166 | Nature | Vol 581 | 14 May 2020
Article
channel can acquire both distance and velocity information simulta-
neously (see Fig. 1d).
This scheme leverages three key properties of DKS; the large (that
is, gigahertz) existence range of the soliton, the fact that the repeti-
tion rate changes associated with laser scanning are small, and the
ability, as detailed below, to very rapidly sweep between stable oper-
ating points without destroying the soliton state or deterioration of
the chirp linearity. Homodyning the reflected signal with the original
comb teeth channel-by-channel, using low-bandwidth detectors and
digitizers, allows the coherent ranging signal to be recovered and
reconstructed for each comb line μ simultaneously, yielding velocity
and distance (xμ, vμ) for each pixel. The scheme presented here thus
enables true parallel detection of dozens and potentially hundreds
of pixels simultaneously. Hence, massively parallel (and high-speed)
coherent lidar becomes possible, while requiring only a single well
controlled laser to generate the carrier-frequency chirped soliton.
This is in contrast with dual-frequency-comb coherent time-of-flight
systems^21 ,^22 , which achieve better distance precision and acquisition
speeds, yet exhibit a limited ambiguity range dictated by the pulse
repetition rate, and are challenging to parallelize because the whole
frequency comb must illuminate a single pixel. In a similar fashion,
recently demonstrated coherent stitching of multiple channels from
an electro-optical frequency comb generator can be used to improve
the distance measurement accuracy of FMCW^23 , yet demands the
spectral overlap of adjacent comb modes and concurrent illumination
of a single pixel.
We demonstrate here the principle of spectral multiplexing in coher-
ent lidar employing a 99-GHz repetition rate DKS in a ultra-low-loss sili-
con nitride (Si 3 N 4 ) microresonator, which is fabricated using the
photonic damascene process^24 (see inset of Fig. 1b and Methods).
Figure 2a shows the optical spectra of the DKS at the extrema of the
soliton existence range with relative laser-cavity detunings Δ 1 = 1.2 GHz
and Δ 2 = 2.9 GHz. Increasing the detuning, we observe the well known
temporal compression (58 fs to 45 fs)^10 and Raman self-frequency
shift (ΩR/2π = 2 THz)^25 of the DKS. Interestingly, despite the fre-
quency excursion greatly exceeding the overcoupled cavity linewidth
(κ 0 /2π = 15 MHz, κex/2π = 100 MHz) the power of comb teeth between
190 THz and 200 THz does not change by more than 3 dB, thus providing
more than 90 channels suitable for coherent lidar. The relative laser
detuning can be inferred from the phase modulation response spec-
trum (see Fig. 2b), wherein the C-resonance peak of the bistable cavity
field solution in the soliton existence range directly reveals the relative00.40.81.21.6Detuning (GHz)Pump power (W)1 2Modulation regionSimulations:CWMIChaosBreathersStable DKSPower (10 dB per div)C
SSCPumpVNA response(10 dB per div)0- π
π0- π
πRelative phase (rad)Power (arbitrary units)Relative phase (rad)6210142–2 20–20Detuning (GHz)Repetiton rate offset (GHz)–25–20–15–10
× 100Repetition rateoffset (MHz)No Raman effectsRaman effects are included01234Time (0.5 μs per div)Δ 1
Δ 2Time (μs)0 0.5 1.01.5 2.02.5165 170 175 180 185 190 195 200 205 210
Frequency (THz)190 192 194 196 198
Frequency (THz)Power
(5 dB per div)ΩRadb cΔ 1Δ 2Δ 1 Δ 2Δ 1 Δ 2Fig. 2 | Dynamics of frequency-modulated soliton microcombs. a, Optical
spectra of a DKS at relative laser cavity detuning Δ 1 = 1.2 GHz and Δ 2 = 2 .9 GHz,
respectively. (‘div’ denotes one division unit of the y axis.) The Raman
self-frequency shift ΩR/2π of the soliton is highlighted. The inset shows the
spectral region of frequency-modulated lidar operation, showcasing
individual line f latness better than 3 dB over the full pump-laser frequency
excursion range. b, Bistable phase modulation response of DKS measured with
the vector network analyser, where the S-resonance is related to the
high-intensity soliton field and the C-resonance is related to the low-intensity
continuous-wave field. c, Numerical simulation results of
frequency-modulated DKS generation. The laser is tuned through the
modulation instability region (blue shaded area) and the breathing region (red
shaded area) into the soliton state and triangular frequency modulation is
imprinted at a chirp rate of (dΔ/dt) ≈ 1 .7 × 10^16 Hz^2. The inclusion of stimulated
Raman scattering into the simulation reveals a modulation of the repetition
rate of up to 10 MHz during the frequency-modulated cycle. d, Simulated
stability chart of the soliton microcomb for the device used in the lidar
experiments. The soliton existence range is highlighted in green and confined
by the stability of the soliton solution. MI, modulation instability.