Investigation of rectangular shaped wave packet dynamics in a high-repetition-rate ultrafast fiber laser

We identify a new regime where laser pulses represent the dynamics of rectangular-shaped wave packets (RSWPs) in a passively mode-locked Tm³⁺-doped fiber laser. In this regime the laser consists of a train of mode-locked pulses underneath a rectangular-shaped envelope. The density of pulses within a RSWP can be as high as 2.8 GHz, which is consistent with cavity fundamental repetition rate. The effects of small-signal gain value, pulse repetition rate, and net dispersion on the RSWP performance are analyzed. These results imply that this new regime particularly favors high-repetition-rate ultrafast lasers. We further reproduce the phenomenon from using numerical simulations and understand such behavior by referring to the nonlinear dynamics.