Abstract: This invention provides an optical fibre (1) incorporating a body (2), and an array of longitudinally extending holes or inclusions (3) formed in the body (2), the holes or inclusions (3) having a different refractive index from the surrounding body (2) and being arranged to form a full or partial ring structure (5) extending generally around a longitudinal axis of the fibre, the ring structure (5) being disposed so as to approximate the refractive or reflective transmission characteristics of a multi-layer optical fibre. The fibre (1) may have a solid core or a hollow air core. The invention also provides a method of forming the microstructured optical fibre (1).