Liquid Refractive Index Sensor Based On Biconical Plastic Optical Fiber

There are more advantages of utilizing plastic optical fiber to make sensor than that ofthe quartz fiber. Usually the numerical aperture of the plastic fiber is bigger, so it,s relativelyeasy to pour the light source into the plastic fiber or coupling with each other. In addition,the plastic optical fiber,s quality per unit is small, and the advantages of chemical resistanceand impact resistance, tensile strength and bending resistance of plastic fibers makes it moresuitable for sensor than quartz fiber.With the consideration of experiment, processing or dealing with plastic optical fiber issimple than quartz fiber, because plastic optical fiber is much flexibility than glass, not easyto break. In view of this we choose the plastic optical fiber for measuring the refractive indexof liquid. The measurement of refractive index is very important in practical such aschemical testing, chemical biology with the growth of the bacteria in real-time, environmentmonitoring of water quality inspection, even in beverage industry also should be demandedto test the refractive index.In this paper we provide a kind of refractive index sensor utilizing the plastic fiber. Bydealing with the plastic optical fiber can achieve the purpose of making the fiber besensitivity with the outside refractive index in order to detect the index changing of outside.In this experiment, heating stretching method is used to reduce the diameter, making theplastic optical fiber appearing the structure of cone, we have successfully made the fiberdiameter of core with200um and5mm in total length and this section of cone is used as thesensor head, the waist size of cone and the diameter of the fiber can be effectively controlledwith the experimental device introduced in this paper.Except that, this paper also use the software of matlab to simulate the ratio changing ofthe power in the cladding with the power of the total in the fiber cases of different indexes.Then we simulated the changing of the ratio in the caces of different core diameter andwavelengths for changing the experimental conditions to explore the characteristics of thesensor in the back.In this experiment, by making a certain volume concentrations of glycerin solution, thefiber refractive index of solution can be changed from1.335to1.417with0.015as aninterval. And with the solution which has been prepared changing the refractive index of the sensing head’s out environment. The light will be modulated before coupling into the opticalfiber, and the output power at the other end of the fiber can be transformed into voltagesignal and displayed on the digital oscilloscope. By observation of the changes of the voltage，the transmission loss of the sensor will also be recognized. Experiment result has shown thatthe increasing of the RI is linearity with the transmission loss. And the sensitivity of thesensor made by the fiber fabricated can be reached to18dB/RIU.In addition to the above experiment, this paper also considered the diameter of thefiber and different wavelengths of light effecting the experimental result, exploring thereaction of the outside index of with different wavelengths, using three different wavelengthsto do the experiment respectively. Experiment shown that with the increasing wavelength ofincidence laser, the experimental curves of the slope increases, too.When exploring the characteristics of the plastic optical fiber sensor in different corediameter conditions, the selection of optical fiber straight core diameter are200um300umand400um, the experimental results show that the sensor with fiber core of smaller indiameter, and the measurement of the index of refraction of the outside world sensitivity willincrease.Considering the influence of temperature on the experiment we first let the temperatureof the sensor area rise up from16to61, from the changing intensity of the lightobserved indicated that the influence of temperature on the experiment is only0.003dB/and can be ignored. To detect the transmission loss of the fiber when detecting the RI of thesolution, we should put the sensor into the solution, and the temperature changing of thesolution can lead the changing of the transmission loss. So we put the taper section into thesolution, heating the solution, and the results show that the loss caused by temperatureexceeded no more than0.04dB/, for the stability of the experiment the factor oftemperature can be ignored.The first part of this paper is the introduction part and introduces the development ofthe fiber optic sensor characteristics and application scope, then simply introduced the statedevelopment of the refractive index sensor. The second part is the presentation of thecharacteristics of plastic optical fiber, mainly on the material loss and this is very importantto make using plastic optical fiber to made refractive index sensor. The third chapterestablishing the basic theory of model about the refractive index sensor using the waveoptics theory, then simulate with the model; Chapter4of this article mainly introduced theexperiment result and analysis, finally discusses the different temperatures and the different wavelengths of light to the influencing of the experiment, through get riding of the influencefactors of the experiment to achieve the purpose of accurate measurement.