Specialty Optical Fibers Handbook

or glassy state. On cooling from T

g

to room temperature, the linear

thermal expansion is the result of changes in bond lengths, whereas above T

g

,

a thermal expansion coefficient, which is at least twice as great, is due to

structural rearrangements and to bond length changes. For materials such as

silica, silicates, and some chalcogenides, upon reheating through T

g

and up to

the melting point, there is little probability of crystallization when heating at

reasonable rates. Other glasses such as the HMFs may rather easily crystallize on

either reheating or on the initial cooling. Thus, in such glasses, care must be

taken to ensure that no crystallization occurs. In particular, surface-nucleated

crystallization may be a problem both mechanically and