Fig. 1. (a) Experimental setup of the mode switchable fiber laser based on a master oscillator power amplifier scheme; (b) schematic of the intra-cavity mode switchable seed fiber laser. AIFG, acoustically induced fiber grating; RF, radio frequency; LD, laser diode; PSC, pump and signal combiner; YDF, ytterbium-doped fiber; CLS, cladding light stripper; QBH, quartz block holder; FLM, fiber loop mirror; MFA, mode field adapter; PC, polarization controller; FBG, fiber Bragg grating.

Fig. 2. (a) Transmission and reflection spectra of the few-mode FBG used in the intra-cavity mode switchable fiber oscillator; (b) transmission spectra of the AIFG under different modulation frequencies.

Fig. 3. (a) Output power of the LP₀₁ and LP₁₁ modes from the fiber amplifier as a function of the pump power (inset: beam profiles of the LP₁₁ mode); (b) output spectra of the LP₀₁ and LP₁₁ modes under the maximum output power (inset: output spectra of the seed laser).

Fig. 4. (a) Output power of the LP₀₁ and LP₁₁ modes from the fiber oscillator as a function of the pump power; (b) output spectra and corresponding beam profiles of different operation modes.

Fig. 5. (a) Output power of the LP₀₁ and LP₁₁ modes from the amplifier as a function of the pump power (insets: mode profiles of the LP₁₁ mode under different pump powers); (b) output spectra of the LP₀₁ and LP₁₁ modes under the maximum output power.

Fig. 6. (a) Switching time between the LP₀₁ and LP₁₁ modes of the seed laser with the AIFG outside the fiber oscillator; (b) switching time between the LP₀₁ and LP₁₁ modes from the amplifier with the AIFG outside the fiber oscillator.

Fig. 7. (a) Switching time between the LP₀₁ and LP₁₁ modes of the seed laser with the AIFG inside the fiber oscillator; (b) switching time between the LP₀₁ and LP₁₁ modes from the amplifier with the AIFG inside the fiber oscillator.

Fig. 8. Output power of the LP₁₁ mode in both configurations recorded in the total duration of 10 min.