Fig. 1. (a) Energy level scheme of ; (b) absorption and emission spectra of Er-doped fibers^[14].

Fig. 2. Experimental setup of the 10 watt-level narrow linewidth tunable all-fiber amplifier. CW: continuous wave; ISO: isolator; WDM: wavelength division multiplexing; EDFC-980-HP: model of erbium-doped fibers; EYDF-12/130: model of erbium–ytterbium co-doped fibers, core diameter , cladding diameter .

Fig. 3. (a) Evolutions of laser output power with the pump diode laser power of the second stage of the fiber amplifier at different wavelengths (inset: maximum laser power as a function of laser wavelength); (b) the optical spectra at different laser wavelengths with maximum pump power, measured using an optical spectral analyzer (OSA, Yokogawa, AQ6375B, spectra 1200–2400 nm) with scanning resolution of 0.05 nm.

Fig. 4. The output laser spectra of different wavelengths as a function of different pump power of the power amplifier: (a)–(f) corresponding to center wavelengths of 1540 nm, 1550 nm, 1560 nm, 1570 nm, 1575 nm and 1580 nm, respectively.

Fig. 5. (a) Measurement setup for the laser linewidth using scanning Fabry–Perot interferometers; (b) the measured line shapes with pump power 15.8 W at center wavelength 1550 nm.