(1) Communications Research Laboratory, Independent Administrative Institution, 4-2-1, Nukui-Kita, Koganei, Tokyo 184-8795, Japan, Phone: +81-42-327-5320, Fax: +81-42-327-7035, E-mail: soba@crl.go.jp

(2) Research Laboratory of Electronics, Massachusetts Institute of Technology, Room 36-323, 77 Massachusetts Avenue, Cambridge, MA 02139, U.S.A., Phone: +1-617-253-8949, Fax: +1-617-253-9611, E-mail: hideyuki@mit.edu

(3) Dept. of Electrical and Electrical Engineering, Sophia University, 7-1, Kioicho, Chiyodaku, Tokyo 102-8554, Japan, Phone: +81-3-3238-3330, Fax: +81-3-3238-3321, E-mail:

I. Introduction
As society is undergoing a fundamental change from an industrial society to an information society, the rapidly increasing demand for bandwidth, driven by the Internet, has led to a paradigm shift in the telecommunications industry to IP centric networks. Photonic networks are thus becoming to play important roles as global information infrastructures. Up to now, the primary generation of photonic networks has thrust into the bandwidth expansion issue. However, the next generation of photonic networking is now emerging, and the main issue is dynamic bandwidth provisioning on demand.

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

Figure 1
Fig. 1: (a) Layered structure of a hierarchical hybrid OTDM/WDM network. And (b) node cut-through and (c) node architecture in 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) [1]. 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
Fig. 2: (a) Principle of simultaneous wavelength-band generation of frequency standardized WDM signal in CS-RZ format using SC generation and spectrum slicing. And (b) experimental setup for wavelength-band signal generation and transmission.

Figure 2(a) shows the operational principle of frequency standardized simultaneous wavelength-band generation [4]. 40 Gbit/s carrier-suppressed return-to-zero (CS-RZ) multiplications are performed by supercontinuum (SC) generation [5], 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 [6]. 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 [4].

Figure 3
Fig. 3: (a) Measured optical spectra of SC at the output of SCF (upper trace), signals before transmission (middle trace), and after transmission (lower trace). Measured optical spectra after 80 km transmission and WDM DEMUX of (b) ch. 1 and (c) ch. 81. And measured eye diagrams of (d) ch.1and (e) ch. 81.
Figure 4
Fig. 4: Operational principle of the photonic conversion of (a) OTDM-to-WDM, and (b) WDM-to-OTDM by using optical time-gating. And (c) experimental setup for format conversions.


VI. Multiplexing format conversion
Conversion of 40 Gbit/s OTDM-to-4 x 10 Gbit/s WDM is done by optical time-gating of a highly linear chirped SC pulse as shown in Fig. 4(a) [7]. By shifting the time-position of the optical time-gating, the center wavelength of the time-gated SC pulse can be tuned. When 40 Gbit/s OTDM signals are used to control the time-gating ON/OFF window, the 10 GHz repetition rate SC pulses are converted to 4 x 10 Gbit/s WDM signals. Fig. 4(b) shows the principle operation of WDM-to-OTDM conversion [7]. Time-aligned 4 x 10 Gbit/s WDM signals are used for controlling the ON/OFF gate window of the 40 GHz repetition rate pulse trains, resulting in a conversion of WDM signals to 40 Gbit/s OTDM signals.

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) [8], 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 [7].

Figure 5
Fig. 5: Experimental results of (a) optical spectrum and (b) eye diagram of input 40 Gbit/s OTDM, (c) optical spectrum and (d) eye diagram of converted 4x 10 Gbit/s WDM, and (e) optical spectrum and (f) eye diagram of reconverted 40 Gbit/s OTDM.

V. Wavelength-band conversion
For wavelength-band path routing, inter-wavelength-band conversions of 640 Gbit/s OTDM signals both from C-to-L-band and L-to-C-band followed by 640-to-10 Gbit/s OTDM DEMUX have been experimentally demonstrated [9]. Highly nonlinear dispersion-shifted fibers (HNL-DSFs), which have high nonlinear coefficients and low dispersion slopes [10], were used as wavelength converters and ultrafast nonlinear optical loop mirror (NOLM) demultiplexings [11].

Figure 6
Fig. 6: Experimental setup for (a) C-to-L wavelength-band conversion and (b) L-to-C 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 [9].

Figure 7
Fig. 7: Experimentally measured 640 Gbit/s OTDM signal after (a),(b) C-to-L wavelength-band conversion, and (c),(d) L-to-C wavelength-band conversion.

IV. Conclusion
Key technologies for hierarchical hybrid OTDM/WDM network, that is, 3.24 Tbit/s frequency standardized continuous C- and L-wavelength-band signal generation, 40 Gbit/s OTDM-WDM mutual multiplexing format conversions, and 640 Gbit/s OTDM wavelength-band conversions have been developed. The proposed schemes based upon fast optical signal processing will become crucial in the future hierarchical hybrid OTDM/WDM network.

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11. H. Sotobayashi, C. Sawaguchi, Y. Koyamada, and W. Chujo, “Ultrafast walk-off free nonlinear optical loop mirror by a simplified configuration for 320 Gbit/s TDM signal demultiplexing,” to be published in OSA Opt. Lett., vol. 27, no. 15 (2002).


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