Transverse mode analysis of bipolar cascade laser

Bipolar cascade lasers (BCL) allow to increase the differential quantum efficiency (DQE) by recycling electrons and holes in multiple active regions connected in series, for instance via epitaxially stacked highly doped Esaki tunnel junctions. However, specific index and gain distribution might lead to peculiar optical field distribution. We report the transverse mode analysis of a BCL. The structure is a 9 μm wide edge emitting InGaAsP-InP BCL, consisting of two stacked active regions of four quantum wells (λ∼1.55 μm) each. The layers which separate the two gain regions are two 0.5 μm thick InP cladding layers sandwiching the highly doped InGaAsP Esaki layers. The center region of the structure is therefore composed of low index, high loss materials. The optical field behavior of this structure has been calculated using an effective index method. The simulation predicts two modes propagating into the structure, the fundamental mode, and the second-order mode. We experimentally visualized the near field figure with an infra-red (IR) camera focusing on the laser output facet.