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A modified plano-convex waveguide structure is studied analytically and successfully applied to InGaAsP/InP lasers in the m wavelength region to realize stable transverse mode operation. The structure of these lasers is characterized by a standard buffer layer between an active layer and an upper cladding layer and a waveguide layer of varying thickness between the active layer and the substrate. A theoretical analysis of this structure showed that, for a given channel depth, increases in buffer-layer thickness give rise to larger maximum channel widths of the substrate for fundamental transverse mode operation. It was also shown that the optical confinement factor in the active layer decreases little as the buffer-layer thickness is increased from 0.1 to 0.3 μm. Buffer-layer loaded plano-convex waveguide lasers in the 1.5 μm range were prepared by liquid phase epitaxy and fundamental transverse mode operation up until was obtained. The dc threshold current was 100-300 mA and the differential quantum efficiency per facet was 10-15 percent. Continuous CW operation for over 2000 h at 25°C has been achieved.