Monitoring technique for a hybrid PS/WDM-PON by using a tunable OTDR and FBGs

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2006 Meas. Sci. Technol. 17 1070

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INSTITUTE OF PHYSICS PUBLISHING MEASUREMENT SCIENCE AND TECHNOLOGY

Meas. Sci. Technol. 17 (2006) 1070–1074 doi:10.1088/0957-0233/17/5/S22

Monitoring technique for a hybridPS/WDM-PON by using a tunable OTDRand FBGsSwook Hann, Jun-sang Yoo and Chang-Soo Park

Department of Information and Communications, Gwangju Institute of Science andTechnology, 1 Oryong-dong, Buk-gu, Gwangju 500-712, Korea

E-mail: swookhann@yahoo.com and csp@gist.ac.kr

Received 18 July 2005, in final form 28 November 2005Published 7 April 2006Online at stacks.iop.org/MST/17/1070

AbstractA monitoring technique for hybrid passive optical networks (PON) ispresented. The technique is based on the remote sensing of fibre Bragggratings (FBGs) using a tunable optical time domain reflectometer (OTDR).The FBG would help discern an individual event during the monitoring ofthe hybrid PON in collaboration with the information provided by theRayleigh backscattered power. The hybrid architecture of passivesplitter-PON and WDM-PON can be analysed by the monitoring method byusing the tunable OTDR and FBGs at the central office under the in-servicestate of PON.

Keywords: passive optical networks, OTDR, monitoring method, remotesensing method

(Some figures in this article are in colour only in the electronic version)

1. Introduction

The passive optical network (PON) [1] is a promisingarchitecture in optical access networks. To guarantee qualityof service through the PON, monitoring techniques shouldbe provided under the in-service state through both faultdiagnosis and maintenance [2–4]. However, the PON has somedifficulties in in-service monitoring and maintenance due to itsinherent point-to-multipoint architecture, not point-to-point.

Recently, the PS/WDM-PON (or hybrid PON) as a hybridform of power-splitter (PS)-PON and WDM-PON has drawnmuch interest from network providers because it can expandnetwork coverage as an overlay structure of the alreadyinstalled PS-PON or from the scalability by the wavelengthto be added. In such a hybrid PON, the monitoring techniqueto diagnose failures from multi-branched service lines is moreimportant [2–4].

In this paper, a diagnosis technique on multiple branchedlines in a hybrid PON will be introduced. For wavelengthscanning in the WDM-PON part and branch line identity in thePS-PON part, an optical time domain reflectometer (OTDR)with wavelength tunability and a wavelength-dependentreference reflector using fibre Bragg gratings (FBGs) are

proposed and experimentally demonstrated. This wavelengthdependence in the reflector blocks the accumulation from otherbranch spans. Therefore, each branch can easily be discernedwhile the branch is being scanned by the tunable OTDR. Also,the influence of Rayleigh scattered power accumulated fromlonger branches to the shortest one is analytically discussedand reflected in the OTDR traces.

2. Monitoring technique for a hybridPS/WDM-PON

The hybrid PS/WDM-PON, composed of an optical lineterminal (OLT) in a central office (CO), a feeder line, a remotenode (RN) of WDM couplers and/or PSs, and distribution fibrelines with multiple optical network units (ONUs) separated byless than 20 km fibre from the OLT, is depicted in figure 1.The OTDR is a well-known instrument for monitoring andmaintaining a fibre line. However, the conventional OTDRwas devised to monitor and analyse the backscattered powerfrom the point-to-point type of the fibre line. Therefore, thisOTDR is not adequate to use for the PON structure due to thebackscattered powers from other branches. Furthermore, in

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Monitoring technique for a hybrid PS/WDM-PON

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Figure 1. Centralized monitoring technique for a hybrid PS/WDM-PON.

