The Theoretical Study of an Optimal Distributed Fiber Moisture Sensor Based on the Periodic Polymer Coating

摘要

In our previous works, we presented a theoretical approach to describe the behavior of a single-mode step-index optical fiber or polymer optical fiber such as poly (methyl methacrylate) or PMMA fiber coated by moisture-sensitive polymers (MSPs) and moisture-insensitive polymers periodically. The periodic structure is surrounded by a high young's modulus material such as stainless steel mesh. Absorption of the moisture by the MSPs causes to deform the optical fiber, such that the optical fiber gratings are induced. Induced fiber gratings are the basis of the proposed relative moisture sensor. Due to the low Young's modulus of polymers, the polymer fiber sensors are more sensitive relative to the glass optical fiber sensors. The difference between the coefficient of thermal expansion of different polymers is the origin of errors in the sensor's outputs. In this paper, a theoretical approximate method is employed to optimize the moisture density in the presence of temperature variation and as a result, the error due to temperature change is investigated, and finally, a structure for temperature-compensated distributed optical fiber moisture sensor is proposed. By the development of this numerical approach, it is shown that the proposed sensor can be employed for moisture and temperature measurement simultaneously. The optimal design of a long length distributed optical fiber moisture sensor makes it a good candidate for application in the agriculture fields and gardens' smart irrigation.