Homodyne Ultra dense WDM-PONs: can they be affordable in access

Homodyne Ultra dense WDM-PONs: can they be
affordable in access?

Josep Prat
`
Josep M. Fabrega
Ronald Freund

`
Universitat Politecnica de Catalunya - BarcelonaTech
Fraunhofer Institute for Telecommunications - Heinrich Hertz Institut
jmfabrega@tsc.upc.edu

June 10, 2010

`
J. Prat, J. M. Fabrega, R. Freund (UPC-HHI) Homodyne UD-WDM PONs June 2010 1 / 27

Outline

Introduction

Transceiver architectures

Case studies

Conclusions

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Introduction

Novel multimedia applications
Voice over IP
Video on demand
HDTV
User bit rate demand expected to be increasing
Nielsen law: bandwidth per user increments in a 50 % per year
In 2020 each user would demand an average bandwidth of 1 Gb/s

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FTTH roadmap and tendencies in PONs

Actual tendencies:
PON standardization bodies pushing towards high capacity
systems by increasing the aggregate bit rate (10 Gb/s)
ONU operates at a very high bit rate in the opto-electronics
transceivers just to use a small fraction of it (≈ 3 %)
High power consumption!
New philosophy proposed, exploiting the pure WDM dimension
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Towards this new philosopy

Advantages Drawbacks
IM-DD
Simplicity sensitivity
optical ﬁlters selectivity

Coherent
Use of advanced modulation formats Image frequency (heterodyne)
Electrical ﬁltering for channel selection Phase noise (Homodyne)
Detection amplitude, phase and polarization
Linear transformation optical ⇒ electrical
Increase of sensitivity

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How to minimize this phase error?

OPLLs: Several architectures proposed
Decision driven [1]
Costas [2]
Balanced [3]
Subcarrier modulated [4]
Phase diversity with zero IF receiver
Analog mutiple differential detection [5]
Digital phase estimation:
Wiener ﬁlter [6, 7]
Regenerative frequency dividers [8]
Viterbi & Viterbi [9, 10]

But many of them are not the cheap solutions we search for!

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How to minimize this phase error?

OPLLs: Several architectures proposed
Decision driven [1]
Costas [2]
Balanced [3]
Phase diversity with zero IF receiver
Analog mutiple differential detection [5]
Digital phase estimation:
Wiener ﬁlter [6, 7]
Regenerative frequency dividers [8]
Viterbi & Viterbi [9, 10]

But many of them are not the cheap solutions we search for!
New solutions have to be proposed

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Lock-In amplified OPLL

Low-cost PLL based on balanced OPLL [3]

Compared to other oPLL architectures:
Costas [2]
Balanced [3]
10 ns loop delay (eq. 20 cm of fiber)

Linewidth tolerance BER 10−9 Linewidth tolerance BER 10−3
Balanced 420 kHz 1.2 MHz
Costas 1.15 MHz 2.65 MHz
SCM 1.35 MHz 2.75 MHz
Lock-In amplified 675 kHz 3.1 MHz

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Lock-In amplified OPLL

Low-cost PLL based on balanced OPLL [3]

Compared to other oPLL architectures:
Costas [2]
Balanced [3]
10 ns loop delay (eq. 20 cm of fiber)

Linewidth tolerance BER 10−9 Linewidth tolerance BER 10−3
Balanced 420 kHz 1.2 MHz
Costas 1.15 MHz 2.65 MHz
SCM 1.35 MHz 2.75 MHz
Lock-In amplified 675 kHz 3.1 MHz
Lock-In amplifier OPLL is a competitive low-cost solution!
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Time switched phase diversity

Phase diversity by switching from I to Q component at each bit
Less components duplicity than standard phase diversity (lower cost)
3 dB penalty respect to an ideal system
Linewidth tolerance for 10−3 BER-ﬂoor: 1.8 % of bitrate [11]
3 GHz channel spacing for 1 dB penalty at BER of 10−9 [12]
A similar structure has been recently integrated in InP substrate [13]
A more simple structure is achieved when driving the laser directly [14]

