Page 409 - Fiber Bragg Gratings

386 Chapter 8 Fiber Grating Lasers and Amplifiers

well as the management of dispersion. The fiber amplifier has enabled
undersea transmission over thousands of kilometers. With the increasing
demand for bandwidth, wavelength division multiplexing (WDM) of opti-
cal channels is seen to be a viable solution for increasing transmission
capacity at a given transmission rate, in point-to-point routes [120],
The important issues in amplified transmission systems for WDM
applications are the available gain, the gain uniformity across the band-
width of an amplifier, the robustness to transient switching of WDM
channels in single and cascaded amplifier chains, and the overall noise
figure. The gain spectrum and upper-state lifetimes of an erbium-doped
fiber amplifier vary as a function of the core dopants. High-germanium
erbium-doped fibers have a highly nonuniform gain with a bandwidth of
—35 nm from 1530 to 1565 nm, peaking at approximately 1535 nm. The
use of aluminum reduces the nonuniformity, making the gain spectrum
flatter, while ytterbium as a codopant with erbium allows an efficient
transfer of energy from the available high-power diode pumps at 980 nm
(a wavelength at which erbium suffers from excited-state absorption,
reducing efficiency) to the required transition in the 1550-nm wavelength
region for amplification. The latter shifts the gain peak to >1540 nm,
while narrowing the gain bandwidth. Along with the nonuniformity in
the gain, the doped fiber amplifier is homogeneously broadened. While
allowing amplification across the wide gain bandwidth, the gain available
at any wavelength is dependent on the simultaneous presence or absence
of other wavelength channels; thus, gain may be depleted from a saturat-
ing signal from an existing channel as another channel is switched on.
As the gain in a single amplifier fluctuates, the problem is exacerbated
with a chain of cascaded amplifiers, leading to severe cross-talk. Nonuni-
form gain across the bandwidth of the amplifier produces a wavelength-
dependent low-frequency cross-talk penalty. Apart from the signal degra-
dation, severe damage to components is a possibility because spiking with
intermittent interruption and resumption of transmission. The inversion
and thus the gain is also pump-power dependent, leading to gain changes
as the pump source ages. For analog transmission systems the problem
is worse, because the local gain slope leads to harmonic distortion, degrad-
ing the received signal. Schemes have been developed to stabilize the
gain of fiber amplifiers as well as flatten the gain variation over a wide
bandwidth. Fiber gratings offer simple solutions to solve both these prob-
lems. In the following sections, we look at a specific example of gain