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JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 23, DECEMBER 1, 2009 5417

Monitoring the Quality of Signal in Packet-SwitchedNetworks Using Optical Correlators

Ruth Vilar Mateo, Jaime García, Guillaume Tremblay, Youngjae Kim, Sophie LaRochelle, Member, IEEE,Francisco Ramos, Member, IEEE, and Javier Martí, Member, IEEE

Abstract—The increasing demand in the Internet network forreal-time multimedia data traffic with high quality of service (QoS)is pushing the limits of existing network structure. Optical packetswitching (OPS) is considered as a possible technology for futuretelecommunication networks due to its compatibility with burstytraffic and efficient use of the network resources. But OPS bringsabout new challenges to the research in optical performance mon-itoring (OPM). In this scenario, each packet follows its own pathalong the network depending on the routing information containedin the label and thereby packets suffer from different signal degra-dations. Therefore, a definitive goal for OPM is to provide compre-hensive signal quality information as part of QoS implementationto keep the level of QoS promised to customers. In this paper, anovel optical SNR (OSNR) monitoring technique based on the useof optical correlation is presented. A fiber Bragg grating-based cor-relator was constructed and used to experimentally demonstratethe successful correlation. Experiments performed on a 40 Gb/ssystem confirm the viability of this approach. By measuring statis-tics from the autocorrelation peak, the monitor is capable of directOSNR monitoring with an error of less than 0.5 dB.

Index Terms—Fiber Bragg gratings, optical packet switching,optical packet-switched networks, optical performance moni-toring, optical SNR (OSNR).

I. INTRODUCTION

T ELECOMMUNICATION networks are experiencing adramatic increase in demand for capacity, much of it

due to the exponential growth of the Internet and associatedservices. In addition to rapidly increasing capacity demands,new multimedia and data services are driving the needs forhigh-performance and high-utilization networks. Optical packetswitching (OPS) technology is particularly attractive as a pos-sible technology for future telecommunication networks, dueto its compatibility with Internet protocol (IP) and efficient

Manuscript received February 17, 2009; revised June 18, 2009. First pub-lished July 21, 2009; current version published October 16, 2009This work(at COPL, Université Laval) was supported by the all-optical packet switchingproject of the Canadian Institute for Photonic Innovations (CIPI). The work ofR. Vilar was supported by the Spanish Government under an FPU grant.

R. V. Mateo, J. García, F. Ramos, and J. Martí are with the NanophotonicsTechnology Centre, Universidad Politecnica de Valencia, Valencia 46022, Spain(e-mail: [email protected]).

G. Tremblay, Y. Kim, and S. LaRochelle are with the Centre d’optique, pho-tonique et laser, Département de génie électrique et de génie informatique, Uni-versité Laval, Québec, Canada.

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JLT.2009.2028034

use of the network resources [1], [2]. Indeed, the benefit ofOPS rises from the higher network utilization at subwavelengthgranularity and from the supporting of more diverse serviceswith bursty traffic patterns. Moreover, OPS technology offers anew capability to process packets directly at the optical layer.At this point, the concept of all-optical label switching (AOLS)appears to be a solution to avoid the bottleneck imposed bythe nodes based on electronic processing [3]. In such an AOLSscenario, all packet-by-packet routing and forwarding functionsof multiprotocol label switching are implemented directly inthe optical domain. By using optical labels, the IP packets aredirected through the core optical networks without requiringO/E/O conversions whenever a routing decision is necessary.The main advantage of this approach is the ability to routepackets/burst independently of bit rate, packet format, andpacket length. In addition to packet routing and forwarding,the processing of optical labels brings about new challengesto the research in optical performance monitoring (OPM).OPM is expected to play an important role in next-generationoptical networks performing some network crucial functionssuch as signal quality characterization for quality of service(QoS) assurance and service level agreement fulfillment [4].QoS refers to the capability of a network to provide betterservice or guarantee a certain level of performance to selectednetwork packets. Therefore, new techniques to monitor thesignal quality directly at the physical layer must be deployedas part of QoS, to insure that networks are performing at thedesired level and thereby realizing a reliable, high-performanceand service-differentiation enabled all-optical network [5].

With the introduction of new-generation multimedia services,OPS networks transport different types of traffic with differentquality requirements so that performance monitoring is espe-cially important to ensure that packets receive appropriate treat-ment as they travel through the network. Packets entering thenetwork must be analyzed to determine their QoS requirementsdepending on the kind of network service, and then, the signalquality must be monitored in each intermediate node to guar-antee certain level of performance. In this scenario, each opticalsignal may traverse different paths and different optical compo-nents; thereby having its own history and quality. This is shownin Fig. 1, where packets coming from different services and withdistinct QoS requirements, and , are generated from sev-eral sources and go through different paths along the network.The three packets pass diverse optical links and switches, thenexit from the same node (node A). The bottom of Fig. 1 de-picts the optical power of the output packets from node A. Wecan see that the ASE noise level in each packet is different and

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5418 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 23, DECEMBER 1, 2009

Fig. 1. Packets with different quality in an OPS network.

thereby the packets have different level of performance. There-fore, in order to determine the health of optical signals in OPSnetworks and to guarantee certain QoS level, the optical packetsin each intermediate node must be monitored.

