Abstract

Harmonical mode-locking (HML) of erbium doped fiber lasers (EDFL's) has proved to be a convenient technique for the generation of high bit-rate soliton trains, whereas passive mode-locking (PML) showed to be promising for ultrashort pulse generation. Active modulation in HML soliton lasers provides high repetition rates but does not permit to control the temporal reshaping of solitons that circulate in the cavity [1], Conversely, in PML lasers the presence of nearly istantaneous nonlinear gain leads to the maximum shortening of the pulse width that is permitted by the laser bandwidth. In HML soliton lasers, the soliton pulsewidth is fixed by the values of the cavity group-velocity dispersion and of the soliton peak power [1], Therefore, whenever the average power of the HML laser is increased, no time compression of the circulating pulses results. In fact, in HML lasers the only mechanism that produces pulse shortening is the soliton compression effect. This compression is inhibited by the finite band width of the laser cavity. As a consequence, the increase of energy in the cavity leads to splitting of the circulating pulse into a number of replicas. A similar quantization effect of the soliton number in the fiber cavity was previously observed in PML-EDFL's [2].

© 1994 Optical Society of America