A novel fiber-optic bending sensor based upon the propagation of LP21 mode is demonstrated. The sensor, comprised of an S-bend fiber on an elastic film, measures LP21 mode specklegram rotation, which increments linearly with bending angle by the stress-optic effect. The sensor is capable of experimentally achieving a sensitivity as high as 4.13 rad/m(-1). The theoretical analysis of the sensor, which is a combination of fiber coupled-mode theory and elastic-optic theory, validates the accuracy of the sensor. The sensor is also shown to be temperature-immune, and can detect both bending direction and bending angle with a large dynamic range. Furthermore, the sensor implementation incorporates inexpensive single-mode fiber at 650 nm for few-mode operation, enabling low-loss transmission and compatibility with existing interfaces.

https://dr.ntu.edu.sg/bitstream/10220/10937/1/26.%20Fiber-optic%20bend%20sensor%20using%20LP21%20mode%20operation.pdf

Fiber-optic bend sensor using LP21 mode operation.

An integrated structure in the original single mode fiber (SMF) which can measure curvature and temperature simultaneously, is proposed and demonstrated The key components of the device include an in-fiber Mach–Zehnder interferometer (MZI) and a fiber Bragg grating (FBG), which are integrally fabricated in a coating-unstripped SMF by femtosecond laser direct writing The curvature sensing of the device is based on the wavelength shift of the MZI interference fringe dips It is found that the bending response is closely related to the bending orientation The measured curvature sensitivities are 2324 and 2491 nm/m−1 for the 0° and 180° orientation in the range of 0–10 m−1, respectively Under different bending states, the temperature response of the FBG resonant wavelength is almost the same and the mean temperature sensitivity is 972 pm/° The advantages of compact, simple fabrication, low insertion loss, no assembly, wide operating range of curvature measurement and real-time temperature sensing make our device very attractive in practical engineering monitoring and somatosensory posture recognition

Compact Fiber Curvature and Temperature Sensor Inscribed by Femtosecond Laser Through the Coating

We report a new method for constructing a single fiber optical tweezers, which can realize multi-dimensional manipulation of trapped yeast cells by using a LP11 mode beam excited in a normal communication single core optical fiber. This allows a simple and convenient orientation control on the trapped yeast cells. The LP11 mode beam, both for generating trap and orientation manipulation, has been modulated by using the tension and twisting loaded on the fiber. We present experimental results of controllable deflection and orientation manipulation of the yeast cells. To the best of our knowledge, it is the first report about the trapped yeast cells being driven by the normal single fiber optical tweezers in multi dimensions, and it constitutes a new development for single fiber optical trapping and makes possible of more practical applications in the biomedical research fields.

Multi-Dimensional Manipulation of Yeast Cells Using a LP $_{11}$ Mode Beam

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.

/pdf/probing-micron-scale-distributed-contortions-via-a-twisted-2a0l7m980y.pdf

Probing micron-scale distributed contortions via a twisted multicore optical fiber

Although experimental advances in the implementation and characterization of fiber speckle sensor have been reported, a suitable model to interpret the speckle-pattern variation under perturbation is desirable but very challenging to be developed due to the various factors influencing the speckle pattern. In this work, a new methodology based on the finite element method (FEM) for modeling and optimizing fiber specklegram sensors (FSSs) is proposed. The numerical method allows computational visualization and quantification, in near field, of changes of a step multi-mode fiber (SMMF) specklegram, due to the application of a uniformly distributed force line (UDFL). In turn, the local modifications of the fiber speckle produce changes in the optical power captured by a step single-mode fiber (SSMF) located just at the output end of the SMMF, causing a filtering effect that explains the operation of the FSSs. For each external force, the stress distribution and the propagations modes supported by the SMMF are calculated numerically by means of FEM. Then, those modes are vectorially superposed to reconstruct each perturbed fiber specklegram. Finally, the performance of the sensing mechanism is evaluated for different radius of the filtering SSMF and force-gauges, what evidences design criteria for these kinds of measuring systems. Results are in agreement with those theoretical and experimental ones previously reported.

Numerical modeling of fiber specklegram sensors by using finite element method (FEM).

Twisting and bending characteristics of low-multimode LP21 mode propagation in optical fibers is presented for the first time. Theoretical fiber mode modeling, combining geometrical rotation with opto-elastic effects, demonstrates that the propagation of the LP21 mode is bending-effect-immune. Experimental testing verifies that the LP21 mode specklegram rotates 0.9112 of the fiber twist angle in a fused silica fiber, independent of any fiber bending. This characteristic allows for the LP21 mode to be highly applicable in fiber specklegram sensors.

George Wu

Papers

Fiber-optic bend sensor using LP21 mode operation.

Effects of twisting and bending on LP21 mode propagation in optical fiber.