Impacts of the distribution of hydrogel spheres attached on bare fiber core on light transmission of the fiber

Impacts of the distribution of hydrogel spheres attached on bare fiber core on fiber transmission are studied by ray-tracing simulation. The aim of the study was to optimize the previously reported configuration and sensitivity of humidity sensors based on bare fiber core attached with humidity-sensitive hydrogel spheres. Four uneven distribution types, namely, asymmetric-geometric, asymmetric-arithmetic, symmetric-geometric and symmetric-arithmetic for sensors in length of 10 cm and attached with six hydrogel spheres are simulated and compared with the uniform distribution. Spheres in diameter of 0.6, 0.8 and 1 mm are simulated to reveal impacts of the sphere size. It is found that the sphere size is a significant factor that determines impacts of the sphere distribution on the sensitivity, which is defined as the slope of fiber transmissions over refractive index of spheres. For sensors attached with 0.6 mm hydrogel spheres, uneven distributions result in sensitivities close to that of the uniform distribution. However, uneven distributions with 0.8 or 1 mm spheres yields much higher sensitivities. The highest sensitivities for sensors with 0.8 and 1 mm are 19% and 39% higher than that of uniform distributions respectively, suggesting that properly distributing hydrogel spheres is a convenient way to enhance sensitivities. Moreover, results show that sensors with 0.8 mm hydrogel spheres have higher sensitivities than that of 1 mm for the same sphere distribution, and the underlining re-collecting effect is revealed to explain this phenomenon. In addition, simulations reveal that optical loss of spheres have a positive but marginal effect on the sensitivity.