Optics and Photonics Journal, 2011, 1, 151-154
doi:10.4236/opj.2011.14025 Published Online December 2011 (http://www.SciRP.org/journal/opj)
Copyright © 2011 SciRes. OPJ
Design of a Silicon Overlay Glass Waveguide Sensor
Kristian E. Medri, Robert C. Gauthier
Department of Electronics, Carleton University, Ottawa, Canada
E-mail: kmedri@doe.carleton.ca
Received August 13, 2011; revised Sep t ember 12, 2011; acc e pt ed September 24, 2011
A glass based slab waveguide, coated with a thin patterned high dielectric overlay, is configured into a re-
fractive index sensor. The asymmetric nature of the waveguide configuration is exploited by keeping the
mode in the slab waveguide while enhancing the field level in the overlay-superstrate. The sensor's response
is examined using the FDTD simulation technique. A sensitivity of up to one part in 105 in the index of re-
fraction discrimination is determined. The nature of the sensor ensures optical fibre compatibility, requires
sub-µL sample volumes and provides a high resolution.
Keywords: Refractive Index Sensor, Slab Waveguide, Patterned High Dielectric Overlay, FDTD, Integrated
1. Introduction
Silica based planar lightwave circuits (PLCs) evolved
from simple passive waveguide structures to complex
active devices containing resonators and filters.[1,2]
When interfaced with optical fibers, PLCs can demon-
strate low insertion losses due to th e compatibility of the
materials, large modal overlap and relaxed manufactur-
ing tolerances. In general, glass based integrated optics
demonstrate a slow response when configured as active
devices and are large due to the limited dielectric con-
trast between guiding and substrate-superstrate layers. In
the near and distant future it is expected that the optical
fiber will continue to dominate as the signal carrying
medium for optical telecommunications and sensing. The
inter-compatibility requirements of active and passive
integrated-optic devices and the optical fibre technology
will remain. This will require low loss, low insertion loss,
and fast IO components. These requirements can be met
by a glass based waveguide design as the light guiding
region, to minimize fibre coupling and insertion losses,
then adding a thin high dielectric overlay of patterned
silicon at the waveguide superstrate interface to retain
the high dielectric contrast and response speed. This re-
sults in a hybrid platform of silica and silicon akin to a
strip loaded waveguide.
Refractive index sensors can be used for many appli-
cations. Previously proposed sensors which have similar
footprints and responsiv ities include a 13 um sensor pro-
viding 90 nm/RIU[3] and an 8 um sensor providing 130
nm/RIU [4] which have small non fiber compatible mode
sizes. The researchers of the former include a compari-
son to alternative sensing structures while the latter men-
tions a much longer 173 um sensor providing a lower
resolution of 33 nm/RIU [5]. This longer sensor has a
larger mode size. They mention the option of using a 10
um plasmon sensor providing 465nm/RIU but with a
large 10 dB loss.
In order to demonstrate the principles involved in this
hybrid waveguide configuration we employ a slab wave-
guide in glass and a silicon overlay patterned as a Bragg
grating [6]. The functionality is explored by examining
the structure’s performance as an index of refraction
sensor sensitized to the sensing region comprised of the
superstrate region above the grating as well as in the
etched regions of the grating. The next section presents
the design of the sensor and discusses the modeling te-
chnique employed. The sensor’s performance is then pre-
2. Design
The simplest planar optical structure is the traditional
three layer glass slab waveguide (superstrate-waveguide
-substrate), modified here into a 4 layer structure by
adding a thin high dielectric overlay at the superstrate-
waveguide interface (superstrate-overlay-wave-guide-
substrate). The basic theory and analytical solution for
the three layer structure is widely described [7,8] and has
been extended to address multilayer structures with di-
electric overlays [9,10]. This enables the modal profiles
and propagation constants for the three layer and four
layer structures to be determined with high accuracy.
However, when the overlay is patterned in the direction
of light propagation, the perturbation approach [11] to
the waveguide analysis does not apply as the dielectric
contrast is very high between waveguide and overlay
layers. The optical properties of such structures can be
modeled using numerical methods such as the Finite-
Difference-Time-Domain (FDTD) [12,13] technique pro-
vided that the thin overlay layer is well resolved in the
discretized grid.
The structure under consideration is shown in Figure
1. The substrate layer is glass with an index of refraction
n4 = 1.500 and extends to - in the x direction. The
waveguide layer is formed from a 1 µm thick slab of
index n3 = 1.600. The superstrate layer is typically air
with an index of n1 = 1.000 and extends to +. The thin
overlay, shown as a dashed line, has an index of n2 =
3.480, a thickness t2 = 17.5 nm and is patterned as a
Bragg grating in the z direction. The sensing region is
considered to be that of the superstrate in the grating
region including the etched regions of the grating. The
grating pitch is designed to meet the Bragg condition for
a wavelength of 1550 nm when the sensing region con-
tains the highest index material to be sensed. Also for
this material, the thickness t2 of the overlay is chosen
such that the overall waveguiding structure remains sin-
gle mode with the maximum valu e of the effective index
below 1.600 for the wavelengths considered. The alter-
nating high and low effective index regions of the 40 µm
long grating are each made a quarter wavelength in
length which is determined by using the propagation
constants of the 3 and 4 layer modal solutions [14].
3. Simulation and Discussion
A 2-D FDTD environment was set up to model the
propagation of light in the waveguide sensor structure.
