6 Introduction planar silica on silicon waveguide fabrication, as the reduced processing temper- ature reduces the deformation of the substrate [31]. Fluorine and trivalent boron (as B 2 O 3 ) are other dopants commonly used in germania-doped silica fiber. A major difference between germanium and fluo- rine/boron is that while the refractive index increases with increasing concentra- tion of germanium, it decreases with boron/fluorine. With fluorine, only modest reductions in the refractive index are possible ($0.1%), whereas with boron, large index reductions (> 0.02) are possible. Boron also changes the topology of the glass, being trivalent. Boron and germanium together allow a low refrac- tive index difference between the core and cladding to be maintained with a large concentration of both elements [32]. On the other hand, a depressed clad- ding fiber can be fabricated by incorporating boron in the cladding to substan- tially reduce the refractive index. The density of the boron-doped glass may be altered considerably by anneal- ing, thermally cycling the glass, or by changing the fiber drawing temperature [33]. Boron-doped preforms exhibit high stress and shatter easily unless handled with care. The thermal history changes the density and stress in the glass, thereby altering the refractive index. The thermal expansion of boron-silica glass is $4 Â 10 À6 C À1 , several times silica (7 Â 10 À7 C À1 ) [34]. Boron-doped silica glass is generally free of defects, with a much-reduced melting tempera- ture. Being a lighter atom, the vibrational contribution to the absorption loss extends deeper into the short wavelength region and increases the absorption loss in the 1500 nm window. Boron with germanium doping has been shown to be excellent for photosensitivity [32, 35­39]. 1.3 ORIGINS OF THE REFRACTIVE INDEX OF GLASS The refractive index, n, of a dielectric may be expressed as the summation of the contribution of i oscillators of strength f i each, as [40] n 2 À 1 4p e 2 X f i ; ð1:1:1Þ ¼ n 2 þ 2 3 me 0 i o 2 À o 2 þ iG i o i where e and m are the charge and mass of the electron, respectively, o i is the res- onance frequency, and G i is a damping constant of the ith oscillator. Therefore, refractive index is a complex quantity, in which the real part contributes to the phase velocity of light (the propagation constant), whereas the sign of the imaginary part gives rise to either loss or gain. In silica optical fibers, far away from the resonances of the deep UV wavelength region that contribute to the background refractive index, the loss is negligible at telecommunications