Zigzag modes in quantum cascade laser emission spectra

We present emission spectra obtained from ridge waveguide quantum cascade (QC) lasers with atypical periodic features that result from optical modes that suffer total internal sidewall reflections and traverse zigzag paths along the length of the waveguide structure. These "zigzag" modes are equally spaced, in frequency, about the single lasing mode that arises at threshold. Moreover, they only exist at relatively low injection currents and with increasing current yield to the broadband Fabry-Perot modes that typically comprise the complicated spectra of QC lasers. A straightforward geometric analysis shows a clear correspondence between the zigzag modes and parameters useful for characterizing the quality of the laser waveguide structure.