Page 712 - Introduction to Information Optics

696 12, Networking with Optics

12.3. DESIGN OF OPTICAL NETWORK TRANSPORT

In the previous section we discussed some critical optical network elements
such as optical fibers, optical amplifiers, transmitters, and receivers. Now we
consider the design of simple optical network transport when these key
components are put together. We first will discuss the impacts of fiber
dispersion, fiber nonlinearity, and signal-to-noise ratio (SNR); then we will
present design examples considering all effects.

12.3.1. OPTICAL FIBER DISPERSION LIMIT

The electric field of a linearly polarized optical signal or pulse can be
expressed as

mm]z <at]
E(x, y, z,t)= n[E(x, y, z, t)e ~ + c.c.] (12. 1 0)

where n is a unit vector, /?(«) is the propagation constant, co is the angular
frequency, and c.c. is the acronym of the complex conjugate. fi((ai) can be
expanded in a Taylor series around the center frequency co 0 of the light,

iti — " 0

where

A,
<a = coo
Chromatic dispersion is caused by the dependence of p(w) on the optical
frequency. Optical pulse consists of many frequency elements that travel at
different speeds due to the frequency dependent fi(co). As they travel along the
optical fiber, the higher-frequency elements of the pulse propagate faster than
the lower-frequency ones. Thus, the optical pulse broadens when it reaches its
destination. This phenomenon, often referred to as chromatic dispersion, is
detrimental to the optical transmission system, as schematically explained by
Fig. 12.19. At the origination of the transmission system, binary sequence 101,
represented by different power levels of optical pulses, is sent to a trunk of
optical fiber. Fiber chromatic dispersion causes the spread of optical pulses.
The receiver (not shown in the figure) may see the symbol 0 as 1. As a result,
a transmission error occurs.