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A simple external cavity is constructed to provide adjustable feedback to a gain-switched distributed feedback laser diode. A 27-dB enhancement in the sidemode suppression ratio (SMSR) and a 0.3-nm reduction in the spectral linewidth have been observed. The improvement shows a saturation behavior when the feedback power reaches about -16 dBm. We report on the evolution and the dynamics of the spectral improvement. The steady state can be achieved as soon as four round-trip feedback cycles are completed. Dependences of the speed of improvement and the SMSR on the feedback pulse arrival time are also investigated. The optimal result can be obtained over a sensitive time window of about 20 ps.