Fig. 1. (a) Molecules absorption spectrum^[1] and (b) atmospheric transmission windows

Fig. 2. The energy levels of different rare-earth ions (a-f) and their emission spectra^[10-11]

Fig. 3. Methods for fiber laser mode-locking. The methods (a-c) have successfully mode-locked the mid-infrared fiber laser

Fig. 4. (a-d) 2,8 μm nonlinear polarization rotation based mode-locking^[29]. (a) Experimental configuration; (b) direct and reconstructed output spectra; (c)autocorrelation trace; (d) second harmonic signal spectrum. (e-g) 3.5 μm nonlinear polarization rotation based mode-locking^[32]. (e) Experimental setup; (f) pulse spectrum; (g) autocorrelation trace

Fig. 5. Mid-infrared ultrafast fiber lasers mode-locked by saturable absorbers. Left to right column: setup, autocorrelation trace and spectra

Fig. 6. Typical Q-switched fiber laser based on saturable absorbers

Fig. 7. Frequency shifted feedback mode-locking^[72]. including pump-output power curve, output spectrum, Q-switched waveform, mode-locked waveform, autocorrelation trace and radio frequency spectrum

Fig. 8. (a-d) 70 fs pulses generation via nonlinear compression^[34]. (a) Experimental setup; (b) output spectra; (c) autocorrelation trace before compression; (d) autocorrelation trace after compression. (e-h) 15.9 fs pulses generation via chirped pulse amplification and high order soliton self-compression^[33]. (e) Experimental setup; (f) spectra of seed and amplified pulse; (g) spectrum of compressed pulse; (h) retrieved temporal intensity and phase of compressed pulse

Fig. 9. Mid-infrared supercontinuum generation directly pumped by mid-infrared ultrafast pulse^[4]. (a) Experimental setup; (b) supercontinuum spectra pumped by different peak power

Table 1. Comparison of results of mid-infrared fiber lasers Q-switched by various saturable absorbers