IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 20, NO. 18, SEPTEMBER 15, 20081533
10-Gb/s Operation of RSOA for WDM PON
K. Y. Cho, Y. Takushima, and Y. C. Chung, Fellow, IEEE
Abstract—We report on the 10-Gb/s operation of the reflective
semiconductor optical amplifier (RSOA) for the next-generation
wavelength-division-multiplexed passive optical network (WDM
PON). The bandwidth of the RSOA used in this experiment is
merely 2.2 GHz. Nevertheless, a clear eye opening is obtained
at 10 Gb/s by using the electronic equalizer processed offline.
We investigate the impacts of the network’s operating conditions
(such as the injection power to the RSOA and the fiber length) on
the performances of these equalizers. The results show that the
RSOA-based WDM PON is operable at 10 Gb/s and the maximum
reach can be extended to ?20 km with the help of the forward
error correction codes.
Index Terms—Passive optical network (PON), semiconductor
capable of providing
10-Gb s data to each subscriber. There
have been numerous attempts to develop practical WDM PONs
diodes (FP-LDs) –. However, until now, these networks
have been implemented to operate at the moderate speed in the
range of 155 Mb/s–5 Gb/s due to the limited bandwidths of
the directly modulated colorless light sources , . A simple
method to overcome this limitation and increase the transmis-
sion speed to
10 Gb s would be the use of high-speed ex-
ternal modulators at the optical network units (ONUs) , .
ulator at every ONU, it is not realistic for the use in cost-sensi-
tiveaccess networks. It has also been proposed to overcome this
limitation by using the FP-LD which is injection-locked to the
seed light provided from the central office (CO) . While this
laser could be directly modulated at 10 Gb/s, the colorless oper-
ation was not possible due to its extremely narrow lock-in range
0.02 nm .
In this letter, we demonstrate the feasibility of implementing
a 10-Gb/s WDM PON by using directly modulated RSOAs to-
gether with electronic equalizers. The modulation bandwidth of
the RSOA, limited by carrier lifetime, is only about 2 GHz ,
. Thus, it is nearly impossible to modulate such a device at
10 Gb/s. However, unlike semiconductor lasers, we note that
the frequency response of the RSOA has a smooth rolloff with
no relaxation oscillation peak, while its modulation has a good
linearity similar to the laser diode. These properties are almost
HE wavelength-division-multiplexed passive optical net-
work (WDM PON) has long been considered as an ulti-
Manuscript received April 23, 2008; revised June 23, 2008.
The authors are with the Department of Electrical Engineering, KAIST, Dae-
jeon 305-701, Korea (e-mail: firstname.lastname@example.org).
Color versions of one or more of the figures in this letter are available online
Digital Object Identifier 10.1109/LPT.2008.928834
Fig. 1. Measured frequency response of RSOA.
ideal for the electronic equalization using the decision feedback
equalizer (DFE) consisting of feedforward and feedback filters
, . Thus, despite its extremely limited bandwidth, it is
possible to operate the RSOA at 10 Gb/s by using the electronic
equalization technique. For a demonstration, we show that the
power penalty caused by the limited bandwidth of the RSOA
can be suppressed to 2.5 dB by using electronic equalizers. We
evaluate the impacts of the optical signal-to-noise ratio (OSNR)
and chromatic dispersion (CD) in the RSOA-based WDM PON
operating at 10 Gb/s using electronic equalizers. These results
show that the maximum reach of this network can be extended
to 20 km with the help of the forward error correction (FEC)
II. ELECTRONIC EQUALIZATION OF LIMITED MODULATION
BANDWIDTH OF RSOA
We first evaluated the frequency response of an RSOA
packaged in a transistor outline metal-can (TO-CAN) shown
in the inset of Fig. 1 . We injected continuouse-wave (CW)
seed light (1550 nm) into the RSOA and measured the small
signal response by directly modulating its injection current.
The optical power incident on the RSOA was -12 dBm. The
bias current of the RSOA was 80 mA. Under these conditions,
the gain and output power of the RSOA were measured to
be 17 and 5 dBm, respectively. As shown in Fig. 1, the 3-dB
bandwidth of the RSOA was measured to be only 2.2 GHz.
However, the rolloff characteristics were smooth (i.e., there
was no peak or zero within the Nyquist bandwidth for 10-Gb/s
signals). When we attempted to operate this RSOA at 10 Gb/s,
a severe penaltywas observed due to thelimited bandwidth. For
example, Fig. 2(a) shows the measured eye diagram when the
RSOA was directly modulated with a 9.95-Gb/s NRZ signal.
There was no eye opening at the decision point. However, we
could still observe the transient responses and the degradation
was not catastrophic.
We evaluated the possibility of overcoming this bandwidth
limitation of the RSOA by using electronic equalizers. The
signal from the RSOA was first filtered by an optical bandpass
filter which has Gaussian-shape passband (1 nm) and detected
1041-1135/$25.00 © 2008 IEEE
1534 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 20, NO. 18, SEPTEMBER 15, 2008
Fig. 2. Eye diagrams of the CW light-injected RSOA operating at 10 Gb/s:
(a) before equaization (just after the front-end of the receiver) and (b) after the
powers incident on the RSOA.
by using a PIN photodiode. We then sampled the received
electrical signal by using a real-time storage oscilloscope at
50 GS/s and processed it offline. The tap coefficient of the
equalizer was determined according to the minimum mean
square error criteria . This setup could measure the bit-error
rate (BER) up to 10
since the data length was 2
We used 21 pseudo-random binary sequence to limit the
offline processing time. However, there was no significant
difference when we used longer patterns. Fig. 2(b) shows an
example of the eye diagram obtained by using the electronic
equalizer consisting of half-symbol-spaced 17-tap FFE and
3-tap DFE. The result showed that the clear eye opening was
recovered by the equalizer.
