Control of Laser Speckle Noise by Using Polymer-Dispersed LC

Fells

Applied Physics Letters

et al. 2016

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. Here, we present an alternative technology based upon a compact, electrohydrodynamic chiral nematic liquid crystal device. A spatially random phase modulation of the incident light is achieved through the electrohydrodynamic instabilities that are induced by an alternating electric field. Using a chiral nematic liquid crystal device that is doped with an ionic compound, we find that the speckle contrast can be reduced by as much as 80%.

mentioning

confidence: 99%

“…[11][12][13][14] For example, Furue et al achieved 40% speckle reduction by applying an alternating squarewave electric field to a ferroelectric LC, combined with a wedge cell and polarizer. 11 They later reported 25% speckle reduction with an electrically addressed polymer-dispersed nematic LC device.…”

mentioning

confidence: 99%

Speckle contrast reduction of laser light using a chiral nematic liquid crystal diffuser

Fells

Applied Physics Letters

et al. 2016

“…To date, there have been numerous reports demonstrating speckle reduction using LC devices and materials. For example, previous studies have shown a reduction in the speckle contrast using: (1) a nematic LC with photo-isomerisable alignment layer that enables two orthogonal polarisation states to be created thereby leading to polarisation diversity 15 ; (2) a chiral smectic ferroelectric LC (FLC) with an alternating field applied that creates a spatially and temporally random refractive index across the cell 16 – 18 ; (3) surface and/or polymer stabilised FLC 19 for polarisation diversity; (4) nematic LCs mixed with photocurable monomers for light scattering 20 , and (5) the use of a LC spatial light modulator (SLM) that applies multiple random phase masks corresponding to the Hadamard orthogonal function to create statistically independent speckle patterns at the observer 21 . These techniques, whilst they show promise, are not without their limitations such as a small amount of speckle reduction, complex electric field profiles, and/or the use of expensive and bulky components (such as an SLM).…”

Section: Introductionmentioning

confidence: 99%

Enhancing laser speckle reduction by decreasing the pitch of a chiral nematic liquid crystal diffuser

Jin

et al. 2021

Sci Rep

The artefact known as speckle can plague numerous imaging applications where the narrow linewidth of laser light is required, which includes laser projection and medical imaging. Here, we report on the use of thin-film chiral nematic liquid crystal (LC) devices that can be used to mitigate the influence of speckle when subjected to an applied electric field. Results are presented which show that the speckle contrast (a quantitative measure of the presence of speckle) can be significantly reduced by decreasing the pitch of the chiral nematic LC from 2700 to 244 nm. Further reduction in the speckle contrast can be observed by operating the diffuser technology at a temperature close to the chiral nematic to isotropic transition. At such temperatures, we observe a simultaneous improvement in the transmission of light through the device and a decrease in the electric field amplitude required for the minimum speckle contrast value. We conclude by presenting a laser projected image of the 1951 USAF target with and without the LC device to demonstrate the visual improvement as a result of the speckle reduction.

“…By virtue of their electro‐optic properties, liquid crystals (LCs) are appealing for the reduction of the speckle contrast and as a result have been considered for this purpose by numerous research teams. [ 6–15 ] An example of previous research in this area involved the use of a nematic LC device with a controllable alignment layer consisting of polyvinyl alcohol (PVA) and an azo dye. [ 6 ] Exposing the device to ultraviolet (UV) light resulted in a reorientation of the LC director through the PVA/azo dye alignment layer enabling access to two orthogonal polarization states and thus giving rise to a reduction in the speckle contrast of

1 / \sqrt{2}

.…”

Section: Introductionmentioning

confidence: 99%

“…have also reported speckle reduction using FLCs that are either surface or polymer stabilized as well as the use of nematic LCs dispersed with a photocurable monomer. [ 10,11,13 ] Other techniques have included the addition of nanoparticles to LC materials. [ 14,15 ] Alternatively, LC spatial light modulator (SLM) technology has been used to generate multiple random phase masks corresponding to the Hadamard orthogonal functions.…”

Section: Introductionmentioning

confidence: 99%

Laser Speckle Reduction Using a Liquid Crystal Diffuser Enhanced with Redox Dopants

Jin

Advanced Photonics Research

et al. 2021

Herein, a large reduction in the speckle noise is observed using a thin electroresponsive film consisting of a chiral nematic liquid crystal (LC) that has been enhanced with the addition of a redox dopant. Two different redox dopants are investigated over a range of concentrations, one being an electron acceptor and the other being an electron donor redox dopant. Results are presented that show that the incorporation of either of these dopants leads to a greater reduction in the speckle contrast than that observed using just the chiral nematic LC host when subjected to electrohydrodynamic instabilities. Furthermore, it is found that the permanent electrochemical reactions typically observed when ionic dopants, such as cetyltrimethylammonium bromide, are used are not observed for these devices, resulting in a considerable improvement in terms of the operating lifetime of the speckle reducer technology. To conclude, results that show that the speckle contrast can be reduced to C = 0.11 ± 0.02 at a temperature of 30 ºC are presented and the improvement of the quality of an image generated using a modified commercial projector fitted with a monochromatic laser source is demonstrated.