Thermally poled fibre devices

Abstract

Poling of silica glass, silica fibre and silica planar waveguides is an exciting research field. The thermal poling technique has been demonstrated to produce large second order nonlinearities and linear electro-optic coefficients in silica glass, silica fibre and silica planar waveguides. In this thesis, efforts have been made both to obtain large, stable and reliable linear electro—optic coefficients in silica fibre using thermal poling and to understand the mechanisms for the thermal poling effects in silica fibre.

Firstly, the thermal poling conditions for boron co-doped germanosilicate fibre were optimized and the dependence of the thermal poling induced linear electro-optic coefficient on the poling voltage, poling temperature and poling time were analyzed. To improve the efficiency of optimizing the poling conditions for different types of silica fibre, an in situ poling technique was deployed to study aluminum co-doped germanosilicate fibre. It is found that the thermal poling history has significant influence on the induced linear electro-optic effect. Furthermore, the in situ technique was also employed to measure and analyze the decay behavior of the induced linear electro-optic coefficient in boron co-doped germanosilicate fibre.

Secondly, an innovative method was proposed to investigate the frozen—in fields existing in the thermally poled fibres. The frozen-in fields are the cause for the induced linear electro-optic effect in silica fibre. This method can be used to measure both the magnitude and the direction of the frozen—in fields. Furthermore, this method is able to measure the third order nonlinearity of the fibre core. It is the first time that it has been observed that the third order nonlinearity of the fibre core increases after thermal poling.

Next, it was found that the time evolution of the linear electro-optic coefficient in thermally poled silica fibre is very different for different polarity of the poling voltage. There are two distinct processes in thermal poling: the faster linear process of charge migration and the subsequent single exponential process of charge ionization. It is the first time that it has been shown that there are two frozen-in fields in thermally poled fibre: the shielding field and the ionization field. Both the fields are able to produce a linear electro-optic effect in silica fibre. Furthermore, It was found that the charge distributions in thermally poled fibre could move during thermal poling. It is the first time that the competition between the shielding field and the ionization field seen by the core has been analyzed and it is shown to be a linear process. The third order nonlinearity of the poled fibre core is fairly constant during this competition, but it is still larger than that of the unpoled fibre core.

Finally, the thermal poling technology and the Bragg grating technology were integrated to demonstrate a single fibre (low frequency) intensity modulator.