Simultaneous 2-channels NRZ –to- RZ format conversion by
using FWM in highly nonlinear fiber (HNLF)
Wai Phyo Aung
Department of Electronic Engineering, Mandalay Technological University
Abstract: We demonstrate 2-channels parallel format conversions from the non return-to-zero (NRZ) to the return-to-zero
(RZ) by using four-wave-mixing (FWM) in a highly nonlinear fiber. The conversions are based on four-wave-mixing (FWM)
effect, operate non return-to-zero (NRZ) data and RZ clock like AND gate switch and output as return-to-zero (RZ) data. Up
to two converted data channels as the wavelength spacing between the NRZ signals and the pump is set to 2-nm and 2.4-nm. 2channel NRZ signals at 10 Gb/s can be converted to the corresponding RZ signals get negative power penalty about 2dB for 2channels.This paper illustrates modulation format conversion NRZ-to-RZ format by using FWM in highly nonlinear fiber
with simulation diagrams, output results and eye diagrams.
Keyword - OOK modulation format, Mach-Zehnder modulator (MZM), Electroabsorption Modulator (EAM), highly nonlinear
fiber (HNLF), four-wave-mixing (FWM).
I. INTRODUCTION
In the past few years, there has been an increasing interest in all-optical format conversion as it may become an important
technology for future optical networks. Optical Network could employ various formats according to their scales and applications. The
most of optical network primarily employed on-off-keying (OOK). Many schemes have been demonstrated to perform different types
of format conversions such as NRZ-OOK to RZ-OOK. The return-to-zero (RZ) format is widely used in OTDM networks due to its
tolerance to fiber nonlinearities in spite of dispersion-induced effects, while the non-return-to-zero (NRZ) is preferred in WDM
networks for its ease of implementation, relatively high spectral efficiency and timing-jitter tolerance. The transmission performance
of a communication link is strongly depend on the pulse width of the return-to-zero(RZ) signals at the transmitter due to the
nonlinearities, influences of dispersion and characteristic of optical receivers[1-2]. NRZ-to-RZ format conversion have been
demonstrated not only single-channel [3] but also multi-channel [4-6]. Only a few papers have demonstrated multi-channel NRZ-toRZ conversions on WDM system, although most of them are with limited channels and large wavelength spacing [7-8]. Multi-channel
optical conversions between non-return-to-zero (NRZ) to return-to-zero (RZ) modulation format with tunable duty cycle are highly
desirable for WDM network and all optical parallel signal processing. Multi-wavelength conversions, most nonlinear signal
processing schemes depend on the Kerr nonlinearity will suffer from a finite in-band crosstalk produced by the undesirable four-wavemixing (FWM) components, which cannot be removed at the output.
In this paper, Modulation format conversion from multi-channel non-return-to-zero (NRZ) to return-to-zero (RZ).The
conversions are based on the four-wave mixing (FWM) in highly nonlinear fiber (HNLF). 2-channel NRZ signals at 10 Gb/s can be
converted to the RZ signals with the help of return-to-zero (RZ) clock signal. Simulation results are determined by the help of
Optisystem software; we demonstrated results with spectrum, waveform, Q factor and bit error rate (BER) measurements.
II. OPERATION PRINCIPLE
As shown in Fig. 1, the principle is based on the four-wave mixing (FWM), in highly nonlinear fiber. If we control the power well,
there are three possible nonlinear effects: (i) four –wave mixing (FWM), (ii) cross-phase modulation (XPM), and cross gain
modulation (XGM). In this research, we mainly used four-wave mixing (FWM) effect. FWM, most likely will occur between (𝜆𝑝 )
and (𝜆𝑠 ) near the zero-dispersion wavelength. The wavelengths of other channels can be expressed as
1
𝜆𝑠
+
1
𝜆𝑝
=
1
𝜆𝐿𝑖
+
1
𝜆𝑅𝑖
(Li and Ri
correspond to the i-th channel of the short and long wavelength side, respectively). NRZ-to-RZ format conversion, we should use at
least one pump channel that is modulated by synchronized CLK signal and NRZ channels (𝜆𝑠 ). Then, the modulated signal is
amplified by EDFA and then followed into 0.5km length of highly nonlinear fiber (HNLF) with the dispersion slop at 0.032
ps/nm^2/k. The output signals are filtered by optical band-pass filter (BPF), then the output as RZ signals.
