All-fibre mode multiplexers

We made two types of three-mode multiplexers with three single-mode input cores and one enlarged output core. Light in each input core evolved adiabatically into just one of the LP01, even-LP11 and odd-LP11 modes of the output core. In the first type, the transition between inputs and output was formed by the controlled collapse of selected holes in a photonic crystal fibre with three cores in an equilateral triangular array. In the second type, three standard single-mode fibres were made dissimilar and tapered together within a fluorine-doped silica capillary. In both cases the output light patterns matched the LP01 and LP 11 distributions as expected, with broadband operation across at least 100 nm in the near-IR, low insertion losses and mode purities exceeding 10 dB.