A method for pumping remote optically-pumped fiber amplifiers (ROPAs) in
fiber-optic telecommunication systems is disclosed which uses cascaded
Raman amplification to increase the maximum amount of pump power that can
be delivered to the ROPA. According to the prior art, high power at the
ROPA pump wavelength, .lamda.p, is launched directly into the fiber and
the maximum launch power is limited by the onset of pump depletion by
Raman noise and oscillations due to the high Raman gain at
.about.(.lamda.p+100) nm. In preferred embodiments of the present
invention, a `primary` pump source of wavelength shorter than .lamda.p is
launched into the delivery fiber along with two or more significantly
lower-power `seed` sources, among which is included one at .lamda.p. The
wavelength and power of the seed source(s) are chosen such that, when
combined with the high-power primary source, a series, n, where
n.gtoreq.2, of Raman conversions within the fiber ultimately leads to the
development of high power at .lamda.p. In another embodiment, one or more
of the seed sources at wavelengths shorter than .lamda.p are replaced by
reflecting means to return, into the fiber, backward-travelling amplified
spontaneous Raman scattered light resulting from high power in the fiber
at a wavelength one Raman shift below the particular seed wavelength. In
either case, the high power at .lamda.p is developed over a distributed
length of the fiber, reaching its maximum some distance into the fiber
and exceeding the maximum power possible at that point with the prior
art.