Liquid Crystal Devices for Optical Communications and Sensing Applications

Abstract

This thesis is focussed on the design and development of liquid crystal based tunable photonic devices for applications in optical communications and optical sensing, with an emphasis on all-fiber device configuration. The infiltration of liquid crystals into photonic crystal fiber provides a suitable common platform to design and fabricate simple and compact all-fiber tunable photonic devices which can be easily integrated with optical fiber networks and sensing systems. Based on the infiltration of liquid crystals – materials with high dielectric anisotropy, into photonic crystal fibers a common platform is developed to address the need for electronically tunable photonic devices with a compact all-fiber device configuration. A ferroelectric liquid crystal based tunable filter is theoretically studied and experimentally demonstrated for applications in the demodulation of multiple Fiber Bragg Grating sensors. The electrical tunability of liquid crystal infiltrated photonic crystal fiber is employed for the development of all-fiber tunable photonic devices for a variety of applications in optical communication systems. A nematic liquid crystal infiltrated photonic crystal fiber based all-fiber broadband electronically controlled variable optical attenuator is demonstrated in the wavelength range from 1500 nm – 1600 nm. With smectic liquid crystal infiltration the electrical tuning of photonic bandgap is achieved and an all-fiber tunable notch filter for applications in optical communication is demonstrated. A novel technology for all-fiber based electric field sensing is developed with the use of nematic liquid crystal infiltrated photonic crystal fibers. A simple and compact all-fiber sensor head is demonstrated, which allows for the accurate measurement of electric field intensity, along with detection and measurement of electric field signal parameters such as frequency, amplitude and also the direction of the electric field. Studies performed on the transmission and reflection responses of the sensor, demonstrate the capability of the simple and compact all-fiber electric field sensor to operate in both in-line and end-point type configurations. The effect of the applied electric field frequency on the propagation properties of liquid crystal infiltrated photonic crystal fiber is studied in the range from 50 Hz to 1 kHz. With the use of a suitable fitting function on the time varying transmission response of the fiber, it is demonstrated that the parameters of the applied electric field such as frequency and amplitude can be measured. Selectively infiltrated photonic crystal fibers are studied in a polarimetric electric field sensing scheme and it is demonstrated that the optimization of the length of the infiltrated section of the photonic crystal fiber subjected to an electric field allows to obtain a linear transmission response for the sensor in a given electric field range. The directional electric field sensitivity of a liquid crystal infiltrated elliptical core photonic crystal fiber is studied and a true all-fiber directional electric field sensor is demonstrated which is capable of simultaneous detection and measurement of the amplitude and direction of an applied electric field.