Complete Analysis of
Erbium-Doped Fiber Amplifiers
Technical Paper
HP 8153A
Lightwave Multimeter
Technical Paper
Christian Hentschel, Edgar Leckel
Böblingen Instruments
Herrenberger Strasse
71034 Böblingen
Germany
© Hewlett-Packard Company 1995
HP 8153A Lightwave Multimeter, Technical Paper, Page 1
Abstract
Optical amplifiers make the communication system
transparent from the beginning to the end, in contrast
to conventional repeater-type systems. This poses
new challenges for the performance of all system
components, including the optical amplifier. As a
consequence, optical amplifiers need to be most
thoroughly tested before their deployment.
This paper discusses methods and instruments for
measuring EDFAs with respect to gain and noise
figure, both in static and dynamic form, polarization
dependence, polarization mode dispersion, wavelength division multiplex (WDM) characteristics and
more. The influence of the source’s spectral width
on the amplifier’s performance is also discussed.
Authors
Christian Hentschel is the manager of the lightwave
standards laboratory at HP’s Böblingen Instruments
Division in Germany. He studied communications
engineering at the University of Aachen and
graduated as Dr. of Engineering in l971. Since joining
HP in 1972, he has been in various R&D positions
including project manager of HP’s first-generation
fiber optic instrumentation. Author of HP’s Fiber
Optics Handbook, he has been active in the
development of fiber optics instrumentation
technology, measurement standardization and
applications since l982.
Edgar Leckel is the project manager in R&D at HP’s
Böblingen Instruments Division in Germany. He
studied communications engineering at the
University of Stuttgart and joined HP in 1988. Since
then he has played a leading role development of the
tunable laser source, optical power sensors and
EDFA test system. He has published several papers
on tunable lasers and other fiber optic
instrumentation.
HP 8153A Lightwave Multimeter Technical Paper, Page 2
1. Introduction
2. Gain Measurement
It is interesting to see that optical fiber amplifiers
(OFAs) are penetrating all lightwave communication
markets and applications. As a consequence, many
different types of OFAs are being developed and built
today.
(1/1)
(PMD)
For example, a booster amplifier in a video
distribution system, designed to drive the many fibers
in parallel over a relatively short distance, differs
substantially from an in-line amplifier in a submarine
system, although their principle of operation is the
same. Some of the tests for these amplifiers are
common and well established today, e.g. gain, output
power and noise figure. Other tests are more
specific, for example measuring the wavelengthdivision-multiplex (WDM) characteristics or
measuring the dynamic gain.
Needless to say, one test will not meet the system
requirements for all these different amplifiers.
This paper discusses EDFA measurement techniques;
both well established and more advanced techniques.
EDFA gain and signal output power are usually
measured as a function of wavelength and input
signal. Consequently, the tunable laser is the most
important test equipment in optical amplifier (EDFA)
testing, in particular because these amplifiers are the
key elements in WDM systems.
The optical input power controls the EDFA
saturation state and gain. Gains of 50 dB are
achieved with modern amplifiers. In deep saturation
(at high input power), this gain will drop to 10 dB or
less.
(2/1)
At low input power, the total output power is mostly
amplified spontaneous emission (ASE). The
amplified signal is small. Therefore, an optical
spectrum analyzer (OSA) must be used to extract the
signal. Even then, the measured signal may contain
some power from the ASE and the amplified source
spontaneous emission (amplified SSE). Therefore,
measuring and subtracting both noise powers is
advisable.
Furthermore, optical spectrum analyzers are not
designed to measure absolute optical power
accurately. Therefore it is also recommended that
the OSA is calibrated using a tunable laser and an
accurate power meter such as the InGaAs large-area
HP 81524A optical head.
HP 8153A Lightwave Multimeter, Technical Paper, Page 3
HP 8168F Tunable Laser
A frequent requirement in testing optical amplifiers is
high input power, for example 0 dBm. Such power
levels are necessary to obtain large signal-to-noise
ratios required in analog systems. The new HP 8168F
tunable laser generates +8 dBm signal power when no
attenuator is included, or +7 dBm with an attenuator.
2.1 Polarization dependent gain (PDG)
The polarization state of optical signals traveling on a
long fiber is subject to continuous and sudden
environmental changes. It is therefore statistical in
nature. When the link consists of a number of
concatenated amplifiers, then these statistics may
lead to changes in received power level and even
system failure. This is due to the fact that it is
difficult to build optical amplifiers with PDGs of less
than 0.1 dB. In any case, testing the polarization
dependence of optical amplifiers is gaining
importance. Two methods, polarization scanning and
the Mueller / Stokes method are commonly used, see
figure 2/3:
(2/3)
The HP 81600 EDFA test system makes use of the
Mueller method because the test system already
includes a waveplate-type polarization controller. Its
polarization dependent insertion loss, specified as
±0.03 dB can be cancelled out with this method. The
method is described in [1].
HP 8153A Lightwave Multimeter, Technical Paper, Page 4
Typical causes of PDG in optical amplifiers are:
a) the input and output isolators, producing
polarization dependent loss (PDL);
b)polarization holeburning (PHB) in the active fiber;
PHB reduces the gain and the ASE in the signal’s
state of polarization [2]. This effect depends on
the degree of saturation and on the relative
orientation of the signal and the pump; typical
PDGs caused by PHB are on the order of
0.1 - 0.2 dBp-p. PHB is detrimental in lightwave
communication systems because it tends to build
up ASE in the state of polarization which is
orthogonal to the signal.
The ”passive“ PDL of the isolators can be measured
with either of the two methods mentioned above.
The ”active“ PDG can be measured by adding a small
probe signal to the saturating signal and changing the
probe’s orientation with respect to the orientation of
the saturating signal. Measuring the amplified probe
signal is possible either by modulating the probe
signal and using frequency selective detection, or by
adding a small wavelength offset to the probe to
make it accessible to an optical spectrum analyzer.
(2/4)
F
Notice that, in addition to polarization holeburning,
the probe signal experiences the same polarization
dependent loss (PDL) as the saturating signal.
Therefore measuring the PDL at low input power
levels, e.g. at -30 dBm, and determining the gain
variation from polarization holeburing is
recommended, e.g. as a function of input power level,
from the difference to the PDL.
2.2 WDM Gain
WDM is considered to be one of the most powerful
methods of increasing the transmission capacity of an
optical fiber system. It is a clear alternative to using
extremely high-frequency modulation and detection,
and it makes use of the thousands of gigahertz of
electrical bandwidth offered by the fiber. Note that
the 40 nm typical optical bandwidth offered by an
EDFA corresponds with 5000 GHz electrical
bandwidth. Of course, today’s WDM system designs
don’t fully exploit these capabilities; even WDM
systems don’t usually occupy more than 50 gigahertz
bandwidth.
An optical amplifier can amplify all of the optical
carriers in parallel. However, the gain for an
individual channel depends on the power and
wavelength of all channels, because all channels
together set the amplifier’s operating point
(compression state). That makes WDM
characterization somewhat complicated. Various
combinations of channel powers should be applied to
obtain complete WDM characteristics.
F
Figure 2/4 shows a measurement setup for
polarization holeburning using two tunable laser
sources, two polarization controllers to set the
polarization states of both the saturating signal and
the probe, and an optical spectrum analyzer. The
best way of using this setup is analyzing the
polarization dependent probe gain with the Mueller
method mentioned previously.
(2/5)
HP 8153A Lightwave Multimeter, Technical Paper, Page 5
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