We examine the electronic structure and optical properties of 1.5-μm InAs/InGaAsP/InP quantum dash-in-a-well (DWELL) and dash-in-a-barrier (DBAR) lasers. Using 1-D and 3-D k.p calculations, we show that the electron states are not confined to the dash layer in the DWELL structures and are poorly confined in the DBAR case, due to the small conduction band offset in InGaAsP systems. The built-in strain induces a large HH-LH splitting, resulting in a significant reduction in the calculated valence band density of states (DOS). Coupled together, these properties can be engineered to give a nearly symmetric conduction and valence band DOS within 0.1 eV of the band edge. The calculated gain due to light polarized along the TE(110) and TE(1-10) directions is anisotropic, with the degree of anisotropy dependent on the dash height and the area density of the dashes. We conclude that the dashes can provide a high modal gain with reduced transparency and threshold carrier density but similar threshold current density compared to equivalent quantum well devices.