AMO-OFDM Signal Delivery of 20 Gbit/S throughput in 20-Km
Single Loopback Fiber Link Employing Baseband I/Q Separation
Adaptive Optical OFDM Modulation and Separate I/Q Baseband Signal
Transmission with Remotely-fed RSOAs for Colorless ONU in Next-generation
WDM Access
Jeong-Min Joo
1
, Moon-Ki Hong
1
, Dung Tien Pham
1
and Sang-Kook Han
1,2
1
Department of Electrical and Electronic Engineering, Yonsei University, 50 Yonsei-ro, Seodaumun-gu, Seoul, Korea
2
Yonsei Institute of Convergence Technology, Yonsei University, Songdo-dong, Yeonsu-gu, Incheon, Korea
Keywords: Adaptively Modulated Optical Orthogonal Frequency Division Multiplexing, Separate I/Q Baseband
Transmission, Remotely-fed, Bandwidth-limited, Reflective Semiconductor Optical Amplifier, Colorless
Optical Network Unit, Next-generation Access, Wavelength Division Multiplexed Passive Optical Network.
Abstract: We demonstrated a novel scheme to transmit 20-Gb/s adaptively modulated optical orthogonal frequency
division multiplexed (AMO OFDM) signal employing separate in-phase (I) and quadrature (Q) channel
baseband delivery in a 20-km single loopback fiber link based on 1-GHz reflective semiconductor optical
amplifiers (RSOAs) for a colorless optical network unit (ONU). Adaptive loading process was applied to
the OFDM signals to overcome the bandwidth limitation of RSOAs. The I and Q channel data streams in the
OFDM signals were separately carried on individual optical carriers with different wavelengths without
Hermitian Symmetry for baseband transmission. Our proposed scheme was experimentally demonstrated
using a periodic property of free spectral range (FSR) in a wavelength multiplexer. The separated optical
“dual” carriers were provided by the same port of a wavelength multiplexer to reduce the number of used
WDM channels for a upstream.
1 INTRODUCTION
Wavelength division multiplexed passive optical
network (WDM PON) has been one of the most
attractive candidates to satisfy explosive growth of
required bandwidth as well as to provide various
advantages for next-generation access networks,
such as transparency of data format, robust security,
and ease of maintenance and upgrading. Its key
challenging issue has been to reduce its inventory
cost, especially in realizing optical network units
(ONUs). A colorless ONU is inevitable for this issue
because the same transceiver can be used regardless
of the wavelength. A reflective semiconductor
optical amplifier (RSOA) is a good example so has
been proposed in many reports, but it is bandwidth-
limited (Lee et al., 2006).
Many technologies have been proposed to deal
with this issue and can be categorized as follows:
using electrical equalization (Papagiannakis et al.,
2008) and a specifically designed RSOA module and
its driving circuit (Cho et al., 2011); (Valicourt et al.,
2011). Electrical equalization techniques are
susceptible to chromatic dispersion and require
optical filter detuning with optical devices. It is
imperative to use complicated design techniques to
fabricate the RSOA module and its circuit to
carefully tune its electrical frequency response. In
addition, most of the proposals are based on a
locally fed RSOA or demonstrated in optical back-
to-back link to avoid Rayleigh backscattering and
therefore, they are far from practical scenarios.
Orthogonal frequency division multiplexing
(OFDM) with multi-level modulation has been
widely used due to its high-spectral efficiency in
overcoming bandwidth limitation of the RSOA. It is
also robust to chromatic dispersion because the
OFDM symbol duration is longer than single carrier
modulated case in which the data is parallel-
distributed into multiple subcarriers. Optical OFDM
with direct-detection has been preferred for access
networks because of cost effectiveness (Schmidt et
399
Joo J., Hong M., Pham D. and Han S..
AMO-OFDM Signal Delivery of 20 Gbit/S throughput in 20-Km Single Loopback Fiber Link Employing Baseband I/Q Separation - Adaptive Optical
OFDM Modulation and Separate I/Q Baseband Signal Transmission with Remotely-fed RSOAs for Colorless ONU in Next-generation WDM Access.
