Refractive index contrast

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, Δ, is often given by $\mathrm{\Delta }=\frac{n_1^2-n_2^2}{2n_1^2}$, where n₁ is the maximum refractive index in the core (or simply the core index for a step-index profile) and n₂ is the refractive index of the cladding. The criterion n₂ < n₁ must be satisfied in order to sustain a guided mode by total internal reflection. Alternative formulations include $\mathrm{\Delta }=\sqrt{n_1^2-n_2^2}$ and $\mathrm{\Delta }=\frac{n_1-n_2}{n_1}$. Normal optical fibers, constructed of different glasses, have very low refractive index contrast (Δ<<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 Δ>1 allowing light to be efficiently guided around corners on the micro-scale, where popular high-Δ material platform is silicon-on-insulator. High-Δ 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 Δ to minimise losses to light scattered outwards.

References

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