UV and thermal poling of germanosilicate fibre

The non-zero second order susceptibility obtained by poling a glass is generally explained by the creation of a permanent frozen electric field E DC which breaks the centro-symmetry of the glass. Multiplying the third order susceptibility tensor terms by E DC gives rise to second order non zero terms according to the relation (1) So, after a poling process applied to a fiber, a linear (Pockels) non-zero electrooptic coefficient r 333 appears.