DOI: 10.5220/0004008903990402
In Proceedings of the International Conference on Data Communication Networking, e-Business and Optical Communication Systems (OPTICS-2012),
pages 399-402
ISBN: 978-989-8565-23-5
Copyright
c
2012 SCITEPRESS (Science and Technology Publications, Lda.)
al., 2008).
In this paper, we experimentally demonstrate a
novel 20-Gb/s adaptively modulated optical OFDM
(AMO OFDM) transmission scheme over 20-km
single fiber loopback link employing bandwidth
limited RSOAs as colorless ONUs. The in-phase (I)
and quadrature (Q) channels are separated in two
different wavelengths and transmitted at baseband.
The optical carriers for upstream I/Q channels are
generated at an optical line terminal (OLT) and
delivered through the 20-km bi-directional link. In
the proposed scheme, these wavelength channels
have a free spectral range (FSR) spacing of an
arrayed waveguide grating (AWGR) to reduce the
number of use WDM-PON channel. Therefore, these
“dual” channels could be transmitted through the
same port of an AWGR.
2 SYSTEM DESIGN
Figure 1: Proposed optical OFDM/WDM PON using
separated I/Q baseband delivery for colorless ONU.
Figure 1 briefly shows the proposed optical
OFDM/WDM-PON system. The adaptive bit and
power loading for each OFDM subcarrier, is used to
overcome the bandwidth limitation of the RSOAs.
This adaptive loading is based on evaluated error
vector magnitude (EVM) in term of the “probe”
signal for each subcarrier. This EVM converts to the
signal-to-noise ratio (SNR) and apply to the
bit/power loading algorithm (Chow et al., 1995).
Then, the OFDM signal is divided into real and
imaginary value for I and Q channel in the time
domain, and each channel signal drives the RSOA 1
and 2, independently. Two continuous wave (CW)
signals has difference wavelength in OLT are fed to
the RSOA 1 and 2 in ONU. These CW signals work
as “dual carriers” for upstream signals. The
wavelength spacing between dual carriers depends
on FSR of AWG because they are required through
the same port of AWG. Each carrier is modulated by
the I/Q channels of the OFDM signal at RSOA 1 and
2, independently. After the modulation at RSOAs,
the dual carriers are combined at blue/red (B/R)
filter which works as red and blue band separators or
combiner, and transmitted to the OLT through a
single mode fiber (SMF) link. The receiver consists
of two photo detectors (PDs) for receiving dual
channel carriers. Received I and Q channel signals
are combined and recovered at digital receiver for
OFDM signal. This scheme should need the training
sequence (TS) because time synchronization
mismatch between the I/Q channels could be
occurred. The TS was compared to received data
stream and find the start point of the I and Q channel
frames, independently.
3 EXPERIMENTAL SETUP,
RESULTS AND DISCUSSIONS
Figure 2: Experimental setup.
Figure 2 represents the experimental setup of the
proposed scheme. The CW optical source was
realized by a tunable light source (TLS) at 1532 to
1572 nm wavelengths and optical power of 1 dBm.
This CW seed light was launched into an RSOA
through a 20-km SMF link. The RSOA was operated
at the bias current of 48 mA and temperature of
20
o
C. Their 3-dB electrical bandwidth was less than
1 GHz as shown in inset of Figure 2. The adaptively
loaded OFDM signal was generated by MATLAB
®
and extracted from an arbitrary waveform generator
(AWG) sampling at 8 GSample/s. This extracted
OFDM signal consists of real-valued data streams of
the I and Q channels. The number of FFT size was
set as 1024. The occupied bandwidth was 4 GHz,
range from DC to 4 GHz. An electrical amplifier and
variable electrical attenuator (VEA) were used for
full modulation of the RSOA. Modulated optical
RSOA1
RSOA2
Wavelength Multiplexer
1
2
3
λ1 λ2
I Q
…
……
OFDM Digital Receiver
PD1
PD2
λ1
λ2
I
FSR for I channel
λ1
λ2
…
B/R
