We implement the strain and temperature sensors based on multimode interference in perfluorinated (PF) graded-index (GI) plastic optical fibers, and investigate their sensing performance at 1300 nm. We obtain strain and temperature sensitivities of À112 pm="=m and þ49:8 nm= C/m, the absolute value of which are 12.9 and over 1800 times as large as those in silica GI multimode fibers, respectively. These ultra-high strain and temperature sensitivities probably originate from the unique core material, i.e., PF polymer.

Abstract:We develop a single-end-access strain/temperature sensor configuration based on multimodal interference in a polymer optical fiber (POF) with an extremely high sensitivity. The light Fresnel-reflected at the distal open end of the POF is exploited. We obtain high strain and temperature sensitivities of -122.2 pm/µε and 10.1 nm/°C, respectively, which are shown to be comparable to those in two-end-access configurations.

It has been reported that temperature sensors based on modal interference in perfluorinated graded-index polymer optical fibers show extremely high temperature sensitivity at room temperature. In this work, we confirm that the temperature sensitivity (absolute value) is significantly enhanced when the temperature increases toward ∼70 °C, which is close to the glass-transition temperature of the core polymer. When the core diameter is 62.5 µm, the sensitivity at 72 °C at 1300 nm is 202 nm/°C/m, which is approximately 26 times the value obtained at room temperature and >7000 times the highest value previously reported using a silica multimode fiber.

We review the recent advances on strain and temperature sensing techniques based on multimodal interference in perfluorinated (PF) graded-index (GI) polymer optical fibers (POFs). First, we investigate their fundamental characteristics at 1300 nm. When the core diameter is 62.5 μm, we obtain strain and temperature sensitivities of −112 pm/με and +49.8 nm/ • C, the absolute values of which are, by simple calculation, approximately 13 and over 1800 times as large as those in silica GI multimode fibers, respectively. These ultra-high strain and temperature sensitivities probably originate from the unique PF polymer used as core material. Subsequently, we show that the temperature sensitivity (absolute value) is significantly enhanced with increasing temperature toward ∼70 • C, which is close to the glass-transition temperature of the core polymer. When the core diameter is 62.5 μm, the sensitivity at 72 • C at 1300 nm is 202 nm/ • C, which is approximately 26 times the value obtained at room temperature and >7000 times the highest value previously reported using a silica multimode fiber. Then, we develop a single-end-access configuration of this strain and temperature sensing system, which enhances the degree of freedom in embedding the sensors into structures. The light Fresnel-reflected at the distal open end of the POF is exploited. The obtained strain and temperature sensitivities are shown to be comparable to those in two-endaccess configurations. Finally, we discuss the future prospects and give concluding remarks.

To date, we have developed a temperature sensor based on multimodal interference in a polymer optical fiber (POF) with an extremely high sensitivity. Here, we experimentally evaluate the influence of annealing (heat treatment) of the POF on the temperature sensitivity at room temperature. We show that the temperature sensitivity is enhanced with increasing annealing temperature, and that, by annealing the POF at 90°C, we can achieve a temperature sensitivity of +2.17 nm/°C, which is 2.9 times larger than that without annealing (+0.75 nm/°C).