Tellurite And Fluorotellurite Glasses For Active And Passive

2. Literature review; MDO 46
4
α = A 1/
λ ) + B exp( B / λ)
+ C exp( −C
λ)
(2.14)
t
0 ( 0 1
0
1
where A0, B0, B1, C0 and C1 are constants and λ is the wavelength of the light. The first
term indicates loss due to light scattering from microscopic density and composition
fluctuations in the material. These effects rapidly decrease with increasing wavelength.
The second and third terms describe, respectively, losses due to ultraviolet absorption
from the electronic band edge (Urbach tail) and infrared edge losses due to multiphonon
absorption. With an assumption of a single component tellurite glass, the first term could
be calculated by equation (2.15) [41-44]:
8 3 8 2 8 3 2 2
A0 = π n p βkTg
≈ π ( n −1)
βkTg
(2.15)
3
3
where n is the refractive index, p is the average photoelastic constant, β is the isothermal
compressibility, Tg is the glass transition temperature and k is Boltzmann’s constant
(1.38×10 -23 J.K -1 ). This expression becomes more complicated, when considering
multicomponent glasses, due to compositional fluctuations [35]. As Tg increases with
quenching rate from the melt, so do scattering losses [39]. The second and third terms of
equation (2.14) are used to fit experimental data on the UV and IR spectra of fibres, bulk
glass and films. The fitting parameters for tellurite glasses are, for the first term (Rayleigh
scattering loss) 0.29/λ 4 dB.km -1 , the second term (UV absorption loss)
6.47×10 -6 .exp(9.84/λ) dB.km -1 , and the third term (IR absorption loss)