Temperature Sensing Characteristics of a Four-Core Sapphire Derived Fiber Based on Supermode Interference

A Mach–Zehnder interferometer (MZI) sensor based on a coupled four-core sapphire-derived fiber (FSDF) for temperature sensing is proposed and demonstrated. The coupled FSDF with a high refractive index (RI) difference between core and cladding can support four LP01-like supermodes (LP01s modes) and eight LP11-like supermodes (LP11s modes). We have fabricated and investigated two sensor samples with a sensing length of 3.1 cm (Sensor I) based on LP01s-LP11s mode interference and with a sensing length of 9.0 cm (Sensor II) based on LP01s-LP01s mode and LP11s-LP11s mode interference. The high-temperature behaviors of both sensors were studied after annealing at temperatures up to 900 °C. The experimental results show that the temperature sensitivity of Sensor I is around 70 pm/°C. For Sensor II, multiple supermode interferences are involved providing different sensitivities. The sensitivity based on the first-order LP01s and second-order LP01s interference is about 136 pm/°C, and the sensitivity based on the first-order LP01s and fourth-order LP01s mode interference is about 36 pm/°C. The sensitivity based on two LP11s mode interference achieves about 64 pm/°C. The experimental results are compared to theoretical simulation. The theoretical simulations give additional information on how the geometry of the coupled multicore fiber affects its temperature properties. Adjusting the core radius and the core pitch is shown to have a considerable influence on the achievable sensitivity and may even change the sign of the sensor sensitivity to temperature. Therefore, designing a specific geometric structure of the coupled fiber is beneficial to optimize the sensing characteristic of this type of sensor.