CD & PMD Fibre Testing: When Fibre Characterisation Matters

FDNS Blogs

Oct 1

3 min read

What are CD and PMD?

Chromatic dispersion (CD) and polarisation mode dispersion (PMD) are fibre transmission characteristics that can matter on high-speed, long-distance, or technically demanding optical links. They differ from ordinary attenuation and cannot be determin

ed by OTDR testing alone.

Chromatic dispersion explained

Chromatic dispersion occurs because different wavelengths travel through an optical fibre at different group velocities. This can broaden optical pulses and affect the available transmission margin, particularly on systems operating at high data rates over longer distances.

Polarisation mode dispersion explained

PMD is caused by different propagation characteristics between the polarisation states of the optical signal. Its impact depends on the fibre, link length, installed components and transmission system.

Why OTDR cannot replace CD/PMD testing

OTDR testing provides valuable information about the physical fibre path, events and attenuation characteristics. It does not measure chromatic dispersion or polarisation mode dispersion. Where the transmission system requires fibre characterisation, specialist CD/PMD testing is therefore a separate measurement.

When should CD testing be considered?

CD testing can be relevant to high-capacity and long-distance optical transmission where dispersion may affect system margin. The requirement should be determined from the optical system design, transmission technology, link length and project specification.

When should PMD testing be considered?

PMD testing may be relevant on links where the transmission rate, distance and system design make PMD a potential performance consideration. The appropriate test requirement should come from the transmission-system and project specifications.

Data Centre Interconnect and fibre characterisation

Modern Data Centre Interconnect networks can use high-capacity optical transmission over inter-building or inter-site fibre. Where the transmission design requires it, CD/PMD testing can form part of the commissioning and acceptance programme.

CWDM, DWDM and amplified networks

Fibre characterisation can be particularly relevant to CWDM, DWDM, and amplified optical networks, where wavelength, distance, and transmission performance must be considered together. The test programme should be designed around the actual optical system rather than applying the same measurements to every fibre installation.

How CD/PMD testing fits with Tier 1 and Tier 2 testing

A structured testing programme can begin with physical inspection and Tier 1 insertion-loss measurements, followed by OTDR characterisation where required. CD/PMD then provides additional transmission characterisation when the system design calls for it.

For a broader view of the workflow, see bi-directional OTDR testing and FDNS Fibre Optic Testing Specialists.

CD/PMD test reporting

A professional report should identify the fibre or link, test conditions, equipment, measurement results and the acceptance criteria against which the result is being assessed. Results should be traceable to the project and transmission-system requirements.

FDNS CD/PMD testing

FDNS provides CD/PMD Fibre Testing & Characterisation for optical networks where specialist fibre measurements are required. CD/PMD can be combined with OTDR, insertion-loss, ORL and high-speed transmission testing as part of a wider commissioning programme.

Frequently asked questions

What is CD testing?

Chromatic dispersion testing measures the wavelength-dependent propagation characteristics of fibre that can contribute to pulse broadening and transmission impairment.