Optics and Photonics Journal, 2013, 3, 337-341
doi:10.4236/opj.2013.32B077 Published Online June 2013 (http://www.scirp.org/journal/opj)
Cascadability of Uniform Fibre Bragg Grating for 40
Gbit/s RZ-OOK to NRZ-OOK Conversion
O. Ozolins, V. Bobrovs, G. Ivanovs
Institute of Telecommunications, Riga Technical University, Riga, Latvia
Email: oskars.ozolins@rtu.lv
Received 2013
ABSTRACT
The cascadability of uniform fibre Bragg grating for 40 Gbit/s return to zero on-off keying to non-return to zero on-off
keying format conversion has been shown using OptSim simulation program. The main idea of this approach is use of
specially designed uniform fibre Bragg grating with appropriate transfer function for shaping of 40 Gbit/s return to zero
on-off keying optical spectrum. Error free performance is achieved after four cascades of uniform fibre Bragg grating
with different reflectivity values.
Keywords: All-optical Devices; Fibre Bragg Grating; Fibre Optic Transmission Systems; Format Conversion; Intensity
Modulation; Optical Signal Processing
1. Introduction
In last year’s photonic technologies for optical transmission
have gained great breakthrough because of high demand
for bandwidth and need to reduce costs per every trans-
mitted bit [1,2]. To overcome the emerging challenges
various optical modulation formats have been utilized in
optical transmission depending on the bit rate, transmis-
sion distance and media [3].
Historical circumstances have made on-off keying
(OOK) in both non-return to zero (NRZ) and return to
zero (RZ) in the modulation format of choice for the
most of optical transmission systems [3]. Optical modu-
lation format NRZ-OOK is simplest in generation. The
Mach Zender modulator (MZM) is biased at 50% of
transmission and driven from minimum to maximum.
NRZ-OOK modulation format is more spectrally effi-
cient comparing to RZ-OOK and thus is appropriate for
metro and access transmission systems because of rather
short span lengths and lower bit-rates [1,3]. In turn the
RZ pulse fills a part of the bit period which can have
three different types of duty cycle: 33%, 50% and 67%.
Shorter pulses results in broad spectrum which is clear
from properties of the Fourier transform. This is the main
drawback of RZ-modulated signals resulting in a reduced
tolerance to chromatic dispersion and a smaller spectral
efficiency. Still, RZ-OOK format is preferred in transport
optical transmission systems thanks to superior tolerance
to nonlinear optical effects (NOE), inter-symbol inter-
ference (ISI) and polarization mode dispersion (PMD) [3,
4]. Interconnections among transport, metro and access
networks are necessary because of different modulation
formats in use. Optical-electrical and electrical-optical
conversion degrades system efficiency. This is mainly
related to increase of time and cost. Therefore it is im-
portant to perform all-optical signal processing [1]. This
will lead to increase of systems overall efficiency. One of
the functionalities of all-optical signal processing is
modulation format conversion in optical domain. It will
be demanded at the gateways to ensure transparent and
efficient interconnection especially for future ubiquitous
transparent optical networks.
Different approaches have been worked out for all-
optical format conversion. One part of this conversion is
related to nonlinear signal processing: semiconductor
optical amplifier (SOA)/distributed feedback laser (DFB-
LD) [5], SOA-loop-mirror [6], SOA/fibre Bragg grating
(FBG) [7], nonlinear optical loop mirror (NOLM) and
four-wave mixing (FWM) [8]. Other part is related to
linear optical signal processing: specially designed silicon
mirroring resonators (MRR) have been used for modula-
tion format conversion [9,10]. The main idea of this ap-
proach is use of optimized MRR with appropriate transfer
function for shaping of input optical spectrum. Different
approach could be based on uniform FBG [11]. In this
paper cascadability of uniform fibre Bragg grating for 40
Gbit/s RZ-OOK to NRZ-OOK conversion has been shown
using OptSim simulation program.
