Last Updated: August 16, 2025
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>> What is PMD versus Differential Group Delay (DGD)?
Polarization Mode Dispersion (PMD) is the average Differential Group Delay (DGD) one expects to see when measuring an optical fiber.
DGD is the time separation or delay between the two principle polarization modes of the transmission link at the receiver.
DGD is an instantaneous event and varies randomly with wavelength and time. This means that DGD is a statistical parameter, obeys the law of probability theory and thus has uncertainty associated with it.
PMD is the average value of a distribution of a large number of independent DGD measurements.
>> What is fiber PMD versus cable PMD?
Fiber PMD is PMD that is generally measured on spool, while cable PMD would be the PMD of the fibers in a cabled/installed mode.
The spool measurement is not an accurate indicator of cabled and installed PMD. Many studies have found that there is no correlation between the two.
Other measurement techniques such as a Low Mode Coupling (LMC) measurement have been developed to make this correlation. However, LMC is destructive and time consuming and thus, predominately performed on a sampling basis.
Fiber customers should insist on data from the cable manufacturer that establishes this correlation and provides a Link Design Value (LDV) needed for their application.
>> What is a Link Design Value?
Link Design Value (LDV) is a useful design parameter for calculating the worst-case contribution of the fiber toward the overall system PMD of a link.
LDV, also referred to as PMDQ,is a term developed in standard bodies used to evaluate the impact of fiber related PMD where cabled fibers are deployed in concatenated sections. This is, in building a network typically 2-10 km sections of cable are spliced together.
The LDV is the worst case PMD of the end-to-end link made up of randomly chosen cable sections spliced together. Thus the LDV represents the worst case PMD of a fiber path in a cabled and deployed span.
PMD standards indicate that LDV should be calculated from nominally 20 to 24 sections and have a maximum cumulative distribution Q of nominally 0.001 to 0.0001. This implies that 0.1% or 0.01% of all spans (made up of concatenated sections) would be above this level of PMD.
>> What is system PMD?
System PMD is the total PMD attributed to the collection of optical components that makes up an optical link between a transmitter and a receiver. Many of these components are illustrated in the following figure.
Indeed, optical fiber may only be one of the components if amplification, power splitting, dispersion compensation or optical multiplexing is used in the link.
With PMD performance of cabled optical fibers continuing to decrease, more emphasis will be place on the PMD performance of the other components in the link.
Typically, in a long haul system, the PMD attributed to the optical fiber itself is given half of the system PMD budget. The system PMD is calculated from the square root of the sum of the squares of each individual component PMD.
>> How much PMD can be tolerated in my system?
The level of system PMD that can be tolerated depends on data rate, distance and how much system outage one is willing to tolerate.
The following figure indicates the average system outage as a function of system PMD for 10, 40 and 80 Gbps systems.
As can be seen in this figure, as the system PMD limit for a given bit rate is approached, slight increases in PMD will cause significant increases in system outage.
International standard guidelines for PMD recommend an outage probability of 6.5 x 10-8, corresponding to 2 seconds of outage per year attributable to PMD.
Coupled with the 50% recommendation of system PMD that should be allocated for optical fiber, a goal for a 10 Gbps system would be a cabled optical fiber with ~ 4 ps of total PMD. This drops to below 1 ps if a 40 or 80 Gbps system is envisioned operating on this fiber in the future.
The PMD requirement on your cabled fiber is now only dependent on knowing the distance of the route. For example if you have a 100 km link at 10 Gbps, then your cabled fiber should have a LDV below 0.4 ps/sqrt(km).
For longer distances, the LDV requirements become even more stringent such that at 1000 km, a 10 Gbps system would require a cabled fiber LDV of < 0.13 ps/sqrt(km) while a 40 Gbps system would need a value < 0.03 ps/sqrt(km).
Errors induced by PMD may be difficult to isolate. Since PMD is statistical in nature and varies with wavelength and time, errors can appear in random, frequent burst or long durations.
Errors may appear later in the life of a link as the cable plant ages and environmental factors change its performance. There are a host of external factors that can influence the PMD behavior of a link, from seasonal changes in temperature, to the effects of wind, to vibrations from proximity to railroad tracks or large building equipment.
>> How do I guard against the transient nature of PMD?
The best way to guard against the transient nature of PMD is to use a fiber that has been manufactured with a process that both lowers PMD and provides PMD stability with environmental changes.
>> Can I fix or compensate for PMD?
Yes, but it is generally an added cost since it must be done on a per channel basis.
There are two ways to compensate for PMD, optically or electronically.
Optical PMD compensation is done by splitting the signal into two polarization states, actively measuring the PMD of a signal and adjusting an optical delay line to retard or advance one of the polarizations. These techniques were generally very expensive and never were widely deployed.
Electronic PMD compensation can be done using adaptive processing techniques, but have limited range of compensation and are difficult to scale to higher data rates, such as at 40 Gbps. It is best to minimize your needs for compensation by installing low PMD optical fiber and components.
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