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Soliton pulse pairs at multiple colors in normal dispersion microresonators
Zhiquan Yuan
1
,
, Maodong Gao
1
,
, Yan Yu
1
,
, Heming Wang
1
,
2
,
, Warren Jin
2
,
3
,
,
Qing-Xin Ji
1
, Avi Feshali
3
, Mario Paniccia
3
, John Bowers
2
,
, and Kerry Vahala
1
,
1
T. J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, CA 91125, USA.
2
ECE Department, University of California Santa Barbara, Santa Barbara, CA 93106, USA.
3
Anello Photonics, Santa Clara, CA 95054, USA.
These authors contributed equally to this work.
Corresponding authors: jbowers@ucsb.edu, vahala@caltech.edu
Soliton microcombs [1] are helping to advance the miniaturization of a range of comb systems [2].
These combs mode lock through the formation of short temporal pulses in anomalous dispersion
resonators. Here, a new microcomb is demonstrated that mode locks through the formation of pulse
pairs in normal-dispersion coupled-ring resonators. Unlike conventional microcombs, pulses in this
system cannot exist alone, and instead must phase lock in pairs to form a bright soliton comb. Also,
the pulses can form at recurring spectral windows and the pulses in each pair feature different optical
spectra. This pairwise mode-locking modality extends to higher dimensions and we demonstrate
3-ring systems in which 3 pulses mode lock through alternating pairwise pulse coupling. The results
are demonstrated using the new CMOS-foundry platform that has not previously produced bright
solitons on account of its inherent normal dispersion [3]. The ability to generate multi-color pulse
pairs over multiple rings is an important new feature for microcombs. It can extend the concept of
all-optical soliton buffers and memories [4, 5] to multiple storage rings that multiplex pulses with
respect to soliton color and that are spatially addressable. The results also suggest a new platform
for the study of quantum combs [6–8] and topological photonics [9–11].
I. INTRODUCTION
Microresonator solitons exist through a balance of op-
tical nonlinearity and dispersion, which must be anoma-
lous for bright soliton generation. Moreover, microres-
onators must feature high optical Q factors for low pump
power operation of the resulting microcomb.
While
these challenges have been addressed at telecommuni-
cations wavelengths using a range of material systems
[1], CMOS-foundry resonators do not yet support bright
solitons as their waveguides feature normal dispersion
[3]. Furthermore, all resonators are dominated by nor-
mal dispersion at shorter wavelengths. For these rea-
sons, there has been keen interest in developing methods
to induce anomalous dispersion for bright soliton genera-
tion in systems that otherwise feature normal dispersion.
Such methods have in common the engineering of dis-
persion through coupling of resonator mode families, in-
cluding those associated with concentric resonator modes
[12, 13], polarization [14] or transverse modes [15].
Here, we engineer anomalous dispersion in CMOS-
foundry resonators by partially-coupling resonators as il-
lustrated in Fig. 1a. This geometry introduces unusual
new features to bright soliton generation. For example,
spectra resembling single pulse microcombs form instead
from pulse pairs as illustrated in Fig. 1a. The pulse pairs
circulate in a mirror-image fashion in the coupled rings
to form coherent comb spectra (Fig. 1b) with highly
stable microwave beat notes (Fig. 1c). The interaction
of the pulses in the coupling section between the rings
is shown to induce anomalous dispersion that compen-
sates for the overall normal dispersion of each ring. This
pairwise compensation spectrally recurs thereby opening
multiple anomalous dispersion windows for the forma-
tion of multi-color soliton pairs. These windows can be
engineered during resonator design. Furthermore, the
spectral composition of each pulse in a pair is different.
Fig. 1b, for example, shows through-port and drop-port
spectra that reflect the distinct spectral compositions of
pulses in cavity A and cavity B of Fig. 1a. This pecu-
liar effect is also associated with Dirac solitons [16] and
it is shown that the 2-ring pulse pair represents a new
embodiment of a Dirac soliton as the underlying dynami-
cal equation (see Methods) resembles the nonlinear Dirac
equation in 1 + 1 dimensions. Pulse pairing is also ex-
tendable to higher-dimensional designs with additional
normal dispersion rings. For example, in Fig. 1d-f 3
pulses in 3 coupled rings alternately pair to compensate
for the normal dispersion of each ring.
In what follows, we first study the dispersion of this
system and compare it to previous mode coupling meth-
ods. Experimental results including dispersion measure-
ment and comb formation are then presented. Pairwise
pulse formation is then studied in the time domain. Fi-
nally, because multi-pulse spectra in these systems resem-
ble conventional single-pulse soliton spectra, it is conve-
nient to resolve this ambiguity by denoting 2 and 3 ring
systems as bipartite and tripartite soliton microcombs,
respectively. The need for this nomenclature becomes
clear by the demonstration of multiple pulse-pair states,
including a 2 ring microcomb state containing 4 pulses
that behaves as a 2-pulse soliton crystal, and a 3 ring
state with 12 pulses that behaves as a 4-pulse soliton
crystal.
arXiv:2301.10976v1 [physics.optics] 26 Jan 2023