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Figure 2. OTDR trace of PON with a 1 × 8 PS: (a) 1 × 8 PS-PON for line monitoring; (b) connection losses reduced by increasing thenumber of branches from i = 8 to i = 1 with a step 1, giving a final total of 8 branches; (c) measured attenuation losses by changing theVOA with a 2 dB step; (d ) loss variation of the fibre end peak on the OTDR trace at real 2 dB VOA loss as the total number of branches isincreased.

the case when wavelength-dependent passive devices such asa WDM coupler are being used at a RN, a tunable opticalsource is required in the OTDR for the scanning of eachbranch. Practically, the measurement value for a 1 × 8 PSas an example shows a value different from the actual one.The 1 × 8 PS has a splitting loss of about 10 dB, butappears to be about 5.4 dB on the OTDR trace due to thepowers accumulated from other branch lines except for the lineconcerned (see figures 2(a) and (b)). This is the reason why theadditional Rayleigh backscattered powers are accumulated tothe measured backscattered power for a shorter branch duringthe flying time of the OTDR pulse. These phenomena appearas a number of stair-like steps on the OTDR trace. The numberof branches involved in the power measurement of the shorterline can be counted by reading the number of stair-like steps

and the amount of accumulated power from the stairs’ heightbetween adjacent branches. This deviation can be confirmedby the following experiment. Figure 2(b) shows the variationin connection loss between the PS and the shortest branchby adding other branches in the steps of 1, starting from asingle branch (i = 8) to a total of 8 branches. Therefore,the measured connection loss between the PS and the shortestbranch is reduced as the number of PS branches increases. Thesevere level drop at the 11.93 km branch is due to the variableoptical attenuator (VOA) (which simulates an environmentalexternal loss) inserted in the shortest branch (i = 1) as shown infigure 2(c). Thereby, the measurement values of the VOA onthe OTDR trace appear to be 1.247, 1.948, 2.629 and 2.793 dB,respectively, even though the actual attenuation values of 2, 4,6 and 8 dB are shown in figure 2(c). This unbalanced branch

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loss from the PS to each fan-out branch on the OTDR trace,C(i), can also be confirmed by the following expression:

C(i) = 5log(M) + [5log(i) + C.L(i)] (i � M), (1)

where i is the natural number ascending from the shortest pathlength of a 1 × M splitter, M is the total number of branchesof the PS, C.L(i) is the excess connection loss of each branchof the PS. From equation (1), the measured OTDR loss ofthe PS-PON gives us the number of branches in operation.Figure 2(d ) shows the relationship between the connectionloss of the branch and the number of branches added in thecase of 2 dB VOA attenuation. The connection loss of theshortest branch decreases with an increase in the total numberof branches and is well matched with the measured values.

Adding a wavelength-dependent Bragg reflector such asFBG to each branch and scanning the returned power with atunable OTDR can avoid the error of line monitoring. This isthe reason why the Bragg reflection is distinguishable from themultiple backscattered powers by unwanted branches. Also,this should be applicable to both PS-PON and WDM-PON.We propose a monitoring coupler that consists of an FBGas a reflector for the specific wavelength and a wavelength-selective coupler to separate a monitoring signal from thedata signal. The FBG is terminated with an angled physicalcontact (APC) connector to suppress the unwanted reflectionat the fibre end. For wavelength scanning, an external cavitylaser is added to a conventional single wavelength OTDR withthe wavelength tuning range of 1520–1620 nm, resolutionof 0.01 nm, output power of +7 dBm and spectral widthof 100 MHz. Monitoring pulses are generated through anacousto-optic modulator with 500 ns pulse width (about 50 mspatial resolution), reflected back by the FBG and induce abackscattered power of −42.2 dBm (for example, −54.2 dBmwith a 1 × 8 PS) from a 20 km single-mode fibre. Thefibre attenuation, 0.21 dB km−1, consists of absorption andscattering losses, 0.0084 dB km−1 and 0.2016 dB km−1,respectively.

