Fig. 1. Experimental scheme for kilowatt-level SC generation in a single-stage RFL with a half-open cavity. MFA, mode field adapter; HR-FBG, high-reflectivity fiber Bragg grating; F-PSC, forward pump/signal combiner; YDF, ytterbium-doped fiber; OC-FBG, output coupler fiber Bragg grating; B-PSC, backward pump/signal combiner; LD, laser diode; CLS, cladding light stripper; GDF, germanium-doped fiber.

Fig. 2. Output spectral evolution of pump laser power in the RFL with a full-open cavity: (a) forward direction and (b) backward direction.

Fig. 3. (a) Laser output power and (b) conversion efficiency of the RFL with a full-open cavity in both directions. The green background area is the growth area of the backward output power; the yellow background area shows that the forward output power stops increasing.

Fig. 4. Compared output spectra of RFLs with a full-open cavity and a half-open cavity under the same pump power of 964 W.

Fig. 5. Output spectral evolution with pump laser power in the RFL with a half-open cavity.

Fig. 6. (a) Laser output power and (b) conversion efficiency in the RFL with a half-open cavity.

Fig. 7. Normalized temporal signal of the RFL with a half-open cavity: (a) spectrum expansion below the ZDW; (b) spectrum expansion near the ZDW; (c) spectrum expansion above the ZDW; (d) maximum spectrum broadening. The recording time window is set to 100 ms.