Fiber Optic PH Sensor Based On Evanescent Wave Absorption

pH value of solution is important that the solution of the chemical parameters pH. pH value measurement and control in chemistry, biochemistry, clinical chemistry, environmental science and other fields are particularly important. pH value of the traditional method of testing has a long time to respond, and is also poor stability, difficult to detect small changes and rapid changes. At the same time, because of large measurement matrix, does not apply to micro-environment and in vivo biology of online testing, more difficult to adapt to the adverse environment applications.Fiber optic pH chemical sensor, is a pH indicator and the combination of optical fiber sensing technology consisting of a new type of pH detection technology. Contrast to electrochemical analysis of the Traditional technology, fiber-optic pH chemical sensor has a series of outstanding advantages, such as small size, is not charged, anti-electromagnetic interference performance, non-polluting and so on. Fiber-optic pH sensor has broad application prospects in various types of chemical reactions, environmental monitoring, biomedical field and underground mines and other dangerous weapons, testing the remote control aspects of occasions. Thereby it has be exposed to a wide range of international importance.In this paper, we set up an evanescent wave absorption-based fiber-optic pH sensors based on wave propagation in the fiber and fiber-optic evanescent field in the theory. After corrosion of the HF, the cladding was removed from the of core of fiber optic, Through the sol-gel method of coating the surface of the core and embedding the pH dye to form a new cladding. The sol-gel membrane, is a porous membrane, the dye-embedded in the membrane can be sensitive to the pH value, and light absorption of evanescent field of the dye depends on the pH value of solution, that is, the concentration of hydrogen ions. So we can detect the light intensity of evanescent waves to examine the changes in the PH value of solution.In this paper, the production of fiber-optic pH sensor and the experimental system of liquid pH measurement method are at the core. At the same time introduce the progress of fiber-optic pH sensor, the evanescent field of fiber-optic, sol-gel coating theory and methods, a variety of reaction conditions affect on the performance of the sensitive membrane and so on. As follows:1 using optical fiber to measure liquid pH value is belong to the scope of sensor, so first of all, in the introduction part of the article there is an overview of fiber-optic sensors. Introduce the development of fiber optic sensor in China and abroad, the basic principles and classification, as well as its prospects for a wide range of applications.2. For the shortcomings of current pH value detection methods and equipment characteristics, According to the characteristics of fiber-optic sensors, we propose to use fiber-optic chemical sensor for detection of liquid pH value. Especially its unique advantage in physiology, medicine in vivo monitoring and industrial process, environmental protection, monitoring of on line. carry out a systematic overview on history of the fiber-optic pH sensor development, classification, response mechanism and applications.3. From the light propagation theory and the wave theory of step-index fiber, we discuss the evanescent wave formation mechanism and evanescent field characteristics, which the evanescent wave transmission depth is important to the production of sensor, so we carry out a detailed analysis. Finally, we establish a Theoretical model of a fiber-optic sensor based on evanescent wave absorption.4. We Introduce the Production of sensor probe, Including two parts, corrosion and coating, Corrosion Part include corrosion principle and corrosion rate control of HF. In the sol-gel coating part, we introduce the principle of sol-gel reaction, discuss reaction conditions and production processes (such as viscosity, pulling rate, aging time and gel drying temperature, humidity, etc.) on affect of membrane performance in details.5. design the sensor experiment system, which including the choice of light source, Photoelectric-detector and coupling method of light. In this paper, we application semiconductor laser as the light source that its wavelength is 650nm, the chose of light wavelength depends on the Characteristics of absorption spectra of dyes. However, temperature changes effect the output power of semiconductor lasers which we used in room temperature, so in experiment, we design double optical paths difference compensation system and subtract amplifier electric circuit to eliminate experimental error because of the temperature drift of light source.The fiber-optic PH sensor which we product for measuring the liquid pH value were studied, through processing and analysis of the experiment data, we calibration the sensor performance. We can measure when pH sensor in the liquid pH value of 4.0 ~ 10.5, the output light intensity (here is the voltage difference) with the liquid hydrogen ion concentration changes, we also fit measurement results. To the repeatability, linearity and sensitivity, hysteresis and response time of the sensor, we carry out a detailed analysis, comparison of the sensor sensitivity when the coating is different length and thickness. We can conclude that in the same conditions, the more thicker of the membrane(the depth within evanescent wave transmission), the more greater of sensitivity.In this paper, compare to other electrode for pH value measurement, the measurement system which we designed has some advantages, such as simple structure, wide measuring range, low demand of light Source property, low production costs, and good flexibility, corrosion resistance, high sensitivity, free from electromagnetic interference, safe, reliable, and may constitute a network of optical fiber sensing, and many other advantages. Because we have received optical signals with the pH changes in liquid, the optical signals then change to electrical signals, so it's facilitate to Connect with the computer, to realize online measurement and bio-sensing. And it has broad application prospects in chemistry, biochemistry, clinical chemistry, environmental science and other fields.we have to improve this experiment as ollows:(1) Because the pH-sensitive membrane is Often in liquid for a prolonged period, the embedded dyes will often leak in liquid, leakage of dyes will affect long-term stability of the sensor, in order to prevent leakage of sensitive membrane, there are a number of relevant reports, but it also needs a long-term research efforts.(2)In this paper, although the fiber-optic pH sensor has basic functions to measure pH value in liquid, but the noise of photoelectric-detector affects the accuracy of the measurement results, so how to design the circuit to improve the sensitivity of detector and reduce the noise need to strive.(3)For the sensor probe, we need to design a better package approach so as to achieve the detection of pH value in scarce liquid ,thus we can improve practical level of the sensor.