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

Many diode laser structures, including those based on group-III nitride system, contain passive waveguide layers of higher refractive index than in the adjacent layers. Modes of such passive waveguides ("passive" modes) can interact 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" modes). The lasing mode is the mode at which laser generation occurs. The lossy ghost modes are parasitic modes of a laser structure that can consume energy 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 ghost 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 of 0.5 mu m of AlGaN-cladding layer is found to be insufficient to prevent rather strong coupling between the modes, Under the resonant coupling conditions, the modal gain is shown to be strongly suppressed, allowing no lasing at all.