Aiming at the problem of high-ring-cavity losses caused by the use of a high-insertion-loss filter components to tune the output wavelengths in a continuous-wave (CW) all-fiber optical parametric oscillator (FOPO), this paper proposes a low ring-cavity-loss tunable CW FOPO based on the cascaded single-mode-multimode-single-mode (SMS) fiber structures. By applying an axial pulling force to the SMS fiber device to adjust the transmission spectrum of the filter device, the cascaded SMS fiber devices were used to achieve a tunable CW output of the double sidebands in the range of 1494~1501 nm and 1638~1629 nm. Among them, the insertion losses of the cascaded SMS fiber devices at the selected wavelengths are less than 1 dB, and the total FOPO ring-cavity losses are no more than 5 dB.
A high-energy, single-frequency fiber laser with long pulse duration of 100 ns has been experimentally investigated in an all-fiber architecture. Only 34-cm long heavily Yb-doped phosphate fiber was employed in power scaling stage to efficiently suppress the Stimulated Brillouin effect (SBS). In the experiment, 0.47 mJ single pulse energy was achieved in power scaling stage at the pump power of 16 W. The pre-shaped pulse was gradually broadened from 103 to 140 ns during the amplification without shape distortion.