Page 535 - Fiber Optic Communications Fund

516 Fiber Optic Communications

ψ x,out
W xx
ˆ
ψ x,in

W xy

W yx
ˆ
ψ x,in

ψ y,out
W yy

Figure 11.20 Polarization mode dispersion compensation using four transversal filters.

Received signal Adaptive ψ in [k]
ˆ
after IF and phase Fixed DCF equalizer,
noise removal W, for PMD

Figure 11.21 Block diagram of the digital equalizer for a polarization-multiplexed fiber-optic system.

∗
′
W [k] (n+1) = W [k] (n) + y,out [n − k]e [n]Δ, (11.108)
x
xy
xy
′
∗
W [k] (n+1) = W [k] (n) + y,out [n − k]e [n]Δ, (11.109)
y
yy
yy
∗
′
W [k] (n+1) = W [k] (n) + x,out [n − k]e [n]Δ (11.110)
yx
y
yx
When a PMD equalizer is used, it is not necessary to have a separate adaptive equalizer for CD, as the diagonal
elements of the matrix H have contributions from CD. Typically, the fixed dispersion-compensating filter
compensates for the mean (non-time-varying) CD and the residual CD is compensated by the transversal
filters W and W . W compensates for the residual CD of the r-polarization component, r = x, y. Fig. 11.21
xx yy rr
shows block diagram of the digital equalizer that compensates for CD and PMD.
11.8 Digital Back Propagation
So far in this chapter, we have assumed that the fiber-optic system is a linear system and focused on the
mitigation of the linear impairments such as chromatic dispersion and polarization mode dispersion. In this
section, we consider the mitigation of fiber nonlinear effects. Fiber dispersion and nonlinear effects can be
compensated using the digital back-propagation techniques [22, 23]. Let us first consider a single-span system