1
Field-Trial of a high-budget, filterless, λ-to-the-user,
UDWDM-PON enabled by an innovative class of
low-cost coherent transceivers
Marco Presi, Massimo Artiglia, Fabio Bottoni, Mario Rannello, Ivan Valdez-Cano, Jaison Tabares, Juan-
Camilo Vel
´
asquez, Saeed Gasemi, Victor Polo, Guang Yong Chu Student Member, IEEE,
Josep Prat, Member, IEEE, Gregorio Azcarate, Robert Pous, Chantal Vil
´
a,
Helene Debregeas, Gemma Vall-llosera, Albert Rafel, and Ernesto Ciaramella, Senior Member, IEEE
Abstract—We experimentally demonstrate an innovative Ul-
tra Dense Wavelength Division Multiplexing (UDWDM) Passive
Optical Networks (PON) that implements the full λ-to-the-user
concept in a filterless distribution network. Key element of
the proposed system is a novel class of coherent transceivers,
purposely developed with a non-conventional technical approach.
Indeed, they are designed and realized to avoid D/A-A/D con-
verter stages and Digital Signal Processing (DSP) in favor of
simple analog processing so that they match system, cost and
power consumption requirements of the access networks without
sacrificing the overall performance. These coherent transceivers
target different use case scenarios (residential, business, fixed,
wireless) still keeping perfect compatibility and co-existence with
legacy infrastructures installed to support gray, Time Division
Multiplexed (TDM) PON systems. Moreover, the availability of
coherent transceivers of different cost/performance ratios allows
for deployments of different quality service grades. In this paper,
we report the successful field trial of the proposed systems in a
testbed where 14 UDWDM channels (and one legacy E-PON
system) are transmitted simultaneously in a dark-fiber network
deployed in the city of Pisa (Italy), delivering real-time and/or
test traffic. The trial demonstrated filterless operations (each
remote node selects individually its own UDWDM channel on
a fine 6.25 GHz grid), real-time GbE transmissions (by using
either fully analog or light digital signal processing), multi-
rate transmission (1.25 and 10 Gb/s/), high Optical Distribution
Network loss (18 ÷ 40 dB) as well as a bidirectional channel
monitoring system.
Index Terms—WDM networks, Access Networks, Passive Op-
tical Networks, Coherent Detection, Homodyning, Heterodyning
M. Presi, M. Artiglia, M. Rannello, Fabio Bottoni and E. Ciaramella are
with Scuola Superiore Sant’Anna, via G. Moruzzi, 1 56127 PISA (ITALY).
E-mail: marco.presi@sssup.it
Fabio Bottoni is now with Oclaro, via ... Milano, ITALY email:
fabio.bottoni@oclaro.com
I.V. Cano, J. Tabares, J. C. Velsquez, S. Ghasemi, V. Polo, G. Y. Chu and
J. Prat are with Universitat Politecnica de Catalunya (UPC), c. Jordi Girona
1-3, 08034 Barcelona, Spain. E-mail ivan.cano@tsc.upc.edu
H. Debrgeas is with III-V Lab, joint laboratory of Nokia Bell Labs,
Thales Research and Technology, and CEA Leti, Palaiseau, France. E-mail:
Helene.Debregeas@3-5lab.fr
G. Vall-llosera is with Ericsson Research, Farogatan 6, 16480 Stockholm,
Sweden. E-mail: gemma.vall-llosera@ericsson.com
R. Pous, G. Azcrate, and C. Vil are with Promax, c. Francesc Moragas 71,
08907 L’Hospitalet de Llobregat, Spain. E-mail: cvila@promax.es
A. Rafel is with BT Technology Service & Operations, Adastral Park,
Martlesham Heath, Ipswich, UK. E-mail: albert.2.rafel@bt.com
This work was partly supported by the European Commission FP7-ICT
program under Grant 318515.
TThe authors would like to acknowledge the Municipality of Pisa (ITALY)
and Agestel for having made available the city fiber system.
