A New Method In Measuring Liquid Concentration Based On Sensing Technique Of Biconical Fiber Taper

The concentration characterizes one of medium colligative property main parameters, solution concentration's survey holds the important status in the modern industry o Solution concentration's measurement and the control in papermaking, chemical industry, sugar manufacturing, dairy products, drugs manufacture and so on have wide applications, it is the important technological means that guarantees and improves the product quality.In this article we use biconical fiber taper for measuring liquid concentration, using the taper region's sensitivity to the external environment, when the external environment changes, the transmission characteristics of light in the taper region will change.We change the refractive index of liquid around the taper region by changing the liquid concentration, in order to influence the transmission characteristics of light in the taper region indirectly, so we can detect the liquid concentration around the taper region via testing the output power of biconical fiber taper.Biconical fiber taper is different from the ordinary cylindrical optical fiber, its structure including three segments—convergent region, taper waist and diffusion zone. The two transition regions look like two cones. With the fast development of optical communication and integrated optics, biconical fiber taper is widely applied as one kind of new optical fiber in optical fiber's beam splitting and connection, medicine, light sensing, optical solition communication and so on, for its unique transmission light way.In the liquid concentration measurements by biconical fiber taper, we used laser diode as light source, used optical power meter as a detector, so it belongs to the optical fiber sensor category. The basic principle of fiber sensor is sending the light source into the modulation region through optical fiber where the measured parameters outside interact with the light to enter the modulation region, so that the optical properties such as light-intensity, wavelength (color), frequency, phase, the polarization state and others changes to modulated signal which will be sent to photodetector, demodulator via fiber , to have an access to the measured parameters.Modulation region of the optical fiber sensor could be fiber itself (internal modulation), or sensitive components made of other materials(external modulation). But as for the principle of modulation, the sdudies are only on the changes in the nature of light causing by external factors, and modulation is the same whether internal or external.the modulation used to measure liquid concentration by biconical fiber taper is internal.At present, the main direction of development of the fiber-optic sensor technology is: (1) multi-purpose. Namely a fiber optic sensor can measure not only one kind of physical quality but also a variety at the same time. (2) Improve the spatial resolution and sensitivity of distributed sensors, reduce its cost, design complexity of sensor network project. Pay attention to the effcts on the fiber made by the parameters of distributed sensor, namely, pressure, temperature, in particular chemical parameters (hydrocarbons, some pollutants, humidity, PH value, etc). (3) The development of novel sensing materials and sensor technology. (4) The development and application of low-cost sensors (stent, connectivity, installation) in bad conditions (high temperature, high pressure, chemical corrosion). (5) Fiber-optic connectors and other micro-combined with micro-optical technology.In this paper, the measurement of liquid concentration using biconical fiber taper is studied experimentally at the core, and at the same time a fiber-optic mode theory, the principle of optical fiber sensors and applications, biconical fiber taper processing methods, and several other areas are introduced. Details are as follows:1. Measuring liquid concentration by biconical fiber taper pertains to sensors as large category, so that in the introductory part of the article, the definition of sensors is presented firstly, meanwhile, optical fiber sensor's hold pivotal position in the sensor field is discussecd on.2. From the basic equation of the optical fiber, we made strict derivation for fiber modal theoretics, they are basis of the analysis carried out to biconical fiber taper. The transmission principle of light in fiber is total internal reflection of light, and fiber taken as sensitive componentssensor of fiber sensor, some of its characteristics, such as loss and dispersion are very important, therefore, the paper also gave details of their presentation.3. Measuring liquid concentration by biconical fiber taper pertains to fiber sensor category, fiber optic sensors have high sensitivity, high-precision, high-speed, high-density, adapt to harsh environments, as well as non-contact, non-destructive, easy to use and others for point. Discussed the principles of fiber optic sensor,starting from the fiber-optic sensor's two categories—functional and non-functional fiber-optic sensors. Fiber have a certain effect for many parameters outside ,in fact,study optical fiber sensing principle is just the study of how apply these effects to achieve "Biography" and "sense" function for measured parameters outside world. This is the core of optical fiber sensor, but also the main difference between fiber-optic system and optical fiber communication systems.4. From three angles—the normalized frequency, die-field radius and the effective refractive , discussed the transmission characteristics of biconical fiber taper and derived its mode coupling equation based on fiber-optic mode theory.Introduced biconical fiber taper processing methods,including the abrasive, chemical etching solution and melting Rafah, and specified the melting Rafah in which CO2 laser,fusion splicer and burners are taken as heat .As were drawn under different time, speed,pull, as well as positions where fiber was placed on flam each time , fibers drawn had different geometric properties. Place the fiber drawn under electron microscope for observation and measure the length of different parts of fiber taper region.5. Made experimental study on measuring liquid concentration with biconical fiber taper, and measured the optical power outputting from biconical fiber taper changing with the concentration sugar and salt solution, obtaining a relatively high sensitivity and a relatively large dynamic range. And compare the conditions that optical transmission ratioes change with liquid concentration for biconical fiber tapers that have different waist lengths.