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Journal ArticleDOI

1.25–2.5 Gb/s simple nyquist transmitters for coherent UDWDM-PON with enhanced spectral efficiency

19 Apr 2018-Fiber and Integrated Optics (Taylor & Francis)-Vol. 37, Iss: 4, pp 219-228

AbstractWe present a coherent ultra-dense wavelength division multiplexing passive optical network (UDWDM-PON) with enhanced spectral efficiency. The benefit of using Gaussian or Nyquist pulse-shaping filters at the transmitter is evaluated through numerical simulations and experiments. The transmitters consist of directly phase modulated distributed feedback (DFB) lasers through beat signals whose duty-cycle and amplitude are digitally adjusted. The results show that transmitting a Nyquist-shaped signal achieves a 25% spectral saving allowing to place 2.5 Gb/s/user data in 6.25 GHz channels. Furthermore, the proposed transmitter tolerates differential link-losses of 15 dB at Rx sensitivity of ?44 dBm at BER=10?3 with intradyne detection.

Topics: Spectral efficiency (53%), Transmitter (52%), Wavelength-division multiplexing (52%), Passive optical network (50%)

Summary (1 min read)

1. Introduction

  • Motivated by the need of high spectral efficiency and aggregated capacity, coherent ultra-dense wavelength division multiplexing solutions have gained interest [1, 2].
  • In [6] the Tx was highly complex because of using an external modulator.
  • The authors analyze the benefit of using Gaussian or Nyquist shaping filters in the Tx to increase the spectral efficiency, extending the results of [8].
  • This is, to the best of their knowledge, the first simple directly phase modulated Tx with spectral confinement for a coherent UDWDM-PON.

2. Numerical Simulations

  • The simulations were performed with VPITranmissionMaker® and MATLAB® following the Monte-Carlo method.
  • For this shaping technique, the lower α, the lower the total filter BW and the higher spectral compression.
  • In the NRZ signal, the authors observe that the sharp transitions between symbols result in high-frequency harmonic components.
  • The values at 1 dB penalty were 2.8 GHz, 2.2 GHz and 1.8 GHz for NRZ, Gaussian and Raised-cosine pulse-shaping respectively.
  • Figure 3. Optical spectra for two users separated by 6.25 GHz for three different pulse-shaping schemes: (a) NRZ, (b) Gaussian, (c) Raised-cosine.

3. Experimental setup

  • Once the technique was evaluated through simulations, the authors implemented the UDWDM-PON scenario depicted in Figure 5.
  • The setup was composed by two identical transmitters (Tx1 and Tx2) based on direct modulated DFBs with linewidths of 4 MHz and 3 MHz and modulation BW of 10 GHz and 2.5 GHz respectively.
  • The data sequences were digitally equalized by means of a 1-tap finite impulse response (FIR) filter with half-bit period delay.
  • The Rx consisted of an intradyne detector based on a 3x3 optical coupler which mixed the incoming optical signal with a LO.
  • The three currents were combined and processed to obtain the inphase (I) and quadrature (Q) parts as in [10].

4. Results

  • Two electrical signals, one with and another without Nyquist-shaping, were digitally generated.
  • The BW of the residual intensity modulation (IM) is narrowed.
  • The use of a matched filter in the Rx can improve the eye-opening and thus the performance [6].
  • Then, the performance of Tx1 was evaluated when Tx2 caused interference at different channel spacing (CS).

5. Conclusions

  • The authors evaluated with numerical simulations and experimentally tested a direct phase modulated DFB laser at 1.25 Gb/s and 2.5 Gb/s with Nyquist shaped DPSK.
  • A coherent UDWDM-PON was emulated with a 25 km SMF link and intradyne detection.
  • The authors initially found that with a DLL of 0 dB, there was no significant benefit when using Nyquist shaping in the Rx sensitivity and CS.

