Experimental Study on Transverse Mode Instability Characteristics of Few-Mode Fiber Laser Amplifier Under Different Bending Conditions

We studied the transverse mode instability (TMI) characteristics of few-mode fiber laser amplifier employing ytterbium-doped fiber (YDF) with core/cladding diameter of 30/400 μm under different bending diameters. The gain fiber is coiled in ellipse shape with minimum bending diameter changed from 9 cm, 10 cm, 11 cm to 12 cm in the bending part, and the experimental results show an anomaly characteristic of TMI. The TMI threshold improved from 772 W, 1149 W and 1458 W to 1641 W when the bending diameter of YDF increase from 9 cm, 10 cm, 11 cm to 12 cm, accompanied by the deterioration of beam quality from 1.60, 1.67, 1.87 to 1.95. This is mainly because tightly bending in few-mode fiber will promote the overlap between modes and lead to mode coupling, which may trigger TMI. Besides, the bending losses of HOMs decrease while bending diameter increasing, so that there is higher output efficiency and deteriorated beam quality when bending diameter increase. Although the phenomenon is not agreed with common TMI with near single-mode fiber laser, the result will be helpful for the design and power scaling of few-mode fiber lasers.