It is demonstrated experimentally that the intrinsic modulation response of injection lasers can be modified by reducing mirror reflectivities, which leads to suppression of relaxation oscillation resonance and a reduction of nonlinear distortions up to multi‐GHz frequencies. A totally flat response with a 3‐dB bandwidth of 5 GHz was obtained using antireflection coated buried heterostructure lasers fabricated on a semi‐insulating substrate. Harmonic distortions were below 40 dB within the entire 3‐dB bandwidth. These results are in accord with theoretical predictions based on an analysis which include the effects of superluminescence in the laser cavity.