In this paper power transient is investigated as function of add/drop wavelength and surviving channel wavelength. We have reported that power excursions varies with different wavelength allocations of the add/drop channels. Transient response is reduced by 73.39% in case when add/drop channels are taken in L band instead of C band. Also power transient response is calculated as a function of wavelengths of surviving channel. It has been observed that at higher wavelengths power excursions are less than at shorter wavelengths of C band.

1. IJSRD - International Journal for Scientific Research & Development| Vol. 1, Issue 9, 2013 | ISSN (online): 2321-0613
All rights reserved by www.ijsrd.com 1919
Power Transient Response of EDFA as a function of Wavelength in the
scenario of Wavelength Division Multiplexed System
Prabhjyot Singh1
Simranjit Singh2
1
M. Tech. Student 2
Assistant Professor
1,2
Department of Electronics and Communication Engineering
1, 2
UCoE Patiala, Punjab, India
Abstract—In this paper power transient is investigated as
function of add/drop wavelength and surviving channel
wavelength. We have reported that power excursions varies
with different wavelength allocations of the add/drop
channels. Transient response is reduced by 73.39% in case
when add/drop channels are taken in L band instead of C
band. Also power transient response is calculated as a
function of wavelengths of surviving channel. It has been
observed that at higher wavelengths power excursions are
less than at shorter wavelengths of C band.
Key words: EDFA transient, Power excursions, WDM
optical networks, add/drop (disturbing) channels.
I. INTRODUCTION
The transmission capacity of long-haul optical networks has
evolved tremendously over the past decades by adding
multiple wavelength channels through the use of wavelength
division multiplexing (WDM) and dense WDM technology.
Recently, there has been a rapidly growing interest in the
study of all-optical wavelength-division multiplexed
networks that provide both switching and transmission the
optical domain. WDM system is an attractive means for
large capacity transmission systems and flexible optical
networks. Such WDM networks have become possible
because of the availability of optical amplification,
particularly erbium-doped fiber amplifiers (EDFA’s).
Although new amplification technologies, such as Raman
and semiconductor optical amplifiers can provide
remarkable performances in terms of gain bandwidth and
flexibility, standard EDFAs are still the most attractive
solution as the best tradeoff between cost end performances
[30,31]. EDFA’s provide low-cost and efficient
amplification in wavelength-division multiplexed networks.
The Performance of an Optical Communication system can
be improved by the use of EDFAs as an Optical Amplifier.
The EDFA is the most deployed fiber amplifier as its
amplification window coincides with the third transmission
window of silica-based optical fiber. EDFAs are reliable for
transmitting data through long distance because of their
wide bandwidth and optimum bit error rate.
Although EDFA’s are a key technology enabling
the realization of transparent WDM communication, and
despite their unsurpassed performance, a number of
significant technological challenges remain. In multi-
wavelength light wave networks, the number of transmitted
channels may vary due to, e.g., network reconfiguration,
network growth to larger number of channels or failure of a
channel that can cause one or more channel to drop out[1,2].
Because these amplifiers are operating near saturation, and
since the total output power of a saturated EDFA is very
nearly constant, independent of the number of channels, the
gain experienced by each channel will, therefore, depend on
the number of channels present. Rapidly changing gain, due
to channel drop or addition, leads to cross-gain saturation in
fiber amplifiers that in turn would induce power transients in
the surviving channels which can seriously degrade system
performance parameters such as bit-error rate (BER) and
signal-to-noise ratio (SNR) during the transient events. They
can result in the drastic deterioration of the surviving
channels performance. Sudden changes in network
configuration, fiber breaks, other failure mechanisms, and
protection switching may cause abrupt changes in optical
power which can also cause transience. The EDFA
transients arise due to two factors: (i) they usually work in
deep saturation and (ii) present long upper state lifetime of
Erbium ions (~ 10 ms) [3]. Although this perturbation will
generally be small in a single amplifier, it will grow rapidly
along a cascade. To cater to the demand for an increasing
number of channels, high signal powers, greater than 20 dB
m, cascaded/multistage amplifiers are used. So small
transient perturbations in a single EDFA grow rapidly along
a cascade [2], [4].
