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In this paper, we present an optimized quantum-well (QW)-semiconductor optical amplifier (SOA) which is capable of regenerating phase-modulated signals, such as return-to-zero differential-phase-shift keying (RZ-DPSK) signal. Based on the optimized QW SOA, the amplitude fluctuations are suppressed while the phase information is preserved. The essential mechanism of the optimized QW SOA is the low linewidth enhancement factor (α-factor), which can be obtained by means of structural optimization. Based on the numerical model, the power fluctuations can be suppressed from 4.26 mW (33% of input peak power) to 1.59 mW (13% of output peak power) at 40 Gb/s. The average phase noise induced by SOA is as small as 50. Furthermore, the DPSK signal regeneration scheme based on a single QW SOA is quite simple and stable.