The construction of a high power single quantum well buried heterostructure InGaAsP/InP (λ=1.3 μm) and InGaAsP/GaAs (λ=0.8 μm) laser diode with reduced values of parasitic capacitance has been proposed and investigated. This design uses a 3.5 μm thick polyimide layer in the current confinement regions of the buried laser. The active area of the laser is surrounded by semiconductor material and the entire structure was grown by liquid phase epitaxy. Lasers with λ=1.3 μm and resonator lengths of 310, 450, and 560 μm exhibited modulation bandwidths of 8.7, 9.2, and 6.6 GHz at operating powers of 41, 72, and 91 mW, respectively. Lasers with λ=0.8 μm and a 600 μm resonator showed a 5.6 GHz modulation bandwidth at an output power of 98 mW. © 1996 American Institute of Physics.