Design of Transmission Fibers and Doped Fiber Amplifiers for Mode-Division Multiplexing

Transmission fibers and erbium-doped fiber amplifiers (EDFAs) for 12 signal modes (including spatial and polarization degrees of freedom) are studied. Modal fields are computed numerically and their effective areas, group delays (GDs), and chromatic dispersion coefficients are determined. Multimode rate equations are solved numerically to calculate mode-dependent gains (MDGs) and pump power requirements in multimode EDFAs. Results are compared for various numerical apertures (NAs), index profiles, and doping profiles. Graded-index depressed-cladding fibers offer an attractive combination of low GD spread in transmission fibers (583 ps/km rms with 0.15 NA) and low MDG in EDFAs (0.14 dB rms at 25-dB mode averaged gain with 0.15 NA). Optimized erbium doping profiles are comprised of a uniform cylindrical region plus an extra annulus.