Symmetry-free tm-doped photonic crystal fiber with enhanced mode area

Thulium-doped fiber lasers have recently attracted a growing interest because of the possibility to combine emission around 2 $\mu$m with the high beam quality and compactness provided by the fiber medium. A key factor for power scaling of these devices, especially if operation in the short-and ultrashort-pulse regime is desired, is the availability of active fibers capable of joining large effective area and robust single-mode guiding. The strong thermo-optic effect originating from the large quantum defect of Tm ions hinders the possibility to easily find a design with such features, strongly improving the confinement of high-order modes and shrinking the mode area of the fundamental one. In this paper, a rod-type double-cladding photonic crystal fiber with inner cladding properly designed without any mirror symmetry in the cross-section is presented, and its guiding properties are thoroughly analyzed by means of numerical simulations, taking into account the effects of thermally-induced refractive index change. The results have shown that, with a careful choice of the structural parameters, the proposed fiber is capable of ensuring effective high-order mode suppression and effective area up to 3800 ² when operating under heat load of about 300 W/m.