Page 720 - Introduction to Information Optics

704 12. Networking with Optics

high. As a result, optical SNR at the output of a chain of N amplifiers is
reduced.
We now present some general but useful engineering rules of designing
amplified DWDM optical transmission systems.
1, Allocate the minimum OSNR required by the optical receiver to achieve
the desired system BER. For example, an OSNR of 21 dB at 0.1-nm
15
measurement bandwidth may be required to achieve 1 x 10~ BER for
a system at 10 Gbits/s transmission rate.
2, Allocate the power penalty from optical fiber nonlinearity. There are many
nonlinear phenomena, including SPM, XPM, FWM, SBS, and SRS, that
may cause impairments to transmission systems, especially at high speed.
Such impairments directly require additional optical power increase in
order for the optical receiver to maintain the same system performance
integrity. Such a power increase in unit decibels is called the power penalty.
3. Allocate the power penalty from optical fiber dispersion. Similar to
nonlinearity, fiber dispersion leads to system performance degradation
that adds additional OSNR requirements.
4. Allocate the OSNR margin, considering the potential penalty from the
component aging effect, polarization dependent effects, and so on.
To conclude this section, we present a typical 10 Gbits per second system
design example in Table 12.4, This example illustrates that system OSNR
requirements may be raised substantially to compensate for various impair-
ments.

12.4 APPLICATIONS AND FUTURE DEVELOPMENT OF
OPTICAL NETWORKS

12.4.1. LONG-HAUL BACKBONE NETWORKS

Previous sections explained that various technologies, especially optical ampli-
fiers and DWDM, have revolutionized backbone long-distance optical net-
works. Now we will briefly review the evolution of the backbone optical
network from yesterday's short-reach system to tomorrow's ultra-long-haul
(ULH), all-optical system. We will also introduce the key enabling technologies
for ULH transmission.

12.4.1.1. Evolution of Backbone Optical Network
A generic schematic of the backbone optical network before the deployment
of WDM and ED FA technologies is presented in Fig. 12.22(a), where TX and