  The chapter is situated in the fields of fiber materials sciences and
technologies (particularly dealing with the fiber glass compositions and
luminescent ion doping, and transparent glass ceramic optical fibers), and
applications such as fiber lasers and amplifiers. We propose to present a
review of research activities on rare-earth (RE) and transition metals (TM)
doped silica-based optical fibers, aiming at understanding theier spectral
properties, and particularly some of their optical transitions that will allow
interesting and alternative applications. Silica, as opposed to alternative,
low temperature materials, is chosen for practical and economical reasons.
Selected RE and TM ions are studied both as probes of their local environment
and for their application potentials. In this chapter, we will focus on erbium
(Er) ions for the potential spectral 'tailoring' of its gain curve at 1.55
$\mu$m, thulium (Tm) as local environment probe along both near infrared
transitions around 0.8 and 1.47 $\mu$m, and chromium (Cr) for the sensitivity
of valency and spectroscopy to the glass composition and its potential as
saturable absorber in lasers. We will present some extensive studies on the
influence of the local environment on dopants: such as (but not limited to)
energy transfer mechanisms between rare-earth ions of same nature (Tm-Tm) or
different nature (Yb-Tm), effect of the local phonon energy on Tm ions emission
efficiency, broadening of Er ions emission induced by oxide nanoparticles grown
in situ during the fabrication process, etc. Knowledge of these effects is
essential for photonics applications, and many studies are under progress or
are still needed in this field. The potential of some applications which could
benefit from the basic studies on materials are also numerically studied :
0.8-$\mu$m thulium-doped fiber laser, so-called S-band (1.47 $\mu$m)
thulium-doped amplifier for telecommunications and laser applications,
sensitisation of Tm-based devices using ytterbium sensitized ions.
