We are going to present an iterative fourier transformation algorithm (IFTA) based computation method for phase retrieval of beam splitter elements. The power distribution between the separate beamlets will be controlled via a camera feedback loop. To start the closed loop an initial phase close to the desired one is needed to ensure the stability of the algorithm. This phase will be computed via a mixed-region-amplitude-freedom (MRAF) type IFTA. After a fixed number of MRAF-iteration cycles the closed loop starts. The initial phase will be given to a spatial light modulator (SLM) located in the entrance plane. A picture of intensity distribution of the exit plane (fourier plane) is then taken via a CCD camera. This distribution will replaces the theoretical one and together with the theoretical phase in this plane will be fed back into a standard type IFTA. The new phase in the entrance plane of each subsequent cycle will then be given to the SLM in the same manor. We will show that this approach can compute the desired phase for beam splitters with close to ideal distribution even if the input beam profile and phase is unknown and regardless of the imaging quality of the used SLM. Furthermore we will show some parallel micro structuring examples with femtosecond laser beams utilizing this algorithm. All experiments will be conducted with imaging close to the resolution limit.
A. Kratsch, F. Ehrenberg, A. Engel, and S. Weißmantel, "Using a camera feedback loop for phase retrieval in an iterative Fourier transform algorithm to calculate beam splitter phase gratings for parallel laser microstructuring (Conference Presentation)," Proc. SPIE 10519, Laser Applications in Microelectronic and Optoelectronic Manufacturing (LAMOM) XXIII, 105190D (Presented at SPIE LASE: January 29, 2018; Published: 14 March 2018); https://doi.org/10.1117/12.2288372.5751434147001.
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