Fig. 1. Experimental setup of the incoherently pumped RFL. ASE, amplified spontaneous emission; FBG, fiber Bragg grating; WDM, wavelength division multiplexer; RDFB, random distributed feedback.

Fig. 2. (a) Bandwidth-tunable spectra of the ASE source at the maximum power level (measured after the WDM). (b) Maximum output power of the ASE source dependence on the bandwidth.

Fig. 3. (a) Evolution of the output spectrum with pump bandwidth of 40 nm. (b) Spectral purity of the first-order Stokes wave dependence on the pump power (inset: output spectrum with maximum OSNR). (c) Output powers as functions of the pump power.

Fig. 4. (a) Spectra with maximum spectral purity at different pump bandwidth. (b) Evolutions of the spectral purity (inset: pump power range with spectral purity >99%). (c) Output powers of the first- and second-order Stokes waves (inset picture) dependence on the pump power.

Fig. 5. Spectral broadening with different pump bandwidth.

Fig. 6. (a) Calculated residual pump with 40 nm pump bandwidth. (b) Theoretical evolutions of the spectral purity. (c) Simulated output powers of the first-order Stokes wave dependence on the pump power. (d) Longitudinal power distributions with 115 W pump power (blue line indicates the pump wave, while red line indicates the forward first Stokes wave).

Fig. 7. (a) Effective Raman gain coefficient as a function of the pump bandwidth. (b) Evolutions of the output power with 0.6, 10, and 40 nm pump bandwidth (cal., calculated results based on the unmodified power balance model with the effective Raman gain coefficient; exp., experimental data).