Ghost modes and resonant effects in AlGaN-InGaN-GaN lasers

Many diode laser structures, including those based on group-III nitride sys tem, contain passive waveguide layers of higher refractive index than in th e adjacent layers. Modes of such passive waveguides ("passive" modes) can i nteract with an active layer mode ("active" mode), giving rise to two kinds of normal modes or "supermodes" of a laser structure. Away from resonance, one of them is localized predominantly in the active region ("lasing" mode ), while the other ones are located mostly in passive waveguides ("ghost" m odes). The lasing mode is the mode at which laser generation occurs. The lo ssy ghost modes are parasitic modes of a laser structure that can consume e nergy from the active region. Resonant coupling occurs when the lasing mode and a ghost mode are in phase synchronism. In this paper, the concept of g host modes is applied to InGaN-based diode lasers, The values for critical thickness are calculated for p-GaN cap layer and for n-GaN buffer/substrate layer for a particular multilayer laser structure. The typical thickness o f 0.5 mu m of AlGaN-cladding layer is found to be insufficient to prevent r ather strong coupling between the modes, Under the resonant coupling condit ions, the modal gain is shown to be strongly suppressed, allowing no lasing at all.