The Fabrication of an Eccentric Three-Core Fiber and Its Application as a Twist Sensor

The fabrication and application for twist sensing of an eccentric three-core fiber (3CF) were demonstrated. The fiber was made by a stack-and-draw technique, in which silica rods and core canes were put in a tube and drawn on a fiber drawing tower. Three cores formed a Mach–Zehnder interferometer (MZI), where the lights transmitted in the three cores interfered with each other, resulting in the formation of envelopes on spectrum. Because two of the cores were off-axis, phase differences among the cores varied with twist due to different stretches on each core, which caused shift of the spectral envelopes of the interference signal. Wide-range twist measurement can be realized with relatively high sensitivity by tracking lower dips of the envelopes. Experimental results revealed that the dips shift quadratically with twist angle, which means that the sensitivity increases with twist. The compensation of temperature influence was also implemented by inscribing a Bragg grating on one of the cores with femtosecond laser. Because the fiber can be mass-produced, it is suitable for twist sensing in practical application for its low cost.