5. Repeaters and Optical Amplifiers 119
amplifiers can be used for very high-bit-rate data systems, for any wave-
length region, being limited only by the available pump sources.
5.2.2 SEMICONDUCTOR AMPLIFIERS
A semiconductor amplifier is similar to a laser diode operating below
its laser threshold. There are two basic types: Fabry-Perot amplifiers
(FPA) and traveling wave amplifiers (TWA). The main difference is in
the amount of light which is reflected back into the amplification media
from the attached mirror facets; facet reflectivities are approximately 0.3
for a FPA and 10
3
–10
4
for a TWA. A schematic of a SLA is shown
in Figure 5.2, along with a graph of how light output is related to pump
current and facet reflectivity.
High facet
reflectivity
Low facet
reflectivity
L
i
g
h
t
O
u
t
p
u
t
Current I
L
d
Injection Current – I
W
Figure 5.2 Schematic of an SLA and its light output vs. injection current
characteristics for two different facet reflectivities.

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