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Laser diodes of InGaAsP/InP are studied with a modified double heterostructure containing an ultrathin ( m) active layer and adjacent guiding layers transparent for laser emission ("three-layer waveguide" structure). Due to improved optical confinement in these structures, the laser action at room temperature has been observed with a threshold current density as low as ∼0.5 kA/cm2. CW operation in broad area diodes has been obtained. The dependence of threshold on the thickness of active layer is discussed in terms of optical confinement and nonradiative loss due to carrier leakage and Auger recombination. It is concluded that the leakage is the probable origin of the threshold increase at smallest thickness of the active layer rather than light diffraction.