Enhancement of second harmonic conversion in optical fibers by hydrogen and heat treatment

Abstract

Efficient second harmonic generation in optical fibers is still a largely unexplained phenomenon.1 In particular, neither the microscopic mechanism nor the defects or impurities that might be involved have been identified yet. We present new results that may contribute toward a solution of this problem. We modified the defect content of a Ge-doped-core fiber by subjecting it to a hydrogen and heat treatment that led directly to enhancement of the achievable second harmonic conversion efficiency. The fiber was placed in a pressure chamber with 3-atm H2 at room temperature for 3 days and was then removed and heated at 250°C for 6 h. Strands of this fiber, 15 cm long, were prepared for second harmonic generation: each was seeded with light at 1.06 μm and 532 nm at fixed power levels for a fixed time, long enough to reach saturation of the conversion efficiency.2 The conversion efficiency of the treated fiber was compared with that of untreated strands of the same fiber seeded in identical conditions. To date, an enhancement by more than a factor of 2 has been observed in such treated fibers.