Refractive index sensor based on multi-mode plastic optical fiber with long period grating

A multi-mode plastic optical fiber (POF) with a long period grating (LPG) was proposed for a refractive index (RI) sensing probe. The LPG was fabricated on the surface of the POF by a simple die-press-print method using a commercial available thread rod as the mould. The RI sensing performances for straight and U-shaped POFs with LPGs were studied. It is found that the straight RI sensing probe with LPG structure was not sensitive enough for RI measurement. After bending the straight POF probes with LPGs into U-shaped probes, the RI sensing performance was improved markedly. By altering the structural parameters, the RI sensing performances of the U-shaped POF probes with LPGs were optimized, a sensitivity of 1130%/RIU with a resolution of 8.44X10(-4) in the RI range of 1.33-1.41 was obtained. The probe is a low cost solution for RI sensing purpose, which has the features of simple structure, easy fabrication, compact size and intensity modulation at visible wavelengths.