Preparation of long-period gratings (LPGs) for biological substances detection

This work discusses methods of applying biological films on the surface of high-sensitive long-period grating (LPG) in the process of developing optical fiber sensors, which allow for label-free detection. Referred techniques prepare the surface of LPG structure for further coating with layers providing specificity of the sensor system. Presented are two methods of fiber coating, based on self-organizing layers, which were applied on two different LPGs. Short and thermally undemanding method was used on UV-written LPG fiber whilst heat requiring method was applied on LPG written by arc method, with silicon nitride (SiNx) nano-coated surface. Sensitivity of fibers was testes through immersing in liquids of various refractive indexes (RIs) and found to be 925.5 nm/RIU for UV-written and nano-coated arc-written LPGs to be -1115.8 nm/RIU, respectively. Shifts of resonance wavelengths occurring during the processes of coating are presented and discussed. Experiments were performed using simple instruments and in short-term proceedings and can therefore be easily repeated as well as applied to industrial production processes. We confirmed effectiveness of both methods and proved that selected fibers were suitable for use in each of those approaches respectively.