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

David J Hansford,Steve J Elston,Stephen M Morris,Yihan Jin

doi:10.1038/s41598-021-83860-3

Abstract

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.

Highlights

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
We showed that a positive dielectric anisotropy chiral nematic liquid crystal (LC) with a pitch of 250 nm and doped with an ionic dopant can cause a spatially and temporally random phase perturbation to incident laser light when operated in a dynamic scattering mode by subjecting the LC to a low frequency (< 100 Hz) square wave electric field of sufficiently large a mplitude[22]
The image appears dark, in accordance with a nematic LC aligned in the homeotropic state when viewed between crossed polarisers, except for the bright circular regions which correspond to the distortion in the director field around the spacer beads

Summary

Introduction

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. 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. The variation in amplitude and phase that arises due to path differences between the observer and these ‘secondary emitters’ leads to a random amount of constructive and destructive interference, which in turn causes a random noise pattern across the observed light field This granular intensity profile has been given the name ‘speckle’[1]. These different methods are not mutually exclusive and can be used in combination to further reduce speckle contrast: the total speckle reduction is typically the product of the speckle reduction

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Conclusion