Fig. 1. Schematic diagram of the single-frequency fiber amplifier and its measurement for power, spectrum, intensity dynamics and linewidth. LD, laser diode; YDF, ytterbium-doped fiber; HR-FBG, high-reflection fiber Bragg grating; OC FBG, output coupler fiber Bragg grating; CPS, cladding power stripper; PDF, phosphorous-doped single-mode fiber; SF, single-frequency; WDM, wavelength division multiplexer; PM EYDF, polarization-maintaining erbium-ytterbium co-doped fiber; PD, photodetector; AOM, acousto-optic modulator.

Fig. 2. The spectra of the 1480 nm pump laser at different output powers. The 1480 nm light is the main component.

Fig. 3. Normalized temporal behavior of the 1480 nm pump laser versus output power. There is a strong fluctuation at the power of 32 W.

Fig. 4. The co-pumped configuration. (a) The output power of 1560 nm laser and the backward power versus pump power. The backward power exhibits an exponential increase. (b) The spectrum of 1560 nm at maximum output power. Inset: output spectrum of the 1560 nm pre-amplifier. The ratio of the 1560 nm signal laser to the residual 1480 nm pump laser is 33.7 dB.

Fig. 5. The co-pumped configuration. (a) Fine spectra of the 1560 nm laser at different output powers. There is an obvious background spectral broadening. (b) The spectra at different output powers measured by the delayed self-heterodyne method. The 4.3, 10.2, 32.2 and 59.1 W output powers of the 1560 nm signal laser correspond to 0.85, 7.7, 32 and 61.7 W powers of the 1480 nm pump laser, respectively.

Fig. 6. Fine spectra of the 1560 nm laser at different pump powers of 1480 nm in a co-pumped manner, when the EYDF gain fiber is replaced by a passive fiber. There is also an obvious background spectral broadening.

Fig. 7. The co-pumped configuration: the RIN of the 1560 nm signal laser and corresponding pump laser at output powers of (a) 32.2 W and (b) 59.1 W. The noise at 657 kHz and the beat noise of the pump laser are transmitted to the signal laser.

Fig. 8. The counter-pumped configuration. (a) The output power of the 1560 nm laser and backward power versus pump power. The backward power exhibits a nonlinear increase. (b) The spectrum of 1560 nm at output power of 47.8 W. The ratio of the 1560 nm signal laser to the Er ASE is 57.9 dB.

Fig. 9. The counter-pumped configuration. (a) Fine spectra of the 1560 nm laser at different powers. There is no obvious background spectral broadening. (b) The spectra at different output powers measured by the delayed self-heterodyne method. The 3.4, 8.2, 25.9 and 47.8 W output powers of the 1560 nm signal laser correspond to 0.85, 7.7, 32 and 61.7 W powers of the 1480 nm pump laser, respectively.

Fig. 10. RIN of the 1560 nm signal laser at different output powers using different pump manners. The 25.9 and 32.2 W powers of the 1560 nm signal laser correspond to 32 W power of the 1480 nm pump laser in the counter-pumping and co-pumping manners, respectively. The 47.8 and 59.1 W powers of the 1560 nm signal laser correspond to 61.7 W power of the 1480 nm pump laser in the counter-pumping and co-pumping manners, respectively.