The angle-polished method for side-pumping technology that is useful in high-power fiber lasers and fiber amplifiers is investigated. By geometric optics, the simple model for calculation of the coupling efficiency is derived. Numerical simulations show that both the polished angle of multimode fiber and the refractive index of index-matching material are pivotal to the side-coupling efficiency. By experiments, coupling efficiency of 80.2% and maximum coupling power of 1.12 W are measured for a 200-μm multimode fiber and a double-clad fiber with a 400/340-μm D-shaped inner cladding. Slope efficiency of 56% and output power of 282 mW are demonstrated in a D-shaped Yb-doped double-clad fiber laser by the side-pumping system with the angle-polished method.
"Coupling efficiency of angle-polished method for side-pumping technology," Optical Engineering 43(4), (1 April 2004). https://doi.org/10.1117/1.1666855