Page 195 - Optical Communications Essentials

Optical Amplifiers

Optical Amplifiers 185

Pump
OI
Signal OI
Signal
in WSC
Tap out
EDFA Tap
Pump laser
(a)
Pump
OI OI
Signal Signal
in WSC Tap out
Tap EDFA
Pump laser
(b)

Pump Pump
OI OI
Signal Signal
in WSC WSC out
Tap Tap
EDFA
Pump laser Pump laser
(c)
OI: Optical isolator
WSC: Wavelength-selective coupler
Figure 11.8. Configuration of an EDFA and possible pumping methods. (a) Codirectional
pumping; (b) counterdirectoinal pumping; (c) dual pumping.

which is known as counterdirectional pumping. One can employ either a single
pump source or use dual-pump schemes, with the resultant gains typically being
17 and 35dB, respectively. Counterdirectional pumping allows higher gains,
but codirectional pumping gives better noise performance. In addition, for oper-
ation in the C-band pumping at 980nm is preferred, since it produces less noise
and achieves larger population inversions than pumping at 1480nm.
When a single pump is used, a copropagating 980-nm laser minimizes the
EDFA noise, which is advantageous in preamplifier applications. On the other
hand, use of a counterdirectional pumping 1480-nm laser optimizes the output
power but results in higher noise, which is a suitable application for a booster
amplifier. Table 11.2 compares the characteristic 980- and 1480-nm pump lasers.
Since the erbium gain spectrum varies by several decibels as a function of
wavelength over the C-band, gain-flattening filters typically are used to atten-
uate wavelengths with higher gain, thereby equalizing the gain across the band.
To maintain a reasonable gain in an EDFA, gain flattening generally involves
the use of multiple stages of EDFA amplification combined with both codirec-
tional and counterdirectional pumping, as shown in Fig. 11.9. Here the first
stage uses a 980-nm codirectional pump whereas the second stage uses a 1480-nm
counterdirectional pump laser.

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