The reference reflection for monitoring is obtained fromthe FBG (−9 dB reflectivity under the saturation level ofthe OTDR receiver, 0.1 nm full width first zeros (FWFZ) byconsidering the tuning resolution of the tunable OTDR, andis sidelobe-flattened, which means a few redundant reflectionsaround the FBG, as shown in figure 3). The reference reflectionshows the end point of each fibre distribution line as a reflectedpulse under the saturation level of the OTDR. The reflectivityof the FBG is considered from the maximum optical sourcepower of +7 dBm within −21 dB fibre link losses (includingthe splitting loss of 1 × 8 PS, 20 km fibre loss and additionallink losses) and a receiver sensitivity of −54.2 dBm. TheFresnel reflection from the end fibre is removed by the APCconnector to suppress unwanted reflection at the multiple fibreends. To multiplex/demultiplex the monitoring band anddata band, a wavelength-selective coupler (WC1) is used infront of the OLT. The RN has the function of distributingthe data signals to all the ONUs. The RN consists of WC2,AWG with a periodic repeatability, and PS. Also the WC2 hasthe function of multiplexing/demultiplexing PS/WDM-PONbands in front of the RN as shown in figure 1. To diagnosethe state of the distribution fibre lines, the monitoring band isseparated into two sub-bands. One is for the PS monitoring

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Table 1. Experimental wavelength plans of the hybridPS/WDM-PON.

Example items Data band (nm) Monitoring band (nm)

AWG 1508–1512 1548–1552PS 1529–1531 1569–1571

band and the other is for the AWG monitoring band. Thewavelength plans of the hybrid PON are shown in table 1.

In the previous work, filters are used to isolate betweenthe service signal and the monitoring signal [5]. However,in this proposed method, each ONU has the WC1 adaptorwithout using any stop-band filters in front of the receiver(RX). With the centralized tunable OTDR and reference FGBs,both optical and physical diagnoses of the hybrid PS/WDM-PON can be performed.

In this paper, we will show the experimental results of thecentralized monitoring technique under the in-service state.

3. Experiment results

The experimental setup is designed for the demonstrationof the in-service line monitoring technique for the hybridPS/WDM-PON from the OLT to the ONU. The proposedcentralized monitoring technique is as follows: (1) the opticaland physical diagnoses are performed; (2) the hybrid PONarchitecture is demonstrated and verified.

The variations of the trace of the tunable OTDR depictboth temperature dependence of the FBG and bending loss ofthe fibre in the PON (as shown in figure 4). The spectrumshift of the FBG and AWG was measured by varying thetemperature from 20 ◦C to 50 ◦C.

Figure 4(a) shows the monotonic increase of the centrewavelength due to the rise in temperature of the AWGand FBG, silica-based passive optical components, of about0.01 nm ◦C−1. Figure 4(b) depicts the bending loss of the fibrewith a mandrel by 10 turns. The bending loss on the OTDRtrace increases as the radius of the mandrel becomes smaller.

The demonstrated configuration of the hybrid WDM/PS-PON is shown in figures 5 and 6. Figure 5 depicts themonitoring step for PS-PON under the in-service state. A1 × 4 passive splitter fans out four ONUs with each marker.While observing the FBG V with λ5 (1569.83 nm), the OLTcan service the data channel to a ONU as shown in figure 5(a),WC1. The OTDR trace shows several events on the PS-PON as shown in figure 5(b). The first peak of the OTDR

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Figure 4. Optical characteristics of passive components in the PON: (a) temperature variation of the FBG and AWG; (b) bending loss of thefibre.

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Figure 5. PS-PON monitoring step of the proposed monitoring method: (a) schematic diagram of PS-PON; (b) OTDR trace of PS-PON.

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Figure 6. WDM-PON monitoring step of the proposed monitoring method: (a) schematic diagram of WDM-PON; (b) OTDR trace ofWDM-PON.

trace is the WDM coupler (marked as WC2 in figure 5(a)).The second peak is the power splitter; the others are thefibre spans of different lengths. The OTDR trace has about3.9 dB drops from the splitter point, not 6 dB (governed byequation (1)).

Figure 6 is the monitoring step for WDM-PON under thedata service for ONU1 through the first port of the AWG. In

this experiment, four ports of the AWG (100 GHz spacing,3 dB bandwidth of 0.4 nm) are used, λ1–λ4 from 1548 nm to1551 nm. The tunable OTDR can diagnose the status of theservice lines with λ1. The OTDR trace of the WDM-PON isshown in figure 6(b). The first port of the AWG has a 1 × 2coupler with the asymmetric fan-out fibre lengths, 2 km and2.3 km (‘(1)’ and ‘(2)’, respectively, in figure 6). Note that

