Reading CD, PMD and DGD Units in Fiber Dispersion Reports

Direct answer

CD concerns wavelength-dependent group delay, while PMD concerns polarization-related delay. Values in ps/nm, ps/(nm·km), ps and ps/√km describe different quantities. Establish wavelength, length, statistical definition and specimen boundary before comparing results with the exact transceiver requirements.

Identify the delay quantity first

EXFO's CD explanation describes pulse spreading from wavelength-dependent propagation. Corning WP5051 discusses polarization-related differential group delay and the statistical meaning of PMD. An insertion-loss pass does not establish either result.

Use this guide for existing-route upgrades, mixed-fiber paths and dispersion-test instructions. Whether a short patch cord needs separate characterization depends on the system and path; long-haul test requirements should not automatically become requirements for every data-center cord.

Reported quantity	Common unit	Meaning to verify
End-to-end CD	ps/nm	Dispersion of the defined path at a stated wavelength
CD coefficient	ps/(nm·km)	Length-normalized and still wavelength-dependent
DGD	ps	Differential delay between principal polarization states
PMD measurement	ps	Requires a method and statistical definition
PMD coefficient	ps/√km	Requires an applicable coupling model and length definition

Separate total dispersion from a coefficient

The EXFO FTBx-570 specifications distinguish end-to-end CD from its normalized expression. Keep original units and the length used before placing results in a comparison sheet.

For an original dimensional example, assume uniform fiber with a CD coefficient of 17 ps/(nm·km) at a specified wavelength and a 2 km length. End-to-end CD is 34 ps/nm. Assuming that coefficient is approximately constant over a narrow interval, a 0.1 nm wavelength separation corresponds to approximately 3.4 ps group-delay difference.

These assumptions are not an actual module spectrum, pulse-broadening result or transmission-margin claim. Mixed fibers, broad wavelength ranges and additional components need an appropriate path model rather than a single nominal coefficient.

Keep PMD separate from inter-fiber skew

EXFO's PMD definition concerns polarization-related propagation. DGD concerns polarization states; inter-fiber skew concerns different physical paths. Both can use ps without measuring the same quantity. A PMD coefficient also carries statistical and model conditions and is not a deterministic bound on instantaneous DGD.

Corning's August 2023 paper distinguishes cabled and uncabled fiber PMD. Identify the specimen state rather than treating raw-fiber data as the measured result of a delivered route. This guide does not adopt the paper's historical transceiver forecasts or thresholds as universal requirements.

Minimum fields for a dispersion-test order

Item	Requested record	Procurement purpose
Boundary	Endpoints, fiber ID, splices and components	Identify the tested route
Wavelength	Scan range and reporting wavelength	Compare like conditions
Length and unit	Measured length, normalization, original quantity	Separate total values and coefficients
Method	Instrument, statistical definition, environment	Understand applicability
System requirement	Exact transceiver part and vendor conditions	Establish acceptance basis

Procurement FAQ

Does insertion-loss acceptance establish CD and PMD acceptance? No. They are separate measurands; testing requirements depend on the system.
Can a PMD coefficient simply be multiplied by length to obtain DGD? Do not use linear per-kilometer arithmetic. Establish the definition, coupling conditions and statistical quantity first.
Can every 800G module share one dispersion limit? No assumption is justified from rate alone. Check the actual optical interface, modulation and vendor conditions.
Must every short patch cord receive CD/PMD testing? This guide sets no such universal requirement. Define the path and acceptance objective before adding tests.
What if reports use different units? Retain the originals and request their wavelength, length and quantity definitions before deciding whether conversion is meaningful.

Sources and review date

Reviewed on 2026-10-16. Source publication dates remain separate from this review date. Procurement checklists are editorial recommendations; numerical examples illustrate assumptions, not actual specimen results.

Sources checked: 2026-10-16