Thermally stable, high-sensitivity π-phase shifted long period fiber gratings near turn around point for dual parameter sensing

This paper presents a novel technique to develop high sensitive and thermally stable π-phase shifted long period gratings (π-PS-LPGs) near turn around point (TAP) in photosensotive fibers, in air and water like environments. The analysis and experimental results showed that evolution and wavelength tuning of loss bands in a π-PS-LPG are highly dependent on change photo-induced refractive index (RI), which grows by UV fluence and decays by thermal relaxation. The wavelength tunability is of the order of 100–200 nm, depends on grating period and wavelength separation of the loss band from TAP. Enhanced thermal stabilty PS-LPGs near TAP were fabricated by annealing of the PS-LPGs written with high UV fluence. The experiments on spectral evolution of two loss bands in a π-PS-LPG near TAP revealed significant difference in their sensitivities, both to change in temperature and surrounding refractive index (SRI). This study provides valuable insights for design and development of thermally stable π-PS-TAP-LPGs in water-like environments, as biochemical sensors for simultaneous measurements of temperature and surrounding refractive index (SRI).