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Large-mode-area (LMA) photonic crystal fiber (PCF) amplifiers are attractive for high-power amplification of single-mode (SM) narrow-linewidth light. Traditionally, LMA PCF designs include air holes but splicing and interfacing of fibers with air holes is complicated. Recently, this was addressed with the development of a LMA PCF amplifier fiber without air holes but with SM operation maintained. This all-solid fiber can be spliced with standard splicers and thus enables integration of PCFs into all-fiber monolithic laser systems. The new fiber, named DC-250/30-PM-Yb-FUD, is demonstrated in a free-space configuration for high-power amplification of 1064 nm light with a narrow linewidth of less than 20 kHz. A seed power of 380 mW is amplified to 40 W, corresponding to a single-stage gain of 20 dB, with an optical to optical efficiency of 69%. No indication of stimulated Brillouin scattering is observed, and the output is stable during frequency modulation of the seed laser.

crystal fiber technology for monolithic single-mode large mode area all-solid amplifier,"