Chirp compensation in mode-locked DFB laser diodes with extended cavity

The spectro-temporal behavior of actively mode-locked fiber-external-cavity DFB lasers at 1.55 μm is analyzed in detail. Experiments are performed with multiquantum-well InGaAsP lasers of different lengths and different phase-amplitude coupling factors. Transform-limited picosecond pulses are generated with pulsewidth adjustable over half a decade by changing the RF conditions and the device length. As in the case of conventional laser diodes with external grating, the smallest time-bandwidth products are obtained for modulation frequencies to the high-frequency side of the mode-locking band. This result is presently described in terms of chirp compensation by the DFB cavity dispersion. An analytical expression of the pulse compressibility is established from a steady-state mode-locking equation including refractive index variations and cavity dispersion effects. Gain parameters entering the expression are separately evaluated from standard rate equations. A good agreement is found between experimental and theoretical results