Erbium-doped twin-core fibre narrow-band filter for fibre lasers

Theoretical analysis of a recently experimentally demonstrated erbium-doped twin-core (ErTCF) fibre saturable-absorber-based narrow-band tracking filter is presented. Basic qualitative characteristics of the filter are deduced from an analogy with an ideally inhomogeneously broadened saturable absorber while the effect of wide band saturation of absorption due to spatial overlap of individual spectral components of the radiation is studied in detail using a numerical model based on coupled mode theory, resolved in space and frequency. The effect of the input power, twin-core fibre length and laser operating wavelength on the spectral characteristics of the filter is investigated.