Impact of the electrical configuration on the thermal poling of optical fibres with embedded electrodes: Theory and experiments

Thermal poling of optical fibres is a well-known technique to create second order nonlinearity inside silica optical fibres, otherwise characterized by negligible nonlinear properties in the electric dipole approximation. Some recent work, realized by F. De Lucia et al., has introduced a new technique, designated as 'Induction poling' [1] and with the adoption of liquid materials as embedded electrodes (both metallic and non-metallic) [2], allows thermal poling of optical fibres with any length and geometry. Despite these advances, thermal poling still represents a technological challenge that needs to be continuously optimized and simplified. In this work we focus our attention on the optimization of the electrical configuration of thermal poling of single mode optical fibres. We consider the single-anode (S-A) configuration, where a single electrode is embedded inside one of the two cladding channels of the optical fibre and connected to the desired electrical potential, and the double-anode (D-A) configuration, introduced for the first time by W. Margulis et al. in 2009 [3] and later commonly adopted by the scientific community. Fig. 1(a) shows the dependence (numerically calculated with COMSOL Multiphysics) of the χ(2)eff on the poling duration for both electrode configurations and at two different positions. The key result of these simulations is that the final value (for extended poling times) of the χ(2)eff in S-A configuration is approximately double with respect to the one obtained in the D-A approach. Furthermore, the value at the centre of the fibre is almost zero in D-A configuration. We hypothesize that this behaviour arises from the mutually competitive evolution of the space-charge formation due to the presence of two anodes. In contrast, the S-A configuration does not suffer from this limitation. Experimentally for the first time the χ(2)eff was measured in a process of second harmonic generation (SHG) at 1550 nm in a fibre periodically poled in S-A configuration. The nonlinearity has been periodically erased via exposure to a UV light generated by a frequency doubled Argon-ion laser (CW, 244 nm). Fig. 1(c) shows the spectrum of the SHG light