The wavelength sensitivity and spectral resolution of Mach-Zehnder fiber interferometers obtained through a combination of two identical uncoated and titanium dioxide $\rm (TiO_{2})$ coated Long Period Fiber Gratings (LPFG) is presented and compared with single LPFGs based refractometric sensors. A set of LPFGs were fabricated in single mode fiber with the resonance band having an amplitude of 3 dB in order to split in half the optical power between the core and the specific cladding modes. The separation between the pair of LPFG written in the fiber was varied between 1 and 3 cm and the thickness of the $\rm TiO_{2}$ coating around the fiber ranged from 20 to 40 nm. A wavelength shift sensitivity of 216 nm/RIU was achieved for the device with 3 cm and a 30 nm thick $\rm TiO_{2}$ coating, which presented a spectral resolution of 1.1x10-4 RIU. Despite the lower wavelength shift sensitivity of 142 nm/RIU, attained for a 2 cm long device and 30 nm thick $\rm TiO_{2}$ coating, a spectral resolution of $1.8\times 10^{-5}$ RIU was measured, which is one order of magnitude lower than a single LPFG.

It is presented the fabrication and characterization of optical fiber sensors for refractive index measurement based on localized surface plasmon resonance (LSPR) with gold nano-islands obtained by single and by repeated thermal dewetting of gold thin films. Thin films of gold deposited on silica (SiO₂) substrates and produced by different experimental conditions were analyzed by Scanning Electron Microscope/Dispersive X-ray Spectroscopy (SEM/EDS) and optical means, allowing identifying and characterizing the formation of nano-islands. The wavelength shift sensitivity to the surrounding refractive index of sensors produced by single and by repeated dewetting is compared. While for the single step dewetting, a wavelength shift sensitivity of ~60 nm/RIU was calculated, for the repeated dewetting, a value of ~186 nm/RIU was obtained, an increase of more than three times. It is expected that through changing the fabrication parameters and using other fiber sensor geometries, higher sensitivities may be achieved, allowing, in addition, for the possibility of tuning the plasmonic frequency.

Biogenic amines (BA) are compounds found in a vast range of food products. In recent years, has been a crescent awareness towards food safety, followed by an increase in food regulations. Long period fiber gratings (LPFG) coated with titanium dioxide (TiO ₂ ) were used to monitor the optical properties of a layer of poly(ethylene-co-vinyl acetate) (PEVA) doped with maleic anhydride (MA) with was polymerized on top of TiO ₂ . This hydrophobic polymeric structure is permeable to BA which cause a steady increase of its effective refractive index causing a wavelength shift in the coated LPFG attenuation band. LPFG wavelength shift was observed and measured for the monoamine tyramine (TYR), to the diamines, putrescine (PUT), cadaverine (CAD), histamine (HIS) and tryptamine (TRYP) and to the polyamines, spermidine (SPED) and spermine (SPEM). It was determined that while PEVA coated devices present a residual sensitivity to BA, the maleic anhydride greatly increases it. In fact, for PEVA only coated LPFGs the sensitivities of 1.45±0.11, 0.97±0.05, 0.46±0.08 and 0.94±0.09 nmM ^-1 , for PUT, CAD, HIS and TYR, respectively, were measured. However, for PEVA doped maleic anhydride coated LPFGs the sensitivities are 3.34±0.13, 3.06±0.11, 2.62±0.14 and 3.65±0.23 nmM ^-1 , for PUT, CAD, HIS and TYR, respectively. Thus, the refractive index of PEVA increase with BAs in-diffusion and MA doping further enhance PEVA sensitivity to BA. The proposed sensor is expected to play a part in further development of a biosensor for the quantification of BA in real foodstuff, providing a methodology for quality control.