ECOC 2014, Cannes - France
PD.1.4
40-Gb/s TDM-PON over 42 km with 64-way Power Split
using a Binary Direct Detection Receiver
(1)
(1)
(2)
(1)
Doutje van Veen , Vincent Houtsma , Alan Gnauck , Patrick Iannone
(1)
(2)
Bell-Labs/Alcatel Lucent, Murray Hill, New Jersey, USA, dora.van_veen@alcatel-lucent.com
Bell-Labs/Alcatel-Lucent, Holmdel, New Jersey, USA
Abstract We demonstrate a 40-Gbps TDM-PON over a 42-km, 64-split fiber plant using optical
duobinary modulation. Experimental results show that our architecture supports 31 dB of power
budget for a differential reach of 26 km at 1550 nm without DSP.
Introduction
Until now, all generations of commercially
deployed passive optical networks (PONs),
whether standardized by ITU-T or IEEE, have
been bi-directional, power-splitter-based lowcost TDM-TDMA systems with downstream data
sharing one wavelength and the upstream data
sharing another (aside from an optional
downstream video enhancement wavelength).
The main challenges associated with
increasing the serial data-rate are achieving the
needed optical power budget, the decreased
dispersion tolerance and the possible increased
cost of implementation at a higher rate.
Therefore for the next ITU-T standard beyond
the 10-Gbps TDM-PON standard, the (Full
Service Access Network) FSAN group has
proposed NGPON2, a time- and wavelengthdivision multiplexed (TWDM) PON that
multiplies the bit rate by stacking 4 or 8
1
wavelengths with 10-Gbps each .
Still, the development of a practical low-cost
single-wavelength solution beyond 10-Gbps,
has the potential to either supplant NGPON2 as
the next-gen solution, or serve as a perwavelength upgrade path for NGPON2. We
previously demonstrated a 26-Gbps TDM PON
over 40 km using the limited bandwidth of a low
cost commercial 10-Gbps receiver to convert
2
NRZ into 3-level duobinary , with more options
for upgrading a 10-Gbps PON discussed in ref.
[3].
In this paper we explore the use of optical
duobinary transmission instead of 3-level
duobinary detection as an upgrade path to 40Gbps TDM PON. ODB moves some of the
40-Gb/s
215-1 PRBS
10-Gb/s
Generator
4:1
40 Gb/s
Mux
ODB
Tx
1550-nm
DFB laser
Transmitter
64
40-Gbps TDM-PON experimental setup
The experimental setup, shown in Fig. 1,
consists of an optical line terminal (OLT)
connected via up to 42 km of feeder fiber to a
1:64 power splitter serving user optical network
units (ONUs). Our focus here is demonstrating
40-Gb/s downstream performance, since a
lower upstream rate is acceptable for most PON
deployments. One viable upstream option is 10Gbps time division multiple access, as is already
5
available for 10G-EPON .
The modulation scheme we applied in this
7
paper is optical duobinary modulation (ODB)
which has the following advantages: A binary
direct detection receiver can still be used,
helping to keep the cost down in the ONU;
Higher tolerance to chromatic dispersion can be
achieved compared to NRZ, thus increasing the
reach. Since ODB requires modulating both
TIA
APD-ROSA
RF
combiner
RF
splitter
10 dB
Real-time
two-tap FFE
40-Gb/s
Amp
Clock
recovery
Recovered
clock
1:4
40 Gb/s
DeMux
MZM
Dual drive MZM
~ 35 GHz 3-dB BW
Biased at the null
ONU
complexity from the ONU (3-level detection) to
the OLT (phase and intensity modulation
needed) compared to 3-level duobinary.
We successfully achieved a 40-Gbps
downstream bitrate using a 25-Gbps APD-based
4
receiver with 31 dB of optical power budget,
and 26 km differential reach at 1550 nm over an
outside plant consisting of 42 km of SSMF and a
64-way power split.
Data
Data
Outside Plant
APD/
TIA
Rcvr
Fig. 1: 40-Gbps PON experimental setup
LPF
Data
ATT
OLT
Data
(4) Quarter-pattern shifted 215-1 PRBS
ATT
1-bit delay
APD
2
Various fiber lengths
EDFA
15-GHz
low-pass
filters
LPF
1
FBG-based
dispersion precompensation
(4) 10-Gb/s 215-1 PRBS
Receiver
Fig. 2: Setup details on the optical duobinary transmitter (left) and the APD-based receiver setup (right).