2. Simulation Method and Model
The OptSim 5.2 simulation program is software tool for
Copyright © 2013 SciRes. OPJ
O. OZOLINS ET AL.
338
the design and simulation of optical communication sys-
tems at the signal propagation level. It was applied for
demonstration of 40 Gbit/s RZ-OOK to NRZ-OOK format
conversion and assessment of cascadability of uniform
FBG impact on conversions efficiency. For uniform FBG
transfer function calculation Bragg Grating Filters Syn-
thesis (BGFS) 2.6 simulation program was used. The
main results of this paper are obtained with two different
simulation tools: for physical layer simulation of optical
transmission systems and other for synthesis of physical
components transfer function depending on different
parameters. Calculation method of simulation program
OptSim 5.2 solves set of complex differential equations
and takes into consideration linear and nonlinear im-
pairments on electromagnetic signal propagation in opti-
cal and electrical components of transmission system.
In this research the time domain split step (TDSS)
method was used which despite its complexity grants a
precise outcome of simulation. This calculation method
is used in all commercial simulation tools to perform the
calculation of the electromagnetic signal propagation
equation in optical fibre:


,,
Atz LNAtz
z

(1)
A (t, z) represents the optical field, L is the linear op-
erator which stands for linear impairments and N is the
nonlinear operator which is responsible for nonlinear
impairments. The equation (1) is calculated over small
spans of fibre by including either linear or nonlinear op-
erator. For instance, on the first span only L is considered,
on the second span only N operator and so on [12].
The BGFS 2.6 simulation program was used to obtain
FBG transfer function. Transfer matrix method is employed
in BGFS 2.6 simulation program to simulate different
configurations of FBG filters. It is applied to solve the
coupled mode equations and to obtain the spectral response
of the FBG filter. The transfer function was for reflection
spectra of FBG filter. The FBG parameters were opti-
mized by altering the length of grating and reflectivity
parameter. Two different transfer functions of FBG filter
reflection spectra were synthesized with uniform apodi-
zation profile and 60% and 97.5% reflectivity values. In
Figure 1 and Figure 2 uniform FBG filter reflection
spectrums with 97.5% and 60% reflectivity are shown
accordingly.
Us it is noticed two simulation programs were used in
simulations. In BGFS 2.6 simulation program calculated
reflection spectra of FBG filter was recorded in the data
file. After simple mathematical calculations compatible
data file format were created for OptSim 5.2 simulation
program. User defined optical filter block was used for
building of 40 Gbit/s RZ-OOK to NRZ-OOK format
converter based on uniform FBG. The synthesis of the
user defined optical filter is based on the Overlap and
Add algorithm [13]. At first the data points are interpo-
lated so to obtain a continuous transfer function. The
algorithms developed to implement user defined filter
component are very accurate and the transfer function is
synthesized. Then it is applied for investigation of uni-
form FBG filter cascadability impact on format conver-
sion.
3. Results and Discussion
The simulation setup for investigating of the uniform
FBG filter cascadability impact on 40 Gbit/s RZ-OOK to
NRZ-OOK format conversion is illustrated in Figure 3.
The setup consists of three parts: optical transmitter,
1549,0 1549,5 1550,0 1550,5 1551,0
-100
-50
0
Reflection (dB)
Wavelength (nm)
First
Second
Third
Fourth
Figure 1. Amplitude transfer function of uniform FBG op-
tical filter reflection spectra with 97.5% reflectivity after
different number of cascades shown in inset.
1549,0 1549,5 1550,0 1550,5 1551,0
-100
-50
0
Reflection (dB)
Wavelength (nm)
First
Second
Third
Fourth
Figure 2. Amplitude transfer function of uniform FBG op-
tical filter reflection spectra with 60% reflectivity after dif-
ferent number of cascades shown in inset.
Copyright © 2013 SciRes. OPJ
O. OZOLINS ET AL.
Copyright © 2013 SciRes. OPJ
339
format converter in cascade configuration and preampli-
fied optical receiver. The optical transmitter for 40 Gbit/s
RZ- OOK optical signal generations consists of two
LiNbO3 MZMs and continuous wave (CW) light laser.
