We experimentally investigate nonlinearity impact in a multi-span uncompensated coherent optical system, transmitting, in the 50GHz grid, Nchx100G CP-DQPSK channels, varying Nch from 3 to 71. A good agreement was found with an analytical model, confirming the progressive growth of interference with Nch. Impacts on network design are evaluate

A. Carena

F. Forghieri

G. Bosco

P. Poggiolini

R. Pastorelli

S. Piciaccia

V. Curri

Pastorelli, R.

Bosco, Gabriella

Carena, Andrea

Poggiolini, Pierluigi

Curri, Vittorio

S. Piciaccia, F. Forghieri

PORTO Publications Open Repository TOrino

Politecnico di Torino
Porto Institutional Repository
[Proceeding] Investigation of the Dependence of Non-Linear Interference on
the Number of WDM Channels in Coherent Optical Networks
Original Citation:
R. Pastorelli; G. Bosco; A. Carena; P. Poggiolini; V. Curri; S. Piciaccia, F. Forghieri (2012).
Investigation of the Dependence of Non-Linear Interference on the Number of WDM Channels
in Coherent Optical Networks. In: European Conference on Optical Communication (ECOC),
Amsterdam, September 2012. pp. 1-3
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We.2.C.2.pdf ECOC Technical Digest © 2012 OSA
Investigation of the Dependence of Non-Linear Interference on 
the Number of WDM Channels in Coherent Optical Networks  
R. Pastorelli
(1)
, G. Bosco
(2)
, A. Carena
 (2)
, P. Poggiolini 
(2)
, V. Curri
 (2)
, S. Piciaccia
(1)
, F. Forghieri 
(1) 
(1)
 Cisco Photonics Italy srl, Via Philips 12, 20900, Monza, Italy, rpastore@cisco.com 
(2)
 Dip. di Elettronica e Telecomunicazioni, Politecnico di Torino, Torino, Italy, gabriella.bosco@polito.it 
Abstract We experimentally investigate nonlinearity impact in a multi-span uncompensated coherent 
optical system, transmitting, in the 50GHz grid, Nchx100G CP-DQPSK channels, varying Nch from 3 to 
71. A good agreement was found with an analytical model, confirming the progressive growth of 
interference with Nch. Impacts on network design are evaluated.   
Introduction 
In a DWDM optical network, the transmission 
systems design requires the balance between 
optical signal-to-noise ratio (OSNR) at the 
receiver and transmission margin allocation, 
where the margin is defined as the extra-OSNR 
required at the receiver after transmission in 
order to get the same BER value as in back-to-
back. Margins in DWDM transmission are 
usually dominated by multichannel impairments; 
this is the reason why multichannel propagation 
has been widely studied, both experimentally 
and via simulations. In literature, due to 
computational cost, split-step simulations often 
explore multichannel impairments considering a 
limited number of interfering channels
1,2
.  
In this work, we study the evolution of 
multichannel impairments for 100Gb/s CP-
DQPSK (Coherent  Polarization-Multiplexed  
Differential  QPSK) DWDM signals with 50-GHz 
channel spacing, over an uncompensated multi-
span link composed of 20x125km spans of Ultra 
Low Loss (ULL) SMF, progressively increasing 
the number of interfering channels up to the full 
C-band. 
The experimental evaluation of the impairment 
evolution from 2 to 70 interfering channels is 
compared to theoretical results based on the 
analytical model presented in 
3
 for the nonlinear 
propagation in uncompensated coherent optical 
systems, finding a good agreement.  
Finally, the impact on network planning of the 
growth of nonlinear interference with the number 
of WDM channels is analyzed and analytically 
extended to other fiber types. 
Experimental set-up description  
The experimental set-up is shown in Fig. 1. It is 
a dual-fibers commercial Cisco DWDM system 
with 12 spans of 125 km of SMF ULL fiber with 
loss of 22.5 dB each, including patch-cords. The 
DWDM signal is folded in the terminal site C 
behaving at optical level as a standard DGE 
node (Fig. 1). In the terminal site A, a 
Wavelength Cross-Connect (WXC) has been 
used as multiplexer and de-multiplexer. On each 
span, EDFA amplification is adopted and a 
dynamic gain equalizer (DGE) is used every 6 
spans to control tilt and ripple. The maximum 
available aggregate power at the amplifier 
output is 20 dBm. In the return path, in the 
section “DGE D – terminal A”, the DWDM signal 
is dropped after 2 spans, in order to obtain a 
total of 20 spans (corresponding to 2500 km). 
The number of spans was limited to 20 in order 
to keep the measured BER values under the 
FEC correction threshold of 4·10
-3
 in the entire 
range of power levels, even in high-nonlinearity 
conditions. 
The 100Gb/s CP-DQPSK signal is generated 
and received with a real-time coherent 
transponder module
4
. The transmitter (TX) runs 
at 28 Gbaud, including 7% overhead for forward 
error control (FEC). All internal module 
parameters, e.g. Mach-Zehnder-modulator 
(MZM) bias-voltages, are fully automatically 
controlled. The channel under measure, in the 
following referred as device under test (DUT), is 
generated by a transponder module, which uses 
a tunable external cavity laser (ECL) with a 
linewidth of ~100kHz. The neighboring channels 
 
