Liquid crystal devices for laser and optical beam-steering applications

Thesis

This thesis explores the properties and applications of liquid crystal (LC) lasers, with a focus on their integration into optical beam-steering systems and the establishment of a free-space optical communication link using LC-based beam-steering. LC lasers, known for their unique optoelectronic properties, offer several advantages, including compactness, flexibility, and tunability, making them promising candidates for next-generation light sources in applications such as display technologies, biomedical imaging, and optical communications. Despite considerable progress, there remains a limited understanding of the polarisation states and coherence characteristics of LC lasers. This research addresses this gap by systematically studying the polarisation state and temporal coherence of band-edge LC lasers, using techniques such as Stokes parameter measurements and Michelson interferometry.

Challenges like pulse-to-pulse variations and wavelength mismatches between LC lasers and measurement systems were successfully overcome. The key findings reveal that band-edge LC lasers are nearly circularly polarised, with the same handedness as the LC helical structure. Several factors, including excitation fluence, LC layer thickness, and alignment quality, were identified as influencing the circularity and degree of polarisation of these LC lasers. Regarding temporal coherence, the longest coherence length of 275.32 µm was observed at a layer thickness of 10.91 µm. Thinner or thicker LC layers resulted in broader spectral linewidths and reduced temporal coherence, although the excitation fluence of the pump laser had little effect on the coherence length.

The insights gained on polarisation properties provide a basis for the development of a two-dimensional optical beam-steering system for LC lasers, employing polarisation gratings and tunable LC waveplates. This system preserves the laser wavelength and characteristics with minimal impact on intensity. The far-field beam-steering pattern was validated using a Jones matrix simulation, which explained the appearance of additional diffraction orders beside the primary beam.

The same beam-steering method was applied to both the transmitter and receiver in a free-space optical communication link, leading to substantial improvements in the field of view and link margin of the system. This approach is low-cost, lightweight, and requires only simple voltage controls. To demonstrate its viability, the thesis presents a link capable of transmitting 50 Mbps pseudo-random binary sequence signals over a distance of 1 metre with a bit error rate below $10^{-3}$ in a point-to-point scenario. The beam-steering system, with a steering bandwidth of 58.82°/sec, can be adapted for different transmission rates and wavelengths, offering significant flexibility for applications such as indoor mobile terminals.

Authors: Zhang, G

• DOI: 10.5287/ora-b7wgp14my
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English