Fibers are the key part of advanced fiber laser systems. Their properties are directly reflected in the output parameters of any experimental or commercial setup. Therefore, understanding and improving the fiber design is an essential and auspicious part of fiber laser development.

Our research group is mainly focused on fiber designs for pulsed high-power fiber lasers and, consequently, mitigating parasitic nonlinear effects is the main objective. In order to achieve this, active fibers with very large mode areas are required. Besides, it is most important to ensure effective single-mode operation and, therewith, excellent beam quality. Micro-structured fibers are the category of choice to reach this goal. Just recently, a novel approach for mode-field-area scaling has been developed in our group: the delocalization of higher-order modes. Delocalization results in a twofold advantage: it reduces the excitation of higher-order modes by the input beam and, additionally, it minimizes their overlap with the active core region. In this way, the fundamental mode experiences an improved excitation and a preferential amplification.

So-called large-pitch fibers are the first fiber designs exploiting the delocalization concept. They employ a photonic structure consisting of few hexagonally arranged air holes with large hole-to-hole distances (Fig. 1) to achieve the delocalization of higher-order modes. The simplicity of this fiber design ensures an excellent reproducibility. As the fiber design is not based on resonant effects, it is easily scalable by increasing the hole-to-hole distance. Values between 30 µm and 75 µm have been realized, which corresponds to core diameters of up to 130 µm. These effectively single-mode fibers raised the bar for the highest average output power obtained with very large-mode-field areas significantly. Hence, large-pitch fibers are called to serve as the backbone of next-generation fiber-laser systems with very high pulse energies and average powers.

Microscope images (all at the same scale) of a) standard step index fiber with 6 μm core and 125 μm outer diameter, b) 85 μm core rod type photonic crystal fiber with 200 μm air-clad diameter, and c) 108 μm core large-pitch fiber with 340 μm airclad diameter. (rights: IAP)

The process of designing a fiber is a challenging task that demands many different skills from a physician: The process starts with the development of suitable numeric tools for the simulation of novel designs, then it continues with the optimization of promising designs to address specific functionalities and finally it involves extensive experimental work aimed at evaluating the actual performance of the new fibers and its inclusion and test in experimental setups.

Selected publications:

[1] F. Jansen, F. Stutzki, C. Jauregui, J. Limpert, and A. Tünnermann, "High-power
     very large mode-area thulium-doped fiber laser," Opt. Lett. 37, 4546 (2012).
     link to jounal

[2] F. Stutzki, F. Jansen, H.-J. Otto, C. Jauregui, J. Limpert, and A. Tünnermann,
     "Designing advanced very-large-mode-area fibers for power scaling of fiber-
     laser systems," Optica 1, 233 (2014)
. link to journal

[3] H.-J. Otto, A. Klenke, C. Jauregui, F. Stutzki, J. Limpert, and A. Tünnermann,
     "Scaling the mode instability threshold with multicore fibers.," Opt. Lett. 39,
     2680-2683 (2014). link to journal

[4] J. Limpert, F. Stutzki, F. Jansen, H.-J. Otto, T. Eidam, C. Jauregui,
     and A. Tünnermann, "Yb-doped large-pitch fibres: effective single-mode
     operation based on higher-order mode delocalisation," Light Sci. Appl. 1,
     e8 (2012). link to journal

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