Optical fiber exposed to high‐pressure hydrogen gas for 3–4 days, and subsequently heated in air at temperatures in the range 250–500 °C for 8–24 h, shows an enhanced second‐harmonic conversion efficiency after seeding with 1.06 &mgr;m and 532 nm light, when compared with untreated fiber seeded under similar conditions. Up to now, an enhancement by as much as a factor of 4.4 has been observed. Some possible correlations between this enhancement and the change in the defect content of the fiber due to the H2‐heat treatment are pointed out.