Holographic multimode fiber-based endoscopes aspire to deliver high-quality in vivo imaging inside previously inaccessible structures of living organisms, amongst other insightful applications. At its core are holographically synthesized light fields which, after propagating through a multimode fiber, lead to diffraction-limited foci at desired positions at the fiber output.

Focussing behind multimode fibers results, however, in a high-intensity peak contaminated by a certain level of undesired speckle that extends across the complete field of view, while carrying a significant portion of optical power. The purity and sharpness of the achieved foci are determinant for the imaging performance, leading most of the times to the loss of contrast: a “showstopper” for applications requiring imaging with a high dynamic range. Therefore, amongst other activities, the researchers investigate all fundamental and technological limitations preventing from achieving a perfect focus, free from any undesired contamination.

In this work, the scientists pursue the perfect diffraction-limited focus generated after propagation through a multimode fiber and explore its limitations. They demonstrate diffraction-limited foci containing in excess of 96 % of optical power delivered by the fiber, which represent, to the best of the teams knowledge, the highest value reported to date. Some of the key factors contributing to this result are the ability to simultaneously shape and control the amplitude, phase, and two orthogonal polarization states of the light field entering the multimode fiber.

In a nutshell, this work provides a guideline to consistently achieve high purity foci with reproducible performance and contains an extensive quantitative and qualitative study on the impact of various conditions of the experimental procedure. Such practical learnings are an essential step towards transferring ideas to instruments. The results have already shortened the path of the technology to the users in neuroscience within the framework of Leibniz IPHTs transfer endeavour “DeepEn”.