Fig. 3 shows the BER curves obtained by using the electronic
equalizer. In this figure, the BER curve of the 10-Gb/s signal
generated by using a LiNbO modulator (bandwidth: 20 GHz)
on the performance of the electronic equalization, we varied the
optical power of the seed light incident on the RSOA. When we
0 dBm, the receiver sensitivity (at BER
the reference was only 2.5 dB, despite the limited bandwidth
of the RSOA (2.2 GHz). In fact, the power penalty induced by
the infinite tap-length DFE calculated by using the measured
frequency response in Fig. 1 was 2.2 dB . Thus, our result
was close to the theoretical limit. As we decreased the optical
power incident on the RSOA, the receiver sensitivity was also
decreased optical power on the 3-dB bandwidth of the RSOA
was negligibly small.) However, even when we set this optical
power to be
12 dBm, the penalty was measured to be as small
as 3.5 dB. Fig. 4 shows the BER curves measured by changing
the fiber length while setting the optical power incident on the
RSOA to be
12 dBm (i.e., the OSNR of the upstream signal
) was measured
Fig. 4. BER curves measured at various transmission distances.
Fig. 5. BER as a function of the number of taps (a) FFE and (b) DFE sections.
was maintained to be constant regardless of the transmission
the fiber length due to the increased CD.
We have also investigated the required number of taps of the
electronic equalizer. Fig. 5 shows the BERs as a function of
the number of taps of the feedforward and decision-feedback
sections measured after 20-km transmission at a fixed received
power ( 9.5 dBm). The dotted line indicates the measurement
limit caused by the number of sampling points. Thus, the data
points below this line represent the BER estimated from the
bathtub curve. The results show that we can achieve the nearly
optimal performance by using 12-tap feedforward and two-tap
III. ERROR-FREE TRANSMISSION OF 10-Gb/s UPSTREAM
SIGNALS IN RSOA-BASED WDM PON
Fig. 6 shows the setup used to evaluate the performance of
10-Gb/s WDM PON implemented by using RSOAs. The up-
stream signal was obtained by directly modulating the RSOA
packaged in a TO-CAN at 10 Gb/s. When we set the output
power of the seed light to be 3 dBm, the optical power inci-
dent on the RSOA was
12 dBm. Under these conditions, the
upstream signal power received by the PIN receiver at the CO
10 dBm, which was not sufficient for the error-free op-
. Thus, to implement a 10-Gb/s WDM
PON with the maximum reach of 20 km, it would be necessary
to utilize the FEC in conjunction with electronic equalization.
We first evaluated the impacts of the FEC overhead by mea-
suring the receiver sensitivities (at BER
tion of the bit rate ranging from 9 to 12 Gb/s. The transmis-
sion distance was either 0 or 20 km. The optical power inci-
dent on the RSOA was set to be
) as a func-
12 dBm. Fig. 7 shows that
CHO et al.: 10-Gb/s OPERATION OF RSOA FOR WDM PON1535
Fig. 6. Configurations of RSOA-based WDM PON.
Fig. 7. Receiver sensitivity (at BER ? ??
bit rate without using FEC.
) measured as a function of the
Fig. 8. Receiver sensitivity (at BER ? ??
types of FEC codes.
) achievable by using various
the receiver sensitivity is quite sensitive to the bit rate, indi-
cating that the slightly increased bandwidth due to the FEC
overhead can seriously deteriorate the system’s performance.
Thus, the effectiveness of FEC is uncertain in this extremely
bandwidth-limited system. To resolve this uncertainty, we cal-
culated the receiver sensitivity (at BER
by using various types of FEC codes. Fig. 8 shows the results.
The code rate was set to be constant at 10.3 Gb/s (considering
the overhead of 64B/66B coding used in 10G Ethernet). The
horizontal axis shows the line rates including the FEC over-
heads and the dotted line represents the optical power avail-
able at the upstream receiver after the transmission over 20 km.
As we increased the line rate (by utilizing larger FEC over-
shows that the use of the stronger FEC code did not necessarily
improve the receiver sensitivity. This was because the power
penalty caused by the increased bandwidth exceeded the im-
an efficient FEC (i.e., small overhead and high coding gain) for
the RSOA-based WDM PON operating at 10 Gb/s. Among the
FEC codes evaluated in Fig. 8, Reed-Solomon (RS) (255, 239),
Bose-Chaudhuri-Hocquenghem (BCH) (4359, 4320), low-den-
sity parity-check (LDPC) (1908, 1697), and the concatenated
scheme using BCH(3860, 3824) as outer and BCH(2040, 1930)
as inner codes could increase the maximum reach of the WDM
PON to 20 km. We note that these requirements can be greatly
relaxed by the slight improvement of the modulation bandwidth
of the RSOA. For example, if the modulation bandwidth is in-
creased by 10% (i.e., from 2.2 to 2.4 GHz), the penalty will be
1.5 dB (at 0 km) and
we could further improve the power margin by optimizing the
structure of the RSOA and its packaging. However, it should be
noted that the coding gain of FEC could be slightly changed if
there are burst errors .
2.5 dB (at 20 km). Thus,
We have demonstrated the feasibility of operating an RSOA-
based WDM PON at 10 Gb/s by using electronic equalization
and FEC techniques. Although the RSOA is an inherently slow
device, its frequency response has a smooth rolloff with no re-
laxation oscillation peak. Thus, we could operate the RSOA
packaged in a TO-CAN (bandwidth: 2.2 GHz) at 10 Gb/s by
using DFE. The maximum reach of this network can be ex-
20 km with the help of FEC codes.
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