Polarization Mode Dispersion: Interpreting the Data
February 21, 2018
Polarization Mode Dispersion (PMD): Interpreting the Data
Issued: February 14, 2018
Author: Gina Paoni
Trans World Fiber Optics, Inc.
PMD Coefficient and PMD Delay – What's the Difference?
Units: ps/km1/2 (picoseconds per square root kilometer)
Numerous experiments with fibers of different lengths have revealed that the PMD Delay of the fiber is proportional to the square root of the fiber length (L) multiplied by a proportionality coefficient. This coefficient is called the PMD Coefficient (PMD Coeff) and is typically measured in units of picoseconds per square root kilometer (ps/km1/2).
PMD Coefficient is the parameter that is typically specified for commercially available fiber. 
Simply put, PMD Coefficient is a number used in the design phase to calculate the parameters of the network in order to obtain a resulting Average Differential Group Delay (PMD Delay) which does not exceed the desired maximum.
PMD Delay (aka DGD - Differential Group Delay)
Units: ps (picoseconds)
PMD Delay occurs when the signal on the "X" axis and the signal on the "Y" axis reach the receiver at different times. That 'time-of-arrival difference' (or delay) results in the receiver being unable to distinguish whether it is seeing a "1" or a "0". Thus, the data becomes unreadable resulting in Bit Errors.
Distinguishing between PMD Coefficient and PMD Delay can get confusing so it may help to look at it like this:
- Meeting PMDCoeff threshold is necessary for design
- Meeting PMD Delay threshold is necessary for successful transmission
Testing it Out
When the design stage is complete, and the network fiber has been installed, qualified technicians can perform Fiber Characterization on the installed fiber and measure the actual PMD Delay (in picoseconds).
Here is where we find out if the calculations used in the design (calculations made using the PMDCoeff (ps/km1/2) supplied by the manufacturer) were adequate to produce the desired results.
With regard to Polarization Mode Dispersion, as long as the resulting PMD Delay is within specifications, transmission can be expected to proceed without error.
In our experience of testing PMD on tens of thousands of fibers, we have found that, in 100% of cases, fibers which have measured PMD Delay that exceeds the threshold will also have corresponding measured PMDCoeff which exceeds the threshold.
However, it does not apply the other way around.
We often see fibers with measured PMDCoeff above the threshold but measured PMD Delays well within the specified threshold.
The threshold which ultimately matters in the successful transmission of data? Wait for it…
… PMD Delay
 An Introduction to the Fundamentals of PMD in Fibers
Issued: July 2006
Author: Sergey Ten and Merrion Edwards
ISO 9001 Registered