WDM-to-OTDM Conversion Using Supercontinuum Generation in a Highly Nonlinear Fiber

Abstract

We demonstrated an all-optical wavelength-division-multiplexing (WDM)-to-optical-time-division-multiplexing (OTDM) conversion using spectral broadening of super continuum (SC) generation in a highly nonlinear fiber. Four 10-Gb/s WDM channels were injected into time delay blocks for controlling the timing among them, and converted to OTDM channel by the SC-based multiplexing. The SC generation had advantages of eliminated the use of any pump in the WDM-to-OTDM conversion. All OTDM demultiplexed optical signals achieved error-free performance with <2-dB power penalty at ({rm BER} = 10^{-9}) compared with the back-to-back signal.

Full text

1882 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 26, NO. 18, SEPTEMBER 15, 2014
WDM-to-OTDM Conversion Using Supercontinuum
Generation in a Highly Nonlinear Fiber
Quang Nguyen-The, Member, IEEE, Motoharu Matsuura, Senior Member, IEEE,
and Naoto Kishi, Senior Member, IEEE
Abstract— We demonstrated an all-optical wavelength-
division-multiplexing (WDM)-to-optical-time-division-multiplex-
ing (OTDM) conversion using spectral broadening of super
continuum (SC) generation in a highly nonlinear fiber. Four
10-Gb/s WDM channels were injected into time delay blocks
for controlling the timing among them, and converted to OTDM
channel by the SC-based multiplexing. The SC generation had
advantages of eliminated the use of any pump in the WDM-
to-OTDM conversion. All OTDM demultiplexed optical signals
achieved error-free performance with <2-dB power penalty at
BER =10−9compared with the back-to-back signal.
Index Terms—Optical fiber communication, wavelength-
division multiplexing, optical time division multiplexing, optical
signal processing, optical Kerr effect.
I. INTRODUCTION
OPTICAL fiber communication systems are characterized
by their extremely high transmission capacity, which
are based on optical time-division-multiplexing (OTDM)
and wavelength-division-multiplexing (WDM) with their own
advantages [1]–[3]. In addition, it is necessary to develop
all-optical signal processing, which replaces optical-electrical-
optical (OEO) conversion with the electronic speed bottleneck
in high-speed WDM and OTDM networks. To provide such
a WDM/OTDM network, the all-optical conversion between
WDM and OTDM signal formats become an important trans-
multiplexing operation at photonic gateways of that networks
[4]–[6]. In all-optical WDM-to-OTDM conversion, lower data
rate signals at different wavelengths from the end users of
access would be aggregated into high speed OTDM data
stream for the backbone or core networks [7]. In combination
with all-optical OTDM-to-WDM conversion [8], [9], a solution
of all-optical processing for interconnection at the photonic
gateway nodes of the WDM/OTDM networks would be com-
pletely provided.
So far, there have been many reports demonstrating
all-optical WDM-to-OTDM conversion using an electroab-
Manuscript received January 7, 2014; revised May 14, 2014; accepted
July 13, 2014. Date of publication July 17, 2014; date of current version
August 28, 2014. This work was supported by JSPS KAKENHI under Grant
24360148.
Q. Nguyen-The and N. Kishi are with the Department of Information and
Communication Engineering, University of Electro-Communications, Tokyo
182-8585, Japan (e-mail: thequang@uec.ac.jp; kishinaoto@uec.ac.jp).
M. Matsuura is with the Center for Frontier Science and Engineer-
ing, University of Electro-Communications, Tokyo 182-8585, Japan (e-mail:
m.matsuura@uec.ac.jp).
Color versions of one or more of the figures in this letter are available
online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LPT.2014.2339932
sorption modulator (EAM) [10], cross-gain modulation
(XGM) effect in semiconductor optical amplifiers (SOAs) [11],
a Mach-Zehnder interferometer wavelength converter based
on SOAs (MZI-SOAs) [12] with the advantage of small
device size as well as the possibility of integration with
the other devices. However, the conversion using EAM or
SOA technology has a limited frequency response. Recently,
highly nonlinear fiber (HNLF) has attracted much attention for
performing optical signal processing [13]. Therefore, another
approach is to use the different nonlinearities in the HNLF
such as four-wave mixing (FWM) [14], and cross-phase mod-
ulation (XPM) [15] for WDM-to-OTDM conversion. The use
of nonlinear optical loop mirror (NOLM) in conjunction with
a hybrid mode-locked laser (HML) [16], time-domain optical
Fourier transformation (OFT) [17] for WDM-to-OTDM con-
version has also been reported. Recently, we have successfully
achieved the conversion of 4×10 Gb/s WDM channels to one
40 Gb/s OTDM channel by using Raman compression [18].
