polarization mode dispersion (original) (raw)
Acronym: PMD
Definition: the polarization dependence of the propagation characteristics of light waves in optical fibers
Categories: 
fiber optics and waveguides, 
lightwave communications
- optical effects
- dispersion
* chromatic dispersion
* group delay dispersion
* intermodal dispersion
* polarization mode dispersion
* waveguide dispersion
* dispersion compensation
* dispersion management
- dispersion
Related: fibersspun fibersfiber-optic linkspolarization-maintaining fibersbirefringence
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DOI: 10.61835/87d Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn
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Contents
What is Polarization Mode Dispersion?
Minimizing the Sensitivity to PMD
What is polarization mode dispersion (PMD)?
What is the difference between PMD and differential group delay (DGD)?
Why is PMD a problem in fiber-optic communications?
How does PMD scale with fiber length?
How can the effects of PMD be reduced?
Summary:
This article provides a detailed explanation of polarization mode dispersion (PMD), a crucial phenomenon in optical fibers that limits performance in high-speed fiber-optic communication systems. It describes how random birefringence leads to a differential group delay (DGD) between different polarization states, causing pulse broadening, inter-symbol interference, and an increased bit error rate.
The text covers the statistical nature of PMD, its scaling with fiber length, and its detrimental impact on both communication links and certain fiber-optic sensors.
Furthermore, it explores various methods for mitigating PMD, including the use of optimized fibers like spun fibers, and techniques for PMD compensation, which are essential for achieving high data rates over long distances.
(This summary was generated with AI based on the article content and has been reviewed by the articleās author.)
What is Polarization Mode Dispersion?
In optical fibers, there is usually some slight difference in the propagation characteristics of light waves with different polarization states. A differential group delay can occur even for fibers which according to the design should have a rotational symmetry and thus exhibit no birefringence. This effect can result from random imperfections or bending of the fibers, or from other kinds of mechanical stress, and is also affected by temperature changes, mainly because those can cause mechanical stress. Mainly due to the influence of bending, the PMD of a fiber cable can be completely different from that of the contained fiber on a spool. By comparing PMD values of different fibers in the coiled state, one can hardly predict how those values were compared to those of the straight fiber; in particular, spun fibers exhibit much lower values in the straight state while not looking much better than unspun fibers on a spool.
Polarization mode dispersion is based on birefringence ā more precisely, the differential group delay (as a measure of PMD) is the derivative of the difference of propagation constants (a measure of the birefringence) with respect to angular optical frequency.
Second-order PMD is the derivative of the differential group delay with respect to angular frequency [6] ā effectively the second-order derivative of the difference of propagation constants.
The terms polarization mode dispersion (PMD) and differential group delay (DGD) are often used interchangeably, but sometimes with slightly different meanings. Some authors call the phenomenon PMD and consider DGD to be its magnitude. Others define PMD as the statistical standard deviation of DGD in some wavelength interval. Note that for optical fibers the DGD can have a substantial and complicated dependence on the optical wavelength and temperature.
Detrimental Effects of PMD
Communication Systems
Polarization mode dispersion can have adverse effects on optical data transmission in fiber-optic links over long distances at very high data rates because portions of the transmitted signals in different polarization modes will arrive at slightly different times. Effectively, this can cause some level of pulse broadening or even pulse splitting. This leads to inter-symbol interference, and thus a degradation of the received signal, leading to an increased bit error rate. In order not to exceed the accepted level of bit error rate, one then has to limit the transmission rate.
Effects of polarization mode dispersion often need to be described statistically because they depend in a complicated way on a substantial number of factors, some of which are hard or impossible to predict. For example, temperature changes can lead to mechanical stress in the fibers, with a substantial dependence on additional factors like the type of buffer layer used in the cable, and those affect PMD. For short fiber sections, where the local birefringence stays approximately constant, the DGD is proportional to the fiber length. For longer sections, however, different portions of fiber contribute uncorrelated amounts to the DGD, and the total r.m.s. value of the differential group delay scales only with the square root of the fiber length. Therefore, PMD is often quantified with units of ps kmā1/2.
Fiber-optic Sensors
There are certain polarimetric fiber-optic sensors where tiny polarization changes of light in fibers need to be detected. For example, there are sensors for electric currents which are based on the Faraday effect, i.e., on the rotation of the polarization direction in proportion to a magnetic field which is generated by an electric current. Obviously, additional polarization changes due to random birefringence of the sensor fiber should be suppressed as much as possible.
