We report travelling-wave 1.5 μm semiconductor laser amplifiers with very low gain ripple. The average facet reflectivity is less than 5 × 10−5 over a 170 A interval. The minimum gain ripple is below the measurement accuracy, 0.2dB, at 27.5dB gain. When used as an optical preamplifier in a 4Gbit/s receiver, a sensitivity of −34.3 dBm is achieved.