To provide amplification characteristics of low noise with a wide band and a flat gain, by constituting an amplification section with three stages of an initial-stage amplification section, an intermediate-stage amplification section and a latter-stage amplification section, and causing an optical amplification medium of at least one of the three stages to be made of an Er3+-added fluorophosphoric acid fiber.
Amplification optical fibers 1-1, 1-2, 1-3, excitation light sources 2-1, 2-2, 2-3, demultiplexers 3-1, 3-2, 3-3, and optical isolators 4-1, 4-2, 4-3, 4-4 constitute a main section. As the amplification optical fiber 1-2, an Er3+-doped fluorophosphoric acid fiber is used. As the excitation light sources 2-1 and 2-2, strained quantum well semiconductor lasers are used. As the demultiplexers 3-1, 3-2 and 3-3, WD fiber couplers are used. As the optical isolators 4-1, 4-2, 4-3 and 4-4, optical isolators of polarization-independent type are used.
YAMADA MAKOTO
SUDO SHOICHI
NAKAGAWA KOICHI
