Researchers at the lab of Tobias Kippenberg at Ecole Polytechnique Federale de Lausanne have found a new way to implement a parallel FMCW lidar engine by using integrated nonlinear photonic circuitry. They coupled a single FMCW laser into a silicon-nitride planar microresonator, where the continuous wave laser light is converted into a stable optical pulse train due to the double balance of dispersion, nonlinearity, cavity pumping and loss. The study has been published in Nature.
Coherent laser ranging, also known as FMCW lidar, emits linear optical frequency chirps. Heterodyne mixing with a replica of the emitted laser light maps the target distance to a radio frequency. Coherent detection has many inherent advantages such as enhanced distance resolution, direct velocity detection via the Doppler effect, and imperviousness to sunlight glare and interference. But the technical complexity of precisely controlling narrow-linewidth frequency-agile lasers has so far prevented the successful parallelization of FMCW Lidar.
“Surprisingly, the formation of the dissipative Kerr soliton does not only persist when the pump laser is chirped, but transfers the chirp faithfully to all the generated comb teeth,” says Johann Riemensberger, postdoc in Kippenberg’s lab and first author of the study. The small size of the microresonator means that the comb teeth are spaced 100 GHz apart, which is enough to separate them using standard diffraction optics. Because each comb tooth inherits the linear chirping of the pump laser, it was possible to create 30 independent FMCW lidar channels in the microresonator.
Each channel is capable to measure distance and velocity of a target simultaneously, while the spectral separation of the different channels makes the device immune to channel crosstalk, as well as a natural fit for co-integration with recently deployed optical phased arrays based on photonic integrated optical grating emitters. This work paves a way for future widespread application of coherent lidar in autonomous vehicle applications.