A liquid crystal device for speckle reduction in coherent light

Thesis

High coherence in laser light causes spatially distributed interference called speckle. In applications such as holographic projection, this undesirable side effect degrades image clarity. The current methods of speckle reduction, such as a rotating ground-glass diffuser, require additional bulky moving parts. In this thesis an alternative technology is presented based upon a compact chiral nematic liquid crystal device. A spatially-random, time-varying phase modulation of the incident light is achieved through the electrohydrodynamic instabilities that are induced by an alternating electric field.

Presented within is a study of material and device properties as a function of speckle reduction and transmission, as well as the electric field conditions required for peak performance. These properties are investigated experimentally and through optical simulation. It is shown that a chiral nematic with short pitch (𝑝 ≈ 250 𝑛𝑚), moderate birefringence (1.5 ≤ Δ𝑛 ≤ 2.0) and minimal rotational viscosity provide maximum speckle reduction while optimising transmission of light through the device. A glass cell thickness of 20 µm is preferred for maximum speckle reduction and cell lifetime, with minimal hysteresis. Increased cell temperature is found to improve speckle reduction and transmission, while slightly reducing the electric field amplitude required for peak performance. This is expected to be largely due to a significant reduction in viscosity with increased temperature. Additional scattering particulates are shown to only improve speckle reduction in materials with low birefringence (Δ𝑛 ≪ 0.2). The ionic dopant CTAB is shown to reduce the threshold for EHDI but has no consistent influence on speckle reduction or field amplitude for peak performance. Furthermore, it is shown that CTAB molecules undergo an irreversible electrochemical reaction at the electrodes, limiting or completely preventing a cell from exhibiting further EHDI.

In this thesis, through material and device optimisation, a LC diffuser is fabricated that is capable of reducing speckle contrast to 𝐶 = 0.07 with transmission of just over 50%.

Authors: Hansford, D

• DOI: 10.5287/ora-xkr2ezxgd
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: dynamic scattering; electrohydrodynamic instability
• Subjects: Liquid Crystals; Laser speckle