The traditional monitoring techniques based on BER estima-tion [6], [7] or Q-factor [8], [9] are not appropriate for the futureultrahigh-speed optical networks due to the O/E conversionsthat limit the bit rate unless it is preceded by optical time de-multiplexing. In fact, new performance monitoring techniquesshould be defined in order to overcome this limitation. Recentworks have demonstrated all-OPM and selective packet drop-ping by monitoring the error of the optical header field to inferthe error of the data payload [10]–[12]. The technique proposedin [10] uses labels at lower bit rates than the payload, so thequality is not monitored at the packet bit rate. With respect to[11] and [12], the basis of the monitoring techniques is different.Both of them are based on “time-to-live” (TTL) field concept, sothey do not measure the real value of the optical SNR (OSNR),but they estimate it as function of the TTL value.

In this paper, we propose and demonstrate a novel techniquebased on the use of optical correlation to assess the signal qualityat the optical domain with relaxed speed requirements. A spe-cific data word, with information associated with QoS or kindof service, is inserted into the packet header and is processedby means of an optical correlator based on fiber Bragg gratings(FBGs). Therefore, the technique uses serial optical labels trav-eling at the same bit rate as the payload allowing performancemonitoring at high speeds as the correlation is performed in theoptical domain. In particular, the OSNR of the incoming packetsis estimated from the noise statistics of the autocorrelation pulsepeak power to ensure that they fulfill the QoS requirements.Using a 10-ps pulsewidth 40 Gb/s return-to-zero ON–OFF keying

(RZ-OOK) system, we show that OSNR can be monitored withan error 0.5 dB.

The paper is structured as follows. In Section II, the principleof operation of the novel monitoring technique is described.Section III provides an overview of different digital correlationimplementations and explains the basis of an FBG-based corre-lator. In Section IV, the monitoring technique is demonstratedexperimentally. Finally, the paper is summarized in Section V.

II. PRINCIPLE OF OPERATION

The signal quality monitoring method is based on sendinga specific data word (monitoring field) inserted into the packetheader, which specifies the QoS requirement of the incomingpackets. This QoS field fixes the quality of signal requirementsfor each packet flow coming from a specific service. Then op-tical headers of packets with the same QoS requirements, i.e.,the same monitoring field associated with the same kind of ser-vice, are processed in each intermediate node to check if theestimated quality fulfills the terms of QoS and to guarantee therequired level of performance. Fig. 2 shows the block diagramof the proposed monitor for optical packets [13]. The monitor iscomposed of correlators, each of which is configured to pro-duce a “match” signal for a specific QoS. The number of corre-lators, , defines the number of type of services or QoS levelsprovided by the carriers. Three basic levels are normally definedin the network: best-effort service, differentiated service, andguaranteed service [14].

When the packet arrives at the node, the monitoring infor-mation field, with or 1 and

, is extracted with a pulse peak power of .The circuit responsible for this task is composed of two blocks:

VILAR MATEO et al.: MONITORING THE QUALITY OF SIGNAL IN PACKET-SWITCHED NETWORKS USING OPTICAL CORRELATORS 5419

Fig. 2. Block diagram of the proposed OSNR monitor.

a packet clock recovery circuit and a high-speed SOA-MZI op-tical gate configured to perform a Boolean AND. These sub-systems were demonstrated in the IST-LASAGNE project [15].Once the monitoring field is extracted, the optical correlatorallows signal quality monitoring with high fidelity and in realtime. In the next section, several typical implementations of anoptical correlator are explained.

In optical correlation, the received signal is correlated inthe optical domain with the sent signal so that the correla-tion function is a measure of how similar and are. Thecorrelation can be implemented in a discrete system as

(1)

where is the number of bits in the correlation sequence,is the bit period, represents the reference signal as theweights that multiply each of the delayed received signals, and

is the received signal delayed by bit times. Let ussuppose that the optical correlation is configured to match with areference signal composed of bits with . If the two sig-nals, and , are identical, (1) becomes an autocorrelationand the output correlation pulse appears as a single sharp peakin the center with an amplitude equal to . If the signals areless well matched, then the peak decreases and the informationon either side of the peak increases. The autocorrelation func-tion is considered in power basis as it will be explained in thefollowing sections. Therefore, from the amplitude of the cor-relator’s output different types of services (QoS requirements)may be separated (Fig. 2). As an example, the output signal fromthe optical correlator corresponding to is shownin Fig. 3.

The OSNR of the incoming packet can be calculated byusing the statistics of the autocorrelation pulse peak power. TheOSNR is defined as

(2)

where “mean” is the mean value and is the standard deviationof the autocorrelation pulse peak power. Equation (2) shows thatthe OSNR monitoring depends on the shape of the correlator’soutput as it has been commented before.

