Speckle- and interference fringes-free illumination system with a multi-retarder plate

Abstract

Our study proposes a novel method for obtaining speckle-free homogeneous illumination using a combination of a multi-retarder plate, a microlens array, a Fourier lens, and a diffraction optical element (DOE) based on pseudorandom binary sequences. The proof-of-concept multi-retarder plate is introduced to generate multiple uncorrelated laser beams, while a mathematical model was developed to explain the method's mechanism and evaluate its effectiveness. In the DOE passive (stationary) mode, the method was found to reduce speckle contrast to 0.167, 0.108, and 0.053 for red, green, and blue laser diodes, respectively. In active mode, the speckle contrast was further reduced to 0.011, 0.0147, and 0.008. The observed differences in speckle contrast in the stationary mode were attributed to variations in the coherence lengths of the RGB lasers. By implementing the proposed method, we successfully generated a square-shaped illumination spot that was free from interference artifacts. The spot obtained exhibited a slow, weak variation in intensity across the screen, attributable to the multi-retarder plate's suboptimal quality. However, this limitation can be readily addressed in future studies through the adoption of more advanced fabrication techniques.

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].…”