Yb-doped Photonic Crystal Fibers (PCFs) have triggered a significant power scaling into fiber-based lasers. However thermally-induced effects, like mode instability, can compromise the output beam quality. PCF design with improved Higher Order Mode (HOM) delocalization and effective thermal resilience can contain the problem. In particular, Fully- Aperiodic Large-Pitch Fibers (FA-LPFs) have shown interesting properties in terms of resilience to thermal effects. In this paper the performances of a Yb-doped FA-LPF amplifier are experimentally and numerically investigated. Modal properties and gain competition between Fundamental Mode (FM) and first HOM have been calculated, in presence of thermal effects. The main doped fiber characteristics have been derived by comparison between experimental and numerical results.
Modal properties of disordered optical structures, including a 1D-like multilayer structure and a 2D planar slab, have been numerically simulated in the Mid-IR region. The amount of scattering and the disorder level have been varied. A Finite Element Method solver has been used to show the modal properties of these structures, highlighting the correlation between the spectral behavior and the amount of disorder. The quality factor has also been investigated. A statistical parameter, based on the definition of photons travel distance, has been proposed to give a measure of the disorder according to the modal properties. With the help of a Monte Carlo based software this parameter has been investigated to verify its suitability.
Innovative Photonic Crystal Fibers (PCF) with optimized air-hole matrix, designed to break the C6ν symmetry of the inner cladding while preserving their feasibility through the well-established stack-and-draw technique, are presented. The possibility to provide stable SM guiding at λ = 2 μm with core diameter up to 80 μm and a coupled pump power exceeding 300 W is analyzed by means of a full-vector modal solver based on the finiteelement method with embedded thermal model, to account for the effects of heating on the mode confinement. Simulation results have shown this approach is effective in providing modal discrimination, allowing selective amplification of the sole fundamental mode due to the delocalization of the high-order modes with mirrorsymmetric field distributions. Effective suppression of the high-order modes under a heat load of 340 W/m, while keeping an effective area exceeding 2500 μm2 has been demonstrated.
Designs of Tm-doped photonic crystal fibers for laser operation must take in account the strong thermo-optical effects due to the Tm quantum defect and the consequent corruption of the single mode guiding properties. A new fiber design with a ∼ 80 μm core diameter, based on the cladding mirror symmetry reduction is proposed and analyzed using a full-vector FEM-based modal solver. The thermal effects are investigated using a computationally efficient model. A large pitch fiber with similar core diameter, which represents the actual state-of-art of Tm-doped laser technology, has been investigated in order to have a basis of comparison. Optimizing some key parameters of the new symmetry free fiber, the possibility to achieve a wide band single mode operation under an heavy heat load of over 300 W/m is demonstrated. In particular a very high modal discrimination value larger than 0.5 is obtained.
The development of low loss, small size and flexible waveguides is one of the most challenging issues of THz research due to the poor characteristics of both metal and dielectrics in this frequency range. Hollow core tube lattice fibers (HCTLFs) have been recently proposed and experimentally demonstrated to overcome this problem. However, they require very large hollow core size leading to big and hardly flexible fibers. Scaling law analysis plays an important role in determining the best trade-off between low loss and small fiber diameter. The dependence of the confinement on frequency and core radius are here numerically investigated. Results show that confinement loss exhibits a stronger dependence on core size and frequency with respect to other hollow core fibers proposed for THz waveguiding, such as Bragg, Tube, and Kagome fibers.