20-G
Gb/s, 20--km WD
DM-PON
N Upstrream Trransmisssion usin
ng
4-P
PAM Modulated
d Free-R
Runningg 1550 n
nm VCS
SEL and
d
Adaptiive SC-F
FDE
Hongg
guang Zhang, Xiaofei Chen
ng, Zhaowen X
Xu
Institu
ute for Infocomm Research,
R
Agency for
f Science, Technnology and Researrch, Singapore, 1338632.
E-maill: [email protected]
Abstract:
A
We propose
p
a noveel 20Gb/s WDM-PON upstreeam transmissiion scheme ussing 4-PAM
modulated
m
freee-running 155
50nm VCSEL
L and adaptiive Single-Caarrier Frequenncy-Domain
Equalization (SC
C-FDE). A tran
nsmission distaance of 20km iis achieved in ddirect detectionn system.
OCIS codes: (140.7
7260) Vertical surfface cavity emittin
ng diodes; (060.45 10) Optical comm
munications
1. Introdu
uction
Vertical Caavity Surface Emitting
E
Laser (VCSEL) is an
n attractive canndidate for low
w-cost, low pow
wer consumptioon,
high-speed
d optical sourcee, and it can bee potentially ap
pplied in Waveelength-divisionn-multiplexed passive opticall
network (W
WDM-PON). However,
H
chrom
matic dispersio
on (CD) limits the transmissioon distance of VCSEL at bit rates
of 10-Gb/ss and above [1]]. Inverse-dispeersion fiber [2]], optical injecttion locking [3] and optical fiilter [4] are alw
ways
used to mittigate the degraading effects of
o CD, and shorrt PRBS word length (27-1) is used in testinng to get longerr
transmissio
on distance [2]. DSP algorith
hms can also bee used to mitigaate the degradaation caused byy CD. In [5], S
SingleCarrier Freequency-Domaain Equalization
n (SC-FDE) is proposed to coompensate chrromatic disperssion (CD)
impairmen
nts in coherent optical transmiission systems. However, it iis ineffective foor direct detecttion systems siince
signal phasse information is not preserveed in square-laaw detection [6 ]. To solve thiss problem, we propose to usee
adaptive SC-FDE to com
mpensate for CD
D distortion in direct detectioon systems.
In ordeer to improve sp
pectral efficien
ncy, multilevel modulation suuch as four-levvel pulse amplittude modulatioon (4PAM) is in
ntroduced in op
ptical communication [7]. In this paper, we propose and ddemonstrate a nnovel 20-Gb/s
upstream trransmission sccheme for WDM
M-PON by usiing SC-FDE annd 4-PAM moddulated 1550nm
m VCSEL. 4-P
PAM
is applied to
t modulate thee VCSEL at a symbol rate off 10 GBaud, annd thus 20Gbit//s IM-DD transsmission system
m is
realized. Experiment
E
resu
ults show that a transmission distance of 200km can be achhieved after appplying the adapptive
SC-FDE allgorithm.
Figure 1: WDM-PON
W
upstreeam transmission scheme
s
using 4-PA
AM signal modulaated VCSEL and aadaptive SC-FDE. RN: remote node;; A/D:
analog-to-diigital conversion; Syn.:
S
synchronizattion; S/P: series to parallel conversioon; Rem. CP: CP rremoval; P/S: paraallel-to-serial convversion;
o.: demodulation.
