Page 320 - Fiber Optic Communications Fund

7

Transmission System Design

7.1 Introduction

So far, we have discussed photonic/optoelectronic components such as lasers, modulators, optical fibers,
optical amplifiers, and receivers. In this chapter, we put together these components to form a fiber-optic
transmission system. Critical system/signal parameters that affect the performance are identified and design
guidelines are provided. In Section 7.2, the performance of a simple fiber-optic system consisting of a trans-
mitter, a fiber, a preamplifier, and a receiver is analyzed. The transmission performance advantage of a
coherent receiver over the direct detection receiver for this unrepeatered system is discussed. Section 7.3
covers the dispersion-induced limitations and provides a simple design rule relating the bit rate, dispersion
coefficient, and reach. In Section 7.4, optical amplifier noise-induced limitations are discussed. For a long-haul
fiber-optic system, optical amplifier noise is one of the dominant impairments. A design rule pertaining to
amplifier spacing, number of amplifiers, and total reach is also discussed in Section 7.4.

7.2 Fiber Loss-Induced Limitations

Let us consider an unrepeatered direct detection system based on OOK, as shown in Fig. 7.1. Let P be the
in
transmitted power when ‘1’ is sent. The received power is
{
P = P exp (−L) when ‘1’ is sent,
1r
in
P = (7.1)
r
P = 0 when ‘0’ is sent.
0r
The variances of shot noise and thermal noise are given by Eqs. (5.72) and (5.76). Ignoring the dark current,
for bit ‘1’, we have
2 = 2qI B , (7.2)
1,shot 1 e
2
= 4K TB ∕R , (7.3)
1,thermal B e L
where
I = RP exp (−L) (7.4)
in
1
is the mean photocurrent of bit ‘1’. The mean photocurrent of bit ‘0’ is
I = RP = 0. (7.5)
0
0r
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.