Polarization mode dispersion

A polarization mode dispersion (PMD ) of the effect is known, in which light with different polarization to propagate at different rates in an optical waveguide. As the size of the polarization mode dispersion ( differential group delay ) is expressed in picoseconds usually the resulting skew. On long distances it is proportional to the PMD coefficient of the optical fiber and the root of the length of the cable. For long fiber optic cable, the polarization mode dispersion is one of the factors that limit the maximum bandwidth of the data transfer, but their influence is typically significantly lower than that of the chromatic dispersion.

Cause

In an ideal optical fiber the fundamental mode has two orthogonal polarizations with identical propagation constants ( degenerate modes). External influences or production errors arise in a real fiber but always disturbances, thereby changing the two propagation constants, and the glass fiber is birefringent. Since the disturbances on the length of the fiber are randomly distributed, the modes couple into each other, what a " random walk " corresponds to random process. Because a single pulse of light in a conventional optical fiber having more optical portions in two planes of polarization, and the differently polarized components arrive due to PMD with a time offset at the receiver widens the light pulse and may overlap in the worst case even adjacent light pulses. This particularly affected the transmission with a high data transfer rate.

The maximum delay time difference, also called differential group delay (english differential group delay, DGD ), is found to be on long journeys with strong mode coupling:

Denotes the PMD coefficient in ps / √ km and the length of the fiber.

In comparison to the group velocity dispersion ( chromatic dispersion ) is the influence of the polarization mode dispersion is less than the transmission quality. Because it but to this day can not be effectively compensated, their disturbing influence is now in the foreground.

For a transmission of 10 Gbit / s, the influence of PMD was negligible and the chromatic dispersion compensation by using NZDS fibers manageable (English non-zero dispersion -shifted ). For transmissions of the next generation at 40 Gbit / s, this is no longer the case and the fibers to be used must be inspected prior to commissioning of systems to PMD and verified accordingly. Fiber links with compensated chromatic dispersion and a PMD of greater than 60 ps remaining can not be used for 40 Gb systems. Likewise, working on new modulation method and forward - correcting codes, which can compensate for a lack of expectant transmission by up to about 6 dB.

In addition, the PMD depends on the deployment of optical fiber and changes its value when train, pressure or torsion is applied to the glass fiber. Temperature fluctuations have an impact. Because like glass fibers are laid on trays of high-voltage lines, may be because of PMD in the planning of high bit-rate transmission links using optical fibers - eg if the fibers are to be laid in earth wires of high voltage lines - not started from the theoretical transmission capacity, but it must for safety's sake reserves are taken into account.