 
 
Fig. 1: Transmission system set-up with 20 in-line fiber spans. 
We.2.C.2.pdf ECOC Technical Digest © 2012 OSA
are generated from a single TX-module, which 
modulates the combined light of up to 70 DFB-
lasers distributed on a 50-GHz grid in the C-
band. The receiver (RX) part of the transponder 
consists of an integrated coherent RX containing 
two polarization diverse 90° hybrids, followed by 
balanced photo-diodes and trans-impedance 
amplifiers (TIA) with automatic gain control. The 
local oscillator is an ECL of the same type used 
for the transmitter. The electrical signals at the 
output of the TIAs are digitized and processed 
by a 65 nm CMOS technology mixed-signal 
ASIC with ~70 MGates. The four ADCs have a 
sample-rate of 56 Gsamp/s each and ~4 
effective bits (ENOB) of resolution. The digital 
signal processing consists of an ITU-compliant 
clock-recovery, static filtering for chromatic 
dispersion compensation (>70,000 ps/nm), 
dynamic filtering for time-varying polarization 
effects such as PMD (17-tap FIR filters, DGD 
tolerance >100ps) and a Viterbi-&-Viterbi carrier 
recovery (with adjustable bandwidth and 
polarization coupling) to mitigate phase 
deviations due to laser linewidth and non-linear 
phase noise. The receiver synchronizes blindly 
onto the received signal and optimizes to the 
current link scenario (CD, PMD, LO-offset, 
linewidth and nonlinear phase noise).  
Experimental results 
The goal of the experiment was the investigation 
of the influence that the number of DWDM 
channels has on multichannel impairments 
affecting the device under test (DUT). To 
evaluate the contribution to the total impairment 
of neighboring channels, the DUT was placed at 
the center wavelength of the DWDM spectrum 
and the C-band grid was populated with other 
70 interfering channels (see Fig. 2, where the 
full C-band transmitted spectrum is presented). 
Amplifiers are working in gain control mode, 
operating with a per-channel target power of 1 
dBm and a NF of about 6 dB. Taking into 
account losses of the patch-cords between 
amplifier output and fiber input, per-channel 
power PinF at the fiber input is about 0.5 dBm, 
while the received OSNR is about 10.5 dB over 
0.5 nm.  
During the experiment, pre-FEC BER of the 
DUT has been measured switching off a subset 
of transmitted channels; the remaining active 
channels symmetrically populate the 50 GHz 
grid around the DUT. In each configuration, the 
DUT pre-FEC BER is obtained as the worst 
value measured after 60 single acquisitions, 
each performed over a time frame of 1 s.  
Fig. 3 shows the pre-FEC BER vs. the number 
of channels (with PinF=0.5 dBm): increasing the 
bandwidth occupation, pre-FEC BER keeps 
growing without saturating, indicating that also 
channels at the edges of the DWDM spectrum 
contribute to multichannel impairments. The Q-
penalty, passing from 2 to 14 neighboring 
channels, is about 0.5 dB, that is only half of the 
total impairment measured with all 70 interfering 
channels. 
This behavior is also due to the fact that the 
optimum transmitted power depends on the 
number of interfering channels. To investigate 
this dependency, for a subset of configurations, 
the pre-FEC BER as a function of the 
transmitted power has been measured. An 
example of the results is shown in the inset of 
Fig. 4. Interpolating these curves we can obtain 
the optimum power Popt, which minimizes the 
BER, as a function of the number of WDM 
channels, as shown in Fig. 4. Increasing the 
 
 
Fig. 3: Pre-FEC BER and Q vs. number of 
interfering channels with PinF =0.5 dBm  
(dots: experimental data; lines: analytical model) 
 
 
 
Fig. 4: Popt vs. Nch (dots: experimental data; lines: 
analytical model). Inset: pre-FEC BER vs. launch 
power for 7, 21, and 63 channels. 
.  
 