Manuscript received April 19, 2005; revised August 26, 2015.
I. INTRODUCTION
T
HE recent standardization of the Next Generation Passive
Optical Network 2 (NG-PON2) [1], the designated candi-
date technology upgrade of currently installed B/E/G/XG/NG-
PON systems, introduced for the first time, after almost a
decade of research, the Wavelength Division Multiplexing
(WDM) technology in optical access systems as an overlay
to the Time Division Multiple Access (TDMA) signals [2].
However being an upgrade of gray Passive Optical Networkss
(PONs) with Optical Distribution Network (ODN) realized
only by power splitters and drop fiber trunks, WDM must be
introduced by preserving the filterless character of deployed
networks plant, i.e. without requiring the installation of optical
filters in the ODN [3], [4]. NG-PON2 solved this issue by
adopting low-cost tuneable optical thin-film filters inside the
Optical Network Units (ONUs). Presently, these filters have
usually large bandwidths, i.e. can provide only wavelength
selection on 100 GHz (or higher) grids [5]–[7]. On the other
hand, upcoming network orchestration technologies favoring
grid-less operation and/or featuring the to the user paradigm
require transceivers capable of finer frequency selectivity [8].
Coherent transceivers [9] are today operated in optical core
networks thanks to their capability of implementing sophisti-
cated Digital Signal Processing (DSP), which allows for for
compensation of transmission impairments [10], high spectral
efficiency [11] and/or software-defined operations [12]. To
attain these functionalities, DSP requires costly hardware (high
bandwidth and high resolution Digital-to-Analog Converter
(D/A) and Analog-to-Digital Converter (A/D) as well as high-
end processors and software units [13]–[15]. Furthermore, ex-
pensive optical devices (e.g. low-line-width lasers) and passive
components (e.g. 90
◦
hybrid couplers) are also needed. All of
these implementation features prevented the use of coherent
detection in access networks so far, because cost requirements
here are very much tight: it is indeed expected that PON
equipment must have cost levels comparable with current
consumer electronics, especially at user premises. However,
many of the functionalities offered by present coherent systems
are not required in the access network, where low data rate
per user is typically expected (well below 10 Gb/s), the
transmission reach does not exceed 100 km and Kerr-induced
non-linear impairments are usually not triggered.
2
Pisa
External
Network
COCONUT - OLT
WDM
MUX
ASK - UDWDM
8x1.25 G
ONU
ASK 10G
E-PON
ONU
ONU
PSK Homodyne
ONU
PSK Heterodyne
ONU
ASK 1G
GbE
Switch
ASK -10 G
PSK - 1.25 G
Homodyne
PSK - 1.25 G
Heterodyne
HR-OSA
IP-TV
Broadcast
E-PON
OLT
CE
Internet
Server
GbE
Switch
WiFi AP
25 km
SMF
10 km SMF
IP-TV
interface
PC
PC
Sant'Anna Laboratories
Sant'Anna Laboratories
MT
MT
MT
Figure 1. Schematic representation of the field trial setup. Gray areas represent the equipment placed in the lab. OLT: Optical Line Terminal; HR-OSA: High-
Resolution Optical Spectrum Analyser; CE: Co-existence Element; SMF: G.652 Single Mode Fiber; ONU: Optical Network Unit; PC: Personal Computer;
AP: Access Point; MT: Mobile Terminal; TV: Television
Therefore the authors believe that coherent detection can
play a significant role in the access network, but the corre-
sponding design must be redefined and optimized for access
requirements: this may allow to take advantage of some key
features of coherent detection. These systems can have very
low sensitivity (thus increasing the system power budget and
the tolerable ODN insertion loss, which are very critical
in the access domain); moreover, they allow for filter-less
and colorless operations [8], [16], [17]. These considerations
stimulated new research lines aiming at determining if co-
herent detection could be effectively revised in order to be
implemented in the optical access network domain. Among
the many different approaches considered, we decided to
focus on the two aforementioned aspects: low-sensitivity and
fine wavelength selectivity, by trading receiver simplicity for
high-spectral efficiency and complex DSP functionalities. By
focusing on these two parameters, we were able to design a
new class of coherent transceivers featuring:
• use of low cost lasers (Distributed FeedBack laser (DFB)
