Measuring a characteristic of a multimode optical fibre

Method and apparatus to measure a characteristic (e.g. temperature, strain) of a multimode optical fibre 102 deployed in a well or along an electrical cable comprises: transmitting a light pulse from a light source 104 into the fibre; passing Brillouin backscattered light that is responsive to the light pulse, from the fibre through a spatial filter 108 (e.g. near or far field spatial filter respectively connected to, and spaced apart from, a proximal end 120 of the fibre), wherein a portion of Brillouin backscattered light is passed through the spatial filter; and directing this portion (e.g. via on optical coupler 504) to optical detection equipment 110 (e.g. optical filter 112 and detector 114) for processing. The spatial filter may be positioned between the fibre and the detection equipment and may include: at least one aperture to pass a portion of a respective speckle pattern; a single mode optical fibre (200); or an opaque layer (300) with a hole (302) therein. A beam combiner (508) may be used to combine light from an optical local oscillator and the Brillouin backscattered light portion. Raman detection equipment may also be incorporated to process Raman backscattered light.