In general, system performance may be degraded
by fiber nonlinearity when the channel powers are too high
and by a small receiver signal-to-noise ratio (SNR) when the
channel powers are too low. These transients occur on the
time scale of microseconds to milliseconds, and could
momentarily and significantly disrupt the system
performance. When channels are dropped, the power of the
surviving channel increases which severely degrades the
performance of the surviving channel because of self-phase
modulation. And when channels are added it degrades
surviving channels due to cross-phase modulation (XPM)
and four-wave mixing (FWM) for up to a few microseconds.
To overcome these problems, the transient effects
in the optical amplifiers must be controlled. . Also To
maintain quality of service for surviving channels, it is
necessary to limit the power excursions they experience. In
the past several control strategies have been proposed to fix
the EDFA gain at a given operating point. Low electronic
cost control suggested in [5] is widely used. Also a
combination of optical and electrical control schemes is
possible [7]. Also proposed is gain clamping via the
construction of ring lasers [6], [7] and insertion of a
compensating signal [8]. Inserting a compensating signal
into the first EDFA has been reported to be effective in
stabilizing chains of six [9] and eight [10] EDFA’s.
Some solutions to mitigate optical amplifiers
transients were successfully demonstrated and are based on
a combination of techniques such as linear gain control, gain

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clamping and fast automatic gain control [13, 14].
Additionally, passive mitigation approaches have also been
proposed, such as the use of EDF with a large active area of
Er3+
[15, 16]. In the optical domain, the idea of re-
circulating amplified spontaneous emission (ASE) as a
clamping mechanism in a feedback loop was presented in
the context of WDM ring networks [17]. The idea of re-
circulating ASE using a band pass filter to select the lasing
wavelength, and a variable optical attenuator (VOA) to
adjust the feedback cavity loss was presented in [18, 19].
The same principle using fiber Bragg grating (FBG) as band
pass filters was demonstrated to flatten and clamp the gain
of L-band EDFA for WDM applications [31]. Other
approaches based on feedback techniques make use of a
DWDM multiplexer [32]. Also novel techniques to
minimize gain-transient time of WDM signals in EDFA in
channel add/drop networks have been introduced. The
newly proposed gain controller is composed of a disturbance
observer and a PID controller. Another approach based on
use of the per-band link control method in cascades of
distributed fiber Raman amplifiers (DFRAs) for
compensating power transients of WDM channels was
introduced. By adding compensation channels the power
excursion is reduced in the surviving channels.
However many of these proposals are not practical
because of high cost, or instability due to electronic control
induced or chaotic behavior [20]. The all-optical scheme has
a drawback, the frequency of channel add/drop should be
less than that of the relaxation oscillation frequency of
EDFA, which is several hundred Hz. Also inserting the extra
channel is not economical in signal bandwidth. While
transient suppression methods using PID controller,
feedback electrical/optical are very complex and use
additional circuitry, so is expensive.
The transient response of EDF is a function of the
wavelength and power of the survival channels, number of
channels, pump etc. In order to gain a better understanding
of the transient response and to reduce the power transient,
analysis of the effect of the factors such as signal
wavelength and power, pump wavelength and power, etc. is
needed to be done. A true understanding of the EDFA gain
dynamics will help in the design of protection and control
schemes against deleterious nonlinear effects in
transmission. So power transient response of surviving
channels can be reduced by analyzing the factors on which
transient depends and then by optimizing them. This method
is very cost effective as it is done without adding any
complex additional circuitry.
Wang et al. [21] investigated Gain transients in
both co-pumped and counter pumped distributed Raman
amplifiers as a function of signal launch power, fiber type,
surviving signal wavelength. All these parameters affect the
magnitude of the pump-depletion level and, thus, determine
the amplifier transients. It is found that gain transients are
much more pronounced in a copumping scenario than in a
counter pumping scenario for the same operating conditions.
Transient suppression technique with better than 0.2-dB
ripple capability was also demonstrated in this study. Chan
et al. [22] simulated how the EDFA transient dynamics
depend on different EDF lengths and erbium concentration.
EDFA model was designed that used optimized length and
erbium concentration to suppress the power transient.
