Effect of high-temperature plasma-deposited nano-overlays on the properties of long-period gratings written with UV and electric arc in non-hydrogenated fibers

This work presents the effect of both high temperature (similar to 350 degrees C) and nano-overlay deposition on the performance of the long-period gratings (LPGs) written in non-hydrogenated standard fiber (Corning SMF28) and in photosensitive fibers (Fibercore PS1250/1500 and Nufern GF1) using UV exposure and electric arc discharge. Silicon nitride (SiNx) nano-coatings were deposited on LPGs at high temperature in a radio-frequency plasma-enhanced chemical-vapor-deposition process. The effect is discussed from a point of view of application of the gratings as platforms for high-temperature overlay deposition to enhance sensing properties of such LPGs. Our experiment shows that when deep resonances (similar to 20 dB) are desired only arc-induced gratings are suitable for high-temperature coating deposition or high temperature processing up to 350 degrees C. It is shown that the temperature-induced degradation effect for UV-written LPGs does not depend on composition of the core of the fiber and takes place for both photosensitive fibers. For some applications, where a resonance notch of only several dB is sufficient, high-temperature coating deposition and processing can still be applied to UV-written gratings. For the LPG written in GF1, a very low temperature sensitivity has been found. Moreover, our experimental results and simulations demonstrate that plasma-deposited high-refractive-index SiNx film (n = 2.37 @ lambda = 1550 nm) with a thickness below only 80 nm can successfully tune the sensitivity of LPGs to a pre-designed external refractive index. Such deposition processes take typically only several minutes.