High-Nonlinearity in Glass Fibers and Cross-Phase Modulation - Optical Signal Processing by Silicon Photonics - page 63

5.6.2 Tellurite (TeO 2 ) Glasses
Tellurite glasses, compared to silicate and fluoride glasses, have a reasonably wide
transmission region, the lowest phonon energy among the common oxide glasses,
larger refractive indices, and high nonlinear-index coefficients. Their large refrac-
tive index and small phonon energy are desirable for radioactive transitions of
rare-earth ions (Er 3 + , Tm 3 + , Nd 3 + , Pr 3 + , Yb 3 + , and so on) and the application of
fiber lasers and amplifiers. The lower photon energy leads to lower nonradioac-
tive transition rate (high fluorescence quantum efficiency) between adjacent rare-
earth energy levels, causing new fluorescence transitions, and laser emission from
additional energy levels. Under normal conditions, tellurium-dioxide (TeO 2 ) has
no vitrification ability without modifiers. Thus, glass-modifiers and/or secondary
glass-formers are necessary in order to obtain tellurite glasses. Incorporation of a
second component to tellurite glasses is expected to extend the Te-O inter atomic
distance, which should increase the mobility of polyhedron thereby provide a
favorable condition for tellurite vitrification. The properties of TeO 2 -based glasses
are summarized below:
(a) High linear refractive index (1.82 ~ 2.27)
(b) High nonlinear-index coefficient (16 ~ 210 × 10 − 20[m 2 /W])
(c) Very wide transmission range (0.35 ~ 6 μ m)
(d) Low transition temperature (250 ~ 400 °C) and melting temperature
(450 ~ 800 °C)
(e) Good glass stability, strength, and corrosion resistance
(f) Good rare-earth ion solubility
(g) Relatively low phonon energy for oxide glasses (600 ~ 850 cm −1 ) to minimize
nonradioactive losses
(h) High electrical conductivity
(i) High resistance to devitrification and atmospheric moisture
(j) Good chemical durability
(k) High homogeneity
(l) Highly capable of incorporating large concentrations of rare-earth ions into
the matrix.
5.6.3 Chalcogenide Glasses
The chalcogenide glasses are composed of one or more chalcogenide elements
such as S, Se, and Te, with other elements such as As, Ga, Ge, In, and Sb to form
a stable glasses. Other elements such as P, I, Cl, Br, Cd, Br, Ba, Si, or Tl can be
added to these glasses for tailoring their thermal, mechanical, and optical prop-
erties. The chalcogenide elements are covalent in nature and can exhibit chain,
ring, and/or network structures. In contrast to oxide glasses, they can depart from
atomic stoichiometry through the partial segregation of chalcogens and/or red ox