In this paper mode instability in a 500 W ytterbium-doped fiber laser is experimentally examined by changing the pumping wavelength, spectral bandwidth of signal light, active fiber temperature and coiling radius. The magnitude of power transfer from the fundamental mode to the higher order mode due to mode instability is measured as a criterion for its incident. The experiments show that the coiling radius of the first few tens of centimeters of the active fiber plays a significant role in controlling mode instability, and shifting the pumping wavelength from 976 to 973 nm can mitigate mode instability.

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In this paper we present a novel method to reliably strip the unwanted cladding light in high-power fiber lasers. Soft metals are utilized to fabricate a high-power cladding light stripper (CLS). The capability of indium (In), aluminum (Al), tin (Sn), and gold (Au) in extracting unwanted cladding light is examined. The experiments show that these metals have the right features for stripping the unwanted light out of the cladding. We also find that the metal-cladding contact area is of great importance because it determines the attenuation and the thermal load on the CLS. These metals are examined in different forms to optimize the contact area to have the highest possible attenuation and avoid localized heating. The results show that sheets of indium are very effective in stripping unwanted cladding light.