Creating large second-order nonlinearity in twin-hole optical fibre with core at the centre of the two holes

A twin-hole optical fibre with the fibre core located at the centre of the two holes was thermally poled to induce second-order nonlinearity (SON) in the core. The SON spatial distribution was measured with second-harmonic microscopy. It was found that the SON was distributed in a narrow layer, which moved towards the cathode for longer poling time. The whole fibre core became second-harmonic active after the SON layer had reached the core-cladding interface with a maximum value of 0.26 pm/V induced under a poling condition of 3.5 kV, 320degC and 40 min