From a phenomenological model of InGaAsP/InP double heterostructures emitting around 1.5 μm we have analysed both theoretically and experimentally the influence of the active-layer thickness, cavity length, mirror reflectivity and scattering losses on the useful laser parameters. Due to the high intervalence band absorption and the small electron effective mass of the quaternary material, the external quantum efficiency and the lasing emission wavelength have been found to vary strongly with the active-layer thickness. It is shown that the external quantum efficiency can be increased by reducing the active-layer thickness to less than 0.1 μm and improving the interfaces. Thus BH lasers have been made with a 50% external quantum efficiency and stable fundamental transverse mode up to 35 mW per facet, for 400 μm long devices operating at 300 K. The lasing wavelength, determined by the threshold carrier density, depends on the different laser parameters. As predicted, experiments show a lasing wavelength shift of 400 Å when the active-layer thickness varies between 0.1 and 0.2 μm. Improvement of the LPE process leading to a better control of the thickness uniformity of the active layer has been found determinant in reducing the lasing-wavelength dispersion.