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Abstract

Apparatus comprising a source of optical radiation (15), an optical fibre (1), and an absorbing material (2), wherein; the optical fibre (1) comprises a core (37), at least one cladding (38), and an optical surface (3); the source of optical radiation (15) provides optical radiation (16) that propagates along the core (37) of the optical fibre (1), and unwanted optical radiation (14) that propagates along the cladding (38) that surrounds the core (37); and the absorbing material (2) is in contact with the optical surface (3) over a length (5) of the optical fibre (1). The apparatus is characterized in that: the absorbing material (2) has a refractive index (101) that is higher than a refractive index (100) of the optical surface (3) within a temperature range (102) thus enabling the unwanted optical radiation (14) to pass from the optical fibre (1) into the absorbing material (2) within said temperature range (102); the absorbing material (2) is such that it can absorb at least some of the unwanted optical radiation (14) that enters into it from the optical fibre (1); the absorbing material (2) is such that its temperature increases upon absorption of the unwanted optical radiation (14); and the absorbing material (2) is such that its said refractive index (101) reduces with increasing temperature. This has the effect of limiting the amount of the unwanted optical radiation (14) that can be removed per unit length of optical fibre (1) to a predetermined absorption per unit length (62). Thus the apparatus is such that it is able to remove the unwanted optical radiation (14) up to a power level (12) substantially equal to the product of the predetermined absorption per unit length (62) and the length (5) over which the absorbing material (2) is in contact with the optical surface (3).

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