Study On Fiber Optic PH Sensor In Long Period Fiber Grating Based On Intelligent Hydrogels

The pH value is a critical factor affecting physical, chemical and biological reactions in the environment. It is of significance to environmental protection, aquaculture and crop-plantation. The conventional instruments of measuring pH values are pH meter, pH test paper and pH indicator. Although these instruments have been widely used, there still limitations such as complexity of calibration and inconvenience of maintenance. Meanwhile, fiber optic sensors have been introduced and actively investigated due to their advantages such as insensitivity to electromagnetic fields. Therefore, a long period fiber optic p H sensor based intelligent hydrogel has been designed and studies in this paper.With a careful theoretical analysis of the theory of long period fiber grating and intelligent hydrogel, we designed a pH sensor based on the deposition of intelligent hydrogel on the cladding of a long period fiber grating. The key material, i.e. intelligent hydrogel, which is mainly made from N-isopropylacrylamid and sodium alginate, can respond to the pH change within the range from 0 to 14. By means of spiral coating under the condition of ultraviolet light, an uninterruptible spiral-coating-layer structure is constructed. Experimental results have confirmed that the LPG-based pH sensor can realize the successful measure of the entire range of pH values, corresponding to the max sensitivity of 148.96pm/pH in this field.In practice, the center wavelength in transmission spectrum will change not only with the pH value but also the temperature, i.e. the cross-sensitivity. The temperature sensitivity of 58.27pm/ ℃ in the experiment confirmed the existence and the interference of cross-sensitivity. To solve the problems mentioned above, several solutions have been proposed in this paper and we choose one of these to design a double long period fiber grating pH sensor. By using VPI software to simulate and data fusion processing, we confirmed that this method can point us in the direction of a good solution to reduce cross-sensitivity.