Demo
2. WDM-PON upstream
m transmissio
on scheme
Fig. 1 illusstrates the prop
posed WDM-PO
ON upstream transmission
t
sccheme using 4--PAM signal m
modulated VCS
SEL
and adaptiv
ve SC-FDE. Th
he 1550-nm VCSELs are dep
ployed at the O
ONUs as low-coost, low powerr consumption,, highspeed upstrream optical so
ources. 4-PAM
M signal with cy
yclic prefix (CP
P) and preambble inserted is ddirectly modulaated
onto the VCSEL. After trransmission ov
ver fiber, uplink
k signals are seent to the receiivers via a DEM
MUX in the central
office (CO
O), and then posst-processed offfline with the adaptive SC-F
FDE algorithm.. As an alternattive to OFDM,, SCFDE has siimilar perform
mance and the saame DSP comp
plexity, but sufffers less nonliinear impairmeents due to its llower
peak-to-av
verage power raatio [8]. In add
dition, because both the Fast F
Fourier Transfoorm (FFT) andd Inverse Fast
Fourier Traansform (IFFT
T) are processed
d at the receiveer, the transmittter of SC-FDE
E system is sim
mpler than that of
OFDM, an
nd therefore it is
i more suitablee to be implem
mented in the O
ONUs of WDM
M-PON systemss.
The DSP
P block structu
ure of the adapttive FDE is alsso shown in Figg. 1. Received signal is digitiized and
synchronizzed, and then itt is framed into
o blocks by S/P
P convertor. Daata sequence iss transformed into frequency
domain by
y FFT, and equaalized block-by
y-block, while feed forward ffilter deals withh the major intter-symbol
interferencce (ISI) and feeedback filter haandle the residu
ual. Both filterrs are adaptivelly updated baseed on low
computatio
onal frequency
y domain least-m
mean-square (L
LMS) algorithm
m [9]. The equualization undeergoes training stage
in the preamble period an
nd decision-dirrected stage in the data periodd. In the processsing, overlap--save method iss used
to perform
m linear convolu
ution effectivelly and fractional spaced equaalization (FSE)) with 2 samplees per symbol iis
used to avo
oid aliasing. After removing CP,
C the equalizzed blocks are changed back to a serial sym
mbol stream by the
P/S converrtor and then deemodulated to binary signal. In our proposeed scheme, no injection lockiing or dispersioon
compensattion fiber is useed. Because mo
ost of the comp
plex DSP is peerformed at thee receiver, the ttransmitter cann be
kept very simple
s
and doeesn’t require high-speed digitaal-to-analog coonverters (DAC
C). Thus, the O
ONUs in SC-FD
DE
system can
n be significanttly less expensiive than that off OFDM.
3. Experim
ment Setup
The experiiment setup is shown
s
in Fig. 2.
2 At the transm
mitter, the 4-PA
AM signal wass generated by combining twoo
binary pseu
udo-random biit sequence (PR
RBS) data sign
nals from a pulsse pattern geneerator (PPG). U
Using MATLA
AB, a
PRBS sequ
uence with worrd length of 2166 was generated
d and grouped into blocks, annd CP was inseerted at the endd of
each block
k. The Preamble sequence waas composed off 8 random binaary sequences with word lenggth of 1024 forr each
sequence. The
T preamble sequence was also grouped in
nto blocks withh CP inserted. After preamble insertion, thee
binary dataa sequence wass loaded into th
he PPG. The diifferential outpputs of the PPG
G were combined with an RF
power com
mbiner and one output was deelayed by a tunaable phase-shiffter. By adjustiing the delay tiime and the
amplitude of differential outputs preciseely, optimized 4-PAM signall with block sizze of 256 bit (116-bit CP was
inserted in each block) is generated from
m the output off RF power com
mbiner.
An un-cooled free-run
nning 10-Gb/s VCSEL was biased
b
at 9.3 m
mA with launchhing power of -5.6 dBm and
wavelength
h of 1544.66 nm. The generated 4-PAM sig
gnal was intenssity modulated onto the VCSE
EL at a symbool rate
of 10 Gbau
ud, and transmiitted over singlle mode fiber (SMF)
(
with disspersion coeffiicient of 17 ps//(nm•km) at 15550
nm. At the receiver, a tun
nable Gaussian
n optical filter (OTF1)
(
with 3--dB bandwidthh of 0.3 nm waas used to mitiggate
the degrading effects of CD.
