W-type fiber design for application in U- and S-band amplifiers by controlling the LP01 mode long wavelength cut-off

We propose a novel passive band-pass filtering scheme to obtain a net gain in a band-competing system using a W-type fiber waveguide, slicing a shorter wavelength gain band out of a given stimulated emission cross-section. By controlling the location of the LP01 mode cut-off, fiber design parameters are presented for obtaining a U-band gain from Tm ions and S-band from Er ions. The tolerance of the LP01 mode cut-off wavelength and the accompanying bending loss are theoretically analyzed in terms of waveguide parameters.