The first MZM was used as a pulse carver driven by a
half clock, while the second one was driven by a 40
Gbit/s pseudo random binary sequence (PRBS) with a
pattern length of 231-1. The optical signal was then cou-
pled into the optical format converter which consists of
uniform FBG filter and the three port optical circulator.
In this investigation four optical format converters were
used to see the impact of cascadability on format conver-
sion efficiency. These converters were connected with
optical circulators in cascade by connecting third fort of
first circulator with first port of next. After each con-
verter 40 Gbit/s NRZ-OOK optical signal was detected in
a preamplified receiver. This receiver consists of erbium
doped fibre amplifier (EDFA), 100 GHz Gaussian optical
band pass filter (OBPF), 45-GHz photodiode, electrical
low pass filter (ELPF), optical spectrum analyser (OSA),
electrical scope and bit error rate (BER) tester.
Figure 4 shows the power spectrum density (PSD) of
the input 33% RZ-OOK, converted NRZ-OOK after dif-
ferent number of uniform FBG with 97.5% reflectivity
cascades and generated NRZ-OOK. Figure 5 shows bit
error rate (BER) as a function of received power for the
input 33% RZ-OOK and converted NRZ-OOK after dif-
ferent number of uniform FBG with 97.5% reflectivity
cascades. Insets: eye diagrams of back to back 33% RZ-
OOK signal and converted NRZ-OOK signals. In this
case uniform FBG with 97.5% reflectivity was applied
for the conversion of signal formats. Increased reflective-
ity for uniform FBG leads to lower insertion loss of the
device and flat top of the pass band (see Figure 1). Ac-
cumulation of insertion loss is lower and full width half
maximum (FWHM) bandwidth reduction effect is also
reduced while filters are cascaded. Due to imperfections
of the employed uniform FBG: (the side lobe minimum
values are not matched to tones of RZ-OOK signal zeros
and pass band shape is not optimized) the RZ-OOK to
Figure 3. Setup for investigating of the uniform FBG filter cascadability impact on 40 Gbit/s RZ-OOK to NRZ-OOK format
conversion.
-36 -33 -30 -27 -24-21 -18 -15-12
10
9
8
7
6
5
4
-log(BER)
Received power (dBm)
RZ-OOK b2b
First FBG
Second FBG
Third FBG
Fourth FBG
193,2 193,3 193,4 193,5 193,6
-30
-20
-10
0
10
20
30
40
50
PSD(dBm/nm)
Frequency (THz)
RZ-OOK
First FBG
Second FBG
Third FBG
Fourth FBG
NRZ-OOK
Figure 5. BER as a function of rec eived power for the input
33% RZ-OOK and converted NRZ-OOK after different
number of uniform FBG with 97.5% reflectivity cascades.
Insets: eye diagrams of back to back 33% RZ-OOK signal
and converted NRZ-OOK signals.
Figure 4. Power spectrum density of the input 33% RZ-OOK
converted NRZ-OOK after different number of uniform
FBG with 97.5% reflectivity cascades and generated NRZ-
OOK. Details are shown in inset.
O. OZOLINS ET AL.
340
NRZ-OOK conversion is not successful. Adding more
filters in cascade leads to better BER performance. After
third and fourth cascade the eye diagrams shows partial
format conversion, but with large amount of amplitude
ripples. These ripples could be reduced by adding addi-
tional Gaussian band pass filter. Therefore this leads to
finding that reflectivity of uniform FBG must not be high.
This also leads to drawback which is related to increased
insertion loss value.
To obtain more convincing results the additional simu-
lations was carried out with device which has lower re-
flectivity and less imperfections for transfer function.
