Polarization mode dispersion

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As polarization mode dispersion (PMD) of the effect is known, in which light of different polarization at different speeds in an optical fiber propagating. As the size of the polarization mode dispersion, the resulting delay difference (differential group delay) is usually specified in picoseconds. Over long distances it is proportional to the PMD coefficient of the fiber optic cable and to the square root of the length of the cable. For long fiber optic cables, polarization mode dispersion is one of the factors that limit the maximum bandwidth of data transmission, but its influence is typically significantly less than that of chromatic dispersion .

root cause

In an ideal optical waveguide , the fundamental mode has two orthogonal polarizations with identical propagation constants (degenerate modes ). External influences or manufacturing errors always cause interference in a real glass fiber, which means that the two propagation constants change and the glass fiber becomes birefringent . Since the disturbances are randomly distributed over the length of the glass fiber, the modes couple over to one another, which corresponds to a " random walk " process. Since a single light pulse in a conventional glass fiber always has optical components in both polarization planes, and the differently polarized components arrive at the receiver with a time delay due to the PMD, the light pulse widens and, in unfavorable cases, can even overlap with neighboring light pulses. This particularly affects the transmission at a high data transmission rate.

The maximum running time difference, and differential group delay called ( English differential group delay , DGD) concerns about long distances in strong mode coupling:

,

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

Compared to the dispersion of the group velocity ( chromatic dispersion ), the influence of the polarization mode dispersion on the transmission quality is smaller. However, because it has not yet been able to be effectively compensated, its disruptive influence is now in the foreground.

In the case of transmissions of 10 Gbit / s, the influence of PMD was negligible and the chromatic dispersion was manageable through compensation with NZDS fibers (English non-zero dispersion shifted ). This is no longer the case with the next generation of transmissions at 40 Gbit / s and the fibers to be used must be examined for PMD and verified accordingly before the systems are put into operation. Fiber links with compensated chromatic dispersion and a remaining PMD greater than 60 ps cannot be used for 40 Gbit systems. At the same time, new modulation methods and forward-correcting codes are being worked on, which can compensate for an inadequate transmission by up to approx. 6  dB .

Furthermore, the PMD depends on the laying of the glass fiber and changes its value when tension, pressure or torsion is exerted on the glass fiber. Temperature fluctuations also have an influence. Since fiber optics are often laid on high-voltage lines, the PMD means that when planning high-bit-rate transmission routes using fiber optics - if z. B. the glass fibers are to be laid in earth cables of high-voltage lines - do not assume the theoretically possible transmission capacity, but reserves must be taken into account to be on the safe side.

literature

  • Fedor Mitschke: 4.6 - polarization mode dispersion . In: Glass fibers: physics and technology . Elsevier, Spektrum, Akad. Verlag, Heidelberg 2005, ISBN 3-8274-1629-9 , pp. 68-72 .
  • Andrea Galtarossa, Curtis R. Menyuk: Polarization Mode Dispersion (Optical and Fiber Communications Reports) . Springer, New York 2010, ISBN 1-4419-2004-8 .
  • Martin Werner: communications engineering. An introduction to all courses, 7th edition, Vieweg + Teubner Verlag, Wiesbaden 2010, ISBN 978-3-8348-0905-6 .
  • Dirk Jansen: Optoelectronics . Basics - components - transmission technology - networks and bus systems, Friedrich Vieweg & Sohn Verlagsgesellschaft mbH, Wiesbaden 1993, ISBN 978-3-528-04714-6 .

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