Optical fiber humidity sensors

Optical fiber sensors (OFS) have evolved mainly from the fiber communications technology and have experienced remarkable growth since the 1970s. OFS is suitable for use as the sensing instrument for applications in extraordinary and harsh environments, such as in civil engineering structures, high-voltage and biomedical instrumentation, mining environments, and agriculture ecosystems. Humidity sensors are among the applications of OFS that have been intensively studied by many researchers to cater to the high specific demand of the sensing industry. This chapter first provides humidity measurement-related parameters and the advantages of the fiber sensor for humidity sensing over the conventional electronic sensors. This is followed by comprehensive reviews of the development of various fiber humidity sensors in accordance with coated and noncoated fiber humidity sensors. Their operating principles, various sensing schemes, and related optical components are described. In addition, the performance and applications of the sensor schemes are discussed to investigate which are the most effective. Finally, the gap in the literature is identified and a possible solution is provided. All the sensors reported in the literature are presented in designated tables to allow a broad evaluation and cross comparison.