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Experimental and theoretical investigations of photonic crystal fiber amplifier with 260 W output.

We report on a polarization-maintaining narrow-linewidth high power ytterbium-doped photonic crystal fiber amplifier with an output as high as 260 W and a slope efficiency of approximately 74%. Measurements of the beam quality yielded M2 values in the range of 1.2-1.3. The linewidth was determined at two different powers using an optical heterodyne detection technique and yielded values that were less than 10 KHz. Our maximum output power was pump limited and measurements of the reflected light indicated that we operated below the stimulated Brillouin scattering (SBS) threshold. Using a pump-probe technique, we estimated the Brillouin gain bandwidth to be approximately 68 MHz. In addition, the Brillouin gain spectrum revealed secondary peaks lying at the high-frequency side. In order to study the power limitations of our amplifier, we developed a detailed model that included a distributed noise source for the SBS process and a temperature gradient obtained via quantum defect heating. Our simulations indicated that for this particular fiber amplifier configuration an output power approaching 1 KW can be achieved. We also found that for forced air cooling the SBS threshold saturates regardless of the operating temperature of the polymer coating. Finally, we show that relatively small enhancement is obtained if a continuous transverse acoustic velocity gradient was implemented in conjunction with the thermal gradient. The latter conclusions drawn from our simulations also hold true for conventional fibers.