Two-mode fringes in planar photonic crystal waveguides with constrictions: a probe that is sensitive to propagation losses

We analyze the transmission of planar photonic crystal channel waveguides, each of which consists of three missing rows in a triangular lattice of air holes and modified at both ends by constrictions. The structures are fabricated into a GaAs/AlGaAs heterostructure in which an internal source consisting of three layers of quantum dots is embedded. The constrictions induce peculiar spectral features that are used to improve the sensitivity of transmission measurements to propagation losses. Two effects are pointed out: (i) The constrictions act as mirrors, inducing Fabry–Perot fringes on the transmitted spectra, (ii) and the constrictions also induce a mode-mixing process, mostly between the fundamental and the third transverse modes of the waveguides. Using the visibility of the resultant two-mode fringes observed on the transmitted spectra, we extract a quantitative value for propagation losses at $\lambda =1 \mathit{\mu }\mathrm{m}:$ ${\alpha }_1=25 {\mathrm{cm}}^{-1}$ (1 dB/100 µm) for the fundamental mode.