1 September 2000

Ž .Optics Communications 183 2000 65–71www.elsevier.comrlocateroptcom

Transparent supervision of optically amplified fibre links withreceived signal and ASE monitoring

Josep Prat), Sergio Ruiz-Moreno( ) ( )Signal Theory and Communications Department TSC UniÕersitat Politecnica de Catalunya UPC , Jordi Girona ETSETB-D5,`

Barcelona 08034, Spain

Received 7 December 1999; received in revised form 17 April 2000; accepted 5 July 2000

Abstract

This paper illustrates that remote fault localisation in all-optical links with concatenated fibre amplifiers can be performedby simple monitoring of the signal and the accumulated ASE noise powers at the receiver end. The method is first presentedin ideal operating conditions, and second, the effect of the spontaneous emission factor variation and the EDFA non-linearbehaviour are taken into account. From the analysis, the requirements of the link design for proper supervision accuracy arederived. The presented supervisory method is a simple solution for links where a fibre break or an amplifier fault is to bedetected and located with the granularity of an amplifier section without local supervisory elements within the transmissionsections. q 2000 Published by Elsevier Science B.V.

PACS: 42.81.-i; 42.79.sz; 42.87.-d.Keywords: Optical fibre; EDFA; Supervision; Optical transmission

1. Introduction

The excellent performances of the Erbium-DopedŽ .Fibre Amplifier EDFA for optical communications

in terms of gain, noise, output power, polarisationsensitivity, bandwidth, transparency and cost havecaused its very wide deployment in optical networks.However, its basic advantages of transparency andsimplicity are difficult to maintain when a supervi-sion technique is installed over an amplifier cascade.Management and operation of optical transmissionsystems involve performancerfault monitoring andthe localisation of defective sections in an optical

) Corresponding author. Tel.: q34-93-4016455; fax: q34-93-4017200; e-mail: jprat@tsc.upc.es

route. In a cost-effective solution, the managementsupervision system should have a much lower costthan the transmission system itself. The problemarises from the fact that in general, it is easy tolocalise a fault with a granularity of an electro-opticregenerator section, but not with the granularity of an

w xoptical amplifierrfibre span section 1 .A technique that uses a dedicated wavelength as a

supervisory channel for optical monitoring at eachamplifier is being established in the new Wavelength

Ž .Division Multiplexing WDM optical transport net-w xworks 2 . Other proposed methods in the literature

use modulated sub-carriers, another fibre, modulatethe polarisation or add fiber Bragg gratings at each

w xamplifier section 3,4 . Reflectometric methods be-come difficult because of the long spans and the

0030-4018r00r$ - see front matter q 2000 Published by Elsevier Science B.V.Ž .PII: S0030-4018 00 00875-0

( )J. Prat, S. Ruiz-MorenorOptics Communications 183 2000 65–7166

presence of optical isolators in the amplifiers mod-w xules 5 .

w xThe technique that we have proposed 6 is sim-pler as no supervision elements are needed withinthe optical route: gain variations are detected, andthe faulty sections are located by means of analysingthe signal and the amplified spontaneous emissionŽ .ASE power levels at the receiver terminal. Themethod is based on the fact that a gainrloss varia-tion in an all-optical section affects in a differentway the signal and the accumulated ASE along thelink. If a section gain decreases, the received ASEpower from the prior amplifiers decreases, but notthe ASE of the following ones. Since the monitoringtechnique is remote and does not need an EDFAsupervisory specific channel or wavelength, the opti-cal path transparency is therefore guaranteed. Theproposed system locates faults in a fibre span – dueto a fibre break, a connector loss increase, etc. – orin an optical amplifier, most probably due to a pumplaser power reduction or failure.

In this paper, the design requirements of thetransmission system are investigated. We first pre-sent the method in its simplest form, in pursuit ofcomprehension. Next, the effect of the spontaneousemission factor is shown, and the non-linear regimeof the all-optical link is modelled and analysed.Finally, the design lines are derived from the results.

