Fast spectral improvement in picosecond pulses generated from a DFB laser diode using a loosely coupled external cavity

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 l inewidth have been observed. The improvement shows a saturation behavior wh en 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 t he speed of improvement and the SMSR on the feedback pulse arrival time are also investigated. The optimal result can be obtained over a sensitive tim e window of about 20 ps.