Minimizing the optical losses in anisotropic single-mode lightguides with elliptical boron germanosilicate cladding

This paper discusses how the damping at wavelength 1.55 µm depends on various factors for anisotropic single-mode lightguides with boron germanosilicate elliptical cladding. It is shown that the refractive index of this cladding in the preform has a substantial effect on the optical losses of the lightguides. The thickness of the cladding that isolates the core from the boron-containing glass of the elliptical cladding is optimized. The optical losses to Rayleigh scattering are reduced by doping the core with small additives of moisture. For a lightguide with a glass-fiber diameter of 125 µm in a two-layer polymeric coating, an optical loss level equal to 0.5 dB/km is achieved, with a polarization maintenance of 5×10⁻⁶m⁻¹.