  We provide detailed insights into a link between structural and optical
disorder of resonant optical structures, in particular, distributed Bragg
reflectors (DBRs) and resonant microcavities. The standard (targeted) DBR
structures have periodic square wave like refractive-index profiles, and their
optical performance is determined by the refractive index ratio of the two
applied materials and the number of DBR periods. It is well known that its
structural disorder strongly affects its optical properties, but, despite that,
this influence has not been quantitatively addressed in the literature. We
propose a precise quantitative definition for a structural disorder of a single
DBR unit cell (disorder factor), completing the set of DBR fundamental
parameters. Then we expose the basis for the effective refractive index
approximation (ERIA), showing that, as long as DBR optical properties are
concerned, the influence of increasing structural disorder is virtually
identical to the influence of decreasing refractive index ratio, with the
latter influence being easily quantified. Making use of the ERIA method, simple
analytical formulas, which enable rapid insights into the reflectivity and stop
band width of DBRs with different types of transient layers at the
heterointerfaces, are derived and the results validated, via both transfer
matrix simulations and direct experimental measurements of imperfect DBRs. The
insights of the ERIA method are then further applied on resonant
microcavitities, providing a comprehensive link between their structural
disorder and subsequent deterioration of their quality (Q) factor.