However, due to the nature of the techniques in the previous
demonstrations, additional pump signal was required for the
conversion.
In this letter, we proposed and experimentally demonstrated
the conversion of 4 ×10 Gb/s WDM channels to one 40 Gb/s
OTDM channel by using supercontinuum (SC) generation in
a HNLF. The SC generation had advantages of eliminated the
use of any pump in the WDM-to-OTDM conversion. There-
fore, the straightforward techniques by using SC generation
for WDM-to-OTDM conversions would decrease cost, size of
the WDM/OTDM network. Four WDM channels were time
interleaved in the delay blocks with appropriate time delays
between each channel before injecting into the HNLF for the
multiplexing process by using SC generation and filtering. The
converted 40 Gb/s signal was demultiplexed to 10 Gb/s for bit
error rate (BER) measurement. Less than 2-dB power penalty
at BER =10−9from the back-to-back was obtained for all
demultiplexed channels.
II. OPERATION PRINCIPLE
The operation principle of the proposed all-optical
WDM-to-OTDM conversion using the SC generation is shown
in Fig. 1. WDM return-to-zero (RZ) data signals at rate
B=N×R(Nchannels at base rate R) configured with N
channels of wavelengths λ1,λ2,…, λNare generated by using
conventional amplitude modulators such as EAM and LiNbO3
modulators. However, those modulators generate around 20 ps
for 10 GHz amplitude modulators, which are not short enough
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NGUYEN-THE et al.: WDM-TO-OTDM CONVERSION USING SUPERCONTINUUM GENERATION 1883
Fig. 1. Operation principle of the proposed scheme.
for the formation of ultrahigh-speed OTDM data streams.
WDM RZ data signals are sent to a delay block to control the
delay of each channel by t for WDM-to-OTDM conversion.
The delay block is built by means of tunable optical delays and
coupled into one output fiber. Insets 1 and 2 in Fig. 1 describe
the formation of the WDM RZ data signals that are temporary
shifted relative to each other by the delay block. These WDM
RZ data signals are sent to the SC generation for broadening
their spectra due to self-phase modulation (SPM). In a medium
with Kerr nonlinearity, after propagation over the distance z,
the maximum SPM-induced spectral broadening of the signal
pulse can then be estimated as [19]
ω =
ω
cn2zI0
τ
where n2is the nonlinear refractive-index coefficient at the
frequency ω,I0is the peak intensity of the input signal pulse,
and τis the pulse width. It can be seen from expression (1)
that the maximum SPM-induced spectral broadening depends
on the peak intensity of the pulse I0and the pulse width τ.
When the WDM RZ data signals are injected into the SC gen-
eration, each of them gets spectrally broadened. Therefore,
the overlapped portion of all WDM RZ data signals can be
achieved by adjusting the power levels of the data signals.
Inset 3 in Fig. 1 shows conceptual overlapped spectra after the
SC generation. These overlapped spectra then are filtered by
the optical filters. In this way, a converted OTDM data signal
at rate B=N×Rwith wavelength of λ0, as shown in inset 4
in Fig.1, can be achieved at the output of the optical filters.
III. EXPERIMENTAL SETUP
Figure 2 shows the experimental setup of 4×10 Gb/s WDM
channels to one 40 Gb/s OTDM channel conversion using
SC generation. A combined four 10 Gb/s WDM nonreturn-to-
zero (NRZ) data signals at wavelength of 1554.94 nm (ch1),
1556.55 nm (ch2), 1558.17 nm (ch3), and 1559.79 nm (ch4)
with channel spacing of 1.6 nm (200 GHz) were produced by
using four external cavity laser diodes (ECLs) and a LiNbO3
Fig. 2. Experimental setup for 4 ×10 Gb/s WDM channels to one
40 Gb/s OTDM channel conversion. ECL: external-cavity laser-diode, LNM:
LiNbO3modulator, EAM: electroabsorption modulation, PPG: pulse pattern
generator, EDFA: erbium-doped fiber amplifier, WDM-PTC: WDM power
and time controller, AWG: array waveguide grating, VOA: variable optical
attenuator, TDL: tunable delay line, HNLF: highly nonlinear fiber, OBPF:
optical bandpass filter, DEMUX: demultiplexing.
modulator (LNM) driven by electrical NRZ data from a
pulse pattern generator (PPG). These WDM NRZ data signals
were amplified by an erbium-doped fiber amplifier (EDFA),
and converted to a combined four 10 Gb/s WDM return-
to-zero (RZ) data signals by utilizing an EAM. An EDFA
was used at the output of the EAM to compensate for the
EAM insertion loss. The WDM channels were manually
synchronized. Before injecting into a SC-based multiplexing,
it is important to adjust the power levels of the WDM RZ data
signals for getting the overlapped spectra of all data signals in
the frequency. In addition, delay blocks are also required to
control the timing among the WDM RZ data signals. To fulfill