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Figure 7. BER curves of the hybrid PS/WDM-PON for back-to-back, about 20 km link without the proposed tunable OTDR (TOTDR)method, about 20 km link with the proposed TOTDR method in figures 5 and 6: (a) BER of PS-PON; (b) BER of WDM-PON.

the OTDR trace can verify the health of each of the multiplepaths of the WDM-PON at different optical wavelengths. Theproposed specification of the FBG is 0.1 nm FWFZ with a−9 dB reflectivity; therefore, the tolerance of the temperatureon the local distribution line for each ONU is 40 ◦C higher thanthat of the RN. In the case of fire through a duct of fibre, thetemperature deviation between the RN and the ONU is morethan 40 ◦C. Therefore, the perturbation of the temperatureon the local distribution line of each ONU can appear as awavelength shift of the FBG at that time.

The service data rate is 10 Gbit s−1 (at a 95% confidencelevel for a 231 − 1 pseudo-random binary sequence) for futurePON environments with 1530.5 nm. The PS-PON has held awavelength source in common among all the ONUs. Underthe proposed monitoring technique, the optical access servicefrom the OLT to each ONU should be isolated. The channelisolation work is in charge of the wavelength-selective couplerwith a 25 dB isolation rate.

In the proposed architecture of WDM-PON, the operationchannel for delivering the data to each ONU has a differentchannel band from the monitoring channel using a differentfree spectral range (FSR) of the AWG. Therefore, the crosstalkbetween the service channel and the monitoring channel canbe minimized as shown in table 1.

The BER measurement for WDM/PS-PON is achievedwith a pulse pattern generator (PPG) and an error rater (ERT)as shown in figure 7. In the case of the PS-PON, the case withthe OTDR has a power penalty of less than 0.2 dB at 10−11

compared with the case without the OTDR in figure 7(a).In the case of the WDM-PON, the case with the OTDR hasa 0.25 dB power penalty at 10−10 compared with the casewithout the OTDR in figure 7(b). The insets of figure 7 showthe eye diagrams for 10 Gbit s−1 at each ONU with the OTDRthrough both the PS-PON and the WDM-PON without usingany optical amplifier.

In this hybrid PON, the power penalty by the spontaneousRaman scattering appeared to be less than 0.2 dB at a BERof 10−10 compared to that without the OTDR. This can beneglected because both the data signal and monitoring powersare less than 10 dBm and this architecture can be well operatedwithin the length of 20 km [6].

4. Conclusion

A monitoring technique on the hybrid PS/WDM-PON hasbeen presented. To monitor PS-PON and WDN-PONseparately and simultaneously, each fibre branch was identifiedand scanned in terms of wavelength by using a wavelength-tunable OTDR and FBG. This method can also be appliedto the overlay structure of WDM-PON and PS-PON. Even inmonitoring PS-WDM, by measuring only backscattered powerreturning from the branch corresponding to the wavelengthto be scanned, it can give us a more accurate measurementvalue through the OTDR trace. Under the in-service state of10 Gbit s−1 data, the experimental results on diagnosis showedonly a 0.25 dB power penalty. This method is also expectedto be useful for an easy expansion of already installed opticalaccess networks.

Acknowledgment

This work was partially supported by Brain Korea 21 Program.

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[2] Nakazawa M, Tokuda M, Washio K and Morishige Y 1981Marked extension of diagnosis length in optical time domainreflectometry using 1.32 µm YAG laser Electron. Lett. 17783–4

[3] Cox J D, Boggis J M, Bryant E G, Hunter C A andStallard W A 1990 First field demonstration of in-servicefault location/supervisory using optical time domainreflectometry Electron. Lett. 26 110–2

[4] Laferriere J, Daget M and Champavere A 1997 Original methodfor analyzing multipaths networks by OTDR measurementProc. OFC’97 (Dallas, TX) TuT4

[5] Araki N, Izumita H, Honda N and Nakamura M 2003 Extendedoptical fibre line testing system using new eight-channelL/U-band crossed optical waveguide coupler for L-bandWDM transmission J. Lightwave Technol. 21 3316–22

[6] Scheerer C 1996 OTDR pulse power limit in on-line monitoringof optical fibres owing to stimulated Raman scatteringElectron. Lett. 32 678–9

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