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Time switched phase diversity

Phase diversity by switching from I to Q component at each bit
Less components duplicity than standard phase diversity (lower cost)
3 dB penalty respect to an ideal system
Linewidth tolerance for 10−3 BER-ﬂoor: 1.8 % of bitrate [11]
3 GHz channel spacing for 1 dB penalty at BER of 10−9 [12]
A similar structure has been recently integrated in InP substrate [13]
A more simple structure is achieved when driving the laser directly [14]

Time-switched phase diversity provides high performances at low cost

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Outline

Introduction


Case studies

Conclusions

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Summary of the performances
Phase noise
Technique Linewidth Penalty Required key Complexity
tolerance component
Decision-drive loop 5 MHz 0 dB 90◦ hybrid High
Costas loop 4.9 MHz 0 dB 90◦ hybrid Medium/High
Subcarrier loop 5.1 MHz 0 dB 90◦ hybrid High
Balanced loop 2.4 MHz 2 dB Optical coupler Low
Lock-In loop 6.4 MHz 1 dB Optical coupler Low
Full phase diversity 5% bitrate 0 dB 90◦ hybrid Medium
Time-switch (Scrambler) 1.8% bitrate 4 dB Phase modulator Medium
Time-switch (Direct drive) 3.4% bitrate 4 dB High-chirp laser Low

Penalty is respect to an ideal system whereas tolerance is for a BER-ﬂoor of 10−3

Three main approaches: Optical phase locked loop, full phase diversity and time-switched diversity

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Summary of the performances
Phase noise
Technique Linewidth Penalty Required key Complexity
tolerance component
Decision-drive loop 5 MHz 0 dB 90◦ hybrid High
Costas loop 4.9 MHz 0 dB 90◦ hybrid Medium/High
Subcarrier loop 5.1 MHz 0 dB 90◦ hybrid High
Balanced loop 2.4 MHz 2 dB Optical coupler Low
Lock-In loop 6.4 MHz 1 dB Optical coupler Low
Full phase diversity 5% bitrate 0 dB 90◦ hybrid Medium
Time-switch (Scrambler) 1.8% bitrate 4 dB Phase modulator Medium
Time-switch (Direct drive) 3.4% bitrate 4 dB High-chirp laser Low

Penalty is respect to an ideal system whereas tolerance is for a BER-floor of 10−3

Three main approaches: Optical phase locked loop, full phase diversity and time-switched diversity

Polarization mismatch
Local control Polarization diversity Polarization switching
Penalty 0 dB 0 dB 3 dB
Key component Polarization actuator Pol. beam splitter Pol. scrambler/switch
Response time 1 ms – 1 s < 10 µs < 10 µs
Complexity High Med./high Low (if placed at CO)

Several transceiver architectures are going to be discussed in the following slides:
The targeted modulation format is DPSK/BPSK, although other multilevel modulations can be used
Channel selection is performed by tuning the local laser to the right wavelength and filtered by the electrical filters
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Operation specifications

Bitrate fixed at 1 Gb/s, to directly transmit the common EPON
protocol over fiber
Downstream modulation format to be used is PSK, because of its
good trade-off between performances and simplicity
Upstream modulation format
PSK preferred
IM can be also used with an asymmetrical up/down data rate to not
to penalize optical power budget
Digital and analog signal processing adopted against impairments
Assume a double fiber network, avoiding Rayleigh backscattering

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Transceiver architectures proposed
A D G

