We experimentally demonstrate an optical node with time-space-and-wavelength domain contention resolution, deflection and dropping capability. The node is composed of an optical buffer based on an optical crossconnect and a wavelength converter. Although the experimental results are shown at 10 Gbit/s the bitrate can be increased substantially. Bit-error rate measurements are shown, sustaining only 3.5 dB power penalty after 10µs of optical buffering and agile wavelength conversion over 18nm span.
© 2006 Optical Society of America
In optical networks, contention occurs when two packets arrive simultaneously to the same node, and have to be routed to the same output. Contention resolution in optical switching can be addressed using the time, the space and/or the wavelength domain . Time domain contention resolution can be implemented using fiber delay lines (FDL), recirculating configurations  or buffering strategies with a combination of traveling and re-circulating delay lines . Recirculating configurations are preferable, since there is an inherent finer degree of granularity in terms of time delay because shorter FDL can be used and packets can be accessed upon each recirculation through the switch fabric. The main drawback of this approach is the patterning effects induced in the active switches, which limit the number of recirculations and heavily impairs the quality of the signal. On the other hand, contention resolution in the wavelength domain is widely used by means of wavelength converters (WC). However, generally speaking, wavelength converters are intended to be used as packet routers, without considering any contention problem.
Space domain contention is mainly resolved by optical crosspoint switches or similar routing devices, which basically route the packets through an alternative path.
Further refinements in optical packet switching (OPS)  also request optical nodes capable of dropping or deflecting packets in case of contention events unsolved by the methods mentioned above . Deflection routing capability is an attractive feature since it can work with limited optical buffering (or even no buffering) because it reroutes (on the fly) the contending bursts to an output port other than the intended output port . Hence, contention by combining all three dimensions (space, time, and wavelength) is attractive solution for congestion-free routing nodes. Combining all these functionalities, an optical node is expected to look that the one depicted in Fig. 1.
In this paper we propose a physical implementation of an optical node with simultaneous time-space-and-wavelength contention resolution, deflection and dropping capabilities. The node is based on a cascaded optical crosspoint switch (OXS), a semiconductor optical amplifier Mach-Zehnder Interferometer (SOA-MZI) and a fast widely tunable sampled grating distributed Bragg reflector (SG-DBR). It is important to stress that space contention resolution, and the deflection and dropping capabilities are achieved by routing the packets to different outputs of the OXS. We demonstrate experimentally the proposed scheme with alloptical buffering up to 10.08 µs and wavelength conversion over a wavelength range of 18 nm for a system operating at a payload bit rate of 10 Gbit/s and label signal at 155 Mbit/s. Despite packet routing is not shown in this paper, we believe that the functionalities shown can provide attractive features to implement highly versatile optical routers.
2. System concept and setup
The integrated 4×4 OXS device consists of two waveguide layers. Two active vertical couplers (AVC) are formed at each cross-point of the switch by having an active waveguide stacked on top of both input and output passive waveguides. The switching mechanism of the OXC is carrier-induced refractive index and gain changes in the AVCs [8–9]. In the ON state, the effective refractive index of the active upper layer is reduced by the presence of injected carriers to equal that of the lower waveguide thereby allowing coupling. The injected carriers in the active layer also provide gain for the signal resulting in a high ON/OFF contrast.
Complementarily, the agile wavelength contention is performed using an SOA-MZI wavelength converter and a SG-DBR tunable laser. The conversion curve of the SOA-MZI device used during the experiments is shown in the Inset (a) of Fig. 2.
In the experimental setup shown in Fig. 2, the continuous wave (CW) probe light (1545.7 nm) generated by a tunable laser source (TLS) was modulated using an intensity modulator (IM) forming optical packets. The packets had 100ns length and were based on a 215-1 pseudo random binary sequence (PRBS). The slot time in which each packet was inserted was 1.12µs. The packets were then amplified using an erbium-doped fiber amplifier (EDFA). Most of the amplified spontaneous emissions (ASE) are rejected out using an optical bandpass filter (BPF). A polarization controller (PC) was used to adjust the polarization of incoming signal when they were launched into the 4×4 OXS. A recirculating loop with a time delay of 1.12µs (equal to the time slot of each packet) is formed by the OXS and a FDL together with in-loop amplification.
The OXC was controlled by a complex programmable logic device (CPLD). This electronic control received the labels of the packets from the same pattern generator that provided the payload data. This CPLD set the switches of the OXC, hence routing the packet either through the recirculating loop or to the exits (dropping, deflection or routing). Thus, all the routing and network control relies on an electronic stage. At the exit port, an EDFA boosts the signal which after a PC is launched into the SOA-MZI wavelength converter. The pumping signal at the other branch of the SOA-MZI was generated by a fast widely tunable SG-DBR laser. An optical BPF was added to filter out the old original signal and remove the amplified spontaneous emission (ASE) noise produced by the SOA-MZI.
