Diffusion and Interface Effects during Manufacturing of Compact Microstructured Optical Fibers

Compact Microstructured Optical Fibers (CMOFs) enable very flexible optical waveguide designs with high geometrical precision by stacking of various doped silica rods. The geometrical parameters (pitch to diameter ratio, core to pitch ratio) shift during sintering of the interstitial cavities of the initially imperfect filled preform. We modeled geometrical shift effects and compared it with manufacturing results. Additionally, refractive index modulating dopants, e.g. Germanium or Fluorine, can develop dissociative reactions. In that case gassing reactions are followed by undesired void formation during the thermal processing steps. The paper shows simulations of thermally treated Germanium and Fluorine doped silica rod arrangements. The model results were confirmed by dopant and void profile measurements of manufactured CMOFs. The investigated geometrical disturbances in stretched canes or drawn fibers emphasize the importance of a sufficient thick barrier layer. The minimal undoped silica barrier layer thicknesses of Germanium and Fluorine doped package rods are approximated to be about 12 µm and 40 µm, respectively, for a typical single drawing step procedure at 1900°C.