Page 408 - Fiber Optic Communications Fund

9

Channel Multiplexing Techniques

9.1 Introduction

Typically, the single-channel symbol rates range from 10 Gsym/s to 40 Gsym/s. A symbol rate beyond
40 Gsym/s is hard to achieve in practice because of the speed of electronic components in transmitter
and receiver circuits. In the low-loss region of the fiber (1530–1620 nm), it has a bandwidth greater than
10 THz. To utilize the full bandwidth of the fiber, several channels can be multiplexed and they can share
the same fiber channel. An EDFA operating in C-band (1530–1565 nm) has a bandwidth of about 4.3 THz
and, therefore, several channels can be amplified simultaneously by a single amplifier. The multiplexing
techniques can be divided into three types: (i) polarization division multiplexing (PDM) or polariza-
tion multiplexing (PM), (ii) frequency or wavelength-division multiplexing (WDM), (iii) time-division
multiplexing (TDM).

9.2 Polarization-Division Multiplexing

PDM is an effective technique to double the capacity. Commercial coherent systems make use of PDM and
WDM to enhance the capacity. A single-mode fiber supports two polarization modes–one with the electric
field aligned with the x-axis and the other aligned with the y-axis (see Section 2.7.5). Therefore, it is possible
to transmit information using each of these polarization modes. A schematic of the PDM or PM is shown in
Fig. 9.1. At the transmitter, a polarization beam splitter is used to split the x- and y-polarization components
of the laser source. The x-(y-)polarization component of the laser is modulated by the electrical data m (m )
x
y
using an optical modulator Mod x (Mod y). If the modulators are operating in the linear region, their outputs
are (see Section 4.6.2.2)
= xA m (t)e −i2f c t , (9.1)
x
x
c
= yA m (t)e −i2f c t , (9.2)
y c y
where f is the laser frequency. The polarization beam combiner (PBC) combines these polarization compo-
c
nents. The output of the PBC is
= + = A e −i2f c t [m (t)x + m (t)y]. (9.3)
x
y
y
x
c
Fiber Optic Communications: Fundamentals and Applications, First Edition. Shiva Kumar and M. Jamal Deen.
© 2014 John Wiley & Sons, Ltd. Published 2014 by John Wiley & Sons, Ltd.