3. Experiment and results
We first assessed the wavelength dependence of our proposed system, by connecting the SG-DBR tunable laser as pumping signal for the SOA-MZI and tuning its signal to four different wavelengths (namely, 1542.14, 1550.11, 1555.74 and 1559.79 nm - Fig. 3(a)) out of the 85 possible ITU 50-GHz spacing C-Band wavelengths. The tuning time is less than 100 ns for 75% of wavelengths and 200 ns at most. Power flatness is 5% and side mode suppression ration (SMSR) is 35 dB. The optical buffer was set to recirculate the packet only once (hence buffering the packet only 1.12 µs). The bit-error rate (BER) measurements are shown in Fig. 3, along with the optical spectra of the pumping CW (Inset (a)). These measurements are based on the Q factor, calculated directly from the waveform trace .
The results show that the wavelength conversion operation provides 2R regeneration with around 8 dB receiver sensitivity improvement at 10-9 of BER . Furthermore, the power penalty for each wavelength remains within less than a 2.5 dB boundary. The scenario considered so far actually reproduces a deflection and space-wavelength contention resolution situation, where an incoming packet is deflected via one circulation through the OXC to the desired output, or just routed to another wavelength/fiber.
Secondly, we assessed the combined operation focusing on one wavelength but for different amounts of optical buffering. In this case, the pumping signal at the MZI-SOA was generated by an SG-DBR, at 1550 nm. The optical spectra at different points of the setup are shown in Fig. 4. As it can be observed, due to the co-propagation operation of the SOA-MZI, the optical spectra at the output port of the SOA-MZI contain the converted signal along with the original one.
Figure 5 shows the waveform of the original packet, the ouput of the OXC buffer, and the output of the wavelength converter. As observed, the waveform traces are quite clean and free of pattern effect after 1 and 5 loops (1.12 µs and 5.6 µs of optical buffering). However, the trace after 9 loops (10.08µs of optical buffering) is blurring and shows some pattern effects.
Figure 6 shows the BER performance of the signal at different points of the setup. The back-to-back signal has a receiver sensitivity of -29dBm at 10-9 BER, improved to -37 dBm after pure wavelength conversion. Hence, the wavelength converter is operating as 2-R regenerator (amplifying and reshaping), which will contribute to overcoming the patterning effects produced by the OXC. The BER performance after one loop (1.12 µs optical buffering) is degraded 0.5 dB. However, after wavelength conversion, it experiences signal regeneration which increases the receiver sensitivity to -32.5 dBm. For five and nine recirculations (5.6 and 10.08 µs of optical buffering), the sensitivity is degraded, but after wavelength conversion, a net power penalty of only 1 dB and 3 dB is experienced, respectively. Hence, packets arriving simultaneously to an optical node can be handled appropriately, optically buffered and wavelength converted in this setup with a low power penalty. It is envisaged that a better performance can be obtained if the insertion loss and polarization dependent losses of the OXC can be further reduced.
We have experimentally demonstrated an optical node with simultaneous time-space-and-wavelength domain contention resolution, deflection and dropping capability at the physical layer. The node is composed of an optical buffer based on an optical crossconnect and a wavelength converter. Although the experimental results are shown at 10 Gbit/s the bitrate can be increased substantially, since the OXC is bitrate independent and by using a high-speed SOA-MZI. BER measurements are shown, sustaining only 3.5 dB power penalty after 10µs of optical buffering and wavelength conversion over 18nm wavelength span. The wavelength converter utilized by the SG-DBR tunable laser and SOA-MZI gives conversion flexibility to any C-Band ITU channel and combined with fast tuning times (less than 100 ns for 75% of lambdas) demonstrates good characteristics for high bandwidth utilization in OPS networks. The scheme is compatible with recent developments showing larger delay variation . It is envisaged that this node can be hybrid-integrated with delay lines using silica-on silicon technology to achieve better compactness and stability. Moreover, it can be further scaled for larger switch matrix, and can be integrated with in-loop optical 3R regenerators to maintain the signal performance for ultra-large optical buffer.
This work was done in the IST projects ePhoton/ONE and LASAGNE (All-optical LAbel-SwApping employing optical logic Gates in NEtwork nodes), funded by the IST Program of the European Commission. The package and electronic interface of the OXS device is funded by the SULIS fund.
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