In this dissertation, liquid crystal materials and devices are investigated in meeting the challenges for photonics and communications applications. The first part deals with polymer-stabilized liquid crystal (PSLC) materials and devices. Three polymer-stabilized liquid crystal systems are developed for optical communications. The second part reports the experimental investigation of a novel liquid-crystal-infiltrated photonic crystal fiber (PCF) and explores its applications in fiber-optic communications. The curing temperature is found to have significant effects on the PSLC performance. The electro-optic properties of nematic polymer network liquid crystal (PNLC) at different curing temperatures are investigated experimentally. At high curing temperature, a high contrast, low drive voltage, and small hysteresis PNLC is obtained as a result of the formed large LC micro-domains. With the help of curing temperature effect, it is able to develop PNLC based optical devices with highly desirable performances for optical communications. Such high performance is generally considered difficult to realize for a PNLC. In fact, the poor performance of PNLC, especially at long wavelengths, has hindered it from practical applications for optical communications for a long time. Therefore, the optimal curing temperature effect discovered in this thesis would enable PSLCs for practical industrial applications. Further more, high birefringence LCs play an important role for near infrared photonic devices. The isothiocyanato tolane liquid crystals exhibit a high birefringence and low viscosity. The high birefringence LC dramatically improves the PSLC contrast ratio while keeping a low drive voltage and fast response time. A free-space optical device by PNLC is experimentally demonstrated and its properties characterized. Most LC devices are polarization sensitive. To overcome this drawback, we have investigated the polymer-stabilized cholesteric LC (PSCLC). Combining the curing temperature effect and high birefringence LC, a polarization independent fiber-optical device is realized with over 30 dB attenuation, ~12 Vrms drive voltage and 11/28 milliseconds (rise/decay) response times. A polymer-stabilized twisted nematic LC (PS TNLC) is also proposed as a variable optical attenuator for optical communications. By using the polarization control system, the device is polarization independent. The polymer network in a PS TNLC not only results in a fast response time (0.9/9 milliseconds for rise/decay respectively), but also removes the backflow effect of TNLC which occurs in the high voltage regime.

An active core fiber optic gas sensing technique has been developed by using a photonic crystal (PC) hollow core fiber (HCF) as a transducer and a tunable diode laser as a light source for multi-gas sensing. The intrinsic optical absorption signal of an analyte molecule in the near nfrared region is monitored for sensing C2H2,CO2 and NH3. Although the overtone absorptions are known to have low absorption cross-sections, this sensor can detect these gas components down to the parts-per-million (ppm) level by using a 1-meter hollow core fiber as a transducer. This sensor is an example of application of PC-HCF to gas sensor design. The sensitivity of this gas sensing technique can be improved by introducing periodic openings along the fiber, decreasing the hole diameter down to 0.5 mm and using a longer hollow optical fibers. Other advantages of this gas sensing technique include less interference, fast response and potential applications like high temperature, remote and corrosive gas sensing.

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Microstructure and photonic crystal fibers with periodic as well as random refractive-index distributions are investigated. Two cases corresponding to fibers with one-dimensional (1D) radial index distributions and two-dimensional (2D) transverse index distributions are considered. For 1D geometries with an arbitrary number of cladding layers, exact analytical solutions of guided modes are obtained using a matrix approach. In this part, for random index distributions, the average transmission properties are calculated and the influence of glass/air ratio on these properties is assessed. Important transmission properties of the fundamental mode, including normalized propagation constant, chromatic dispersion, field distributions, and effective area, are evaluated. For 2D geometries, the numerical techniques, FDTD (Finite-Difference Time-Domain) method and FDM (Finite Difference Method), are utilized. First, structures with periodic index distributions are examined. The investigation is then extended to microstructure optical fibers with random index distributions. Design of 2D microstructure fibers with random air-hole distributions is undertaken with the aim of achieving single-mode guiding property and small effective area. The former is a unique feature of the holey fiber with periodic air-hole arrangement and the latter is a suitable property for nonlinear fiber devices. Measurements of holey fibers with random air-hole distributions constitute an important experimental task of this research. Using a section of a holey fiber fabricated in the draw tower facility at Virginia Tech, measurements of transmission spectra and fiber attenuation are performed. Also, test results for far-field pattern measurements are presented. Another objective of this dissertation is to explore new applications for holey fibers with random or periodic hole distributions. In the course of measuring the holey fibers, it was noticed that robust temperature-insensitive pressure sensors can be made with these fibers. This offers an opportunity for new low-cost and reliable pressure fiber-optic sensors. Incorporating gratings into holey fibers in conjunction with the possibility of dynamic tuning offers desirable characteristics with potential applications in communications and sensing. Injecting gases or liquids in holey fibers with gratings changes their transmission characteristics. These changes may be exploited in designing tunable optical filters for communication applications or making gas/liquid sensor devices. / Ph. D.

Research towards the development of a gas-discharge fibre laser using noble gases, with target emission wavelengths in the mid-IR. Additional and separate work on gas treatment methods for managing the formation of photo-induced defects in silica glass.

The absence of appropriate media to cultivate photons efficiently at the micro or nano scale has hindered taking the full advantage of processing information with light. The proposal of such a medium for light, known as photonic crystals (PCs)--multi-dimensional artificially periodic dielectric media--brings the possibility of a revolution in communications and sensing much closer. In such media, one can manipulate light at a scale on the order of the wavelength or even shorter. Applications of PCs other than in communication include bio-sensing because of the peculiar properties of PCs such as the capability of enhance field-matter interaction and control over the group velocity. As a result, PC waveguide (PCW) structures are of interest and it is expected that PC sensors offer the feasibility of multi-analyte and compact sensing schemes as well as the ability of the detection of small absolute analyte quantities (nanoliters) and low-concentration samples (picomoles), which may be advantages over conventional approaches such as fiber optic and slab waveguide sensors. Depending on the nature of the analyte, either dispersive or absorptive sensing schemes may be implemented. Light propagation is controlled fully only with 3D PCs. One of the problems arising due to reducing the dimension to 2D is that PCs become strongly polarization sensitive. In many cases, one wants to implement polarization insensitive devices such that the PC provides a full band gap for all polarizations. To address this problem, a novel type of PC called annular PC is proposed and analyzed. The capability of tuning the TE and TM polarizations independently within the same structure provides great flexibility to produce polarization-independent or polarization-dependent devices as desired. PCW bends are expected to be the essential building blocks of photonic integrated circuits. Sharp corners having small radii of curvature can be obtained. To enhance the low-loss and narrow-band transmission through these bends, PC heterostructures waveguide concept is introduced. We show that in PCWs formed by joining different types of PCs in a single structure, light can flow around extremely sharp bends in ways that are not possible using conventional PCWs based on a single type of PC.