ECAI 2016 – International Conference 8th Edition [600275]

ECAI 2016 – International Conference – 8th Edition
Electronics, Computers and Artificial Intelligence
30 June -02 July, 2016, Ploiesti, ROMÂNIA
Optical Similaritons in Optical
Communication Systems

Leila GRAINI, Kaddour SAOUCHI
Laboratory of Study and Research in
Instrumentation and Communication of Annaba
(LERICA), Department of Electronics, Badji
Mokhtar University, Annaba 23000, ALGERIA.
E-mail : [anonimizat]
Silviu IONITA, Grigore-Adrian
IORDACHESCU
Center of Modeling and simulation of the systems,
Faculty of Electronics and Communications,
University of PITESTI, ROMANIA.

Abstract – In this paper, we numerically study the
performance of a multi-wavelength source achieved b y
spectral slicing the spectrum of similariton pulses . In
this aim, we propose to generate a single continuum
source capable of providing all the necessary chann els,
enhancing thus the efficiency of a WDM system. The
continuum source we used is based on the generation of
similaritons in a special highly nonlinear and norm al
dispersion Photonic Crystal Fiber. The resulting
spectrum will span the C-band and will have a power
spectral density as flat as possible. Q-factors and eye-
diagrams have been calculated to investigate the sy stem
performance.
Keywords: Similariton spectrum; WDM
communication system; Q factor; Eye-diagram
I. INTRODUCTION
In a wavelength division multiplexing (WDM)
system, many channels of information are carried
over the same fiber, each channel using a separate
wavelength delivered by a laser diode. However,
when several closely spaced channels are
simultaneously propagated through the same fiber, t he
wavelength stability of the source becomes an issue of
prime importance, thus increasing the economical
factor of these systems and making it available onl y
for long-haul applications with their high capacity
demand.
To avoid this limit, the continuum slicing techniqu e
replaces laser diodes to generate multi-wavelength
optical sources [1, 2, 3]. In this technique one w ould
prefer to have a high power continuum with good
spectral flatness over a narrow bandwidth. Optical
similariton has the potential to generate this kind of
continuum owing to its characteristics, such as
parabolic waveform, resistance to optical wave
breaking, self-similarity in shape, chirp linearity , and
a flat and broad spectrum [4, 5, 6, 7].
II. 10-GH Z WDM COMMUNICATION SYSTEM
BASED ON SIMILARITON SPECTRUMS
A. Numerical model of similarion pulse propagati on
Similariton pulse is a result of the interaction
between normal dispersion, nonlinearity and gain.
The numerical model of the similariton propagation in
optical fiber is the well-known nonlinear Schröding er equation (NLSE) with gain expressed in the followin g
form [6]:
(1)
Where A is the slowly varying amplitude of the
pulse, β2 is the second order dispersion of the fiber, γ
is the nonlinear coefficient, and g is the distributed
gain coefficient.
It has been proved that the solution of Eq. ( 1) is a
self-similar asymptotic solution characterized by a
parabolic intensity profile [6], and it is possible to
solve it numerically by using the split step Fourie r
method (SSF) [8].
B. System description
The block diagram of the WDM transmission
system at 10 Gb/s is shown in Figure 1. The
transmitter block is designed to produce 10 GHz tra in
of 2.4 ps pulses at a wavelength of 1550 nm. These
pulses are used to generate similariton spectrums b y
propagating in a normal dispersion and highly
nonlinear photonic Cristal fiber (PCF), and are
subsequently sliced in the spectral domain to creat e
32 – channel WDM multi-wavelength source
(wavelengths λ1-λ32 ). The wavelengths range from
1536.8 nm to 1562.2 nm, with 0.8 nm wavelength
spacing (100 GHz). The carrier signals are thus
modulated by 10 Gbit/s 2 7-1 Pseudo Random Binary
Sequences (PRBS) signals of user data in order to
compose a 10 Gbit/s RZ transmitter. An intensity
Mach-Zehnder modulator is used for on-off keying
modulation (OOK) to modulate each channel
according to the RZ electrical data. The resulting
signal is in the RZ-OOK type format and its pulse
duration is equal to 0.33 % of the bit time (duty
cycle). The duty cycle is mainly determined by the
optical demultiplexer parameters. The encoded data
from all users are multiplexed by optical multiplex er
and then passed through a standard single mode
optical fiber (SMF) followed by a dispersion
compensating fiber (DCF) and a loss compensating
optical amplifier which is EDFA (Erbium-Doped
Fiber Amplifiers). The multiplexing signal finally
arrives at the optical receiver. The receiving bloc k is
designed to convert the optical signal that carries the

GRAINI et all
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information into electrical pulses. It is composed of a
demultiplexer of 50 GHz bandwidth and 100 GHz
channel spacing between channels, PIN photodiodes
performing the optoelectronic conversion, followed
by an electrical low-noise amplifier. A Bessel low
pass filter then sets the electrical bandwidth of t he
receiver, and is followed by the decision circuits. The
values of the fibers parameters used in our calcula tion
are given in table 1.

