Optical modelling vs. FTIR reflectance microscopy. Characterization of laser treated ceramic fibres

Ceramic silicon carbide (SiC) based fibres treated by a laser induced chemical vapour deposition process (LCVD) have been investigated by MIR-FTIR reflectance microscopy. A significant change of the spectral behaviour has been detected after laser irradiation and after fibre coating. A strong SiC resonance appears and the damped interference waves in the short wavelength region vanish simultaneously. Spectra modelling has been used to interpret the measured near-normal specular reflectance spectra. The dielectric function of the fibre material is described by a Lorentzian oscillator model. Because of the cylindrical shape of the fibrewith a diameter close to the wavelength of the used light the Maxwell based algorithm for spectra simulation of flat layered samples with smoothinterfaces has been extended by a term of interface irregularities which modifies the phase of the reflected beam. Based on this model the measuredspectra have been well described and the dielectric functions of the fibreshave been derived. The interference-like waves result from retro-reflections inside the fibre. Spectral changes due to laser treatment of fibres have been attributed to re-crystallization of the originally amorphous SiC based material resulting in scattering on the crystallite interfaces.