Laser Spatial Coherence Suppression With Refractive Optical Elements Toward the Improvement of Speckle Reduction by Light Pipes

Zhaomin Tong,Mei Wang,Suotang Jia,Xuyuan Chen,Yifei Ma,Changyuan Sun

doi:10.1109/access.2019.2956517

Zhaomin Tong, Mei Wang + Show 4 more

Open Access

https://doi.org/10.1109/access.2019.2956517

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Abstract

We study laser speckle reduction by light pipes, and we propose a new method to improve its efficiency. Proof-of-concept refractive optical elements (ROEs) with a staircase-like structure are introduced before a holographic diffuser to split a laser beam into laser sub-beams. Optical paths of the laser sub-beams after transmitting through the ROEs are different, and these partially correlated (or uncorrelated) laser sub-beams are added in intensity basis because of the folded mirror reflections by the light pipe. Thus, laser spatial coherence is suppressed, which helps to reduce speckle. We demonstrate this method in a simplified laser projection system, where subjective speckle contrast is reduced from 0.33 to 0.24 before and after introducing a two-dimensional ROE, respectively. Comparing with other improved speckle reduction methods by light pipes, the proposed method is motionless and simpler.

Highlights

With the development of high-power and low-cost laser diodes (LDs), especially for green and red colors, laser display industry grows fast during the past years [1], [2]
In cases after introducing the ROE1DH, ROE1DV and ROE2D, the spatial coherence of the LD is destroyed because the temporal coherence of the LD is suppressed after bringing optical path differences among the laser sub-beams defined by the refractive optical elements (ROEs) cells [3]; a larger value of Diris introduce more laser spatial coherence suppression, and the objective speckle contrast ratio decreases with the increase of the diameter of the aperture of the iris Diris
We can find that by using the same holographic diffuser with a diffusing angle of θd = 5◦, the lowest value of Nexp corresponds to the condition when LLP = 140 mm at a constant diameter of the aperture of the iris Diris. Based on these experimental results, it is clear that the improvement of speckle reduction with the helps of ROEs is affected by the parameters of the light pipe

Summary

INTRODUCTION

With the development of high-power and low-cost laser diodes (LDs), especially for green and red colors, laser display industry grows fast during the past years [1], [2]. LD modules with more primary wavelengths require the customizations of laser design and fabrication, which are costly; time-average of different speckle patterns is commonly used and is efficient for speckle reduction. The problem for speckle reduction achieved by time-average of different speckle patterns is that actuators or motors are required to drive the device, which are normally noisy and increase the complexity of the projection system. Light pipes are essential optical devices in projection displays to homogenize light [18] Besides this function, we find that with the enhancement of the modal dispersion among the guided wave modes by increasing the working numerical aperture and/or the length of a light pipe, laser speckle at the exit facet of the light pipe can be reduced. Comparing with the time-averaging methods, the method presented in this paper is motionless and simpler, making it more suitable for speckle reduction application in laser displays

DEVICE FABRICATION AND EXPERIMENTAL SETUP

IMPROVEMENT OF SPECKLE REDUCTION USING THE ROEs

CONCLUSION