Page 475 - Fiber Optic Communications Fund

456 Fiber Optic Communications

1st order
echo pulse
2nd order
50 echo pulse
0
Power (dBm) –100
–50

–150 4000
3000
–200
–200 2000
–150 –100
–50 1000 Distance (km)
0
Time (ps) 50 100 150 200 0
Figure 10.22 Interaction of three signal pulses leading to echo pulses. The parameters are the same as in Fig. 10.20.

12

Input
10

8
output
Power (mW) 6

4

2 1st order
Echo pulse 1st order
echo pulse
0
–2 –1.5 –1 –0.5 0 0.5 1 1.5 2
Time (ps) × 10 –8
Figure 10.23 Optical power distributions at the fiber input and output. Fiber length = 3200 km.

to FWM sidebands at f + f − f and f + f − f . Similarly, in the case of IFWM, the nonlinear interaction
1
3
2
2
3
1
between pulses centered at t , t , and t leads to echo pulses at t + t − t and t + t − t . The difference
3
2
1
3
2
1
1
3
2
between FWM and IFWM is that echo pulses appear in the time domain instead of in the frequency domain,
as shown in Fig. 10.24(b). Hence, it is also known as time-domain FWM. The nonlinear interaction between
signal pulses leads to first-order echo pulses, as shown in Figs. 10.22 and 10.23, and the nonlinear interaction
of signal pulses and first-order echo pulses leads to second-order echo pulses. However, the amplitudes of the