This example demonstrates the design of a distributed Raman amplifier for ultra-wideband WDM transmission, using multiple pumps to achieve a gain flatness over an 80-nm signal bandwidth as designed after the work of Kidorf et al. As mentioned earlier, a very wideband flat amplification can be achieved by selecting launched powers and emission wavelengths of the Raman pumps properly. Figure 7 shows the general design setup.

Figure 7. Design Setup for Wideband Raman Amplifier Evaluation

One hundred test carriers are used to sample the Raman gain response over a bandwidth of approximately 82 nm. Each are launched with an average power of –3 dBm into 60 km SSMF. The accumulated fiber attenuation is completely compensated using the SRS effect of eight counter-propagating Raman pumps.

At the receiver, 100 power detectors are used to evaluate the Raman gain response at the output of the fiber. Figure 8 shows the optical spectrum at the receiver. The gain ripple is less than 2 dB over 81 nm. Note that there is still enough power margin to introduce a gain-flattening filter at the output of the fiber span to achieve a total gain ripple of less than 0.5 dB.

Figure 8. Optical Output Spectrum after Propagation over 60 km SSMF
Applying Backward Raman Amplification Using Eight Raman Pumps

The average launch powers of the eight Raman pumps vary only between 19.5 and 21.5 dBm. The flat gain response of the amplifier is achieved by selecting the emission frequencies of the pumps carefully. The eight Raman pumps are spaced unequally over about 86 nm, with an offset to the WDM signal band of 77 nm to 163 nm, as depicted in Figure 9. Note that the four pumps emitted at the smallest wavelengths are about equally spaced and that the other four are spaced much wider.

Intuitively, there are two reasons for this spacing arrangement. First, the Raman gain response is strongly asymmetrical. It has an almost linear increase for wavelength offsets between signal and pump of less than 100 nm and then a sudden decrease after the gain peak (see Figure 3).

Figure 9. Spectral Distribution of the Eight Raman Pumps
at the Backward Input of the Fiber (Red) and the Forward Output of the Fiber (Blue)

Second, there are strong pump-to-pump interactions, as the Raman pumps are spaced over 86 nm for which the Raman efficiency is already very large. Pumps emitted at the very low wavelengths amplify the WDM signal band as well as the pumps at the higher wavelengths.

Figure 10. Propagation of the Eight Raman Pumps over the Fiber

Figure 10 shows the pumps' power profile along the fiber. Starting with almost equal pump powers at the far end of the fiber, the pumps at the higher wavelengths are first amplified by the pumps at the lower wavelengths. Further down the fiber, when the power of the low-wavelength pumps is reduced due to energy transfer to high wavelengths and fiber attenuation, the effect of pump-to-pump amplification is reduced. As can be concluded from Figure 10, interaction of the different pump waves is nonnegligible.