1820 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 23, NO. 23, DECEMBER 1, 2011

Cognitive Routing in Converged Access-MetroEnvironment via -Selective SOA-MZI Switch

Alexandros Maziotis, Bernhard Schrenk, Marios Bougioukos, and Hercules Avramopoulos

Abstract—A ring+tree architecture for optical intranetworkdata transmission between different users is presented. Thenodes at the ring contain a packet envelope detection to extracta wavelength-dependent control signal for a semiconductor op-tical amplifier-based Mach–Zehnder interferometer (SOA-MZI)switch. The scalability of the architecture and the feasibility forfield deployment are discussed. A tree split of 1:1000 has beenfound to be compatible with data transmission over 50 hops.

Index Terms—Access networks, all-optical switching, opticaldata communication, optical signal processing.

I. INTRODUCTION

O PTICAL access networks have become a commer-cial reality in the past years. Different multiplexing

methods such as time or wavelength division multiplexing(TDM, WDM) have been adopted to share the fiber plant forcost-effective deployment. In course of a further access-metroconvergence, ring-like metro topologies can further collectthe traffic of local tree-like access segments [1]. Means ofdirect intranetwork data routing between optical network units(ONU) at different trees is then enabled and beneficial for theoverall network efficiency, since the energy-hungry conversionfrom the optical to the electrical domain at the optical lineterminal (OLT) and also part of the traffic aggregation at thecentral office is avoided. Earlier works have demonstratedthis functionality by porting the intelligence of central officesat the network nodes [2] or by applying a dense wavelengthrouting scheme at the network nodes in combination with atunable laser at the ONU [3]. These approaches, however,require either exhaustive higher layer analysis with data de-and re-encapsulation or a complex remote node design.In this work we present a method for selective data routing

within a ring+tree access network, with network nodes thatare capable to perform wavelength-determined interferometricswitching. The scalability of the architecture is evaluated interms of compatible user density and noise accumulation.

II. NETWORK ARCHITECTURE AND DATA PACKET SWITCHING

The key elements for the data routing are the network nodes(NN) along the ring, responsible for adding and dropping data

Manuscript received July 15, 2011; revised September 01, 2011; acceptedSeptember 14, 2011. Date of publication September 22, 2011; date of currentversion November 16, 2011. This work was supported in part by the EU FP7APACHE project.The authors are with the School of Electrical and Computer Engineering,

National Technical University of Athens, 15773 Athens, Greece (e-mail:maziotis@mail.ntua.gr).Color versions of one or more of the figures in this letter are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/LPT.2011.2169398

Fig. 1. Network architecture with node design and wavelength allocation.

from the corresponding trees (Fig. 1). The routing is based on aninterferometric SOA-MZI switch that is controlled by the exactwavelength of the incident data packet. The cross-phase modu-lation between the data signal and a control pulse is thereby usedto switch the data to one of the two interferometer ports. A va-riety of SOA- or SOA-MZI based switches has been presentedin earlier works, including label switching with headers or sig-nalling at auxiliary wavelengths [4]–[7], showing also cascadedoperation at high data rate [8].In this work, we aim at a reduction of subsystems complexity

and component count thereof, in line with cost considerationsfor access networks. The burst-like data from the ONUs is trans-mitted at a particular wavelength inside a WDM channel ,as depicted in Fig. 1, precisely with a specific wavelength shift

from the nominal center of this channel. The required ac-curate and reproducible wavelength emission has been alreadydemonstrated with commercial light sources [3]. The deviation

inside the channel addresses the destination NN at whichthe data packet is dropped. A packet envelope detection (PED)at the NN, which responds due to an initial set-and-forget align-ment only to incoming signals with a certain deviation , isthen responsible for extracting a control signal for the SOA-MZIswitch out of the wavelength-aligned data packet. An incomingpacket that is inline with the spectral response of the PED isdropped by the SOA-MZI switch while other packets that do notmatch in their deviation are forwarded to the next NN. Si-multaneous operation at multiple wavelengths can be achievedwith a free spectral range property of the PED (Fig. 1). This al-lows also to replace simple TDM trees with WDM/TDM trees,which further enables intranetwork routing between the samewavelengths (i.e., subnetworks) at different network trees.To complete the NN design, the incoming data packets from

either the East ring side or the add-port of the tree are fed to thePED and the SOA-MZI switch simultaneously, to generate thecorrect switching condition for the SOA-MZI switch. The signal