Filter
Q
|λ1 - λ2| = FSR of wavelength multiplexer
Bidirectional
SMF Link
Q
λ2
I
λ1
Q
λ2
I
λ1
WDM Laser
Source For ONU
OLT
ONU
…
λ2n-1
λ2n
B/R
Filter
FSR for Q channel
λ2n-1
λ2n
… …
OFDM Signal
Generator
…
…
I
λ1
λ2
… …
Q
λ2n-1 λ2n
Wavelength Multiplexer
0.5 1 1.5 2 2.5 3 3.5 4 4.5
-25
-20
-15
-10
-5
0
Frequency(GHz)
Normalized Power(dB)
Frequency Response
TLS
1
2
3
Link type - A:<1m,
B:<1m, C:20km
SMF
PC
VOA
RSOA
AWG
VEA
AMP
48mA, 20°C
Link type - A:<1m,
B:20km, C:<1m
SMF
Pre-amp
EDFA
isolator
PD
DSA
Q ch
50GSample/s
OBPF
VOA
VOA
I ch
8GSample/s
Offline
Digital Processing
1 2
3
1
2
3
Pin_PD=-5dBm
1532nm ~ 1572nm
Link Type – A: Optical Back-
to-back, B: 20km Locally Fed,
C: 20km Remotely Fed, TLS :
tunable laser source, SMF:
single mode fiber, PC:
polarization controller, EDFA:
erbium doped fiber amplifier,
VOA: variable optical
attenuator, OBPF: optical band-
pass filter, AMP: Electrical
amplifier, VEA: variable
electrical attenuator, AWG:
arbitrary waveform generator,
DSA: digital serial analyzer
OLT
ONU
1dBm
RSOA
VEA
AMP
Q ch
PD
I ch
OPTICS 2012 - International Conference on Optical Communication Systems
400
AMO-OFDM Signal Delivery of 20 Gbit/S throughput in 20-Km Single Loopback Fiber Link Employing Baseband I/Q
Separation - Adaptive Optical OFDM Modulation and Separate I/QQ Baseband Signal Transmission with Remotely-fed RSOAs
for Colorless ONU in Next-generation WDM Access
signals from the RSOA were transmitted through 20-
km SMF link. A preamplifier was realized by
employing an erbium-doped fiber amplifier (EDFA)
and optical bandpass filter (OBPF) to enhance the
receiver sensitivity. This whole process was
consecutively repeated for counterpart of the
previous channel in different wavelength. For
example, the Q channel part of the baseband OFDM
symbol was consecutively transmitted and recovered
after doing the same process for the I channel part of
the baseband OFMM symbol. The received two
channel electrical signals were sampled by a digital
serial analyzer (DSA) at 50-GSample/s sampling
speed. Then, these signals were digitally combined
to construct the complex-valued OFDM symbol and
evaluated by an offline process from MATLAB
®
.
Figure 3: Probe EVM, adaptive bit and power loading
profile, BER each data subcarrier and constellation of
maximum bit loading subcarriers: (a) optical back-to-back
(data rate: 29.68 Gb/s, average BER: 9.79x10
-4
, (b) 20-km
bi-directional (data rate: 21.59 Gb/s, average BER:
4.57x10
-4
); (c) recovered signal constellation for the 20-
km bi-directional case.
In first, a “probe” signal with the uniform bit and
power allocation of 16 quadrature amplitude
modulation (QAM) was transmitted to evaluate the
channel response. The SNR converted from
evaluated EVM was used to decide the response of
each subcarrier. According to this response, adaptive
bit and power loading were applied to each OFDM
subcarrier. Figure 3 (a) and (b) represent the
evaluated EVM of probe signal, the adaptively
loaded bit and power allocation profile, bit error rate
(BER) performance for each OFDM subcarrier, and
constellation of maximum bit loading subcarriers in
optical back-to-back and bi-directional 20-km
transmission, respectively. These results verified that
more bit were allocated at subcarriers of lower EVM
and less bit were allocated at subcarrier of higher
EVM of probe signal. The pre-emphasis values
depended on difference between real number value
and round number for realistic bit loading of bits.
Compare to the optical back-to-back, some
subcarriers at high frequency region allocated zero
bits because the EVMs at these subcarriers were not
enough to transmit even a one bit.
Figure 4: Maximum achievable throughput as a function
of: (a) input optical power of the RSOA, (b) input optical
power of preamplifier, (c) various wavelength, (d) I/Q
channel wavelength spacing.
