Page 409 - Fiber Optic Communications Fund

390 Fiber Optic Communications

electrical data,
m x
x ψ x
Mod x
ψ Fiber
Laser channel
PC ψ
PBS y y PBC
Mod y
electrical data,
m y
Figure 9.1 Polarization-division multiplexing. PC = polarization controller, PBS = polariaztion beam splitter, PBC =
polarization beam combiner, Mod = optical modulator.

I x,I
IQ Rx1
x I x,Q
PBS m x
Recieved signal
ESP
m y
y I y,I
IQ Rx2 I y,Q

Figure 9.2 Block diagram of the polarization demultiplexing receiver. ESP = electrical signal processing.

These polarization components propagate as two polarization modes in a single-mode fiber. At the receiver,
two IQ receivers are used to detect the x- and y-polarization components as shown in Fig. 9.2 (see
Section 5.6.5). Let the complex currents corresponding to x- and y-polarization components be

I (t)= I (t)+ iI (t), (9.4)
x x,I x,Q
I (t)= I (t)+ iI y,Q (t). (9.5)
y
y,I
Note that the x- and y-axes at the receiver may not be the same as those at the transmitter and, in addition,
there is a coupling between the polarization modes during the fiber propagation due to random fluctuations
in the refractive index. So, in the absence of noise, the complex currents in the frequency domain may be
written as
̃ x x ̃ xx ̃ xy y (9.6)
I ()= ̃m ()M ()+ M ()̃m (),
̃ ̃ ̃ (9.7)
I ()= ̃m ()M ()+ M ()̃m ()
y x yx yy y
or
̃ ̃ (9.8)
I = M̃ m,
[ ]
̃
I
̃ I = x , (9.9)
̃ y
I