Second-harmonic generation (SHG) from a twin-hole fiber by thermal poling was studied. Quasi-phase-matching (QPM) was performed with a low-pressure mercury lamp. Erasure of the second-order nonlinearity was studied by uniform exposure, and after that QPM was performed using a photomask erasing the nonlinearity periodically. SHG was measured with a Q-switched Nd3+: YAG laser. The second-harmonic power increased by a factor of 7 after QPM. A ray-tracing analysis on the light distribution of the lamp after the photomask was performed, and the effect on QPM is discussed. A model for the second-order nonlinearity by charge layers is also discussed. (C) 2013 Elsevier B.V. All rights reserved.