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Massively parallel coherent laser ranging using a soliton microcomb

Abstract

Coherent ranging, also known as frequency-modulated continuous-wave (FMCW) laser-based light detection and ranging (lidar)1 is used for long-range three-dimensional distance and velocimetry in autonomous driving2,3. FMCW lidar maps distance to frequency4,5 using frequency-chirped waveforms and simultaneously measures the Doppler shift of the reflected laser light, similar to sonar or radar6,7 and coherent detection prevents interference from sunlight and other lidar systems. However, coherent ranging has a lower acquisition speed and requires precisely chirped8 and highly coherent5 laser sources, hindering widespread use of the lidar system and impeding parallelization, compared to modern time-of-flight ranging systems that use arrays of individual lasers. Here we demonstrate a massively parallel coherent lidar scheme using an ultra-low-loss photonic chip-based soliton microcomb9. By fast chirping of the pump laser in the soliton existence range10 of a microcomb with amplitudes of up to several gigahertz and a sweep rate of up to ten megahertz, a rapid frequency change occurs in the underlying carrier waveform of the soliton pulse stream, but the pulse-to-pulse repetition rate of the soliton pulse stream is retained. As a result, the chirp from a single narrow-linewidth pump laser is transferred to all spectral comb teeth of the soliton at once, thus enabling parallelism in the FMCW lidar. Using this approach we generate 30 distinct channels, demonstrating both parallel distance and velocity measurements at an equivalent rate of three megapixels per second, with the potential to improve sampling rates beyond 150 megapixels per second and to increase the image refresh rate of the FMCW lidar by up to two orders of magnitude without deterioration of eye safety. This approach, when combined with photonic phase arrays11 based on nanophotonic gratings12, provides a technological basis for compact, massively parallel and ultrahigh-frame-rate coherent lidar systems.

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Fig. 1: Massively parallel frequency-modulated continuous-wave lidar using soliton microcombs.
Fig. 2: Dynamics of frequency-modulated soliton microcombs.
Fig. 3: Time–frequency analysis of a chirped soliton microcomb.
Fig. 4: Demonstration of massively parallel velocity measurement using a soliton microcomb.
Fig. 5: Parallel distance measurement and imaging.

Data availability

The data used to produce the plots within this paper are available at https://doi.org/10.5281/zenodo.3603614.

Code availability

The code used to produce the plots within this paper is available at https://doi.org/10.5281/zenodo.3603614.

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Acknowledgements

We thank A. S. Raja for his contribution with microresonator testing. Samples were fabricated at the Center of MicroNanoTechnology (CMi) with the assistance of R. N. Wang. This work was supported by funding from the Swiss National Science Foundation under grant agreement number 165933 and by the Air Force Office of Scientific Research (AFOSR), Air Force Material Command, USAF, under award number FA9550-15-1-0250. Sample fabrication and process developement was funded by contract HR0011-15-C-055 (DODOS) from the Defense Advanced Research Projects Agency (DARPA), Microsystems Technology Office (MTO). J.R. and W.W. acknowledge support from the EUs H2020 research and innovation program under the Marie Sklodowska-Curie IF grant agreement numbers 846737 (CoSiLiS) and 753749 (SOLISYNTH), respectively. We acknowledge interactions with A. Zott from ZEISS AG.

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Authors

Contributions

A.L. and J.R. conducted the various experiments and analysed the data. E.L. assisted with laser linearization, W.W. performed the numerical simulations, A.L. designed the samples and J.L. fabricated the samples. All authors discussed the manuscript. J.R., T.J.K., M.K. and E.L. wrote the manuscript. T.J.K. supervised the work and conceived the experiment.

