Direct-detection optical OFDM superchannel for
long-reach PON using pilot regeneration
Rong Hu,1 Qi Yang,1 Xiao Xiao,1 Tao Gui,2 Zhaohui Li,2 Ming Luo,1 Shaohua Yu,1 and
Shanhong You3,*
1
State Key Laboratory of Optical Communication Technologies and Networks, Wuhan Research Institute of Posts &
Telecommunications, Wuhan 430074, China
2
Institute of Photonics Technology, Jinan University, Guangzhou 510632, China
3
School of Electronic & Information Engineering, Soochow University, Suzhou 215006, China
* shyou@suda.edu.cn
Abstract: We demonstrate a novel long-reach PON downstream scheme
based on the regenerated pilot assisted direct-detection optical orthogonal
frequency division multiplexing (DDO-OFDM) superchannel transmission.
We use the optical comb source to form DDO-OFDM superchannel, and
reserve the center carrier as a seed pilot. The seed pilot is further tracked and
reused to generate multiple optical carriers at the local exchange. Each
regenerated pilot carrier is selected to beat with an adjacent OFDM sub-band
at ONU, so that the electrical bandwidth limitation can be much released
compared to the conventional DDO-OFDM superchannel detection. With the
proposed proof-of-concept architecture, we experimentally demonstrated a
116.7 Gb/s superchannel OFDM-PON system with transmission reach of
100 km, and 1:64 splitting ratio. We analyze the impact of
carrier-to-sideband power ratio (CSPR) on system performance. The
experiment result shows that, 5 dB power margin is still remained at ONU
using such technique.
©2013 Optical Society of America
OCIS codes: (060.2330) Fiber optics communications; (060.4250) Networks.
References and links
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#195576 - $15.00 USD Received 12 Aug 2013; revised 20 Sep 2013; accepted 11 Oct 2013; published 28 Oct 2013
(C) 2013 OSA
4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026513 | OPTICS EXPRESS 26513
13. W. R. Peng, I. Morita, H. Takahashi, and T. Tsuritani, “Transmission of high-speed (>100 Gb/s) direct-detection
optical OFDM superchannel,” J. Lightwave Technol. 30(12), 2025–2034 (2012).
14. W. R. Peng, H. Takahashi, I. Morita, and H. Tanaka, “Transmission of a 213.7-Gb/s single-polarization
direct-detection optical OFDM superchannel over 720-km standard single mode fiber with EDFA-only
amplification,” in Proc. ECOC’10, paper., PDP2.5 (2010).
15. C. Xi, A. Li, D. Che, Q. Hu, Y. Wang, J. He, and W. Shieh, “High-speed fading-free direct detection for
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1. Introduction
The long-reach passive optical network (LR-PON) enables network operators and service
providers to deliver wide-bandwidth traffic to a vast number of end users at a low cost by
exploiting optical amplification in combination with wavelength division multiplexing
(WDM). The consolidation of metro/access networks reduces the number of active optical
network interfaces and elements in the field, thus minimizes network planning. This in turn
lowers capital expenditure (CAPEX) and operational expenditure (OPEX) of the integrated
network [1–3].
As a technique well employed in wired and wireless communications, orthogonal frequency
division multiplexing (OFDM) has recently attracted many research focuses in the optical
communications and access networks. The direct-detection optical OFDM (DDO-OFDM)
[4–8] is thought to be one of the most promising candidates for next-generation optical access
due to several advantages, such as low cost, resistance to dispersion, and modulation/spectra
flexibility. J. M. Tang et al. demonstrated a real-time 10.375 Gb/s optical OFDM-PON
upstream transmission with adaptive dynamic bandwidth allocation (DBA) and colorless
ONUs in IM/DD architectures [9]. D. Y. Qian et al. proposed and experimentally demonstrated
a single-side band 43.6-Gb/s downstream OFDMA-PON using 64/32/16QAM signals and
direct detection [10]. J.H. Yan et al. proposed a double-sided DDO-OFDM downstream
scheme, which transmitted an aggregated data rate of 120 Gb/s using less than 25 GHz
single-end receiver [11].
In this paper, we conduct a proof-of-concept experiment of a regenerated pilot assisted
DDO-OFDM superchannel LR-PON downstream system. We firstly employ the optical comb
source to form an OFDM superchannel loading with high data rate optical signal. Unlike the
works in [12–14], we reserve one carrier unloaded so that we can reuse it at local exchange to
generate multiple pilot sources. The pilot sources are then beat with corresponding modulated
sub-bands at ONUs. In such manners, the bandwidth requirement of the photo detectors (PDs)
at ONUs can be greatly reduced. We experimentally demonstrate a 116.7 Gb/s downstream
transmission of 100 km reach and 1:64 splitting ratio using the proposed scheme. More than 5
dB power margin is obtained at ONUs under an optimized carrier-to-sideband power ratio
(CSPR). Our demonstration shows that the proposed regenerated pilot assisted OFDM-PON
would be a feasible solution for the future LR-PON configuration.
