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28 Gb/s duobinary signal transmission over 40
km based on 10 GHz DML and PIN for 100 Gb/s
PON
Zhengxuan Li, Lilin Yi,* Xiaodong Wang, and Weisheng Hu
State Key Lab of Advanced Optical Communication Systems and Networks, Shanghai Jiao Tong University,
Department of Electronic Engineering, Shanghai 200240, China
*lilinyi@sjtu.edu.cn
Abstract: In this paper, we demonstrate the direct modulation and direct
detection of 28-Gb/s duobinary signal for the future downstream capacity
upgrade in next generation passive optical network (PON). Commercial 10-
GHz directly modulated laser (DML) and PIN with a combined modulation
bandwidth of ~7 GHz are used as transmitter and receiver respectively. In
order to mitigate the chromatic dispersion induced signal distortion, an
optical delay interferometer (DI) is employed to narrow down the signal
spectrum, thereby realizing 40-km single mode fiber (SMF) transmission in
C-band. Besides, the chirp-induced spectral broadening of the directly
modulated signal enables a higher launch power than external modulation
schemes, which increases the loss budget of the system. As a result, 31-dB
loss budget is achieved, supporting 64 users with 40-km reach. Also, as the
transceivers in both optical line terminal (OLT) and optical network unit
(ONU) are commercial l0-GHz devices, the proposed scheme is compatible
with 40-Gb/s time and wavelength division multiplexing passive optical
network (TWDM-PON) systems, providing a cost-efficient alternative for
the development of 100G PON.
©2015 Optical Society of America
OCIS codes: (060.2330) Fiber optics communications; (060.2360) Fiber optics links and
subsystems.
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Received 28 Apr 2015; revised 14 Jul 2015; accepted 15 Jul 2015; published 27 Jul 2015
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1. Introduction
Time and wavelength division multiplexing passive optical networks (TWDM-PON) has been
admitted by both researchers and industries as the most promising solution for the
construction of next generation PON stage 2 (NG-PON2). Based on the wavelength stacking
technique, TWDM-PON can reuse the optical distribution network (ODN) and some of the
already mature techniques in the current 10-Gb/s PON systems, thereby enabling a smooth
and efficient upgrade. During the past few years, a lot of proposals on 40-Gb/s TWDM-PON
have been presented for discussion [1–3], which promoted the standardization and
commercialization process of NG-PON2. However, the 40-Gb/s capacity would not provide a
long-term satisfaction for the desire of users, so researches for 100G-EPON come out right on
the heels of NG-PON2. In order to relax the bandwidth requirement on devices, advanced
modulation formats with higher spectral efficiency are required to realize high data rate
modulation and detection on bandwidth-limited transceivers. Hongguang Zhang et al. have
demonstrated 30-km transmission of 25-Gb/s four-level pulse amplitude (4-PAM) signal
based on 10-Gbps transmitter optical subassembly (TOSA) and receiver optical subassembly
(ROSA) [4], where electrical equalizer is required in the receiving side to equalize the
frequency response and compensate the chromatic dispersion during fiber transmission.
Yuanbao Luo et al. presented a double sideband (DSB) orthogonal frequency division
multiplexing (OFDM) based symmetric 100-Gb/s TWDM-PON, where 26.7-km fiber
transmission is realized by using Mach-Zehnder modulator (MZM) and digital signal
processing (DSP) in both optical line terminal (OLT) and optical network unit (ONU) sides
[5]. Duobinary format has also been proposed by using a bandwidth-limited device either at
the transmitter [6] or receiver side [7], where the high data rate signals are required to be
#239820
Received 28 Apr 2015; revised 14 Jul 2015; accepted 15 Jul 2015; published 27 Jul 2015
© 2015 OSA
10 Aug 2015 | Vol. 23, No. 16 | DOI:10.1364/OE.23.020249 | OPTICS EXPRESS 20250
externally modulated or transmitted in O-band to avoid the chromatic dispersion induced
signal distortion.
In this paper, we demonstrate 40-km downstream transmission of 28-Gb/s duobinary
signal in C-band using transmitter and receiver both operating at 10-GHz bandwidth. Instead
of external modulation, the signal is directly modulated on a distributed feedback (DFB) laser.
Taking advantage of the low pass filtering characteristic of the directly modulated laser
(DML) and the PIN detector, the 28-Gb/s on-off-keying (OOK) signal is converted into
duobinary format at the receiving side. And in order to mitigate the signal distortion induced
by the interaction between frequency chirp of the DML and the chromatic dispersion in fiber,
an optical delay interferometer (DI) is employed to reshape the signal spectrum [8, 9], thereby
realizing 40-km single mode fiber (SMF) transmission in C-band with only 1.2-dB penalty
compared to back-to-back (BtB) case. Moreover, due to the chirp-induced spectral
broadening, the directly modulated signal shows a high tolerance to fiber nonlinearity, and no
distortion is observed until the launch power exceeds 16 dBm. Finally, 31-dB loss budget is
achieved, which could support 64 users within 40-km reach.
