The design method of high-efficiency coupling of fiber for the high-power solid laser

The high-power solid laser outputs multimode beam. When we used the classical method defining beam radius, which is used to define 86.5% of the beam spot energy as the radius of basic mode, to design the coupling system, the coupling efficiency was less than 80%. Even when we used 2/3 of the core diameter to design coupling system, the coupling efficiency was still less than 85%. By analyzing the structure of multimode beam, we embedded an equivalent basic mode into the multimode beam, defined 98% of the beam spot energy as the radius of the equivalent basic mode, and designed the coupling system whose coupling efficiency is more than 95%. In order to prove this method, we designed fiber coupling system, and optimized numerical value for the lens’ aperture and focus. We got the coupling efficiency more than 94% when the input power is 2000W.