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Estimation of the SRS crosstalk on pilot-tones in WDMsystems using a dither .transfer matrix

Saeid Seydnejad, James Harley, Frederic SimardPhotonic Systems DesignNovtel Networks1341 Baseline Road, O ttawa ON K2C OA7 Canadae-ma il: saeidsey @nortelnetworks.com

. AbstractThe effect of Stimulated Raman Scattering (SRS) on pilot-tones .and optical performance monitoring ina WDM system is considered by small signal analysis of the SRS equation and introducing a newconcept- a Dither Transfer Matrix (DTM). ,

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8/8/2019 Estimation of the SRS Crosstalk on Pilot-Tones in WDM Systems Using a Dither Transfer Matrix

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Estimation of the SRS Crosstalk on Pilot-Tonesin WDMSystems Using Dither Transfer Matrix

Saeid Seydnejad, James Harley, Frederic SimardPhoto nic System s Desig n, Nortel Networks1341 Baseline Road, Ottawa ON K2C OA7 Canadae-mail: saeidse y @nortelnetworks.com

I. IntroductionAdding small signal sinusoidal dithers (pilot-tones) to optical carriers provides a simple tool for

optical performance monitoring and channel identification in WDM systems. Presence of a specificdither at a particular point in the network indicates the presence of its corresponding wavelength andpower level. This is true when each dither travels solely with its optical carrier. However, theStimulated Raman Scattering (SRS) causes the transfer of the dither from one wavelength to the othersand consequently an inaccuracy in the power level estimation [1][2]. Furthermore, the dither can betransferred back and forth between different channels in a multi-span system in a phenomenon wewould like to call higher-order effects. To consider the effect of SRS on pilot-tones and opticalperformance monitoring, we introduce a new concept called a Dither Transfer Matrix (DTM). TheDTM provides an effective tool for compensating for the performance degradation arising fromSRS onpilot-tone optical monitoring.11. Dither Transfer Matrix

The evolution of SRS power exchange between channels is governed by the following set ofordinary differential equations:

In (1) p , (z) is the power of the nth channel as a function of propagation distance z, a is the fiberattenuation coefficient, g = dg / df represents the slope of the Raman gain profile, Af is the inter-channel frequency spacing, and A is the effective cross-sectional area of the fiber. It has been shownthat the nonlinear system of equations in (1) exhibits the following general solution [11,

1- -mwhere, G =-IAf , J o = pm o Z, =- p n o in the above equation denotes thenth channel2 A m= l ainput power. For pilot-tone analysis, a s-mall signal variation in the optical power due to thesuperimposed dither is considered.If, for the kth channel, we definethen,

8/8/2019 Estimation of the SRS Crosstalk on Pilot-Tones in WDM Systems Using a Dither Transfer Matrix

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Assuming Ap, , ( t ) = a , sin(w,t+e,)yields,N

&k (z, t ) = &a, sin(wit+si ( 5 )1 4

in which a , denotes the amplitude of the superimposed dither. Equation ( 5 ) reveals that the ditherpower for channel k is the summation of all other dithers with the scaling factor given by &ai . It caneasily be shown that:

i + k (a)fk:.= ( (6)

N G ( m - l ) Z , J ,in which, U = , , I = exp(G(k- ) Z ,J , ) , v = P m O e 9m= lN+c m o G ( m ) Z e e C ( m - l ) Z e J aEquation (5) can be written as:G (I - l )Z , Jov = e

m= l

The N x N matrix shows the dither interaction between different channels in the system. Each f k jdemonstrates the amount of SRS dither power transferred from channel i to channel k.,hence the nameDither Transfer Matrix (DTM).If the fiber loss ( e -"")s disregarded, DTM will be an identity matrix in the absence of SRS. In general

f k , = 1 is due to the conservation of power for every dither. In other words, the total amount ofpower over all channels at every point of the system is a constant, and equal to the original dither. DTM istherefore a positive definite matrix. When SRS is not severe, f k , > 0 for i < k and f k , C 0 for i > k .When SRS becomes severe, higher order effects appear and f k i

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111.Experimental ResultsThe most important feature of DTM is its ability to evaluate the amount of dither transfer from onewavelength to other wavelengths. The dither transfer function is clearly different from the SRS averagepower transfer from shorter to longer wavelengths. In order to correct the power readings given by pilot-tones, DTM should be used in conjunction with the SRS average optical power transfer (eq. (2)).Comparing the dither power provided by DTM and the average power given by equation (2) enables us toestimate the SRS error and compensate for the inaccuracy in the power readings of the pilot-tones. Figure(1)displays the error estimated by this procedure in a 4-span 40-channel system with 80km LEAF fiberper span. The experimental data is represented by an asterisk and the model prediction is displayed as asolid line. For this experiment, the system was first equalized with OSNR 23-24.5. A combination ofdual and booster amplifiers with design flat gain was used in every span. Booster amplifiers were set topeak power control mode with a 3.5dBm peak target. The frequency'of the dithers was adjusted in therange of 300-4OOkHz with 2% modulation depth. If the power readings of the pilot-tones are corrected bythe values predicted by the DTM, the inaccuracy in power readings due to SRS can be well compensated.

40-Channel E L E A F with 3 .5dBm Launch Power

Fig. 1.

IV. Summary

I I I I I II 1 I I I . II I I I I II I 8 , I ,

-31530 1535 1540 1545 1550 1555 1560WavelengthExperimental data ('*') shows the difference between the averagepower recordedby anOSAand the power measured by pilot-tones. The solid line displays thisdifference by comparing the DTM and the SRS average power transfer.

Although the stimulated Raman scattering causesan energy transfer from shorter to longer wavelengthswhen power measurement by pilot-tones is aimed, we must consider how the superimposed dithers areexchanged between channels. DTM, as introduced here, not only provides an effective tool for thisconsideration, but also gives insight into other situations that might arise in the dither domain.However, in its current form, DTM does not include the effect of dispersion and walk-off, but can bemodified if required.References[1]-Z. Wang, et. al., IEEE Photonics Technology Letter, Vol. 7 ,No. 12, pp. 1492-1494, 1995.[2]- H.S. Chung, et. al.Proc. OFC 2000, WK7,2000.[3]- D.N. Christodulides, R.B. Jander, IEEE Photonics Technology Letter, Vol. 8 , No. 12, pp. 1722-1724,1996.