Fig. 3. Autocorrelation pulse.

Fig. 4. Simulation results of OSNR for different bit rates.

At high speeds (above 40 Gb/s), the correct evaluation of BERrequires optical demultiplexing. However, the proposed tech-nique alleviates this speed requirement, as it only measures theautocorrelation pulse, so the system works well at the packetrate. Moreover, the correlation is performed all optically, thusreducing the hardware requirements.

Fig. 4 shows the simulation results of the OSNR estimationusing the autocorrelation pulse for different bit rates. As it can beseen, the estimated OSNR adjusts perfectly to the “real” OSNRvalue and it is independent of the bit rate. Now that the proposedtechnique has been validated at high speeds by means of simu-lations, in the next section we demonstrate it experimentally at40 Gb/s.

III. OPTICAL CORRELATORS

Now that the optical correlation as an OSNR monitor has beenintroduced, this section presents several implementations at theoptical domain. A common implementation of an optical corre-lator is the tapped delay line [16]. In this implementation, thereceived signal is sent to a tapped delay line, which requiresone tap for each bit in the desired sequence that are weighted bythe factors “1” or “0” depending on the value of the bit of thesequence. The weights can either be phase shifter or amplitudeweights or both (amplitude weights of 1 s or 0 s are implementedby placing a switch in each path that is closed forand opened for ). The received signal is equally


split among the delay lines. Each successive delay line adds oneadditional bit of delay to the received signal before the com-biner, where the signals are added to yield the correlation outputfunction. The problem of this implementation is that the opticaltapped-delay-line structure requires a separate fiber branch andan optical switch for each bit in the desired bit pattern, making itimpractical to construct bank of correlators of many bits. More-over, the length of each fiber branch must be cut precisely to pro-vide the requisite 1-bit delay between successive branches andthat it is extremely difficult at high speeds. Another implementa-tion is based on planar lightwave circuits [17]. But this solutionis disadvantageous in cost and packaging losses (input/outputcoupling losses). A simpler, easily manufactured, and manage-able correlator may be constructed by writing a series of FBGmirrors into a single length of fiber [18]. In this paper, we designan FBG-based correlator in order to validate the OSNR moni-toring technique.

A. FBG-Based Correlator

In this type of correlator, a series of FBGs is written into asingle length of fiber. In these applications, the array of grat-ings reflects back part of the signal at different times, resultingin multiple replicas of the input signal spaced in time with thedelay increment . The reflectivities of the FBGs provide thesame weighting function as the optical correlation, and theirvalues are designed so that the light reflecting of each FBG hasequal power when it exits the correlator, following this recursiveequation:

(3)

Since the light makes a double pass through the array, thespacing between FBGs must match half the bit period to producea round-trip delay of 1-bit.

An FBG-based correlator can be constructed by using ther-mally controlled FBGs as tunable-reflectivity mirrors [18]. Thethermal control consists of a series of thin-film microheatersonto the surface of a single uniform grating, which cause a shiftin the grating spectrum toward longer wavelengths in responseto the rise in temperature. Then, FBGs are fabricated to actas high reflectivity mirrors. By tuning the FBG thermally, thegrating passband is shifted so that the signal wavelength inter-sects with the rising or falling edge of the grating response to ob-tain the desired reflectivities given by (3). However, tuning thegrating reflectivity by operating it in the edge of the reflectivityis quite sensitive and has some disadvantages associated withpolarization dependence, time-delay variation with reflectivity,and dispersion. As each subgrating is tuned to a different reflec-tivity by operating them at the band edge, each grating will intro-duce some non-negligible polarization dependent loss, causingdistortion in the correlation output. Moreover, the time delay ofa signal reflected from an FBG varies for different points alongthe edge. These unwanted time delay variations between sub-grating degrades the correlation. Also, as the edge is steep, thesignal can be affected by dispersion.

To solve these problems, our correlator was also constructedby writing a series of FBG into a single length of fiber but eachgrating had the desired reflectivity thereby avoiding the thermal

Fig. 5. Insertion loss and polarization dependence of our FBG-based correlator.

tuning and the inconvenience associated with operating at theedge of the FBG spectrum. Regarding the polarization depen-dence Fig. 5 shows the polarization dependence loss of our cor-relator, which is almost zero from 1553.5 to 1554.2 nm. TheFBG-based correlator is centered at 1553.8 nm with a 0.6-nmbandwidth so that the correlator passband is just located in theband where polarization dependent loss is zero.

The response of the array of gratings was designed by usingthe matrix transfer approach [19], [20]. This approach is basedon identifying 2 2 matrices for each uniform grating sectionas defined in [21] and calculating the FBG response taking intoaccount the field amplitude of modes traveling in anddirections and thus the phase of the signals. For the spacing be-tween gratings, we insert a phase-shift matrix to model the fiberspans between them. During the simulations, the autocorrelationfunction was considered in complex amplitude basis and conse-quently optical interferences can be observed. For this reason, inour design, we properly adjusted the spacing between gratings,i.e., the phase difference, to maintain the pulse phase along thecorrelator avoiding the interference effects commented previ-ously [22].

