Gain-flattened fiber Raman amplifiers with nonlinearity-broadened pumps

Gain flattening of an E-band (1360–1460-nm) fiber Raman amplifier was achieved by pump spectral broadening to a 16.3-nm linewidth by use of nonlinearity in a 6.2-km standard fiber. With the broadened pump, the characteristic Raman gain ripple was removed and the 3-dB gain bandwidth was increased by 10.6 nm relative to the nonbroadened pump case. The experimental results agreed well with a gain profile simulation. The use of a 100-m holey fiber for pump broadening was also investigated near $1.56 \mathit{\mu }\mathrm{m}$ for U-band (1625–1675-nm) Raman amplifiers. Much less broadening was observed, which is believed to be a result of less four-wave mixing.