Multi-parameter optochemical sensing based on coated cascaded long-period fiber gratings and frequency division multiplexing

We propose a multi-parameter optochemical sensing technique based on coated cascaded long-period fiber gratings (CLPFG) and frequency division multiplexing (FDM). Three CLPFGs, each with identical period Λ and length L but different separation S, are fabricated and connected with in-between fibers whose polymer jackets are not striped, which produces a composite spectrum that contains three fringe patterns with different frequencies. After a Fourier transform of the output spectrum, the characteristic peaks in Fourier spectrum are extracted with a filtering operation. The phases recovered by inverse transform of the respective filtered peaks allow the determination of the responses to respective measurands. A detailed investigation of sensing principle and methodology is developed. In experiment, the sensing system is composed of CLPFG1 (S1 = 10 mm) coated with TiO2/SnO2 composite film, CLPFG2 (S2 = 30 mm) coated with PAH/PAA film and bare CLPFG3 (S3 = 80 mm), which act to measure relative humidity, pH value and concentration of NaCl solution, respectively. The results show that the responses to three parameters can be readily extracted and the sensitivities reach 0.025 rad/%, 0.687 rad/pH unit and 0.113 rad/%, respectively.