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arXiv · 2412.13869

Decoherent Focusing Design and Advanced Passive Speckle Reduction for Laser Illumination Systems

A. Lapchuk·O. Prygun·D. Manko·I. Gorbov·Y. Morozov
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Abstract

To effectively apply passive speckle reduction methods, it is essential to use an illumination system that maximally exploits the non-ideal temporal coherence and angular diversity (spatial coherence reduction) of laser light. This study examines the necessary conditions for these factors to act independently to achieve maximum speckle reduction. A novel design for decoherent focusing of laser illumination into a rectangular, uniformly lit spot is proposed. The design is based on an array of rectangular prisms of varying heights and a Fresnel lens comprising crossed 1D cylindrical Fresnel lenses with flat facets. Mathematical modelling demonstrates that such a lens can effectively focus a Gaussian beam into a rectangular uniform illuminated spot, even for lenses with a high numerical aperture (NA = 0.2). A planar implementation of this lens is also proposed. The comb-like spectrum of a laser diode is shown to limit the capabilities of passive methods, as it is challenging to generate a significant number of decorrelated laser sub-beams beyond the available spectral modes. Experimental results confirm that applying a slight modulation (~15%) to the laser diode current during intensity integration transforms the comb spectrum into a continuous one. This enables the generation of an unlimited number of decorrelated laser beams, independent of the laser's spectral modes, to achieve the desired number for a specific optical device.

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A. Lapchuk, O. Prygun, D. Manko, I. Gorbov, Y. Morozov. 2024-12-18. Decoherent Focusing Design and Advanced Passive Speckle Reduction for Laser Illumination Systems. https://doi.org/10.1088/2040-8986%2Fadccd6

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