Fig. 1. Schematic of the experimental setup. CPS, cladding power stripper; ISO, fiber isolator; LD, laser diode; QBH, quartz block holder; YDF, Yb-doped double-cladding fiber.

Fig. 2. (a) Seed powers and backward powers under different pump powers. (b) Total output powers and backward powers under different pump powers at the seed power of 260 W.

Fig. 3. (a) The far-field beam profile before this abrupt point (695 W). (b) The far-field beam profile after this abrupt point (774 W).

Fig. 4. (a) Forward spectra near the abrupt point (at the output port). (b) Backward spectra near the abrupt point (at the rear port).

Fig. 5. (a) The total output powers and backward powers under different pump powers at the seed power of 181 W. (b) Beam quality at the total output power of 905 W (before the abrupt point) at the seed power of 181 W.

Fig. 6. Threshold powers of the SRS-induced mode distortion under different seed powers.

Fig. 7. (a) Total output powers and backward powers under different pump powers at the seed power of 260 W. (b) Beam quality at the total output power of 1001 W (before the abrupt point) at the seed power of 260 W.

Fig. 8. Threshold powers of the SRS-induced mode distortion under different forward feedback coefficients.