Investigation On The Refractive Index Sensing Performance Of Macrobending Plastic Optical Fibers

The refractive index (RI) is an important optical parameter of a material because otherparameters, such as concentration, purity, temperature, stress can be deduced from the RIs. Sothe RI sensing technology has many applications in a range of areas, including food science,medical testing, biochemistry analysis, and environment monitoring. Optical fiber sensors aremain tools to measure RIs, which has a lot of advantages including immunity toelectromagnetic interference, rapid response, compact size and compatible with the opticalfiber communication networks. With the deepening of studying and the expanding ofapplication, many new requirements were proposed, such as simplify manufacturingtechnology and low cost equipment. Therefore, the plastic optical fibers (POFs)，with thefeatures of good flexibility, low cost, operations in the visible light regime, and easy toprocess and connect, attracted considerable attention in RI sensing technology. Thus, the RIsensors based on POFs have significant research value and good application prospects.In this paper, for achieving a fast, efficient and low cost RI measurement, we proposedtwo types of RI sensor structures which are based on macrobending POF, one ismacrobending Micro-/Nano POF RI sensor to realize miniature RI sensor, and another one isside-polished macrobending POF to realize high sensitivity, simple structure and low cost RIsensor. We studied the performance of the sensors in the constant and varied (10~70°C)temperature, optimized the structural parameters, and improved the sensing performancetheoretically and experimentally. Moreover, the temperature effect on the light transmission inthe macrobending commercial POF was investigated. The main results we obtained asfollows:By a heating and pulling method, the diameter of commercial POFs was decreased from1mm to tens of micrometers to obtain the Micro-/Nano POFs. The macrobending structure of RI sensing function device was prepared by the packaging process. A Ray tracing method wasused for optimizing the structure of macrobending Micro-/Nano POFs and simulating the RIsensing performance. The simulation results coincide well with the experimental results. Akey structure parameter of R/ρ was found, which is the ratio of the radius of curvature of themacrobending fiber to the radius of the fiber itself. It was proposed that the sensor withstructure parameter of R/ρ=20shows the best RI sensing performance. In this condition, therange of RI measurement is from1.33to1.45, and a linear RI sensing trend is observed, witha sensitivity of50dB/RIU. The scale of the RI sensing probe is very small, about (2×1) mm2.The temperature dependence of macrobending Micro-/Nano POFs was studied bytheoretically and experimentally, and the theoretical simulation coincide well with theexperimental results. When the probe working in varied temperature, it is found that thechanges in bending loss induced by temperature are of the same order of magnitude as thoseinduced by RI changes. As a result we successfully extracted the temperature dependent lossof the sensor itself. The RI measurement deviation and temperature correction method of thesensor were presented.Combining the macrobending with the side-polished structure, a POF probe with aside-polished macrobending structure was proposed. A thermal setting method was used tosolve the problem of the mechanical stability of the probe with smaller curvature radius afterside-polishing process. It is found that the performance of the sensor is affected by not onlycurvature radius and polished depth, but also the polished position as well. When thecurvature radius, the polished depth, and the polished position are5mm,500μm, and60°,respectively, the best performance of the probe is obtained. The maximum sensitivity is154dB/RIU, and a linear sensing trend is obtained as well.The temperature dependence of the side-polished macrobending POFs RI sensors wasinvestigated. By extracting the RI variation induced by the thermo-optic effect of liquid, theinfluence of temperature to the RI sensor was obtained, and a temperature correction methodof the sensor was given. It is found that there is a temperature insensitive substance for the side-polished macrobending POF RI sensors. When the fiber was coated with the substance,its transmission characteristics would not be influenced by temperatures, and the phenomenonmay have potential applications in other sensing areas.The temperature dependence of macrobending commercial POF was investigated. It isfound that, for the curvature radius less than20mm, the temperature effect to the transmissionloss of the POF increases with the turns increasing, and the maximum value is0.011dB/°C.For the curvature radius greater than25mm, the effect of temperature on the transmission lossis less. It is less than0.002dB/°C, and for the different turns the temperature dependentmacrobending loss of the POF become same. It is shown that, for reducing the temperatureeffect to the transmission performance, the curvature radius of the POF should be greater than25mm.