Page 192 - Optical Communications Essentials

Optical Amplifiers

182 Chapter Eleven

TABLE 11.1 Typical Performance Values for SOA Parameters
Parameter O-band device C-band device Conditions
Operating peak wavelength range 1280–1340nm 1530–1570nm —
3-dB bandwidth 60nm 40nm —
Small-signal gain 25dB 20dB Fiber-to-fiber gain
Gain ripple 0.5dB 0.5dB Across 3-dB bandwidth
Operating current 250–300mA 250–300mA —
Saturation power 10dBm 8dBm 250-mA drive current
Noise figure 9dB 9dB 25-dBm input power
Polarization-dependent gain (PDG) 1dB 1dB —

11.4. Erbium-Doped Fiber Amplifier (EDFA)

The active medium in an optical fiber amplifier consists of a nominally 10- to
30-m length of optical fiber that has been lightly doped (say, 1000 parts per
million weight) with a rare-earth element, such as erbium (Er), ytterbium (Yb),
neodymium (Nd), or praseodymium (Pr). The host fiber material can be stan-
dard silica, a fluoride-based glass, or a multicomponent glass. The operating
regions of these devices depend on the host material and the doping elements.
The most common material for long-haul telecommunication applications is
a silica fiber doped with erbium, which is known as an erbium-doped fiber
amplifier or EDFA. Originally the operation of an EDFA by itself was limited to
the C-band (1530- to 1560-nm region), since the gain coefficient for erbium
atoms is high in this region. This fact actually is the origin of the designation
conventional band or C-band. Outside of this region the erbium gain peak drops
off rapidly, and in the L-band it is only 20 percent of that in the C-band.
However, recent improvements in erbium-doped fiber designs and the use of
high-power pump lasers operating at wavelengths that are different from those
used by C-band pump lasers have allowed the extension of EDFAs into the
L-band.
In addition, a combined operation of an EDFA together with Raman amplifi-
cation techniques for the L-band (see Sec. 11.5) has resulted in a hybrid ampli-
fier that can boost the gain over the 1531- to 1616-nm region with a 3-dB gain
bandwidth of 75nm.

11.4.1. Amplification mechanism
Whereas semiconductor optical amplifiers use external current injection to excite
electrons to higher energy levels, optical fiber amplifiers use optical pumping. In
this process, one uses photons to directly raise electrons into excited states.
The optical pumping process requires the use of three energy levels. The top
energy level to which the electron is elevated must lie energetically above the
desired lasing level. After reaching its excited state, the electron must release

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