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MAZIOTIS et al.: COGNITIVE ROUTING IN CONVERGED ACCESS-METRO ENVIRONMENT 1821

Fig. 2. Experimental setup for packet switching in an access network node.

exits then correspondingly at the drop port of the tree or theWestring interface of the NN. Recall that though these NNs requirean electrical power supply, they avoid electrooptic conversionsor heavy signal processing and act as extender boxes for the lossbudget at the same time. It shall be also noted that the OLT canbe seen as one of the NNs and that the NN design allows the datato be dropped back to the source tree at the same NN where itwas received.

III. EXPERIMENTAL VALIDATION AND DISCUSSION

For the proof-of-concept (Fig. 2), two ONU transmitterswere emulated by two laser diodes at nm and

nm and a common electroabsorption modulator(EAM). The data bursts at 10 Gb/s were generated with apseudorandom bit sequence of length . The 0.4 slong packets had a repetition time of 1.2 s. An Erbium-dopedfiber amplifier (EDFA) acts as booster amplifier and sets theONU output power to 0 dBm . The packets at the differentwavelengths have been interleaved with the help of WDMmultiplexers and a fiber delay line (FDL). The loss in the treewas adjusted with the attenuator (and ) and noise wasloaded to the signals in their optical reception bandwidth toemulate a degraded optical signal-to-noise ratio (OSNR) at theNN input for validating the scalability of the scheme later on.The use of a chirped transmitter at the ONU validates at the

same time the correct operation of the PED at theNN,whichwascomposed by a Fabry–Pérot filter (FPF) with consecutive SOA.While the exploitation of the optical memory effect of the FPFallows to fill the space bits with light, the saturated SOA sup-presses remaining transients in the packet envelope. An EDFAwith a gain of 10 dB was added before the SOA to compensatefor the low SOA gain of 16.4 dB. The fiber-based FPF had afinesse of 1350 and a free spectral range of 396 GHz, compat-ible with the two particularly chosen wavelength channels. Thecombwas aligned at GHz with respect to the center of thesechannels to drop the packet at (Fig. 3(a)).The SOA-MZI switch was biased commonly for both wave-

length channels to pass incoming packets to the through portalong the ring. The common, colorless bias did not cause sub-optimal operation for one of the wavelengths. The SOA gainrecovery time of 20 ps is fast enough for switching operationat 10 Gb/s without additional holding beam. Each SOA con-sumed a current of 260 mA. The control signal from the PED

that is used to switch the packet to the drop port of the MZIwas fed in counter-propagating direction with the data packetssince it is allocated at the same wavelength as the packets andwould cause crosstalk. At the NN input, a 50/50 coupler ( )relays the incoming data packets to the PED and SOA-MZIswitch, while it allows to choose either a packet that is addedfrom the tree or received from the ring. For proper switchingoperation, only one packet at a single wavelength and at one ofthe ports should be present at a given time slot. This impliesthat the OLT should provide enough empty slots and means ofhigher layer signalling to allow and synchronize the communi-cation between the ONUs. Finally, two EDFAs were added tothe inputs of the NN to balance the losses along the ring spansand compensate for the rather high fiber-to-chip coupling lossesof the SOA-MZI switch. With these, the net gain of the NN was

dB. Note that NNs with EDFAs in the fiber plant are reason-able for next-generation optical access networks [9]. Broadbandnoise filters (BNF) centered at 1558.17 nm with a bandwidth of4 nm were further included as colorless filtering element to re-ject the peak noise emission of the EDFAs. Practical deploymentof the NNs requires a fully integrated design without polariza-tion dependent loss and wider BNFs to allow efficient multi-wavelength operation.The ONU receiver was based on a combination of EDFA and

