End-caps of different types are widely used in fiber laser systems to reduce the fiber facet damage threshold and the back reflection. Most commercial end-caps consist of a cylindrical rod of fused silica with a flat or angle polished end-surface. By fusion splicing the end cap to the output fiber, the laser beam diverges and the spot size enlarges inside the end-cap. Thus, the power density at the air-silica interface is much lower compared to the case without end-capping. However, with the traditional cylindrical rod shape, the divergent beam angle exiting from the end-cap may not be desired, but will be the result due to the flat (or angled) end surface of the end-cap. For many applications (such as medical surgery, metal cutting, etc.), less divergent, collimated, or focused beams are required. Spherical shaped end-caps are developed to meet the requirements for lower back-reflection, higher reliability, and flexible beam (convergent or divergent) angle. In this paper, general ball shaped end-caps are studied and illustrated. Different ball shaped end-caps were tested and measured for a variety of applications. For example, a 2.2 mm diameter ball shaped end-cap was manufactured on the end of the output LMA fiber and tested with a 1.7 kW fiber laser system.
Wenxin Zheng and Gongwen Zhu, "Ball-shaped end caps for fiber laser systems," Proc. SPIE 10525, High-Power Laser Materials Processing: Applications, Diagnostics, and Systems VII, 105250A (Presented at SPIE LASE: January 31, 2018; Published: 15 February 2018); https://doi.org/10.1117/12.2286174.
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