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'''Polarization mode dispersion''' ('''PMD''') is a form of [[modal dispersion]] where two different [[Polarization (waves)|polarizations]] of light in a waveguide, which normally travel at the same speed, travel at different speeds due to random imperfections and asymmetries, causing random spreading of [[optical pulses]]. Unless it is compensated, which is difficult, this ultimately limits the rate at which data can be transmitted over a fiber.
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==Overview==
 
In an ideal optical fiber, the core has a perfectly circular cross-section. In this case, the fundamental mode has two orthogonal polarizations  (orientations of the [[electric field]]) that travel at the same [[group velocity|speed]]. The signal that is transmitted over the fiber is randomly polarized, i.e. a random superposition of these two polarizations, but that would not matter in an ideal fiber because the two polarizations would propagate identically (are [[degeneracy (mathematics)|degenerate]]).
 
In a realistic fiber, however, there are random imperfections that break the circular symmetry, causing the two polarizations to propagate with different speeds. In this case, the two polarization components of a signal will slowly separate, e.g. causing pulses to spread and overlap. Because the imperfections are random, the pulse spreading effects correspond to a [[random walk]], and thus have a mean polarization-dependent time-differential &Delta;''&tau;'' (also called the [[differential group delay]], or DGD) proportional to the square root of propagation distance&nbsp;''L'':
 
:<math>\Delta\tau = D_\text{PMD} \sqrt{L} \, </math>
 
''D''<sub>PMD</sub> is the ''PMD parameter'' of the fiber, typically measured in [[picosecond|ps]]/√km, a measure of the strength and frequency of the imperfections.
 
The symmetry-breaking random imperfections fall into several categories. First, there is geometric asymmetry, e.g. slightly elliptical cores.  Second, there are stress-induced material [[birefringence]]s, in which the refractive index itself depends on the polarization. Both of these effects can stem from either imperfection in manufacturing (which is never perfect or stress-free) or from thermal and mechanical stresses imposed on the fiber in the field &mdash; moreover, the latter stresses generally vary over time.
 
==Compensating for PMD==
 
A '''PMD compensation''' system is a device which uses a [[polarization controller]] to compensate for PMD in [[fiber]]s. Essentially, one splits the output of the fiber into two principal polarizations (usually those with ''d&tau;''&nbsp;''d&omega;''&nbsp;=&nbsp;0, i.e. no first-order variation of time-delay with [[frequency]]), and applies a differential delay to bring them back into synch.  Because the PMD effects are random and time-dependent, this requires an active device that responds to [[feedback]] over time.  Such systems are therefore expensive and complex; combined with the fact that PMD is not yet the limiting factor in the lower data rates still in common use, this means that PMD-compensation systems have seen limited deployment in largescale telecommunications systems.
 
Another alternative would be to use a polarization maintaining fiber ([[PM fiber]]), a fiber whose symmetry is so strongly broken (e.g. a highly elliptical core) that an input polarization along a principal axis is maintained all the way to the output. Since the second polarization is never excited, PMD does not occur. Such fibers currently have practical problems, however, such as higher losses than ordinary optical fiber and higher cost.   An extension of this idea is a [[single-polarization fiber]] in which only a single polarization state is allowed to propagate along the fiber (the other polarization is not guided and escapes).
 
==Related phenomena==
 
A related effect is '''polarization-dependent loss''' (PDL), in which two polarizations suffer different rates of loss in the fiber due, again, to asymmetries. PDL similarly degrades signal quality.
 
Strictly speaking, a circular core is not required in order to have two degenerate polarization states. Rather, one requires a core whose symmetry group admits a two-dimensional irreducible [[group representation|representation]].  For example, a square or equilateral-triangle core would also have two equal-speed polarization solutions for the fundamental mode; such general shapes also arise in [[photonic-crystal fiber]]s.  Again, any random imperfections that break the symmetry would lead to PMD in such a waveguide.
 
==References==
 
* Rajiv Ramaswami and Kumar N. Sivarajan, ''Optical Networks: A Practical Perspective'' (Harcourt: San Diego, 1998).
* Jay N. Damask, ''Polarization Optics in Telecommunications'' (Springer: New York, 2004)
 
== See also ==
* [[Optical polarization multiplexing]]
 
[[Category:Polarization (waves)]]

Revision as of 03:36, 3 March 2014

Msvcr71.dll is an significant file that assists support Windows procedure different components of the program including significant files. Specifically, the file is chosen to aid run corresponding files in the "Virtual C Runtime Library". These files are important in accessing any settings which help the different applications and programs in the system. The msvcr71.dll file fulfills various significant functions; yet it's not spared from getting damaged or corrupted. Once the file gets corrupted or damaged, the computer may have a hard time processing plus reading components of the program. However, users need not panic considering this problem might be solved by following several procedures. And I might show you several tricks regarding Msvcr71.dll.

Another answer is to provide the computer program with a new msvcr71 file. Often, once the file has been corrupted or damaged, it can no longer be capable to function like it did before thus it's just all-natural to replace the file. Just download another msvcr71.dll file within the web. Often, the file may come in a zip structure. Extract the files from the zip folder plus spot them accordingly inside this location: C:\Windows\System32. Afterward, re-register the file. Click Start and then choose Run. When the Run window appears, type "cmd". Press Enter plus then sort "regsvr32 -u msvcr71.dll" followed by "regsvr32 msvcr71.dll". Press Enter again plus the file must be registered accordingly.

Perfect Optimizer also offers to remove junk files and is fully Windows Vista compatible. Many registry product simply don't have the time and cash to research Windows Vista mistakes. Because perfect optimizer has a big customer base, they do have the time, cash and factors to support fully help Windows Vista.

There are tricks to make your slow computer work effective plus fast. In this particular article, I will tell you just 3 best tricks or techniques to avoid a computer of being slow plus instead of which make it quicker and function even much better than before.

Google Chrome crashes on Windows 7 when the registry entries are improperly modified. Missing registry keys or registry keys with improper values could cause runtime errors plus thereby the issue occurs. You are recommended to scan the entire program registry and review the result. Attempt the registry repair process utilizing third-party tuneup utilities software.

Software errors or hardware mistakes that happen when running Windows and intermittent mistakes are the general reasons for a blue screen bodily memory dump. New software or motorists that have been installed or changes inside the registry settings are the typical s/w causes. Intermittent errors refer to failed system memory/ hard disk or over heated processor plus these too may cause the blue screen bodily memory dump error.

The 'registry' is simply the central database which stores all a settings and choices. It's a really significant piece of the XP system, that means which Windows is consistently adding plus updating the files inside it. The problems occur whenever Windows actually corrupts & loses a few of these files. This makes your computer run slow, as it attempts difficult to obtain them again.

There is a lot a wise registry cleaner will do for your computer. It can check for and download updates for Windows, Java and Adobe. Keeping updates current is an important piece of good computer health. It will furthermore protect a individual plus company confidentiality in addition to a online security.

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