III. PERFORMANCE EVALUATION
During the propagation and amplification throu gh
the normal dispersion and highly nonlinear photonic
crystal fiber (PCF with high nonlinear coefficient
γ=51 [W.Km] -1 at 1550 nm ), the pulse waveform
becomes parabolic (similariton) [9]. The spectrum o f
such pulse could thus be broadened in a flat and
rectangular way.
In our analysis, we assume the incident pulse to be
of Gaussian shape. The electric field A (0, t)
corresponding to such a pulse can be expressed in t he
following form:
(2)
Where P0 is the power of the input pulse and T0 is the
width, and it is related to the full width at half
maximum (FWHM) of the input pulse by
TFWHM ≈1.665 T0.
Figure 2 is given for P0 = 0.02 W, TFWHM = 2.4 ps ,
and g = 1.9 m-1. The output spectrum of similariton
pulses (continuum) shows a widening from 5 nm to
100 nm with good spectral flatness in the center of the
pulse spectrum (over a range of 40 nm ). This results
in smaller peak power variation (around 6 dBm)
across the 32 sliced channels as shown in Figure 3. TABLE I: FIBER PARAMETERS USED IN THE CALCULATIONS
Parameters SMF DCF PCF

β2 (ps 2/Km) -29.7 +102 +1.1
n2 (m 2/W) 3.2 *10 -20 7*10 -20 2.2*10 -20
α (dB/Km) 0.2 0.5 neglicted
Aeff (µm2) 50 30 2.5
L (Km) 50 11 0.03

The peak power in each individual channel is found to
be sufficient for direct modulation without further
amplification (15dBm to 21dBm).
The generated continuum, centered on the
wavelength of 1550 nm with spectral broadening of
40 nm at – 3 dB spectral width, is subsequently sliced
by an optical demultiplexer with a bandwidth of 50
GHz and 100 GHz (0.8 nm) channel spacing in order
to best limit the interference. As a result, we obt ain a
spectral superposition of 32 channels. Each channel is
a pulse train having the same repetition rate as th at of
the initial pulse (10 GHz). The pulse widths produc ts
are almost constant at ~6 ps with sub-band spectral
spacing of 0.7 nm across all the channels.
For an easier interpretation of the results, w e
consider the propagation of four central channels a t
10 GHz repetition rate with 100 GHz channel
spacing. The pulse shapes of four central and
consecutive channels (channels 15 to 18) are shown in
Figure 4.a. The channel wavelengths are 1548.8 nm,
1549.6 nm, 1550.4 nm and 1551.2 nm. It can be see
that there is a time delay (timing jitter) in addit ion to
the peak power variation (0.03 W). This timing jitt er
Figure 1. Block diagram of a WDM system: transmitt er part, transmission line, and receiver part

Optical Similaritons in Optical Communication Syste ms
3
(of 0.4 ps ) across the channels is due to the nature of
the chirp induced by the similariton pulse at the
output of the PCF. The frequency chirp induced by a
similariton pulse is linear, positive and increases as
the fiber length increases [6], i.e., it increases with the
increase in time. So we can say that the higher
wavelength channel appears first (in time) compared
to the lower wavelength channel. As we can see from
Figure 4.a, and Figure 4.b, the wavelength channel of
1551.2 nm appears at an earlier time compared to th e
wavelength channel of 1548.8 nm.
The time variation across the channels is observed
to be small but the power variation is greater whic h
suggests the need for a gain equalizer before
propagation.
The generated channels have been tested in a
transmission setup, illustrated in Figure 1. This s etup
consists of a 50 km span of single mode fiber. The
dispersion compensation fiber of 11 Km length is
used to compensate for the dispersion. The losses a re
compensated by Erbium Doped Fiber Amplifiers. The
EDFA used gain is 30dB.
Figure 5 shows the calculated eye-diagrams at
1548.6, 1549.4, 1550.6 and 1551.4 nm wavelengths
(channels 15 – 18). We found that the eye-diagrams of
all side channels are opened clearly. Eye-degradati on
for channels at 1550.6 and 1551.4 nm is smaller
compared to channels at 1548.6 and 1549.4 nm. The
larger eye-degradation in these channels is due to the
decrease in the peak power. The obvious common
characteristic between these eye-diagrams is the
presence of timing jitter. This can be understood b y
noting that the small timing jitter induced by the
sliced pulse has an effect on the pulse trains duri ng
the propagation, resulting in a degradation of chan nel
performance. We note that we can avoid this problem
by the compensation of the chirp inducing during th e
similariton formation.

