IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 20, NO. 18, SEPTEMBER 15, 2008 1533
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: email@example.com).
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
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