Effect of annealing on creation of second-order nonlinearity in twin-hole optical fibre of pure silica

The effect of thermal pre-annealing on the creation of second-order nonlinearity in a pure twin-hole optical fibre thermally poled at 3.5 kV and 280degC for 30 min has been investigated. In unannealed as-drawn fibres, a SON of ~0.18 pm/V was induced at the core-cladding interface. The induced SON showed little polarisation dependence. In fibres pre-annealed at 400degC for 3 h, the induced SON was smaller, decreasing to ~0.09 pm/V in magnitude. In fibres pre-annealed at 800degC for 3 h, no detectable SON was found at the core-cladding interface. Electron-migration along connected existing defects at the core-cladding interface was postulated to be responsible for the frozen-in space-charge field that generated the effective SON observed. Thermal annealing is proposed to have caused defect-annihilation, which resulted in a smaller SON.