For an optical inverse taper, for example, for a $\rm SiN$ taper, material of core is $\rm Si_3N_4$ while cladding is oxide. At the facet, there is a polish angle theta to reduce the backreflection. The question is duiring we calculate output ray angle in the air at the facet, should we use the effective refraction index $neff$ instead of material refraction index $n$ of $\rm Si_3N_4$ in snell's law since most of power is in cladding? why? How to prove this in mathematics? Also to cacluate backreflection power at the facet, should we also use Neff replacing n in the formual $(n1-n2)^2/(n1+n2)^2$
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