In this paper, a helical structure is fabricated on a commercial plastic optical fiber (POF) by heat pressing and twisting method. The helical POF is proposed as a RI sensor. A POF is firstly heated and pressed to form a flat-shape with a thickness of 60% of the diameter of the POF. After that, the flat-shape is twisted into a helical structure. The helical structures fabricated on POFs (with diameters of 0.25, 0.50, and 1.00 mm) and with lengths (of 10, 15, and 20mm) are experimentally evaluated for liquid RI measurement, and the results show that when the helical structure is fabricated on the POF with a diameter of 1.00 mm, a helical pitch of 2mm, and a length of 15mm, the RI sensitivities of 2292%/RIU, 4128%/RIU, and 1750%/RIU are obtained in the RI ranges of 1.33-1.37, 1.37-1.40, and 1.41-1.45, respectively. The experiment results have demonstrated that the proposed sensor is a low-cost solution for RI measurement, and with the features of high sensitivity, simple structure, easy fabrication, compact size and intensity modulation at visible wavelengths.
The temperature dependence of a refractive index (RI) sensor based on a core-offset in-line fiber Mach-Zehnder interferometer was investigated in order to eliminate the temperature and RI cross sensitivity caused by the thermo-optic effect. The RI sensitivity of the sensor was around -20.00 nm/RIU, and the RI correction was given for the sensor in the temperature range of 25-60℃. The experimental results showed that the temperature variation might lead to deviation in the measurement of RI, but hardly affected the RI sensitivity.