Page 289 - Optical Communications Essentials

Optical Link Design

Optical Link Design 279

16.7. Summary
The design of a high-quality transmission link involves a series of tradeoffs
among the many interrelated performance variables of each component based
on the system operating requirements. Thus, the link analyses may require sev-
eral iterations before they are completed satisfactorily. Since performance and
cost constraints are very important factors in a transmission link, the designer
must choose the components carefully to ensure that the link meets the oper-
ational specifications over the expected system lifetime without overstating the
component requirements.
Key topics related to link design include these:
■ A link power budget analysis wherein one first determines the power margin
between the optical transmitter output and the minimum receiver sensitivity
needed to establish a specified BER and then allocates this margin to fiber,
slice, and connector losses, plus any additional margins required for other
components, possible device degradations, transmission-line impairments, or
temperature effects
■ A link margin which is an optical-power safety factor for link design that
involves adding extra decibels to the power requirements to compensate for
possible unforeseen link degradation factors
■ Power penalties which relate to a reduction in the signal-to-noise ratio (SNR)
of the system from the ideal case due to signal impairment factors, such as
modal noise, chromatic dispersion, polarization mode dispersion, reflection
noise in the link, low extinction ratios in the laser, or frequency chirping
■ A rise-time budget analysis for determining the dispersion limitation of an
optical link
■ The format or encoding of the transmitted digital signal so that the receiver
can extract precise timing information from the incoming signal
■ Forward error correction, which is a mathematical signal processing tech-
nique that encodes data so that errors can be detected and corrected. In FEC
techniques, redundant information is transmitted along with the original
information. If some of the original data are lost or received in error, the
redundant information is used to reconstruct them
Associated with this book is an abbreviated version of the VPItransmission-
Maker tool from the VPIsystems, Inc. suite of software-based design tools. This
simulation engine module is called VPIplayer and has all the tools needed to
explore, design, simulate, verify, and evaluate active and passive optical compo-
nents, fiber amplifiers, dense WDM transmission systems, and broadband
access networks. Familiar measurement instruments offer a wide range of set-
table options when displaying data. The VPIplayer module and a wide variety of
interactive demonstration examples are available via Internet access for educa-
tional use and concept demonstrations. See Chap. 1 or See. 16.6 for details on
how to download the demonstration software.

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