We design and carry out the optical fiber sensors employing discoloration ink, which are particularly suitable for deploying in the optical fiber networks for water-immersion and temperature -overload sensing.
A conventional water immersion fiber sensor is based on the fiber bending mechanism, and it is monitored by an optical time-domain reflectometer (OTDR). However, such sensor immersed in water has a remarkably high bending loss. Thus, many events of water immersion occurring could not simultaneously be monitored by an OTDR. In general, temperature fiber sensors are somewhat complicated and expensive.
In this thesis, we propose two types of OTDR-based optical fiber sensors, which are especially suitable for simultaneous multipoint sensing. To install them in the quasi-distributed branch networks can form mesh sensing networks, reaching their sensing ranges and avoiding the failure of sensing points introducing an interruption to the rear sensing networks. Once the branched sensing points are damaged or immersed in water/overloaded in temperature, their respective states can quickly be observed from an OTDR, and then those sensing points can easily be located and repaired to facilitate the creation and maintenance.
The proposed sensors, which possess low-cost, compact and easily do-it-yourself (DIY) features, consist of a fiber connector with either 8-degree angled physical contact (APC) facet or physical contact (PC) facet and a 3-mm thickness of discoloration ink, for example, soaking discoloration ink and temperature discoloration ink. Because of the inner structure of discoloration ink, it naturally presents a state of diffuse reflection, easily leading the reflected light into the fiber; therefore, the OTDR can obtain a relatively high pulse-height. However, when the sensors were in a temperature-overload or water-immersion state, the ink changed its color and presented a transparent state, which leads to the reduction of diffuse reflection and produces a higher return loss for sensors to make the pulse-height measured by an OTDR decrease.