 
Fig. 2: Transmitted WDM spectrum (71 channels). 
. 
We.2.C.2.pdf ECOC Technical Digest © 2012 OSA
number of channels from 3 to 15, Popt is reduced 
by about 0.6 dB. The same power reduction is 
obtained when passing from 15 to 71 channels: 
this means that channels at a distance equal or 
greater than 400 GHz from the DUT appreciably 
affects the DUT transmission performance. 
 
Analytical model and network design 
In this section, the experimental measurements 
are compared to results obtained resorting to an 
analytical model for nonlinear propagation over 
uncompensated optical systems proposed in 
3
. 
This model is based on the assumption that the 
non-linear interference acts as additive 
Gaussian noise in excess with respect to ASE 
noise. It can be shown that the non-linear noise 
power (PNLI) can be analytically evaluated
3
 and 
the system BER can be then directly derived 
from the equivalent “non-linear” OSNR, defined 
as: OSNRNL=(PTX)/(PASE+PNLI), where PTX is the 
transmitted power per channel and PASE is the 
power of ASE noise introduced by EDFAs.  
We applied the analytical model to the 
experimental setup, obtaining a good agreement 
with the measurements, as shown in Figs. 3 and 
4. When applying the analytical equations, we 
took into account the measured penalty of the 
system in linearity with respect to the theoretical 
matched filter performance, which was about 2.3 
dB over the all range of measured BERs. The 
parameters of the ULL SMF fiber used in the 
analytical evaluation are: = 0.17 dB/km, 
D=15.5 ps/nm/km and =1.1 W
-1
km
-1
. The 
values of and Dwere obtained from 
experimental measurements, while for  we 
used a typical value for ULL SMF fibers derived 
from the data-sheets. Data from analytical 
model confirm the experimental trend of growth 
of multichannel impairments as a function of Nch. 
The obtained results are also in good agreement 
with the simulation study performed in 
5
.  
To evaluate the impact of such a behavior in the 
network design, a theoretical estimation of the 
OSNR penalty as a function of the bandwidth 
occupancy was performed (Fig. 5). The full C-
band with 100 channels is considered. The 
OSNR penalty is evaluated using the optimum 
value of the fiber launch power, i.e. the one 
minimizing the BER, for 15 transmitted 
channels. The OSNR penalty is then defined as 
the increase (or decrease) in OSNR required to 
obtain the same BER as in the 15 channels 
case. Note that optimizing the system 
transmission performances with 15 channels is 
equivalent to consider as negligible the 
impairment contributions coming from channels 
at more than 400 GHz from the DUT.  
In the described experimental set-up, the 
allocation of OSNR margins able to recover the 
multichannel impairments with Nch=15 implies 
an underestimation of about 0.95 dB in case of 
full C-band transmission (see solid line in Fig.5).  
By using the analytical model we extended the 
study to two other network scenarios with 
relevant application interest: in the first one the 
ULL SMF is replaced with SSMF fiber (= 0.2 
dB/km, D=16.7 ps/nm/km and =1.3    W
-1
km
-1
) 
while in the second one it is replaced with E-
LEAF fiber (= 0.2 dB/km, D=3.8 ps/nm/km and 
=1.5 W
-1
km
-1
). In both cases, the length of each 
spool is 106 km, to keep constant the span 
budget with respect to the ULL SMF set-up. The 
OSNR penalty has been evaluated as a function 
of Nch using a fiber launch power equal to the 
optimum one for the Nch=15 case. As shown in 
Fig. 5, OSNR margin allocation based on 15 
transmitted channels causes an underestimation 
of the required margin at full channel capacity of 
0.95 dB in case of SMF and 1.25 dB in case of 
E-LEAF transmission fiber.  
Conclusions 
Both experimental and theoretical results 
highlight the non saturated growth of the impact 
of non-linear interference with the number of 
WDM channels, demonstrating that a network 
design based on the presence of few WDM 
channels may underestimate the required 
OSNR margin in a densely populated WDM 
configuration.    
The authors would like to thank Corning for providing 
optical fibers, M. Pierpaoli, S. Colombo and G. 
Calabretta for the experimental transmission set-up. 
 
