Temperature as a guiding mechanism for high-power very-large-mode-area active fibers

Summary form only given. Pulsed Ytterbium-doped fiber laser systems have increased significantly in average output power, pulse energy and peak power within the last two decades. This progress was enabled by the development of large mode area fiber designs that inhibit nonlinear effects. Recently, single-mode fiber lasers delivering 100 W of average power at a record mode-field diameter of 100 μm have been demonstrated. In very large mode area (mode field diameter > 50 μm) fibers operating at high average powers, even the small quantum defect of Ytterbium is able to produce sufficient thermal load to substantially affect the modal properties of the waveguide [1]. In this contribution we show that these thermal waveguide changes alone are strong enough to create guidance for a fundamental mode in an air-clad fiber with an index-antiguiding core.This thermally guiding fiber possesses an Yb-doped region with a diameter of 42 μm that has an index of refraction that is about 2·10-5 smaller than the surrounding cladding (see Fig. 1a). The air-clad has a diameter of 170 μm, the outer diameter amounts to 630 μm. This 1.3 m long straight fiber was used as a counter-propagating amplifier by cladding-pumping it around 976 nm. The near-field image of the output beam was monitored with a camera. In passive and low power (cold) operation, the fundamental mode was delocalized from the core (leftmost image in Fig. 2). However, when increasing the power, the thermal load caused by quantum defect heating starts to imprint a parabolic index profile onto the core that decays logarithmically in the cladding [2], as illustrated (Fig. 1c). This thermally-induced index profile localizes the energy in the fiber core. This process starts at the pump-coupling end of the fiber and propagates towards the seed-coupling end.For powers up to 129 W robust single-mode operation was observed. The mode-field diameter of the output mode decreased more strongly than in the case of fibers with a guiding structure. Exceeding 129 W of average output power led to the onset of mode instabilities predominantly with radially symmetric modes, making this the first demonstration of mode instabilities with this class of modes. Furthermore, these experiments emphasize the advantages of higher-order-mode delocalization and show the ultimate limit of mode-area scaling with standard step-index fibers. Furthermore, the relation to gain-guiding index-antiguiding fibers is discussed in detail.