Fig. 1. Schematic of the experimental setup. OC, output coupler.

Fig. 2. (a) Profile of square pulse; (b) optical spectrum; (c) RF spectrum of the square pulse under a pump power of 508 mW.

Fig. 3. Oscilloscope traces of square pulses under different pump powers.

Fig. 4. Measured pulse widths and output powers versus pump powers.

Fig. 5. Values used in the simulation of three figures are the same as the instruction of the passage except unless otherwise specified. (a) Dynamic evolution of wave-breaking-free pulses as a function of round-trip number; (b) pulse widths with different nonlinearity: red is γ=1.2  W−1 km−1, blue is γ=2.2  W−1 km−1, and green is γ=3.2  W−1 km−1; (c) dynamics of the pulse width at different cavity lengths. The dots represent the values of fiber length that enable the generation of square pulses.