MEASUREMENT AND ANALYSIS OF PUMP-DEPENDENT REFRACTIVE-INDEX AND DISPERSION EFFECTS IN ERBIUM-DOPED FIBER AMPLIFIERS

In a doped fiber, at wavelengths close to an absorption, the refractiv e index, and hence the dispersion, is expected to be a strong function of wavelength, as described by the Kramers-Kronig relationship. Furth ermore, this spectral variation itself will be a function of pumping. This paper describes an accurate and sensitive experimental determinat ion of the pump dependence of the refractive index in an erbium-doped fiber amplifier. A Mach-Zehnder interferometric measurement is describ ed where only one arm is comprised of doped fiber. Particular attentio n is paid to accounting for the significant difference in power levels in the two arms and the effects of polarization and incoherent light. The measured refractive-index change matches well with that theoretic ally predicted. The pumped and unpumped dispersion of the amplifier ar e calculated. While the extra dispersion is of similar magnitude to th at of silica fiber, the impact on long telecommunications systems is l ikely to be negligible. However, this extra dispersion may be importan t in shorter systems such as soliton lasers. Given the good agreement between theory and experiment, the variation of dispersion with fracti onal inversion is calculated from absorption and gain measurements. Th e presence of codopants is known to alter the absorption and emission spectra; the effect on the dispersion is calculated, and a near linear dependence on germanium concentration is observed.