1. Crosstalk in WDM Systems Paul G. Eitner ECEE-641 6 March 2003

2. 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

3. PGE - ECEE6413 WDM wavelengths recommended by Int’l Telecommunications Union  G.694.1DWDMat 12.5, 25, 50, or 100 GHz spacing around 193.1 THz (100 GHz spacing shown)  G.694.2CWDMat 20 nm spacing from 1270 to 1610 nm

4. 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)

5. 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

6. 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

7. PGE - ECEE6417 Four-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 fiber

8. PGE - ECEE6418 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 distances FWM Mitigation with NZDSF ( Reference 3) FWM component

9. 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

10. PGE - ECEE64110 References 1.G. P. Agrawal, Fiber-Optic Communication Systems, 2 nd 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.

11. 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