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

Abstract

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.