An optical wavelength-multiplexing system allowing long distance transmission
with reducing effects of noise and fiber nonlinealities is disclosed. First wavelength-multiplexing
light and second wavelength-multiplexing light within a wavelength bandwidth of
100 nm or more are propagated in opposite directions through the same optical fiber.
The wavelength band of the first wavelength-multiplexing light is set to a shorter
wavelength side as compared to that of the second wavelength-multiplexing light.
In addition, excitation light having a wavelength shorter than the wavelength band
of the first wavelength-multiplexing light is propagated through the optical fiber
in the same direction as the second wavelength-multiplexing light. The excitation
light is shorter in wavelength than the first wavelength-multiplexing light by
an amount determined depending on Raman scattering characteristic of the optical fiber.