DEMONSTRATION OF DOWN-CHIRPED AND CHIRP-FREE PULSES FROM HIGH-REPETITION-RATE PASSIVELY MODE-LOCKED LASERS

Knowledge and control of the chirp parameters of semiconductor lasers is a prerequisite to obtaining transform-limited pulses and/or to comp ensate for group velocity dispersion in fiber, Here, we report measure ments of the sign and magnitude of chirp in high-repetition-rate mode- locked semiconductor lasers. The chirp of these monolithic lasers is m easured in the frequency domain, using filtering and cross-correlation techniques, For different injection currents, a range of different ch irp values is measured, including strongly down-chirped pulses at high er injection currents and transform-limited pulses to slightly up-chir ped pulses at lower injection currents, The pulse chirp and the result ing broadening are due to the algebraic addition of opposite-signed ch irps due to saturation of the absorption section and the gain section, These may cancel each other under some conditions, leading to a solit on-like transform-limited pulse.