B
H
E

C
F I

Architectures A, C, E and G are intended for BPSK modulation
DPSK modulation format has to be used in architectures B, D, F, and I
Polarization is managed at OLT for architectures A, B, E, F, G, H and I
In transceivers C and D a PBS used for achieving polarization diversity
For digital approaches (A, C, E, G), Inside digital I and Q post-processing several basic operations are performed:
phase estimation
frequency estimation and control
data estimation
polarization switching combination
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Transceiver comparison
Arch. Phase handling Polarization handling Processing Sens. penalty Linewidth tolerance Cost
A 90◦ hybrid Switch at CO Digital 3 dB 5 MHz Med./High
B 90◦ hybrid Switch at CO Analog 4 dB 5 MHz Med./High
C 90◦ hybrid PBS Digital 0 dB 5 MHz Very high
D 90◦ hybrid PBS Analog 1 dB 5 MHz Very high
E Switch (Scr.) Switch at CO Digital 6 dB 1.8 MHz Medium
F Switch (Scr.) Switch at CO Analog 7 dB 1.8 MHz Medium
G Switch (Dir.) Switch at CO Digital 6 dB 3.4 MHz Low
H Switch (Dir.) Switch at CO Analog 7 dB 3.4 MHz Low
I OPLL Switch at CO Analog 4 dB 675 kHz Low

Architecture with 90◦ hybrids a PBS and DSP (C):
No additional penalty with respect to an ideal system
Achieves high linewidth tolerance
It is costly because it implies the duplication of many components needed
Its cost can be low if novel fabrication techniques are used [15]

Direct-drive time-switching (G):
High penalty (6 dB)
High linewidth tolerance
Reduced complexity and cost
Requires a fully engineered laser capable to be phase modulated

OPLL approach (I):
Low complexity
Lower linewidth tolerance (675 kHz)
Delay associated to the optical path length
Local laser should be embedded with the optical reception front-end

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Outline

Introduction


Case studies

Conclusions

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Case studies

Two cases of future deployment were tested in the laboratory:
Subband WDM tree PON, featuring wavelength grooming [16]
Ring-tree ultra-dense WDM-PON, with transparent remote nodes [17]
Both networks are based on the ultra-dense WDM concept, aiming to give
service to a high number of users (around 1000), at very high speed (1 Gb/s)

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Subband WDM tree PON

4 GHz channel spacing and 1 Gb/s data rate
32 channels accommodated in an ITU-T G.694.1
100-GHz D-WDM channel
Serve 40 x 32 = 1280 users ⇒ more than 1 Tb/s
25 km ﬁber spool simulated the access trunk ﬁber
Losses at the AWG were measured to be 6.47 dB
1:32 power splitter, adding 16 dB losses
Total network losses were measured to be 27.67 dB
9 dBm optical output power at CO
−38.7 dBm of sensitivity (BER=10−9 )
Power budget calculated to be 47.7 dB

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Ring-tree ultra-dense WDM PON

Totally passive and transparent
Simple and resilient architecture
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Experimental evaluation

Network designed to offer connectivity to 1024 users
4-node conﬁguration
8 secondary trees with 1:128 splitting factor
4 GHz channel spacing
Three different cases were investigated: RN1, RN2 and RN4

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Results and discussion

BER = 10−9 BER = 10−3
RN1 RN2 RN4 RN1 RN2 RN4
Sensitivity −43 dBm −41.3 dBm - −49.1 dBm −49.3 dBm −49.1 dBm
Link Losses 39.4 dB 41 dB 44.2 dB 39.4 dB 41 dB 44.2 dB
Power Budget 42.9 dB 41.2 dB - 49 dB 49.2 dB 49 dB

In normal operation (BER = 10−9 ), the maximum power budget reached is 41.2 dB arriving to RN2
In resilient mode FEC codes are used to overcome the possible ﬁber cut
Using FEC codes BER = 10−3 is operable and RN4 can be reached, featuring a power budget of 49 dB

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Outline

Introduction


Case studies

Conclusions

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Conclusions

The overall goal of this study was to find whether homodyne
transceivers are affordable or not for upgrading the current
standard PONs
It was found that they are more power and bandwidth efficient
Several transceiver architectures have been proposed and
discussed
Trade-off between performances and cost is difficult to overcome
Time-switched diversity transceiver has been implemented
Upgrading of PON architectures has been discussed for
implementing full ultra-dense WDM networks
Laboratory testbeds have been developed for two network concepts
The proposed topologies have been demonstrated to be feasible,
achieving transmission of up to 1 Gb/s in links higher than 25 km

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Future lines

Some improvements to be achieved, resulting in a step forward:
Compact coherent transceiver
State of polarization mismatch between local oscillator and
received signal
Careful design of the modulation formats to be used
Full bidirectionality over a single fiber
Rayleigh backscattering
Light reflections
Spectrum management
Spectral efficiency maximization
Wavelength monitoring, control and stabilization
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Thank you!!
Time for questions...

