As the bit rate of optical fiber communication channel increases, optical communication networks become sensitive to polarization mode dispersion, polarization-dependent loss, polarization-dependent gain, and polarization-dependent modulation. Communication systems with these polarization-induced problems need dynamic polarization controllers that may be implemented by liquid crystal(LC) cells, $LiNbO_3$ waveguides, fiber squeezers, and Faraday rotators.
Liquid-crystal-based devices may offer some potential advantages: high electro-optic response, low power consumption, and low-cost fabrication. In this thesis, we have proposed and fabricated an in-line polarization controller that uses a hollow optical fiber (HOF) filled with a nematic liquid crystal, fabricating the thin-film electrodes on the cladding of the HOF. The proposed polarization controller is a phase-retardation-control type in which the principal axis of phase retarders is fixed. The controller consists of three control sections that operate as phase retarders. The principal axis of the second control section is rotated by 45 ˚with respect to that of the first section and the third section, while the principal axis of the first section is parallel to that of the third section. The principal axes of the three control sections are formed by an average arrangement of LC directors. Therefore the principal axes are determined by the position of the thin-film electrodes fabricated on the cladding. The rotation of LC directors gives the phase retardation in each section, which is controlled by the applied electric field.
The initial alignment of LC directors depends highly on the surface interaction between LC and the hole of the HOF. We used a photo-alignment method that LC directors are aligned parallel to the axis of the HOF. In the photo-alignment, 2 wt% poly(vinyl cinnamate) is used as a photo-polymer, and the light source is 320nm linearly polarized ultraviolet light. The thin-film electrodes were fabricated on the cladding by using a diamond-shaped hole structure formed in the silicon substrate by deep reactive ion etching and anisotropic wet etching. The fabricated device was pigtailed with standard single-mode fibers by use of UV-curable adhesive. The fiber-to-fiber insertion loss of the 35-mm-long device was 8 dB. The measured propagation loss of the HOF filled with the LC, ZLI1800-100, was 1 dB/cm. From the result we estimate that the loss due to mode mismatch and misalignment was 2 dB on average at the input and output ends. The performance of the proposed device was demonstrated on the $Poincar\acute{e}$ sphere monitored by a polarization analyzer. The full wave retardation voltage of the polarization controller is 85 V and the response time is 25 ms. The transformations of polarization from an arbitrary state to a desired state were demonstrated by using the control sections.
