Pump-induced refractive index modulation and dispersions in Er/sup 3+/-doped fibers

A novel measurement system provides determination of pump induced phase shifts in erbium doped fibers with an accuracy of /spl sim//spl pi//20. Using this system, a systematical analysis of the pump induced modulation of the refractive index and dispersions for a signal at 1550 nm and a pump at 980 nm is reported. The analysis contains measurements of pump induced refractive index changes as function of wavelength, pump power, and doping concentration. A model taking account of the contribution to the refractive index changes from optical transitions between /sup 4/I/sub 15/2/ states and /sup 4/I/sub 13/2/ states in Er/sup 3+/ yields good agreement to experimental results apart from a wavelength independent offset. The offset is interpreted to originate from high energetic optical transitions. The results show that for a large refractive index modulation, a short and highly doped fiber should be used with limited amplified spontaneous emission effect. In optical communication systems comprising erbium doped fiber amplifiers, a tradeoff between dispersion and amplification must be made.