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The low luminescence light levels radiating from the side of the fibre prevented a side luminescence spectrum from being measured. Instead, counter-propagating lumines- cence spectra were obtained using a 2×2 optical fibre coupler in the experimental arrangement shown in Figure 3.6.

laser doped fibre ~ 50 mm index matched splice Avtech laser driver DC PC GPIB interfaced index matched optical spectrum analyser bare fibre adapter 2x2 coupler

Figure 3.6: Experimental arrangement to measure the counter-propagating luminescence spectrum from an optical fibre sample.

The excitation source was operated in continuous wave mode at the maximum oper- ating current of the source. The doped fibre section was spliced to the coupler arm which contained the majority of pump light. The resulting fluorescence which propa- gates in the forward direction is termed co-propagating fluorescence and the fluores- cence which propagates back through the 2×2 coupler is termed counter-propagating fluorescence. The drawback to measuring the co- or counter-propagating fluores- cence is that the fluorescence is subjected to reabsorption as it propagates through the doped sample, often producing dips at wavelengths where a strong ground state transition exists. The counter-propagating luminescence spectra reported in this work were recorded using an Ando AQ6310B optical spectrum analyser with a res- olution of 5 nm.

THULIUM-DOPED SILICA FIBRES

4.1

Introduction

The Tm3+ _{ion offers a range of fluorescence wavelength bands extending from 0.475}

up to 3.5µm which have, or have the potential for, applications in a variety of fields such as telecommunications, medicine, atmospheric sensing and defence, to name but a few. More notably, the 1.47 - and 2 - µm transitions of Tm3+ _{have attracted}

the most attention due to their potential application as optical amplifiers and high powered lasers. Unfortunately, in silica glass these transitions are dominated by non- radiative decay, due to the relatively high phonon energy of the glass, which limits the quantum efficiency of both transitions to a few percent. For thulium doped silica based fibres to become a viable solution for the aforementioned applications, considerable improvement in the quantum efficiency of these transitions is required.

Improvements in the quantum efficiency of optical transitions can be achieved by modifying the local environment surrounding the rare earth ion [87, 88, 39]. Such modifications are often achieved by breaking up the structure of atoms surrounding a rare earth ion with other, often larger elements, termed network modifiers. Dis- cussing the role of network modifiers requires an understanding of the basic structure of silica glass. Silica is built from basic structural units, the most common of which is the network former (SiO4)2−. This network former consists of a silicon atom at

the centre of a tetrahedron with an O atom bonded to each corner, as shown in Figure 4.1.

Figure 4.1: Tetrahedral structure of SiO42−, showing 4 oxygen atoms surrounding the

central silicon atom. [89].

The formation of the glass structure was first proposed by Zachariasen who sug- gested a set of four rules for glass formation in an oxide in order to obtain a random network [90]. The four rules are listed below:

1. oxygen atoms are linked to no more than two atoms of type A (A = B, Si, As, Ge, etc.);

2. the oxygen co-ordination around A is small (i.e. 3 or 4); 3. oxygen polyhedra share corners but not edges or faces and 4. at least three corners are shared.

This commonly accepted structure of silica glass is therefore tetrahedral units that are tightly connected by their corners through oxygen atoms (bridging oxygens); these random connections form a 3 dimensional structure similar to that shown in Figure 4.2.

The strong electron bond which exists between the silicon and oxygen atoms give silica glass its impressive mechanical strength and thermal properties. The drawback to the silica glass network is that it can accommodate only a small number of rare earth ions before clustering occurs [91]. One explanation of this phenomenon is that

Figure 4.2: 3 - dimensional structure of SiO2, showing the interconnection of the tetra-

hedral units [89].

rare earth ions require co-ordination of a sufficiently high number of non-bridging oxygens to screen the electric charge of the ion, but the highly rigid silica glass network cannot sufficiently co-ordinate non-bridging oxygens resulting in a system with a higher enthalpy state. Therefore, rare earth ions tend to share non-bridging oxygens to reduce the excess enthalpy, resulting in the formation of clusters [92]. To accommodate greater amounts of rare earth ions, network modifiers are required to increase the number of available non-bridging oxygen ions in the silica glass network. Network modifiers such as Na3+ _{and Al}3+ _{are often used to facilitate the incorpora-}

tion of rare earth ions, as their size is substantially greater than the basic network. These modifiers act to break the bridging oxygens to form non-bridging oxygens which can be used to co-ordinate the rare earth ions. Of the network modifiers stud- ied in silica glass to date, aluminium has shown the most favourable characteristics. In studies of Nd3+ _{-doped silica glass, clustering of 0.29 mol% of Nd}

2O3 ions was

eliminated with the incorporation of 2.87 mol % of Al2O3 [92]. Aluminium has also

been used to improve the efficiency of Er3+ _{-doped fibre amplifiers by eliminating the}

quenching effects from the 4_I

Tm3+ _{is not significantly different from that of Er}3+ _{and Nd}3+_{, one could speculate}

that similar effects may be observed in Tm3+ _{-doped silica glasses.}