Optical fibres: atomic spectroscopic methods for the determination of fluoride glasses composition

Abstract

Three different methods for the evaluation of the composition of optical fibres employed in long distance telecommunications have been developed. Such fibres are obtained from fluoride glasses containing very low impurity levels. The developed methods are based on inductively coupled plasma atomic emission spectrometry coupled with:

• wet chemical dissolution;

• alkali fusion;

• direct solid slurry sampling.

The procedures are compared with reference to the optimization of sample preparation, loss of components after sublimation, normalization with standards, accuracy, precision.

Introduction

Fluorozirconate glasses discovered in 1975 by Poulain [1]are now widely considered as excellent candidates for manufacturing optical fibres. The interest in this material began with long-distance telecommunication applications because of its low theoretical background loss 1×10⁻³ dB/km (two orders of magnitude lower than silica glass). At present fluorozirconate fibres became more attractive for the production of optical fibre amplifiers operating at the wavelength of 1.3 and 1.5 μm 2, 3; moreover, they can also find widespread applications in medical and military fields.

These fibres consist of an inner part (core) having a higher refractive index and an outer shield (cladding) surrounding the core. Both core and cladding are multicomponent glasses transparent in the visible and infrared region, constituted by fluoride compounds such as ZrF₄, BaF₂, LaF₃, YF₃, AlF₃, NaF, and LiF. During the glass synthesis, which is carried out at 850°C, some components like ZrF₄, AlF₃, NaF and LiF sublime, evaporate and may carry others away. The result is a real glass composition different from the nominal one [4]. Since physical and optical properties are so strongly dependent on glass composition, it is very important to develop an analytical procedure to determine the fluorides concentration in the glass.

Only few papers on the analytical determination of fluorides in glasses have been published 5, 6, 7; these studies refer only to ZBLAN glass composition (ZrF₄, BaF₂, LaF₃, AlF₃, NaF).

In this work, inductively coupled plasma atomic emission spectroscopy is used to determine the composition of both core and cladding glasses. Three different analytical procedures have been used. The first involves the dissolution of cladding and core by means of acid digestion, the second proceeds through the fusion with a flux agent, the third was optimized for the direct analysis of the solid material in the form of an aqueous suspension (slurry-sampling analysis).

Slurry sampling analysis presents a series of advantages that make this procedure more and more attractive; in particular, it allows the direct analysis of solid samples insoluble with common treatments such as some fluorides used for producing glasses or lead-based glasses with remarkable analysis time savings. In addition, the absence of a dissolution step with chemical reagents allows on one hand to reduce the risk of chemical alterations by impurities from the reagents, and on the other hand to reduce the effect of chemical interference during the atomic emission determination.