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Bibliography I

[1] L. G. Kazovsky.
Decision-driven phase-locked loop for optical homodyne receivers: performance analysis and laser linewidth requirements.
IEEE / OSA Journal of Lightwave Technology, 3:1238, 1985.
[2] S. Norimatsu, K. Iwashita, and K. Sato.
Psk optical homodyne detection using external cavity laser diodes in costas loop.
IEEE Photonics Technology Letters, 2(5), 1990.
[3] L. G. Kazovsky.
Balanced phase-locked loops for optical homodyne receivers: performance analysis, design considerations, and laser
linewidth requirements.
IEEE / OSA Journal of Lightwave Technology, 4:182, 1986.
[4] S. Camatel, V. Ferrero, and P. Poggiolini.
2-psk homodyne receiver based on a decision driven architecture and a sub-carrier optical pll.
In Proceedings of the Conference on Optical Fiber Communication and the National Fiber Optic Engineers Conference,
2006 (OFC/NFOEC 2006), Anaheim (CA), March 2006.
[5] Y. H. Cheng, T. Okoshi, and O. Ishida.
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IEEE / OSA Journal of Lightwave Technology, 7:368–374, 1989.
[6] M. G. Taylor.
Phase estimation methods for optical coherent detection using digital signal processing.
IEEE / OSA Journal of Lightwave Technology, 27:901–913, 2009.
[7] M. G. Taylor.
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Bibliography II
[8] R. Noe.
Phase noise-tolerant synchronous qpsk/bpsk baseband-type intradyne receiver concept with feedforward carrier recovery.
IEEE / OSA Journal of Lightwave Technology, 23(2), 2005.
[9] D. Van den Borne, C. R. S. Fludger, T. Duthel, T. Wuth, E. D. Schmidt, C. Schulien, E. Gottwald, G. D. Khoe, and
H. de Waardt.
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In Proceedings of 33th European Conference on Optical Communications (ECOC 2007), Berlin (Germany), September
2007.
[10] M. Seimetz.
Laser linewidth limitations for optical systems with high-order modulation employing feed forward digital carrier phase
estimation.
2008 (OFC/NFOEC 2008), San Diego (CA), March 2008.
[11] J. M. Fabrega and J. Prat.
Homodyne receiver prototype with time-switching phase diversity and feedforward analog processing.
OSA Optics Letters, 32(5), 2007.
Experimental investigation of channel crosstalk in a time-switched phase diversity optical homodyne receiver.
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[13] A. Ramaswamy, L. A. Johansson, J. Klamkin, C. Sheldon, H. F. Chou, M. J. Rodwel, L. A. Coldren, and J. E. Bowers.
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In Proceedings of 33th European Conference on Optical Communications (ECOC 2007), Berlin (Germany), September
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Bibliography III

[15] N. Keil, H. Yao, C. Zawadzki, W. Doldissen, W. Schlaak, M. Mohrle, and D. Schmidt.
Polymer as integration platform for low-cost devices in future optical networks.
In Proceedings NOC06, Berlin (Germany), July 2006.
[16] C. Bock, J. M. Fabrega, and J. Prat.
Ultra-dense wdm pon based on homodyne detection and local oscillator reuse for upstream transmission.
In Proceedings of 32th European Conference on Optical Communications (ECOC 2006), Cannes (France), September
2006.
Ultra-dense, transparent and resilient ring-tree access network using coupler-based remote nodes and homodyne
transceivers.
In Proceedings of International Conference on Transparent Optical Networks ICTON’09, Ponta Delgada (Azores, Portugal),
July 2009.

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