  In this paper, for the first time, analytical expressions for determination
of the wavelength of defect modes (DMs) in one-dimensional (1D) photonic
crystals (PCs) with two defect layers (DLs) were obtained from the condition of
zero reflection. Analytical formulas on the conditions for the merging of two
DMs and on the DMs with zero value of the reflection coefficient were obtained.
Different typical 1D PC structures with two DLs were considered and compared,
including their DMs behavior. The distribution of electromagnetic field within
the first photonic bandgap (PBG) was analyzed for several cases of DM merging
and for the different orders of DMs. The results of this research permit the
simplification of the analysis of optical sensors and filters based on 1D PCs
with two DLs, as well as a more comprehensive understanding of the DM behavior
within the PBGs of such PC structures. Furthermore, this paper helps to explain
the results obtained by other researchers earlier in this field, too.
