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Continuous measurement of small length scale contortions along an arbitrary path is a highly relevant goal within many branches of engineering and technology. An optical fiber-where the probing light propagates within a confined and shielded region-presents an ideal platform for developing the distributed contortion-sensors. In the past, significant progress has been made in developing optical fiber sensors, but a robust and high-resolution distributed contortion-sensor has not been reported in detail. Here, we report the first distributed measurements of fiber contortions with an ultrahigh sensitivity-≤0.3 µm in the transverse plane, 40 µm longitudinal spatial step size, and ≤8 µm resolution for periodic contortions in the longitudinal plane-via a Bragg-grating-inscribed twisted multicore optical fiber. The results are in excellent agreement with the predictions from the Euler-Bernoulli beam-bending model that relates the applied force with the fiber microcontortions. Our distributed-sensor holds promise for a widespread application within a diverse range of fields including biotechnology, robotics, transportation, geology, and security.

A critical limitation for optical fiber sensor technology is the complexity of the interrogators used in such measurements, which has driven continued interest in enhanced optical fibers and fiber assemblies that will simplify interrogator design. In this work, we report on a novel multicore fiber shape sensor utilizing a distal graded index (GRIN) fiber micro-turnaround. We show that four offset cores of this fiber can be interrogated simultaneously with a single high performance optical frequency domain reflectometry measurement. The GRIN turnaround is 498 µm in length and reflects signal from one offset core to an opposite core with a 2 dB roundtrip attenuation. We show that the bend sensing accuracy of our single measurement system is similar to the accuracy of sequential measurements of four individual cores. We also demonstrate fiber shape reconstruction with a single measurement over 0.55 m with 80 µm spatial resolution when the fiber is wrapped around two posts.