Fiber coupler for mode selection and high-efficiency pump coupling.

A pump/signal fiber coupler with functions of pump light coupling and high-order-mode suppression is reported. This coupler is composed of pump fibers and a double-clad fiber (DCF) with high pump-coupling efficiency. The core of DCF is microdeformed in the coupling region, which serves mode selection. Theoretical analysis and experimental results verify these functions. In the experiment, the pump-coupling and signal-transmitting efficiency of the coupler were ∼98% and ∼96%, respectively. This coupler can prevent mode distortion with fundamental mode launching, and improve the fiber laser beam quality with high-order-mode launching.