References 
[1] C. Xie, PTL 21, 274 (2009) 
[2] D. Rafique et al., Opt. Exp. 19, 9044 (2011)  
[3] A. Carena et al. JLT 30, 1524 (2012)  
[4] R. Pastorelli et al., NFOEC’12, NTh1I (2012) 
[5] C. Xia et al., OFC’11, OWO1 (2011) 
 
 
 
Fig. 5: OSNR penalty vs. Nch using three different 
fiber types. Fiber launch power is the Popt in case of 
Nch=15 (ULL SMF: Popt=0.3 dBm, SSMF: Popt=0.2 
dBm, E-LEAF: Popt= -2 dBm). 
 


Investigation of the Dependence of Non-Linear Interference on the Number of WDM Channels in Coherent Optical Networks

Crossref

We experimentally investigate nonlinearity impact in a multi-span uncompensated coherent optical system, transmitting, in the 50GHz grid, Nchx100G CP-DQPSK channels, varying Nch from 3 to 71. A good agreement was found with an analytical model, confirming the progressive growth of interference with Nch. Impacts on network design are evaluated

BOSCO, GABRIELLA

CARENA, Andrea

POGGIOLINI, PIERLUIGI

CURRI, Vittorio

PORTO@iris (Publications Open Repository TOrino - Politecnico di Torino)

