We experimentally discovered and theoretically explain a novel turnkey regime for operation of soliton microcombs, wherein a new operating point enables the direct access of the soliton state by simple turn-on of the pump laser

Bowers, John E.

Chang, Lin

Guo, Joel

He, Jijun

Ji, Qing-Xin

Kinghorn, David

Kippenberg, Tobias J.

Liu, Junqiu

Liu, Tianyi

Shen, Boqiang

Vahala, Kerry

Wang, Heming

Wang, Rui Ning

Wu, Lue

Xiang, Chao

Xie, Weiqiang

Yang, Qi-Fan

English

Caltech Authors - Main

SF3O.4.pdf CLEO 2020 © OSA 2020
Integrated turnkey soliton microcombs operated
at CMOS frequencies
Boqiang Shen1,†, Lin Chang2,†,∗, Junqiu Liu3,†, Heming Wang1,†, Qi-Fan Yang1,†,
Chao Xiang2, Rui Ning Wang3, Jijun He3, Tianyi Liu3, Weiqiang Xie2,
Joel Guo2, David Kinghorn2, Lue Wu1, Qing-Xin Ji1,4,
Tobias J. Kippenberg3,∗, Kerry Vahala1,∗ and John E. Bowers2
1T. J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, CA 91125, USA
2ECE Department, University of California Santa Barbara, Santa Barbara, CA 93106, USA
3Institute of Physics, Swiss Federal Institute of Technology Lausanne (EPFL), CH-1015 Lausanne, Switzerland
4School of Physics, Peking University, Beijing 100871, China
†These authors contributed equally to this work.
∗linchang@ucsb.edu, tobias.kippenberg@epfl.ch, vahala@caltech.edu
Abstract: We experimentally discovered and theoretically explain a novel turnkey regime 
for operation of soliton microcombs, wherein a new operating point enables the direct ac-
cess of the soliton state by simple turn-on of the pump laser. 
 OCIS codes: (130.3120) Integrated optics devices; (140.3945) Microcavities; (190.0190) Nonlinear optics
Optical frequency combs have found a remarkably wide range of applications in science and technology. And
a recent development that portends a revolution in miniature and integrated comb systems is dissipative Kerr
soliton formation in coherently pumped high-quality-factor (high-Q) optical microresonators [1]. Also, the recent
integration of microresonators with lasers has revealed the viability of fully chip-based soliton microcombs [2,3].
However, their operation requires complex startup and feedback protocols that necessitate difficult-to-integrate
optical and electrical components. Here, a regime for turnkey operation of soliton microcombs co-integrated with
a pump laser is demonstrated and theoretically explained. Significantly, a new operating point is shown to appear
from which solitons are generated through binary turn-on and turn-off of the pump laser, thereby eliminating all
photonic/electronic control circuitry. These features offer compelling advantages for high-volume production.
In the experiment, integrated soliton microcombs whose fabrication and repetition rate are compatible with
CMOS circuits are butt-coupled to a commercial distributed-feedback (DFB) laser via inverse tapers (Fig. 1a). The
microresonators are fabricated using the photonic Damascene reflow process [4] and feature Q factors exceeding
16 million, resulting in a low milliwatt-level parametric oscillation threshold. Up to 30 mW of optical power is
launched into the microresonator. Given its compact footprint and the absence of control electronics, the pump-
laser/microcomb chip set was mounted into a butterfly package (Fig. 1b) to facilitate measurements and also enable
portability. Fig. 1c shows spectra of multi-soliton states produced by the laser/microcomb chips with 20 GHz and
15 GHz repetition rates. The coherent nature of these soliton microcombs is confirmed by photodetection of the
soliton pulse streams, and reveals high-contrast, single-tone electrical signals at the indicated repetition rates.
a b c
Wavelength (nm)
P
ow
er
 (1
0 
dB
/d
iv
)
fr = 20 GHz
1520 1540 1560 1580 1600
fr = 15 GHz
-0.5 0 0.5
40 dB
R
F 
po
w
er
Frequency (MHz
+19.787 GHz)
RBW
100 Hz
R
F 
po
w
er
40 dB
-0.5 0 0.5
Frequency (MHz
+15.056 GHz)
RBW
100 Hz
Fig. 1: Integrated soliton microcomb chip and soliton spectrum. (a) Rendering of the soliton microcomb chip
that produces soliton pulse signals. Zoom-in panel: Images of a chip of 10 DFB lasers and Si3N4 microresonator
chip devices. (b) Image of a pump/microcomb in a compact butterfly package. (c) Optical spectra of multi-soliton
states at 20 GHz and 15 GHz repetition rates. Insets: Electrical beatnotes showing the repetition rates.
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SF3O.4.pdf CLEO 2020 © OSA 2020
a
Detuning
In
tra
ca
vi
ty
 p
ow
er
MI comb Soliton
c.w. state
0
Detuning
b
In
tra
ca
vi
ty
 p
ow
er
MI comb Soliton
0
0
1
C
om
b 
po
w
er
 
