Optical fibre amplifier

An optical fibre amplifier including a first length of single mode optical fibre laterally optically coupled in a first coupling region with a second length of single mode optical fibre, and in a second coupling region with a third length of single mode optical fibre, wherein said second and third lengths of optical fibre have substantially matching index profiles that are mismatched with respect to that of said first length of optical fibre, thereby providing said first length of optical fibre with a spot size mismatched with respect to that of said second and third lengths of optical fibre, and wherein included in said first length of optical fibre, between said first and said second coupling regions, is a length of optical fibre that is rendered optically amplifying to light of a first frequency when optically pumped with light of a second frequency, and wherein each of said first and second coupling regions is provided by a 2.times.2 tapered fused fibre coupler in which one of the two constituent fibres of that coupler is tapered more than the other by an amount providing the two constituent fibres with substantially matching propagation constants within its coupling region.