Gurkan et al. [23] presented a more extensive study of the
differences between C and L-band EDFA dynamics for a
40-channel WDM system for varying numbers of cascaded
EDFAs. Results have shown that the L-band EDFA transient
response time is -5 times slower than for the C-band. Also
longer transient time for L-band provides the necessary time
to prevent the degrading transmission penalties.
Kar´asek et al. [2] analyzed and compared
surviving channel power excursions resulting from
switching ON–OFF of one, three, and six out of eight WDM
channels in a cascade of concatenated strongly inverted and
standard two-stage EDFA’s. It follows from the performed
analysis that the rise time of the surviving channel power
transients in the strongly inverted cascade is approximately
twice as fast as in the cascade of standard amplifiers The
effect of pump power, EDF length and span loss on the
characteristics of the cascade of six strongly inverted
EDFA’s has been analyzed. Lee et al. [24] investigated
steady state and transient behavior of a C-band EDFA has
been for the various add/drop channel allocations. The
channels are located at around short wavelengths, long
wavelengths, and in the middle of C-band. The measured
transient behavior in each case is calculated. Sugaya et al.
[25] illustrated transient response of discrete Raman
amplifier in case of channel add-drop through comparing
co- and counter-propagating pump schemes by experiment
and calculation. Karlsek et al. [26] investigated, both
experimentally and theoretically, the effect of channel
removal/addition on surviving channel power transients in
distributed Raman fiber amplifier (RFA). The effect of
pumping scheme, pump power, the length and type of
Raman fiber, and number of added and/or dropped channels
on the dynamics of surviving channel power fluctuations has
been studied. Gest et al. [27] analyzed the dynamic response
of nine different cascades of DFRAs. Three cases of
cascades – a cascade of all unclamped DFRAs, a cascade of
all gain-clamped DFRAs, and a cascade of mixed
unclamped and gain-clamped DFRAs is compared and
analyzed in terms of gain excursions and overshoot &
undershoot. The evolution of the rise and fall times in the
surviving channel after each amplifier in the cascade is also
monitored. Also investigation of the influence of the
surviving channel location in the amplification band is done.
Kaler [28] investigated Effect of channel adding/dropping
on EDFA transients. Also further comparison of the
transient response of Compact EDFAs and Transient EDFAs
is done. Tian et al. [29] studied the transient dynamics of the
EDFAs responding to channel adding/dropping events. The
differences in the responses of the EDFAs pumped at 1480
and at 980 nm are compared and analyzed. It is also
observed that EDFA pumped at 980 nm has much faster
transient response than the one pumped at 1480 nm.
Already many papers have been presented on the
pump control method. Also effects of number of channels
add/drop pump power, wavelength and different pumping
scheme have been extensive studied. But the impact of the
wavelength allocation of the add/drop channels in C and L
bands on the survivor channels power transient have not
clearly investigated. Since L-band transmission is becoming

3. Power Transient Response of EDFA as a function of wavelength in the scenario of wavelength division multiplexed system
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commercially available, optical ADM operations will be
required for both the L and C bands. The differences
between C- and L band dynamics have been recently
studied.
In this paper, we have measured the power
transient of the surviving channel for different wavelength
allocations of add/drop channels in C and L band. Also
effect of surviving channel wavelength in terms of power
excursions is observed. Q factor is also calculated for
surviving and add/drop channel different wavelength
allocations. And we have shown that the gain of the EDFA
depends on the channel assignment in the steady state.
This paper is divided into 4 sections. In Section 2,
the optical simulation setup is described. In Section 3,
simulation results have been reported and discussed for the
different wavelengths of surviving and disturbing channels.
And finally in Section 4, conclusions and future work are
made.
II. SIMULATION SETUP
In this work, an analysis is presented regarding power
transient excursions resulting from channel drop/add in the
system of transmission using EDFA amplifier with as many
as 25 WDM channels. In order to gain a better
understanding of the transient response and, analyze the
effect of the factors such as signal wavelength and add/drop
channel wavelength we have made this system as shown in
figure 1.
