Crosstalk in WDM Systems

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

2. 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 - ECEE641

3. 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 nm PGE - ECEE641

4. WDM Crosstalk • Crosstalk if not removed results in unwanted signal at detector for channel l1 • 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

5. Examples • Sources in fiber • Four-wave mixing • Cross-phase modulation • Stimulated Raman scattering (constrains power) • Stimulated Brilluoin scattering (constrains l-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 - ECEE641

6. Crosstalk in WDM Switch R1, G1, B1 R2, G2, B2 R3, G3, B3 R1, G2+DG1, 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 lG1 = lG2 PGE - ECEE641

7. Four-Wave Mixing • Four-wave mixing: spurious signal at a nearby frequency, generated in response to refractive index nonlinearity • nFWM = n1 + n2 - n3 • nFWM = 2n1 – n2 (degenerate case) • Reduces power in desired channel and introduces crosstalk at other frequencies • Most efficient when n1iszero-dispersion wavelength in fiber • Mitigation options include • Unequal wavelength spacing • Wider wavelength spacing • Trade off dispersion using non-zero dispersion-shifted fiber PGE - ECEE641

8. FWM component FWM Mitigation with NZDSF(Reference 3) • 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 PGE - ECEE641

9. 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 - ECEE641

10. References • G. P. Agrawal, Fiber-Optic Communication Systems, 2nd Edition, Wiley-Interscience, 1997. • ITU-T Recommendations G.694.1 and G.694.2, June 2002. • M. Yadlowsky, E. DeLiso, and V. da Silva, “Optical Fibers and Amplifiers for WDM Systems”, Proc. IEEE 85(11), 1997. • M. J. O’Mahony, “Optical Multiplexing in Fiber Networks: Progress in WDM and OTDM”, IEEE Communications Magazine, December 1995. PGE - ECEE641

11. 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 PGE - ECEE641