2. Principle
Figure 1 illustrates the system architecture of the proposed DDO-OFDM superchannel
downstream scheme based on the optical comb regeneration technique. At OLT, a laser source
is firstly fed into the optical comb generator to produce n optical carriers (wavelength of
λ (i,.., n)) with frequency spacing of Δf. Then, an arrayed waveguide grating (AWG),
which is denoted as AWG1, is employed to separate the optical combs. Except for the central
carrier ( λc ), all the others are used to carry the OFDM signals. The central carrier ( λc ) is
reserved as the seed carrier for further comb regeneration. Such seed carrier and the modulated
#195576 - $15.00 USD Received 12 Aug 2013; revised 20 Sep 2013; accepted 11 Oct 2013; published 28 Oct 2013
(C) 2013 OSA
4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026513 | OPTICS EXPRESS 26514
optical OFDM signals are combined by AWG2, as shown in Fig. 1(a), and then sent to the local
exchange through the feeder fiber. Since the seed carrier ( λc ) is un-modulated, it could be
extracted and reused to produce the optical pilots using another comb generator at the local
exchange. Before the multi-band signal is relatively de-multiplexed into n channels by AWG3,
a polarization tracking is indispensable to align the polarization state of the seed carrier with the
following comb generator, and also maintain the polarization states of the regenerated carriers
the same as that of the modulated sub-bands. The comb regenerator is also driven with the same
radio frequency ( Δf. and produce n optical carriers with wavelength of λi (i = 2,..n +1) ). The
wavelength of the regenerated carrier is one carrier spacing shifted compared to the comb
source at OLT. The regenerated comb sources are separated by AWG4. Each comb source is
then coupled with a corresponding OFDM sub-band to satisfy the pilot assisted single side band
(SSB) DDO-OFDM configuration, as illustrated in Fig. 1(b). Supposed that the bandwidth of
each OFDM sub-band is B, the minimum electrical bandwidth requirement of each
photo-detector at ONUs is only 2·B [10, 15], as shown in Fig. 1(c). Each coupled pilot carrier
and sub-band is fed into a distribution path. Furthermore, each distribution path is split to
support multiple ONUs. The main advantages of such proposed scheme come from two
aspects: i) The superchannel configuration is used to carry high data rate signal; ii) Compared
to the conventional method [11], the electrical bandwidth limitation for each photo-detector is
only related to the bandwidth of each sub-band, instead of the entire superchannel. Based on the
proposed method, much more ONUs could be supported by one set of devices. Although the
cost of local exchange is increased slightly, the overall cost has been well reduced from the
aspect of CAPEX and OPEX. Additionally, the cost of devices in local exchange can be further
reduced by employing integration technology such as silicon photonics [16].
Local-Exchange
OLT
λ1
CW
OFDM
IQMn
a
AWG3
BPF
ONU1
coupler
…
BPF
…
λ1
…
λc
coupler
λn
λn
split
λn+1
ONUk
Comb Generator & AWG4
pilot-carrier
λ1 λ2
λn-1
λn
b
…
Polarization Tracker
λn
λn-1
…
OFDM
IQMn-1
ONUk
λc
…
λn-1
λ3
…
λc
λc
AWG2
…
…
Comb
Generator
& AWG1
λ2
OFDM
IQM2
split
coupler
λ2
…
λ2
λ2
λ1
OFDM
IQM1
…
λ1
ONU1
coupler
λ2
BPF
λn-1 λn
…
…
λc-1
λc λc+1
…
λ1
λ2
c
pilot-carrier
λn-1 λn
…
…
λc-1
λc λc+1
λn-1 λn
sub-band
BPF
λn+1
guard band
≥B
B
Fig. 1. System architecture of the proposed DDO-OFDM LR-PON downstream scheme. (a)
Optical spectrum of the generated DDO-OFDM superchannel; (b) coupling scheme for the
filtered pilot-carriers and sub-bands; (c) the coupled signal transmitted to ONUs. BPF: Band
Pass Filter.
