09-07-2012, 10:21 AM
Eigen Value Equation and Core-Mode Cut-off of Weakly Guiding Tapered Fibre as Three Layer Optical Waveguide and used as Biochemical Sensor
Eigen Value Equation and Core-Mode Cut-off of Weakly Guiding Tapered Fibre as Three...doc (Size: 802.5 KB / Downloads: 42)
Abstract
The core-mode cut-off plays a major role in the evanescent field absorption based sensors. A method has been proposed to calculate the core-mode cut-off by solving the eigen value equations graphically of a weakly guiding three layer optical waveguide. The variation of normalized waveguide parameter (V) is also calculated with different wavelengths at core-mode cut-off. At the first step, theoretical analysis of tapered fibre parameters has been performed for core-mode cut-off. Taper angle of an adiabatic tapered fibre is also analyzed using length-scale criterion. Secondly, single mode tapered fibre has been developed to make a precision sensor element suitable for chemical detection. Finally, the sensor element has been used to detect absorption peak of ethylenediamine (EDA). The results are presented in which the absorption peak at 1540nm is observed.
Index Terms—Eigen value equation, Evanescent wave absorption, Tapered fibre, Chemical Sensor, Ethylenediamine detection.
I. INTRODUCTION
N recent years, the single mode tapered fibre (SMTF) is used in several sensor applications such as chemical sensing, refractive index sensing etc. The basic concept behind the fibre sensor using SMTF is the evanescent field interaction [1-3]. The fabrication of SMTF can be done by heating and pulling the fibre, after removing the outer jacket. It is now considered that the fibre is with three layers consisting of core, cladding and the external medium (air). When such an optical waveguide tapers down, the effective index ( ) of the fundamental mode (LP01) decreases monotonically. Because of the finite cladding, LP01 mode becomes cladding mode when is below the cladding refractive index (ncl). When becomes equal to the ncl, the fibre is said to be core-mode cut-off [4]. Among the various important aspects of optical waveguides, cutoff conditions play an important role in the design of SMTF based evanescent field absorption sensors.
Evanescent wave absorption sensing and spectroscopy [5-10] are the widely used techniques for the detection of unknown chemicals. Optical fibres can be used in evanescent wave absorption sensing applications, due to the fraction of modal power extended into the external environment as evanescent wave. The basic idea behind the evanescent wave absorption spectroscopic technique is the wavelength selective absorption of guided power in an optical fibre by the chemicals present in external environment. For the exposure of evanescent wave to the chemicals, suitable modification of the fibre structure is required. For this requirement, the uncladded and tapered fibres can be used. The tapered fibres are more sensitive than the uncladded fibres because of the enhancement of the modal field by tapering. Previously in many literatures evanescent wave absorption by using multimode tapered fibres was reported [5-10].
In this paper, SMTFs are used for evanescent wave absorption sensing applications. The ethylenediamine (EDA) is used as the chemical, which has an absorption peak at 1540nm. The transmission spectrum of the tapered fibre, when EDA is present in the external environment, shows a peak at 1540nm. This result shows the SMTFs can be used as a sensor element in evanescent wave absorption sensing applications.