Self-Organization in Ge-Doped Silica Fibers and Second Harmonic Generation

Abstract

Sometimes routine experiments discover effects which nobody wants, and only study by numerous groups explains what the effects were. It is interesting that this happens even with well-known and widely used materials. In 1986, the Swedish physicists Osterberg and Margulis observed a fascinating effect: a spontaneous, rather efficient (efficiency was reported to be about 0.1) second harmonic generation in a Ge-doped optical fiber after 5h illumination by a fundamental wave of YAG:Nd³⁺-laser [88]. This process is forbidden in glass due to inversion symmetry of the material; therefore, the experimental result attracted a lot of attention. A weak generation had been observed earlier [89]; and further it was shown [90] that the intensity of this generation is proportional to the surface area of the optical fiber. Owing to the violation of inversion symmetry in the near-surface layer, there is nothing to forbid the generation, and therefore a weak effect comes as no surprise. Against that, the medium is centrally symmetric in the bulk of the fiber, so that the nonlinear susceptibility tensor Ξ⁽²⁾ is identically equal to zero and frequency doubling is impossible. This explains why the report of highly efficient generation aroused considerable interest in this problem, and several theories followed.