An
optical fiber for transmitting
radiation comprising two or more core regions, two or more core regions, each core region comprising a substantially transparent core material and having a core
refractive index, a core length, and a core
diameter, wherein said core regions are arranged within a cladding region, said cladding region comprising a length of first substantially transparent cladding material, having a first
refractive index, wherein said first substantially transparent cladding material has an array of lengths of a second cladding material embedded along its length, wherein the second cladding material has a second
refractive index which is less than said first refractive index, such that
radiation input to said
fiber propagates along at least one of said core regions. The cladding region and the core regions may be arranged such that
radiation input to said
optical fiber propagates along one or more said lengths of said core regions in a single mode of propagation. The
optical fiber may be used as a bend sensor, a spectral filter or a directional coupler. The invention also relates to a method of manufacturing a multicore optical
fiber.