Fig. 1 shows the simulation setup to analyze the
EDFA transients. The simulation tool used in this work is
the Optisystem (version 7.0), a software of Optiwave
Corporation. A 25 channel WDM system was simulated in
Optisystem. We consider twenty five WDM channels. This
is sufficient to show the dynamics of an EDFA after
channels dropping and adding. 24 channels out of 25 are
added dropped. Powers of all 25 channels are kept constant
with each channel given input power of -20 dBm. The
add/drop of 24 channels was simulated by modulating the
optical signal by a square wave at a low bit rate of 4000
bits/s. A 32-bit pseudorandom or user defined bit pattern is
encoded on each add/drop channel using non-return-to zero
pulse generator. Twenty four channels are added at t = 0.1
ms and dropped back at t =0.3ms and then is repeated with a
period of 4ms. This 24 channel light source is modulated by
an Optical Modulator (OM).
Fig. 1: Simulation setup to calculate and analyze power
transient of the surviving channel.
(OTDV: Optical time domain visualizer; BER: Bit error
rate)
And these 24 channels are combined with a CW-
probe (surviving) channel to simulate adding–dropping
twenty four out of twenty five channels (worst case
condition) using a multiplexer. Add/drop channels
(disturbing channels) are equally spaced with a uniform
channel spacing of 1 nanometer (nm). The surviving
channel is kept at constant wavelength of 1532nm; while we
have given disturbing channels two different wavelength
allocations in different set of measurements. In 1st
case
disturbing channels are given wavelengths in C band from
1538 to 1561nm and in 2nd
case they are allocated
wavelength in L band from 1566 to 1589nm.
These channels are coupled into the dynamic
EDFA. The amplification stage is composed by a WDM
(coupler), that combines the input and pump powers,
followed by a 5-m erbium doped fiber and isolator. The
pump laser at 980 nm wavelength and with constant power
of 20 dBm is used for forward (co) pumping the signal
(channels). The pump laser with wavelength of 1480 nm can
be used as well.
We have used the dynamic erbium-doped fiber
with the erbium ion density of 1000 ppm-wt. Its core radius
is assumed to be 2.2 µm and numerical aperture is assumed
to be 0.24. The metastable life time of 10 ms is used. The
insertion loss at 1550 and 980 nm is taken as 0.01 and 0.015
dB/m respectively. Power amplified signal is then passed
through a fiber. For the sake of simplicity, only one section
of 40 km standard fiber is used with one EDFA. At the
receiver each channel passes through photo detectors and
then through electrical low pass filter with a bandwidth of
bit rate. System performance is evaluated by BER analyzer
by calculating the factors like Q factor, eye diagram and bit
error rate.
III. SIMULATION RESULTS AND DISCUSSION
We examine and analyze the relevant factors which affect
the transient power excursion of the surviving channel in a
single all-optically stabilized EDFA, such as add/drop
channels wavelengths, surviving channel wavelengths.
Power Transient as a Function of Add/DropA.
(Disturbing) Channels Wavelength
Firstly, the system impact of the disturbing channels
wavelengths on the power transients of EDFA is observed.
To study and investigate the effect of disturbing channels
wavelength all the other factors like surviving channel
power & wavelength, pump power & wavelength, EDFA
length are kept constant. 24 channels are added at 1 ms and
then dropped at 3 ms. the surviving channel wavelength is
taken at 1532 nm. The disturbing channels wavelengths are
varied and two cases are made. When the wavelengths of
add drop channels are changed, the output transient of
surviving channel is affected In first case 24 disturbing
channels are taken in C band from 1538 to 1561 nm. While
in 2nd
case their wavelengths are taken in L band from 1566
to 1589 nm. Now corresponding to two cases power
transient is calculated by simulation. The results are as
shown in the fig. 2. It shows analysis of the results for the
different disturbing channel wavelengths after the EDFA.

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Fig. 2: Transient response comparison of surviving channel
for disturbing channels wavelengths in C and L band.
Now, power excursion, ∆P is given by:
∆P = P (0) – P (∞).
Where P (0) is power before the channel add or
drop and P (∞) is power after the channel add/drop.
Simulation shows a different power transient behavior under
different disturbing channels wavelengths although the
output gain of the surviving channel is observed to be
almost same. As it can be seen that power excursion when
disturbing channels are in C band is 43.5388 mW. While
when we take disturbing channels in L band the power
excursion is dropped to 11.583 mW, that is 31.9558 mW
less than what it was in case of C band disturbing channels.
