I review recent progress on optical fiber amplifiers and their applications in fiber-optic communication systems.
This study focuses on rare-earth doped fiber amplifiers (RDFAs) and fiber Raman amplifiers (FRAs). There are three
types of RDFA, namely erbium, thulium, and praseodymium doped fiber amplifiers (EDFAs, TDFAs, and PDFAs,
respectively) and EDFAs have been widely deployed in trunk networks for about a decade. EDFAs have wideband, low
noise, and high pumping efficiency characteristics, and are key components of high-capacity and cost-effective
wavelength-division-multiplexing (WDM) transmission systems in the low loss 1.5 μm band. In contrast, distributed
Raman amplification (DRA) has been recognized as a powerful and practical technology in long-haul trunk networks in
recent years. DRA/EDFA or DRA/lumped FRA hybrid-amplification systems yield significantly higher signal-to-noise
ratios than lumped optical amplification systems that use EDFAs or FRAs. The latter are hybrid systems called
all-Raman systems. As regards the bandwidth enhancement of optical fiber amplifiers, which is indispensable for
cost-effectively realizing a rapid increase in communication traffic, silica Raman amplifiers have seamless single band
bandwidths (Δλ) up to ~100 nm wider than those of RDFAs (~30 to ~80 nm). Further bandwidth enhancement can be
achieved by using tellurite-based Raman amplifiers with Δλ up to ~160 nm or multi-band amplifiers. Each multi-band
amplifier uses plural single-band RDFAs and/or FRAs in the parallel configuration in the low loss wavelength region
(1.3-1.6 μm band) of silica transmission fibers.