Pulse repetition rates in passive, selfstarting, femtosecond soliton fibre laser

Rare-earth doped optical fibres offer wide gain bandwidths and provide an ideal medium for the generation of ultra-short optical pulses. To date research in mode-locked fibre lasers has centered on active mode-locking schemes incorporating fast phase and amplitude modulators. However, the large non-linear effects obtainable in optical fibres make passive mode-locking an attractive proposition. A self-starting, passive mode-locking scheme based on the reflection properties of the Non-linear Amplifying Loop Mirror (NALM) has recently been reported. The system is capable of both picosecond/nanosecond duration square pulse operation and ultra-short femtosecond soliton generation. In this paper we discuss the various modes of operation of the laser and present results on pulse repetition rates, an important characteristic for a practical source of ultra-short solitons.