Enhanced pump absorption efficiency in coiled and twisted double-clad thulium-doped fibers

Results of the first experimental demonstration of the recently proposed technique for improvement of the pump absorption in double-clad fibers by their simultaneous coiling and twisting are reported. The peak absorption (14 dB) of 3-m long hexagonal thulium-doped fiber was increased by 8 dB by its simultaneous coiling and twisting. Explanation of the effect is given by numerical modelling of the pump absorption in hexagonal and panda-type fibers. Improvement of fiber laser performance was also proved. The slope efficiency increased from 19.6 % of the straight fiber to 23.9 and 29.4 % of the coiled only and simultaneously coiled and twisted fiber, respectively.