Inner cladding microstructuration based on symmetry reduction for improvement of singlemode robustness in VLMA fiber

Abstract

Very large mode area, active optical fibers with a low high order mode content in the actively doped core region were designed by removing the inner cladding symmetry. The relevance of the numerical approach is demonstrated here by the investigation of a standard air-silica Large Pitch Fiber, used as a reference. A detailed study of all-solid structures is also performed. Finally, we propose new kinds of geometry for 50 μm core, all-solid microstructured fibers enabling a robust singlemode laser emission from 400 nm to 2200 nm.

Single-Mode Propagation in Yb-Doped Large Mode Area Fibers With Reduced Cladding Symmetry

“…Modal discrimination is defined as the overlap difference between FM and HOM. According to previous works [12], [18], fiber is assumed to be Single-Mode (SM) for ≥ 0.3, while a FM overlap higher than 0.8 is requested to grant efficient amplification. Finally, effective area A e f f has been calculated for different pump power values, with the aim to evaluate the thermal lensing effect on the FM confinement [16], [17].…”

“…Manuscript Innovative fiber designs, capable of providing robust singlemode guiding under severe heat load, together with large mode area are thereby needed to push forward the current limits of fiber laser systems. One promising solution appears to be the use of PCF designs with reduced symmetries, that is without any mirror symmetry in the inner cladding, to disfavor the propagation of the High-Order Modes (HOMs) [12]- [15]. In this letter three different designs of double-cladding Yb-doped rod-type PCFs with reduced cladding symmetry are numerically compared through simulations, carried out by means of a full-vector modal solver based on the finite-element method, with integrated thermal model [16], [17].…”

Double-cladding photonic crystal fibers with reduced cladding symmetry for Tm-doped lasers

“…Three more air-holes have been kept to further weaken symmetry of the crosssection, similarly to what was done previously with an all-solid fiber. 12 The air-holes have normalized diameter d/Λ = 0.24. The 19 innermost unit cells have been Tm-doped.…”

“…W/m for both fibers and it corresponds to effectively SM regime. 12 One possible way to allow SM propagation under a heavier heat load is to apply core down-doping, which precompensates for the refractive index increase due to thermo-optic effect. Fig.…”

“…Recently, both air-silica and all-solid Ybdoped Large Mode Area (LMA) with inner cladding specifically designed to break the C 6ν symmetry have been proposed, showing remarkable results in the capability to effectively delocalize the HOMs while maintaining the Fundamental Mode (FM) well confined in the doped core. 11,12 In this paper two designs of TM-doped DC-PCFs with Symmetry-Free cladding (SF-PCFs) and core diameter of ∼80 μm are presented, and their guiding properties are analyzed and compared by means of a full-vector modal solver based on the finite-element method. 13, 14 A thermal model is applied to investigate the effects of thermally-induced refractive index change on the SM properties.…”