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For Peer Review Only
1.25
-
2.5 Gb/s Simpl
e Nyquist Transmitters for Coherent
UDWDM-PON with Enhanced Spectral Efficiency
Journal:
Fiber and Integrated Optics
Manuscript ID
UFIO-2018-2188.R2
Manuscript Type:
Original Article
Keywords:
beat phase modulation, DFB, Nyquist shaping, UDWDM-PON, coherent
detection
URL: http:/mc.manuscriptcentral.com/ufio Email: h2@alum.mit.edu
Fiber and Integrated Optics

For Peer Review Only
1.25 - 2.5 Gb/s Simple Nyquist Transmitters for Coherent UDWDM-
PON with Enhanced Spectral Efficiency
A coherent ultra-dense wavelength division multiplexing passive optical network
(UDWDM-PON) with high spectral efficiency is experimentally tested. The transmitters
consist of directly phase modulated distributed feedback (DFB) lasers through beat signals
whose duty-cycle and amplitude are digitally adjusted. The benefit of using Gaussian or
Nyquist pulse-shaping filters at the transmitter is evaluated through numerical simulations
and experiments. The results show that transmitting a Nyquist-shaped signal achieves a
25% spectral saving allowing to place 2.5 Gb/s/user data in 6.25 GHz channels. The
proposed transmitter tolerates differential link-losses of 15 dB with Rx sensitivity of -44
dBm at BER=10
-3
with intradyne detection.
Keywords: beat phase modulation, DFB, directly modulated laser (DML), Nyquist
shaping, UDWDM-PON, coherent detection.
1. Introduction
Motivated by the need of high spectral efficiency and aggregated capacity, coherent ultra-dense
wavelength division multiplexing (UDWDM) solutions have gained interest [1, 2]. These systems
are attractive because they can inherently filter the optical signal by selecting the wavelength, and
provide higher sensitivities compared with direct detection thus avoiding optical amplifiers [3].
However, for the competitive access market, low complexity transceivers with simplified digital
signal processing (DSP) need to be designed while keeping high the performance [4].
In UDWDM-PON, a major concern is the unwanted spectral side-lobes resulting when
modulating the optical carrier. Since the frequency grid is very narrow, these spectral components
cause interference to adjacent channels, yielding to Bit Error Rate (BER) degradation. The use of
spectral shaping filtering at the transmitter (Tx) thus can play an important role reducing the
modulated spectral width and suppressing interference from adjacent channels [5]. As a result, it
can improve the network spectral efficiency as demonstrated in [6]. However, in [6] the Tx was
highly complex because of using an external modulator.
In previous work, we proposed direct phase modulation of a distributed feedback (DFB)
laser for simplifying the Tx. The modulating data was coded and had three levels. In addition, its
amplitude and duty cycle (d-c) were digitally adjusted to achieve the phase variations [7]. In this
paper, we analyze the benefit of using Gaussian or Nyquist shaping filters in the Tx to increase
the spectral efficiency, extending the results of [8]. We first do an evaluation through numerical
simulations. Then we present the experimental validation in an UDWDM-PON. This is, to the
best of our knowledge, the first simple directly phase modulated Tx with spectral confinement for
a coherent UDWDM-PON. At the receiver (Rx), an intradyne detector recovered the data
achieving sensitivity values of -49 dBm and -44 dBm for bitrates (Rb) of 1.25 Gb/s and 2.5 Gb/s
respectively at BER of 10
-3
. Furthermore, sensitivity penalties < 1 dB are observed in channels
separated only by 6.25 GHz and tolerating a differential link-loss of 15 dB [9].
2. Numerical Simulations
The simulations were performed with VPITranmissionMaker® and MATLAB® following the
Monte-Carlo method. The simulation setup was composed of two users with Tx consisting of an
ideal phase modulator (PM) which generated a 0 - 180º phase shift keying (PSK) signal. Both
users were combined in a 3dB optical coupler as presented in Figure 1a.
The modulating data was a 2
15
-1 Pseudo Random Binary Sequence (PRBS) differentially
encoded at
of 1.25 Gb/s with rise-time
0.1
, where T
b
is the bit period.
Additional electrical filtering was applied to data before optical modulation to modify the
dynamics of the electrical pulses and shaping its spectrum.
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Figure 1. (a) Simulation Setup. (b) Sensitivity penalty @BER=10
-3
vs normalized filter BW for the three
pulse-shaping schemes.
We considered two shaping filter types: Gaussian and Raised-cosine. In the first one, the
sharp symbol transitions were smoothed to Gaussian dynamics with a rise-time
depending on
the 3-dB filter bandwidth (BW) calculated as  0.35
. On the second hand, the Raised–
cosine adjusted the excess filter BW beyond the Nyquist BW: 12
. The parameter that defined
the filter was the roll-off (α) which ranged between 0 and 1. For this shaping technique, the lower
α, the lower the total filter BW and the higher spectral compression. The filter BW was expressed
as:  󰇛1 α󰇜
2
.
The optical power of each generated differential PSK (DPSK) signal was set to 0 dBm
and launched through 25 km of single-mode fiber (SMF). Then a variable optical attenuator
(VOA) emulated further splitting and adjusted the power at the Rx. The state of polarization
(SOP) of the transmitted signal was matched with that of the local oscillator (LO) by means of a
polarization controller (PC). The signal was detected with a coherent intradyne Rx based on a 3x3
optical coupler [10].
In order to find the optimal parameters (
andα󰇜for the shaping filters, we set the input
power at R
X
at the value that produced BER = 10
-3
for Non-return-to-zero (NRZ) format. Then,
we computed the power penalty due to the Gaussian filter BW. The results are shown in Figure
1b. A power penalty larger than 1 dB was observed for 1.2
. Therefore, the Gaussian
filter was set at  1.2
(corresponding to
0.29
󰇜. For the Raised-cosine filter, the
power penalty at maximum bandwidth 1
(corresponding to α = 1) was 3.5 dB.
Figure. 2 depicts the original NRZ electrical spectrum before and after the filters. In the NRZ
signal, we observe that the sharp transitions between symbols result in high-frequency harmonic
components. There is significant power beyond 1.25 GHz and the main to secondary spectral
lobes power suppression (MSPS) is 14dB. With electrical Gaussian filtering, the symbol
transitions are smoothed and the MSPS is 18 dB. When using the Raised-cosine filter, the
electrical pulses are sinc-shaped and ideally exhibit a square spectrum. As seen in Figure 2, the
MSPS is >60 dB.
Figure 2. Electrical spectra of 1.25 Gb/s PRBS data and eye diagrams for each case: (a) NRZ, (b) Gaussian,
(c) Raised-cosine pulse shaping at the Tx.
Intradyne
Rx
1544.9nm
LO
25kmSMF
VOA
PC
DPSKTx
2

