New simplified coupling system of 15MW Nd:YAG laser pulse with 600μm optical fiber by use of a taper fiber

Abstract

Optical fiber delivery of high peak power Nd:YAG Q-switched laser pulse is required for applications such as material processing, medical or military. The maximal transmitting power is variously limited by optical breakdown of air in the focal point and the damage threshold of fiber. A coupling system of 600μm all-silica optical fiber fused with taper fiber coupler and 15MW, 8ns Nd:YAG laser pulses is presented, consisting of an inversed telescope and a taper fiber coupler without the use of a vacuum chamber. The taper fiber delivers light basing on the principle of total reflection. The influence of incidence angle and taper length on the reflection angle are theoretic analyzed based on geometric optics. The coupling efficiency of the 12 tapered fibers fused with 1meter fiber is measured under different incidence angle of, respectively, 20mrad, 40mrad and 60mrad. The experiment results show that the coupling efficiency decreases gradually with the increase of taper length for larger modal loss. The larger incidence angle also decreases the coupling efficiency, which is consistent with above theoretic analyze. For 20mrad incidence angle, the coupling efficiency reaches as 95.53%. No surface damage or bulk damage is observed for 20meters fiber. In conclusion, the system could transmit 15MW Nd:YAG laser pulses by 600μm all-silica optical fiber stably. Intensity of 600μm fiber output surface is 4.42GW/cm 2 . The simplified coupling system resolves air breakdown of focal area and surface damage of fiber surface.