The image distortions caused by the inherent mode dispersion and coupling of the multimode fiber (MMF) lead its output light field to be scattered and prevent it from applicating in endoscopy. Although various wavefront shaping methods have been proposed to overcome these image distortions and form the focused spots through the MMF, they a re usually time-consuming due to the multiple iterations and tedious calculation. In this paper, we present a binary amplitude-only modulation parallel coordinate algorithm for focusing and scanning light through a multimode fiber (MMF) based on the digital micro-mirror device (DMD) in a reference-free multimode fiber imaging system. In principle, our algorithm is capable of efficiently calculating the masks to be added to DMD for yielding a series of tightly focused spots; and for the same number of modulation sub-regions, our method is more than M (the number of focused spots) times faster than the amplitude iterative optimization algorithm. In the experiment, efficient light focusing and scanning at the distal end of the MMF without the iteration process are demonstrated. Furthermore, we demonstrate that the proposed method can also be extended to focus and scan light at multiple planes along the axial direction by just modifying the input wavefront accordingly. We predict the high-speed focusing method through the MMF might have the potential application for fast spot-scanning imaging.
At present, there are some problems in the digital scanning imaging methods of multimode optical fiber, such as poor quality of focused spot, large amount of calculation and long time in the forming process of focused spot. In order to solve the above problems, a parallel phase compensation method based on liquid crystal spatial light modulator (LCSLM) is proposed to achieve fast point-focused scanning of multimode fiber (MMF). The parallel algorithm, which includes the two processes of collecting online specklegrams and calculating offline phase masks, calculates the compensation phase masks so that the phase masks can be loaded on SLM in turn to generate a series of focused spots at different predefined positions, which greatly saves time. Experiments demonstrate that this method doesn’t need many iterations, and the phase compensation mask used for focusing spot at a predefined position on the MMF output facet can be obtained by using a two-step phase-shifting technique, and a series of phase compensation masks can be quickly obtained by using the parallel method, so as to form focused spots at different predefined positions. Experimentally, we obtained 100 focused spots, the average focused efficiency was 30.12%, the average focused diameter was 2.3382μμm, and the quality of the focused spot was improved compared with previous reports.
The image distortions caused by the inherent mode dispersion and coupling of the multimode fiber (MMF) lead its output light field to be scattered and prevent it from applicating in endoscopy. Although various wavefront shaping methods have been proposed to overcome these image distortions and form the focused spots through the MMF, they are usually time-consuming due to the multiple iterations and tedious calculation. In this paper, based on the phase-only modulation of the spatial light modulator (SLM) and the parallel algorithm, we experimentally demonstrate the highspeed focusing through the MMF without the iteration process. Our parallel algorithm, including the online speckle recording process and the offline compensated phase calculation process, allows it possible to calculate the compensated phase masks for generating several focused spots at different predefined positions at one time, which is quite timesaving. Experiment shows that a series of focused spots at arbitrary target positions at the MMF output facet are efficiently generated by just one-loop accessing the SLM and the speckle capturing-device and one-time phase retrieval. Besides, focusing at different depths along the output light axis of the MMF is demonstrated by modifying the compensated phase masks. We predict the high-speed focusing method through the MMF might have the potential application for fast spotscanning imaging.
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