3. What is second-order PMD? Is SOPMD important?
Second-order PMD (SOPMD) is the change of the DGD (first-order PMD) with changes in wavelength. Since PMD is a vector quantity, SOPMD has two components. The first and dominant component of SOPMD is known as depolarization. Depolarization is the change in the direction of the PMD vector with a change in wavelength. The second component of SOPMD is the polarization dependent chromatic dispersion (PDCD). PDCD measures the change in the DGD with a change in wavelength.
t is the PMD vector at optical frequency w. tw is the SOPMD vector composed of the parallel PDCD component, tw||, and the orthogonal Depolarization component, tw^
SOPMD can not be ignored when considering the impact of PMD on system availability.
Statistically, the SOPMD level positively correlates with the DGD level. The magnitude of the depolarization component of SOPMD increases with increasing DGD in a linear manner. As DGD gets worse, SOPMD also gets worse. Moreover SOPMD depends on the modulation bandwidth of the signal. As bit rates increase the SOPMD also increases.
Therefore, when measuring an optical system for PMD tolerance, SOPMD cannot be neglected. Measuring only the DGD tolerance of a system is an oversimplification of the PMD impairment and can underestimate the PMD outage of a network. NRT’s products integrate the testing of SOPMD and DGD are integrated into all of NRT’s products allowing for the complete determination of system tolerance.