mixing between the seed comb and the harmonics of the pump laser. Subcombs with comb spacing identical to cavity FSR appear in both seed comb and signal comb when further decreasing the detuning. The subcombs will eventually merge together in all three bands. The formation process of broadband microcomb is well reproduced in our numerical simulation of coupled Lugiato–Lefever (LL) equations (see Methods) and is presented in Fig. 3b. Note that the numerical simulation of intracavity light field is performed by employing a circular cavity without the effects of chaotic propagation. This configuration is an approximation for the weakly deformed highQ cavity to reduce computational complexity.
A comparison of comb output between phasematched coupling and chaosassisted coupling is demonstrated. In this experiment, a microfiber and a nanofiber are simultaneously coupled to the deformed cavity (Fig. 4a). The diameter of the microfiber is set to 1.5 μm to achieve phasematched coupling with the whisperinggallery mode at 1550 nm. The 0.5μmdiameter nanofiber has effective refractive index close to 1, and it is used to collect the comb light through chaosassisted coupling. The coupling efficiency can be simulated by 3D FDTD method, and the result is presented in Fig. 4b. The efficiency for phasematched coupling decreases rapidly with the increase of optical frequency, while the efficiency for chaosassisted coupling maintains very well from nearIR to visible wavelengths. The dimension of the cavity is scaled down in this simulation and the detailed is described in the Methods. In the experiment, 300 mW pump power is sent into the cavity through the microfiber. The optical spectra are measured at both the through port and drop port and are shown in Fig. 4c, d. No comb signal in nearvisible or visible band can be seen for the phasematched coupling output. In comparison, intense comb lines are detected with chaosassisted coupling.
Observation of soliton formation steps
Soliton formation in microresonators is critical to many microcomb applications for its high coherence and predictable spectrum envelope^{16}. Here, we show that soliton formation could be feasible in a deformed cavity for the first time. In the experiment, the laser wavelength is scanned from bluedetuned to reddetuned regime, while the comb power is recorded on the oscilloscope. When the laser scanning rate is increased to ~10 nm/s, the distinct soliton formation steps are observed. Upon entering the soliton regime, the comb power shows a “step” feature and the noise on the comb power disappears. Both single and multiple steps are observed and presented in Fig. 5. This points to evidence of dissipated Kerr cavity solitons in a microresonator^{10}. It shall be noted that the transition from noisy comb to soliton regime is often complicated by thermal effect. The abrupt change in comb power leads to a rapid decrease of cavity temperature and thus a blueshift of the resonator frequency. As a result, the cavitylaser detuning quickly exits the soliton existence range and thus the soliton steps in our experiment are only ~1 μs in duration. Several methods exist to overcome this thermal complexity, including pump power kicking^{47}, rapid frequency scanning by using singlesideband modulator^{48}, and auxiliary laser for thermal compensation^{49,50}. Further investigation using these methods is needed in the future to stably access the soliton state in our cavity and to verify soliton modelocking.
Discussion
In summary, we have demonstrated microcomb with twooctave span ranging from 450 to 2008 nm in a deformed cavity. The microcomb wavelength range is useful for optical clock, medical imaging, and calibrating radial velocity for exoplanet detection. The deformation of cavity lifts the circular symmetry and introduces a new degree of freedom to the microcomb systems. This could be used as an additional knob for broadband dispersion engineering of microresonator. Preliminary indication of soliton formation is also shown in the deformed cavity. The combination of temporal solitons and chaos in deformed cavity in the future may provide new possibilities for optical soliton physics studies and microcomb applications.
Methods
Deformed cavity design
In a deformed cavity, the angle of reflection and the angle of following incidence are different when a light beam is reflected on the boundary of the cavity. As a result, the incident angle χ of reflection is not conserved, nor is the angular momentum quantity \(\sin \chi\). This varying χ makes the intracavity ray dynamics highly sensitive to initial conditions, and the chaotic channel forms accompanied by rapid and broadband momentum transformation.
By using this momentum transformation, the effective mode index of light can be reduced from 1.44 (close to that of the WGMs) to nearly 1 (close to that of the nanofiber) in the chaotic channel. As a result, the light can be easily coupled into the nanofiber from the chaotic channel through the wavelengthindependent refraction. This chaosassisted broadband momentum transformation is provided in the cavity with its boundary designed as
where R_{0} is the size parameter, and a(b)_{2,3} are optimized as a_{2} = −0.1329, a_{3} = 0.0948, b_{2} = −0.0642, and b_{3} = −0.0224.
