Soliton Microcomb Range Measurement
Myoung-Gyun Suh and Kerry Vahala
∗
T. J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, California 91125, USA.
∗
Corresponding author: vahala@caltech.edu
Laser-based range measurement systems (LI-
DAR) are important in many application areas
including autonomous vehicles, robotics, manu-
facturing, formation-flying of satellites, and ba-
sic science. Coherent laser ranging systems us-
ing dual frequency combs provide an unprece-
dented combination of long range, high preci-
sion and fast update rate. Here, dual-comb dis-
tance measurement using chip-based soliton mi-
crocombs is demonstrated. Moreover, the dual
frequency combs are generated within a single mi-
croresonator as counter-propating solitons using
a single pump laser. Time-of-flight measurement
with 200 nm precision at 500 ms averaging time is
demonstrated. Also, the dual comb method ex-
tends the ambiguity distance to 26 km despite
a soliton spatial period of only 16 mm. This
chip-based source is an important step towards
miniature dual-comb laser ranging systems that
are suitable for photonic integration.
The invention of the optical frequency comb has had
a major impact on laser ranging systems.
In ad-
dition to providing a highly accurate frequency cal-
ibration source in methods such as multi-wavelength
interferometry
1
and frequency-modulated continuous
wave laser interferometry
2
, frequency combs have en-
abled a new method of ranging called dual-comb LIDAR
3
(DCL). In this method, two frequency combs having
slightly different repetition rates are phase locked. In
the detection process, the resulting intercomb beats cre-
ate a repetitive interferogram that is able to attain sub-
nanometer range precision. At the same time, the am-
biguity range of the combined comb system is greatly
extended beyond the pulse-to-pulse separation distance
of each comb. Applications of DCL systems would bene-
fit from more compact and miniature comb systems, and
the recent development of a miniature frequency comb
(microcomb
4,5
) suggests that chip-integrated DCL sys-
tems may be possible. Microcombs have been demon-
strated in several material systems
4,6–10
and many im-
plementations are monolithic on a silicon wafer so that
integration with both other photonic components
11–13
as
well as electronics is possible. If applied to DCLs, full in-
tegration would enable scalable manufacturing for mass
market applications.
A recent advancement in microcombs has been the
realization of soliton mode-locking
14–18
, which provides
phase-locked femtosecond pulses with GigaHertz to Ter-
aHertz repetition rates.
Soliton microcombs are be-
ing studied in several frequency comb applications, in-
cluding optical frequency synthesis
19
, secondary time
standards
20
, and dual comb spectroscopy
21–23
. In this
work, we demonstrate time-of-flight distance measure-
ment using a chip-based dual-soliton source. Beyond
the demonstration of microcomb LIDAR, the two soli-
ton streams are generated as counter-propagating soli-
tons within a single resonator
24
. This simplifies the sys-
tem by eliminating the need for two resonators and two
pump lasers. It also improves the mutual coherence be-
tween the two combs.
The experimental setup is shown in figure 1a. As de-
scribed there, two pump fields are coupled to the mi-
croresonator along the clockwise (CW) and counterclock-
wise (CCW) directions of a common whispering-gallery
resonance. The microresonator is a silica wedge disk fab-
ricated on a silicon wafer
26
. The resonator had an un-
loaded quality factor of approximately 300
∼
500 mil-
lion and a 7 mm diameter corresponding to a 9.36 GHz
free spectral range (FSR). With the high circulating
power, frequency combs are initiated by way of paramet-
ric oscillation
27,28
and are broadened by cascaded four-
wave mixing
4,5
. Solitons are generated in both CW and
CCW directions and stabilized using a feedback loop as
described elsewhere
25
. The servo control holds the fre-
quency detuning of one pump direction fixed relative to
the cavity resonant frequency, while the second pump fre-
quency can be independently tuned using an AOM. The
generated CW/CCW soliton streams are coupled in op-
posing directions to the optical fiber and then transferred
towards the LIDAR setup by using circulators.
To characterize the solitons the CW and CCW soli-
tons are also combined by tapping power from the main
fiber using the dotted paths in figure 1a. Superimposed
optical spectra of the CW and CCW solitons (measured
on the OSA) are shown in figure 1b. The characteristic
hyperbolic-secant-square function (green dotted curve) is
fit to the spectral envelope and a soliton pulse width of
200 fs is determined from this fitting. The combined soli-
ton streams were also detected by two PDs. The output
voltage of one of the PDs measured using an oscilloscope
is shown in the figure 1c inset. The periodic pulses in this
time trace reflect the periodic interference of the dual
soliton pulse streams as they stroboscopically interfere
on account of the slight difference in their respective rep-
etition rates (∆
f
rep
∼
18 kHz). This voltage time trace
signal (or interferogram) is Fourier transformed to obtain
the electrical spectrum in figure 1c main panel. The CW
and CCW pumps were offset in frequency by ∆
f
pump
= 2.53 MHz in this measurement and their correspond-
ing beatnote is indicated in figure 1c main panel. The
arXiv:1705.06697v3 [physics.optics] 28 Jun 2017