Free Space Fibre-To-Fibre Coupling Using Optical Fibres Below Cut-Off

A method of free space fibre-to-fibre coupling includes providing a first optical fibre 105 and a second optical fibre 110 spaced apart from one another by a free space distance having a pair of spaced-apart achromatic collimating lenses 111, 112 within the free space distance. Essentially single mode light is launched from the first fibre and propagates as free space light through said free space distance. The light is then received by, and propagates in, the second optical fibre as essentially single mode light. The first optical fibre and second optical fibre are both < 50 m and sufficiently short to provide essentially single mode operation. Essentially single mode light is described in paragraphs [0002] and [0006] of the application, as well as claims 1 & 8. Essentially single mode light means light of a range of wavelengths, where at least a portion of that range is below the cut-off wavelength for the first fibre (the portion above the cut off would be Single Mode Light [SML] and the portion below would be Multi-Mode Light). The system may also include an optical device between the lenses.