Time evolution of the second-order nonlinearity layer in thermally poled optical fiber

Time evolution of the second-order nonlinearity induced in a single-hole D-shaped fiber thermally poled at 3.5 kV and 320 degrees C has been investigated with second-harmonic microscopy. It was found that the nonlinear layer moved towards the cathode with increasing poling time until it finally reached the cathode surface. The magnitude of the nonlinear coefficient first increased, saturated at similar to 0.32pm/V between 5 and 20 min after poling, and then began to decrease for longer poling time. The nonlinearity remained large even when the nonlinearity layer reached the centerline between the two electrodes. In samples poled long enough (> 45 min), apart from the main nonlinearity layer at the cathode, weak second-order nonlinearity was also induced at the core-cladding interface sections that were nearly parallel to the poling direction. (c) 2006 American Institute of Physics.