Cladding light handling in thulium-doped fiber amplifiers for kW-class spectral beam combination

Abstract

Power scaling of fiber lasers has always been pursued, being limited by nonlinear effects and heat generation in the active fiber and various components. Among the most critical components are cladding light strippers (CLS) between amplifier chains, removing light from leaked higher order modes, the unabsorbed pump or losses from splices and components. Polymer-based CLS work sufficiently well for the near-IR including the pump wavelength at 793 nm but suffer from high absorption at the signal wavelength near 2 µm and have not been evaluated in detail in this regime. Therefore, it is necessary to examine different acrylates and siloxanes at both the pump and signal wavelengths individually concerning their performance as CLS and test their limits. We present a CLS with an improved design which can withstand 7.5 W at 2039 nm while stripping >46 dB. For higher powers to >800 W, we examine CO2-laser inscribed CLS at the pump wavelength, reaching 21 dB stripping efficiency within only 15 mm of length.