Figure 4 (a) and (b) represent the maximum
achievable data rate with the input optical power of
the RSOA and the preamplifier for optical back-to-
back, 20-km unidirectional (locally-fed), and 20-km
bi-directional (remotely-fed) cases. The 20-km bi-
directional transmission system supports higher than
20 Gb/s at upper the -10 dBm RSOA input optical
power and -11 dBm preamplifier input optical power
as shown Figure 4 (a) and (b). This system has about
6 Gb/s penalty compare to the 20km unidirectional
case. This degradation was mainly come from the
Rayleigh backscattering noise. The maximum
achievable data rate for all cases is measured with
BER < 10
-3
.
The maximum achievable data rate with various
-4000-3500-3000-2500-2000-1500-1000-500 0 5001000150020002500300035004000
0
0.5
1
1.5
2
Frequency(MHz)
Power level[A.U]
-4000-3500-3000-2500-2000-1500-1000-500 0 5001000150020002500300035004000
0
0.5
1
1.5
2
Frequency(MHz)
Power level[A.U]
128 256 384 512 640 768 896 1024
-4
-3
-2
-1
log(BER)
-4000-3500-3000-2500-2000-1500-1000-500 0 5001000150020002500300035004000
20
40
60
80
100
Frequency(MHz)
EVM (%) of Probe
128 256 384 512 640 768 896 1024
-4
-3
-2
-1
log(BER)
-4000-3500-3000-2500-2000-1500-1000-500 0 5001000150020002500300035004000
20
40
60
80
100
Frequency(MHz)
EVM (%) of Probe
-4000-3500-3000-2500-2000-1500-1000-500 0 5001000150020002500300035004000
0
2
4
6
Frequency(MHz)
Bit Allocation
-4000-3500-3000-2500-2000-1500-1000-500 0 5001000150020002500300035004000
0
2
4
6
Frequency(MHz)
Bit Allocation
-2 -1 0 1 2
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
2.5
Quadrature
In-Phase
4QAM – 2bits
-2 -1
0
1 2
-2 -1
0
1 2
Imaginary
Real
-2 -1 0 1 2
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
Quadrature
In-Phase
BPSK– 1bits
-2 -1
0
1 2
-2 -1
0
1 2
Imaginary
Real
-6 -4 -2 0 2 4 6
-6
-4
-2
0
2
4
6
Quadrature
In-Phase
32QAM – 5bits
Imaginary
Real
0
2 4-4 -2
6
-6
0
2 4-4 -2
6
-6
-4 -3 -2 -1 0 1 2 3 4
-4
-3
-2
-1
0
1
2
3
4
Quadrature
In-Phase
16QAM – 4bits
-4 -2
0
2 4
-4 -2
0
2 4
Imaginary
Real
-4 -3 -2 -1 0 1 2 3 4
-4
-3
-2
-1
0
1
2
3
4
Quadrature
In-Phase
8QAM – 3bits
-4 -2
0
2 4
-4 -2
0
2 4
Imaginary
Real
(c)
(c)
(d)
0 5 10 15 20 25 30
16
18
20
22
24
26
I/Q Channel Spacing(nm)
Maximum Data Rate(Gb/s)
Measured data
Low performance channel (Q ch)
High performance channel (I ch)
1535 1540 1545 1550 1555 1560 1565 1570 1575
12
14
16
18
20
22
24
Wavelength(nm)
Maximum Data Rate(Gb/s)
(a)
(b)
-19 -17 -15 -13 -11 -9 -7 -5 -3 -1
13
16
19
22
25
28
31
RSOA Input Power(dBm)
Maximum Data Rate(Gb/s)
Locally-fed
Optical BTB
Remotely-fed
-22 -19 -16 -13 -10 -7 -4 -1 2
10
13
16
19
22
25
28
31
34
Preamp Input Power(dBm)
Maximum Data Rate(Gb/s)
Locally-fed
Optical BTB
Remotely-fed
Preamplifier input optical power : - 7 dBm
RSOA input optical power : -7 dBm
401
wavelength and spacing also measured to verify
colorless operation and applicability of the FSR
periodicity of AWG as shown in Figure 4 (c) and (d).
The measured optical carrier range from 1532.5 nm
to 1572.5 nm (equivalent to almost entire C band),
the maximum achievable data rate was maintained
almost 20 Gb/s at the RSOA and preamplifier input
optical power of -7 dBm and -3 dBm, respectively.