of pre-allocated wavelength) with coarse wavelength con-
trol rather than expensive low-line-width tuneable Exter-
nal Cavity Laser (ECL) [18]
• direct laser modulation analogue processing (or very light
DSP in few selected cases) for reliable and low-latency
real-time operation
• low-cost receiver front-end optical couplers (either 3 ×
3 or 2 × 2 fused fiber couplers) rather than costly 90
◦
hybrids [19], [20]
• wavelength reuse (when useful)
• low-cost, coherent-based monitoring system to inspect
and manage the multi λ system and the channel acti-
vation/deactivation [21]
This new class of coherent receivers was developed over the
last three years within the COCONUT project [22], demon-
strating a low power consumption level, and moderate cost per
bit, if compared to other competitive coherent technologies
[23], [24]. They thus show a potential for being effectively
considered in real access network deployments. This new
class of receivers has been developed initially to address the
issue of upgrading legacy infrastructure the capabilities; yet,
most of their features (channel selectivity at the receiver,
flexible wavelength allocation, low receiver sensitivity) can be
advantageously exploited in new deployments (the so called
“green fields”) or even exploited in alternative or novel ODNs
designs (for example those that could be considered in FSAN).
Here we provide a detailed description of the field trial, which
was also open to public visitors, where these new transceivers
have been put together showing the full capabilities of the
proposed λ-to-the-user system on a real fiber network de-
ployed in the city of Pisa (Italy), connected directly to/from
Scuola Superiore Sant’Anna laboratories. The fiber plant has
been made available by courtesy of Agestel (a local company).
High-power budget (up to 40 dB) filterless Ultra Dense Wave-
length Division Multiplexing (UDWDM) operations (14 WDM
channel with spacing as low as 6.25 GHz), different data
rates (1.25 to 10Gb/s), co-existence with legacy PON systems,
real-time traffic delivery in different use case scenarios were
demonstrated in eight concurrent experiments through the four
flavors of COCONUT coherent transceivers. Although the
performances of the single systems may have been already
reported in reference of lab environments [25]–[27], we pro-
vide here an extended report of the first demonstration of
their joint operation in a field trial condition, also showing
for the first time real-time traffic operation, with Ethernet-
based applications and high-definition video transmission. In
addition coexistence with E-PON system is proven, in a
typical evolution case. These results demonstrate for the first
time that when all the systems are operated simultaneously
in a real filterless network scenario, providing real service
distribution their performances are not altered significantly,
demonstrating the colorless property of the receivers, the
resilience to their interaction and to the network reflections.
This paper is organized as follows: in section II we provide a
description of the trial network and introduce the five coherent
transmission systems tested. In section III we summarize the
testbed results, while in section IV we discuss the potential
applications of the single transceivers to possible use cases.
Finally in Section V we draw the conclusions.
3
Figure 2. Map of the dark fiber deployment in loop configuration connecting
the Scuola Superiore Sant’Anna laboratories (top right in the map) laboratories
to the Pisa city center.
II. FIELD TRIAL DESCRIPTION
The setup of the field trial is schematically represented
in Figure 1. At the Scuola Sant’Anna lab, we simulated
a central office hosting two Optical Line Terminal (OLT)
systems: the COCONUT OLT and a commercially available
E-PON. A co-existence WDM filter combined the two OLTs.
The COCONUT OLT consisted of four different groups of
transceivers, based on different modulation formats (either
Amplitude Shift Keying (ASK) or Phase Shift Keying (PSK)),
modulation types (direct or external), detection schemes (het-
erodyne or intradyne) and bit-rate (1.25 or 10 Gb/s). We used
an E-PON system because of availability of the equipment,
but this is perfectly equivalent to a G-PON, since E-PON and
G-PON share the same wavelength plan and they differ mainly
for the framing, so from co-existence point of view this test
is equivalent of testing a G-PON.
