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 ${\displaystyle \Delta ={n_{1}^{2}-n_{2}^{2} \over 2n_{1}^{2}}}$, 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.^[1] The criterion n₂ < n₁ must be satisfied in order to sustain a guided mode by total internal reflection. Alternative formulations include ${\displaystyle \Delta ={\sqrt {n_{1}^{2}-n_{2}^{2}}}}$ ^[2] and ${\displaystyle \Delta ={n_{1}-n_{2} \over n_{1}}}$.^[3][4] 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.^[5] Integrated optics can make use of higher core index to obtain Δ>1 ^[6] allowing light to be efficiently guided around corners on the micro-scale, where popular high-Δ material platform is silicon-on-insulator.^[7] 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.^[7][8]

References

•  This article incorporates public domain material from Federal Standard 1037C. General Services Administration. Archived from the original on 2022-01-22. (in support of MIL-STD-188).

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This page was last edited on 25 July 2023, at 07:26