Hindering effect of the core-cladding interface on the progression of the second-order nonlinearity layer in thermally poled optical fibers

The spatial distribution of the second-order nonlinearity layer induced in thermally poled Ge-doped optical fibers has been investigated with second-harmonic microscopy. It was found that the core-cladding interface acts as an insurmountable barrier to the nonlinear layer at lower poling voltages. At higher poling voltages (>2.5kV), the nonlinear layer can partially overcome this barrier and reach the fiber core, in which it is isolated from the rest of the nonlinear layer in the cladding.