Refractive index contrast, in an optical waveguide, such as an optical fiber, is a measure of the relative difference in refractive index of the core and cladding. The refractive index contrast, ÃÂ, is often given by , where n<sub>1</sub> is the maximum refractive index in the core (or simply the core index for a step-index profile) and n<sub>2</sub> is the refractive index of the cladding. The criterion n<sub>2</sub> < n<sub>1</sub> must be satisfied in order to sustain a guided mode by total internal reflection. Alternative formulations include and . Normal optical fibers, constructed of different glasses, have very low refractive index contrast (ÃÂ<<1) and hence are weakly-guiding. The weak guiding will cause a greater portion of the cross-sectional Electric field profile to reside within the cladding (as evanescent tails of the guided mode) as compared to strongly-guided waveguides. Integrated optics can make use of higher core index to obtain ÃÂ>1 allowing light to be efficiently guided around corners on the micro-scale, where popular high-ÃÂ material platform is silicon-on-insulator. High-ÃÂ allows sub-wavelength core dimensions and so greater control over the size of the evanescent tails. The most efficient low-loss optical fibers require low ÃÂ to minimise losses to light scattered outwards.