We also emulated a scenario where asymmetrical bit-rates
are used (e.g. 10 Gb/s downstream and 1.25 Gb/s upstream).
Similarly, different experiments used three different wave-
length allocation plans for the Upstream (US)/Downstream
(DS) transmission: ultra-narrow spacing (2.5 GHz), UDWDM
spacing (6.25 GHz) and large spacing (100 GHz ITU grid).
This allowed to emulate different field deployments (green
or brown field). In all cases, a cascade of power splitters
and a WDM multiplexer combined these signals [28]. The
WDM Multiplexer (MUX) had 100 GHz channel spacing and
100 GHz Full Width at Half Maximum (FWHM) on each
channel. For UDWDM operations, up to 8 channels fitted
one MUX port on a 6.25 GHz grid. The channel monitor-
ing system, provided by a low-cost High-Resolution Optical
Spectrum Analyser (HR-OSA), was placed at the MUX output
through an optical by-pass allowing the monitoring of both
upstream and downstream traffic. The combined OLT output
was then directly connected to an external feeder fiber. The
feeder is composed of dark fibers that are part of an operating
fiber network deployed in the city of Pisa: it was made
available by a Agestel, and was composed by a series of
dark G.652 Single Mode Fiber (SMF) fiber trunks in a loop
configuration from Sant’Anna facilities to downtown and back,
as shown in Figure 2. The loop length was approximately
10 km. This external section included several connections
Table I
WAVELENGTH ALLOCATION PLAN
System Link Frequency (THz) Wavelength (nm)
ASK Intradyne US 193.0 1551.72
PSK Homodyne US/DS 193.2 1550.92
ASK 4 × 10 Gb/s DS 194.3 − 193.7 1550.12 − 1547.12
DPSK Heterodyne US/DS 193.5 1549.32
ASK Intradyne DS 193.9 1546.12
(8 × 1.25 Gb/s)
(SC/PC connectors) in outside cabinets, with a total insertion
loss of 10 dB. The end of this fiber loop reentered the lab
and connected to a drop fiber system realized by a cascade of
power splitters and fiber spools to reach five ONUs, each one
communicating with its own corresponding transceiver at the
OLT side (either E-PON or the COCONUT ones). The full
ODN had a maximum reach of 35 km, a maximum loss of
40 dB and ONUs maximum differential loss of 22 dB. We
note that the maximum differential and absolute loss values
exceeded by 7 and 5 dB, respectively, those recommended in
current PON standards [29]. For the sake of the simplicity,
each communication system used its own wavelength band.
The exact wavelength allocation is reported in Table I. As will
be seen, some systems had a very low frequency separation
between downstream and upstream signal, which allowed
to allocate both into a single ITU-T WDM frequency slot;
other systems were configured so that two different ITU-T
slots were used for upstream and downstream. Both at the
OLT and the ONU sides, some of the transceivers have
been connected to Ethernet switches or multimedia streaming
servers to demonstrate network operations in a heterogeneous
distribution of real-time traffic. In the following sections, those
coherent systems will be reviewed in detail.