3. Experimental setup
The experimental setup of the proposed 116.7 Gb/s DDO-OFDM superchannel transmission
for LR-PON is shown in Fig. 2. At OLT, an external cavity laser (ECL) with 100 kHz linewidth
operating at 1549.6 nm is firstly split into two tributaries by a 3 dB optical coupler. The upper
branch is fed into an optical phase modulator, which is driven by strong RF sine wave (~1.5 W)
#195576 - $15.00 USD Received 12 Aug 2013; revised 20 Sep 2013; accepted 11 Oct 2013; published 28 Oct 2013
(C) 2013 OSA
4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026513 | OPTICS EXPRESS 26515
at 15 GHz. A programmable wavelength selective switch (WSS) is then used to select and
reshape 6 optical carriers, while the central optical carrier is suppressed. All of the 6 optical
carriers are modulated with OFDM-QPSK signal using the optical IQ modulator (IQM). The
lower branch is directly coupled with the upper path via a variable optical attenuator (VOA).
The optical spectrum of the coupled signal is shown in Fig. 2(a). The OFDM-QPSK signal is
generated from a high speed digital-to-analog converter (DAC) of 10 GS/s sampling rate with a
resolution of 6 bits. The inverse fast Fourier transform (IFFT) size is 512, in which 498
subcarriers are filled with payload. Cyclic prefix with length of 1/32 FFT size per OFDM
symbol is adopted to avoid the inter-symbol interference. Each sub-band has a line rate
of:19.45 Gb/s. The data rate of the superchannel is 116.7-Gb/s.
OLT
15 GHz
OFDM TX
I
3 dB
ECL
WSS
PM
Q
IQM
EDFA
EDFA
PD
3 dB
Sub-Band
1 2 3
-40
Pilot-Carrier
4 5 6
-60
-80
c
1549.3
1549.8
1550.3
-40
-60
-80
1549.3 1549.8
Wavelength (nm)
1550.3
TOF
3 dB
WSS
Agilent
N7788 Pol.
Analyzer
Local Exchange
-10 b
-30
-50
-70
1548.8 1549.3 1549.8 1550.3
Wavelength (nm)
Wavelength (nm)
-20
1548.8
Intensity (dBm)
a
1548.8
Intensity (dBm)
c
Intensity (dBm)
Intensity (dBm)
0
PM
WSS
VOA
-20
80 km
b
20 km
Oscillator
Scope
Offline
processing
d
a
EDFA
VOA
ONU
3 dB
-10
d
-30
-50
-70
-90
1548.8
1549.3 1549.8 1550.3
Wavelength (nm)
Fig. 2. Experimental setup of the proposed 116.7-Gb/s DDO-OFDM superchannel transmission
for LR-PON. VOA: Variable Optical Attenuator; WSS: Wavelength Selective Switch; TOF:
Tunable Optical Filters; PM: Phase Modulator; IQM: IQ Modulator; ECL: External Cavity
Laser.
The feeder fiber between OLT and local exchange consists of a single span of 80 km
standard single mode fiber (SSMF). At the local exchange, an Agilent N7788 polarization
analyzer is used to track and align the polarization status of the seed pilot carrier with the
subsequent optical phase modulator to regenerate the pilot carriers. Currently, the polarization
tracking modules/devices have been well designed and commercialized supporting up to 140
krad/s polarization variation rate [17]. The optical signal is then split into two tributaries by a 3
dB optical coupler. In the upper path, a tunable optical filters (TOF) is used to filter the seed
#195576 - $15.00 USD Received 12 Aug 2013; revised 20 Sep 2013; accepted 11 Oct 2013; published 28 Oct 2013
(C) 2013 OSA
4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026513 | OPTICS EXPRESS 26516
carrier, and then feed it into another phase modulator (driven by the same 15 GHz RF frequency
source). The spectrum of the regenerated comb source, without any optical shaping, is shown in
Fig. 2(b). Two WSSs are used to flatten the regenerated carriers, and also perform the function
of the AWG3 and AWG4 as described in the Section II. The selected sub-band and pilot carrier
are coupled by the proposed criterion, as shown in Figs. 2(c) and 2(d), and then fed into the
distribution path. Note that all the devices at local exchange should be polarization maintained.
The distribution path consists of a 20 km SSMF and a VOA, in which VOA is used to
emulate the optical split. At the ONU, a single-end photo detector (U2T photo receiver,
XPRV2021) of 40 GHz bandwidth is used to detect the optical signal. The RF signal is then fed
into a Tektronix real-time scope, sampled at 50 GS/s, and processed off-line.
4. Results and discussion
Firstly, we conduct the back-to-back and 80 km transmission measurements, as shown in Fig.
3. The optimum Q factor in the back-to-back measurement is 15.6 dB at a CSPR of ~7 dB. After
80 km transmission, the optimum Q factor reaches 14.4 dB at a CSPR of ~6.5 dB. The launch
power into the feeder fiber is fixed at 0 dBm. Compared to back-to-back, there is about 1.2 dB
degradation for the optimum Q factor, which is mainly due to the increased amplified
spontaneous emission (ASE) noise from the EDFA. Additionally, the optimum CSPR after
transmission decreases slightly, which agrees with the results reported in [13]. Note that, the
CSPR in this paper is obviously greater than those in the reported papers [12, 13], because only
one sub-band is supported by each regenerated pilot carrier.