IV. EXPERIMENTAL RESULTS AND DISCUSSION

To demonstrate the feasibility of the system, a 40 Gb/s cor-relator configured to match the pattern [10001011] was chosenwhose design is shown in Fig. 6. Each “1” of the pattern is repre-sented by a uniform FBG. The optical fiber length between twosuccessive FBGs is proportional to the bit period in order to rec-ognize the desired pattern, i.e., fiber length proportional to ,2 , and 4 , where is the bit period. One aspect of nov-elty of our correlator is the design with unequal grating spacing,i.e., with different time intervals, in order to reduce the side-lobes and the effect of secondary reflections over the autocorre-lation pulse. By means of simulations, a study of the correlatoroutput as function of the pattern was carried out in order to de-cide which one provided better performance and less overhead.Fig. 7(a), (c), (e), and (g) show the correlator response to a singleinput pulse for the patterns [1111], [101011], [10001011], and


Fig. 6. FBG-based correlator design.

Fig. 7. Correlator output as function of the pattern. (a), (b) Correlation responseto a single pulse and correlation output for the pattern [1111], respectively; (c),(d) Correlation response to a single pulse and correlation output for the pat-tern [101011], respectively; (e), (f) Correlation response to a single pulse andcorrelation output for the pattern [10001011], respectively; (g), (h) Correlationresponse to a single pulse and correlation output for the pattern [10000010011],respectively.

[10000010011], respectively, all with . As the spacing in-creases, secondary reflections have less amplitude and less im-pact on the autocorrelation pulse. Fig. 7(b), (d), (f), and (h) show

Fig. 8. Correlation output sensitive to fabrication process.

the autocorrelation output for the patterns mentioned earlier. Inthe cases (b) and (d), the presence of secondary reflections causethe increase of the sidelobes, reducing the contrast between thecentral peak and these sidelobes and even hiding it. The cases (f)and (h) provide good performance at the expense of increasedoverhead. Therefore, the pattern [10001011] was set to obtainthe better performance with less overhead.

The FBG-based correlator was realized by exposure of thefiber to ultraviolet laser light with the phase mask scanningmethod [23], [24]. All the FBGs were tuned to the same wave-length . Bragg wavelength shift can occur dueto environmental conditions and due to technological consid-erations, so all the FBGs cannot be tuned to the same wave-length. Concretely, FBG wavelength is sensitive to temperature.To avoid this effect, the FBG-based correlator should be pack-aged and stabilized by using thermal paste and heat sink. Inthose conditions, the temperature variations have no significantimpact on correlator response. As far as technological consid-erations are concerned, Bragg wavelength mismatches are pro-duced by writing the FBGs with different refractive-index mod-ulation amplitude. Due to this fabrication process, Bragg wave-lengths are detuned from each other. Moreover, different errorsor imperfections during the writing could also produce detuning.This mismatch can impact on the autocorrelation pulse reducingthe optical power and therefore increasing the monitor error.To overcome Bragg wavelength detuning, the FBG should bewritten with the same index modulation depth and the reflec-tivity can be adjusted by varying the FBG physical length as itis explained in [25].

The reflectivities were fixed to 16%, 23%, 38%, and 100%[using (3)]. Fabricating low reflectivity gratings with high pre-cision is not an easy task because of the reflectivity depends onthe function. This means that when the grating is not sat-urated, a slight deviation of the refractive index will have a largeimpact on the reflectivity. Fig. 8 shows the sensitive of the cor-relation to fabrication process when the reflectivity variationsare 10%. As it can be seen, these variations cause a slight re-duction in the amplitude of the autocorrelation pulse.

The grating with the highest reflectivity FBG4was firstly written. Then, the fiber was moved along its axisover a length corresponding to the required “0” in the patternand the FBG3 was written. The grating FBG2


and FBG1 were written followinga similar process. The optical fiber length between the succes-sive gratings were , , and

, measured from the beginning of one gratingto the beginning of the following one (Fig. 6). These lengthscorrespond to time intervals of , 2 , and 4 at 40 Gb/s.The length of the gratings were

and .The accuracy of the FBG positioning along the fiber was in

the micrometer range. The bandwidth of the FBGs correspondedto a full-width at half-maximum of 10 ps.