Corresponding author

Correspondence to 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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Extended data figures and tables

Extended Data Fig. 1 Pump frequency sweep linearization via the heterodyne method.

a, Setup for pump-laser frequency measurement via heterodyne beat note and chirp linearization feedback. b, Initial frequency modulation, when the VCO is driven with a triangular ramp. The measured frequency is compared with the targeted ideal modulation. The ramp frequency is 100 kHz. c, Final triangular frequency modulation pattern, after four iterations. d, Evolution of the root-mean-square (RMS) frequency deviation during the optimization loop. e, Evolution of the deviation between measurement and target sweep, at each iteration of the loop.

Extended Data Fig. 2 Linearization results at different modulation frequencies.

a, c, e, The evolution of the root-mean-square frequency deviation during the optimization loop for modulation frequencies of 10 kHz, 1 MHz and 10 MHz, respectively. b, d, f, Corresponding evolution of the deviation between the measurement and the target sweep, at each iteration of the loop.

Extended Data Fig. 3 Channel-by-channel analysis of heterodyne chirp characterization.

a, Time–frequency maps obtained with short-time Fourier transform of the heterodyne beat detection of the individual FMCW channels. Top left to bottom right panels denote optical carriers between 192.1 THz and 196 THz. Modulation frequency is 100 kHz. The pump channel at 193 THz is outlined in purple. b, As for a, but for modulation frequency 10 MHz.

Extended Data Fig. 4 Frequency-dependent transduction of carrier modulation from pump to comb sidebands.

a, Time-dependent frequency of pump laser at 193 THz (grey) and 195 THz comb sideband (μ = 20, dark green) and modulation frequency 100 kHz. b, As for a, but for modulation frequency 10 MHz. c, Power spectral density of frequency modulation Sff for pump (grey) and sideband (dark green). The markers denote the positions of harmonics, which are used in the transduction analysis. The lower panel shows the power spectral density of sideband frequency modulation harmonics normalized to the corresponding modulation power spectral density of the pump laser (see Fig. 3). d, As for c, but for modulation frequency 10 MHz.

Extended Data Fig. 5 Pump frequency sweep linearization via the delayed homodyne method.

a, Setup for pump-laser frequency measurement via delayed homodyne detection and chirp linearization feedback. Calibration of the MZI is performed by fitting the frequency-dependent phase modulation response of the MZI. b, Initial frequency modulation, when the VCO is driven with a triangular ramp, determined using a Hilbert transform. The measured frequency is compared with the targeted ideal modulation. The ramp frequency is 100 kHz. The red-shaded regions around the extremal points are excluded from the linearization update. c, Final triangular frequency modulation pattern, after 20 iterations. Convergence achieved after four iterations. d, Evolution of the root-mean-square frequency deviation during the optimization loop. e, Evolution of the deviation between measurement and target sweep, at each iteration of the loop.

Extended Data Fig. 6 Calibration of channel dependent frequency excursion bandwidth for distance and velocity measurements.

a, Measurement setup. The linearized frequency-modulated microcomb (see Extended Data Fig. 5 for setup schematic) is amplified and individual channels are selected by connecting the local oscillator path of the measurement setup to a calibrated imbalanced MZI (8.075 m). b, The top panel shows the frequency-excursion bandwidth Bμ determined from independent measurement of the length of imbalanced MZI. Linear fit related to Raman self-frequency shift ΩR. The bottom panel shows the residuals of the linear fit.

Extended Data Fig. 7 Channel-by-channel analysis of proof-of-concept lidar demonstration.

a, Time-frequency maps obtained with short-time Fourier transform of the delayed homodyne beat detection of the individual FMCW channels back-reflected from the rotating flywheel. Top left to bottom right panels denote optical carriers between 192.1 THz and 195.2 THz. The pump channel at 193 THz is outlined in purple. Modulation frequency is 100 kHz. b, As for a, but for static flywheel.

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Riemensberger, J., Lukashchuk, A., Karpov, M. et al. Massively parallel coherent laser ranging using a soliton microcomb. Nature 581, 164–170 (2020). https://doi.org/10.1038/s41586-020-2239-3

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