5-kW-Level Bi-Directional High-Efficiency Pump and Signal Combiner With Negligible Beam Quality Degradation

(6+1)×1 pump and signal combiner with high power handing capability, negligible beam quality degradation and bi-directional signal transmission ability has been developed by employing signal-fiber non-taper fabrication approach and in-line active splicing process. The influences of lateral core offset and angular misalignment between the signal input and output fibers in combiner have been studied theoretically, showing that M
²
factor rather than signal efficiency is more suitable to be the evaluation criterion. Three splicing methods have been compared experimentally, revealing that the M
²
-based active splicing is the most effective to preserve beam quality and obtain good efficiency simultaneously. Signal laser transmitted backward through 25-μm fiber core has shown efficiency of 95.5% and M
²
factor deterioration no more than 10% at 1 kW of power. Combined pump from laser diodes into 400-μm fiber clad has reached beyond 5 kW of power at efficiency of 98.2%, with the maximum temperature on combiner being 70.5°C, which was tested in the condition without cooling on fiber pigtails and with flat-cleaved fiber termination aggregating thermal load. Both results indicate the effectiveness of the proposed fabrication technique to make bi-directional (6+1)×1 combiner for high power and superior beam quality fiber laser systems.