A Novel Method for Measuring the Concentration of Liquids using Helical Long-Period Fiber Gratings

This research involves a novel liquid concentration measurement of helical long-period fiber grating (H-LPFG) by dual-wavelength difference. The grating with a period of 782 μm is spirally processed via the commercial welding machine. The resonant peak appears around 1520 nm. Coupled mode theory is used to study the transmission strength as a function of liquid concentration. Theoretically, the transmission intensity depends only on the liquid concentration. Then, we experimentally researched the relationship between the transmission intensity and the concentration as a function of the wavelength at 1495 and 1518 nm. Transmission intensity at these two places is e-exponential with respect to the liquid concentration. Due to the transmission intensity measurement concentration is influenced by the fluctuation of the light source, since the fluctuation of the light source will affect the transmission intensity. Finally, a demodulation system for H-LPFG is proposed using only filters and light detector. This allows a new way to develop a high-precision, low-cost, high-potential liquid concentration sensor.