04 August 2020POLITECNICO DI TORINORepository ISTITUZIONALEInvestigation of the Dependence of Non-Linear Interference on the Number of WDM Channels in Coherent OpticalNetworks / R. Pastorelli; G. Bosco; A. Carena; P. Poggiolini; V. Curri; S. Piciaccia, F. Forghieri. - ELETTRONICO. -(2012), pp. 1-3. ((Intervento presentato al convegno European Conference on Optical Communication (ECOC) tenutosi aAmsterdam nel September 2012.OriginalInvestigation of the Dependence of Non-Linear Interference on the Number of WDM Channels inCoherent Optical NetworksPublisher:PublishedDOI:Terms of use:openAccessPublisher copyright(Article begins on next page)This article is made available under terms and conditions as specified in the  corresponding bibliographic description inthe repositoryAvailability:This version is available at: 11583/2503765 since:The Optical Society of America (OSA)We.2.C.2.pdf ECOC Technical Digest © 2012 OSAInvestigation of the Dependence of Non-Linear Interference on the Number of WDM Channels in Coherent Optical Networks  R. Pastorelli(1), G. Bosco(2), A. Carena (2), P. Poggiolini (2), V. Curri (2), S. Piciaccia(1), F. Forghieri (1) (1) Cisco Photonics Italy srl, Via Philips 12, 20900, Monza, Italy, rpastore@cisco.com (2) Dip. di Elettronica e Telecomunicazioni, Politecnico di Torino, Torino, Italy, gabriella.bosco@polito.it Abstract We experimentally investigate nonlinearity impact in a multi-span uncompensated coherent optical system, transmitting, in the 50GHz grid, Nchx100G CP-DQPSK channels, varying Nch from 3 to 71. A good agreement was found with an analytical model, confirming the progressive growth of interference with Nch. Impacts on network design are evaluated.   Introduction In a DWDM optical network, the transmission systems design requires the balance between optical signal-to-noise ratio (OSNR) at the receiver and transmission margin allocation, where the margin is defined as the extra-OSNR required at the receiver after transmission in order to get the same BER value as in back-to-back. Margins in DWDM transmission are usually dominated by multichannel impairments; this is the reason why multichannel propagation has been widely studied, both experimentally and via simulations. In literature, due to computational cost, split-step simulations often explore multichannel impairments considering a limited number of interfering channels1,2.  In this work, we study the evolution of multichannel impairments for 100Gb/s CP-DQPSK (Coherent  Polarization-Multiplexed  Differential  QPSK) DWDM signals with 50-GHz channel spacing, over an uncompensated multi-span link composed of 20x125km spans of Ultra Low Loss (ULL) SMF, progressively increasing the number of interfering channels up to the full C-band. The experimental evaluation of the impairment evolution from 2 to 70 interfering channels is compared to theoretical results based on the analytical model presented in 3 for the nonlinear propagation in uncompensated coherent optical systems, finding a good agreement.  Finally, the impact on network planning of the growth of nonlinear interference with the number of WDM channels is analyzed and analytically extended to other fiber types. Experimental set-up description  The experimental set-up is shown in Fig. 1. It is a dual-fibers commercial Cisco DWDM system with 12 spans of 125 km of SMF ULL fiber with loss of 22.5 dB each, including patch-cords. The DWDM signal is folded in the terminal site C behaving at optical level as a standard DGE node (Fig. 1). In the terminal site A, a Wavelength Cross-Connect (WXC) has been used as multiplexer and de-multiplexer. On each span, EDFA amplification is adopted and a dynamic gain equalizer (DGE) is used every 6 spans to control tilt and ripple. The maximum available aggregate power at the amplifier output is 20 dBm. In the return path, in the section “DGE D – terminal A”, the DWDM signal is dropped after 2 spans, in order to obtain a total of 20 spans (corresponding to 2500 km). The number of spans was limited to 20 in order to keep the measured BER values under the FEC correction threshold of 4·10-3 in the entire range of power levels, even in high-nonlinearity conditions. The 100Gb/s CP-DQPSK signal is generated and received with a real-time coherent transponder module4. The transmitter (TX) runs at 28 Gbaud, including 7% overhead for forward error control (FEC). All internal module parameters, e.g. Mach-Zehnder-modulator (MZM) bias-voltages, are fully automatically controlled. The channel under measure, in the following referred as device under test (DUT), is generated by a transponder module, which uses a tunable external cavity laser (ECL) with a linewidth of ~100kHz. The neighboring channels    Fig. 1: Transmission system set-up with 20 in-line fiber spans. We.2.C.2.pdf ECOC Technical Digest © 2012 OSAare generated from a single TX-module, which modulates the combined light of up to 70 DFB-lasers distributed on a 50-GHz grid in the C-band. The receiver (RX) part of the transponder consists of an integrated coherent RX containing two polarization diverse 90° hybrids, followed by balanced photo-diodes and trans-impedance amplifiers (TIA) with automatic gain control. The local oscillator is an ECL of the same type used for the transmitter. The electrical signals at the output of the TIAs are digitized and processed by a 65 nm CMOS technology mixed-signal ASIC with ~70 MGates. The four ADCs have a sample-rate of 56 Gsamp/s each and ~4 effective bits (ENOB) of resolution. The digital signal processing consists of an ITU-compliant clock-recovery, static filtering for chromatic dispersion compensation (>70,000 ps/nm), dynamic filtering for time-varying polarization effects such as PMD (17-tap FIR filters, DGD tolerance >100ps) and a Viterbi-&-Viterbi carrier recovery (with adjustable bandwidth and polarization coupling) to mitigate phase deviations due to laser linewidth and non-linear phase noise. The receiver synchronizes blindly onto the received signal and optimizes to the current link scenario (CD, PMD, LO-offset, linewidth and nonlinear phase noise).  Experimental results The goal of the experiment was the investigation of the influence that the number of DWDM channels has on multichannel impairments affecting the device under test (DUT). To evaluate the contribution to the total impairment of neighboring channels, the DUT was placed at the center wavelength of the DWDM spectrum and the C-band grid was populated with other 70 interfering channels (see Fig. 2, where the full C-band transmitted spectrum is presented). Amplifiers are working in gain control mode, operating with a per-channel target power of 1 dBm and a NF of about 6 dB. Taking into account losses of the patch-cords between amplifier output and fiber input, per-channel power PinF at the fiber input is about 0.5 dBm, while the received OSNR is about 10.5 dB over 0.5 nm.  