Tiny hologram inside a fiber lets scientists control light with incredible precision

In a new study published in Light: Science & Applications, researchers led by Professor Markus A. Schmidt from the Leibniz Institute of Photonic Technology and Friedrich Schiller University, Jena, Germany have introduced a novel solution: the tunable Metafiber. This fully fiber-integrated device uses a 3D nanoprinted phase-only hologram directly on the end face of a dual-core fiber to achieve remote focus control by simply adjusting the relative power between the fiber's guided modes.

The hologram is designed to be sensitive to changes in the interference pattern of the light emitted from each core, enabling a shift in the focal spot's position without the need for any moving parts. Experimental results confirm that precise and continuous focus modulation of over 3 microns can be achieved while maintaining high beam quality.

This new approach allows for compact, robust, and fast tunable focusing using optical fibers, significantly advancing the field of reconfigurable photonics. Potential applications include high-speed optical trapping, integrated endoscopic tools for minimally invasive diagnostics or surgery, and improved signal routing in fiber communication systems.

The Metafiber's tunability arises entirely from power modulation -- a method much faster than traditional mechanical or liquid-crystal-based approaches -- and is compatible with existing fiber systems. This makes it ideal for rapid implementation in both research and industrial applications.

The study marks a milestone in on-fiber photonic integration and opens exciting avenues for developing next-generation fiber-based optical systems.

An illustrative image of the power-controlled, fully fiber-integrated spatial focusing using a phase-only 3D nanoprinted hologram coupled to a single-mode dual-core fiber is shown in Fig. 1. To illustrate the functional principle, two focusing light beams related to two relative power differences of the guided modes (green and red) are shown by the yellow and red magenta areas (dashed dark blue line: central fiber axis). The middle left inset shows an example of the intensity distribution of the interfered Gaussian beams in the hologram plane when the power in the modes is equal. The top right (scale bar: 20µm) and center (scale bar: 100µm) insets show images of the nanoprinted 3D hologram and the expansion section on the end face of the dual-core fiber, respectively. The bottom inset shows the spatial focus tuning with a total focus shift of more than 3 µm by adjusting the relative power difference of the two fiber cores.