Fig. 1. Acousto-optic filtering spectra at different wavelengths.

Fig. 2. Reflection spectra of the FM-FBG under (a) acousto-optic non-resonance and (b) acousto-optic resonance; the laser output spectra in the CW state under (c) acousto-optic non-resonance and (d) acousto-optic resonance.

Fig. 3. Schematic diagram of mode-field switchable narrow linewidth mode-locked fiber laser. WDM, wavelength-division multiplexer; Cir, fiber circulator; PC, polarization controller; MS, mode stripper; RF, radio frequency generator; SESAM, semiconductor saturable absorber mirror.

Fig. 4. Mode-locked output spectrum under the non-resonant state.

Fig. 5. (a) Mode-locked pulse sequence under the non-resonant state and (b) the corresponding RF spectrum within the frequency range of 100 Hz, with the RBW of 1 Hz (inset: RF spectrum in the 2 GHz range).

Fig. 6. Mode-locked spectrum under the resonant state.

Fig. 7. (a) Mode-locked pulse sequence under the resonant state and (b) the corresponding RF spectrum within the frequency range of 100 Hz, with the RBW of 1 Hz (inset: RF spectrum in the 2 GHz range).

Fig. 8. Optical intensity distribution of output 2 under the resonant state.