We investigate the phase coherence between a seed laser and a laser amplified by a tapered semiconductor amplifier (TSA) when the seed laser is either continuous wave (CW) or pulsed. The phase fluctuations in the time domain are employed to describe the degradation of phase coherence induced by a TSA. The amplified laser is measured to be approximately 99.98% coherent with the seed, when the CW or pulsed laser is seeded, at different supplying currents of the TSA. Furthermore, the phase coherence is measured when the seed laser is modulated. The results reveal that the phase coherence degradations induced by the TSA remain the same for a seed laser with and without modulation, when different supplying currents of the TSA are applied.