1884 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 26, NO. 18, SEPTEMBER 15, 2014
Fig. 3. Optical spectra of the 4 ×10 Gb/s WDM RZ data signals at the
(a) input and (b) output of the HNLF.
these two requirements, we constructed a WDM power and
time controller (WDM-PTC), which consisted of an arrayed
waveguide grating (AWG), variable optical attenuators (VOAs)
in series with tunable delay lines (TDLs), and a coupler. At the
output of the WDM-PTC, the pulsewidth of the combined
four 10 Gb/s WDM RZ data signal was approximately 20 ps.
These WDM RZ data signals, which are temporary shifted
relative to each other by the WDM-PTC were amplified by
an EDFA, then injected into the SC-based multiplexing for
WDM-to-OTDM conversion. The total power of the combined
four 10 Gb/s WDM RZ data signals before injecting into the
SC-based multiplexing was set to 21.5 dBm.
The SC-based multiplexing was based on SC generation,
which contained 2 km of HNLF and optical bandpass filters
(OBPFs). The HNLF had the second-order dispersion of
−2.2 ps/nm/km and third-order dispersion of 0.032 ps/nm2/km
at 1550 nm, loss of 0.55 dB/km, nonlinear coefficient of
12.6 W−1·km−1, effective area of 11 μm2. The spectra
of the all four data channels at the output of HNLF were
broadened over the SC generation. By adjusting the power
levels of the WDM RZ data signals, the overlapped spectra
of all data signals in the frequency could be achieved. These
overlapped spectra were filtered by two 3-nm OBPFs. In this
way, conversion to a 40 Gb/s OTDM data signal was achieved
at the output of the SC-based multiplexing. The converted
40 Gb/s OTDM data signal was demultiplexed to 10 Gb/s
base rate by using a FWM-based demultiplexing. A spectrum
analyzer and 30 GHz-bandwidth digital sampling oscilloscope
were used to observe the spectrum and eye pattern of the
signals, respectively. The demultiplexed signals were also sent
to a 10 Gb/s receiver for BER measurement.
Fig. 4. (a) Output spectra of the converted 40 Gb/s OTDM data signal at the
output of SC-based multiplexing and (b) corresponding eye pattern captured
by a 30 GHz-bandwidth digital sampling oscilloscope (20 ps/div.).
IV. EXPERIMENTAL RESULTS AND DISCUSSION
Figure 3(a) and (b) show optical spectra of 4×10 Gb/s
WDM RZ data signals at the input and output of the HNLF,
respectively. Compared to the input signals, the spectra of
the all data channels at the output were broadened over the
SC generation. Therefore, the overlapped portion of the spectra
of all data channels can be obtained. The 40 GHz tones in the
spectra after the SC generation at the output of the HNLF
show the successful converting to 40 Gb/s OTDM data signal.
The output spectra and eye pattern of 40 Gb/s OTDM
data signal at the output of SC-based multiplexing stage
after filtering at ∼1557.3 nm are shown in Fig. 4(a) and
(b), respectively. Clear eye-opening in Fig. 4(b) shows that
the present system successfully performs the conversion of
4×10 Gb/s WDM channels to one 40 Gb/s.
Figure 5(a) shows the BER performance of all demulti-
plexed channels. We obtained the error free operation for
all channels. Compared to the back-to-back signal, less than
2-dB power penalty at BER =10−9was obtained for
all demultiplexed channels. The penalty variation among all
channels was obtained to be less than 1-dB. Here the input
WDM channels overlapped each others in the time, causing
crosstalk effects between adjacent channels and increasing
signal distortions. However, the 2R Mamyshev regenerator
[20] can improve the degraded signals significantly. The inset
in Fig. 5(a) shows the eye pattern of 10 Gb/s back-to-back
signal for one of the original 10 Gb/s RZ channels. The eye
patterns of all demultiplexed channels are shown in Fig. 5(b).
All eye patterns show clear and open eyes, which indicates

NGUYEN-THE et al.: WDM-TO-OTDM CONVERSION USING SUPERCONTINUUM GENERATION 1885
Fig. 5. (a) Bit-error rate (BER) measurements of demultiplexed channels;
inset is eye pattern of 10 Gb/s back-to-back signal for one of the original
10 Gb/s RZ channels, and (b) eye patterns of all demultiplexed channels
(20 ps/div.).
good performance of the WDM-to-OTDM conversion for all
channels.
In this work, input WDM channels with pulsewidth of 20 ps
were converted to 6 ps OTDM channel. Therefore, the obtained
pulsewidth is suitable for 40 Gb/s OTDM transmission. Two
and four channels could be used for conversion of 2×20 Gb/s
WDM channels and 4 ×10 Gb/s WDM channels to a 40 Gb/s
OTDM channel, respectively. Due to the limited of charac-
teristics of the HNLF with small nonlinear coefficient and
high third-order dispersion, the current demonstration faces
a challenge to be functional for higher OTDM bit-rates over
40 Gb/s. One possible solution is the use of the Raman
compression [18] or a suitable HNFL.
V. CONCLUSION
We have proposed and experimentally demonstrated
all-optical format conversion scheme from 4 ×10 Gb/s
WDM channels to one 40 Gb/s OTDM channel by using
SC-based multiplexing. Less than 2-dB power penalty at
BER =10−9was obtained all channels after the OTDM
demultiplexing in comparison with the 10 Gb/s back-to-
back signal. Small penalty variation among all demultiplexed
channels was achieved with less than 1-dB. The straightfor-
ward technique by using SC generation made our proposed
technique distinguished from the previous WDM-to-OTDM
conversion. In combination between the demonstrations in
OTDM-WDM conversion in Ref. [9] and WDM-to-OTDM
conversion in this letter can be done in transmultiplexer at
the photonic gateway nodes of the WDM/OTDM networks.
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