National Physical Laboratory

Numerical Aperture (NA)

Calibration of the numerical aperture (NA) of multimode fibre at 850 nm. Both multimode fibre types, 50/125 µm and 62.5/125 µm, can be measured.

Light is transmitted in multi-mode fibres by many different paths or modes. Only light incident on the end of the fibre up to a certain maximum angle relative to the fibre axis can be guided by the fibre structure. The sine of this maximum acceptance angle is called the numerical aperture, NA, of the fibre. The larger the acceptance angle and NA, the larger the proportions of light coupled into the fibre for transmission. The system used to make the measurement is essentially the same as that used for the far field scan Mode Field Diameter measurement system, except for the launch optics which ensure that the light launched into the fibre is uniform over the fibre core and exceeds the NA of the fibre. The measurements are made at 850 nm.

Both 50/125 µm and 62.5/125 µm multi-mode fibre types can be measured. A minimum of 4 m of fibre is required, of which typically 2.1 m is supplied to the customer.

A copy of a typical Calibration Certificate is available.

Measurement Range Typical Uncertainty
0.18 to 0.29 ± 0.0046


Customer Service tel: +44 20 8943 8637

Last Updated: 25 Mar 2010
Created: 26 Jul 2007


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