A. UD-WDM 8 × 1.25 Gb/s ASK Intradyne Real-Time analog
system
PBS
(LPF)
~
~
~
90°
X
X
X
COCONUT ODN
BER
DFB-2
TEC
PBS
(LPF)
~
~
~
90°
X
X
X
DFB-4
TEC
DFB-3
TEC
VOA
OC
MZM
PPG
BER
DFB-1
TEC
MZM
PPG
Figure 3. ASK system schematics. PPG: Pulse Pattern Generator; TEC:
Thermo-Electric Cooler; MZM: Mach-Zendher Modulator; PBS: Polarization
Beam Splitter; LPF: Low-Pass Filter; OC: Optical Circulator; ODN: Optical
Distribution Network
We setup a bidirectional 1.25 Gb/s ASK system, which
makes use of two identical transceivers exploiting two different
wavelength bands to realize simultaneous upstream and down-
stream transmissions. This is achieved by using two distinct
Arrayed Waveguide Router (AWG) ports at the OLT side (one
for downstream and one for the upstream), and an optical
circulator at the ONU side (see Figure 3). The transceivers
4
comprise a transmitter based on external modulation (due to
the limited number of available directly modulated lasers)
driven either by a Pulse Pattern Generator (PPG) for Bit
Error Ratio (BER) measurements or by a Gigabit Ethernet
(GbE) interface. Both transmitters use commercial DFB lasers.
The receivers use a polarization independent scheme based
on intradyne operation [30] and featuring a 3 × 3 optical
hybrid coupler [19]. The received signal undergoes first in a
polarization split and rotate stage. After this, its two orthogonal
polarization components are injected in the 3 × 3 coupler
together with the output of the Local Oscillator (LO), which is
also a DFB laser. As a result of the first stage, all these three
fields enter the coupler with same State of Polarization (SoP).
The local oscillator is tuned to obtain proper intradyne regime
(detuning between signal and LO around 80% of the bit-rate,
900 MHz in this case) [30]. The coupler output lights are then
detected by three PIN photodiodes. These three photocurrents
are squared by using off the shelf analog multipliers, added and
then low-pass filtered (4
th
order Bessel, 933 MHz bandwidth)
to yield the recovered ASK signal [31]. The analog processing
stage output is directly connected either to a BER tester or to a
GbE module in case of real-time application demos. UDWDM
operation has been also tested, by multiplexing 8 downstream
ASK 1.25 Gb/s channels onto a single AWG input port at
the OLT as shown in Figure 4. The two setups differ mainly
in the presence of the 8 downstream channels which are first
grouped by using a cascade of power splitters and then coupled
into a single AWG port. Since the AWG has a pass-band
of 50 GHz and Gaussian shape, suitable pre emphasis was
used to make the channels, spaced by 6.25 GHz, leave the
AWG with a uniform power level (about 3 dBm/channel, see
also Figure 8). Due to the lack of 8 independent PPGs we
modulated the channels at groups of four, dividing them in
odd and even combs driven by PRBS sequences of different
lengths (2
15
− 1 and 2
31
− 1) produced by two independent
PPGs, so that adjacent channels are fully uncorrelated. At the
ONU side, the LO was thermally tuned to receive the selected
individual channel and measure its performance.
PBS
(LPF)
~
~
~
90°
X
X
X
DFB-4
TEC
DFB-3
TEC
OC
MZM
PPG
BER
COCONUT ODN
VOA
PRBS 2 -1
MZM
MZM
PRBS 2 -1
31
15
Co-RX
Figure 4. UDWDM experimental setup. MZM: Mach-Zendher Modulator;
OC: Optical Circulator; PBS: Polarization Beam Splitter; PPG: Pulse Pat-
tern Generator; TEC: Thermo-Electric Cooler; LPF: Low-Pass Filter; ODN:
Optical Distribution Network; VOA: Variable Optical Attenuator
B. 4x10 Gb/s ASK Intradyne system
This system used 4 × 10 Gbit/s directly-modulated lasers
(Directly Modulated DFB Laser (DML)), placed on a 100 GHz
grid with λ between 1550.12 and 1547.12 nm. These lasers
were modulated by 4 PRBS streams obtained by exploiting the
direct and inverted outputs of two independent PPGs (peak-
to-peak modulation around 1.3 Volt). At the ONU, we used a
COCONUT ODN
X
X
X
BER
DFB
TEC
VOA
PPG
PPG
PPG
PPG
~
~
~
~
~
~
(LPF)
~
~
~
RTO
DFB
Figure 5. 4 × 10 Gb/s System Setup. LPF: Low-Pass Filter;TEC: Thermo-
Electric Cooler; ODN: Optical Distribution Network; VOA: Variable Optical
Attenuator
version of the ASK receiver, which was polarization dependent
because of the limited number of available components, and
was realized by using only a 3 × 3 optical coupler, a DFB
as LO and 3 PIN photodiodes (15 GHz bandwidth). The LO
wavelength was tuned to receive each channel (with a sig-
nal-LO detuing at about 3 GHz), while its polarization was
manually controlled. Polarization independent operation of the
receiver has been recently reported [32]. The signal processing
was realized offline, using a real-time oscilloscope (Real Time
Oscilloscope (RTO), 13 GHz analog bandwidth, 40 GSa/s): it
emulated 3 parallel processing stages, each having a 3 GHz
low-pass filter (for frequency-to-amplitude conversion) and a
squaring function: the three processed signals were then added
to recover the ASK signal. BER of the received signal was also
computed within the RTO by a custom developed routine [33].
