Researchers at NASA's Marshall Space Flight Center have devised a method for the creation of crystal-free non-oxide optical fiber preforms. Non-oxide fiber optics are extensively used in infrared transmitting applications such as communication systems, chemical sensors, and laser fiber guides for cutting, welding, and medical surgery. Some of these glasses, however, are very susceptible to crystallization. Even small crystals can lead to light scatter and a high attenuation coefficient, limiting their usefulness. NASA has developed a new method of nonoxide fiber formation that uses axial magnetic fields to suppress crystallization. The resulting non-oxide fibers are crystal-free and have lower signal attenuation rates than silica-based optical fibers.
Signal propagation through traditional silica-based optical fibers is limited to visible and near-infrared wavelengths. In contrast, non-oxide glasses such as chalcogenide and heavy metal fluoride glasses (e.g. ZBLAN) are highly transparent from near ultraviolet to mid-infrared wavelengths, and have lower theoretical signal attenuation rates, resulting in the ability to transmit wideband signals over significant distances with minimal losses.
Theoretical estimates for signal attenuation losses for non-oxide fibers yield values of approximately 0.001 dB/km for a transmitted wavelength of 2.55 micrometers. The theoretical loss for the state-of-the-art fused silica is two orders of magnitude higher at 0.12 dB/km at 1.55 micrometers. There are a number of crystals and defects that can serve as scattering centers in non-oxide glass preforms and fibers. These include ZrF4, LaF3, AlF3, ZrO2, platinum particles from crucible reactions, carbon from organic impurities, and crucible reactions and bubbles due to contraction, cavitation, and gas precipitation. Using this patented preform formation technology, the fibers heated in the 0.1-T magnetic field did not show evidence of crystallization under optical microscopy or Scanning Electron Microscopy (SEM).
As seen in the figures, an axial magnetic field has the effect of suppressing crystallization in ZBLAN. The experiments indicate that the combination of a vertical magnetic field and a rapid cool-down from the crystallite melting temperature will ensure that no crystals are present in the preform after processing.