2. 10-GHz devices based OOK-Duobinary format conversion
Duobinary format has been discussed a lot during the construction of 100G Ethernet
networks. As a partial-response signal, duobinary format has a narrower bandwidth compared
to binary format, making it more suitable for high speed modulation and transmission.
Generally, the duobinary signal is generated by low-pass filtering or delay-and-add filtering of
a pre-encoded data sequence to obtain three-level pulses [10, 11]. The 3-dB bandwidth of the
low-pass filter should be 0.25 times of the signal bit rate to realize an effective format
conversion. The green curve in Fig. 1 depicts the frequency response of the 5th order Bessel
low-pass filter required for generating 28-Gb/s duobinary signal, which has a 3-dB bandwidth
of ~7 GHz. Inset (i) is the corresponding eye diagram of the filtered signal. On the other hand,
using bandwidth-limited devices for data modulation or detection can also convert the binary
signal to three levels [6, 7]. Fig. 1 shows the frequency responses of a 10-GHz directly
modulated DFB laser (XGT-9005A-P08-C1) with a 30-GHz PIN and a 10-GHz PIN
(Conquer-KG-PR-10G) with a 20-GHz MZM measured by a vector network analyzer. The
combined frequency response of the 10-GHz DFB and 10-GHz PIN is also depicted, which
well matches with the response of the ideal Bessel low-pass filter within the 0-10 GHz
frequency region. Therefore, when we modulate 28-Gb/s OOK signal with the 10-GHz DML
and then detect it with the 10-GHz PIN, the received signal can be successfully converted into
duobinary format, as inset (ii) shows. After detection, the received duobinary signal can be
decoded back to binary sequence by a real-time decision circuit [12] or by analog to digital
converter (ADC) with DSP. In this way, 28-Gb/s direct modulation and direct detection can
be realized using the same transmitters and receivers as in the 4 × 10-Gb/s systems [13] and
no high-frequency components are required, providing a cost-effective solution for the further
capacity upgrade of next generation PON systems. In the following section, we will further
investigate the fiber transmission and bit error ratio (BER) performance of the proposed
scheme by experiment.
#239820
Received 28 Apr 2015; revised 14 Jul 2015; accepted 15 Jul 2015; published 27 Jul 2015
© 2015 OSA
10 Aug 2015 | Vol. 23, No. 16 | DOI:10.1364/OE.23.020249 | OPTICS EXPRESS 20251
Fig. 1. Frequency response and the corresponding eye diagram of the 10-GHz DML combined
with the 10-GHz PIN
3. Proposed 100-Gb/s TWDM-PON scheme
Fig. 2. Proposed 100-Gb/s TWDM-PON system. CMD: chirp management device. PS: power
splitter; TOF: tunable optical filter.
Figure 2 depicts the network architecture of the proposed 100-Gb/s TWDM-PON system. In
the OLT side, 4 DMLs operating at different wavelengths are used as transmitters. Note that
the data rate on each channel is 28 Gb/s instead of 25 Gb/s because some overheads should be
reserved for signal processing such as forward error correction (FEC) etc. The outputs of
DMLs are combined by an optical multiplexer (MUX). Then an erbium doped fiber amplifier
(EDFA) boosts the signal power before being launched into the fiber for loss budget
improvement. After fiber transmission, the signal is distributed to all ONUs by a power
splitter. The tunable optical filter (TOF) in each ONU selects the specified wavelength and
then launches the signal into the PIN for detection. Finally, the received electrical signal in
duobinary format is demodulated back to binary signal by DSP. The coarse wavelength
division multiplexers (CWDMs) in the network are used for separating upstream and
downstream wavelengths. In this scheme, as DML is used for transmitting 28-Gb/s signal, a
chirp management device (CMD) is required to eliminate the chromatic dispersion induced
signal distortion during fiber transmission.
As for the upstream link, due to the traffic asymmetry, the data rate requirement is
expected to be lower than the downstream link. Therefore we suppose the 10-Gbps techniques
#239820
Received 28 Apr 2015; revised 14 Jul 2015; accepted 15 Jul 2015; published 27 Jul 2015
© 2015 OSA
10 Aug 2015 | Vol. 23, No. 16 | DOI:10.1364/OE.23.020249 | OPTICS EXPRESS 20252
are sufficient [14, 15]. Therefore, we just focus on the downstream link in the following
experimental demonstrations and performance investigations.
