OrbitFlySens - Optical manipulation of flying microparticles in the hollow core of a photonic crystal fiber by orbital angular momentum modes for innovative fiber sensing applications

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Conventional fiber optic sensors rely mostly on backscattering processes or discrete sensing elements, e.g., fiber Bragg gratings, in sensor fibers. The research project OrbitFlySens pursues a novel sensor concept based on optically trapped microparticles orbiting in hollow core fibers. By exploiting the influence on the particle motion of physical quantities such as temperature, pressure, electric field or impurities along the hollow core fiber, the "flying" microparticles can be used as moveable microsensors that in principle can be propelled and read out optically over kilometer-long hollow core fibers. The extension of the previous concept of axially-moved particles by an additional orbital motion decouples the axial motion used for particle positioning from the actual sensing mechanism. This enables an improved spatial resolution in the µm range, a more flexible application and the vectorial determination of electric fields. In the course of the project HiFlySens excellent results have been achieved for the axial localization of particles and the control of their motion along the fiber. For stationary particles a spatial resolution in the range of the particle size was accomplished, for moving particles, for example in the context of temperature measurements up to 200°C, in the sub-mm range. The achievable sensor resolution is significantly limited by the axial particle motion required for the previous sensing mechanism. Therefore, within the project OrbitFlySens an orbital particle motion shall be used for sensing. For this purpose, the orbital angular momentum (OAM) of a laser beam will be transferred to an optically levitated particle in a hollow core fiber for the first time. For the overall goal of developing an orbiting flying particle sensor, the following research objectives are proposed: (I.) theoretical investigation of an orbiting particle optically trapped in the hollow core fiber by an OAM beam, (II.) generation of OAM modes and design of a twisted hollow core fiber that preserves OAM, (III.) exploration of a rotation detection scheme to measure the instantaneous orbital frequency of an optically levitated particle and its position in the fiber, (IV.) simultaneous control and measurement of the orbital and axial particle trajectories, and (V) demonstration of a sensing application. The two project partners plan to continue the cooperation of the HiFlySens project. The obtained knowledge as well as the expertise in particle localization (Schmauss) and hollow core fibers (Joly), will allow the sensor concept from HiFlySens to be extended with a 10-times enhanced spatial resolution. In addition, the novel principle also enables measurements of the electric field in terms of direction, magnitude, and phase in parallel to temperature measurements. For demonstration purposes, a sensor for combined temperature and electric field measurements with an application perspective in the energy sector is built and characterized.