The output of the fabricated correlator at 40 Gb/s is shownin the Fig. 9. The monitoring field, [10001011], associated witha certain QoS requirement is processed in the node to performthe correlation function. When this field matches with the pat-tern stored in the array of FBGs, an autocorrelation pulse ap-pears [Fig. 9(b)]. The amplitude level of the sidelobes is al-most the same at both sides of the autocorrelation pulse andsome residual peaks appear at the end of the correlation func-tion due to the secondary reflections produced inside the sub-gratings. These residual peaks suffer from larger time delaysand attenuation. From Figs. 7 and 8 (simulation results), it canbe noticed that the contrast between center peak and sidelobesis very high. In this case, the pulses remain phase correlated sothat they are combined in field amplitude. Concretely, the au-tocorrelation function is composed of four pulses ( , where

is the field amplitude of one pulse). Then, in power magni-tude, it corresponds to a contrast equal to , where is thepulse power. This result is in agreement with the contrast ob-served in Figs. 7 and 8. In contrast, in the experimental resultswe observed that the correlator was not sensitive to the coher-ence issues and the pulses were incoherently combined. This isthe consequence of incoherent operation of output laser pulses.From Fig. 9(b), we can see that the autocorrelation pulse is al-most four times the sidelobes, which corresponds to consideringautocorrelation function in power basis (the contrast is approx-imately 3.7). Moreover, as it has been mentioned before, thespacings between gratings were carefully chosen to avoid thatthe autocorrelation pulse was combined with multiple reflectedpulses so that the autocorrelation function was less sensitive tocoherence time effects. In particular, we used unequal spacingbetween gratings. This solution was pointed out in [22].

On the other hand, when the monitoring field, i.e., theQoS requirement, is different (cases (c), [10011001], and (e),[10100001]), the peak decreases and the secondary pulsesincrease [Fig. 9(d) and (f)].

Therefore, each intermediate node can monitor packet flowswith the same QoS requirement independently, i.e., the autocor-relation peak activates the OSNR monitoring process, thus pro-viding a consistent treatment for each QoS class at every hop(Fig. 2). The optical correlator is sensitive to the fluctuations inthe received signal due to the fiber impairments and it can beused to estimate the OSNR of the optical packets from the sta-tistics of the autocorrelation pulse shown in the Fig. 10.

The principle of operation of the quality monitoring tech-nique was validated experimentally by means of the setup shownin Fig. 11. The monitoring field, , wasgenerated through external modulation of a RZ Gaussian pulsesource at 1553.8 nm. The modulating signal driving the Mach-

Fig. 9. (a) Input sequence: 10001011; 3 mV/div; 50 ps/div; (b) Output of theoptical correlator for (a); 1 mV/div (AVG); 100 ps/div; (c) Input sequence:10011001; 3 mV/div; 50 ps/div; (d) Output of the optical correlator for (b); 1mV/div (AVG); 100 ps/div; (e) Input sequence: 10100001; 3 mV/div; 50 ps/div;(f) Output of the optical correlator for (b); 1 mV/div (AVG); 100 ps/div.

Fig. 10. Statistics of the autocorrelation pulse.

Zehnder modulator was obtained from a 40 Gb/s electrical pseu-dorandom binary sequence equipment. An optical circulator isplaced at the array input to route the counter-propagating cor-relation output to the sampling scope. The signal out from thetransmitter is coupled with a second EDFA to simulate the linknoise. Thus, the OSNR of the optical signal can be changed bycombining the signal with different ASE noise levels adjustingthe gain of the EDFA pump laser. The response of the EDFAfor different gain values is shown in Fig. 12. The optical signaldegraded with the noise passes through a 1-nm bandwidth filterand enters the optical correlator. The OSNR can be evaluatedby measuring the statistics of the autocorrelation peak, morespecifically using the mean value and the standard deviation.These values are extracted by means of a high-speed samplingscope. Despite using a sampling scope, the technique allowshigh-speed bit rate operation as it only measures the autocorre-lation pulse, so the system works at the packet rate and therebyalleviating speed requirements with respect to typical BER tech-niques as it was mentioned previously.


Fig. 11. Experimental setup.

Fig. 12. EDFA response for different gain values (ASE noise source).

To demonstrate the feasibility of the proposed monitoringtechnique, Fig. 13 depicts the variation of the measured OSNRfrom the input pulse as a function of the actual OSNR measuredby the OSA. The OSNR measured using the OSA is calculatedas follows:

(4)

where is the signal wavelength peak power and is theASE noise floor level for the channel bandwidth. As it can beseen, the estimated and measured OSNR are practically equal.

Once confirmed that the noise statistics agree with the OSNRvalue, the proposed technique should be validated. Fig. 14shows the OSNR values for the incoming signal versus theestimated OSNR using the autocorrelation pulse and confirmsthe OSNR monitoring functionality of our proposed scheme.The OSNR monitoring errors are the difference between theOSNR measured from the input pulse and from the autocor-relation output pulse. From the curve, the maximum error forthe measurement of different OSNR values is 0.5 dB in the15–25 dB OSNR range (Fig. 14). The slight deviation in lowerOSNR values can be explained by the noise filtering behaviorof the FBG-based correlator. The optical signal degraded withthe noise passes through a 1-nm bandwidth filter and enters the

Fig. 13. OSNR monitoring: OSNR measured using an OSA and using the pro-posed technique.

