- Source: 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
Δ
=
n
1
2
−
n
2
2
2
n
1
2
{\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. The criterion n2 < n1 must be satisfied in order to sustain a guided mode by total internal reflection. Alternative formulations include
Δ
=
n
1
2
−
n
2
2
{\displaystyle \Delta ={\sqrt {n_{1}^{2}-n_{2}^{2}}}}
and
Δ
=
n
1
−
n
2
n
1
{\displaystyle \Delta ={n_{1}-n_{2} \over 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
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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