UV-Induced Absorption in All-Fiber Frequency Doublers: Characterization and Photobleaching

Periodic UV exposure of thermally poled optical fibers is usually used to produce efficient all-fiber frequency doublers based on second-order nonlinearity. To date, no data regarding UV-induced absorption in poled fibers have been reported. In this study, we measured this absorption and showed how it affects the conversion efficiency of second harmonic generation at wavelengths below ~800 nm. Color centers responsible for induced absorption in poled GeO₂-doped fibers were identified and photobleached by continuous-wave radiation at 532 nm. We demonstrated that photobleaching does not affect the value of nonlinearity but improves the conversion efficiency of all-fiber frequency doublers. The results obtained can be used to improve the efficiency of all-fiber second harmonic generation in the blue-green part of the spectrum.