C. 1.25 Gb/s DPSK heterodyne Real-Time analog system
COCONUT ODN
DFB-1
DFB-3
OC-1
VOA
TEC
TEC
Heterodyne
RX
RSOA
~
~
~
x
T
~
~
~
x
T
+
~
~
~
PBS
OC-2
OC-3
PSK SYSTEM ONU
PSK SYSTEM OLT
BER
Tester
HPF
HPF
LPF
DFB-2
TEC
PPG
PPG
EQ
Figure 6. PSK system setup. PPG: Pulse Pattern Generator; OC: Optical
Circulator; TEC: Thermo-Electric Cooler; EQ: Equalizer; LPF: Low-Pass
Filter; R-SOA: Reflective Semiconductor Optical Amplifier; PBS: Polarization
Beam Splitter; HPF: High-Pass Filter; BERT: Bit Error Ratio Tester
This system exploits a heterodyne transceiver whose de-
tailed scheme is depicted in Figure 6. At the OLT side, the
transmitter consisted of a pre-equalized DML (DFB-1), which
produced a phase-modulated signal [26]. It was driven either
by a PPG or from a GbE interface. The DFB was temperature
controlled and emitted at 1549.32 nm. A circulator separated
the down/upstream directions before being multiplexed by the
WDM MUX. The downstream signal reached the COCONUT
ODN with 0 dBm optical power. At the ONU side, the
receiver consisted in a polarization independent heterodyne
mixer [26]. The ONU front-end used a network of optical
circulator and 3 dB couplers/splitters separating US/DS traffic
and allowing to use a single DFB laser (DFB-3 in Figure 6)
simultaneously as LO and seed for the upstream Reflective
Semiconductor Optical Amplifier (R-SOA) transmitter [34]
[35]. The LO emission frequency was shifted by +2.5 GHz
from the receiving signal and its power was set to 0 dBm. The
two polarization diversity branches consisted in a single ended
5
photodiodes (7 GHz E/O bandwidth) followed by a high-pass
filter of 1.2 GHz cutoff frequency and an analogue delay and
multiply circuit to decode the signal. The two branches were
finally combined, low-pass filtered at 1 GHz, amplified and
sent to the Clock and Data Recovery (CDR) circuit for BER
measurements performed by a common Bit Error Ratio Tester
(BERT). The US transmitter used a directly-modulated TO-
CAN R-SOA, biased at 70 mA with 18 dB linear gain although
here under saturation for phase modulation, and was coupled
to the ODN with 3 dBm optical power. At the OLT, the
US signal was detected by a polarization dependent coherent
heterodyne receiver with another DFB laser (DFB-2) as LO,
which implemented the same analogue processing described
above but using balanced detection [34].