2. Basic method

Fig. 1 shows the in-line amplifier systemschematic, with N sections that are composed of anEDFA plus a fibre span. The signal and ASE powerlevels are monitored in a supervisory system at thereceiver end. They can be obtained by directly mea-suring the photo-detected currents in the ones and

Ž .zeros extinction ratio in a digital system, or fromthe signal amplitude and the mean optical power

Žlevel using an RF rectifier plus a DC photo-detector.current sensor , or by filtering a portion of the ASE

spectrum out of the signal band.In order to illustrate the method and to obtain

analytical expressions, some hypotheses are first as-sumed: identical N sections, each optical amplifier

Žgain compensates the following fibre span loss i.e.,.Gs1rL , ideal transmitter extinction ratio and lin-

Ž .ear regime optical amplifiers below saturation ; alinear EDFA behaviour can be physically achievedby reducing the doped fibre length as later will beshown. The optical power detected in the receiver is

PsP qP sP qP N , 1Ž .i ase i ase K

where P is the link input power, P is the accumu-i ase

lated ASE along the link, P is the generated ASEase K

in each individual amplifier. P is proportional toase KŽ . Ž .the spontaneous emission factor n and to Gy1sp

w x7 .We now consider the occurrence of a failure in

Ž .the link. The faulty section the ith section wouldsuffer a net gain reduction, of magnitude gsG L /i i

1. This is readily measured as the ratio between thedetected signal levels after and before the fault oc-currence respectively, then revealing the existence ofa fault when surpassing a defined threshold. Thereceived power when the fault occurs is correspond-ingly decreased, and can be expressed as

P sg P qPF i aseF

isg P qP 1y 1yg qx g , 2Ž . Ž . Ž .i ase N

Ž .where x g is an error term that is null if the fault isin the fibre span, or otherwise depends on the nsp

variation with g of the faulty amplifier. This will be

Fig. 1. Link schematic.

( )J. Prat, S. Ruiz-MorenorOptics Communications 183 2000 65–71 67

ŽFig. 2. Received ASE power normalised to its value before the.fault as a function of the faulty section and the gain variation

Ž .magnitude with Ns10 sections .

later considered, showing that it has no effect in thefault localisation, hence it is not now considered forsimplicity. A failure in a fibre span is then notdistinguished from the failure in the optical amplifierof the same section.

The result is plotted in Fig. 2, that shows thereceived ASE power after the fault occurrence as afunction of the faulty section and the gain variation

Ž .magnitude g . The ASE power decreases linearly asthe faulty section increases, due to the reduced accu-mulation of the EDFA ASE preceding the fault. Theslope increases with the gain reduction g , meaningthat severe faults are better identified.

This result illustrates the supervision method ba-sis. The necessary information to localise the faultalong the link is deduced from the comparison of the

ŽASE power after and before the fault occurrence 1,.2 . The faulty section number is straightforwardly

obtained, with the following expression:

1yP rPaseF aseisRound N . 3Ž .ž /1yg

Ž .The Round function nearest integer number es-tablishes the location threshold between adjacent sec-tions, that is needed because of the presence of noiseand other non-ideal effects that are later analysed.

The precision of the method is related to the differ-ence between the expected ASE corresponding totwo adjacent faulty sections; this is defined as thelocation window

DP 'P i yP iq1 sP 1yg .Ž . Ž . Ž .ase aseF aseF ase K

4Ž .

3. Spontaneous emission factor variation effect

As mentioned above, if the fault occurs in theŽ .optical amplifier there is an error term x g in the

Ž .expression 2 . When the gain of an optical amplifierŽ .changes, its spontaneous emission factor n maysp

also change. Their relationship depends on the physi-cal origin of the fault cause. A fault in the opticalamplifier is most likely to result from a reduction inthe pump power due to a laser degradation, a DCsupply breakdown, etc. A fault in a passive element

Ž .of the amplifier mux, isolator, ASE filter is equiva-lent to a variation in the section loss, and then thereis no error term.

In the former case, the variation from the initialŽ .n to the faulty EDFA n g appears in the namedsp spF

Ž .error term x g as

n g g Gy1Ž .spFx g sP yg . 5Ž . Ž .ase K ž /n Gy1sp

This term would produce a location error if itwere higher than the location window D P . Whenase

Table 1EDFA parameters

Index core radius 1.2 mmIndex core step 0.035

3qEr core radius 1.2 mmAmplifier length 18.8 mMetastable lifetime 10 msASE bandwidth 10 nmPump wavelength 1480 nmPump absorption 1.6 dBrmPump gain 0.5 dBrmPump excess loss 0.03 dBrmSignal wavelength 1550 nmSignal absorption 2.6 dBrmSignal gain 3.6 dBrmSignal excess loss 0.03 dBrm

y2 5 2Signal absorption s 3 10 m

( )J. Prat, S. Ruiz-MorenorOptics Communications 183 2000 65–7168

Ž .the pumping power decreases, n g increases butspŽ .g Gy1 decreases, hence the overall effect on theASE power has been investigated.

