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In this paper we present our recent result on utilizing resonant/bandgap fiber designs to achieve high performance ytterbium doped fiber amplifiers for achieving diffraction limited beam quality in large mode area fibers, robust bending performance and gain shaping for long wavelength operation of Yb-doped amplifiers.

Summary form only given. Ultrafast ps and fs laser systems using ROD fiber amplifiers delivering megawatts of peak power and several hundreds of watts in average power have attracted significant academic and industrial interest in recent years. However, the power scaling has reached a temporary limit caused by thermal effects, which has set a limit of average power scaling of these amplifiers due to so called modal instabilities (MIs) [1]. New types of fiber designs with very large cores have been suggested to increase the average power limit [2, 3]. One way of achieving this, is using resonant structures [3], however it is often speculated that these structures are sensitive to temperature causing unstable beam quality under high thermal load (high power extraction pr unit length).A 25-ps mode-locked linearly polarized seed source at 1032 nm (40MHz rep rate, 16 W average power) is used to seed a distributed mode filtering ROD fiber with 90μm core diameter. The ROD fiber shows excellent power conversion efficiency as shown in Fig. 1 with almost linear slope and the MI threshold level is observed at 314 W of average output power corresponding to ~320kW of peak power. Optical to optical conversion efficiency is 69% and the mode quality is stable below the MI threshold. No non-linear effects are observed and the ASE is suppressed by ~35dB, shown in Fig.1. The experiments are modeled using a semi-analytical approach [4, 5], where a nonlinear coupling constant that depends on the thermo-optical effect is determined. χ is combined with FEM-calculated mode distributions to predict the onset of modal instabilities for this particular fiber design.We demonstrate that a ROD fiber design having a resonant structure can deliver efficient and linear amplification to high average output power without suffering from thermal load induced beam quality degradation. In addition, we evaluate the influence of the noise of the seed source on the MI threshold level. We model the MI threshold for this fiber and show good agreement between measurements and simulations, when the MIs are seeded by system dependent noise.

The effects of thermally-induced refractive index change on distributed modal filtering rod-type photonic crystal fibers are numerically investigated. Results have shown a significant blue-shift of the single-mode range for increasing temperature.

The effects of thermally-induced refractive index change in Yb-doped Distributed Modal Filtering (DMF) photonic crystal fibers are investigated, where high-order mode suppression is obtained by resonant coupling with high index elements in the cladding. The temperature distribution on the fiber cross-section is calculated with an analytical model, for different pump power values. The consequent refractive index change, due to the thermo-optical effect, is applied to the cross-section of the DMF fiber, whose guiding properties are studied with a full-vector modal solver based on the finite element method. A DMF fiber design, which is single-mode in the 1030 nm–1064 nm region, is considered, and the effects of thermal load on the transmission characteristics are evaluated. Results show a blue-shift of the single-mode window and the single-mode bandwidth narrowing as the absorbed pump power becomes higher.

High power harmonic generation of visible (green) and UV light have been dominated mainly by solid-state lasers but, in recent years, fiber laser based systems have started to challenge them [1]. In most applications, high peak and pulse energies are typically required. Nonlinearities need, therefore, to be minimized and the actual trend is to use large mode area fibers with very large core sizes [2, 3]. Such solution typically requires a master oscillator power amplifier architecture in order to achieve enough average output power or pulse energy without compromising the stability and the quality of the output beam.

We experimentally demonstrated that photodarkening is not uniformly distributed in the cross-section of bent LMA fibers. Photodarkening distribution depends on coiling diameter, and affects the use of fibers in applications and their photodarkening propensity measurement.