10-Gb/s
Analyzer
ECOC 2014, Cannes - France
PD.1.4
-2
-17
NRZ w/o EQ
ODB w/o EQ
NRZ w/ EQ
-3
ODB w/ EQ
ODB w/ EQ
-3
Log (BER)
Received Power for BER= 1×10
ODB w/o EQ
-4
-5
-6
-7
-8
-9
-10
-26
-24
-22
-20
-18
-16
-14
-18
NRZ w/ EQ
-19
-20
-21
-22
-23
-300
-12
-200
-100
Received Power (dBm)
200
300
NRZ
10
ODB
0
NRZ
ODB
0
-10
Relative Power (dB)
optical amplitude and phase, a Mach Zehnder
Modulator (MZM) biased at the null is used at
the OLT side to modulate light from a 1550-nm
DFB laser. Use of a MZM can be justified
because it is a shared component so it is much
less cost sensitive compared to components at
the ONU side.
Fig. 2 shows more detail on the transmitter
and receiver setup. The MZM electrical drive
signals are derived from four 10-Gbps PRBSs of
15
length 2 -1, multiplexed together to form a 4015
Gbps 2 -1 PRBS signal. The differential
duobinary drive signals are generated by lowpass filtering the 40-Gbps data and inverteddata with 15-GHz low-pass filters (instead a 15GHz MZM could also be used). To avoid error
propagation and to simplify the receiver circuitry
when using real data, a precoder is needed at
the
transmitter
when
using
duobinary
modulation. In the case of a pseudorandom
binary sequence (PRBS) pattern, like in our
experiments it is not needed.
Simultaneously satisfying the stringent link
budget and reach requirements for the latest
PON standards creates a tension in the choice
of wavelength band between C-band (low loss
and high chromatic dispersion) and O-band
(higher loss and low chromatic dispersion). For
our experiment we have selected the C-band,
however the results can be easily translated to
O-band as well, where the increased fiber loss
will limit the split to 1:32 or less for the
maximum-reach case. To compensate for the
accumulated chromatic dispersion as a function
of fiber length, we propose using Fiber Bragg
Grating (FBG)-based dispersion compensation,
thus enabling 42 km reach (or more) in the Cband. The FBG-based dispersion compensator
is followed by an erbium doped fiber amplifier
(EDFA), which is used to boost the launched
optical power to +12 dBm. Naturally, an O-band
implementation must utilize an SOA, which are
also available with these output powers.
100
Fig. 4: Dispersion tolerance for ODB with and without
the 2-tap equalization scheme, and for NRZ with 2-tap
equalization, with 0 dBm launched power.
Reflected Power (dBm)
Fig. 3: APD/TIA Back-to-Back performance for NRZ and
ODB, with and without 2-tap equalizer (EQ).
0
Dispersion (ps/nm)
-10
NRZ
ODB
-20
-30
-40
-50
-100
-50
0
50
100
Frequency (GHz)
-20
-30
0
5
10
15
20
Launch Power (dBm)
Fig. 5: SBS Measurements for NRZ and ODB in 42 km
of SSMF
In conjunction with the transmitter’s high
4
launch power, a 25-Gbps APD-based ONU
receiver was used to achieve the large optical
power budget typically required in standardized
PONs, at a lower cost and lower power
consumption relative to an optically preamplified receiver. This 25-Gbps APD-TIA
receiver optical subassembly (ROSA) was
6
developed for the 100G Ethernet standard , a
technology that is expected to mature and
become low cost in the near future.
The receiver’s measured -3 dB bandwidth of
15 GHz is smaller than the required ~28 GHz,
resulting in a receiver power penalty. To
ameliorate the bandwidth limitation of the
receiver a simple 2-tap equalizer was
implemented, combining the differential outputs
of the receiver with one output attenuated and
delayed by one bit. After the equalizer, the clock
was recovered and the 40-Gbps data stream
was de-multiplexed to four 10-Gbps streams,
each with virtually identical performance, for
real-time bit-error-rate (BER) measurements.