The distribution path is composed of a 20 km SSMF and a VOA. The VOA is set to 18 dB to
emulate the 1:64 split. Since no amplifier is allowed at the ONU side, it is crucial to find the
optimum launch power for the distribution path, which will determine the maximum achievable
optical power budget between local exchange and ONUs. The measurement is taken under
three different CSPRs (2, 7, and 12 dB), and the results are shown in Fig. 4. The maximum Q
factor is ~13.7 dB, which is achieved when CSPR is 7 dB and launch power is ~12.5 dBm. With
a lower CSPR of 2 dB, the optimum launch power is found to be similar and the optimum Q
factor has ~1 dB degradation relative to that of CSPR of 7 dB. A further increase in CSPR (i.e.
CSPR = 12 dB) would sacrifice the signal quality with a drastically enhanced optimum launch
power.
15
14
13
Scatter plot
2
1.5
12
1
0.5
Quadrature
Q Factor(dB)
16
0
back-to-back
80-km
-0.5
11
-1
-1.5
-2
-2
10
0
2
-1.5
-1
4
-0.5
0
0.5
In-Phase
1
6
1.5
2
8
10
12
14
CSPR (dB)
Fig. 3. The impact of CSPR on system performance for back-to-back and 80 km transmission.
#195576 - $15.00 USD Received 12 Aug 2013; revised 20 Sep 2013; accepted 11 Oct 2013; published 28 Oct 2013
(C) 2013 OSA
4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026513 | OPTICS EXPRESS 26517
Q factor (dB)
15
13
11
9
CSPR = 12 dB
7
CSPR = 7 dB
5
CSPR = 2 dB
3
8
10
12
14
16
18
Launch Power (dBm)
Fig. 4. The optimum launch power measured at three CSPR of 2 dB, 7 dB, and 12 dB.
The receiver sensitivity at the ONU is also investigated by tuning the VOA, as shown in
Fig. 5. The VOA is located right after the 20 km distribution fiber. According to the previous
results, CSPR is set to 7 dB and launch power is set to 12.5 dBm. Firstly, the VOA is set to 18
dB (1:64 split), and the corresponding Q factor is about 13 dB. With the consideration of 7%
FEC limit [18], a maximum receiver sensitivity of about −16 dBm could be achieved. After the
transmission of 100 km length with 1:64 split, more than 5 dB system power margin is achieved
using the proposed method.
We further measure the performances of all the 6 sub-bands with the same experiment
setting (CSPR be 7 dB, launch power be 12.5 dBm). As shown in Fig. 6, the average Q factor of
each sub-band has a 4.5 dB margin above the FEC limit, which indicates that higher data rate
signal or longer reaches could be potentially achieved.
14
Q Factor(dB)
13
12
11
10
9
7% FEC limit (8.5dB)
8
power margin (5 dB )
7
6
-18
-16.5
-15
-13.5
-12
-10.5
Received Power (dBm)
Fig. 5. The measured receiver sensitivity by tuning the VOA.
#195576 - $15.00 USD Received 12 Aug 2013; revised 20 Sep 2013; accepted 11 Oct 2013; published 28 Oct 2013
(C) 2013 OSA
4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026513 | OPTICS EXPRESS 26518
Q Factor(dB)
16
14
12
margin>4.5 dB
10
8
7% FEC limit (8.5 dB)
6
1
2
3
4
5
Sub-band Index (n)
6
Fig. 6. The measured performance of each sub-band when CSPR is 7 dB and launch power is
12.5 dBm.
5. Conclusion
We proposed a novel downstream transmission scheme using DDO-OFDM superchannel for
LR-PON. The superchannel is composed of multiple sub-bands and one pilot-carrier, which is
then used as the seed for comb regeneration at the local exchange. The regenerated pilot
carriers are assigned to beat with the adjacent sub-band at the ONU side. Our demonstration
showed an excellent and low cost PON architecture with significant Q factor margin (~4.5 dB)
to the FEC limit under an appropriate CSPR, which would be a feasible solution for the future
LR-PON.
Acknowledgments
This work was jointly supported by the National Basic Research (973) Program of China
(2010CB328300), 863 Program of China (2012AA011302) and the Open Foundation of OCTN
(2012OCTN-02).
#195576 - $15.00 USD Received 12 Aug 2013; revised 20 Sep 2013; accepted 11 Oct 2013; published 28 Oct 2013
(C) 2013 OSA
4 November 2013 | Vol. 21, No. 22 | DOI:10.1364/OE.21.026513 | OPTICS EXPRESS 26519