Symmetry-free large-mode area rod-type photonic crystal fibers

Abstract

Recently, rare-earth doped fiber lasers become an attractive option for several applications requiring emission of high power radiation in different spectral regions [1]. The development of active fibers capable of providing large mode area is needed for power scaling of these systtems, as well as excellent beam quality, which can be obtained in the Single-Mode (SM) regime by efficiently suppressing the High-Order Modes (HOMs). Rod-type Photonic Crystal Fibers (PCFs), which have demonstrated to provide record effective area and excellent SM amplification properties at ∼1030 nm [2], are a promising solution also for mid-infraded sources, such as Tm-doped fiber lasers operating at ∼2000 nm, which are interesting for medical, industrial, sensing and defense applications [3]. However, good beam properties are more and more difficult to preserve as the power increases, since it is necessary to handle a significant thermal load. This problem is more severe in Tm-doped fiber lasers. In fact, the much larger quantum defect causes a stronger refractive index gradient along the cross-section, which favors HOM confinement [4]. As a consequence, particular care should be taken in designing Tm-doped rod-type PCFs, with respect to the more popular Yb-doped ones. In this work two symmetry-free designs have been considered for Yb-doped rod-type PCFs, as well as for Tm-doped ones. The mirror symmetries in the fiber inner cladding have been removed with the aim to worsen the HOM confinement in the doped core [5]. The HOM suppression, to the advantage of the Fundamental Mode (FM), has been numerically analyzed in different power regimes with a full-vector modal solver based the finite-element method [6], combined with a thermal model to account for refractive index change due to fiber heating [7]. As shown in Fig. 1, both designs are based on triangular lattice with pitch Λ = 14.4 μm. The doped core is formed by the 19 innermost cells, thus resulting in a hexagonal area with edge a = 36 μm. In the first design the inner cladding mirror symmetry removal has been obtained by rotating two rings of air-holes with respect to their position in the triangular lattice. In the second fiber the air-holes are arranged in three main arms, providing a clockwise “spin” to the cross-section. Three more air-holes have been added to both designs, in order to improve the asymmetry. Finally, 6 μm-thick air-cladding, with inner diameter of 260 μm and 240 μm, respectively, provide pump confinement. The confinement of the FM and the first HOM has been evaluated by considering their overlap integral over the doped area Г, taking into account different values of heat load and air-hole diameter. Moreover, the overlap difference AT between FM and HOM has been considered to study the SM properties. The same analysis has been carried out at 1032 nm and 2000 nm, that is for Yb-doped PCFs and Tm-doped ones, respectively. Simulation results have shown that both designs can provide effective HOM suppression, even under severe heat load. A complete discussion of design optimization for Yb-doped and Tm-doped fibers will be made at presentation time.