Generation rate scaling: the quality factor
optimization of microring resonators
for photon-pair sources
KAI GUO,
1,
*XIAODONG SHI,
2
XIAOLIN WANG,
1
JUNBO YANG,
3
YUNHONG DING,
2
HAIYAN OU,
2
AND YIJUN ZHAO
1
1
College of Advanced Interdisciplinary Studies, National University of Defense Technology, Changsha 410073, China
2
Department of Photonics Engineering, Technical University of Denmark, Kgs. Lyngby 2800, Denmark
3
Center of Material Science, College of Liberal Arts and Sciences, National University of Defense Technology, Changsha 410073, China
*Corresponding author: guokai07203@hotmail.com
Received 17 January 2018; revised 27 March 2018; accepted 29 March 2018; posted 4 April 2018 (Doc. ID 320070); published 23 May 2018
To achieve photon-pair generation scaling, we optimize the quality factor of microring resonators for efficient
continuous-wave-pumped spontaneous four-wave mixing. Numerical studies indicate that a high intrinsic quality
factor makes high pair rate and pair brightness possible, in which the maxim ums take place under overcoupling
and critical-coupling conditions, respectively. We fabricate six all-pass-type microring resonator samples on a
silicon-on-insulator chip involving gap width as the only degree of freedom. The signal count rate, pair brightness,
and coincidence rate of all the samples are characterized, which are then compared with the modified simulations
by taking the detector saturation and nonlinear loss into account. Being experimentally validated for the first time
to the best of our knowledge, this work explicitly demonstrates that reducing the round-trip loss in a ring cavity
and designing the corresponding optimized gap width are more effective to generate high-rate or high-brightness
photon pairs than the conventional strategy of simply increasing the quality factor.
© 2018 Chinese Laser Press
OCIS codes: (190.4380) Nonlinear optics, four-wave mixing; (270.0270) Quantum optics; (190.4360) Nonlinear optics, devices.
https://doi.org/10.1364/PRJ.6.000587
1. INTRODUCTION
A quantum-correlated photon-pair source is a key resource in
research of quantum optics such as quantum information
processing [1] and quantum communication [2]. Thereinto,
the most mature technology is quantum key distribution
(QKD), where the nature of correlated photon pairs is applied
to suffice high-security communication [3–7]. Sources capable
of QKD are required to emit single photons in a probabilistic
manner with low noise, preferably in the telecom wavelength
range, to benefit from the compatibility of optical fiber net-
works [8]. Moreover, the single photon generated from sponta-
neous nonlinear processes has a naturally correlated twin
photon, which makes it possible to apply the detection of
one photon (signal) to herald the existence of the other (idler).
While the initially heralded photon-pair sources have been
demonstrated via spontaneous downconversion in optical crys-
tals [9] or quasi-matched waveguides [10], and via spontaneous
four-wave mixing (SpFWM) in optical fibers [8,11], a number
of experiments were carried out in the past decade via SpFWM
in integrated waveguide platforms, of which the material can be
crystalline silicon [12–14], amorphous silicon [15], silica [16],
silicon nitride [17], and AlGaAs [18]. Integrated waveguides
often have large refractive index contrast leading to strong light
confinement and high nonlinear interaction, which enables
efficient photon-pair generation within a few millimeters.
Moreover, by employing either butt-coupled [19] or vertical-
coupled approaches [20], integrated waveguides are compatible
with fiber-based systems; thus, it can be applied as the nonlin-
ear medium of photon-pair sources instead of optical fibers, to
avoid broadband spontaneous Raman scattering noise [11]. In
addition, thanks to the mature fabrication methods of semicon-
ductors and integrated circuits, which enable a variety of func-
tionalities [21], efficient quantum communication systems are
in the progress of on-chip integration [22,23].
Although photon-pair sources using SpFWM in integrated
waveguides are often driven by a pulsed pump, the continuous-
wave (CW) pump, with advantages of cheaper, more stable, and
especially easier on-chip integration, is also widely employed.
The narrow linewidth of CW pumps gives rise to strong spec-
tral anticorrelation and projects the generated photon pairs into
a classi cal spectral mixture [24,25], which is desired for special
applications such as time–energy entanglement [26–28],
wavelength-multiplexed quantum communication [29,30],
and covert quantum communication [31,32]. Moreover, pho-
ton-pair sources capable of long-distance quantum communi-
cation are required to have a high pair rate, which corresponds
Research Article
Vol. 6, No. 6 / June 2018 / Photonics Research 587
2327-9125/18/060587-10 Journal © 2018 Chinese Laser Press
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