Design of a Silicon Overlay Glass Waveguide Sensor

doi:10.4236/opj.2011.14025

Paper Menu >>

Journal Menu >>

Optics and Photonics Journal, 2011, 1, 151-154

doi:10.4236/opj.2011.14025 Published Online December 2011 (http://www.SciRP.org/journal/opj)

Design of a Silicon Overlay Glass Waveguide Sensor

Kristian E. Medri, Robert C. Gauthier

Department of Electronics, Carleton University, Ottawa, Canada

E-mail: [email protected]

Received August 13, 2011; revised Sep t ember 12, 2011; acc e pt ed September 24, 2011

Abstract

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

Optics

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-

sented.

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-

K. E. MEDRI ET AL.

152

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





 

 (1)

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.

153

K. E. MEDRI ET AL.

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.

K. E. MEDRI ET AL.

154

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.

6. References

[1] M. Kawachi, “Recent Progress in Silica-Based Planar

Lightwave Circuits on Silicon,” IEE Proceedings: Opto-

electronics, Vol. 143, No. 5, 1996, pp. 257-262.

doi:10.1049/ip-opt:19960493

[2] J. Broquin, “Glass Integrated Optics: State of the Art and

Position toward Other Technologies,” Proceedings of

SPIE, Vol. 6475, No. 7, 2007, pp. 1-13.

doi:10.1117/12.706785

[3] P. Prabhathan, V. M. Murukeshan and J. Zhang, “Com-

pact SOI Nanowire Refractive Index Sensor Using Phase

Shifted Bragg Grating,” Optics Express, Vol. 17, No. 17,

2009, pp. 15330-15341. doi:10.1364/OE.17.015330

[4] S. Mandal, R. Akhmechet, L. Chen, S. Nugen, A. Baeum-

ner and D. Erickson, “Nanoscale Optofluidic Sensor Ar-

rays for Dengue Virus Detection,” Nanoengineering:

Fabrication, Properties, Optics and Devices IV, The In-

ternational Society for Optical Engineering (SPIE), 2007.

[5] V. M. N. Passaro, R. Loiacono, G. D’Amico and F. De

Leonardis, “Design of Bragg Grating Sensors Based on

Submicrometer Optical Rib Waveguides in SOI,” IEEE

Sensors Journal, Vol. 8, No. 9, 2008, pp. 1603-1611.

doi:10.1109/JSEN.2008.929068

[6] K. E. Medri and R. C. Gauthier, “Patterned Overlays:

Thin Silicon Layer Applied to Glass Waveguides,” Pro-

ceedings of SPIE, Vol. 7943, No. L, 2011, pp. 1-12.

doi:10.1117/12.873426

[7] K. Okamoto, “Fundamentals of Optical Waveguides,” Se-

cond Edition. Academic Press, Cleveland, 2005.

[8] C. Chen, “Foundations for Guided-Wave Optics,” John

Wiley, Hoboken, 2006. doi:10.1002/0470042222

[9] C. Chen, “Development and Implementation of Novel

Numerical Techniques for Integrated Optics and Micro-

wave Planar Structures,” Ottawa-Carleton Institute for

Electrical and Computer Engineering, Ottawa, 2000.

[10] A. Yimit, A. G. Rossberg, T. Amemiya and K. Itoh,

“Thin Film Composite Optical Waveguides for Sensor

Applications: A Review,” Talanta, Vol. 65, No. 5, 2005,

pp. 1102-1109. doi:10.1016/j.talanta.2004.06.045

[11] A. Yariv, “Coupled-Mode Theory for Guided-Wave Op-

tics,” IEEE Journal of Quantum Electronics, Vol. 9, No.

9, 1973, pp. 919-933. doi:10.1109/JQE.1973.1077767

[12] A. F. Oskooi, D. Roundy, M. Ibanescu, P. Bermel, J. D.

Joannopoulos and S. G. Johnson, “Meep: A Flexible

Free-Software Package for Electromagnetic Simulations

by the FDTD Method,” Computer Physics Communica-

tions, Vol. 181, No. 2, 2010, pp. 687-702.

doi:10.1016/j.cpc.2009.11.008

[13] I. Giuntoni, M. Krause, H. Renner, J. Bruns, A. Gajda, E.

Brinkmeyer and K. Petermann, “Numerical Survey on

Bragg Reflectors in Silicon-On-Insulator Waveguides,”

5th International Conference on Group IV Photonics,

17-19September 2008, pp. 285-287.

[14] M. Hammer and O. V. Ivanova, “Effective Index Appro-

ximations of Photonic Crystal Slabs: A 2-to-1-D Assess-

ment,” Optical and Quantum Electronics, Vol. 41, No. 4,

2009, pp. 267-283. doi:10.1007/s11082-009-9349-3

[15] B. E. A. Saleh and M. C. Teich, “Fundamentals of Pho-

tonics,” John Wiley, Hoboken, 1991.

doi:10.1002/0471213748

[16] A. H. Harvey, J. S. Gallagher and J. M. H. L. Sengers,

“Revised Formulation for the Refractive Index of Water

and Steam as a Function of Wavelength, Temperature and

Density,” Journal of Physical and Chemical Reference

Data, Vol. 27, No. 7, 1998, pp. 761-74.

[17] F.J. Bates, et al., “Polarimetry, Saccharimetry and the

Sugars,” National Bureau of Standards C440, U.S. Gov-

ernment Printing Office, Wahington, DC, 1942.

[18] R. C. Weast, “Handbook of Chemistry and Physics,”

CRC Press, Cleveland, 1977.

[19] A. Iadicicco, A. Cusano, A. Cutolo, R. Bernini and M.

Giordano, “Thinned Fiber Bragg Gratings as High Sensi-

tivity Refractive Index Sensor,” IEEE Photonics Tech-

nology Letters, Vol. 16, No. 4, 2004, pp. 1149-1151.

doi:10.1109/LPT.2004.824972

[20] A. Iadicicco, S. Campopiano, A. Cutolo, M. Giordano

and A. Cusano, “Refractive Index Sensor Based on Mi-

crostructured Fiber Bragg Grating,” IEEE Photonics

Technology Letters, Vol. 17, No. 6, 2005, pp. 1250-1252.

doi:10.1109/LPT.2005.846570

[21] L. Poladian, F. Ladouceur and P. D. Miller, “Effects of

Surface Roughness on Gratings,” Journal of the Optical

Society of America B (Optical Physics), Vol. 14, No. 6,

1997, pp. 1339-1344.