7.5GHz


DPSKTx
1
PM
CW
Pulse
shapingfilter
PRBS
Differential
encoder
(a) (b)
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A second user was added into the network with identical pulse shaping. Figure 3 plots
the optical spectra of both channels separated 6.25 GHz. The NRZ, Gaussian shaped and Raised-
cosine filtered signals show an MSPS of 12 dB, 13 dB and 19 dB respectively. The latter thus is
expected to reduce the channel spectral separation. It is interesting to note that the optical MSPS
values are lower than the electrical because of the non-linear characteristic of the phase
modulation. This produced strong Bessel harmonic components in the optical domain.
Next, the minimum channel spacing was evaluated. For this, the second user optical
frequency was shifted from -7.5 GHz to 7.5 GHz with respect to the other user emission
frequency. The BER degradation was measured in the fixed user (Figure 4a). These values were
translated into power penalty at a sensitivity of BER=10
-3
considering the single channel as
reference (Figure 4b). The values at 1 dB penalty were 2.8 GHz, 2.2 GHz and 1.8 GHz for NRZ,
Gaussian and Raised-cosine pulse-shaping respectively. Notably, there was no penalty at a 6.25
GHz spacing and even less than 2.5 GHz could be achieved when shaping the pulse.
Figure 3. Optical spectra for two users separated by 6.25 GHz for three different pulse-shaping schemes:
(a) NRZ, (b) Gaussian, (c) Raised-cosine.
Figure 4. (a) BER against Channel Spacing for Tx
1
using three different pulse-shaping filters. (b) Sensitivity
penalty @BER=10
-3
for the three pulse-shaping schemes.
3. Experimental setup
Once the technique was evaluated through simulations, we implemented the UDWDM-PON
scenario depicted in Figure 5. The setup was composed by two identical transmitters (Tx
1
and
Tx
2
) based on direct modulated DFBs with linewidths of 4 MHz and 3 MHz and modulation BW
of 10 GHz and 2.5 GHz respectively. The DFBs were biased at 75 mA, value at which we
observed that the adiabatic chirp dominated.
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The data consisted of two uncorrelated differentially encoded 2
15
-1 Pseudo Random
Binary Sequences (PRBS) at Rb of 1.25 Gb/s and 2.5 Gb/s. Each sequence modulated a separate
DFB. The data sequences were digitally equalized by means of a 1-tap finite impulse response
(FIR) filter with half-bit period delay. The result was a Dicode Return-to-zero (Dicode RZ) signal
whose duty-cycle was adjusted to 50%. Then, its spectrum was shaped with a Raised-cosine FIR
filter of 64-taps. The resulting samples were uploaded to a 20 GSa/s arbitrary waveform generator
(AWG). The outputs were amplified with an 8 GHz (SHF98P) and 12 GHz (TBI-781) BW
electrical amplifiers for Tx
1
and Tx
2
respectively. The waveform amplitude was optimized to
ensure 0-π phase changes when directly modulating the DFB and producing, as a result, optical
DPSK signals [7]. The optical signals were combined with a 3-dB optical coupler and sent through
25 km of single mode fibre (SMF) with a launched power of 0 dBm. A variable optical attenuator
(VOA) reproduced splitting losses and limited the power into the Rx.
The Rx consisted of an intradyne detector based on a 3x3 optical coupler which mixed
the incoming optical signal with a LO. The latter was a 100 kHz linewidth external cavity laser
(ECL) with 0 dBm optical power emitting at λ
LO
= λ
1
= 1544.9 nm. The three outputs of the optical
coupler were detected with 10 GHz p-i-n- photodiodes (PDs) followed by low-noise electrical
amplifiers. The electrical signals were low-pass filtered, sampled and processed with a 50 GSa/s
