Fig. 1. (a) Experimental setup. YDF, ytterbium-doped fiber; LD, laser diode; ISO, isolator; BA-TOF, bandwidth-adjustable tunable optical filter; Pre-amp., pre-amplifier; CLS, cladding light stripper; QBH, quartz block head. Inset: output spectrum of the broadband SFS seed. (b) Wavelength-tunable spectra after the BA-TOF. The legend indicates the central wavelength of the filter. (c) Seed power as a function of the filtering wavelength.

Fig. 2. (a) Wavelength-tunable spectra after pre-amplifier 2. The filter passband for 1070–1085 nm is fixed at 4 nm, whereas that for 1068, 1090, and 1092 nm is increased to 6 nm. (b) Central wavelength difference between the output spectra of the filtered seed and pre-amplifier 2. (c) Wavelength-tunable spectra after the main amplifier. (d) Central wavelength difference between the output spectra of the filtered seed and the main amplifier. (e) Power evolution at 1070 nm. (f) Maximum output power as a function of the operating wavelength.

Fig. 3. (a) Linewidth tunable spectra after the BA-TDF. The legend represents the FWHM linewidth of the filtered spectrum. (b) Output powers of the filtered SFS seed with different FWHM linewidths.

Fig. 4. (a) Linewidth tunable spectra after pre-amplifier 2. The legend represents the FWHM linewidth of the filtered SFS seed. (b) Spectral broadening factor as a function of the seed linewidth. (c) Linewidth tunable spectra after the main amplifier. (d) Spectral broadening factors of the main amplifier depending on the output power. (e) Power evolutions with the seed linewidths of 0.5, 4, and 10 nm. (f) Maximum output power of the main amplifier versus the seed linewidth.

Fig. 5. (a) Simulated spectra of the unfiltered broadband SFS seed and the filtered SFS seed. Temporal intensity profile of (b) the unfiltered SFS seed and (c) the filtered SFS seed. (d) Intensity autocorrelation functions (ACFs) and (e) intensity probability density functions (PDFs) of the unfiltered SFS seed and the filtered signal. (f) Simulated wavelength-tunable spectra after pre-amplifier 2. (g) Simulated central wavelength difference between the output spectra of the filtered seed and pre-amplifier 2. (h) Simulated wavelength-tunable spectra after the main amplifier. (i) Simulated central wavelength difference between the output spectra of the filtered seed and the main amplifier.

Fig. 6. (a) Simulated linewidth tunable spectra after pre-amplifier 2. (b) Broadening factors of the spectral width versus the seed linewidth (after pre-amplifier 2). (c) Simulated linewidth tunable spectra after the main amplifier. (d) Broadening factors of the spectral width after the main amplifier depending on the seed linewidth. (e), (f) Simulated output spectra of the main amplifier with and without Kerr nonlinearity (initial seed linewidth: (e) 0.5 nm; (f) 8 nm). The output spectrum of pre-amplifier 2 is also provided for the sake of comparison.