Kerr frequency combs in large-size,
ultra-high-Q toroid microcavities with low
repetition rates [Invited]
JIYANG MA,
1
XIAOSHUN JIANG,
1,
* AND MIN XIAO
1,2
1
National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences, and School of Physics,
Nanjing University 210093, China
2
Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701, USA
*Corresponding author: jxs@nju.edu.cn
Received 21 July 2017; revised 26 October 2017; accepted 31 October 2017; posted 31 October 2017 (Doc. ID 302809);
published 22 November 2017
By overcoming fabrication limitations, we have successfully fabricated silica toroid microcavities with both large
diameter (of 1.88 mm) and ultra-high-Q factor (of 3.3 × 10
8
) for the first time, to the best of our knowledge. By
employing these resonators, we have further demonstr ated low-threshold Kerr frequency combs on a silicon chip,
which allow us to obtain a repetition rate as low as 36 GHz. Such a low repetition rate frequency comb can now be
directly measured through a commercialized optical-electronic detector.
© 2017 Chinese Laser Press
OCIS codes: (140.3945) Microcavities; (190.4390) Nonlinear optics, integrated optics; (190.4380) Nonlinear optics, four-wave mixing.
https://doi.org/10.1364/PRJ.5.000B54
1. INTRODUCTION
Optical frequency combs act as frequency “rulers,” providing
evenly spaced frequency lines. They have become useful tools
in wide ranges of advanced scientific and applied fields [1–6].
The traditional optical frequency comb [1,2] is based on a large
femtosecond mode-locked laser system that is difficult to inte-
grate on a chip. Over the past decade, the rapid development of
high-Q microresonators has led to the rise of a new type of
optical frequency comb based on Kerr-nonlinearity, known
as the Kerr comb [7,8]. Formed via the process of cascaded
four-wave mixing, Kerr combs have been implemented in dif-
ferent kinds of microresonators [7]. Since their invention, Kerr
combs have attracted tremendous attention and have been gen-
erated in several material systems [8–16], including several
on-chip platforms [17–20]. On-chip operation is essential
for integration with other optical components. Because of their
small mode volumes and high-Q factors, these microresonator-
based Kerr combs usually consume far less pump power,
whereas typical threshold powers of several tens of microwatts
have been reported for such optical frequency combs [8].
Because of their unique advantages over traditional optical fre-
quency combs, as well as their potential for use in future ap-
plications, a number of significant studies have been conducted
on such combs. These include comb noise characterization and
dynamics [14], octave spanning [21,22], and time-domain
characterizations, such as pulse shaping [19] and soliton forma-
tion inside the resonators [23,24]. In addition, Kerr combs have
been demonstrated under various wavelength regimes, including
visible light [25–30] and the near-infrared [8–16]tomid-
infrared [31,32] ranges, each of which provides further potential
for unique applications. Numerical simulations of Kerr combs
have also been developed in depth [33,34]. One of the most
significant applications of Kerr combs is providing a link between
microwave and optical frequencies, such as in optical clocks
[3,35]. To realize practical implementation in such cases, the
comb needs to be fully stabilized [36]. Moreover, both a low
repetition rate that is detectable with commercial detectors
and octave spanning are required in the comb. A recent research
study has shown that, by using silica disk resonators, one can
achieve the lowest possible repetition rate of 2.6 GHz and
near-octave comb spanning with a 66 GHz repetition rate [
20].
Although the Kerr optical frequency comb was first achieved
in silica toroid microca vities [8], due to the fabrication diffi-
culty, microtoroid cavities are conventionally limited to low
quality factors at large sizes [36]. As such, both the repetition
rate and the threshold powers of the generated Kerr combs are
quite high, which is undesirable for practical applications and
makes this platform gradually unpopular in the community. In
this paper, we present a method of producing low-threshold
Kerr combs in large toroid cavities with diameters up to
1.88 mm and Q factors greater than 3 × 10
8
. So far, the largest
toroid cavity obtained in previously reported studies had a
diameter of just 760 μm, and its corresponding optical Q factor
was 2 × 10
7
[36] due to the fabrication limitations on large-
diameter toroids. By optimizing the fabrication process,
B54
Vol. 5, No. 6 / December 2017 / Photonics Research
Research Article
2327-9125/17/060B54-05 Journal © 2017 Chinese Laser Press