real-time oscilloscope (RTO). The three currents were combined and processed to obtain the in-
phase (I) and quadrature (Q) parts as in [10]. Each component was then differentially demodulated
with a bit-delay and multiply operation, and then both were added. Afterwards, the samples
passed through a 4th order low-pass filter with
cut-off frequency and the BER was computed.
Figure 5. Experimental setup; the inset shows the data stream with Nyquist shaping and the RF spectra of
both Txs spaced 6.25 GHz.
4. Results
Two electrical signals, one with and another without Nyquist-shaping, were digitally
generated. Figure 6a shows the electrical data at Rb = 1.25 Gb/s along with the Nyquist shaped
waveform for a roll-off factor (α) of 0.25 (upper) and 1 (lower). Figures 6b, 6c, 6d and 6e present
the photo-detected spectra at a relative intermediate frequency (IF) of 10 GHz for signals at
bitrates of 1.25 Gb/s and 2.5 Gb/s. We observe that the lateral lobes are eliminated, but the main
lobe is not compacted as expected with externally modulated Txs [6]. This is because we are
directly modulating the phase of the laser, which has nonlinear dynamics, and leads to harmonics
due to the Bessel function solution. However, the BW of the residual intensity modulation (IM)
is narrowed. This is particularly noticed for lower α value. In contrast with PM, the IM residual
term is generated with a linear modulation. Finally, we decided to use α = 1 since the effect of
lower α is barely perceived.
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Citations
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Journal ArticleDOI
TL;DR: Results indicate that this new class of CoTRX enables effective implementation of wavelength-to-the-user PON with dedicated 1.25–20 Gb/s per user, in an ultra-dense 6–25 GHz spaced WDM optical grid, with >30 dB loss budget, outperforming the current competing technologies for access networks.
Abstract: Coherent technologies, along with digital signal processing (DSP), have revolutionized optical communication systems, significantly increasing the capacity of the fiber channel owing to transmission of advanced modulation formats and mitigation of propagation impairments. However, commercial solutions for high-capacity core networks are too complex and costly, and therefore hardly feasible, for access networks with high terminal density, where cost, power budget, and footprint are the main limiting factors. This article analyzes the key enabling techniques to implement a complexity-reduced coherent transceiver (CoTRX) by exploiting photonic integration, simplified optical modulation, low-cost DFB lasers, consumer electronics, and low-complexity DSP. Bulk optical modulators are replaced by direct amplitude-and-phase modulation of an integrated electro-absorption modulated laser (EML) with a smaller footprint, generating up to 8-ary modulation formats. Hardware-efficient DSP algorithms for the coherent transmitter and receiver, including pulse-shaping for direct phase modulation, differential detection for optical phase recovery, and digital pre-emphasis with enhanced tolerance to quantization noise, are investigated to face the challenges imposed by low-cost photonic and electronic devices, such as strong phase noise, wavelength drifts, severe bandwidth limitation, and low resolution data converters. Through numerical simulations and real-time experiments, the results indicate that this new class of CoTRX enables effective implementation of wavelength-to-the-user PON with dedicated 1.25–20 Gb/s per user, in an ultra-dense 6.25–25 GHz spaced WDM optical grid, with >30 dB loss budget, outperforming the current competing technologies for access networks.

