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We present a combined theoretical and experimental analysis of InAs/InGaAsP/InP quantum dash lasers. Calculations using an 8 band k.p Hamiltonian show that electron states, due to the low effective mass and small conduction band offsets, are not confined in the dash in the case of dash-in-a-well structures and are only weakly confined in dash-in-a-barrier structures. The shape of the dashes leads to an experimentally observed enhancement of spontaneous emission (SE) and therefore of gain for light polarized along the dash long axis, with the measured SE enhancement in excellent agreement with the theoretical calculations. An analysis of the variation of the integrated spontaneous emission rate with total current and with temperature reveals that, despite the reduced dimensionality of the active region, the threshold current of these lasers, and its temperature dependence, remain dominated by Auger recombination.