  We investigate the parity of transmission spectra in a photonic crystal (PhC)
waveguide with a side-coupled cavity and a partially blocking element. We
demonstrate, by means of numerical calculations, that by varying the
cavity-block distance the spectra vary from being asymmetric with blue parity,
to being symmetric (Lorentzian), to being asymmetric with red parity. For
cavity-block distances larger than five PhC lattice constants, we show that the
transmission spectrum is accurately described as the transmission spectrum of a
Fabry-Perot etalon with a single propagating Bloch mode, and that the parity of
the transmission spectrum correlates with the Fabry-Perot roundtrip phase.