D. 1.25 Gb/s PSK Homodyne Real-Time digital system
COCONUT
ODN
PPG
DFB-1
OC-1
EQ
TEC
DFB-2
TEC
PSK SYSTEM ONU
PSK SYSTEM OLT
PPG
DFB-3
EQ
TEC
I/Q
builder
A/D
FPGA
A/D
DSP
BER
~
~
~
~
~
~
LPF
Figure 7. PSK Homodyne system setup. PPG: Pulse Pattern Generator; OC:
Optical Circulator; TEC: Thermo-Electric Cooler; EQ: Equalizer; LPF: Low-
Pass Filter; A/D: Analog-to-Digital Converter; DSP: Digital Signal Processing;
ODN: Optical Distribution Network
1.25 Gb/s homodyne PSK system was also implemented
by using directly phase-modulated DFB lasers (see Figure 7).
However, in this case the OLT upstream receiver exploited a
dedicated FPGA platform for real time detection. The ONU
upstream transmitter used also a directly phase-modulated
laser (DFB-3), whose line-width was ∆ν = 4 MHz and
emitted at 1550.9 nm (see table I). At the OLT, a digital
coherent receiver comprised a 3 × 3 optical hybrid coupler to
mix the incoming signal with the LO (DFB-2), emitting 3 dBm
output power. The State of Polarization (SOP) was manually
optimized at the receiver input. The three output signals
reached three single-ended TO-CAN packaged photodetectors
(BW=2 GHz). The electrical signals were then amplified and
properly combined to obtain the in-phase (I) and quadrature
(Q) components, which were then low-pass filtered at 1 GHz
by two micro strip Bessel filters for anti-aliasing and noise
rejection. Then, each electrical signal was A/D converted at
the Nyquist rate of 2.5 GSa/s at 8 vertical bits and sent to
a FPGA unit (Xilinx Virtex-6) for real time processing. The
digital signal processing (DSP included a de-skew stage for
channel alignment and a data demodulation stage using two
samples per symbol. In addition, a frequency estimation stage
generated an error signal driving continuously the LO (DFB-
2) to match its frequency with the frequency of the incoming
signal, with an error below ±50 MHz [36]. Such an error
signal was D/A converted by a 1-bit serial converter based on
pulse width modulation and sent to the LO control circuit
for thermal and current tuning. Finally, data decision was
performed by selecting the best sample for each bit and the
BER was measured inside the FPGA by direct error counting
on the recovered data sequence.
E. High-Resolution Optical Spectrum Analyser
~
~
~
Balanced
Receiver
Reference
ByPass
TL-1
TL-2
A/D
DS
US
OLT
PON
P
HR-OSA
Figure 8. HR-OSA setup. OLT: Optical Line Terminal; DS: Downstream; US:
Upstream; A/D: Analog-to-Digital Converter; PON: Passive Optical Networks
The monitoring equipment exploited a high-resolution op-
tical spectrum analyzer (HR-OSA), which is able to detect
individual channels also when operating on the 6.25 GHz
UDWDM grid. Indeed, the HR-OSA has a resolution band-
width as low as 100 MHz. This HR-OSA was developed ad-
hoc to test the COCONUT system. It is based on coherent-
detection and its structure is shown in Figure 8. Each tuneable
laser scans a 5 nm window in the C-band. By stacking
more tuneable lasers, each one covering 5 nm, it is possible
to widen the analysis band. The HR-OSA also provides a
management user interface able to monitor all the active
channels in the network, either in upstream or downstream,
and to identify active transmitters (located either in ONUs or
at the OLT side). The identification system is meant not only
to monitor the health status of the active channels, but also to
assist new ONU activations (i.e. new lasers to be turned on),
by contributing with detailed information about the network
status. Bidirectional monitoring is achieved by tapping 1 dB
power at the OLT ingress/egress port.
Figure 9. Top: optical spectrum of downstream channels; Bottom: C-band
COCONUT channels. a) pilot; b) 8 × 1.25 Gb/s ASK; c-e) 10 Gb/s ASK; f)
1.25 Gb/s DPSK heterodyne; g) 10 Gb/s ASK; h) 1.25 Gb/s DPSK homodyne
