Transmission of 11 x 224 Gb/s POLMUX-RZ-16QAM over
1500 km of LongLine and pure-silica SMF
Citation for published version (APA):
Alfiad, M. S., Kuschnerov, M., Jansen, S. L., Wuth, T., Van Den Borne, D., & De Waardt, H. (2010).
Transmission of 11 x 224 Gb/s POLMUX-RZ-16QAM over 1500 km of LongLine and pure-silica SMF. In
ECOC
2010 - 36th European Conference and Exhibition on Optical Communication, Proceedings
(pp. We.8.C.2-1/3).
[5621370] Institute of Electrical and Electronics Engineers. https://doi.org/10.1109/ECOC.2010.5621370
DOI:
10.1109/ECOC.2010.5621370
Document status and date:
Published: 31/12/2010
Document Version:
Publisher’s PDF, also known as Version of Record (includes final page, issue and volume numbers)
Please check the document version of this publication:
• A submitted manuscript is the version of the article upon submission and before peer-review. There can be
important differences between the submitted version and the official published version of record. People
interested in the research are advised to contact the author for the final version of the publication, or visit the
DOI to the publisher's website.
• The final author version and the galley proof are versions of the publication after peer review.
• The final published version features the final layout of the paper including the volume, issue and page
numbers.
Link to publication
General rights
Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners
and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.
• Users may download and print one copy of any publication from the public portal for the purpose of private study or research.
• You may not further distribute the material or use it for any profit-making activity or commercial gain
• You may freely distribute the URL identifying the publication in the public portal.
If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license above, please
follow below link for the End User Agreement:
www.tue.nl/taverne
Take down policy
If you believe that this document breaches copyright please contact us at:
openaccess@tue.nl
providing details and we will investigate your claim.
Download date: 09. Aug. 2022
Transmission of 11 x 224-Gb/s POLMUX-RZ-16QAM over
1500 km of LongLine and pure-silica SMF
M. S. Alfiad
(1)
, M. Kuschnerov
(2)
, S. L. Jansen
(3)
, T. Wuth
(3)
, D. van den Borne
(3)
, H. de Waardt
(1)
(1)
COBRA institute, Eindhoven University of Technology, The Netherlands (m.s.alfiad@tue.nl)
(2)
University of the Federal Armed Forces Munich, EIT-3, D-85577 Neubiberg Germany
(3)
Nokia Siemens Networks GmbH & Co.KG, St-Martin Str. 76, D-81549, Munich, Germany
Abstract We demonstrate transmission of 11 x 224-Gb/s POLMUX-RZ-16QAM over 1500 km with a
channel spacing of 50 GHz. A hybrid configuration of LongLine and pure silica fiber is used to optimize
both nonlinear tolerance and Raman gain.
Introduction
Recent developments in transponder
technology, such coherent detection and digital
signal processing [1], have enabled solutions
with close to optimum OSNR threshold and a
near-perfect compensation of linear
impairments. Such transponder therefore enable
for the first time a performance close to
theoretical limits and recently a great deal of
research has therefore been dedicated to
understanding the ultimate transmission
capacity of single mode fiber [2]. In order to
achieve a high spectral efficiency (SE) while not
sacrificing too much transmission reach, several
further development are required in the
components and transmission technology of
optical networks: (1) new single mode fiber
(SMF) types with a large core size [3-5] and
lower attanuation [6] to increase the OSNR
margin and consequently increase the
transmission distance, (2) more optimized
amplifiers architectures with hybrid
EDFA/Raman amplification in order to improve
the received OSNR at the end of the link and
finally (3) more advanced digital signal
processing algorithms and forward error
correction (FEC) codes [7].
A suitable candidate to realize both an ultra-high
spectral efficiency, but still maintaining a
sufficiently long feasible transmission distance is
28-GBaud (224 Gb/s) polarization-multiplexed,
16-level quadrature amplitude modulation
(POLMUX-16QAM). Recently, transmission of
224-Gb/s POLMUX-16QAM has been
demonstrated over 1200 km of SMF [4]. In this
paper we show transmission of 11 x 224-Gb/s
POLMUX-RZ-16QAM over 1500 km using a
combination of LongLine (LL) [3] and pure silica
core fiber (PSC) [6] in order to optimize both
nonlinear tolerance and Raman gain.
