  In this paper we present the fluorescence decay profiles of vanadium and
titanium doped gallium lanthanum sulphide (GLS) glass at various doping
concentrations between 0.01 and 1% (molar). We demonstrate that below a
critical doping concentration the fluorescence decay profile can be fitted with
the stretched exponential function: exp[-(t/{\tau})\b{eta}], where {\tau} is
the fluorescence lifetime and \b{eta} is the stretch factor. At low
concentrations the lifetime for vanadium and titanium doped GLS was 30 {\mu}s
and 67 {\mu}s respectively. We validate the use of the stretched exponential
model and discuss the possible microscopic phenomenon it arises from. We also
demonstrate that above a critical doping concentration of around 0.1% (molar)
the fluorescence decay profile can be fitted with the double exponential
function: a*exp-(t/{\tau}1)+ b*exp-(t/{\tau}2), where {\tau}1and {\tau}2 are
characteristic fast and slow components of the fluorescence decay profile, for
vanadium the fast and slow components are 5 {\mu}s and 30 {\mu}s respectively
and for titanium they are 15 {\mu}s and 67 {\mu}s respectively. We also show
that the fluorescence lifetime of vanadium and titanium at low concentrations
in the oxide rich host gallium lanthanum oxy-sulphide (GLSO) is 43 {\mu}s and
97 {\mu}s respectively, which is longer than that in GLS. From this we deduce
that vanadium and titanium fluorescing ions preferentially substitute into high
efficiency oxide sites until at a critical concentration they become saturated
and low efficiency sulphide sites start to be filled.