So we can see that power transient is reduced by 73.396%
when disturbing channels are taken in L band instead of C
band.
The reason behind this transient reduction can be
attributed to the difference in wavelengths between the
surviving and disturbing channel. A power transient in the
surviving channels is mainly induced by cross gain
saturation and four wave mixing when channels are added or
dropped. The cross gain saturation and four wave mixing in
WDM system depends directly on the how closely channels
are spaced. Channels with small channel spacing or gap are
found to be more affected by cross gain saturation & four
wave mixing than the channels which have more channel
spacing. Thus when disturbing channels are taken in C band
their difference in wavelength with surviving channel at
1532 nm is less so more is the power transient.
As we have seen just by optimizing the disturbing
channel wavelength we can reduce the unwanted power
transient effect in the amplifier. Optimizing basic parameter
like add/drop channel wavelength can reduce the transient
behavior which can be used in future reconfigurable
networks. Moreover reduction in transient behavior is not
obtained at the expense of the gain or output power. Output
power remains at same level but transient effect is reduced.
Fig. 3: Q-factor comparison for the wavelength allocation in
C and L band of disturbing channels.
The Q factor at the receiver side with the
corresponding system is also calculated. It is shown in the
figure 3. It can be seen that Q factor is higher when the
disturbing channels are in L band. As power transient effects
in case of channel add/drop in L band is less as compared to
when they are in C band. So Q factor is better when channel
add/drop takes place in L band rather than in C band. This
confirms our experimental analysis that when disturbing
channels are allocated wavelengths in L band, while
surviving channel is in C band, it reduces the power
transient and Q factor is improved.
Power Transient as a Function of Surviving ChannelB.
Wavelength
We have also calculated the effect of surviving channel
wavelengths on the power transient and power excursions.
The wavelengths of add/drop channels are kept. They are
allocated wavelengths from 1557 to 1564 nm. For sake of
simplicity 8 channels are added or dropped. While we
change the wavelength of the survival channel across the C
band and get the wavelength dependence. This is shown in
the figure 4.
Fig. 4: Power excursions as a function of surviving channel
wavelength.
The power excursions depend on the amplifier
gain. And amplifier gain is wavelength dependent. Thus
transient response of EDF is a function of the wavelength.
Higher gain or power leads to higher power transient. Power
excursions behavior is closely related to the shape of the
EDFA gain response. The longer wavelengths receive less
power gain than the shorter ones. The shorter wavelength
channels exhibits faster and stronger transient as compared
to longer wavelengths. It is observed that the channels
centered around 1550 nm exhibit almost constant power
excursions.
Fig. 5: Comparison of power excursions of surviving
channel for channel dropping and adding.
Also power excursions are calculated for channel
dropping and channel adding separately. As can be seen
from the figure, amplitude of the transient power excursions
of the surviving channel are lower in the case of adding
channels versus that of dropping channels. These simulation
results are in good qualitative agreement with the
experimental results reported in [11], [12]. Both simulation
and experiment indicate that dropping of channels results in
much more severe effects on the surviving channels.

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IV. CONCLUSION
Surviving channel power excursion resulting from 24
channels add/drop out of 25 total WDM channels is
investigated and simulated for various wavelength allocation
of disturbing (add/drop) channels in C and L band. It is
found that power transient is reduced by 32.9558 mW or
73.39% when disturbing channels wavelengths are taken in
the L band instead of C band. Also Q factor is found to be
better and higher when wavelength of adding/dropping
channels is in L band. Power transient of EDFA is also
calculated as a function of surviving channel wavelength. It
is observed that power excursions are less in case of higher
wavelengths of C band. Power excursions are observed to be
almost constant for surviving channels wavelengths near the
1550 nm region (from 1442-1458 nm). Thus by just
optimizing the disturbing and surviving channels
wavelengths the power excursions or power transient can be
reduced by a large factor.
ACKNOWLEGEMENT
The author appreciate the help given by the guide Mr.
Simranjit Singh, Electronics and Communication
Engineering Department, Punjabi University, Patiala for
their technical assistance.
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