4. Experimental setup and results
Fig. 3. Experimental setup
The experimental setup is shown in Fig. 3. Only a single channel is demonstrated for the
proof of concept. In order to avoid error propagation, the original binary data are pre-encoded
to remove the correlations between the received bits. The encoded data is imported into a
pulse pattern generator (PPG) operating at 28 Gb/s to get the OOK signal output. Then the
signal is modulated onto the commercially available DML operating at 10 GHz. The output
wavelength of the DML is ~1543 nm under room temperature, and it can be thermally tuned
within ~3 nm range. Before being launched into the fiber, the signal passes through a DI
based chirp management device for chirp elimination, and an EDFA follows behind to boost
the launch power. After 40-km SMF transmission, the signal is attenuated by a variable
optical attenuator (VOA), which emulates the function of power splitter in the ODN and
varies the optical power for bit-rate error (BER) testing. Then the signal is launched into the
receiver for detection. As previously illustrated in section 2, the receiver in the ONU is a 10-
GHz PIN. And the combination of the transmitter and receiver gives a 3-dB frequency
response bandwidth of ~7 GHz, as depicted in Fig. 1. We can see from the eye diagrams in
Fig. 3 that the original OOK signal is converted into duobinary format by the bandwidth-
limited modulation at the transmission side, while the low-pass filtering property of the PIN at
the receiving side further closes the middle eye. The received duobinary signal is sampled by
digital storage oscilloscope (DSO) with 80-GS/s real-time sample rate. The 3-level to 2-level
signal demodulation and BER calculation are realized by off-line DSP in Matlab. Note that
the off-line DSP is required just for level determining and BER calculating, and no signal
recovery module such as equalization, dispersion compensation is applied. Therefore the
function of ADC together with the off-line DSP equals to a real-time duobinary signal
detection circuit [16], which is not available in our lab at present. As duobinary format has
been discussed a lot during the development of 100G Ethernet, the related techniques are
already quite mature [17, 18]. Note that due to the high data rate of the signal, the detection
circuit requires fast electronics, and the same problem exists for PAM-4 format, too. The
decision for OOK format is simpler but the bandwidth requirement on devices is higher,
which also increases the cost. Therefore, a comprehensive comparison between these formats
should be made for an optimal choice.
In the proposed scheme, the chirp management device is a key component to enable the
distortion-free transmission of directly modulated signal. Generally, direct modulation results
in strong frequency chirp, which interacts with the chromatic dispersion of the fiber to change
#239820
Received 28 Apr 2015; revised 14 Jul 2015; accepted 15 Jul 2015; published 27 Jul 2015
© 2015 OSA
10 Aug 2015 | Vol. 23, No. 16 | DOI:10.1364/OE.23.020249 | OPTICS EXPRESS 20253
the pulse shape of the signal during transmission. As a result, the signal is severely distorted
after long distance fiber transmission. To alleviate the influence, dispersion compensating
techniques should be used, such as dispersion pre-compensation [19], electronic equalization
[20], etc. Considering the electronic bottleneck, optical processing is preferred for high data
rate signal processing. In our scheme, a DI is employed as the optical chirp management
device. Figure 4 depicts the transmission property of the DI, which performs as a notch filter
for signal spectral reshaping. When we position the central frequency of the DML on the
falling edge of the filter, the signal spectrum will be narrowed and the long-wavelength
spectral components will be reduced. It has been verified that after spectral reshaping, the
frequency profile of the bit sequences will become flat-topped, thereby creating nearly chirp-
free pulses [21]. In this case, the signal will have a higher tolerance to chromatic dispersion.
Experimental results show that after 20-km and 40-km transmission, almost no signal
distortion is observed as Figs. 5(d) and 5(f) show, proving the feasibility of the spectral
reshaping based chirp management solution for high bit rate directly-modulated duobinary
signals. We also investigate the requirement on the transmission property of the DI. It turns
out that only when the central frequency of the signal spectrum is aligned with the falling
edge of the filter, the dispersion induced signal distortion is eliminated, meaning that the chirp
management performance is quite sensitive to wavelength drifts just like in 10-Gb/s situations
[22, 23]. Experimental results show that the sensitivity penalty is lower than 1-dB when the
frequency offset drifts within ± 1.5 GHz. But ~3-dB penalty is observed when the frequency
deviation is ± 3 GHz. Therefore, the central frequency of the DML and DI should be precisely
controlled, or a feedback circuit is required to keep the wavelength offset stable as in the
commercial chirp-managed laser (CML) design. In spite of this, the requirement on the free-
spectral range (FSR) of the DI is proved to be quite loose, because similar performances are
obtained when we vary the FSR from 0.4 nm to 2 nm. In this way, as long as the FSR of the
DI is set as an integral multiple of the channel space, multi-channel chirp management can be
realized by a single device.