Coupled LL equations in the presence of χ ^{(2)} and χ ^{(3)} nonlinear processes
The comb formation process is simulated with three normalized coupled LL equations, which include multiple χ^{(2)} and χ^{(3)} nonlinear effects, e.g. SFG, different frequency generation, selfphase modulation, and crossphase modulation. In the equations, μ_{ab} and μ_{ac} describe coupling strength for χ^{(2)} and χ^{(3)} SFG; μ_{ba} and μ_{ca} for the reverse process; ζ_{ab} and ζ_{ac} denote crossphase modulation for χ^{(2)} and χ^{(3)} combs, and the inverse process is described by ζ_{ba} and ζ_{ca}. Selfphase modulation of χ^{(2)} and χ^{(3)} combs is described by ζ_{b} and ζ_{c}. The coupled LLEs are as follows:
where the electromagnetic field in the cavity is described by three slowlyvarying field envelopes Φ_{a,b,c} for (a) infrared, (b) χ^{(2)}, and (c) χ^{(3)} combs, respectively. The envelope Φ_{a,b,c} is normalized to \({\left{\Phi }_{{a,b,c}}\right}^{2}=2{g}_{{{a,b,c}}}{N}_{{{a,b,c}}}/{\kappa }_{{{a,b,c}}}\), where N_{a,b,c} is photon number in the corresponding fields. κ_{a,b,c} is the decay rate of the comb modes and \({g}_{{{a,b,c}}}={n}_{2}c\hslash {\omega }_{{{a,b,c}}}^{2}/({n}_{{{a,b,c}}}^{2}{V}_{{{a,b,c}}})\), where ℏ is the Planck’s constant, c is the speed of light in vacuum, n_{2} is the Kerr nonlinear index, ω_{a,b,c} is the resonance frequency at the center of each frequency band, n_{a,b,c} is the refractive index, and V_{a,b,c} is the effective mode volume. The envelopes are functions of τ and ϕ, where τ is the normalized time, τ = κ_{a}t/2, t is the lab time, and ϕ is the cavity polar angle. α_{a,b,c} denotes normalized detuning and f_{0} is the normalized pump input. β_{a,b,c} denotes normalized secondorder chromatic dispersion, and Δk_{b,c} is the FSR mismatch between infrared combs and χ^{(2)} (χ^{(3)}) combs in the unit of half cavity linewidth.
Parameters used in the LLE simulation are: κ_{a}/2π = 1.21 × 10^{7} Hz, κ_{b}/2π = 2.03 × 10^{7} Hz, κ_{c}/2π = 3.05 × 10^{7} Hz, β_{a} = 1.8565, β_{b} = −4.1258, β_{c} = −6.0227, Δk_{b} = −1.0179 × 10^{3}, Δk_{c} = −3.2310 × 10^{3}, ζ_{b} = 1.6840, ζ_{c} = 2.5274, ζ_{ba} = 1.8738, ζ_{ca} = 1.1729, ζ_{ab} = 9.2780, ζ_{ac} = 13.0269, μ_{ba} = 0.0058, μ_{ca} = 0.2139, μ_{ab} = 0.0143, μ_{ac} = 0.7921, f_{0} = 26.1562. Parameters derived from experimental measurements are κ_{a,b,c}, while other parameters are derived from 2D finite element method (FEM) simulation with rotation symmetry.
3D FDTD simulations of broadband coupling efficiency
The full 3D simulations of coupling efficiency are performed by a commercial simulator based on FDTD method. The microtoroid resonator is placed on the x–y plane. To make the simulation time affordable, the principal and minor diameters of the microresonator are scaled down to 24 and 4 μm, respectively. The thickness of the central disk is 2 μm. The nanofiber and the microfiber are represented by cylinders with diameters of 0.5 and 1.5 μm. A probe light source is applied to one side of the fiber. The excited mode in the fiber is set to be a fundamental mode. The material of the resonator is set to be silica with an additional imaginary part of 10^{−5} to its refractive index, and the material of the fiber is set to be silica without extra loss. Coupling efficiency is calculated as the difference of the base value and the dip value of the transmission spectrum. Different resonant modes are identified by field monitors inside the cavity. In Fig. 4b, a fundamental TE mode is used to calculate the coupling efficiency.
Data availability
Source data for Fig. 2 to Fig. 5 can be accessed at https://doi.org/10.6084/m9.figshare.12030408. Additional information is available from the corresponding authors upon reasonable request.
Code availability
The codes that support the findings of this study are available from the corresponding authors upon reasonable request.
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Acknowledgements
The authors thank K. Vahala, B. Shen, L. Wu, Y. Zhi, P. Del’Haye, X.C. Yu, L. Wang, Y.J. Qian, L. Shao, C.H. Dong, J.h. Chen, X.F. Jiang, S.X. Zhang, S.J. Tang, L. Yao and L.K. Chen for helpful discussions. This project is supported by the National Key R&D Program of China (Grant No. 2016YFA0301302 and No. 2018YFB2200401), the National Natural Science Foundation of China (Grant Nos. 11825402, 11654003, 61435001, and 12041602), Beijing Academy of Quantum Information Sciences (Y18G20), Key R&D Program of Guangdong Province (2018B030329001) and the Highperformance Computing Platform of Peking University. X.Y. is supported by U.S. National Science Fundation (award no. 1842641).
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Y.F.X. and X.Y. conceived the idea and designed the experiments. H.J.C. fabricated the devices and built the experimental setup. H.J.C., Q.X.J., H.W., and Q.F.Y. performed the measurements. Q.X.J. and Q.T.C. built the theoretical model and performed the simulations. All authors analyzed the data, participated in preparing the manuscript, and contributed to the discussions. Y.F.X., X.Y., and Q.G. supervised the project.
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Chen, HJ., Ji, QX., Wang, H. et al. Chaosassisted twooctavespanning microcombs. Nat Commun 11, 2336 (2020). https://doi.org/10.1038/s41467020159145
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