Fig. 1. Experimental setup of the Raman laser. PM: power meter, OSA: optical spectrum analyzer, CLS: cladding light stripper, YDFL:Yb-doped fiber laser.

Fig. 2. Forward and backward output power as a function of pump power (only forward pumping in part I and both forward and backward pumping in part II).

Fig. 3. (a) Forward spectrum and (b) backward spectrum. The red and blue lines represent the numerical simulation and experimental results, respectively.

Fig. 4. $M^{2}$ factor of output laser.

Fig. 5. Raman gain spectrum in a $25/400~\unicode[STIX]{x03BC}\text{m}$ gain fiber.

Fig. 6. (a) Calculated power distributions of the (a) pump laser, signal laser and Raman laser along the fiber in the multi-frequency model ($L=60$ m), and (b) backward Raman laser in the dotted box in (a).

Fig. 7. Transmission and amplification of Raman laser from 40 to 60 m (SRS has not been generated from 0 to 40 m).

Fig. 8. Temperature distribution at the input end of two situations. (a) The experiment; (b) only 976 nm LDs were used.

Fig. 9. Output spectra under different powers (the length of YDF is 60 m).

Fig. 10. Exponential of gain coefficient versus wavelength (the length of YDF is 60 m).

Fig. 11. Generation of the new laser wavelength (the forward pump power is 1800 W).

Fig. 12. Experimental setup when the GDF is spliced after the YDF.

Fig. 13. Output spectra and total power ($P$) at different lengths (LG) of splicing GDF: LG, $P$ are (a) 50 m, 3900 W, (b) 70 m, 3610 W, (c) 80 m, 3300 W and (d) 100 m, 2440 W, respectively.