Pump-induced dispersion of erbium-doped fiber measured by Fourier-transform spectroscopy

We report the measurement of group index and dispersion in an erbium-doped fiber amplifier by Fourier transformation of low-coherence interferograms. In a germania-codoped fiber whose background dispersion was - 14 ps/(km nm) we measured resonant gain-induced changes as high as 9 and -12 ps/(km nm) near 1.536 microm. The interferometric measurements agree with calculations based on a Kramers-Kronig transformation of absorption and emission spectra.