In the second case, coating the fiber with a thick polymer drastically changes spectral response of the LPG and better makes it highly sensitive to variations in properties of the overlay versus a number of parameters. Moreover, a hysteresis in Inhibitors,Modulators,Libraries pressure response would appear in such a case, due to the relative softness of polymer materials, as well as due to the changes of the overlay��s optical properties in time, temperature and pressure.In order to overcome problems mentioned above we applied a high-refractive-index (high-n) thin coating that can tune the intrinsic sensitivity of the LPG devices to a certain range of external RIs, while reducing the sensitivity in other ranges. A specially designed coating, in terms of its thickness and optical properties, optimizes the interactions of the light guided in the fiber and in the coating [10�C13].
Inhibitors,Modulators,Libraries By modifying the guiding conditions of the cladding modes in this way, we can make the LPG insensitive to the RI of the liquid used in the pressure system. In order to fulfill durability requirements, the coating cannot change its properties Inhibitors,Modulators,Libraries under either long-term presence of the liquid or the applied pressure. The dense silicon nitride (SiNx) nanofilms deposited by the radio-frequency plasma-enhanced chemical-vapor-deposited (RF PECVD) method seem to be a perfect choice for this type of application. Due to their high hardness (~19 GPa), excellent optical properties and very good adhesion to the silicon-related substrates, SiNx films are typically applied as antireflective coatings for solar cells and as light-guiding layers in planar optical waveguide systems [14].
They exhibit high refractive index (n typically from 2 to 3.5) and negligible absorption in the infrared spectral range [15], both of which qualities are required in LPG-based sensors [16]. To the best of our knowledge, the nanocoating presented in this paper is the first ever applied to reduce Inhibitors,Modulators,Libraries the RI sensitivity of LPG-based AV-951 pressure sensing devices.2.?Experimental Section2.1. LPG FabricationIn this experiment we used commercially available Fibercore PS 1250/1500 photosensitive fiber, where refractive indices of core and cladding defined at �� = 1,550 nm were 1.44937 and 1.4402, respectively. Gratings based on this fiber show very high pressure sensitivity [6]. Each of our samples contains a 10-cm section of the photosensitive fiber with the ends spliced to Corning SMF28 fibers.
A set of LPGs was written with a computer-assisted precision arc-discharge apparatus, described in [4] and [17]. The LPGs were written only in the PS 1250/1500 fiber section. The discharge current was adjusted to be low enough to currently heat the fiber locally and not to produce any visible tapers due to the axial tension applied to the fiber. The arc discharge time was established at �� = 300 ms. The grating period was �� = 375 ��m and the length of the gratings was L = 20�C25 mm.