A method and an apparatus for measuring a profile of a beam output from a randomly-coupled multi-core fibre are provided. An apparatus includes a light source 10, a measurement unit 20, and an analysis unit 30. Light output from the light source is input in one or more of a plurality of spatial modes of the fibre at an input end of the fibre. Themeasurement unit measures a sum of intensity profiles of individual light components output from respective ones of the plurality of spatial modes by averaging a component of interference between the plurality of spatial modes in a beam profile of combined light output from the plurality of spatial modes at an output end of the fibre. The analysis unit calculates a mode field diameter (MFD) and/or an effective area (Aeff) of the fibre based on the sum of the intensity profiles of the individual light components obtained by the measurement unit.
Optical-fiber output beam profile measurement method and optical-fiber output beam profile measurement apparatus