Figure 2. Comparison between the input spectrum (gr een trace) and
the similariton spectrum (blue trace)

Figure 3. Power variation across the 32 sliced cha nnels

(a)

(b)
Figure 4. Sliced pulse trains of four central conse cutive channels
having 10 GHz repetition rates (a), and their spect ra (b)

GRAINI et all
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The Q values are plotted in Figure 6 and performed
on the system before and after transmission. To
evaluate the noise of the sliced pulses, we calcula ted
the Q factor as a function of the wavelength (green
line). A high Q factor of over 20 was realized in
almost all of the channels. After transmission, the
value of the Q factor varies from 15 to 16 (blue li ne)
for short wavelength to long wavelength respectivel y.
These channels align themselves with the top of a
ripple in the continuum spectrum also shown in
Figure 3. The location and magnitude of these rippl es
lead to power variation. Small changes in power can
produce relatively large amplitude variations aroun d
these spectral features, resulting in a degradation of
channel performance. Careful control of the pulse
parameters and a judicious choice of fiber length a re
necessary to obtain sufficient noise properties for all
channels. As can be seen in Figure 4, channel 1548. 6
nm, which overlaps directly with the spectrum of th e
input pulses, shows a significant reduction in
performance as well, due to the high value of the p eak
power. When the pulse power is bigger than a
determined value such as 15 dBm in Figure 3, the
nonlinear effects turn stronger and the short pulse
sequence suffers stronger nonlinear influences whic h
will result in the decrease of system performance.

Figure 5. Eye-diagrams for the four central channel s:
ch 15) λ=1548.6, ch16) λ=1549.4 ch17) λ= 1550.6
and ch18) λ=1551.4
1548.5 1549 1549.5 1550 1550.5 1551 1551.5 10 15 20 25 30
wavelength (nm) Q(dB)
Figure 6. The Q factor versus the four central cha nnels before
(green line) and after transmission (blue line) VI. CONCLUSION
We have studied the performance of a multi-
wavelength source based on a similariton spectrum,
because of its utility in a WDM high bit rate
communication system. Firstly, we showed the
characteristics of pulses sliced from a similariton
spectrum. Clean pulse trains at 10 GHz repetition r ate
are achieved by using an optical demultiplexer with
100 GHz channel spacing and 50 GHz channel
bandwidth, resulting in pulses duration of 0.33 % o f
the duty cycle. The pulse widths variation is obse rved
to be negligible across all the channels.
The Q-factors and eye-diagrams have been calculated
to investigate the system performance. We found tha t
the channels with short-wavelength and high power
levels showed eye-degradation due to the stronger
nonlinear effects. Short pulse sequences will also
suffer from nonlinearity influences which will resu lt
in a decrease of the system’s performance. The
Cross-phase modulation effect (XPM) is neglected
because the inter-channel spacing is large.

REFERENCES
[1] Y. Takushima, K.” Kikuchi, 10-GHz, over 20-channel
multiwavelength pulse source byslicing super-contin uum
spectrum generated in normal-dispersion fiber”. IEE E
Photon.Technol. Lett. Vo. 11, pp.322–424. 1999.
[2] Y. Ozeki, K. Taira, K. Aiso, Y. Takushima, K. Kikuc hi,
“Highly flat super-continuum generation from 2 ps p ulses
using 1 km-long erbium-doped fibre amplifier”. Elec tron.
Lett. 38(25), pp. 1642–1643. 2002.
[3] Y. Ozeki, Y. Takushima, K. Aiso, K. Taira, K. Kikuc hi,
“Generation of 10 GHz similariton pulse trains from 1.2 Km-
long erbium doped fiber amplifier for application t o
multiwavelength pulse sources”. Electronics Letter s 40, no.
18. 2004.
[4] M. E. Fermann, V. I. Kruglov, B. C. Thomsen, J. M. Dudley,
and J. D. Harvey, “Self-Similar Propagation and
Amplification of Parabolic Pulses in Optical Fibers ”, Physical
Review Letters, vol. 84,no 26,pp 6010-6013,2000.
[5] V. I. Kruglov, A. C. Peacock, J. D. Harvey, and J. M. Dudley,
“Self-similar propagation of parabolic pulses in no rmal-
dispersion fiber amplifiers,” Journal of the Optica l Society of
America, vol. 19,no 3, pp. 461-469, 2002.
[6] C. Finot, G. Millot, and J. M. Dudley, “Asymptotic
characteristics of parabolic similariton pulses in optical fiber
amplifiers,” Optics Letters, vol. 29, no 21, pp. 25 33-2535,
2004.
[7] G. Q. Chang, H. G. Winful, A. Galvanauskas, and T. B.
Norris, “Self-similar parabolic beam generation and
propagation,” Physical. Review. E. 72, 016609, 2005 .
[8] G. P. Agrawal, Nonlinear Fiber Optics, 3rd edition, 2001.
[9] L. Graini and K. Saouchi, "WDM Transmitter Based on
Spectral Slicing of Similariton Spectrum," Lecture Notes on
Photonics and Optoelectronics, Vol.1, No.1, pp.30-3 4, March
2013