Stretching the spectra of Kerr frequency combs
with self-adaptive boundary silicon waveguides
Jianhao Zhang,
a,
* Vincent Pelgrin,
a
Carlos Alonso-Ramos,
a
Laurent Vivien,
a
Sailing He,
b
and Eric Cassan
a,
*
a
Université Paris-Saclay, CNRS, Centre de Nanosciences et de Nanotechnologies, Palaiseau, France
b
Zhejiang University, Centre for Optical and Electromagnetic Research, State Key Laboratory for Modern Optical Instrumentation, Hangzhou, China
Abstract. Dispersion engineering of optical waveguides is among the most important steps in enabling the
realization of Kerr optical frequency combs. A recurring problem is the limited bandwidth in which the nonlinear
phase matching condition is satisfied, due to the dispersion of the waveguide. This limitation is particularly
stringent in high-index-contrast technologies such as silicon-on-insulator. We propose a general approach to
stretch the bandwidth of Kerr frequency combs based on subwavelength engineering of single-mode
waveguides with self-adaptive boundaries. The wideband flattened dispersion operation comes from the
particular property of the waveguide optical mode that automatically self-adapts its spatial profile at
different wavelengths to slightly different effective spatial spans determined by its effective index values. This
flattened dispersion relies on the squeezing of small normal-dispersion regions between two anomalous spectral
zones, which enables it to achieve two Cherenkov radiation points and substantially broaden the comb,
achieving a bandwidth between 2.2 and 3.4 μm wavelength. This strategy opens up a design space for
trimming the spectra of Kerr frequency combs using high-index-contrast platforms and can provide benefits
to various nonlinear applications in which the manipulation of energy spacing and phase matching are pivotal.
Keywords: nonlinear optics; effective boundary; subwavelength grating; silicon frequency comb.
Received Feb. 23, 2020; revised manuscript received May 26, 2020; accepted for publication Jun. 1, 2020; published online
Jun. 23, 2020.
© The Authors. Published by SPIE and CLP under a Creative Commons Attribution 4.0 Unported License. Distribution or
reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.
[DOI: 10.1117/1.AP.2.4.046001]
1 Introduction
Third-order nonlinear optical processes in photonic waveguides
have raised increasing interest in recent years, due to their
unique capabi lities for on-chip light generation and for the
manipulation of guided light in spectral/temporal domains, with
an immense potential for the implementation of light sources
based on optical parametric amplification, supercontinuum, or
frequency comb generation processes.
1–9
Kerr frequency comb
generation has great potential for applications in a wide range of
fields, from metrology-on-a-chip to the synthesis of radiofre-
quency signals or integrated spectroscopy.
10–19
Comprising dis-
crete and equally spaced frequency lines, the Kerr frequency
combs require precise control of the waveguide dispersion and
nonlinearity, the gain and loss of optical waveguides, especially
when temporal patterns with few solitons are expected.
20
Controlling the chromatic dispersion of optical waveguides is
a key preliminary step for the exploitation of Kerr frequency
combs. As such, the problem is quite simple in principle: it is
essentially a question of opti mizing the geometrical dimensions
of the waveguides in order to compensate for the material natu-
ral dispersion as well as that provided by the nonlinear effect
itself, in order to satisfy, as a whole, the energy and momentum
conservation conditions. To balance the nonlinearity-induced
wavevector phase mismatch, anomalous dispersion is generally
envisaged;
10,11,20
it can be suppor ted direc tly from the materials
(e.g., silica at telecom wavelengths) or induced by the wave-
guide dispersion with well-designed waveguide cross-sections.
As a result, toroidal-shape cavities using silica
10
or MgF
2
12,13
are
frequently used for frequency comb generation due to their
ultrahigh Q factors of up to a few millions and to the low
dispersion in these structures. Another classical material platform
used for frequency comb demonstrations is based on silicon
nitride (SiN) waveguides. Due to the high quality-factors of
SiN microring resonators (up to ∼10
6
) and a nonlinear Kerr index
*Address all correspondence to Eric Cassan, E-mail: eric.cassan@universite-paris-
saclay.fr; Jianhao Zhang, E-mail: jianhao.zhang@universite-paris-saclay.fr
Research Article
Advanced Photonics 046001-1 Jul∕Aug 2020
•
Vol. 2(4)