Fig. 14. Experimental OSNR monitoring.

optical correlator whose bandwidth is approximately 0.6 nm.Therefore, for lower OSNR values, the output of the correlatoris less sensitive to the degradation than the input, primarily dueto the narrower optical passband of the FBG-based correlatorproviding a smaller noise background. In contrast, for higherOSNR values, where the noise background is practically negli-gible, the difference is due to the insertion loss of the gratings,which slightly affects the power of the correlator response.Therefore, monitoring errors are caused by noise at lowerOSNR values and by insertion loss at higher OSNR values.

As it has been commented before, this technique allows thequality monitoring for a packet flow with specific QoS require-ments. Moreover, from the OSNR value estimated with the cor-relator, each intermediate node can control whether the trans-mission meets the QoS and performance requirements specifiedby the carrier and in case of great signal degradation, the nodecan take immediate actions to restore the connection. Therefore,the beauty of using performance monitors close to the inter-mediate switching nodes is that we could associate the moni-tored parameters with the switch controls and label information.Therefore, it is possible to integrate the optical performancemonitors to switching nodes for the intelligent network and fastresponse to mitigate transmission impairments.


V. CONCLUSION

Future high-speed networks will be characterized by dynamictraffic scenarios. In this context, optical packet-switched net-works promises to bring the flexibility and efficiency of the In-ternet to transparent optical networking with bit rates extendingbeyond that available with electronic router technologies. How-ever, optical impairments degrade the performance of opticalnetworks and, therefore, monitoring optical signal quality is be-coming critically important.

Unlike circuit switching, OPS does not need to establish anyconfirmed connection before communication and packets arerouted over independent paths to their destination. In this sce-nario, QoS allows the network to provide better service and toguarantee a certain level of performance to selected networktraffic. Therefore, in OPS networks, a monitoring system mustbe deployed as part of QoS to insure that networks operate atthe desired QoS level.

In this paper, a novel monitoring technique based on opticalcorrelation is proposed and demonstrated experimentally. Aspecific monitoring field, with QoS information included,is inserted into the optical label and is processed in eachintermediate node. By measuring the noise statistics of theautocorrelation pulse peak power, the OSNR can be estimatedwith an error lower than 0.5 dB. Thereby, the autocorrelationpeak can be a feasible indicator of the signal impairments inhigh-speed optical networks. The advantages of the proposedOSNR monitoring technique are in-band measurements, re-laxed speed requirements with respect to typical techniques,simple implementation, and possibility of integration withother functions in the packet switching node to take immediateactions when the signal quality does not fulfill the terms of QoS.In this context, information obtained from the optical monitorcould be included when calculating new routes for data signalsor backup routes when signal failures occur in next-generationnetworks. Indeed, signal quality monitoring information couldbe added as an additional factor in the routing decision in orderto sustain reliability requirements of future networks and keepthe level of QoS promised to customers.

ACKNOWLEDGMENT

The authors thank R. García-Olcina from the Optical andQuantum Communications Group for the loan of an FBG-basedoptical filter and also the Project TEC2007-68065-C03-01.

REFERENCES

[1] S. J. B. Yoo, “Optical packet and burst switching technologies forthe future photonic internet,” J. Lightw. Technol., vol. 24, no. 12, pp.4468–4492, Dec. 2006.

[2] C. Develder, A. Stavdas, A. Bianco, D. Careglio, H. Lonsethagen, J. P.Fernández-Palacios Jiménez, R. Van Caenegem, S. Sygletos, F. Neri,J. Solé-Pareta, M. Pickavet, N. Le Sauze, and P. Demeester, “Bench-marking and viability assessment of optical packet switching for metronetworks,” J. Lightw. Technol., vol. 22, no. 11, pp. 2435–2451, Nov.2004.

[3] D. J. Blumenthal, B. Olsson, G. Rossi, T. E. Dimmick, L. Rau, M.Masanovic, O. Lavrova, R. Doshi, O. Jerphagnon, J. E. Bowers, V.Kaman, L. A. Coldren, and J. Barton, “All-optical label swappingnetworks and technologies,” J. Lightw. Technol., vol. 18, no. 12, pp.2058–2075, Dec. 2000.

[4] D. C. Kilper, R. Bach, D. J. Blumenthal, D. Einstein, T. Landolsi, L.Ostar, M. Preiss, and A. E. Willner, “Optical performance monitoring,”J. Lightw. Technol., vol. 22, no. 1, pp. 294–304, Jan. 2004.

[5] G. Bendilli, C. Cavazzoni, R. Girardi, and R. Lano, “Optical perfor-mance monitoring techniques,” in Proc. 26th Eur. Conf. Opt. Commun.(ECOC 2000), Munich, Germany, Sep. , vol. 4, pp. 113–116.

[6] L. Ding, W.-D. Zhong, C. Lu, and Y. Wang, “A new bit error ratemonitoring method based on histograms and curve fitting,” Opt. Exp.,vol. 12, no. 11, pp. 2507–2511, 2004.