5 citations


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Abstract: We present a lookup table (LUT)-free carrier recovery architecture for intradyne optical DPSK receivers that reduce the required digital signal processing hardware resources, power consumption as well as total process clocks, aimed at cost-effective transceivers for access networks applications. The proposed architecture simplifies frequency compensation algorithm to avoid using m th-power operation and LUTs. We prototyped the proposed carrier recovery on a commercial FPGA for real-time evaluation with data at 1.25 Gb/s. The optical transmission system is implemented by direct-phase modulation of commercial DFB lasers, 25 km of single-mode fiber, and a coherent receiver with low-cost optical front-end based on 3 × 3 coupler and three photodiodes providing phase-diversity operation. Results show high performance in real-time, achieving –54 dBm sensitivity at BER = 10–3 as well as feed-forward frequency error correction, high robustness against the fast frequency laser drifts, and high tolerance to optical phase noise in a 6.25 GHz spaced ultradense wavelength division multiplexing grid.

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Proceedings ArticleDOI
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TL;DR: By means of pulse shaping, direct laser modulation reveals as an efficient low-cost option for future coherent ultra-dense-WDM-PON access systems, achieving sensitivities below −40 dBm with channel spacings of 6.25 GHz only.
Abstract: By means of pulse shaping, direct laser modulation reveals as an efficient low-cost option for future coherent ultra-dense-WDM-PON access systems, achieving sensitivities below −40 dBm with channel spacings of 6.25 GHz only.

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01 Jan 2019
TL;DR: By designing very simple but efficient clock recovery, a symbol-rate DSP architecture, which process data using only one sample per symbol, for polarization diversity (POD) structure, becomes possible and makes the DSP independent from state-of-polarization (SOP), even in the case of low-cost optical front-end and low-speed analog-to-digital converters (ADCs).
Abstract: Nowadays, optical access networks provide high capacity to end users with growing availability of multimedia contents that can be streamed to fixed or mobile devices. In this regard, one of the most flexible and low-cost approaches is Passive Optical Network (PON) that is used in Fiber-to-the-Home (FTTH). Due to the growing of the bandwidth demands, Wavelength Division Multiplexing (WDM), and later on ultra-dense WDM (udWDM) PON, with a narrow channel spacing, to increase the number of users through a single fiber, has been deployed. The udWDM-PON with coherent technology is an attractive solution for the next generation optical access networks with advanced digital signal processing (DSP). Thanks to the higher sensitivity and improved channel selectivity in coherent detection with efficient DSP, optical networks support larger number of users in longer distances. Since the cost is the main concern in the optical access networks, this thesis presents DSP architectures in coherent receiver (Rx), based on low-cost direct phase modulated commercial DFB lasers. The proposals are completely in agreement with consept of wavelength-to-the-user, where each client in optical network is dedicated to an individual wavelength. Next, in a 6.25 GHz spaced udWDM grid with the optimized DSP techniques and phase-shift-keying (PSK) modulation format, the high sensitivity is achieved in real-time field-programmable-gate-array (FPGA) implementations. Moreover, this thesis reduces hardware complexity of optical carrier recovery (CR) with two various strategies. First, based on differential mth-power frequency estimator (FE) by using look-up-tables (LUTs) and second, LUT-free CR architecture, with optimizing the power consumption and hardware resources, as well as improving the channel selectivity in terms of speed and robustness. Furthermore, by designing very simple but efficient clock recovery, a symbol-rate DSP architecture, which process data using only one sample per symbol (1-sps), for polarization diversity (POD) structure, becomes possible. It makes the DSP independent from state-of-polarization (SOP), even in the case of low-cost optical front-end and low-speed analog-to-digital converters (ADCs), keeps the performance high as well as sensitivity in real-time implementations on FPGA.

