Οπτικές ίνες φωτονικού κρυστάλλου με συντονιζόμενες ιδιότητες πόλωσης

Abstract

This doctoral thesis focuses on the theoretical study and analysis of novel types of photonic crystal fibers (PCFs) with tunable polarization properties. Four different PCF designs are proposed, which are demonstrated to support single-polarization (SP) and/or polarization-maintaining (PM) guidance, as well as the thermal and electrical dynamical control of their polarization properties, such as the value of modal birefringence, the polarization axes and the transition among different operational states. The first PCF type consists in an index-guiding PCF enhanced with a hollow core capillary, which is infiltrated by a nematic liquid crystal (LC). By proper selection of the material refractive indices SP guidance is achieved, while the allowed polarization axis is controlled via the application of an external electric field. Both uniform and more realistic nematic direct profiles are investigated. The reduction of the central defect hole radius leads to PM properties, with finely adjustable high values of modal birefringence. A final example also demonstrates the capability of switchable operation between an off-state and SP/PM guidance. The impact of the infiltration of the cladding capillaries in a honeycomb photonic bandgap fiber with a nematic material is investigated in the next section. The selection of the material indices in this case as well is proven to be crucial in terms of the fiber’s polarization properties, owing to the different effective indices sensed in the cladding holes by light of orthogonal polarization. Both SP and PM guidance is predicted, with very high levels of birefringence in the later case. The possibility to thermally tune the value of birefringence and to operate between zero- and high-birefringence states is also assessed. The third example of the proposed polarizing PCFs is based on a hybrid mechanism approach supported by an accordingly designed liquid-crystal infiltrated PCF. By letting the refractive index of the glass lie between the indices of the nematic material a tunable electrical transition is demonstrated in terms of the fiber’s polarization state. As the intensity of the applied field increases, the fiber progressively passes through a zero- to high-birefringence state, followed by a SP interval, and finally extremely-high birefringence operation. The fiber’s electro-optic response is investigated both considering constant tilt angles of the LC molecules, and realistic director patterns calculated for various values of the applied voltage. A concurrent combination of favorable characteristics is demonstrated, namely guidance in a solid glass core, the elimination of the need for selective infiltration or specific anchoring conditions, moderate LC molecule switching values, and both wavelength- and voltage-selective polarization properties. In a different context, a fourth design is finally proposed, considering long period gratings (LPG) inscribed in an index-guiding highly-birefringent PCF with liquid-infiltrated holes. The geometrical birefringence of the PCF induces the splitting of the resonance LPG wavelegths corrsponding to the two orthogonal polarizations of the fundamental mode. Thus, a simple polarizing filter results, whose operation wavelength may be adjusted by properly selective the LPG period. The impact on the polarization properties of such fibers is thoroughly studied with respect to the infiltration of the cladding capillaries with an isotropic optical liquid. The thermooptic response of the liquid is shown to lead to extensive thermal tunability of the resonance wavelengths. Two different designs, aiming at maximizing the grating’s thermal sensitivy and at providing linear low-loss response are presented. The fiber’s sensitivity is also discussed in the context of highly accurate refractometric applications. In conclusion, four different designs of tunable polarizing PCFs have been presented, based on liquid-crystal infiltrated and long period grating liquid-infiltrated fibers. The utilization of such fibers might prove indispensable in a wide range of applications where extensive control of light’s polarization state is a prerequisite. These include, for instance, coherent communication systems, fiber-based gyroscopes and interferometers, fiber laser systems, photonic switching devices and optical coherence tomography or quantum cryptography applications.