Fiber lasers have become very popular in the recent two decades due to numerous advantages compared to other solid-state lasers, e.g. their robustness, efficient cooling, high power compatibility, flexibility, large gain bandwidth and comfortable handling. Visible fiber lasers are increasingly becoming a new research focus, since they can be pumped very efficiently with new powerful, commercially available high-power GaN diodes in the UV wavelength range. Potential and important applications, especially in the biomedical field, are spectroscopy, microscopy and microsurgery. So far, fluoride glasses, e.g. ZBLAN, InF and AlF, are the most promising host glasses to realize stable visible fiber lasers. Most of them are originally designed for the MIR spectral range, but they also have the potential to cover the VIS. Pr3+ in ZBLAN glass is very attractive as it offers many possible laser transitions in the VIS and NIR range. In [1] a wide tunability in the VIS and NIR is demonstrated by core-pumping of a very short Pr3+-doped ZBLAN fiber. Regarding the visible spectral range, laser output powers up to 2.3 W around 635 nm were reported recently in a monolithic ZBLAN fiber laser with a Fiber- Bragg grating, but with reduced slope efficiency of 14% [2]. An alternative approach, where authors demonstrated up to 5 W laser power and 25.7% efficiency around 635 nm, is the direct and spliceless connection of pump and laser fiber using an end-facet coating mirror as input coupler [3]. Using this approach emission wavelength is power dependent and not locked.
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