Real-time χ^(2) evolution in twin-hole fiber during thermal poling and repoling

We report the real-time evolution of the second-order nonlinearity χ(2) induced in a twin-hole fiber during its first thermal poling and the subsequent repoling after thermal erasure. The considerable difference in the χ(2) evolution in the two poling processes indicates that the first poling has a strong history effect on the repoling. We use a two-charge dynamics model involving a fast charge carrier Na+ and a slow charge carrier H+ to model the χ(2) evolution in both poling processes. We quantitatively show that the difference in χ(2) evolution is due to the different initial charge distributions before each poling. The extra H+ ions injected into the fiber during the first poling change the initial charge distribution in the second poling, which leads to a different χ(2) dynamic.