The pattern-dependent amplitude modulation in a semiconductor optical amplifier driven by return-to-zero (RZ) data is experimentally shown to be reduced using a delay interferometer (DI). Pulses of this format are amplified with far better performance than without the DI, as quantified by the measured reduction of the amplitude modulation from 1.6 dB to 0.29 dB and the enhancement of the eye diagram extinction ratio from 10.5 dB to 13. 1 dB. © 2012 IEEE.