Chalcogenide double index fibers: fabrication, design, and application as a chemical sensor

Abstract

Double index chalcogenide fibers, based on tellurium, arsenic, and selenium, have been made by an original technique. This technique, based on the build-in-casting method, is achieved in a sealed silica ampoule. In view of the low attenuation obtained in the mid-infrared (IR), these fibers are used to implement Fiber Evanescent Wave Spectroscopy (FEWS). As the IR light is only propagated through the core of the waveguide, a chemical etching is applied in order to remove the glassy cladding of the sensing zone. IR spectra of ethanol and chloroform, recorded with such sensor, are presented showing the high sensitivity of the method.

Introduction

Chalcogenide glasses are based on the chalcogen elements, such as S, Se, and Te, and the addition of other elements, such as Ge, As, and Sb, that facilitate the formation of stable glasses [1]. This category of glasses shows the tendency to transmit the mid-infrared (IR) light due to their low phonon energy associates with the combinations of heavy elements. Moreover, the addition of halogens X (Cl, Br, I) in some compositions increases the transmitting window, which can extend from 2 to 20 μm in case of few millimeters thick samples [2].

Among the numerous chalcogenide glassy systems, we have selected the Te–As–Se based compositions, called TAS glasses. They have favorable viscosity–temperature dependence near the drawing temperature that allows fabrication of optical fibers with a special design. In the form of fibers, the spectral transparency of these glasses lies approximately from 2 to 12 μm. This wavelength region is particularly attractive for optical sensing because it contains the IR fingerprints of the chemical and biological molecules. Based on the general concept of Fiber Evanescent Wave Spectroscopy (FEWS) [3], [4], [5], the use of fibers allows remote, sensitive, and in situ measurements of the IR fingerprints. Our chalcogenide fibers are very suitable candidates for such spectroscopy in the mid-IR [6]. Until now, a single index fiber was used as sensor and as waveguide. It has been demonstrated that the sensitivity is greatly improved using a tapered fiber [6]. On one hand, these fibers have a large diameter, greater than 400 μm, to allow the transmission of the maximum IR light from the source (black body) until the detector (Hg–Cd–Te), and, on the other hand, on a short length of a few centimeters corresponding to the sensing zone, the diameter must be as thin as possible, i.e. smaller than 100 μm. This goal is achieved using a chemical etching solution to locally and congruently dissolve the TAS glass [7].

In this paper, we will describe the first FEWS experiments carried out with double index chalcogenide fibers. To record such spectra, we have to dissolve up the cladding glass at the level of the sensing zone. The use of core/cladding fibers as sensor will be doubly interesting. Firstly, going towards single mode fiber should greatly improve the sensitivity of the sensor. Secondly, these fibers isolate the core, where the light propagates, from the coating polymer, which surround the cladding. The polymer makes the sensor easier to handle by improving its mechanical behavior and increasing its flexibility. But the consequence with a single mode fiber is the presence of the polymer absorption band on the FEWS spectra. The use of double index fiber avoids these artifacts. The first part of the article is devoted to the double index TAS fiber fabrication by a new method and to the implementation of the sensing zone design. Secondly, the IR spectroscopy experiments carried out with these core/cladding fibers are shown and discussed.