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Abstract: Lightwave communications is a necessity for the information age. Optical links provide enormous bandwidth, and the optical fiber is the only medium that can meet the modern society's needs for transporting massive amounts of data over long distances. Applications range from global high-capacity networks, which constitute the backbone of the internet, to the massively parallel interconnects that provide data connectivity inside datacenters and supercomputers. Optical communications is a diverse and rapidly changing field, where experts in photonics, communications, electronics, and signal processing work side by side to meet the ever-increasing demands for higher capacity, lower cost, and lower energy consumption, while adapting the system design to novel services and technologies. Due to the interdisciplinary nature of this rich research field, Journal of Optics has invited 16 researchers, each a world-leading expert in their respective subfields, to contribute a section to this invited review article, summarizing their views on state-of-the-art and future developments in optical communications.

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TL;DR: A single-ended colorless coherent receiver using symmetric 3×3 coupler hybrids is demonstrated, allowing for colorless reception of fifty-five 112-Gb/s PDM-QPSK channels with 1-dB penalty and the receiver works well in transmission over 2560-km TrueWave-REACH-fiber.
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Journal ArticleDOI
Abstract: New technologies for ultradense WDM-PON (udWDM-PON), enabled by coherent techniques and low-cost devices, are developed for an efficient utilization of the optical spectrum, revealing that the “Wavelength-to-the-User” concept can be feasible. In this paper, an udWDM-PON with only 6.25-GHz channel spacing is implemented with conventional DFB lasers, for a splitter-based PON infrastructure with 256 ONUs. The results of the analysis of udWDM access network architecture with respect to their associated complexity, cost, and migration scenarios, exhibit the potential for higher aggregate throughput, higher split ratios, and node consolidation, when compared to competing technologies.

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Proceedings ArticleDOI
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TL;DR: A new ultra-dense WDM-PON with coherent techniques and low cost devices is developed for an efficient utilization of the optical spectrum, showing that the “Wavelength-to-the-User” concept can be feasible with 6.25 GHz channel spacing in a splitter-based PON.
Abstract: A new ultra-dense WDM-PON with coherent techniques and low cost devices is developed for an efficient utilization of the optical spectrum, showing that the “Wavelength-to-the-User” concept can be feasible with 6.25 GHz channel spacing in a splitter-based PON.

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Journal ArticleDOI
TL;DR: The successful field trial of the proposed systems in a testbed where 14 UDWDM channels are transmitted simultaneously in a dark-fiber network deployed in the city of Pisa (Italy), delivering real-time and/or test traffic is reported.
Abstract: We experimentally demonstrate an innovative ultradense wavelength division multiplexing (UDWDM) passive optical networks (PON) that implements the full $\lambda \text {-to-the-user}$ concept in a filterless distribution network. Key element of the proposed system is a novel class of coherent transceivers, purposely developed with a nonconventional technical approach. Indeed, they are designed and realized to avoid D/A–A/D converter stages and digital signal processing 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 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 $\text{6.25}\text{-}\text {GHz}$ grid), real-time GbE transmissions (by using either fully analog or light digital signal processing), multirate transmission ( $\text{1.25}\; \text{and}\; \text{10}\,\text {Gb/s}$ ), high optical distribution network loss (18–40 dB) as well as a bidirectional channel monitoring system.

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Frequently Asked Questions (1)
Q1. What are the contributions in this paper?

In this paper, the authors proposed direct phase modulation of a distributed feedback ( DFB ) laser for simplifying the Tx.