Gain competition in Yb-doped symmetry-free photonic crystal fibers under severe heat load

In the last few years Yb-doped Photonic Crystal Fibers (PCFs) have become the key component for the development of reliable and high-performance lasers [1]. Despite an effective cooling of the fiber medium, a significant heat density is generated when high pump power is involved, which alters the propagation of the modes, causing unwanted coupling and destroying the output beam quality [2]. Symmetry-Free PCFs (SF-PCFs), characterized by the absence of any mirror symmetry in the inner cladding, have shown interesting properties in terms of resilience to thermal effects [3]. In fact SF-PCFs provide a strong delocalization of the Higher-Order Mode (HOM) even under severe heat load, thus preventing its thermally-driven reconfinement in the core. As an unwanted side effect, a poor confinement of the Fundamental Mode (FM), due to its coupling with cladding modes, is obtained in the absence of thermal effects, which can negatively affect the fiber single-mode regime and potentially compromise the effectiveness of the amplification process.In this work the amplification properties of air-silica Yb-doped SF-PCFs have been analyzed with a simulation tool which comprises a FEM-based full-vector modal solver, to calculate the propagating mode fields, an amplifier model, which accounts for the power evolution, and a thermal model, to obtain the temperature distribution and the consequent refractive index change in the fiber cross-section [4]. The 19-cell core SF-PCF considered in this analysis, whose cross-section is shown in the inset of Fig. 1(a), is built over a triangular lattice with pitch Λ = 15 μm and normalized air-hole diameter d/Λ = 0.5. The interaction between FM and first HOM at 1032 nm, induced by both thermal effects and amplification process, has been studied for fibers with inner cladding diameter dIC between 15Λ and 19Λ, while the air-cladding thickness is fixed at 7 μm. A co-propagating pumping scheme, with 400 W of pump power at 976 nm, has been considered for the 1-m long fiber amplifier with a pump absorption of 27 dB/m. The input power for the FM and the first HOM, defined as the LP11 mode or the LP11-like mode with the highest core overlap integral Γ value at a certain heat density Q0, is 5 W and 50 mW, respectively.