The (x, z) plane of a simulation configuration is shown in
Figure 1 and consists of a 3-layer segment of 1 µm for
the source plus 12 µm followed by a 40 µm long grating
and terminated by a 12 µm 3-layer waveguide. The entire
domain was bordered by a 1 µm PML with matched di-
electric values. The y-direction was taken as infinite and
out of the page. The entire structure was discretized on a
Figure 1. Slab waveguide in glass with thin patterned sili-
con grating overlay. Source, reflection and transmission
planes used in FDTD simulation shown.
square grid at 160 points per micron resolution to ensure
suitable structure resolution and minimal numerical dis-
persion as confirmed through convergence tests. The
input source cons isted of a short time dur ation pulse with
a profile and field components matched to those for the
3-layer geometry obtained by solving the field equation
for the field polarized in the y direction (EY). The multi-
wavelength source is defined as the discrete-time deriva-
tive of a Gaussian,
exp/ 2
Ei itttw
 
with a central wavelength of 1550 nm and w = 0.334.
The source plane was spaced 12 µm from the grating
input and the detecto r planes are located 11 µm from the
grating end. The power reflectance and transmittance,
normalized to source output, were computed on each
detector plane following the technique presented in [15].
The simulated spectrums for this structure when pure
water is in the sensing region, n1 = 1.315 for 25˚C [16],
are shown in Figure 2. In the transmission spectrum the
deepest dip at just under 1550 nm corresponds to the
Bragg wavelength of the grating and the secondary dip at
a lower wavelength is associated with power coupling to
substrate and radiation modes. A band pass region for
wavelengths beyond the Bragg wavelength is also ob-
served. The reflection spectrum is a dual of the trans-
mission spectrum. In the analysis of the sensor the trans-
mission spectrums are examined and attention is directed
at the Bragg dip’s wavelength and depth dependence on
the sensing region’s index.
The structure’s response with pure water in the sen-
sing region is used as a reference for the response with
the index of the sensing region increased to a maximum
of n1 = 1.465 (a change of 0.15) in steps of 0.025. The
index values and range were chosen to be representative
of the wavelengths, temperature, and solutes considered.
The highest refractive index of 1.465 corresponds to a 77%,
Figure 2. Transmittance and reflectance spectrums for the
sensor with pure water in the sensing region. Bragg wave-
length denoted by star.
Copyright © 2011 SciRes. OPJ
The highest refractive index of 1.465 corresponds to a
77%, or 77 Brix [17], sucrose solution given a refractive
index shift similar to that of water due to change of
wavelength and the few degree temperature change from
the 20˚C data [18]. As the refractive index in the sensing
region is increased the Bragg dip shifts to a longer wave-
length as shown in Figure 3. In addition, the increasing
index has the effect of modifying the grating’s duty cycle
and modal overlap between the cascade of three and 4
layer waveguide regions making up the grating. The
grating being designed for the maximum index in the
sensing region ensures that the Bragg dip gets mono-
tonically deeper over the sensing range of index values.
The sensor’s response can be evaluated using two cri-
teria. The first is to track the change in the Bragg dip
wavelength and the second is to monitor the change in
signal strength for a fixed operation wavelength. The
sensor’s performance in both modes of operation can be
determined from the family of traces in Figure 3. In or-
der to consider the use of the shift in the Bragg dip ver-
sus refractive index, th e locatio ns of th e mini mu ms in th e
primary dip are marked with stars in Figure 3. A wave-
length shift is 85 nm/RIU is determined and assuming a
wavelength resolution of 1 pm [19] a refractive index re-
solution of 10–5 is possible.
When the output intensity is monitored directly, it is
instructive to use the pure water trace as a reference and
then determine the change in output intensity as a func-
tion of wavelength and sensing region index relative to
the chosen reference. The families of traces in Figure 4
represent change in output intensity derived from the
data of Figure 3. The maximum of the change in trans-
mittance is used to start the sensor waveleng th selection.
Figure 3. Transmittance for different superstrate refractive
indices. The solid line represents the pure water trace with
n1 = 1.315, while two each of the dashed, dashed and dotted,
and dotted lines represent n1 = 1.340, 1.365, 1.390, 1.415,
1.440, and 1.465 respectively. Star points on each trace
indicate the location of the Bragg wavelength of the grating.
Next the response for wavelengths slightly above and
below this were compared to find a wavelength with the
strongest change in response matching the center of the
index range considered.
The sensor response for a wavelength of 1556 nm is
shown in Figure 5. Depending on the desired sensing
index range, the sensing wavelength can be optimized for
the maximum slope and linearity for that range. Since
amplitude detection units are able to resolve 0.1% inten-
sity changes [20], this provides a sen sitivity of 1.8 × 10–4
for a 40 µm sensor. Fabrication roughness can reduce the
sharpness and strength of a grating dip which can be
compensated by increased length [21].
4. Conclusions
A refractive index sensor has been presented based on
the optical properties of a slab waveguide coated with a
patterned thin high dielectric overlay. The hybrid nature
of the entire waveguide configuration makes it possible
to keep the waveguid e mode in the slab wavegu ide com-
patible to optical fiber while enhancing the evanescent
Figure 4. Change in transmittance. Two each of the dashed,
dashed and dotted, and dotted lines represent n1 = 1.340,
1.365, 1.390, 1.415, 1.440, and 1.465 versus the n1 = 1.315
simulation respectively.
Figure 5. Sensor normalized power response for source
wavelength of 1556 nm.
Copyright © 2011 SciRes. OPJ
Copyright © 2011 SciRes. OPJ
wave-overlay interaction using the high dielectric.The
sensitivity of the guided light to variations in the clad-
ding refractive index was examined us ing FDTD simula-
tions and transmission spectrums are obtained for various
superstrate refractive indices. The response of the sensor
is examined using both the change in the Bragg dip
wavelength and transmitted signal strength and is shown
to provide an index resolution of up to one part in 105.
5. Acknowledgements
The authors gratefully acknowledge NSERC, CIPI, and
the reviewers for their support.
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