Chemistry Reference
In-Depth Information
glass (ZrF
4
-BaF
2
-LaF
3
-AlF
3
-NaF), because of their potential in the development of
optical fibres for long-distance telecommunication links. These applications require
the absence of optical attenuation from scattering by crystals and particulates, but also
ultrahigh purity materials containing no light-absorbing impurities such as transition
metal (Fe
2
þ
,Cu
2
þ
, etc) hydroxyl (OH
) ions. To date, the ultimate predicted ultralow
optical loss of 0.01 dB/km compared to 0.2 dB/km for silica glasses [3] has not been
reached, the biggest obstacle being in reducing transition-metal impurities and inhibiting
formation of crystallites during the melt cooling operation or after reheating above the
glass transition temperature (Tg). Nevertheless, fluoride glass remains an attractive
material in shorter optical devices with applications lying in the visible and mid-IR
spectral range, including lasers and amplifier operating at wavelengths not accessible
with silica-based glasses, thank to their low phonon energy (
500-600 cm
1
) compared
to silica (
1100 cm
1
). In particular, the development of optical communication neces-
sitates the design and manufacture of integrated optic lasers and amplifiers, especially of
those based on erbium-doped glasses. Integrated optical amplifiers (IOA) are used to
bring the fibre to the home because they gather two complementary criteria: the high
debit rate by use of wavelength distribution multiplexing (WDM) system and local
network architecture. Indeed they inherit all the advantages of erbium doped fibre
amplifiers (EDFA) - weak noise, weak polarization effect, absence of interference
between channels in WDM application, in contrast with semiconductor optical ampli-
fiers (SOA) [4]. The short length of integrated amplifiers imposes higher Er
3
þ
concen-
tration and higher pump-power density than fibre so the choice of the glass matrix is
particularly critical. Actually, the rare-earth solubility is strongly related to the crystal
chemistry of the glass. Except BeF
2
, fluoride glasses offer high coordination sites for
rare earth ions and thus represent a unique optical host for rare earth ions; solubility can
reach 10mol%, depending on the ion size [5] while solubility is only a few hundred ppm
in the tetrahedra-based network of silica.
In this chapter, the current status of the processing technologies in the planar wave-
guides fabrication and their performance as integrated optical amplifier and laser is
reviewed. A special part will be dedicated to waveguides based on transparent glass
ceramics, an emerging material in the field of active optics, as it may offer macroscopic
glass properties and crystal-like spectroscopic characteristics [6].
11.2 Rare Earth in Fluoride Glasses
The composition and thermal and physical properties (namely, Tg, stability criteria
DT, refractive index and vibration frequency of metal-F bond) of some fluoride glasses
that have been used for waveguide fabrication are given in Table 11.1. One can note
the relatively high concentration of rare-earth fluorides, at least 5mol%. (with YF
3
assimilated to the lanthanide family REF
3
). Such a high concentration allows more
flexibility in rare-earth combinations for active optical applications, especially for
upconversion processes and enhanced pump absorption due to energy transfer between
lanthanide ions.
