Fiber Photonics

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 high refractive index 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 how the geometry of the coupled multi-core fiber affects its temperature properties. Adjusting core radius and 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.