[7] D. Ye and W.-D. Zhong, “Improved BER monitoring based on ampli-tude histogram and multi-Gaussian curve fitting,” J. Opt. Netw., vol. 6,pp. 584–598, 2007.

[8] I. Shake, H. Takara, and S. Kawanishi, “Simple Q factor monitoringfor BER estimation using opened eye diagrams captured by high-speedasynchronous electrooptical sampling,” IEEE Photon. Technol. Lett.,vol. 15, no. 4, pp. 620–622, Apr. 2003.

[9] D. C. Kilper, A. Azarov, W. Weingartner, and P. Vorreau, “Q-factormonitoring for fault management applications,” in Proc. Opt. FiberCommun. Conf. Exhibit (OFC 2004), Feb. , vol. 2, p. 3.

[10] J. Yang, M. Y. Jeon, J. Cao, Z. Pan, and S. J. B. Yoo, “Performancemonitoring in transparent optical networks using self-monitoring op-tical labels,” Electron. Lett., vol. 40, no. 21, pp. 1370–1372, 2004.

[11] J. Q. Yang, Z. Zhu, H. Yang, Z. Pan, and S. J. B. Yoo, “All-opticaltime-to-live using error-checking labels in optical label switching net-works,” in Proc. 30th Eur. Conf. Opt. Commun. (ECOC 2004), Stock-holm, Sweden, 2004.

[12] R. Vilar, J. M. Martínez, F. Ramos, and J. Martí, “All-optical decre-menting of a packet’s time-to-live (TTL) field using logic XOR gates,”Opt. Exp., vol. 16, no. 24, pp. 19734–19740, 2008.

[13] R. Vilar, F. Ramos, and J. Marti, “Optical signal quality monitoringusing fibre-Bragg-grating-based correlators in optical packet-switchednetworks,” in Proc. 33rd Eur. Conf. Opt. Commun. (ECOC 2007), Sept., vol. 5, pp. 239–240.

[14] Internetworking Technologies Handbook. Indianapolis, IN: CiscoSystems, Inc. Cisco Press, 2003, ch. 59.

[15] C. Bintjas, N. Pleros, K. Yiannopoulos, G. Theophilopoulos, M. Ka-lyvas, H. Avramopoulos, and G. Guekos, “All-optical packet addressand payload separation,” IEEE Photon. Technol. Lett., vol. 14, no. 12,pp. 1728–1730, Dec. 2002.

[16] G. W. Euliss and R. A. Athale, “Time-integrated correlator based onfiber-optic delay lines,” Opt. Lett., vol. 43, pp. 149–151, 1983.

[17] K. Takiguchi, T. Shibata, and M. Itoh, “Encoder/decoder on planarlightwave circuit for time-spreading/wavelength-hoping opticalCDMA,” Electron. Lett., vol. 38, no. 10, pp. 469–470, 2002.

[18] M. C. Hauer, J. E. McGeehan, S. Kumar, J. D. Touch, J. Bannister, E.R. Lyons, C. H. Lin, A. A. Au, H. P. Lee, D. S. Starodubov, and A. E.Willner, “Optically assisted internet routing using arrays of novel dy-namically reconfigurable FBG-based correlators,” J. Lightw. Technol.,vol. 21, no. 11, pp. 2765–2778, Nov. 2003.

[19] T. Erdogan, “Fiber grating spectra,” J. Lightw. Technol., vol. 15, no. 1,pp. 1277–1294, Aug. 1997.

[20] M. Yamada and K. Sakuda, “Analysis of almost-periodic distributedfeedback slab waveguides via a fundamental matrix approach,” Appl.Opt., vol. 26, no. 16, pp. 3474–3478, 1987.

[21] R. Kashyap, Fiber Braggs Gratings. San Diego, CA: Academic,1999, pp. 178–185.

[22] I. Fsaifes, C. Lepers, M. Lourdiane, R. Gabet, and P. Gallion,“Pulsed laser source coherence time impairments in a direct detectionDS-OCDMA system,” in Proc. Conf. Lasers Electro-optics (CLEO2006), Long Beach, CA, , paper CWH6.

[23] K. Hill and G. Meltz, “Fiber Bragg grating technology fundamentalsand overview,” J. Lightw. Technol., vol. 15, no. 8, pp. 1263–1276, Aug.1997.

[24] D. Z. Anderson, V. Mizrahi, T. Erdogan, and A. E. White, “Productionof in-fiber gratings using a diffractive element,” Electron. Lett., vol. 29,no. 6, pp. 566–568, 1993.

[25] I. Fsaifes, C. Lepers, A. Obaton, and P. Gallion, “DS-OCDMA en-coder/decoder performance analysis using optical low-coherence re-flectometry,” J. Lightw. Technol., vol. 24, no. 8, pp. 3121–3128, Aug.2006.

