Biosensors & Bioelectronics 15 (2000) 283–289
High sensitivity waveguide grating sensor based on radiative losses
Vladimir Kochergin *, Ivan Avrutsky, Yang Zhao
Department of Electrical and Computer Engineering, Wayne State Uni6ersity, Detroit, MI
48202
, USA
Received 20 October 1999; accepted 17 May 2000
Abstract
A scheme of a waveguide sensor utilizing an output grating coupler with the grating length less than the coupling length is
analyzed. This is the new method for the evaluation of average thickness of the adsorbed layer. This method, based on
measurements of radiation loss coefficients, is proposed. Sensitivity limits of such a scheme are discussed. The new scheme has
strong response on formation on the adsorbed layer and relatively weaker response on the fluctuations of temperature and analyte
concentration. New possibilities for multiparameter sensing are shown. © 2000 Elsevier Science S.A. All rights reserved.
Keywords
:
Waveguide; Grating coupler; Radiation losses; Sensors
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1. Introduction
Applications of grating couplers in biological and
chemical sensing are well known Lukosz et al., 1991;
Nellen and Lukosz, 1993; Brandenberg and Gombert,
1993; Clerc and Lukosz, 1997. High sensitivity and real
time data analysis Brecht and Gauglitz, 1998 provide the
great interest in this type of sensors nowadays. However,
increasing of sensitivity and getting a clear sensor re-
sponse in the presence of fluctuations of temperature and
chemical composition of the analyte are still important
issues Stamm et al., 1998. In this paper we suggest the
use of new detection parameters and show that they can
improve quality of gating sensor.
The traditional approach in grating based sensors is
measurement of angular position of incident beam corre-
sponding to maximal optical power coupled to the
waveguide. One can also use the direction of outcoupled
beam as sensing parameter. Light coupling/out coupling
is described by phase-matching (Eq. (1)):
2y
u
n*=
2y
u
m
i
sin(q)+ l
2y
\
(1)
where u is the wavelength of light, m
i
is the dielectric
permittivity of medium into or from which light is
coupled or outcoupled, n* is the modal index of the
waveguide, q is the angle of incoupling or outcoupling, \
is the grating period and l=91, 92… is the diffraction
order. In the experiment, the wavelength, the grating
period, and the diffraction order are known, so that the
angle q is unambiguously related to the modal index. In
the real time measurements it is more convenient to
measure the deviation of the angle from its initial
position. This deviation is associated with the modal
index change. Possibilities for high accuracy angle mea-
surement provide high sensitivity of grating coupler
based sensors. Operation of all grating coupler sensors is
based on strong dependence of effective refractive in-
dexes of waveguide modes on the thickness of sensing
layer on the top of grating (Fig. 1). However, very high
resolution can not be exploited in practice because
refractive index changes associated with chemical com-
position and/or temperature variations of the sample
solution also contribute to the sensor response Stamm et
al., 1998. Although the modal index depends on refrac-
tive indices and thickness of all the layers forming the
waveguide, there are only three major factors that may
affect the modal index change in sensing experiment.
They are: (1) sensing layer formation; (2) change of the
refractive index of the solution due to chemical reac-
tions; and (3) change of refractive indices of all the
waveguide layers due to temperature variations. For
extraction of pure response produced by sensor layer
thickness dual wavelength scheme has been proposed
* Corresponding author. Tel.: +1-313-5770245.
E-mail address
:
kocherg@ece.eng.wayne.edu (V. Kochergin).
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