Trains of 250 fs pulses at sub-intrinsic-cavity roundtrip intervals from chirped mode-locked laser diodes

To our knowledge, fine periodical evolutions of the pulse structure with the length of the external cavity have never been reported in short-pulse mode-locked extended-cavity laser diodes. In this communication, we demonstrate a strong inter-dependence between the external and intrinsic cavity lengths in these mode-locked laser systems when an external chirp compensation is used. We found that the length of the external cavity (l/sub e/) must be equal to a multiple of the intrinsic laser diode cavity length (l/sub i/) to produce standard pulse structures with one sub-pulse per intrinsic cavity roundtrip time. Similarly, the external cavity length must be equal to (m+1/n)*l/sub i/, with m being an integer, to obtain n sub-pulses per intrinsic cavity roundtrip time. In our experiments, we observed pronounced pulse structures for n up to 8. This latter value corresponds to the generation of /spl sim/250 fs pulse trains with a pulse repetition rate of /spl sim/300 GHz and a total energy /spl sim/20-30 pJ.