Investigation On Structural Modification Of The Plastic Optical Fiber And Its Refractive Index Sensing Performance

Refractive index(RI)is one of the important optical parameters of materials and substances,other parameters,such as concentration,temperature,and pressure can be deduced by measuring the RI.The RI sensing technology can be widely applied in biology,chemistry,health care,and many other fields.The RI measurements based on optical fiber sensing technology have the advantages of fast response,immunity to electromagnetic interference,and remote operation ability,which have been widely investigated.Although many kinds of optical fiber based RI sensors have been proposed,most of them are based on glass optical fibers.With the pursuit of low cost and a single disposable for the biochemical sensing,plastic optical fibers(POFs)based RI sensors have attracted a lot of attention.They are the ideal fibers for the low cost solution of RI sensing,due to the merits of low price,easy to process and connect,and operation in the visible light,etc.In this paper,in order to realize the low cost and high sensitive measurement of RI,the POFs were structural modified by the methods of tapering,side-polishing and pressing,respectively.We proposed and fabricated the POFs RI sensors based on three different structures: the tapered POFs based RI sensors,the side-polished POFs based RI sensors,and the POFs long period grating(LPG)based RI sensors.The RI sensing performances of these sensors were investigated,by optimizing the structural parameters,the RI sensing performances were improved.The main contents of this paper are as follows:The tapered POFs with different taper waist were fabricated by the heat and pulling method.The thermal setting process was used to obtain a U-shaped sensing probe.The RI sensing performance was improved by combining the tapering and macrobending structures.The effects of the taper waist,macrobending curvature radius,and the fiber with cladding or without it were analyzed.The experimental results showed when the macrobending curvature radius and the taper waist were 2.0mm and 250 ?m,respectively,the highest sensitivity of the probe with cladding reached 937%/RIU with the resolution of 7.11×10-4 RIU in the RI range of 1.333-1.410.By removing the cladding,the RI sensing range was expanded to 1.333-1.450,but the sensitivity was about 800%/RIU which was lower than the ones with cladding,and the resolution was 5.55×10-4RIU.In addition,the temperature dependence of the sensor was investigated,and a temperature correction method was given.A side-polished POF was fabricated and proposed for liquid's RI measurement.The RI sensing performances of the straight and U shaped side-polished POFs were studied.The experiment results showed the straight side-polished POFs were not sensitive enough for the RI sensing,but after bending it into U-shaped probes,the sensitivity was improved markedly.Thus,the RI sensing performances of U-shaped side-polished POFs were detailed studied.Not only the RI sensitivity was analyzed by changing the structural parameters,but also the resolution,linearity,repeatability and temperature characteristics were studied.When the polished depth was 400 ?m,polished length was 10 mm,and curvature radius was 2.0 mm,the RI sensing performance was the best,the sensitivity and the resolution were 864%/RIU and 3.3×10-4RIU in the RI sensing range of 1.333-1.450,respectively.The effects of the polishing roughness on the RI sensing performance were also analyzed.A simple die-press-print methd for fabricating the multi-notched LPG on the POFs was explored.The RI sensing performances for the straight and macrobending POFs based LPG were investigated.The results showed that the straight POFs with LPG were not sensitive enough to the RI changes,but they could obtain a greater RI measurement range.In the RI range of 1.333-1.410 and 1.410-1.450,the sensitivities were 143%/RIU and 287%/RIU,respectively.After bending them into U-shaped probes,the RI sensing performances were improved obviously in the RI range of 1.333-1.410.And the RI sensing performance could be optimized by changing the structure parameters of the POF.When the curvature radius was 3 mm,the LPG depth was 200 ?m,the LPG length was 10 mm,and the LPG period was 1 mm for the U-shaped probe,the RI sensitivity and resolution were 1130%/RIU and 8.44×10-4RIU,respectively.In addition,the linearity,repeatability and temperature stability of the POF probe were given.