System Setup
The experimental setup is depicted in Fig. 1. As
shown in the figure, ten distributed feed back
(DFB) lasers and one external cavity laser (ECL)
with wavelengths on the 50 GHz ITU grid, and
ranging from 1548.5 nm and 1552.5 nm are
grouped into odd and even channels using two
array wave guides (AWG). The ECL laser is
used for the channel under test and the DFB
lasers are used for the co-propagating WDM
channels. After the AWG, the two channels
groups are first pulse carved using two Mach-
Zehnder modulators (MZM) driven with a
28-GHz clock signal. Subsequently, the two
wavelength combs are modulated with
28-GBaud 16QAM using two IQ modulators.
The Fujitsu FTM7961EX modulators used have
a V
pi
of ~2.2 V as well as an optical bandwidth of
>33 GHz. In order to generate the 28-GBaud
16QAM optical signal, the IQ modulators are
driven with a 4 level pulse amplitude modulated
(PAM) signals, which are generated using the
two bit DACs [4] shown in Fig. 1c. The input
Fig. 1:
Experimental setup; (a) Transmitter, (b) Re-circulating loop; (c)Generation of the 4-PAM driving signal (c)
16QAM eye diagrams
ECOC 2010, 19-23 September, 2010, Torino, Italy
978-1-4244-8535-2/10/$26.00 ©2010 IEEE
We.8.C.2
signals to the DACs consist of 28-GBaud binary
PRBS signals with a pattern length of 2
15
-1 bits.
The amplitude of the 4-PAM signals is ~2.8 V
p-p
.
Due to the cascade of many discrete
components in the DACs with an electrical
bandwidth in the order of 25-26 GHz, the
extinction ratio for the 28-GBaud 4-PAM signals
is decreased significantly, and the rise and fall
times are strongly increased. In order to
alleviate this problem, we applied RZ pulse
carving to the signal. The non-return to zero
(NRZ) and RZ eye diagrams in Fig. 1d exhibit
the improvement in the signal quality obtained
through pulse carving. The two wavelength
combs of RZ-16QAM modulated channels at the
output of the two IQ modulators are combined
on a 50-GHz channel grid using a wavelength
selective switch (WSS) which is used as well to
equalize the channels powers. Finally, a
polarization multiplexing stage, consisting from a
50/50 splitter a delay line and a polarization
beam splitter (Fig. 1a), is used to polarization
multiplex the signals at the output of the WSS.
The two polarizations of the POLMUX signal are
interleaved in order to enhance the signal’s
tolerance to nonlinear effects (Fig. 1d). Fig. 2
illustrates the optical spectrum of the eleven
POLMUX-RZ-16QAM channels, at the
transmitter side. The biasing point for the RZ
pulse carver has been adjusted to confine the
spectrum of each channel as evident from Fig. 2
which reduces slightly the cross-talk between
the neighboring channels.
The optical transmission link consists of five
spans of 100 km SMF built in a re-circulating
loop. Each span in the loop is composed from
75 km of LL [3] fiber followed. by 25 km of PSC
[6] fiber. Hybrid EDFA / Raman amplification
scheme has been employed in this link (Fig. 1b)
with an average ON/OFF Raman gain of
~10 dB. LL fiber has a core size of 120 μm
2
which reduces its nonlinear coefficient into
~0.8 1/W.km and consequently allows for higher
launch powers. Therefore LL fiber is used
directly after the EDFA amplifiers. However, this
large core size results in a reduction in the
Raman gain for the fiber. Consequently, we use
a 25 km section of PSC (which is a conventional
SSMF with a loss factor reduced to
0.168 dB/km) fiber in the end of each span to
enhance the gain from the back-ward pumping
Raman amplifier.
At the receiver a coherent detection is realized
using an ECL local oscillator, and a polarization
diversity IQ-mixer with balanced photodiodes.
The four outputs from the coherent receiver are
sampled at a sampling rate of 50 GSample/s
using a real time digital sampling scope (DSA
72004), and 10
6
samples (~4 x 10
6
bits) are
saved for offline processing of each measuring
point. In the offline processing, first a frequency
domain equalizer is used to compensate for the
total accumulated chromatic dispersion (CD) on
the received signal, and afterwards a time
domain equalizer is employed for equalizing all
other linear effects in the signal. The time
domain equalizer consists of four FIR filter
banks in a butterfly structure. Each of these FIR
filters has 21 taps, and their coefficients are first
initialized using the constant modulus algorithm
(CMA) followed by the least mean square
algorithm (LMS). Phase lock loop (PLL) based
carrier recovery is employed for the carrier and
phased recovery.
