Tellurite And Fluorotellurite Glasses For Active And Passive

2. Literature review; MDO 39
Extrinsic scattering centres in the glass include crystals, undissolved batch, bubbles,
and refractory material. In the optical fibre, imperfections at the core / clad boundary can
also result in losses [35]. In fibre optic networks, splicing the fibre together is often
necessary. This will result in some degree of loss, as well as misalignment of spliced
cores [34]. As light passes from one medium to the fibre (e.g. air to fibre, or other
material of dissimilar refractive index to fibre), losses will result, due to Fresnel
reflection at the boundary [34] (see section 6.1.2.2.).
The electronic absorption edge is a result of electronic transitions in the glass, and
known as the Urbach edge, its position defined by, αUV.
⎡ E − Eg
⎤
α UV = Aexp⎢
⎥
(2.6)
⎣ hf ⎦
where A is a constant, E the energy of the incident photons, Eg the photon energy
corresponding to the electronic band gap of the material, h Planck’s constant
(6.62608×10 -34 J.sec), and f the frequency of the incident photons [35].
The infrared-edge and other absorption bands in multicomponent inorganic glasses are
due to the vibrational modes of bonds in structural units in the glass. The infrared edge
corresponds to the low frequency cut-off of transmission. This absorption edge tails off to
higher frequencies exponentially. The absorption coefficient, αIR, is given by equation
(2.7) [40].
lnα ( f ) = B − Af
(2.7)
IR