During the experiment, pre-FEC BER of the DUT has been measured switching off a subset of transmitted channels; the remaining active channels symmetrically populate the 50 GHz grid around the DUT. In each configuration, the DUT pre-FEC BER is obtained as the worst value measured after 60 single acquisitions, each performed over a time frame of 1 s.  Fig. 3 shows the pre-FEC BER vs. the number of channels (with PinF=0.5 dBm): increasing the bandwidth occupation, pre-FEC BER keeps growing without saturating, indicating that also channels at the edges of the DWDM spectrum contribute to multichannel impairments. The Q-penalty, passing from 2 to 14 neighboring channels, is about 0.5 dB, that is only half of the total impairment measured with all 70 interfering channels. This behavior is also due to the fact that the optimum transmitted power depends on the number of interfering channels. To investigate this dependency, for a subset of configurations, the pre-FEC BER as a function of the transmitted power has been measured. An example of the results is shown in the inset of Fig. 4. Interpolating these curves we can obtain the optimum power Popt, which minimizes the BER, as a function of the number of WDM channels, as shown in Fig. 4. Increasing the   Fig. 3: Pre-FEC BER and Q vs. number of interfering channels with PinF =0.5 dBm  (dots: experimental data; lines: analytical model)    Fig. 4: Popt vs. Nch (dots: experimental data; lines: analytical model). Inset: pre-FEC BER vs. launch power for 7, 21, and 63 channels. .    Fig. 2: Transmitted WDM spectrum (71 channels). . We.2.C.2.pdf ECOC Technical Digest © 2012 OSAnumber of channels from 3 to 15, Popt is reduced by about 0.6 dB. The same power reduction is obtained when passing from 15 to 71 channels: this means that channels at a distance equal or greater than 400 GHz from the DUT appreciably affects the DUT transmission performance.  Analytical model and network design In this section, the experimental measurements are compared to results obtained resorting to an analytical model for nonlinear propagation over uncompensated optical systems proposed in 3. This model is based on the assumption that the non-linear interference acts as additive Gaussian noise in excess with respect to ASE noise. It can be shown that the non-linear noise power (PNLI) can be analytically evaluated3 and the system BER can be then directly derived from the equivalent “non-linear” OSNR, defined as: OSNRNL=(PTX)/(PASE+PNLI), where PTX is the transmitted power per channel and PASE is the power of ASE noise introduced by EDFAs.  We applied the analytical model to the experimental setup, obtaining a good agreement with the measurements, as shown in Figs. 3 and 4. When applying the analytical equations, we took into account the measured penalty of the system in linearity with respect to the theoretical matched filter performance, which was about 2.3 dB over the all range of measured BERs. The parameters of the ULL SMF fiber used in the analytical evaluation are: = 0.17 dB/km, D=15.5 ps/nm/km and =1.1 W-1km-1. The values of and Dwere obtained from experimental measurements, while for  we used a typical value for ULL SMF fibers derived from the data-sheets. Data from analytical model confirm the experimental trend of growth of multichannel impairments as a function of Nch. The obtained results are also in good agreement with the simulation study performed in 5.  To evaluate the impact of such a behavior in the network design, a theoretical estimation of the OSNR penalty as a function of the bandwidth occupancy was performed (Fig. 5). The full C-band with 100 channels is considered. The OSNR penalty is evaluated using the optimum value of the fiber launch power, i.e. the one minimizing the BER, for 15 transmitted channels. The OSNR penalty is then defined as the increase (or decrease) in OSNR required to obtain the same BER as in the 15 channels case. Note that optimizing the system transmission performances with 15 channels is equivalent to consider as negligible the impairment contributions coming from channels at more than 400 GHz from the DUT.  In the described experimental set-up, the allocation of OSNR margins able to recover the multichannel impairments with Nch=15 implies an underestimation of about 0.95 dB in case of full C-band transmission (see solid line in Fig.5).  By using the analytical model we extended the study to two other network scenarios with relevant application interest: in the first one the ULL SMF is replaced with SSMF fiber (= 0.2 dB/km, D=16.7 ps/nm/km and =1.3    W-1km-1) while in the second one it is replaced with E-LEAF fiber (= 0.2 dB/km, D=3.8 ps/nm/km and =1.5 W-1km-1). In both cases, the length of each spool is 106 km, to keep constant the span budget with respect to the ULL SMF set-up. The OSNR penalty has been evaluated as a function of Nch using a fiber launch power equal to the optimum one for the Nch=15 case. As shown in Fig. 5, OSNR margin allocation based on 15 transmitted channels causes an underestimation of the required margin at full channel capacity of 0.95 dB in case of SMF and 1.25 dB in case of E-LEAF transmission fiber.  Conclusions Both experimental and theoretical results highlight the non saturated growth of the impact of non-linear interference with the number of WDM channels, demonstrating that a network design based on the presence of few WDM channels may underestimate the required OSNR margin in a densely populated WDM configuration.    The authors would like to thank Corning for providing optical fibers, M. Pierpaoli, S. Colombo and G. Calabretta for the experimental transmission set-up.  References [1] C. Xie, PTL 21, 274 (2009) [2] D. Rafique et al., Opt. Exp. 19, 9044 (2011)  [3] A. Carena et al. JLT 30, 1524 (2012)  [4] R. Pastorelli et al., NFOEC’12, NTh1I (2012) [5] C. Xia et al., OFC’11, OWO1 (2011)    Fig. 5: OSNR penalty vs. Nch using three different fiber types. Fiber launch power is the Popt in case of Nch=15 (ULL SMF: Popt=0.3 dBm, SSMF: Popt=0.2 dBm, E-LEAF: Popt= -2 dBm).  

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Investigation of the Dependence of Non-Linear Interference on the Number of WDM Channels in Coherent Optical Networks

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