Fig. 1. Experimental setup of the all-fiberized fiber amplifier.

Fig. 2. Schematic of the assembled active fiber.

Fig. 3. Measured refractive index profiles for the two YDFs.

Fig. 4. (a) EMFAs of FM for the two YDFs under different coiling radii. (b) SRS enhancement when the power ratio of the backward pump is different.

Fig. 5. Loss coefficient of transverse modes for the two YDFs under different coiling radii: (a) LP₀₁ mode, (b) LP₁₁ mode.

Fig. 6. Measured spectra of the modulated MO and after the PAM.

Fig. 7. Output properties of the fiber amplifier with backward pumping: (a) power curve; (b) typical spectra; (c) Fourier transformation of the temporal trace.

Fig. 8. Output properties of the fiber amplifier with forward pumping: (a) power curve; (b) typical spectra; (c) Fourier transformation of the temporal trace.

Fig. 9. (a) Output power of the signal laser at different pump powers with bidirectional pumping; (b) backward power ratio (R_b) at different output powers.

Fig. 10. Output properties of the fiber amplifier with bidirectional pumping: (a) typical spectra; (b) Raman ratio; (c) temporal trace; (d) Fourier transformation (inset: the beam profile at focal length of M2-200s).

Fig. 11. Measured 3 dB spectral linewidth for different output powers and pump processes.