Fig. 1. Bi film characterization results: (a) Scanning electron microscope images for the taper region of the microfiber coated with the bismuth film (the inset shows the surface morphology of the bismuth film); (b) optical fiber end face with bismuth coating; (c) thickness of bismuth thin film deposited on optical fiber; (d) Raman spectrum of bismuth film; (e) XRD diagram of the bismuth film; (f) linear transmittance of bismuth thin film.

Fig. 2. Nonlinear characterization of micro-nano fiber-bismuth SA: Optical microscope images of the waist region of the sample (a) without and (b) with the guiding 650 nm light; (c) saturable absorption property of SA.

Fig. 3. (a) Normalized open-aperture Z-scan traces with different excitation powers; (b) normalized close-aperture/ open-aperture Z-scan trace.

Fig. 4. Experimental device diagram.

Fig. 5. Mode-locking characteristics at 1.5 μm: (a) Mode-locking optical spectrum; (b) RF spectrum at a fundamental frequency of 19.0 MHz with 10 Hz resolution; the inset shows the RF spectrum of 100 MHz span; (c) autocorrelation trace for an output pulse with a pulse duration of 357 fs with sech² fit; the inset is the oscilloscope trace of the output pulse train; (d) relationship between the input power and laser output power/pulse energy.