Extension of the double-clad Yb-doped fiber laser oscillation range thanks to long-period fiber grating filters

In this paper the possibility of extension of the shorter wavelength oscillation spectral range by the inscription of long-period fiber gratings filters into readily available double-clad ytterbium doped fibre laser is investigated. By using a spectrally and spatially resolved rate-equation model of the double-clad YDF, the effect of localized filtering is demonstrated numerically. Even with a small number of bandstop filters covering the strong amplified spontaneous emission gain the slope efficiency increases significantly towards the optimum design consisting of fully distributed filtering (>80%).