Cross-phase modulation adds to PMD challenges
Nonlinear effects of polarization-mode dispersion (PMD) occur faster than standard compensators can track, report researchers from Lucent Technologies Bell Labs (Holmdel, NJ) at the IEEE LEOS Annual Meeting in Rio Grande, Puerto Rico (Nov. 13-16, 2000). For DWDM systems based on 10- and 40-Gbit/s channels, PMD is a serious problem. Although compensation techniques are available for linear effects of PMD, the nonlinear effects that occur at higher powers-specifically cross-phase- modulation-induced polarization scattering between channels-can occur on the timescale of a bit length. This is faster than conventional devices can compensate for. Because PMD compensators adapt to a specific polarization state, they cannot work correctly when the polarization changes rapidly.
Cross-phase-modulation-induced polarization scattering causes the polarization state of the channel to change rapidly. Researchers Lother Möller and colleagues demonstrated that the power fluctuations can result depending on the symbols trans- mitted in the WDM channels.
To demonstrate the effects, the researchers used an experimental setup in which signals propagated twice through the transmission line with orthogonal states of polarization, thereby correcting the differential group delay between the two states in the linear propagation regime of the system. Eight wavelengths, spaced by 100 GHz, were each encoded with 5- or 10-Gbit/s nonreturn-to-zero (NRZ) data. The channels were combined in a 17-km long single-mode fiber. The channel under test was extracted using a tunable narrow-bandpass filter and sent to a detector with a clock recovery circuit.
As the power launched into each channel is increased, the detrimental nonlinear effects increase as well (see figure). The researchers found that holding the launched power steady at 6.5 dBm and keeping the same states of polarization but changing the PMD dispersion significantly improves the quality of the detected signal.
Methods to reduce the nonlinear effects include reducing the overall degree of polarization of the WDM signal. Information-channel coding (for example, forward-error correction) can reduce the bit-error rate caused by nonlinear polarization-mode dispersion. For more information, contact Lother Moeller at lmoeller @lucent.com.
Yvonne Carts-Powell is a freelance science writer based in Belmont, MA.
Eye diagrams are detected with a 30-GHz receiver (50 ps/division). The input signal (a) degenerates when transmitted and demultiplexed with low enough power to suffer only linear effects (b), but more when transmitted with high enough powers to suffer nonlinear effects (c). When the PMD is removed (d), the signal improves. The importance of cross-channel effects is shown by (e)-when only one channel is transmitted, the signal is not degraded.