Silicon Nitride Overlays Deposited on Optical Fibers with RF PECVD Method for Sensing Applications: Overlay Uniformity Aspects

This work discusses uniformity of overlays deposited with radio frequency plasma enhanced chemical vapor deposition method on a core of partly uncoated multimode polymer-clad silica optical fiber. Although the method provides uniform films on flat surfaces, uniformity of deposition on optical fibers suspended in plasma is questionable. In this work we investigate optical-fiber-based structures coated with high-refractive-index ($n_{D}$> 2.3) silicon nitride ($SiN_{x}$) where lossy mode resonance phenomenon appears. Experimental studies, where the fiber is rotated by 90° and 180°, are compared to those where the sample has not been rotated between the depositions. Deposition process time for all samples has been adjusted in order to obtain the same thickness of overlays on all the fibers. The experimental data has been supported by numerical simulations. The experiment has shown that the rotation modifies transmission of the $SiN_{x}$-coated fiber structure, as well as its response to external refractive index. As an effect of rotation we observed shift of the resonance appearing at about λ =600 nm towards shorter wavelengths and reduction in amount of the resonances. However, the resonances appearing in transmission spectrum for sample with no rotation show the highest refractive index sensitivity reaching about 690 nm per refractive index unit.