National Physical Laboratory

Mode Field Diameter (MFD)

Calibration of the mode field diameter (MFD) of single-mode optical fibres. Both shifted and unshifted fibres can be measured. Standard calibration at 1310 nm and 1550 nm.  Calibration at other wavelengths may be possible on request. UKAS accredited service.

The mode field diameter is a measure of the radial intensity distribution of radiation propagating within a fibre. It is a key parameter for the specification of single-mode fibre and can be used to predict fibre properties such as splice loss, microbending loss, cut-off wavelength and waveguide dispersion.

Mode field diameter is measured by the ITU-T reference test method based on the far field scan technique. Radiation from a laser diode is launched into one end of a short length of fibre under test and a second fibre connected to a photodiode is rotated in an arc centred on the other end of the test fibre and typically 10 mm to 20 mm away from it. The intensity of the radiation reaching the photodiode is recorded as a function of angle and from this data the mode field diameter is calculated. The Peterman II definition of mode field diameter is used in the analysis; this definition specifies a particular weighted mean of the angular radial intensity distribution as the mode field diameter.

Measurements can be performed on conventional, dispersion shifted and dispersion flattened fibres. Standard wavelengths used are 1310 and 1550 nm. Measurements in the range 1275 - 1625 nm are available on request. Approximately 10 m of primary coated fibre is required for the measurement (3 m absolute minimum). Bare fibre and fibre mounted in SRM-style holders can be accommodated. Holders can also be provided if required. NPL can provide both standard and dispersion shifted fibre types and can measure customer supplied samples.

A copy of a typical Calibration Certificate is available to download.

Measurement Range Typical Uncertainties
Mode field diameter - 3.5 to 13 µm ± 0.6 %
Mode field non-circularity - 0 to 1 % ± 0.1 %


Customer Service tel: +44 20 8943 8637

Last Updated: 25 Mar 2010
Created: 23 Jul 2007


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