For this purpose, parameters of common Al:Ge:Silicate Erbium doped fibre amplifier have been usedw x Ž .8 as a typical example see Table 1 , also for modelvalidation purposes. Signal and pump wavelengthsare fixed to 1550 and 1480 nm respectively, andforward pumping direction is used. As was presented

w xin Ref. 6 , the ASE power increases with the pumppower but with a lower rate than the signal gain. g

Ž .ranges from almost 0 in absence of pump power toŽ .1 no variation of pump power . The error term and

the location window D P were calculated and com-ase

pared, revealing that the error term is always muchbelow the decision window, and therefore the loca-

Ž .tion Eq. 3 is exact also for any pump powerdecrease.

4. Link simulation

Once the feasibility of the method has been illus-trated in an ideally linear amplifier system, the realnon-linear behaviour of the EDFA amplifiers is nowconsidered.

When the optical powers become limited by thesaturation of the EDFAs, due to high signal power orASE accumulation, the supervision method faces aproblem, and since no analytical resolution can beobtained for the non-linear link, it is numericallymodelled and analysed. The EDFA is simulated im-plementing the numerical steady-state spectral

w xmethod of Giles and Desurvire 8 , splitting thedoped fibre into a high number of length steps wherethe differential equations are discretised. The sameEDFA with the parameters listed in Table 1 has beenused.

Ž . Ž .Fig. 3. Signal thick lines and ASE thin lines EDFA output powers along the link for different nominal signal powers. The EDFA lengthl is 21 m.ini

( )J. Prat, S. Ruiz-MorenorOptics Communications 183 2000 65–71 69

We illustrate the method with its application to aspecific link design, representing a possible practicalcase. From the initial design, the key parameters willbe varied and optimised, and finally some conclu-sions will be derived.

The initial design comprises a five-section EDFAcascade with fibre spans of 20 dB of loss. The EDFAneat pump power is P s5 mW, presenting an opti-p

mum doped fibre length of l s21 m for maximumini

gain, a 1 dB compression output power of y11dBm.

Fig. 3 illustrates the evolution of the signal andASE EDFA output powers along the link for differ-ent designs, with signal powers from y15 to 5 dBm.In each case, the pump laser powers are adjusted inorder to have the same net gain of 20 dB, such thatthe last amplifier output power equals the link inputpower. We observe that in the lower signal powerdesigns, the different amplifiers present slightly dif-ferent gains, because of the saturation affected by theASE noise accumulation.

Next, the total break of a section is consideredŽ .gs0 . The ASE now accumulates starting from thefollowing amplifier without the compression thatpreviously had in presence of the signal power. Thereceived ASE power as a function of the faultysection for the different power designs is plotted in

Ž .Fig. 4. The power is normalised % to its maximumvalue, that occurs when the failure is in the initial

Ž .fibre span 0th . It is noted that the linearity is

Fig. 4. Received ASE power normalised to the maximum value asa function of the faulty section number for the different powerdesigns. The EDFA length l is 21 m.ini

Fig. 5. Necessary pump power as a function of the nominal signalpower for three different doped fibre lengths and a net gain of 20

Ž . ŽdB, in five-section links solid lines and ten-section links dashed.lines .

mainly lost when the fault occurs in the first sec-tions, due to the gain saturation caused by the ASEaccumulation. In this case, it is not possible to locatethe section fault, particularly in the high powerdesigns, and therefore more linear approaches haveto be implemented.

A procedure to linearise the operation of an EDFAamplifier consists of decreasing the doped fibrelength, and increasing the pump power to maintain

w xthe same gain 9 . By doing it, the populations of theerbium ions are held well inverted along the entiredoped fibre for a wider range of EDFA outputpowers, and consequently the saturation power in-creases.

Specifically, shorter EDFA with 3r4 and 1r2 ofthe initial active length l are now considered. Fig.ini

5 shows the corresponding necessary increase of thepump power as a function of the nominal link signalpower for the different doped fibre lengths. At lowinput signal power, the necessary pump power in-crease is about 0.7 dB and 6 dB for the respectivereduced lengths, revealing a sharp increase when thelength is decreased from a certain value, around 3r4l , as the complete population inversion is ap-ini

proached. Fig. 5 also shows the case of 10 EDFAŽ .section designs dashed lines . We observe that the

increase in pump power when doubling the linklength is not substantial.

