Tellurite And Fluorotellurite Glasses For Active And Passive

2. Literature review; MDO 40
where f = frequency, and A and B are materials constants. Multiphonon absorption occurs
when a transverse optical mode couples weakly with a high energy phonon [40]. This
mode decays into two or more lower energy phonons of frequencies which corresponds
to fundamental vibrations in the glass [40].
As the position of the infrared edge is dependent on lattice vibrations, the masses of
the atoms / ions present and the strength of the forces between the atoms / ions define the
position of this edge [35]. The frequency of the edge, f, is given by equation (2.8), known
as the Szigeti relationship [35].
1 k
f = (2.8)
c µ
where c is the velocity of light in a vacuum (2.998×10 8 m.s -1 ), k the bond force constant,
and µ the reduced mass of the bonding atoms (see section 6.1.1.2). Therefore, for a glass
system to transmit further in the infrared than SiO2 (≈ 1.55 µm), would require [35]:
• Replacement of Si by heavier cations (increase µ).
• Replacement of Si by cations with a higher charge (decrease k).
• Replacement of O by heavier anions (increase µ).
• Replacement of O by monovalent anions (decrease k).
One advantage of working in the infrared, is that Rayleigh scattering losses are
proportional to A0/λ 4 [35]. Therefore they are greatly reduced at longer wavelengths (Mie