Fig.1. Principle of the proposed scheme for NRZ-to-RZ modulation format conversion
III. SIMULATION SETUP
The experimental set up for NRZ-to-RZ format conversion scheme is shown in Fig 2. CW laser 1 with (-10dB) at 𝜆𝑠1 = 1548.6𝑛𝑚 is
modulated at 10-Gb/s by a Mach-Zehnder modulator (MZM1) with pseudorandom bit sequences (PRBS) to generate first NRZ-OOK
signal. Then, CW laser 2 with (-10dB) at 𝜆𝑠2 = 1549𝑛𝑚 is modulated at 10-Gb/s by a Mach-Zehnder modulator (MZM2) with
pseudorandom bit sequences (PRBS) to generate second NRZ-OOK signal .The channel spacing between these NRZ channel is
0.4nm. The pump from CW laser 3 is modulated at 10-Gb/s by Electroabsorption Modulator (EAM) with (10dB) at 𝜆𝑝 = 1551𝑛𝑚 to
generate the 10-GHz clock signal. Multiplex, these 2-channel NRZ signal by 2×1 WDM multiplexer. Two NRZ channels and RZ
clock signal are lunch into Coupler. Then, amplified with gain 13dB by Erbium-doped fiber amplifier (EDFA) then followed by 0.5km
highly nonlinear fiber (HNLF) with the dispersion slop at 0.032 ps/nm^2/k and dispersion at -0.08ps/nm/km. Came out signal from
four-wave mixing (FWM) process after passing through the highly nonlinear fiber (HNLF) is filtered with 0.2nm bandwidth at
1553.4nm and 1553 by Band-pass filter (BPF1 & BPF2).Then, output as 2-channel RZ signals and tested by BER analyzer. This
illustration is set up with the help of optisystem software.
Fig.2. Simulation setup for NRZ-to-RZ modulation format conversion
IV. PERFORMANCE OF FOUR-WAVE-MIXING (FWM) AND RESULTS
Fig.3 (a) .spectra before HNLF, (b) spectra after HNLF
Fig.3 (a) Show the measured spectra before highly nonlinear fiber (HNLF). Original NRZ signals at 1548.6nm and 1549 nm
and pump signal at 1551nm.Fig.3 (b) Show the spectra after HNLF. The pump and signals are act like AND gate switch function by
FWM and filter at 1553nm and 1553.4nm with 0.2nm bandwidth by optical band-pass filter.
Table1. Parameter values used in Simulation
NRZ-OOK to RZ-OOK format converter by using Four-wave Mixing (FWM)
Signal 1
Signal 2
Pump
EDFA
HNLF
BFP 1
BFP 2
Wavelength
Laser Power
Wavelength
1548.6nm
-10dB
1549nm
Laser Power
Wavelength
Laser Power
Gain
-10dB
1551nm
10dB
13dB
Lengths
0.5km
Attenuation
Dispersion
Filter Frequency
Filter Bandwidth
Filter Frequency
Filter Bandwidth
0.2dB/km
-0.08ps/nm/km
1553.4nm
0.2nm
1553nm
0.2nm
Simulation parameter for NRZ-to-RZ modulation format conversion for four-wave-mixing (FWM), Show in Table 1
Fig 4 measured waveform of NRZ-to-RZ (a) input NRZ signal at 1548.6nm, (b) input NRZ signal at 1549nm, (c) filtered RZ signal at
1553nm, (d) filtered RZ signal at 1553.4nm
Fig 4.(a) and (b), Show the waveform of the input NRZ signals. To get NRZ signals, we modulated at 10-Gb/s by a Mach-Zehnder
modulator (MZM) with pseudorandom bit sequences (PRBS). Fig 4.(c) and (d), Show the waveform of the output RZ signals after
filtered by optical band-pass filter (BPF) at 1553 nm and 1553.4nm, respectively.
Fig 5, Eye diagram for (a) input NRZ signal, (b) converted RZ signal at 1553nm, (c) converted RZ signal at 1553.4nm
Figure 5 (a), (b) and (c), show the eye diagram of the NRZ-to-RZ modulation format conversion at 10 Gb/s by four-wave-mixing
(FWM) in highly nonlinear fiber (HNLF).The power of these three signals must be same level to compare each other. The greater the
Q-factor, the better the system is .The eye diagram of the two output RZ signal get better result than Back-to-Back NRZ signal
because of increased eye performance.
Fig 6, measured BER of the converted RZ signal and back-to-back NRZ signal by using four-wave-mixing (FWM) in highly nonlinear
fiber (HNLF)
Figure 6.Show the bit-error-rate (BER) measurement of the four-wave-mixing in highly nonlinear fiber (HNLF).The receiver
sensitivity of the converted RZ signals at the BER of 10−9 have the negative power penalty of 1.8dB for first channel and 2dB for
second channel, respectively.
V.
CONCLUSION
In optical networks, different modulation formats are chosen depending on the network size and bit rate. In this research, NRZ-to-RZ
modulation format conversion is mainly based on nonlinear effect such as four-wave-mixing (FWM) in highly nonlinear fiber. Result
RZ signal by using FWM gets better performance than original NRZ signal when tested in BER analyser.In this research, OOK
modulation format conversion (NRZ-to-RZ) has fully illustrated with simulation results with the help of Optisystem software.
ACKNOWLEDGEMENT
The author would like to express special thanks to his supervisor for his valuable suggestion and sharing their experience to write
this research. And also, the author is also thankful to the members and all of his teachers from Department of Electronic Engineering,
Mandalay Technological University.
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