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Introduction to
BER testing of WDM systems
Application note 1299
Wavelength division multiplexing
(WDM) is a new and exciting
technology for migrating the core
optical transmission network to
higher bandwidths. The ability to
transfer multiple optical carriers
over the same span of fiber
promises almost unlimited
bandwidth.
However, the ultimate test for any
transmission medium is its bit
error ratio (BER) performance.
How WDM overlays
onto the network
If a network hotspot produces capacity shortfall problems, network
operators can now opt to deploy a WDM system to quickly expand
capacity on existing fiber links. For example, WDM equipment has
already been deployed to multiply the capacity of existing STM-16/
OC-48 links--by combining and carrying up to 16 STM-16/OC-48 signals
along the existing fiber path. At the same time, upgrading of the
existing STM-16/OC-48 line terminal mux to WDM operation is also
readily achieved, providing increased bandwidth while retaining
existing tributary access and connections. The operation of a WDM
system can best be explained by looking at the sub-system level.
Figure 1.
Simplified
WDM system
configuration.
Figure 2.
Example output
spectrum for four
2.4 Gb/s laser
sources.
Schemes are proposed carrying up to 32 Laser sources and remodulators Optical multiplexer
STM-16/OC-48 signals. The transmitter of a WDM system produces The multiplexer (mux) couples together
Currently, ITU-T draft recommen- laser signals at specific wavelengths and different wavelengths then combines them
dations O.mcs specify 43 WDM with a nominal spectral line spacing for transmission into a single mode fiber--
wavelengths. between them (Figure 2). Frequency maintaining the wavelength integrity of
separation is carried out using laser sources each optical carrier.
of specific frequencies, or using parallel
1550 nm laser sources with a remodulator Fiber path and EDFAs
to obtain the required frequencies. Laser If an optical link is short, the transmission
sources are usually distributed feedback path will consist of nothing more than
(DFB) lasers, typically working in the range optical fiber. If the path is longer, say 50 to
1530 to 100 km, then erbium doped fiber amplifiers
1565 nm. They offer good stability and a (EDFAs) are used for pre- and post-
narrow spectral width which is a pre- amplification. Where longer links are
requisite for dense-WDM (DWDM), where deployed, over 100 km, then EDFAs are
the spectral line spacing can be as narrow also used as intermediate amplifiers.
as 0.8 nm.
Optical demultiplexer and wavelength
If the WDM system is deployed within selection
existing STM-16/OC-48 fiber links then the The receive side employs a demultiplexer
remodulator also accepts the existing (demux) or decoupler to distribute the
1550 nm signal and remodulates it to the optical signal to the wavelength selectors.
chosen WDM wavelength, ready for the These devices define the optical bandwidth
wavelength multiplexing process. to recover the original tributary and remove
unwanted components.
2
Impairments
affecting WDM
system performance
This application note focuses on the requirement for BER testing a
WDM system. There are several potential sources of impairments
associated with WDM components and optical fiber links. The main
impairments that affect BER performance are listed below.
Amplifier spontaneous emissions from Dispersion
EDFAs The characteristics of a fiber can cause
An accumulation of naturally occurring wavelengths to propagate at different
emissions that may cause a reduction in velocities through the fiber. This leads to
overall signal-to-noise ratio. pulse broadening and, ultimately, pulse
merging which results in errors on the
Gain flatness of EDFAs receiver detection circuit.
A measure of how flat the optical spectrum
remains after passing through the amplifier. Crosstalk between adjacent channels
Ideally all wavelengths in the WDM signal This is the interaction between adjacent
are amplified equally. Non-linearity needs to channels in a WDM line signal. Because of
be compensated for, because ultimately it the closeness of channel spacing, the
could lead to channel failure. contents of one channel can cause
interference in an adjacent channel,
Gain competition in EDFAs introducing errors at the receiver after
This is associated with the allocation of demultiplexing and channel selection.
power to channels. Each EDFA has a
defined amount of optical power available Four wave mixing
for amplifying incoming signals. Increasing This occurs if components of existing
the bandwidth of the amplifier adds more optical signals combine to produce a new
channels but leads to an overall reduction optical signal at a new wavelength.
of power in existing channels.
Simulated Brillouin scattering and
Intrinsic and timing jitter Raman scattering
Jitter is the phase variation of a signal from A description of the interaction between the
its correct position in time. It can optical signal and acoustic waves in the
accumulate in a transmission network, fiber, and between the optical signal and the
leading to errors. The remodulation stage of fiber.
a WDM system employs a clock recovery
and re-clocking stage which can contribute
to jitter on an incoming signal.
As any of these impairments could adversely affect the future fail-safe
operation of the network, they need to be evaluated properly and
corrected when implementing a WDM system design. For parametric
measurements such as power level and optical spectrum checks use a
wavelength meter and an optical spectrum analyzer.
For information on diagnosing
individual WDM impairments,
refer to the DWDM Components
Test Guide 5965-3124E.
3
Evaluating BER
performance
Conclusive testing of BER performance (and other impairments) in a
WDM system requires the duplication, as close as possible, of an
in-service situation. Loading up the tributaries of a WDM system with
dynamic, uncorrelated test signals gives a good simulation of live traffic.
A typical test setup requires multiple 2.4 Gb/s optical sources of network
quality, using DFB lasers or equivalent. Testing also requires wavelengths
from the ITU-T WDM grid in order to mimic or test beyond system
designs. An OC-48/STM-16 BER analyzer with SONET/SDH frame
structures will simulate the traffic of a real network, and if it offers a
modular, scaleable transmit/receive measurement capability would be
ideal for WDM system testing.
Figure 3.
Ideally, BER
performance checks
on a WDM system
would use
uncorrelated,
parallel PRBS test
signals to verify
transmission.
1. Tributary-side BER testing
Once assembled, WDM systems are usually 'soak tested'. That is, an
end-to-end BER test is performed across all tributaries of the WDM
system. Each tributary test is typically 3