Experimental Results
The back-to-back OSNR requirement for the
224 Gb/s POLMUX-RZ-16QAM signal is shown
in Fig. 3 (OSNR measured within 0.1 nm
resolution bandwidth). Compared to the
theoretical limits, the measured OSNR
sensitivity curve is shifted by approximately 4 dB
at a bit error rate (BER) of 10
-3
and has an error
floor at around a BER of 2x10
-5
. We conjecture
that this is the result of the electrical bandwidth
limitation of the 4-PAM electrical driving signals,
the nonlinearity in the MZM transfer function and
to a small 50 ohm mismatch at the input of
optical modulators. Fig. 3 shows as well the B2B
sensitivity for the central WDM channel of the
50 GHz wavelength comb (at 1550.5 nm).
Compared to the single channel case, the WDM
curve shows a penalty of 1.5 dB at a BER of 10
-3
and furthermore the error floor shifts upwards to
around 1x10
-4
. This penalty is due to the
introduction of additional electrical components
Fig. 3: Measured back-to-back OSNR requirement
of POLMUX-RZ-16QAM
Fig. 2: 11 x 224-Gb/s POLMUX-RZ-16QAM optical
spectrum
We.8.C.2
with a BW of 25 GHz in order to split the
electrical driving signal between the two parallel
modulators which further degrades the quality of
the electrical driving signal. Note that in the
single channel configuration a 50 GHz
interleaver has been used to band limit the
signal, and the difference between the single
channel and WDM configuration is therefore not
due to narrowband optical filtering penalties.
In Fig. 4 the launch power for the
11 x 224-Gb/s POLMUX-RZ-16QAM channels is
varied between -4 dBm and +4 dBm, and the
BER is calculated for the 1550.5 nm channel at
each of the measured launch powers. This
power variation measurement has been carried
out after transmission distances of 1000 km.
The optimum launch power is found to be
around 0 dBm, which will be used for all of the
following measurments to be reported in this
paper. Transmission results for the same signal
over a 670 km SSMF link with the same
amplification technique are depicted as well in
Fig. 4. It is evident from these results that the
optimal launch power for the SSMF link is
reduced by around 3 dB compared to the
LL+PSC link which proves the ability of LL fiber
to effectively reduce nonlinear effects and to
increase transmission distance by around 50%.
The BER of the received signal at a wavelength
of 1550.52 nm is calculated at different
transmission distances and reported in Fig. 5.
The figure illustrates that a maximum
transmission distance of 1500 km is feasible
with a BER below the FEC limit
[7]. The BER for
the 11 channels has been measured after a
transmission distance of 1500 km with the
optimum launch power of 0 dBm (Fig. 6). During
this measurement, the ECL laser has been
switched such that it is used for each channel
under test. The BER of all measured WDM
results is below the FEC threshold (which is
assumed to be at a BER of 5x10
-3
using a 7%
overhead [7]).
Finally, the constellation diagrams for the
POLMUX-RZ-16QAM signal are shown in Fig. 7
both in the single channel and multi-channel
B2B configuration, as well as after 1500 km of
transmission. The constellation diagram for the
multi-channel B2B configuration confirms the
degradation of the signal quality.
Conclusions
In this paper, we demonstrate the transmission
of 11 x 224-Gb/s polarization-multiplexed
16-level quadrature amplitude (POLMUX-RZ-
16QAM) modulation over 1500 km of LongLine
and pure silica core SMF, with a channel
spacing of 50 GHz and a SE of 4.2 b/s/Hz. This
shows the feasibility of ultra-high spectral
efficiency transmission over a long-haul
transmission distance.
Acknowledgment
The authors would like to thank Draka
Communications, France for providing us with
the LongLine fiber and Fujitsu Optical
Components Limited, Japan for providing us
with the IQ modulators used in this experiment.
References
1 K. Roberts, et al., JLT, Vol. 27, 2009.
2 R. J. Essiambre.et al., JLT, Vol 28, 2010.
3 G. Charlet, et al.,OFC 2009, paper PDPB6..
4 A. Gnauck, e al., OFC 2010, paper PDPB8.
5 X. Zhou, et al., OFC 2010, paper PDPB9.
6 X. Zhou, et al., OFC 2009, paper PDPB4.
7 M. Scholten, et al, ECOC 2009, WS1.
Fig. 7: POLMUX-RZ-16QAM constellation diagrams
Fig. 4:
Launch power variation results
Fig. 6: BER results for the eleven channels after
1500-km transmission
Fig. 5: BER results for the central channel versus
transmission distance
We.8.C.2
