Light in an optical fiber is like a stream: it flows inside a glass thread and doesn't spill out. A thick thread carries many streams at once, a thin one only one. Astronomers often need to pour light from a thick fiber into several thin ones to split it into colors. For this, they have long used a device called a photonic lantern: it gently, without splashing, guides light from a single thick fiber into a bundle of thin ones.

Now engineers have made the lantern work in reverse. They took several thick fibers and joined them into one thick fiber. The light from all the fibers merges into a single flow, like several streams joining a river. Losses are less than ten percent — almost all the light reaches the end. They built such a device and tested it at different wavelengths, and it works equally well.

Why is this needed? Large telescopes collect light over a big area, but getting it into instruments is tricky. The new lantern allows combining light from several mirror segments without loss and without complex alignment. This is a step toward telescopes that don't require perfectly uniform light at the input — making them easier to build and maintain.