High-speed light focusing through multimode fiber for spot-scanning imaging

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.