A side-pump coupler with a refractive index valley was investigated theoretically and experimentally. The high pump coupling efficiency and the suppression of backward light propagation due to the refractive index valley are illustrated. Based on the analysis, the $(2+1) \times 1$ pump and signal coupler with both pump coupling efficiency and signal transfer efficiency of $> 96\%$ and the BR of 2.82 was implemented, which accompanied with the maximum pump power of 398 W. And based on these couplers, an all-fiber bi-directionally pumping MOPA laser with an output power of 256 W and optical-optical efficiency of 80.5% was achieved in the experiment.
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Qirong Xiao, Ping Yan, Haichui Ren, Xiao Chen, and Mali Gong, "A Side-Pump Coupler With Refractive Index Valley Configuration for Fiber Lasers and Amplifiers," J. Lightwave Technol. 31, 3015-3022 (2013)