Fig. 4. Optical spectrum of the duobinary signal before and after chirp management
#239820
Received 28 Apr 2015; revised 14 Jul 2015; accepted 15 Jul 2015; published 27 Jul 2015
© 2015 OSA
10 Aug 2015 | Vol. 23, No. 16 | DOI:10.1364/OE.23.020249 | OPTICS EXPRESS 20254
Fig. 5. Eye diagrams measurment
We have demonstrated in our previous work that due to the chirp induced spectral
broadening, the directly modulated binary signal has a high tolerance to fiber nonlinearity,
thereby enabling a high launch power [24]. Similarly, for the three-level duobinary signal, no
distortion is observed until the launch power exceeds 16 dBm. Then we evaluate the BER
performance of the received duobinary signal. After the decision and decoding processes, the
signal is converted from duobinary into binary format. And by comparing with the original
sequence, the BER of the received signal is obtained. We vary the attenuation values of the
VOA and measure the BER in each case respectively. The calculated BER curves are shown
in Fig. 6. The sensitivity in BtB case is about −16.3 dBm when we take the FEC threshold of
3.8 × 10−3 for evaluation. 20-km fiber transmission introduces ~1-dB penalty, resulting in a
sensitivity of ~-15 dBm. And the signal experiences an extra 0.2-dB penalty when the
transmission distance is increased to 40 km. Taking the 16-dBm launch power into
consideration, the proposed scheme has 31-dB loss budget, which could support 1:64 splitting
ratio with 40-km fiber reach. Note that the receiving power refers to the power launched into
the PIN. For multi-channel applications, a TOF with ~3-dB insertion loss is required to be
considered, thereby reducing the loss budget by ~3 dB. Besides, the launch power is generally
limited because of the relevant eye-safety regulations, which also restricted the loss budget of
the system. To solve this problem, we can replace the PIN with an APD for sensitivity
improvement. Otherwise, using a semiconductor optical amplifier (SOA) for pre-
amplification has also been proposed as a potential solution for increasing receiving
sensitivity [25]. But for some special scenarios that require a higher loss budget, the high
nonlinear tolerance property of directly modulated signal will be a significant advantage
providing that some protection techniques are introduced.
Fig. 6. Measured BER curve
#239820
Received 28 Apr 2015; revised 14 Jul 2015; accepted 15 Jul 2015; published 27 Jul 2015
© 2015 OSA
10 Aug 2015 | Vol. 23, No. 16 | DOI:10.1364/OE.23.020249 | OPTICS EXPRESS 20255
By the experimental demonstration, we have verified the feasibility of using DML as
transmitter for 28-Gb/s data modulation. Experimental results demonstrate that thanks to the
spectral reshaping function of the DI, the directly modulated signal shows a chromatic
dispersion tolerance even better than the external modulated optical duobinaty signal. Only
1.2-dB sensitivity penalty is observed for 40-km fiber transmission in our experiment while
the penalty for external modulated optical duobinary signal is 5 dB [26]. Thus, the
combination of DML and spectral reshaping filter provides a potential solution for the
capacity upgrade in future PON systems.
5. Conclusions
In this paper, we demonstrate the direct modulation and direct detection with 40-km fiber
transmission of 28-Gb/s signal. Taking advantage of the bandwidth-limited characteristic of
the devices, the 28-Gb/s OOK signal is converted into duobinary format when a commercial
10-GHz transmitter and 10-GHz receiver are used for modulation and detection. To eliminate
the strong frequency chirp originated from direct modulation, we use a DI to reshape the
spectrum of the signal. As a result, almost chirp-free signal is obtained with only 1.2-dB
penalty after 40-km fiber transmission. Also, due to the chirp-induced broad spectrum
characteristic of directly modulated signal, the launch power can be as high as 16 dBm. The
loss budget of the system is evaluated to be 31 dB, which could support 64 users within 40-
km reach. The proposed scheme provides a low-cost solution for a smooth capacity upgrade
of next generation PON systems.
Acknowledgments
This work was supported by National Basic Research Program of China (2012CB315602),
National Natural Science Foundation of China (61322507 and 61132004) and Program of
Excellent PhD in China (201155)
#239820
Received 28 Apr 2015; revised 14 Jul 2015; accepted 15 Jul 2015; published 27 Jul 2015
© 2015 OSA
10 Aug 2015 | Vol. 23, No. 16 | DOI:10.1364/OE.23.020249 | OPTICS EXPRESS 20256
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