17-01-2013, 04:52 PM
103-Gb/s Long-Reach WDM PON Implemented by Using Directly Modulated RSOAs
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Abstract—
We propose and demonstrate a long-reach wavelength-
division-multiplexed passive optical network (WDM PON)
capable of providing 100-Gb/s service to each subscriber, for the
first time to the best of our knowledge. For cost-effectiveness,
this network is implemented in loopback configuration by using
directly modulated reflective semiconductor optical amplifiers
(RSOAs) at 25.78 Gb/s. For the modulation of the RSOA at such a
high-speed, we have to minimize the electrical parasitics by using
the butterfly package. Also, to overcome the limited bandwidth
of the RSOA, we utilize the electronic equalization technique at
the receiver. We use four RSOAs at each optical network unit for
the 103-Gb/s upstream transmission. The operating wavelengths
of these RSOAs are separated by the free-spectral range of the
cyclic arrayed waveguide gratings used at the central office and
remote node (RN) for (de)multiplexing the WDM channels. We
extend the maximum reach of this WDM PON to be >120 km by
using Erbium-doped fiber amplifiers at the RN. The results show
that the error-free transmission can be achieved for all WDM
channels in the wavelength range of >35 nm with sufficient power
margins.
Index Terms—Passive optical network, semiconductor optical
amplifiers, wavelength division multiplexing.
I. INTRODUCTION
DUE TO the recent standardization activities of
100 Gigabit Ethernet (100 GbE), there have been
growing interests in the 100-Gb/s passive optical network
(PON) [1]. For example, it has been already demonstrated
that 100-Gb/s PON can be realized by using the optical
orthogonal-frequency-division- multiple access (OFDMA)
technique [2]–[4]. However, it should be noted that these
100-Gb/s OFDMA PONs are not intended to deliver 100-Gb/s
service to each subscriber. In other words, the term “100 Gb/s”
in these reports merely indicates the maximum per-wavelength
transmission speed. Thus, if we assume that this network is
Manuscript received August 28, 2011; revised October 30, 2011; accepted
November 7, 2011. Date of publication December 6, 2011; date of current
version January 18, 2012. This work was supported in part by the IT Research
and Development Program of MKE/IITA, 2008-F017-04, 100 Gb/s Ethernet
Fig. 1. Measured frequency responses of the TO-can packaged (dotted line)
and butterfly-packaged (solid line) RSOAs.
consisted of 10 optical network units (ONUs), it can provide
only 10-Gb/s service to each subscriber on average.
In this letter, we propose and demonstrate a long-reach
wavelength-division-multiplexed (WDM) PON capable of
providing 100-Gb/s service to each subscriber. For the costeffectiveness
(as well as the colorless operation of ONUs), we
implement this network in loopback configuration by using
directly modulated reflective semiconductor optical amplifiers
(RSOAs) operating at 25 Gb/s. Thus, the 100-Gb/s upstream
signal is obtained by combining the outputs of four RSOAs at
each ONU using the coarse WDM (CWDM) technique, as in
the 100GBASE-LR4 specifications [5]. To operate the RSOA
at 25 Gb/s, we mount it in a butterfly package (to minimize
the electrical parasitics) and utilize the electronic equalization
technique at the receiver [6]. The long-reach operation over
>120-km long single-mode fiber (SMF) link is accomplished
by using Erbium-doped fiber amplifiers (EDFAs) at the remote
node (RN). The results show that we can achieve the error-free
transmission of the 100-Gb/s signals (obtained by combining
four 25-Gb/s CWDM channels) in the wavelength range of
>35 nm with sufficient power margins. To the best of our
knowledge, this result represents the first demonstration of the
WDM PON capable of providing 100-Gb/s service to each
subscriber.
PROPOSED ARCHITECTURE OF 100-Gb/s
LONG-REACH WDM PON USING
DIRECTLY MODULATED RSOAS AT 25 Gb/s
the measured frequency response of the RSOA
used in this work. When we mounted this RSOA in a TO-can
package, its modulation bandwidth was measured to be only
∼2.2 GHz [7]. However, when we utilized a butterfly package
to minimize the electrical parasitics, the modulation bandwidth
Upstream link of the proposed long-reach WDM PON capable of
providing 100-Gb/s service to each subscriber.
was increased to ∼3.2 GHz, which was still not sufficient
for the operation at the speed of >10 Gb/s. However, the
reduced roll- off characteristics achieved by using the butterfly
package (i.e., from −40 dB/decade to −20 dB/decade) could
play an important role if we attempt to overcome this limited
bandwidth of RSOA by using the electronic equalization
technique. For example, Fig. 2 shows the power penalties
caused by operating the RSOA at the speed faster than its
modulation bandwidth. In this calculation, we assumed that
an ideal decision-feedback equalizer with an infinite number
of taps was utilized [8]. The result shows that, when we used
the TO-can packaged RSOA, the power penalty increased
rapidly as the operating speed was increased. As a result,
it was practically impossible to operate this RSOA at the
speed of >15 Gb/s. However, when we used the butterflypackaged
RSOA, this penalty was estimated to be <4 dB even
at 25 Gb/s. We have experimentally confirmed that it is indeed
possible to operate the butterfly-packaged RSOA at >25 Gb/s
[6]. Thus, for the realization of the cost-effective WDM PON
capable of providing 100-Gb/s service to each subscriber, we
propose to use the butterfly-packaged RSOAs operating at
25 Gb/s together with the CWDM technique as in the IEEE
100GBASE-LR4 specifications [5]. Fig. 3 shows the upstream
link of the proposed 100-Gb/s long-reach WDM PON. (Here,
we only show the upstream link since the downstream link
is relatively simple. We assume that 100-Gb/s downstream
links are realized simply by using integrated electro-absorption
modulator lasers [9]). The proposed network is implemented
in loopback configuration. To generate the 100-Gb/s upstream
signal, we use four butterfly-packaged RSOAs operating at
25 Gb/s at the ONU. Thus, we need to send a set of four
seed light to each ONU from the central office (CO). For this
Measured BER curves of four upstream CWDM channels after the
transmission over 123.67-km-long SMF. The inset shows the eye diagram of
the 25.78-Gb/s signal operating at 1530.08 nm.
purpose, we assume to use cyclic arrayed waveguide gratings
(AWGs) at the CO and RN, and the operating wavelengths of
the seed light are separated by the free-spectral range (FSR)
of the cyclic AWGs. Thus, a set of four seed light can be sent
to each ONU through a single drop fiber. At the ONU, this
set of four seed light is then separated by using a CWDM
filter and directed to each RSOA (which is modulated at
25 Gb/s using the upstream signal). The modulated outputs
of these RSOAs are combined again by the same CWDM
filter, and sent back to the CO. However, we assume to use
a pair of feeder fibers between the CO and RN to avoid the
effects of Rayleigh backscattering [10]. To secure the sufficient
power budget needed for the long-reach application, we use
EDFAs at the RN. The effect of chromatic dispersion (CD)
is suppressed by designing the transmission link to have a
slightly negative dispersion value by placing a dispersioncompensation
module (DCM) in front of the demultiplexing
AWG at the CO [6].