Figure 2. A typical hybrid Raman/Erbium-doped fiber amplifier. RPU stands for Raman pumping unit

From

Hybrid Raman/Erbium-Doped Fiber Amplifiers for WDM Transmission Systems

Sunil P. Singh

Journal of Optoelectronics Engineering. 2016, 4(1), 1-4 doi:10.12691/joe-4-1-1