Crosstalk in WDM Systems Paul G. Eitner ECEE March 2003

PGE - ECEE6412 Wavelength 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  Switches

PGE - ECEE6413 WDM wavelengths recommended by Int’l Telecommunications Union  G.694.1DWDMat 12.5, 25, 50, or 100 GHz spacing around THz (100 GHz spacing shown)  G.694.2CWDMat 20 nm spacing from 1270 to 1610 nm

PGE - ECEE6414 WDM 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 - ECEE6415 Examples 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 length

PGE - ECEE6416 Crosstalk in WDM Switch R1, G1, B1 R2, G2, B2 R3, G3, B3 R1, 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 = G2

PGE - ECEE6417 Four-Wave Mixing Four-wave mixing: spurious signal at a nearby frequency, generated in response to refractive index nonlinearity  FWM =  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 fiber

PGE - ECEE 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 distances FWM Mitigation with NZDSF ( Reference 3) FWM component

PGE - ECEE6419 Summary 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 alone

PGE - ECEE64110 References 1.G. P. Agrawal, Fiber-Optic Communication Systems, 2 nd Edition, Wiley-Interscience, ITU-T Recommendations G and G.694.2, June M. Yadlowsky, E. DeLiso, and V. da Silva, “Optical Fibers and Amplifiers for WDM Systems”, Proc. IEEE 85(11), M. J. O’Mahony, “Optical Multiplexing in Fiber Networks: Progress in WDM and OTDM”, IEEE Communications Magazine, December 1995.

PGE - ECEE64111 Questions 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 switch