This maximum achievable data rate slightly
decreased at about 1570 nm because gain
characteristic of EDFA in preamplifier has poor gain
characteristic at this wavelength. For this reason, the
optical receiver sensitivity also became worse and
led to the performance penalty. The I/Q wavelength
difference has no influence on the maximum
achievable data rate as shown in Figure 4 (d). These
data rates are similar to the average of low and high
wavelength channel performance.
Figure 5: Maximum achievable data rate with various
input optical power deviation of the preamplifier power
between I /Q channels.
The signal performance was also evaluated as a
function of the input preamplifier power deviation
between I/Q channels as represented in Figure 5. In
this figure, “high” channel means either I or Q
channel which had higher input optical power at the
preamplifier, and vice versa. The measurement
curves labeled “measured” represents the signal
performance after combining I/Q channels (in other
words, “high” and “low” channels) with a given
input preamplifier power deviation. It was verified
that the signal performance was degraded as the
power deviation was increased. As a whole, the
performance degradation followed the “low”
channel’s characteristic. However, this performance
degradation was almost negligible when the
deviation was less than 12 dB, which led to 1-Gb/s
performance penalty compared to the case of no
power deviation. This 12-dB deviation margin was
high enough to provide consistent data rate
transmission in the proposed scheme because both
I/Q channels indeed shared the same link and
consequently, there was no significant input
preamplifier power difference between I/Q channels.
4 CONCLUSIONS
We successfully demonstrated the novel
transmission scheme to provide 20-Gb/s optical
OFDM signal through the 20-km SMF based on the
1-GHz bandwidth-limited RSOAs for source free
and colorless ONU. The I and Q channels were
separated in wavelength domain and independently
transmitted at baseband without Hermitian
Symmetry. The CW optical sources for upstream
signal were generated at OLT pass through bi-
directional link for source free ONU. We reported a
proof-of-concept experiment to show the
applicability of FSR periodicity of AWG. The
OFDM symbol with adaptive bit and power loading
was applied to overcome the bandwidth limitation of
the RSOA. This solution is promising for low-cost
20 Gb/s upstream transceiver for colorless ONU of
>10G-PON.
REFERENCES
Lee, C.-H., Sorin, W. V., Kim, B.-Y., 2006, Fiber to the
home using a PON infrastructure, Journal of
Lightwave Technology, 24(12), pp. 4568-4583.
Papagiannakis, I., Omella, M., Klonidis, D., Birbas, A., N.,
Kikidis, J., Tomkos, I., Prat, J., 2008, Investigation of
10-Gb/s RSOA-based upstream transmission in
WDM-PONs utilizing optical filtering and electronic
equalization, IEEE Photonics Technology Letters,
20(24), pp. 2168-2170.
Cho, K. Y., Choi, B. S., Takushima, Y., Chung, Y. C.,
2011, 25.78-Gb/s operation of RSOA for next-
generation optical access networks, IEEE Photonics
Technology Letters, 23(8), pp. 495-497.
Valicourt, G. de, Make, D., Fortin, C., Enard, A., Van Dijk,
F., Brenot, R., 2011, 10 Gbit/s modulation of
reflective SOA without any electronic processing, In
PROCEEDING’11, OFC 2011, paper OThT2.
Schmidt, B. J. C., Lowery, A. J., Armstrong, J., 2008,
Experimental demonstrations of electronic dispersion
compensation for long-haul transmission using direct-
detection optical OFDM, Journal of Lightwave
Technology, 26(1), pp. 196-203.
Chow, P. S., Cioffi, J. M., Bingham, J. A. C., 1995, A
practical discrete multitone transceiver loading
algorithm for data transmission over spectrally shaped
channels, IEEE Transactions on Communications,
43(234), pp. 773-775.
0 2 4 6 8 10 12 14 16 18 20
10
12
14
16
18
20
22
24
26
28
Preamp Input Power Difference(dB)
Maximum Data Rate(Gb/s)
Local Fed - High Ch
Local Fed - Low Ch
Remote Fed - High Ch
Remote Fed - Low Ch
Remote Fed - Measured
Local Fed - Measured
RSOA input optical power : -7 dBm
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