  Coherent ranging, also known as frequency-modulated continuous-wave (FMCW)
laser based ranging (LIDAR) is currently developed for long range 3D distance
and velocimetry in autonomous driving. Its principle is based on mapping
distance to frequency, and to simultaneously measure the Doppler shift of
reflected light using frequency chirped signals, similar to Sonar or Radar.
Yet, despite these advantages, coherent ranging exhibits lower acquisition
speed and requires precisely chirped and highly-coherent laser sources,
hindering their widespread use and impeding Parallelization, compared to modern
time-of-flight (TOF) ranging that use arrays of individual lasers. Here we
demonstrate a novel massively parallel coherent LIDAR scheme using a photonic
chip-based microcomb. By fast chirping the pump laser in the soliton existence
range of a microcomb with amplitudes up to several GHz and sweep rate up to 10
MHz, the soliton pulse stream acquires a rapid change in the underlying carrier
waveform, while retaining its pulse-to-pulse repetition rate. As a result, the
chirp from a single narrow-linewidth pump laser is simultaneously transferred
to all spectral comb teeth of the soliton at once, and allows for true
parallelism in FMCW LIDAR. We demonstrate this approach by generating 30
distinct channels, demonstrating both parallel distance and velocity
measurements at an equivalent rate of 3 Mpixel/s, with potential to improve
sampling rates beyond 150 Mpixel/s and increase the image refresh rate of FMCW
LIDAR up to two orders of magnitude without deterioration of eye safety. The
present approach, when combined with photonic phase arrays based on
nanophotonic gratings, provides a technological basis for compact, massively
parallel and ultra-high frame rate coherent LIDAR systems.
