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Abstract

An apparatus and method are described for exploiting almost the full almost 25.sup.TH % of bandwidth available in the low-loss window in optical fibers (from 1430 nm-1620 nm) using a parallel combination of optical amplifiers. The low-loss window at about 1530 nm-1620 nm can be amplified using erbium-doped fiber amplifiers (EDFAs). However, due to the inherent absorption of the erbium at shorter wavelengths, EDFAs cannot be used below about 1525 nm without a significant degradation in performance. For the low-loss window at approximately 1430-1530 nm, amplifiers based on nonlinear polarization in optical fibers can be used effectively. A broadband nonlinear polarization amplifier (NLPA) is disclosed which combines cascaded Raman amplification with parametric amplification or four-wave mixing. In particular, one of the intermediate cascade Raman order wavelengths .lambda..sub.r should lie in close proximity to the zero-dispersion wavelength .lambda..sub.0 of the amplifying fiber. For this intermediate Raman order, spectral broadening will occur due to phase-match 15 with four-wave mixing (if .lambda..sub.o <.lambda..sub.0) or phase-matched parametric amplification (if .lambda..sub.r <.lambda..sub.0) . In further cascaded Raman orders, the gain spectrum will continue to broaden due to the convolution of the gain spectrum with the spectrum from the previous Raman order.

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