PIN diode to emulate an avalanche photo diode. Its sensitivitywas normalized to dBm at a BER of .The spectra of the signals are shown in Fig. 3(a). Thanks

to the high finesse of the FPF, the PED suppresses the secondwavelength by 38.7 dB. The extinction of this second packetafter switching by the SOA-MZI is then as high as 13.7 dB.The traces of the transmitted packets at the ONU output ( ),

the recovered packet envelope ( ) and the dropped packet atwith suppressed ( ) are shown in Fig. 2. The control

pulse had residual patterning corresponding to an extinctionratio (ER) of only 1.2 dB. The pulse itself had an ER dB.To assess the switching penalty of the NN, the tree loss was

fixed to 10 dB and no additional noise was loaded at the NNinput. The bit error ratio (BER) was measured via the attenuator. In case that the packet is switched manually by adjustment

of the SOA-MZI bias, there is no visible penalty in respect to theback-to-back transmission without SOA-MZI (Fig. 3(b)). Theforwarded packet at the second MZI port had a similar perfor-mance.When the PED is used to automatically generate the con-trol pulse for the SOA-MZI switch from the incoming packet,the penalty is 1.7 dB at a BER of . This degradation de-rives mainly from the reflection of the control signal into thesignal propagation direction at the chip facet. This reflectionwas dB in respect to the injected signal power, leading toa signal-to-crosstalk ratio of dB.Considering that the optical losses between NNs are balanced

with the net gain thereof, Fig. 3(d) assesses the network scala-bility in terms of tree loss budget (between ONU and NN, andvice versa) and OSNR degradation at the NN input. The pre-sented BER was taken for the data packets that are switchedby the recovered control pulse, including also imperfectionsarising at the PED, such as unsaturated SOA operation in caseof low packet power. Since no recirculating loop was availablefor evaluating this network scenario, a brief theoretical analysis

1822 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 23, NO. 23, DECEMBER 1, 2011

Fig. 3. (a) Spectra, (b) BER due to switching at an NN, (c) simulated OSNR degradation due to multiple NN hops, and (d) BER in case of a network scenario.

Fig. 4. Optimization of the packet extinction via controlling SOA-MZI bias bypower monitoring. The power is normalized to the particular peak levels.

follows to derive the dependence of the OSNR at the NN inputas function of the hops along the ring.After noise accumulation over multiple NNs, the delivered

OSNR to the signal-dropping NN can be derived with

(1)where OSNR is referenced to the ONU output, noise figure(NF) is the effective noise figure of the amplification inside theNN, is the accumulated noise spectral density ( dBmfor a resolution bandwidth of nm), the numberof passed NNs, and and is the input power atthe NN from tree and ring port, respectively (Fig. 2). For ex-ample, having a NF of 6 dB and 10 passes along the ring with

dBm and dBm, OSNRbecomes 28.2 dB at nm (Fig. 3(c)) and is still ac-ceptable in terms of BER performance when looking at the ex-perimentally obtained BER contour in Fig. 3(d). Since for thelatter the input conditions into the NN ( and OSNR )have been worsened at the same time, the experiment shows aworst-case scenario since the signals would arrive with strongeroptical power to the NN in a realistic scenario (i.e., from the ringside with due to loss balancing in the ring)rather than with a degradedOSNR and a low at the sametime. Nevertheless, a tree loss budget of 22 dB, meaning a splitof 1:128, is compatible with an OSNR of 23 dB, which wouldmean transmission over hops. Error-free operation canbe maintained over a range of dB for the tree budget.The SOA-MZI bias can be stabilized by power monitoring

of the switch ports (Fig. 2). Considering that the majority ofpackets will be forwarded, a minimization of the power levelat the drop port leads to a maximization of the packet levels

at the through port and, consequently, to a minimization of thecrosstalk from the passed packets at the drop port. A control loopcan then be applied to stabilize the SOA-MZI bias, as it was ver-ified in its principle by amanual control. As can be seen in Fig. 4,the normalized packet power at the through port ( ) follows thereceived power at the monitor ( ), while the unswitched packetsat the drop port are extinct („).

IV. CONCLUSION

An user-user routing scheme in a ring+tree access-metro net-work based on wavelength-selective SOA-MZI switches hasbeen demonstrated and shown to be feasible for high tree bud-gets of 22 dB and OSNR degradation along the ring nodes.

ACKNOWLEDGMENT

The authors gratefully acknowledge the Centre for IntegratedPhotonics, U.K., for providing the SOA-MZI switch.

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