Fig. 1. Experimental setups of the (a) bismuth-doped fiber laser (BDFL) and (b) bismuth-doped fiber amplifier (BDFA). BDF, bismuth-doped fiber; ISO, isolator; LD, laser diode; OC, optical coupler; PC, polarization controller; PD-ISO, polarization-dependent isolator; WDM, wavelength division multiplexer.

Fig. 2. Characteristics of the BDFL operating in the square-wave NLP regime. (a) Temporal profile of the single-square-wave pulse. Inset: autocorrelation trace (AC). (b) Optical spectrum. Inset: ASE spectrum of the 36-m BDF. (c) Oscilloscopic trace of the pulse train. (d) RF spectrum measured with a 10-Hz RBW. Inset: RF spectrum over a 5-GHz frequency span measured with 1-kHz RBW.

Fig. 3. Versatile performances of the BDFL operating in the square-wave NLP regime with different pump powers. (a) Pulse duration and output power. (b) Pulse energy and peak power. (c) Temporal profiles of the single-square-wave pulse. (d) Optical spectra. Inset: closeup of the central wavelength range. The red oval indicates the mode-locked state at a pump power of 440 mW, corresponding to the results in Fig. 2 and Fig. 4(b).

Fig. 4. Basic performance of the BDFA with the BDFL operating in the square-wave NLP regime. (a) ASE spectrum. (b) Optical spectra of the seed (blue curve, corresponding to the case with a pulse duration of 844 ps, i.e., the same as Fig. 2) and after amplification (green curve) at a maximum output power of 102 mW. (c) Output power with increasing pump power. Different colors show the performance under different arrangements of bidirectional pumping, i.e., increasing to the max power of the forward pumping LD₁ first for the blue, while to the max power of backward pumping LD₂ first for the red. (d) Maximum output power and slope efficiency under different pulse durations.

Fig. 5. Characteristics of the BDFL operating in multi-pulse soliton regime. (a) The seed output power as a function of the seed pump power. (b) Temporal profiles of the 1 to 15 odd pulses with different seed pump powers. The number of pulses is marked on the bottom right for each trace. (c) AC trace at a pump power of 300 mW (corresponding to 4-pulse mode-locking) measured with an output power of 30 mW and amplified by the BDFA. Red curve, AC trace measured by the autocorrelator. Green curve, Sech² fitting. (d) Corresponding optical spectra of the seed (red curve) and after amplification (green curve) at the maximum output power of 85 mW. Inset: amplified spectrum with a wider wavelength range.

Fig. 6. Temporal dynamics of the BDFL operating in different regimes. Upper: temporal profile. Lower: temporal evolution. (a) Single noise-like square-wave at a pump power of 440 mW. (b) Multiple noise-like square waves at a pump power of 620 mW. (c) Consecutive 4-pulse mode-locked at a pump power of 300 mW. (d) Separated 4-pulse mode-locked at a pump power of 300 mW. Inset: closeup.