A high-power Er/Yb polarization-maintaining fiber amplifier is described. The amplifier operates on a single wavelength of 1546 nm and consists of a pre-amplifier with a gain up to 64 dB followed by a double-clad 25/300-m Large Mode Area (LMA) Er/Yb fiber booster stage, counter-pumped through a (2+1):1 PM combiner by two 140-W 915-nm laser diodes. The design of the amplifier required developing new splicing and re-coating approaches for the LMA PM fibers and characterizing mode-field adaptors for single- mode (SM) and LMA fibers and low-loss high-power PM pump-signal combiners maintaining a high Polarization Extinction Ratio (PER). The amplifier provides a robust diffraction-limited output of more than 50 W from true single-mode PM1550 fiber with an M2 < 1.03 and a long-term (~200 hours) stability better than 5% in constant current mode (i.e., no power control feedback loop). With low-loss, high-quality splices between fibers with different MFDs, the maximum local fiber temperatures were kept below 60 oC. The amplifier design ensured that, even when operating at the 50-W level, the parasitic ASE in the 1-micron region was less than 2 mW. The output PER of <15 dB was limited by the quality of the pump-signal combiner which has a high probability of being improved in the future.
The main goal of the work was the analysis of approach, based on the use of pulse compression technique in laser systems for inertial confinement fusion. This approach promises improvement of parameters of existing `direct amplification' laser systems. The compression technique provides the additional instrument for control of spatial and temporal structure of the pulse, necessary for ISF realization.
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