Reducing the Effects of PMD
Optimized Fibers
The first measure for reducing PMD is to choose an optical fiber with reduced PMD and ideally also a reduced sensitivity of PMD to external factors. Modern telecom fibers have fairly stringent PMD specifications, but fibers laid in the early 1990s often exhibit much stronger PMD, which is often not even specified. Note also that details of the deployment of such cables have some influence. Furthermore, aging effects can substantially deteriorate polarization properties of fiber cables, e.g. related to changed elasticity of aged polymer materials [10].
In principle, the problem could be solved by using well-defined polarization states in polarization-maintaining fibers, but this approach is usually not practical for various reasons: it would not only be necessary to use the more expensive and more lossy polarization-maintaining fibers for all components (including e.g. fiber amplifiers), but also the polarization directions would have to be aligned at many interfaces.
Another theoretical possibility would be to determine the so-called principal polarization states of a fiber span, and inject the optical signals only into one such state. For a sufficiently narrow optical bandwidth, there would then be no pulse broadening, although for larger bandwidths there is a polarization-related contribution to chromatic dispersion (with its sign being different for the two principal states). However, this method is usually not practical, partly because the principal polarization states change with time.
A common (because more practical) solution is to use spun fibers, where the fiber is twisted during the fiber drawing process. That way, telecom fibers with substantially improved PMD performance can be obtained. See the article on spun fibers for details.
PMD Compensation
To achieve very high bit rates ā particularly with older fibers and in long fiber-optic links ā it is often necessary to compensate polarization mode dispersion (PMD). Practical PMD compensators introduce a controlled differential group delay to counteract the PMD of the transmission fiber. A typical first-order PMD compensator may contain
- a fiber polarization controller for aligning the signal with fixed axes,
- a polarization-selective splitter, usually a polarization beam splitter (PBS) or a high-birefringence element, which separates the two principal polarization components with low loss,
- a variable optical delay line applied to one of the polarization paths, and
- a corresponding polarization-maintaining combiner (often another PBS or coupler) to recombine the two paths.
By adjusting the delay, the induced differential group delay can be matched to the PMD of the link, minimizing its overall effect.
Because temperature variations and mechanical stresses make PMD time-dependent, high-speed systems often require an automatic feedback loop to adjust the compensator in real time. In systems employing multiple wavelength channels (ā wavelength division multiplexing), compensation may need to be performed separately for each channel because PMD is wavelength dependent.
Minimizing the Sensitivity to PMD
Another strategy can be to limit the capacity of each transmission channel, but using many different channels in a single fiber, e.g. with the technique of wavelength division multiplexing.
There are also advanced modulation schemes with reduced symbol rate (for a given bit rate), which are less sensitive to PMD.
Frequently Asked Questions
This FAQ section was generated with AI based on the article content and has been reviewed by the articleās author (RP).
What is polarization mode dispersion (PMD)?
Polarization mode dispersion is an effect in optical fibers where light waves with different polarization states travel at slightly different speeds. This results in a differential group delay, which can be caused by random imperfections, bending, or mechanical stress in the fiber.
What is the difference between PMD and differential group delay (DGD)?
The terms are often used interchangeably. However, DGD strictly refers to the magnitude of the propagation time difference between polarization modes at a given moment, while PMD can refer to the overall phenomenon or a statistical average of the DGD.
Why is PMD a problem in fiber-optic communications?
PMD causes parts of an optical signal in different polarization modes to arrive at different times, leading to pulse broadening or splitting. This creates inter-symbol interference, which degrades signal quality, increases the bit error rate, and ultimately limits the maximum data transmission rate.
How does PMD scale with fiber length?
For short fiber sections with nearly constant birefringence, the differential group delay (DGD) is proportional to the length. For long fibers, where different sections contribute randomly, the total r.m.s. DGD scales only with the square root of the fiber length.
How can the effects of PMD be reduced?
PMD effects can be mitigated by using fibers designed for low PMD, such as spun fibers. For demanding applications, particularly with older fibers, active PMD compensation devices can be used to counteract the dispersion in the fiber link.
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Bibliography
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| [12] | W. T. Ireeta, V. Musara, L. Wu and A. W. R. Leitch, āStatistical characterization of the output state of polarization in deployed single-mode fibersā, Opt. Lett. 35 (12), 2049 (2010); doi:10.1364/OL.35.002049 |
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