Figure 6 shows the power spectrum density (PSD) of the
input 33% RZ-OOK, converted NRZ-OOK after different
number of uniform FBG with 60% reflectivity cascades
and generated NRZ-OOK. Figure 7 shows bit error rate
(BER) as a function of received power for the input 33%
RZ-OOK and converted NRZ-OOK after different num-
ber of uniform FBG with 60% reflectivity cascades. Insets:
eye diagrams of back to back 33% RZ- OOK signal and
converted NRZ-OOK signals. As one can see from Fig-
ure 2 uniform FBG with 60% reflectivity has not so flat
top at the pass band. After number of cascades this pass
band shape tends to be Gaussian like. Also the change in
FWHM bandwidth have been observed due to the fact,
that cascading of band pass filters leads to usable band-
width reduction. In the case of RZ-OOK to NRZ-OOK
format conversion these properties gives some benefits
and limitations. Main contribution from the cascading of
uniform FBG with 60% reflectivity could be seen in
Figure 7 eye diagrams for converted 40 Gbit/s NRZ-
OOK optical signals. After first filter eye diagram (red
colour) has amplitude ripples due to imperfections in
filter pass band shape. Still the eye diagram opening is
rather high to give error free performance. After adding
more uniform FBG filters with 60% reflectivity the eye
diagrams were with less amplitude ripples and with better
BER performance. This trend was observed till third num-
ber of cascade and mainly is caused due to Gaussian like
pass band shape which arises from cascading of the filter.
However at the fourth number of cascade the effect of
FWHM bandwidth reduction leads to over filtering of the
converted NRZ-OOK signal and decreased BER per-
formance.
Figure 8 shows the power penalty (at BER = 10-9) in-
duced by cascading of a uniform FBG versus number of
cascades for uniform FBG with 60% and 97.5% reflec-
tivity. As it can be seen from the results obtained from
BER performance for both 40 Gbit/s RZ-OOK to NRZ-
OOK cascaded converters that power penalty decreases
with the number of cascades. In the case of uniform FBG
with 60% reflectivity minimum point of power penalty is
achieved after third cascade and after more cascades it
increases significantly due to optical signal over filtering.
193,2 193,3 193,4 193,5 193,6
-30
-20
-10
0
10
20
30
40
50
PSD(dBm/nm)
Frequency (THz)
RZ-OOK
First FBG
Second FBG
Third FBG
Fourth FBG
NRZ-OOK
Figure 6. Power spectrum density of the input 33% RZ-
OOK converted NRZ-OOK after different number of uni-
form FBG with 60% reflectivity cascades and generated
NRZ-OOK. Details are shown in inset.
-36 -33 -30-27 -24 -21 -18-15-12
10
9
8
7
6
5
4
-log(BER)
Received power (dBm)
RZ-OOK b2b
First FBG
Second FBG
Third FBG
Fourth FBG
Figure 7. BER as a function of rec eived power for the input
33% RZ-OOK and converted NRZ-OOK after different
number of uniform FBG with 60% reflectivity cascades.
Insets: eye diagrams of back to back 33% RZ-OOK signal
and converted NRZ-OOK signals.
1234
0
2
4
6
8
10
Power penalty (dB)
Number of FBG
Reflectivity 60 %
Reflectivity 97.5
Figure 8. Power penalty (at BER= 10-9) induced by cascad-
ing of a uniform FBG versus number of cascades for uni-
form FBG with differ ent reflectivity shown in inset.
Copyright © 2013 SciRes. OPJ
O. OZOLINS ET AL.
Copyright © 2013 SciRes. OPJ
341
4. Conclusions
The cascadability of uniform fibre Bragg grating of 40
Gbit/s RZ-OOK to NRZ-OOK format conversion has
been demonstrated numerically for the first time. The
best BER performance is obtained after three cascades of
uniform FBG with 60% reflectivity. It has been found
that uniform FBG RZ-OOK to NRZ-OOK format con-
verter must have Gaussian like pass band which is con-
nected with rather low reflectivity value to reduce am-
plitude ripples in converted signal waveform and side
lobe minimums must be chosen to match side tones of
RZ-OOK signal for conversion.
5. Acknowledgements
This work has been supported by the European Regional
Development Fund within the project Nr. 2010/0270/
2DP/2.1.1.1.0/10/APIA/VIAA/002.
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