Research On The Piastic Optical Fiber Liquid Refractive Index Sensing Technology Based On Evanescent Field Effect

Refractive index is a basic optical parameter and the measurement of it has important applications in many areas of chemical, biological, environmental monitoring and food safety and so on. The optical fiber based refractive index sensors have advantages of small size, light weight, immunity to electromagnetic interference, corrosion resistance and remote operation, which has attracted great research interests. As regards the plastic optical fiber, which is more suitable for use in refractive index sensors with merits of big numerical aperture, high elastic strain range, good flexibility, low cost and ease of processing characteristics. In recent years, research on refractive index sensing technology based on plastic optical fiber is becoming significant.The basic optical parameters and model theory of plastic optical fiber is firstly introduced in this thesis and the characteristic equation of the model field is simulated by using MATLAB software to study the electric field of LP01 and LP11. In addition, the field distribution under different normalized frequencies is also numerically calculated. Then, the optical fiber sensor based on the principle of evanescent field effect was discussed and a variety of optical fiber tapers can be fabricated by a special designed home-made optical fiber tapering machine. A series liquid refractive index sensing experiments were conducted by using the tapered fiber as the sensing head. In contrast, a D-shaped plastic optical fiber sensing head was fabricated by using side-polishing techniques. The sensing properties of these two refractive index sensors were studied. The main experimental results are as follows:(1) For the tapered plastic optical fiber refractive index sensor:the transmitting optical power will increase with the refractive index of the external environment decreasing since the travelling light in the fiber will attenuate during the tapered region. The transmitted light power was measured under three wavelengths (532nm,633nm and 780nm). Experimental results show that the optimum wavelength is 633nm since the sensor has the highest sensitivity compared with the other two wavelengths. Further studies showed that when the diameter of the tapered region is smaller, the linearity of the sensor will increase and the sensitivity of this sensor will also improved to a certain degree. A double-tapered structure of plastic fiber was also studied and the sensitivity and linearity of the sensor will be better improved, e.g. the sensitivity will be up to 955uw/RIU for double tapered structure.(2) For the D-shaped plastic optical fiber refractive index sensor:when the refractive index of the external environment changes from 1.33 to 1.44, the transmission loss and refractive index exhibited a good linear relationship. Four different depths of D-shaped groove ware used to implement this experiment to find the optimal depth for the sensor. When the groove depth of D-shaped fiber was 500um, we got the highest sensitivity of the sensor. Different curvature radius of D-shaped fiber was realized by winding the fiber around a round bar in order to find the optimal sensitivity rate. And experimental results showed that the smaller curvature radius of D-shaped fiber, the higher sensor sensitivity.Therefore, in order to improve the sensitivity and linearity of the sensor, we can properly reduce the tapered region’s diameter and increase the number of the tapered region for the case of tapered plastic optical fiber refractive index sensor. And for the D-shaped plastic optical fiber refractive index sensor, by changing the depth and curvature radius of the optical fiber sensing head, the sensor sensitivity can be dramatically optimized.