Ruth Vilar Mateo received the B.S. and M.S.degrees in telecommunications from the UniversidadPolitecnica de Valencia, Valencia, Spain, in 2004and 2007, respectively. She is currently pursuing thePh.D. degree.

She joined the Nanophotonics Technology Center,Valencia, Spain, as a Research Associate in 2004.Her current research activities include opticalpacket-switched networks, optical performancemonitoring, optical networking, optical signal pro-cessing, and packet switching.


Jaime García was born in Valencia, Spain, in 1979.He received the M.Sc. and Ph.D. degrees in telecom-munications from the Universidad Politécnica de Va-lencia, Valencia, in 2002 and 2008, respectively.

He is currently an Associate Professor at this uni-versity and member of the Valencia NanophotonicsTechnology Center, Valencia. His research interestsinclude photonic biosensors, planar photonic crystalsand periodic structures, slow-light propagation, sil-icon photonics, and all-optical signal processing.

Guillaume Tremblay is currently working toward the Bachelor’s degree inelectrical engineering at the Université Laval, Québec, Canada.

He joined the Centre d’Optique, photonique et laser (COPL) as a summerstudent, in 2008.

Youngjae Kim received the Bachelor’s degree in physics from Sogang Univer-sity, Seoul, Korea, in 1999, and received the Master’s and Ph.D. degree in infor-mation and communications engineering from the Gwangju Institute of Scienceand Technology (GIST), Gwangju, Korea, in 2001 and 2006, respectively.

He is currently with the Centre d’Optique Photonique et Laser (COPL), De-partment of Electrical and Computer Engineering, Université Laval, Québec,Canada, as a Research Professional. His recent research interests include designand development of active and passive optical components for optical sensors,optical imaging systems, optical communication systems, and wireless indoorcommunication systems.

Sophie LaRochelle (M’00) received the Bachelor’sdegree in engineering physics from the UniversitéLaval, Québec, Canada, in 1987, and the Ph.D.degree in optics from the University of Arizona,Tucson, in 1992.

From 1992 to 1996, she was a Research Sci-entist at the Defense Research and Develop-ment Canada—Valcartier, where she worked onelectro-optical systems. She is now a Professor at theDepartment of Electrical and Computer Engineering,Université Laval, where she holds the Canada Re-

search Chair in Optical Fibre Communications and Components. Her currentresearch interests include active and passive fiber optics components for optical

communication systems including fiber Bragg gratings, optical amplifiers,multi-wavelength, and pulsed fiber lasers. Other research interests includepacket-switched networks with photonic code processing, transmission ofradio-over-fiber signals, and OCDMA.

Dr. LaRochelle is a member of OSA and of IEEE-LEOS.

Francisco Ramos (S’98–A’00–M’04) was born in Valencia, Spain, on April2, 1974. He received the M.Sc. and Ph.D. degrees in telecommunication en-gineering from the Polytechnic University of Valencia, Valencia, in 1997 and2000, respectively.

Since 1998, he has been with the Department of Communications at the sameuniversity, where he is now an Associate Professor. He has participated in sev-eral national and European research projects on areas such as optical accessnetworks, broadband wireless systems, and optical networking. His current re-search interests include nonlinear photonics and chaos. He has coauthored morethan 100 papers in international journals and conferences, and he has acted asReviewer for the IEE, IEEE, Elsevier, and Taylor and Francis publishers.

Dr. Ramos is also the recipient of the Prize of the Telecommunication En-gineering Association of Spain for his thesis dissertation on the application ofoptical nonlinear effects in microwave photonics.

Javier Martí (S’89–M’92) received the Ingeniero de Telecomunicación degreefrom the Universidad Politécnica de Catalunya, Barcelona, Spain, in 1991, andthe Doctor Ingeniero de Telecomunicación degree (Ph.D.) from the UniversidadPolitécnica de Valencia, Valencia, Spain, in 1994.

From 1989 to 1990, he was an Assistant Lecturer with the Escuela Universi-taria de Vilanova, Barcelona. From 1991 to 2000, he was a Lecturer and Asso-ciate Professor with the Telecommunication Engineering Faculty, UniversidadPolitécnica de Valencia, where he is currently a Professor and leads the Radioover Fiber Group. In 2003, he was appointed as the Director of the ValenciaNanophotonics Technology Center, Universidad Politécnica de Valencia. He hasauthored or coauthored more than 200 papers in referred international technicaljournals and more than 100 papers in international conference proceedings in thefields of broadband hybrid fiber-radio systems and microwave/millimeter-wavephotonics, fiber-based access networks, terabits-per-second optical time-divi-sion multiplexing/wavelength-division multiplexing optical networks, advancedoptical processing techniques for microwave signals and ultrahigh-speed datatransmission, and planar photonic crystals. He is participating very actively inthe European Research Framework (FP5, FP6, and FP7) and leading severalprojects in the areas of broadband access (OBANET, GANDALF), all-opticalprocessing in photonic networks (LASAGNE), and nanophotonic logic gates(PHOLOGIC).

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