National Physical Laboratory

Optical Length of Optical Fibres

Calibration of the optical length (time of flight) of both single-mode and multimode optical fibres. Standard measurements are made at 1310 nm and 1550 nm for single-mode fibre and at 850 nm and 1300 nm for multimode fibre. UKAS accredited service (single-mode fibres only).

The measurement of optical length is made using a time of flight technique where the time for an optical pulse to propagate through the fibre under test is measured. The optical length is then calculated using the speed of light and an assumed group refractive index of 1.46000. The measurements are made using lasers with centre wavelengths near 1310 nm and 1550 nm and the optical time of flight corrected using the chromatic dispersion characteristic of the fibre to give certified values at 1310 nm and 1550 nm. Other wavelengths, including 1490 nm and 1625 nm are available on request. Single-mode fibres up to approximately 100 km can be calibrated. 

Both optical time of flight and optical length (for N=1.46000) are quoted on the certificate. If the fibre is used to test an OTDR then providing the OTDR group index is also set to 1.46000 the optical length and the OTDR measured length are directly comparable.

Bare fibre can be measured, but fibre with connectors is preferred as this maintains the length of the fibre under test. FC/PC or FC/APC connectors are preferred. Multimode fibre up to ~ 4.4 km can be calibrated at 850 nm and 1300 nm, the measurements are not UKAS accredited.

Fibre Type Measurement Range Typical Uncertainties
Single-mode 100 m to 15 000 m
15 000 m to 105 000 m
± (0.04+1.75*10-5*L) m
± (0.10+1.75*10-5*L) m
Multimode 1000 m
4400 m
± 0.3 m
± 0.5 m

where L is the fibre length in metres.

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


Customer Service tel: +44 20 8943 8637

Last Updated: 25 Mar 2010
Created: 26 Jul 2007


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