Study of the optical parameters of a silica-polymeric optical fiber with a reflective coating made of a thermoplastic fluoropolymer

Silica optical fibers (OF) having a core diameter of 400 – 800 μm made of biocompatible materials are widely used in laser medicine. The results of studying the optical parameters of novel silica-polymeric optical fiber with a reflective thermoplastic copolymer coating (tetrafluoroethylene – ethylene) and the influence of coating conditions on these optical parameters are presented. Coatings from polymer melt were applied to the silica fiber surface by orifice drawing. The numerical aperture of the drawn OF was measured by distribution of the laser radiation emerging from OF in the far field. The optical losses were determined by the distribution of the radiation scattered by the reflective coating along the OF length. The scattering parameters of the laser radiation transmitted through OF were estimated by the intensity and indicatrix of scattering. We studied OF samples up to 50 m in length with a silica core of about 400 μm in diameter and reflective coating with a thickness of 70 – 90 μm, the reflective coating also performed a protective function. The quality of applied coating and optical parameters of the OF samples depended on the speed of fiber drawing (coating speed) V_d. A smooth coating was obtained at V_d ≤ 2 m/min. When V_d > 2 m/min the coating became rough, turning into the so-called «shark skin» at V_d = 6 m/min. Observed scattering of radiation passing through the studied OF samples was attributed to the polymer structure which contained both crystalline and amorphous phases with different values of the refractive index. The smallest scattering was observed in a smooth-coated OF. The total optical loss at a wavelength λ = 532 nm amounted to 300 – 720 dB/km (a nominal numerical aperture was 0.44). Short (1.5 – 3 m) OF samples were shown to provide a transmission of 80 – 93% of the input power.

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