Monolithic piezoelectric control of soliton microcombs

Abstract

High-speed actuation of laser frequency1 is critical in applications using lasers and frequency combs2,3, and is a prerequisite for phase locking, frequency stabilization and stability transfer among optical carriers. For example, high-bandwidth feedback control of frequency combs is used in optical-frequency synthesis4, frequency division5 and optical clocks6. Soliton microcombs7,8 have emerged as chip-scale frequency comb sources, and have been used in system-level demonstrations9,10. Yet integrated microcombs using thermal heaters have limited actuation bandwidths11,12 of up to 10 kilohertz. Consequently, megahertz-bandwidth actuation and locking of microcombs have only been achieved with off-chip bulk component modulators. Here we demonstrate high-speed soliton microcomb actuation using integrated piezoelectric components13. By monolithically integrating AlN actuators14 on ultralow-loss Si3N4 photonic circuits15, we demonstrate voltage-controlled soliton initiation, tuning and stabilization with megahertz bandwidth. The AlN actuators use 300 nanowatts of power and feature bidirectional tuning, high linearity and low hysteresis. They exhibit a flat actuation response up to 1 megahertz—substantially exceeding bulk piezo tuning bandwidth—that is extendable to higher frequencies by overcoming coupling to acoustic contour modes of the chip. Via synchronous tuning of the laser and the microresonator, we exploit this ability to frequency-shift the optical comb spectrum (that is, to change the comb’s carrier-envelope offset frequency) and make excursions beyond the soliton existence range. This enables a massively parallel frequency-modulated engine16,17 for lidar (light detection and ranging), with increased frequency excursion, lower power and elimination of channel distortions resulting from the soliton Raman self-frequency shift. Moreover, by modulating at a rate matching the frequency of high-overtone bulk acoustic resonances18, resonant build-up of bulk acoustic energy allows a 14-fold reduction of the required driving voltage, making it compatible with CMOS (complementary metal–oxide–semiconductor) electronics. Our approach endows soliton microcombs with integrated, ultralow-power and fast actuation, expanding the repertoire of technological applications of microcombs.

Fig. 1: Principle of monolithic piezoelectric control of chip-based soliton microcombs.
Fig. 2: Piezoelectric voltage-controlled soliton initiation and tuning.
Fig. 3: High-speed piezoelectric actuation for on-chip PDH error signal generation and soliton repetition rate stabilization.
Fig. 4: Hybrid AlN-Si3N4 soliton lidar engine.

Data availability

The data that support the plots within this paper and other findings of this study are available on Zenodo (https://doi.org/10.5281/zenodo.3903724). All other data used in this study are available from the corresponding authors upon reasonable request.

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Acknowledgements

We thank J. Riemensberger, A. Lukashchuk and M. Karpov for discussions. This work was supported by contract HR0011-15-C-055 (DODOS) from the Defense Advanced Research Projects Agency (DARPA), Microsystems Technology Office (MTO), by the Air Force Office of Scientific Research, Air Force Materiel Command, USAF, under award no. FA9550-19-1-0250, and by the Swiss National Science Foundation under grant agreement no. 176563 (BRIDGE). E.L. acknowledges support from the European Space Technology Centre under ESA contract no. 4000116145/16/NL/MH/GM. Samples were fabricated in the EPFL Center of MicroNano Technology (CMi) and in the Birck Nanotechnology Center at Purdue University. AlN deposition was performed at OEM Group Inc.

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Contributions

J.L., H.T. and R.N.W. designed and fabricated the samples. J.L., H.T., J.H. and T.L. tested the samples and fabrication yield. E.L., A.S.R., J.L., G.L., M.H.A. and W.W. performed experiments. J.L., E.L., A.S.R. and G.L. analysed the data. J.L., T.J.K. and S.A.B. wrote the manuscript, with input from others. T.J.K. and S.A.B. initiated and supervised the collaboration.

Corresponding authors

Correspondence to Sunil A. Bhave or Tobias J. Kippenberg.

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Competing interests

T.J.K. is a co-founder and shareholder of LiGenTec SA, a start-up company that is engaged in making Si3N4 nonlinear photonic chips available via foundry service.

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Peer review information Nature thanks Matt Eichenfield and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Fig. 1 Experimental characterization of resonance frequency versus applied voltage.

a, Experimental set-up. OSA, optical spectrum analyser; PD, photodiode. b, Resonance tuning data with error bars. The voltage applied to the AlN actuator is varied in the range ±100 V in order to reveal the hysteresis. The standard deviation (s.d.) of the measured frequency at each voltage value is evaluated using the measured frequency with respect to the fitted frequency on the linear curve. The error bars show ±1 s.d. Inset, magnified view highlighting the difference in scale between the observed hysteresis (that is, the frequency difference between two fitted curves) and the error bars. The overall frequency measurement uncertainty is estimated to be below 15 MHz, smaller than the observed hysteresis.

Extended Data Fig. 2 Characterization of resonance linewidth and microresonator dispersion.

a, Comparison of loaded resonance linewidths with different applied voltages. No voltage-dependent linewidth change is observed. b, Comparison of microresonator dispersion with different applied voltages. No dispersion change is observed.

Extended Data Fig. 3 Long-term soliton stabilization via AlN feedback actuation.

a, Experimental set-up. OSC, oscilloscope; BPF, bandpass filter; FBG, fibre Bragg grating. b, Soliton stabilization over 5 h, realized by locking the resonance to the laser and maintaining the soliton detuning. Three selected comb lines that exist for more than 5 h are shown here.

Extended Data Fig. 4 On-chip generation of PDH error signals using the HBAR modes induced by AlN actuation.

a, Experimental set-up. LPF, low-pass filter; Amp., RF power amplifier. b, The measured S21(ω) response of the AlN actuator on a linear frequency scale. Measurements are taken when the laser is on-resonance and off-resonance. c, PDH error signals modulated at the four HBAR frequencies marked with stars in b.

Extended Data Fig. 5 Experimental set-ups used to characterize the soliton lidar engine and soliton locking.

a, Experimental set-up for the synchronous scan of the laser frequency and the microresonator resonance, using the feed-forward scheme. b, Experimental set-up used to characterize the in-loop phase noise of the beat signal between line no. −1 of the microcomb and line no. −13 of the EO comb. QPSK, quadrature phase shift keying; DSO, digital storage oscilloscope; MZM, Mach–Zehnder modulator; PNA, phase noise analyser.

Extended Data Fig. 6 Original, unprocessed optical micrograph data used to prepare Fig. 1b in the main text

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Extended Data Fig. 7 Original, unprocessed SEM data used to prepare Fig. 1c in the main text

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Liu, J., Tian, H., Lucas, E. et al. Monolithic piezoelectric control of soliton microcombs. Nature 583, 385–390 (2020). https://doi.org/10.1038/s41586-020-2465-8

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