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

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 subbeams 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.INDEX TERMS Speckle reduction, light pipe, refractive optical element.

Spatial coherence research and passive speckle suppression for edge-emitting broad-area semiconductor lasers

“…In recent years, passive speckle suppression methods have attracted much attention. These methods do not need moving elements or much time to suppress speckle [21][22][23][24][25][26][27][28], so they are suitable for high-speed imaging and easy to implement. These methods are based on phase delay [21][22][23][24][25][26] or orthometric phase modulation [27,28] to weaken the spatial coherence of the laser.…”

“…These methods do not need moving elements or much time to suppress speckle [21][22][23][24][25][26][27][28], so they are suitable for high-speed imaging and easy to implement. These methods are based on phase delay [21][22][23][24][25][26] or orthometric phase modulation [27,28] to weaken the spatial coherence of the laser. Mode dispersion in MMF is often used to produce partially coherent beams [22][23][24], but more than 10 m of MMF (0.22 NA) is needed to suppress the speckle contrast below 10% for a 0.5 nm 1 e −1 bandwidth laser [22].…”

Record-High Efficiency Speckle Suppression in Multimode Fibers Using Cascaded Cylindrical Piezoelectric Ceramics

“…Numerous endeavors have contributed to suppressing speckle, primarily consisting of static and dynamic approaches. Static speckle suppression approaches involve utilizing wide-spectrum light sources to disrupt the spatial coherence of a laser [6] or employing fiber bundles of varying lengths [7] or refractive optical elements to disturb the temporal coherence of a laser [8]. Dynamic speckle suppression approaches involve the dynamic generation of uncorrelated speckle patterns within the integration time of the detector or eye and, consequently, suppress speckle through time averaging [2].…”

Compound Speckle Reduction in Laser Imaging Systems by Using a Vibrating Multimode Fiber and a Tracked Moving Flexible DOE Loop

“…Angular diversity is one of the most effective techniques, and speckle reduction can be achieved by a diffractive optical element (DOE), diffuser, multimode fiber, light pipe, etc. [10]- [16]. Using a static DOE or diffuser can only produce a finite number of diffracted subbeams, and its speckle reduction ability is insufficient.…”