(a
rb
.u
ni
t)
Time (20 ms/div)
0
2
Fr
eq
ue
nc
y
 (M
H
z)
Laser on
fr = 19.787 GHz
10 Power (a.u.)
c
Fig. 2: Turnkey soliton microcomb operation. (a) Phase diagram, hysteresis curve and dynamics of the conven-
tional pumping system. The blue curve is the intracavity power as a function of cavity-pump frequency detuning.
Laser tuning (dashed red) accesses multiple equilibria (middle point is unstable as indicated by blue arrows). (b)
Phase diagram, hysteresis curve and dynamics of the new pumping system. A modified laser tuning curve (dashed
red) intersects the intracavity power curve (blue) to establish a new operating point from which solitons form. The
feedback phase φ is set to 0 in the plot. Solid black curve is the simulated evolution upon turning-on of the laser
at a red detuning outside the soliton regime but within the locking bandwidth. (c) Measured comb power (upper
panel) and detected soliton repetition rate signal (lower panel) with laser turn-on indicated at 10 ms.
In conventional pumping of microcombs, the laser is optically-isolated. Intracavity power as a function of
pump-cavity detuning features a Kerr-effect bistability. And dynamics can be described using a phase diagram
comprising continuous-wave (c.w.), modulation instability (MI) and soliton regimes that are accessed as the pump
frequency is tuned across a cavity resonance (Fig. 2a). Tuning through the MI regime seeds the formation of
soliton pulses. On account of the thermal hysteresis and the abrupt intracavity power discontinuity upon transition
to the soliton regime, special tuning or active capture techniques are required to compensate thermal transients.
Now consider removing the optical isolation so that backscatter feedback occurs from pumping a resonator
mode. In prior work semiconductor laser locking to the resonator mode as well as laser line narrowing have been
shown to result from backscattering of the intracavity optical field. These attributes as well as mode selection
when using a broadband pump have also been profitably applied to operate microcomb systems without isolation
[2, 3, 5, 6]. However, these prior studies of feedback effects have considered the resonator to be linear so that
the detuning between the feedback-locked laser and the cavity resonance is determined solely by the phase φ
accumulated in the feedback path [7]. In contrast, here the nonlinear behavior of the microresonator is included
and is shown to have a dramatic effect on the system operating point. The relationship between the laser-cavity
detuning δω and intracavity power of the pump mode P0 can be shown to be approximately given by,
δω
κ/2
= tan
φ
2
+
3
2
P0
Pth
(1)
where κ is the resonance power decay rate and Pth is the parametric oscillation threshold for intracavity power. A
single operating point at the intersection of Eq. (1) and the soliton hysteresis occurs as shown in Fig. 2b. Control
of feedback phase shifts the x-intercept of Eq. (1) to adjust the operating point. The system converges to this
equilibrium once the laser frequency is within a locking bandwidth. As verified both numerically (Fig. 2b) and
experimentally (Fig. 2c), this behavior enables soliton mode locking by simple power-on of the pump laser (i.e.,
no triggering or complex tuning schemes). Indeed, the measured comb power shows that a steady soliton power
plateau is reached immediately after turn-on of the laser. And the stable soliton emission is further confirmed by
monitoring the real-time evolution of the soliton repetition rate signal (Fig. 2c). Finally, the system is observed to
be robust with respect to temperature and environmental disturbances.
This work is supported by the Defense Advanced Research Projects Agency, DARPA (HR0011-15-C-055).
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4. J. Liu, A. S. Raja, M. Karpov, B. Ghadiani, M. H. Pfeiffer et al., Optica 5, 1347–1353 (2018).
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Integrated turnkey soliton microcombs operated at CMOS frequencies

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Integrated turnkey soliton microcombs operated at CMOS frequencies

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