100 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 44, NO. 1, JANUARY 2008
Dual Waveguide Coupled Microring Resonator
Sensor Based on Intensity Detection
Zhixuan Xia, Student Member, IEEE, Yao Chen, and Zhiping Zhou, Senior Member, IEEE
Abstract—This paper systematically analyzes the device sen-
sitivity of a dual waveguide coupled microring resonator sensor
based on intensity detection. The relation between the device
sensitivity and transmission coefficient, coupling coefficient, and
operating wavelength has been derived and simulated, showing
that the sensitivity can be enhanced by proper adjustment of the
coupling coefficient and the operating wavelength. The explicit
expressions of optimal self coupling coefficient
and operating
wavelength are derived, serving as the basis for design and opti-
mization. Simulations performed based on the experimental data
lead to much smaller detection limit in terms of the effective index
change
e
. With a 30-dB signal-to-noise ratio measurement
system, the optimized
e
reaches
10
7
for both structures in
this study, two orders of magnitude smaller than the previous re-
sults. In conjunction with the analysis of the waveguide sensitivity,
design guidelines are generalized and presented.
Index Terms—Intensity detection, microresonator, microring,
microring resonator, optimization, sensitivity analysis, sensors.
I. INTRODUCTION
M
ICRORING resonators have become a key building
block for photonic integrated circuits due to their ver-
satility in function and their capability of integration. They
have been utilized in various applications including filtering
[1], [2], switching [3], modulation [4], and wavelength conver-
sion [5], mainly in the optical communications domain. Their
compactness and compatibility with mature semiconductor fab-
rication platforms facilitates integration with microelectronic
devices, leading to a great opportunity for mass-production and
commercialization.
In parallel, due to their small size and potential high sensi-
tivity, sensors based on microring resonators have been proposed
to detect mechanical displacement [6] or low concentration
of chemicals and biomolecules [7]–[14]. By implementing
large index-contrast materials, such as GaAs–AlGaAs [6], [15],
Si–SiO
[14], [16]–[18], Si N –SiO [7]–[9], Low-loss Hydex
material [10], and polymers [11], [13], the dimensions of mi-
croring resonators have been significantly reduced, making it
possible to detect minute amounts of analytes with a large dy-
namic range. The high sensitivity achieved is based on the ring
Manuscript received April 23, 2007; revised July 6, 2007. This work was
supported in part by the National Natural Science Foundation of China under
Grant 60578048.
Z. Xia and Y. Chen are with the Wuhan National Laboratory for Optoelec-
tronics and Huazhong University of Science and Technology, Wuhan 430074,
China (e-mail: xiazhix@163.com; yaochen9898@163.com).
Z. Zhou is with the Wuhan National Laboratory for Optoelectronics and
Huazhong University of Science and Technology, Wuhan 430074, China, and
also with the School of Electrical and Computer Engineering, Georgia Institute
of Technology, Atlanta, GA 30332 USA (e-mail: wnlo8@mail.hust.edu.cn).
Digital Object Identifier 10.1109/JQE.2007.909519
structure, which enhances the photon lifetime, or equivalently,
the finesse of the spectrum, based on the resonance within the
ring [19]. In these sensors, the analyte causes a change in the
refractive index of the cladding, which is probed by the evanes-
cent tail of the modal field, and in turn changes the transmission
behavior of the light propagating in the ring. This change can be
monitored by direct spectral scan for resonance wavelength shift
or detection of the output intensity at a fixed wavelength [20].
The sensing capability of these microring resonator sensors is
frequently characterized by two figures of merit: sensitivity and
detection limit. The sensitivity is defined as the rate of change
of the detected signal under a shift of the target parameter, such
as the displacement, cladding index, or concentration of the an-
alyte, depending on the operation principle of that device. On
the other hand, the detection limit is relatively independent of
the specific application, for it is generally defined as the min-
imal cladding index shift that is detectable, which is determined
by the sensitivity and signal-to-noise ratio (SNR) of the en-
tire system including the laser source and the signal-processing
component.
In principle, the overall sensitivity consists of two contribu-
tions: waveguide sensitivity and device sensitivity [20], both
of which can be analyzed individually. On the one hand, de-
termined by the geometry and the index profile of the wave-
guide, the waveguide sensitivity is independent of the device
configuration. It has been thoroughly studied elsewhere [21],
[26], aiming for the appropriate transverse structure providing
the optimal distribution of the evanescent wave based on the
wave theory. On the other hand, Chao and Guo [20] have ex-
plored the dependence of sensing capability on device-configu-
ration-related parameters, such as the waveguide width and the
radius of the ring, concluding that it is the quality factor
that
dominates the detection limit for the microring resonator sensor
operating by the resonance shift scheme. For a sensor based on
resonance shift, the distinguishable shift of wavelength becomes
smaller as the bandwidth of the resonance decreases, leading to
a lower detection limit.
As an alternative, microring resonator sensors based on in-
tensity detection may be advantageous over those based on res-
onance shift because they eliminate complex spectrum measure-
ment setup, potentially possess higher sensitivity [13], and re-
quire less assay time. Clearly, the device sensitivity for these
sensors is determined by the slope of the spectrum rather than
the spectral bandwidth. Therefore, instead of focusing on
,we
have thoroughly analyzed the explicit relation between the de-
vice sensitivity and three key parameters: the transmission co-
efficient, the self-coupling coefficient, and the operating wave-
length, which has not been reported before to the best of our
knowledge. As will be shown, the first two parameters are of
vital importance because it is through them that the geometric
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