Sc. Fleming et Tj. Whitley, MEASUREMENT AND ANALYSIS OF PUMP-DEPENDENT REFRACTIVE-INDEX AND DISPERSION EFFECTS IN ERBIUM-DOPED FIBER AMPLIFIERS, IEEE journal of quantum electronics, 32(7), 1996, pp. 1113-1121
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.