Experimental investigation and characterization of fabrication shaped clad optical fiber by thermally polishing optical fiber preforms with CO2 laser

Abstract

We experimentally investigated loss of multimode optical fibers (MMFs) drawn of thermally shaped optical fiber preforms (OFPs). Such preforms are typically used for fabrication of double clad active fibers. The investigation involved undoped shaped MMFs coated with a low refractive index polymer. The fibers were drawn of silica rod, prepared by collapsing a pure silica tube (Heraeus F300, OH content is 0.2 ppm) in the MCVD lathe. Background losses of undoped MMFs with inner cladding of various geometries shaped by CO2 laser were measured via cut-back method. Losses of the shaped MMFs were compared to the loss of the circular and of the mechanically shaped MMF. Constraints, drawbacks and advantages of shaping the fiber preform using the CO2 laser are discussed. Shaping OFPs with CO2 beam provides advantages of quick polishing, smooth surface, and freeform shape. Results show that mechanical polishing technique leads to significant OH content elimination, which is expressed as reduced absorption peaks at wavelengths of 0.945 μm and 1.24 μm, which correspond to the third and second overtone, respectively. The average perimeter length of fibers cross section governs absorption at polymer-glass interface.