Novel configuration of combined Er/Er-Yb fiber amplifier is proposed to join advantages of both approaches (high gain and high efficiency). Co-propagating pump at 976 nm and signal at 1556 nm were coupled into a short piece of Er-doped fiber, where signal was amplified to a few Watts level. Then signal together with unabsorbed pump passed to LMA Er- Yb fiber. In this configuration, 2 ns pulses were amplified to 20 W of average power and 37 kW of peak power (limited by four-wave mixing). Differential efficiency reaches 37%, which is a record for fiber lasers with this peak power level.
Single-mode Er-Yb fibers based on phosphorosilicate glass matrix highly doped with fluorine have been fabricated using modified all-gas phase MCVD technology. Fibers have core doped by 6.5 mol.% of P<sub>2</sub>O<sub>5</sub>, 0.9 wt.% of F and different concentration of Er and Yb. The core NA was about 0.07-0.08 relative to the pure silica level. Slope efficiency of more than 19% was achieved using amplifier scheme with co-propagating pump at 976 nm and signal at 1555 nm (input signal power was about 0.6W). Slope efficiency in the laser configuration has reached 34% relative to the input pump power.
An all-fiber pulsed erbium laser with pulse width of 2.4 ns working in a MOPA configuration has been created. Cladding pumped double clad erbium doped large mode area fiber was used in the final stage amplifier. Peculiarity of the current work is utilization of custom-made multimode diode wavelength stabilized at 981±0.5 nm – wavelength of maximum absorption by Er ions. It allowed us to shorten Er-doped fiber down to 1.7 m and keep a reasonably high pump-to signal conversion efficiency of 8.4%. The record output peak power for all-fiber amplifiers of 84 kW was achieved within 1555.9±0.15 nm spectral range.
Simple method to increase stimulated Brillouin scattering (SBS) threshold in MCVD fiber based on design with few concentric layers having different compound has been proposed. Two sets of fibers with core consisting of three layers with different alumina and germania concentrations have been fabricated. First set of fibers was designed for Raman amplifiers and had a relatively small mode field area of 23-28 μm<sup>2</sup>. The second set of fibers was designed for high peak power pulse delivery and had mode area of 225-325 μm<sup>2</sup>. SBS suppression (as compared to the Ge-doped fibers) was estimated from SBS gain spectra and direct observation of SBS threshold to be more than 6 dB and 3.3 dB for the first and the second set of fibers.