Crosstalk in WDM SystemsWavelength Division MultiplexingPowerPoint PresentationWDM CrosstalkExamplesCrosstalk in WDM SwitchFour-Wave MixingFWM Mitigation with NZDSF (Reference 3)SummaryReferencesQuestionsCrosstalk in WDM SystemsPaul G. EitnerECEE-6416 March 2003PGE - ECEE641 2Wavelength Division Multiplexing•Several wavelengths on single fiberHandles multiples of single wavelength data rateAll wavelengths share single optical amplifier at given point along fiberEnables wavelength routing•WDM network componentsCombinersSplittersFiltersSwitchesPGE - ECEE641 3ITU Wavelengths01500 1520 1540 1560 1580 1600Wavelength (nm)G.694.2 G.694.1• WDM wavelengths recommended by Int’l Telecommunications Union G.694.1 DWDM at 12.5, 25, 50, or 100 GHz spacing around 193.1 THz (100 GHz spacing shown) G.694.2 CWDM at 20 nm spacing from 1270 to 1610 nmPGE - ECEE641 4WDM Crosstalk•Crosstalk if not removed results in unwanted signal at detector for channel Induced by one or more of the other wavelengthsOr, mixing of channels with same wavelength due to leaks in network components (may include multipath)PGE - ECEE641 5Examples•Sources in fiberFour-wave mixingCross-phase modulationStimulated Raman scattering (constrains power)Stimulated Brilluoin scattering (constrains -spacing)•Device effectsImperfect channel separation at splitting nodesImperfect filtering•Fiber effects are non-linear often in response to total power as opposed to power at single wavelengthConstrains power along entire fiber lengthPGE - ECEE641 6Crosstalk in WDM SwitchR1, G1, B1R2, G2, B2R3, G3, B3R1, G2+G1, B3• Imperfect separation of R1 and G1 at input demux means G1 mixes with G2 on output• Can’t be removed by spectral filter if G1 = G2PGE - ECEE641 7Four-Wave Mixing•Four-wave mixing: spurious signal at a nearby frequency, generated in response to refractive index nonlinearityFWM = 1 + 2 - 3FWM = 21 – 2 (degenerate case)•Reduces power in desired channel and introduces crosstalk at other frequencies•Most efficient when 1 is zero-dispersion wavelength in fiber•Mitigation options includeUnequal wavelength spacingWider wavelength spacingTrade off dispersion using non-zero dispersion-shifted fiberPGE - ECEE641 8•8 channels at 10 Gbps transmitted through 360 km of NZDS single-mode fiber•Channels separated by 200 GHz (~1.6 nm)•Fiber dispersion approx –3.5 ps/km-nm is large enough to minimize FWM but small enough to support 10 Gbps over long distancesFWM Mitigation with NZDSF(Reference 3)FWM componentPGE - ECEE641 9Summary•Crosstalk causes fundamental limitations on transmit power and distance-bandwidth product on WDM networksCan be present in a single fiber linkAlso arises due to imperfections in network components•Fiber and optical components can be optimized to minimize effects•FWM mitigated by non-zero dispersion-shifted fiber Fiber for use in DWDM systems may not be optimized by minimizing dispersion and attenuation alonePGE - ECEE641 10References1. G. P. Agrawal, Fiber-Optic Communication Systems, 2nd Edition, Wiley-Interscience, 1997.2. ITU-T Recommendations G.694.1 and G.694.2, June 2002.3. M. Yadlowsky, E. DeLiso, and V. da Silva, “Optical Fibers and Amplifiers for WDM Systems”, Proc. IEEE 85(11), 1997.4. M. J. O’Mahony, “Optical Multiplexing in Fiber Networks: Progress in WDM and OTDM”, IEEE Communications Magazine, December 1995.PGE - ECEE641 11Questions•What wavelength spacing does a DWDM frequency spacing of 50 GHz translate to at 1.55 microns?•Name three methods of reducing four-wave mixing•Name two network components (besides fiber) that can cause (or allow) crosstalk•Name two advantages of WDM•Sketch how crosstalk can occur in a WDM