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An optical fibre with an erbium (Er)-doped germanosilicate ring structure around the anode hole has been fabricated for stabilising the thermal poling induced second-order nonlinearity (SON). The SON spatial distribution was visualised with second-harmonic microscopy. It was found that the SON layer was trapped in the Er-doped ring under normal poling conditions. Only at higher poling voltages and for much longer poling time could some SON be formed at the core-cladding interface, while most of the SON layer was still firmly confined within the Er-doped ring.