Preparation of Bragg mirrors on silica optical fibers and inner walls of silica capillaries by employing the sol–gel method, and titanium and silicon alkoxides

Abstract

The paper presents results on sol–gel preparation and characterization of multilayered coatings, Bragg mirrors, on silica slides, silica fibers, and inner walls of silica capillaries. In the coatings titania was employed for high-index layers and silica for low-index ones. Coatings with up to three pairs of titania and silica layers have been fabricated from alkoxide input sols. A sol based on tetramethylorthosilicate with RW = 1.75 and tetramethylorthosilicate concentration of 2 mol/l for used for silica layers. A sol of titanium butoxide with RW and alkoxide concentration of 1.58 and 0.315 mol/l, respectively, was employed for titania layers. Modified dip-coating techniques have been developed for the application of gel layers from the sols onto fibers or inside silica capillaries. Single gel layers were dried at 200 °C, final coatings were heat-treated at 450 °C. Relative velocities of substrates and sols have been controlled to obtain layers with an optical thickness of about 140 nm. Prepared single layers have been characterized by measuring their thicknesses by optical profilometry and refractive indices of titania layers by spectral ellipsometry. Homogeneity and appearance of multilayered coatings have been characterized by scanning electron microscopy and morphology of coatings by X-ray spectrometry. Optical properties of coatings on silica slides have been determined from their UV–VIS-NIR transmission and reflection spectra. Transmission optical properties of coated optical fibers and capillaries have been characterized by angular distributions of the output power from the fibers at a wavelength of 650 nm and by measuring their transmission spectra. Obtained results show that Bragg mirrors fabricated on silica slides can exhibit a minimal transmittance of about 17 % around a wavelength of 650 nm and that prepared fibers and capillaries provided with Bragg-mirror coatings are capable of guiding light and have lowest transmission losses around a wavelength of 550 nm.