Refractive Index Sensing Based on a Long Period Grating Imprinted on a Multimode Plastic Optical Fiber

A corrugated surface long period grating (LPG) is fabricated on a commercial multimode plastic optical fiber (POF) by a simple mechanical die press print method. This so-obtained LPG is proposed as a refractive index (RI) sensor. The LPGs imprinted on the POFs with different diameters are evaluated, and it shows that an LPG on a thin POF (with a diameter of 0.25 mm) exhibits better RI sensing performance than that of the ones on thick POFs (with diameters of 0.5 and 1.0 mm). By altering the structural parameters of the LPG on the POF with a diameter of 0.25 mm, the RI sensing performance is studied. When the grating period is $100~\mu \text{m}$ , the groove depth is $65~\mu \text{m}$ , and the tilted angle is 20°, the optimum sensitivity of 2815%/RIU with a resolution of $1.39\times 10^{\mathbf {-4}}$ RIU is achieved in the RI range of 1.33–1.45. In addition, the characteristic of the temperature dependence of the sensor is presented. The proposed sensor is a low-cost solution for RI measurement and with the features of simple structure, easy fabrication, and intensity modulation at visible wavelengths.