Investigation On Periodic Structure Modification Of The Plastic Optical Fibers And Their Refractive Index Sensing

Refractive index(RI)is one of the most important optical parameters of materials,and its measurement can effectively determine the physical properties of materials such as concentration,sugar content,pH values,and so on.Therefore,the RI sensing technology can be widely exploited for the identification of specific substances,detection of solution concentration,the control of food quality,and so on.Over the years,fiber optic RI sensors have been widely developed due to their advantages of light weight,high sensitivity,immunity to electromagnetic interference,and remote operation.At present,most of the fiber optic RI sensings are realized on glass optical fibers(GOFs).However,the rigidity and poor flexibility of GOFs make it difficult to be modified.The plastic optical fibers(POFs)are with the features of softness,strong plasticity and easy to modify,which have attracted much attention in RI sensing technology.In addition,the larger diameter of the commercial POF makes it easy to couple and align with the light source and detector.It is an ideal fiber for achieving a simple structure and low price RI sensors.Fiber long period grating(LPG)can couple the core mode transmitted in the fiber into the cladding modes,which causes the losses at a specific wavelength.The cladding modes are highly susceptible to changes of the external environment.Therefore,the LPG is an optical device which is very sensitive to the changes of environmental RI.In this paper,the periodic structure modification of commercial POF was carried out,and two types of POF RI sensors which are based on the corrugated surface LPG and the screw-shaped structure were proposed.The RI sensing performances of the two kinds of POFs were investigated,respectively.The main results obtained are as follow:A corrugated surface LPG was fabricated on POF by a mechanical die press print technology,and the optical transmissions of the fabricated LPGs were characterized by the transmission spectrum and the scattering light photographic method.The results showed that there were not significant resonant characteristic loss peaks in the transmission spectrum of the LPG fabricated on multi-mode POF,and a loss took place when the light transmitted through the LPG.The LPG fabricated on a thin POF and with a short period exhibited a higher loss,which was basically consistent with the theoretical simulation results.The research results showed that an LPG fabricated on a thin POF and with a short period exhibited a better RI sensing performance,and increasing the groove depth could improve the RI sensitivity.In addition,the RI sensing performance of a tilted LPG was also investigated.The results showed that the RI sensitivity could be further improved by increasing the tilted angle.For the LPG fabricated on the POF with a diameter of 0.25mm case,when the grating period was 100?m,the groove depth was65?m,the length was 20mm,and the tilted angle was 20°,the highest sensitivity of2815%/RIU with a resolution of 1.39×10~-44 was obtained in the RI range of 1.33-1.45.Additionally,the temperature dependence of the LPG based RI sensor in the temperature range from 20 to 60?was studied as well,and a temperature correction method was given.By introducing a corrugated surface LPG on the D-shaped and U-shaped POFs,the D-shaped and U-shaped POFs assisted by LPGs were prepared and their RI sensing performances were studied.The results showed that the RI sensitivity could be improved significantly by adding the LPG to D-shaped and U-shaped POFs.For the D-shaped POF assisted by an LPG case,the D-shaped structure assisted by an LPG fabricated on a thin POF exhibited a better RI sensing performance,and the RI sensitivities of 2676%/RIU and 9786%/RIU were obtained in the RI ranges of1.33-1.40 and 1.40-1.45,respectively.But for the case of the U-shaped POF assisted by an LPG,the U-shaped structure assisted by an LPG fabricated on a thick POF exhibited a better RI sensing performance.When the probe with cladding,the highest RI sensitivity reached up to 1087%/RIU in the RI range of 1.33-1.40.For the probe without cladding,the RI sensitivity was 708%/RIU which was lower than that of the ones with cladding.But the RI measurement range was expanded from 1.33 to 1.45.In addition,the influences of temperature on RI sensing performances of the D-shaped and U-shaped POFs assisted by an LPG was also investigated,respectively.A heat pressing and twisting method was used to modify a POF into a screw shape.The RI and liquid-level sensing performances of the so-obtained screw-shaped POFs were studied,respectively.It was found that the screw-shaped POF fabricated by twisting a thin flat-POF exhibited a better sensing performance,and the RI and liquid-level sensing sensitivity could be improved markedly by removing the cladding.For RI sensing,the highest RI sensitivity reached up to 4399%/RIU when the length of the screw-shaped POF was 15mm and the screw pitch was 2mm.For liquid-level sensing,when the screw pitch was 3mm,the highest liquid-level sensitivity and the resolution were 0.52dB/mm and 0.02mm,respectively,as the sensor was immersed into the water.In addition,the effect of water absorption for the screw-shaped POF on the liquid-level sensing performance was studied.The results showed that the material water uptake cannot be ignored.