Gain-tailored SBS suppressing photonic crystal fibers for high power applications

We present experimental studies of PM Yb-doped photonic crystal fibers possessing acoustic and Yb-ion concentration tailoring. In the initial design, the concentration of dopants in two regions of the core were selected such that the corresponding Brillouin shifts were sufficiently separated to allow for further stimulated Brillouin scattering suppression through thermal effects. The Yb-ion concentration was maintained uniformly throughout the entire core. When this fiber was utilized in a counter-pumped amplifier configuration, ~500 W of single-frequency (kHz linewidth) output was obtained in a 10 m long fiber. Further power scaling with good beam quality beyond 500 W was limited by modal instabilities. A second fiber design was developed in which the Yb-ion concentration was modified to have preferential overlap with the fundamental mode as well as reduced pump absorption. The onset of the modal instabilities was sufficiently suppressed to allow for an output of 990 W with a nominal linewidth of 300 MHz and good beam quality.

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