Abstract
When a coherent beam transmits through a scattering medium, a random intensity distribution known as speckle is generated. Generally, such speckles can be modulated by feedback-based wavefront shaping, as exemplified by the generation of a bright focal spot. However, it is more challenging to generate a specific focal pattern with uniform intensity distribution. In this study, the Hadamard–Genetic algorithm (HGA), namely a hybrid phase modulation scheme of Hadamard encoding algorithm (HEA) and genetic algorithm (GA), is proposed to face these challenges. This algorithm takes advantages of the high intensity of HEA and the high uniformity of GA and outperforms traditional optimization schemes, especially in suppressing intensity fluctuations of the focal pattern. This research is expected to be applied in the field of laser processing and particle manipulation.
Highlights
Focal beams with designed pattern have been widely applied in areas ranging from laser processing to particle manipulation [1,2], such as laser fusion and optical tweezers
Laser beams with uniform intensity distribution are crucial in precise laser processing because the smoothness of processing depends on the uniformity of the intensity distribution
To modulate the random speckle into a uniform focal spot with designed pattern, we introduced two parameters, namely the intensity enhancement η describing the intensity magnitude of the focal spot, and the coefficient of variation characterizing the uniformity of the intensity distribution of the focal spot, which expresses the intensity fluctuation of the focus area
Summary
Focal beams with designed pattern have been widely applied in areas ranging from laser processing to particle manipulation [1,2], such as laser fusion and optical tweezers. Compared with the generation of a single focal spot, in which only the intensity enhancement in a specific point is considered, the balance between the intensity enhancement and uniformity of the whole focal pattern should be considered in the modulation of the speckle into a uniform designed pattern. It can achieve a high SNR [26], and has been applied in suppressing the intensity of a speckle field [27] This method functions suitably for modulating the wavefront to focus the scattering light into a bright focal spot [28]. This algorithm in modulating the speckle into a uniform focal region is examined to confirm whether it outperforms previously-reported algorithms
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