Hierarchical Hybrid OTDM/WDM Network Based on Fast Optical Signal Processing
Hideyuki Sotobayashi(1)(2), Wataru Chujo(1), and Takeshi Ozeki(3)
In this paper, we propose a hierarchical hybrid OTDM/WDM network as
the future core network. In order to achieve hierarchical hybrid OTDM/WDM
networks, ultrafast optical signal processing would be key technologies.
As enabling technologies for the hierarchical hybrid OTDM/WDM network,
we experimentally demonstrated three kinds of key functions such as
wavelength-band generation, all-optical OTDM/WDM mutual format conversion;
and the inter-wavelength-band conversion.
II. Hierarchical hybrid OTDM/WDM network
The hierarchical optical path architecture consisting of WDM bands
and channels has been proposed to be suitable for large-scale WDM backbone
networks as shown in Fig.1(a) . The grouping of wavelength optical
paths, that is, forming a wavelength-band path, is one way to reduce
the complexity and size of the optical path cross connect (OPXC). Hierarchical
OPXCs have the following merits: (1) economical network by multi-protocol
label switching node-cut-through at wavelength-band path level, (2)
the modularity that means that relatively small-scale modular OPXCs
make up construct large-scale systems, (3) the superior cross-talk specification
of demultiplexing, (4) the economical installation according to the
demands, and (5) the reduced network management complexity through the
use of the two-layered hierarchical network management [2,3]. The node-cut-through
of the wavelength-band path is the key concept in the hierarchical OTDM/WDM
network architecture as shown in Fig. 1(b). The wavelength-band is used
to transmit ultrahigh bit-rate OTDM signals instead of a group of WDM
channels. One of the aims of using OTDM in the wavelength-band path
is possibility to monitor error-free transmission in the physical layer,
which is necessary to cut through the high-level nodes. Figure 1(c)
illustrates the node structure of the hierarchical hybrid OTDM/WDM networks.
The input wavelength-band signals are demultiplexed by a band demultiplexing
(DEMUX). Each band should be first checked to guarantee error-free conditions.
The bands are then switched according to the cross-connect program.
Node cut-through bands are switched to the output band-multiplexing
(MUX) after passing through the all-optical signal processing unit,
which may transform them to the new wavelength of the bands for virtual
wavelength-band path scheme. The other bands are switched to drop-ports.
The size of OPXC space switch is remarkably small compared to that of
WDM channel OPXC. The dropped bands are then assembled to WDM channels
to feed channel-based OPXCs. IP-routers and ATM-switches process the
packet-based routing and switching and forward the packets to individual
channel-based OPXC. An important issue regarding the hierarchical hybrid
OTDM/WDM network nodes is how to realize fast optical signal processing
to construct efficient and economical node structures. The functions
that must be provided by all-optical means are (1) wideband signal generation,
(2) multiplexing format conversion, (3) wavelength-band conversion,
(4) equalization of wavelength and polarization-mode dispersions, (5)
improvement of spectral efficiency, and (6) error monitoring for node-cut-through.
The first three key functions are described in the following sections.
III. Wavelength-band signal generation
Figure 2(a) shows the operational principle of frequency standardized
simultaneous wavelength-band generation . 40 Gbit/s carrier-suppressed
return-to-zero (CS-RZ) multiplications are performed by supercontinuum
(SC) generation , which is directly pumped by a 40 Gbit/s CS-RZ signal
and followed by spectrum slicing using an arrayed waveguide grating
(AWG). The advantage of this method is that the channel spacing is strictly
fixed by the microwave mode-locking frequency of the source laser .
Figure 2(b) shows the experimental setup. 40 Gbit/s CS-RZ format signals
were generated by using time-delayed optical multiplexer with phase
shifter. The SC spectrum was sliced and recombined by AWGs with 100
GHz channel spacing. Tellurite-based erbium-doped fiber amplifiers (T-EDFAs)
were used for amplifying the continuous signal band in the C- and L-bands.
Figure 3(a) shows the optical spectra of SC at the output of SC fiber,
signals before transmission, and after transmission. The power difference
in all WDM channels after 80 km transmission was about 7 dB. Figures
3(b) and 3(c) respectively show the measured optical spectra after WDM
demultiplexing of ch. 1 (1535.04 nm) and ch. 81 (1600.60 nm). These
figures clearly show that optical carriers were suppressed even after
80 km transmission. Figures 3(d) and 3(e) respectively show the measured
eye diagrams for WDM ch. 1 and ch. 81. The eye diagrams for each of
the WDM channels have a good eye opening with BERs less than 10-9. It
is thus concluded that a frequency standardized 3.24 Tbit/s (81 WDM
x 40 Gbit/s) wavelength-band signal in CS-RZ format can be simultaneously
generated by using a single SC source .
VI. Multiplexing format conversion
Figure 4(c) shows the experimental setup. A 10 GHz SC pulse with a
highly chirped, wide pulse width, and rectangular shape was optically
time-gated in a semiconductor saturable absorber (SA) , which was
pumped by 40 Gbit/s OTDM signal. Conversion to WDM was done by WDM demultiplexing
using a 4 ch. AWG (l11544.1 nm
- l4:1552.5 nm) because the center
wavelengths of time-gated SC pulses depend on the time position of time-gating.
For WDM-to-OTDM conversion, 40 GHz pulse trains of l0 are optical time
gated in the SA pumped by 4 x 10 Gbit/s WDM data. The experimentally
measured mutual format conversions are shown in Figs. 5(a)-5(e). These
figures show that 40 Gbit/s OTDM of l0
is converted to 10 Gbit/s WDM channels of l1-l4
and reconverted into 40 Gbit/s OTDM of l0
with clear eye opening and BERs less than 10-9 .
V. Wavelength-band conversion
Fig. 6(a) shows the experimental setup for C-to-L-wavelength-band conversion
of a 640 Gbit/s OTDM signal. A 10 GHz, 700 fs pulse train at 1550 nm
was split into two, one was used for a 640 Gbit/s C-band OTDM signal
and the other was used for the control pulse for demultiplexing of the
wavelength-band converted L-band 640 Gbit/s OTDM signal. After polarization
optimization, the signal and pump were coupled in HNL-DSF #1 in order
to generate four wave mixing (FWM). The pump wavelength lp
was set to 1565.0 nm. A wavelength-band converted 640 Gbit/s signal
at 1580 nm was extracted by using an L-band EDFA, and then optical demultiplexed
to 10 Gbit/s in a NOLM controlled by a 10 GHz, 700 fs C-band pulse train.
Fig. 6(b) shows the experimental setup of L-to-C wavelength-band conversion.
A 10 GHz, 780 fs pulse train at 1580 nm was split into two, one was
used for a 640 Gbit/s L-band OTDM signal and the other was for the control
pulse in demultiplexing of the wavelength-band converted C-band 640
Gbit/s OTDM signal. A wavelength-band converted 640 Gbit/s signal at
1550 nm by FWM was extracted using a C-band EDFA and demultiplexed.
Figures 7(a)-7(d) show the experimental results. In both wavelength-band
conversions, 640 Gbit/s OTDM signals were wavelength-band converted
without significant pulse width broadening with BERs less than 10-9
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