Evolutions of Q-switched mode-locked square noise-like pulse with different cavity lengths

A $Q$-switched mode-locked square noise-like pulse (QMLSNLP) is generated in a nonlinear polarization rotation passively mode-locked fiber laser. When the pump power changes from 154 mW to 414 mW, the frequency of the $Q$-switched envelope varies from 21.7 kHz to 38.9 kHz, while the duration of the $Q$-switched envelope decreases from $5.1\,\,{\unicode{x00B5}\rm s}$ to $3.2{\unicode{x00B5}\rm s}$, correspondingly. In the meantime, QMLSNLP keeps the fundamental repetition rate constant, and the pulse duration increases from 3.4 ns to 6.7 ns. By inserting different lengths of single-mode fiber into the ring cavity, the evolutions of QMLSNLP are measured and analyzed. According to the cavity parameters and experimental results, impacts of the cavity length on QMLSNLP are investigated in detail.