( )J. Prat, S. Ruiz-MorenorOptics Communications 183 2000 65–7170

Fig. 6. Received ASE power normalised to the maximum value asa function of the faulty section for the different power designs,with an EDFA length of 3r4 l .ini

The results in terms of detected ASE for theselinearised designs when the total break occurs arepresented in Figs. 6 and 7. As expected, a betterregular behaviour is now observed. The first or thelast stages are the worst located. The design with the

Ž .shortest doped fibre half the initial length and withlowrmid-signal powers approaches the ideal case oflinear characteristic initially presented in Section 2,where the location window D P is maximum andase

similar for all the stages.As mentioned above, the linearity of the curves is

an indication of the validity of the location method.In general, other corrupting effects, like the analysedspontaneous emission factor variation and the system

Fig. 7. Received ASE power normalised to the maximum value asa function of the faulty section for the different power designs,

Ž .with an EDFA length of 1r2 l , in five-section links solid linesiniŽ .and ten-section links dashed lines .

noises, will deviate the measurement from the ex-pected value. Hence, the worst location windowD P for each design is taken as the location crite-ase

rion to be maximised. The different designs arecompared in Fig. 8 in these terms. From the curves,the satisfactory designs and requirements for locationpurposes can be established. We note the consider-able improvement achieved when reducing the length,whilst slightly incrementing the pump power.

Specifically, if we establish as the location crite-rion a validity limit at the 50% of the linear ideal

Ž .D P ASE slope reduction less than one half , wease

can derive that an amplifier length of 3r4 l can beini

feasibly used along with operating signal powers upto 0 dBm in the five- section link; the correspondingnecessary pump power is slightly increased from 5 to7 dBm. A higher EDFA output signal power of 10dBm may be used if the pump power is increased to20 dBm with the 1r2 length design. These parame-ter specifications are commercially available in pre-sent state-of-the-art EDFA technology. For the ten-

Ž .section link dashed lines , we observe that the linkdesign with 3r4 length EDFAs is no longer valid; ithas to be reduced to the 1r2 length for a validsupervision, up to signal powers of 2.5 dBm. For ahigher number of sections, the proposed methodwould be limited by the excessive necessary pump

Fig. 8. Minimum detected ASE difference from two differentŽ .consecutive faulty sections D P as a function of the nominalase

signal power for the three different EDFA lengths, in five-sectionŽ . Ž .links solid lines and ten-section links dashed lines .

( )J. Prat, S. Ruiz-MorenorOptics Communications 183 2000 65–71 71

power, and the links could not be accurately moni-tored with this transparent method.

The location accuracy is related to the gain de-compression suffered by the link when the signalfails, as observed with the noise saturation in Figs. 4,6 and 7. The higher input signal the higher compres-sion occurs. The link compression is calculated by

Žcomparing the actual gain which equals the attenua-.tion , to the gain of the amplifiers with negligible

Ž .input signal y80 dBm and without noise accumu-lation. The results show that the method is validŽ .ASE slope reduction lower than 50% for any com-pression level below 6.5 dB for the five-section link,and 8 dB for the ten-section links. From the analysisof a variety of links with different parameter combi-nations for this type of EDFA, we can summarizethat the allowable compression level bound is about6 dB. This may constitute a practical rule in thedesign of links with transparent supervision. Further-more, this non-linearity figure is generally sufficientfor the self-stabilization of the power evolution alongthe link, regarding possible practical loss deviationsw x9 .

In the analysis we have considered identical opti-cal amplifier and fibre span sections. As an extensionof the work to a general link design with differenti-ated sections, the practical parameters would be in-cluded in the link simulator, so that when a faultoccurs, the simulator would reveal the faulty sectionnumber that matches the measured signal and ASEpowers, and would provide an updated knowledge ofthe link parameters.

5. Discussion

The results obtained in this work illustrate that theproposed technique is constitutes a feasible simplesupervision solution to remotely locate faults inmulti-repeated optical links with erbium-doped fibreamplifiers. Specifically, the requirements that haveto be satisfied for proper fault supervision refer tothe accumulated ASE noise and the nominal signalpower at the initial link operation design, which mustbe below the saturation power of the amplifiers. Ithas been shown that if the conditions are satisfied,the link will be correctly supervised regardless the

Žmagnitude of the failure i.e., variation of attenuation.or EDFA pump power . A precise location of the

fault from signal and ASE measurement in the re-ceiver is consequently achieved.

Essentially, the results do not establish a severelimitation in the practical specifications of the EDFAof the link to be supervised, as long as they can besatisfied with current pump laser technology. Thelinearity requirements that have been established inthis work can also be specially useful for otherapplications, such as WDM systems, to keep thechannel signal power constant regardless of the num-ber of inserted wavelengths, or in burst traffic trans-mission.

Acknowledgements

This work has been supported in part by theEuropean Commission within the ACTS Project AC-209 ‘Mephisto’. The content of this paper is solelythe responsibility of the authors.

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