C It can also represent the AWG
A
in WDM
M-PON system
m [4]. For BER measurementss, an
erbium dop
ped fiber ampliifier (EDFA) was
w used as a pre-amplifier
p
inn the receiver aand it is follow
wed by a flat-topp
tunable opttical filter (OT
TF2) with 3-dB bandwidth of 0.58 nm to supppress the EDF
FA noise. A vaariable optical
attenuator was used beforre the photodettector (PD) to adjust the opticcal power for B
BER measurem
ment. The opticcal
signal was detected by th
he PD and samp
pled by a 50 GS/s
G digital osciilloscope (Tekk DSA72004B)). The digitizedd
signal was post-processed
d offline wheree adaptive FDE
E, 4-PAM signnal demodulatioon and BER caalculations werre
carried outt.
Figure 2: Experiment setup for 20-Gb/s WDM
M-PON upstream transmission
t
usingg 4-PAM modulateed VCSEL with addaptive SC-FDE. O
OTF:
optical tunable filter;
f
Att.; optical attenuator.
4. Experim
ment results and
a discussion
n
Fig. 3 givees the generated
d 4-PAM signaal at the outputt of RF power ccombiner and ooptical back-too-back (BTB) eeye
diagram. Because
B
the am
mplitude of com
mbiner output iss larger than thhe measuremennt range of osciilloscope, a 3-ddB RF
attenuator was used beforre the oscillosccope. The optiical spectra forr these two optiical filters are aalso shown in Fig. 3.
Before OTF 1
After OTF 1
OTF 1
-20
-40
-60
-80
Optical Power (dBm)
Optical Power (dBm)
Before EDFA
After OTF2
OTF 2
-20
-40
-60
-80
1544.0 1544.4 1544.8 1545.2 1545.6
1544.0 1544.4 1544.8 1545.2 1545.6
Wavelength (nm)
Wavelength (nm)
Figure 3: Eye diagrams (Electrical 4-PAM signal and optical BTB) and optical spectra (before and after tunable filter 1 & 2).
Fig.4 gives the BER results before and after adaptive FDE with different transmission distance. If we use the
Forward Error Correction (FEC) threshold (2×10-3) as the criterion to analyze the system performance, it can be seen
that without equalization, the BER can't reach the FEC threshold when the transmission length is 10 km. In contrast,
after equalization, the BER is below FEC threshold even when the transmission distance is 20 km with optical
power of -13 dBm.
Without Equalization
With Equalization
-2
FEC Threshold
-3
-4
-5
-22
-1
BER (Log)
BER (Log)
BER (log)
-1
BER without Equalization
BER with Equalization
BER without Equalization
BER with Equalization
-2
-3
FEC Threshold
-4
-5
-20
-18
-16
-14
-12
Optical Power (dBm)
(a)
-10
-1
-2
FEC Threshold
-3
-4
-20
-18
-16
-14
-12
-10
Optical Power (dBm)
(b)
-8
-6
-5
-18
-16
-14
-12
-10
-8
-6
Optical Power (dBm)
(c)
Figure 4: measured BER results using different transmission distance. (a) BTB (b) 10 km (c) 20 km
5. Conclusions
We proposed and demonstrated a novel 20-Gb/s WDM-PON upstream transmission scheme using 4-PAM
modulated free-running VCSEL and adaptive SC-FDE in IM-DD system. In the scheme, adaptive FDE is applied at
post-processing stage to compensate CD, and no injection locking or dispersion compensation fibre is used. The
transmission distance for 4-PAM modulated VCSEL is extended to 20 km with our adaptive FDE algorithm. Due to
the simplicity of the intensity modulation used at the transmitter which can result in low cost ONUs, this scheme is
suitable for upstream transmission in WDM-PON systems.
6. Reference
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Modulated Free-Running 1.55-μm VCSELs”, in Proc. ECOC 2008, paper: P.6.02 (2008).
[2] K. Prince, M. Ma, T. B. Gibbon, C. Neumeyr, E. Ronneberg, M. Ortsiefer, I. T. Monroy, “Free-Running 1550 nm VCSEL for 10.7 Gb/s
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