Page 221 - Photonics Essentials an introduction with experiments

Optical Fibers and Optical Fiber Amplifiers

Optical Fibers and Optical FIber Amplifiers 215

Many rare-earth elements can be dissolved in glass to make optical
amplifiers. Some common examples are neodymium, praseodymium,
holmium, and erbium. However, the laser transition in erbium-doped
glass occurs at a wavelength that is very close to the wavelength of
minimum attenuation of glass fibers, and this gives erbium special
importance.
The relatively long spontaneous lifetime of the 13/2 state compared
to other transitions in this schematic means that it is possible to build
up a substantial electron population in this state, and this feature fa-
cilitates the population inversion that is required for laser action. As
indicated in Fig. 9.14, this state is not characterized by a single well-
defined energy level, but rather a distribution of energy levels result-
ing from variations in the local environment of glass molecules that
surround the erbium ions. This distribution is advantageous because
it makes amplification possible over a relatively large band of wave-
lengths.
Pumping excitation of the state is achieved by coupling the light
from a GaAs-based laser into the optical fiber, as shown in Fig. 9.15.
The pump light ( = 980 nm) and the signal light ( = 1550 nm) prop-
agate in the same fiber core. The pump power is typically hundreds of
milliwatts, whereas the entering signal is typically in the microwatt
regime. The two light beams do not interfere with each other in the
amplifier section to any significant degree. The erbium-doped fiber is
spooled into a coil and pumped from both ends.
The passage of the signal through the pumped erbium-doped fiber
provokes stimulated emission that amplifies the signal. This occurs at
the speed of light, that is to say, nearly instantaneously. The amplifi-
cation is thus independent of the modulation rate. A signal consisting
of different wavelengths can be amplified using one erbium-doped
fiber amplifier because the amplifier does not mix or change the wave-
length. These are the two key features of optical amplification. In the
case of electronic amplification, the situation is different. Electronic
amplification starts by optical detection. This conversion erases all

Figure 9.15. Schematic diagram of an erbium-doped fiber amplifier. The pump light is
coupled into the erbium-doped section where it is strongly absorbed, preparing the er-
bium ions in the 13/2 state. The signal travels through the same section and is ampli-
fied by laser action, causing a transition by electrons from the 13/2 state to the ground
state.

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