As is the ONU in the 40-Gbps TDM-PON
needs to be able to process a data-rate of 40Gbps. We therefore propose to use Bit
Interleaving (BI) to relax the requirements
ECOC 2014, Cannes - France
PD.1.4
-5
Experiments
The back-to-back receiver performance is
depicted in Fig. 3, where red squares and blue
circles represent BER data for NRZ and ODB
modulation, respectively. In both cases, the
addition of the equalizer (solid symbols)
improves performance by 1.3 dB or greater at a
-3
BER of 1.0x10 , the raw BER corresponding to
-12
a post-FEC BER of 1.0x10 .
Fig. 4 uses the same symbols to plot the
-3
variation in received power at a BER of 10 as a
function of dispersion. Positive dispersion points
were measured with various lengths of standard
single-mode fiber (SSMF). Negative dispersion
points were measured with a -330-ps/nm FBG
concatenated with SSMF. For received powers
of approximately –19 dBm or less, as required
for our system, the range of dispersion (and
therefore maximum reach) for equalized optical
duobinary exceeds that for NRZ modulation by
over a factor of 2.
Fig. 5 plots backscattered power as a
function of launch power for both NRZ and
duobinary modulation in a 42-km spool of
SSMF. The linear shape of the duobinary curve
(blue
circles)
indicates
that
Rayleigh
backscattering is the dominant process, with the
total backscattered power approximately 30 dB
below the launch power for launch powers up to
20 dBm. The red squares show that the onset of
stimulated Brillion scattering (SBS) limits the
maximum launch power for NRZ modulation to
approximately 10 dBm. This launch power
advantage of ODB relative to NRZ is due to the
fact that the ODB spectrum does not have a
carrier, as seen in the inset in Fig. 5.
Fig. 6 shows the 1:64 PON transmission
results with +12 dBm of optical launch power,
which was found to be the optimum power for
maximum margin at 42.4 km (fiber nonlinearity
limited). From this it can be seen that we have a
-3
31-dB power budget at a BER of 1.0x10 . We
demonstrate two different reaches (0-26.9 km or
15.7-42.4 km) by adjusting the dispersion
precompensation: The -330-ps/nm FBG was
concatenated with either a length of SSMF or
dispersion-compensating fiber to obtain the
required precompensation values, -224 ps/nm
and -488 ps/nm. Obviously, an FBG could be
Open Symbols: -224 ps/nm Precompensation
Solid Symbols: -488 ps/nm Precompensation
Maximum Received Power
RX Sensitivity BER=1.0×10-3
-10
Power (dBm)
needed to process the user data. The bit
8
interleaving PON scheme can be used to
reduce the bitrate that an ONU needs to handle
down to a lower rate, for example to 10-Gbps
which enables the use of commercially available
10G PON electronic ONU parts, thus minimizing
cost and also enabling a lower power
implementation.
-15
Margin
-20
-25
0
10
20
30
40
50
SSMF Length (km)
Fig. 6: Transmission results at 1550 nm over the 1:64
PON for two values of dispersion precompensation.
designed for a certain required dispersion value.
Conclusions
We successfully demonstrated a 40-Gbps TDMPON over a 42-km, 64-split fiber plant.
Experimental results show that our proposed
system supports 31 dB of power budget, for a
-3
BER of 1.0x10 with a differential reach of 26
km at 1550 nm without any form of digital signal
processing (DSP) using a low cost 25-Gbps
APD/TIA receiver. We used a booster EDFA to
launch up to +12 dBm into the fiber and a fiber
Bragg grating to offset dispersion in the outside
fiber plant for operation in the C-band.
The demonstrated dispersion tolerance of
more than 200 ps/nm also enables operation
over a large window in the O-band (about 1275 1355 nm) over at least 0 - 40 km SSMF without
needing to offset dispersion with the FBG.
Acknowledgements
The authors thank Peter Vetter and Ed Harstead for
enlightening discussions and Atul Srivastava and the
NTT Electronics Corporate team for their technical
contribution.
References
[1] ITU-T G.989.1, “40-Gigabit-capable passive optical
networks (NG-PON2): General requirements”
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Detection with a Low Cost 7-GHz APD-Based Receiver,"
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[3] D. van Veen, D. Suvakovic, H. Chow, V. Houtsma, E.
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[6] IEEE Std 802.3ba, clause 88 (2010).
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