Algorithm for Magnitude & Phase of the reflection & transmission coefficient of a Fabry–Perot etalon

Magnitude and phase of the reflection (S11) and transmission (S21) coefficients
of a Fabry–Perot etalon as a function of the cavity round-trip phase relative to an integer
multiple of 2π (in the lower-right plot, the phase is relative to an even multiple of 2π).
Three mirror reflectivities are plotted assuming r = r1 = r2 and zero loss (a cleaved-facet
cavity corresponds to r = 0.565).