460
11 Basics of Optical Amplifiers
Drill Problem 11.4 To achieve a BER = 10
−15 the factor Q must be 8. (a)
Show that the OSNR required for this BER is 37.7 dB. (b) If the EDFA noise
figure is F EDFA = 5 dB, show that for this OSNR the maximum allowed input
power is P in = − 5.3 dBm (29 μW).
11.6 Fiber Link Applications
When designing an optical fiber link that requires optical amplifiers, there are three
possible locations where the amplifiers can be placed, as shown in Fig. 11.1. Although
the physical amplification process is the same in all three configurations, the various
uses require operation of the device over different input power ranges. This in turn
implies use of different amplifier gains. This section will look at simple conceptual
analyses and present generic operational values for the three possible locations of
EDFAs in an optical link.
11.6.1 Power Amplifier Functions
For the power amplifier, the input power is high because the device immediately
follows an optical transmitter. High pump powers are normally required for this
application [17]. The amplifier inputs are generally −8 dBm or greater, and the power
amplifier gain must be greater than 5 dB in order for it to be more advantageous than
using a preamplifier at the receiver.
Example 11.5 Consider an EDFA used as a power amplifier with a 10-dB gain.
Assume the amplifier input is a 0-dBm level from a 1540-nm laser diode transmitter.
If the pump wavelength is 980 nm, what is the pump power?
Solution: From Eq. (11.16), for a 10-dBm output at 1540 nm, the pump power must
be at least
P p,in ≥
λ s
λ p
P s,out − P s,in
=
1540
980
(10 mW − 1 mW) = 14 mW
11 Basics of Optical Amplifiers
Drill Problem 11.4 To achieve a BER = 10
−15 the factor Q must be 8. (a)
Show that the OSNR required for this BER is 37.7 dB. (b) If the EDFA noise
figure is F EDFA = 5 dB, show that for this OSNR the maximum allowed input
power is P in = − 5.3 dBm (29 μW).
11.6 Fiber Link Applications
When designing an optical fiber link that requires optical amplifiers, there are three
possible locations where the amplifiers can be placed, as shown in Fig. 11.1. Although
the physical amplification process is the same in all three configurations, the various
uses require operation of the device over different input power ranges. This in turn
implies use of different amplifier gains. This section will look at simple conceptual
analyses and present generic operational values for the three possible locations of
EDFAs in an optical link.
11.6.1 Power Amplifier Functions
For the power amplifier, the input power is high because the device immediately
follows an optical transmitter. High pump powers are normally required for this
application [17]. The amplifier inputs are generally −8 dBm or greater, and the power
amplifier gain must be greater than 5 dB in order for it to be more advantageous than
using a preamplifier at the receiver.
Example 11.5 Consider an EDFA used as a power amplifier with a 10-dB gain.
Assume the amplifier input is a 0-dBm level from a 1540-nm laser diode transmitter.
If the pump wavelength is 980 nm, what is the pump power?
Solution: From Eq. (11.16), for a 10-dBm output at 1540 nm, the pump power must
be at least
P p,in ≥
λ s
λ p
P s,out − P s,in
=
1540
980
(10 mW − 1 mW) = 14 mW