• crystal
• fiber
• laser
• microsphere
• photonic
• trapping

This work covers the first steps in the development of a high resolution, fast temperature sensor suitable for operation in harsh environment, as hydrogen combustion engines
The proposed system consists of a hollow-core photonic crystal fiber (HC-PCF) guiding a micron-sized dielectric sphere through optical forces, that are then used to excite and detect oscillations around trapping positions. Accurate information on the local temperature along the fiber will be inferred from the microsphere motion, by observing the light scattered off the probe at the fiber ends, either through intensity modulation or Doppler shift.
To accurately predict the optical forces on the microsphere inside the hollow core, the guiding properties of the HC-PCF have been characterized through analytical and numerical models. The HC-PCF is usually modeled in literature as a hollow dielectric waveguide, in our analysis we investigated the possibility of modeling it as a capillary step index fiber with properly chosen parameters. As a separate approach, we    computed the optical modes of a numerical toy-model of the HC-PCF fiber, using a commercial software (COMSOL) and compared them to the analytical models. Finally, an accurate numerical model starting from a scanning electron microscope image of a commercially available fiber is presented, with an optimization procedure based on commercially available software.
After a proper characterization of the predicted optical modes propagating in the fiber has been performed, the optical forces necessary to trap a spherical probe in the core of the HC-PCF are presented: both the ray optics regime and Mie scattering have been considered, with the analytical determination of the multipole coefficients for cylindrical waveguide modes. The different treatments have been used to characterize two different optical traps: a standing-wave trap suitable for nano-spheres (a few hundreds of nm radius) and an intermodal beating-based trap, which is suitable for micro-spheres (in the range of a few micrometers radius). These traps can be used to move the probe along the fiber core and then excite longitudinal or transverse oscillations by changing the laser parameters.
The intermodal beating-based optical trap requires a higher order optical mode to be generated and coupled into the fiber separately: the optical mode generation in free space has therefore been investigated and tested experimentally through different diffractive optics-based technologies: a liquid crystal spatial light modulator (LC-SLM) and a digital micromirror device (DMD). The working principles of the two devices are presented, together with various applications that have been tested experimentally: phase only holograms for the LC-SLM, binary amplitude holograms and gray scale amplitude encoding for the DMD.