First demonstration of a multimode-to-multimode photonic lantern for astronomy

Telescopes are the great eyes of humanity, allowing us to peer into the deepest corners of the universe. To see the faintest, most distant objects, astronomers have long sought to build instruments with ever-larger mirrors. However, constructing a single, massive mirror is incredibly difficult and expensive. A more flexible approach involves building a modular telescope, which combines light from several smaller, separate mirrors into one powerful beam. The challenge lies in how to gather that light and merge it without losing any of it. In the past, scientists often tried to squeeze light from these large, blurry views into very thin, single-mode fibers, but the turbulence of Earth's atmosphere scrambles the light so much that it cannot fit into such narrow channels without expensive correction systems. A newer idea involves using a device called a photonic lantern, which acts like a traffic director for light, guiding it smoothly from many paths into one. While these devices have been used to split light into many paths, they had not been successfully used to combine multiple beams of light into a single, larger beam for astronomy until now.

A team of researchers has now demonstrated the first working version of a multimode-to-multimode photonic lantern designed specifically to combine light from several large fibers into one. Instead of trying to force light into a tiny, restrictive channel, their device takes light from seven separate input fibers and fuses them together into a single, larger output waveguide. The team built these devices using a process that slowly tapers the seven fibers together, melting them into a single, solid structure where the light can travel as one unified beam. They tested these new lanterns by shining light through them at different colors, measuring how much light made it from the entrance to the exit. The results were striking: the devices allowed more than 90 percent of the light to pass through with very little loss. This high efficiency proves that the light moves smoothly through the transition, staying confined within the new, combined structure rather than leaking out.

The researchers constructed these lanterns to support the MARCOT-Pathfinder telescope, a project currently operating at the Calar Alto Observatory in Spain. This telescope is made of seven individual tubes, each collecting light from the sky. To work effectively, the system needed a way to merge the light from all seven tubes into a single stream that could be fed into a high-resolution spectrograph. The team selected input fibers with a core diameter of 25 micrometers, a size chosen to capture a wide view of the sky, roughly twice the size of the typical blur caused by atmospheric turbulence. They fused these seven fibers into a single output fiber with a core diameter of 50 micrometers. By carefully calculating the number of light paths, or modes, that each fiber could carry, they ensured that the output fiber was large enough to hold all the light coming from the seven inputs without forcing any of it out.

To verify that their design worked, the team measured the performance of four different lanterns at various wavelengths, including green light at 520 nanometers and red light at 685 nanometers. They used two different testing methods to ensure their results were accurate. In one method, they measured the light before and after splicing the lantern to other fibers, while in the other, they compared the output directly to a reference. Across all tests, the lanterns consistently delivered efficiencies above 90 percent. The only exceptions were two specific ports on the very first device they built, where the manufacturing process was not yet fully refined. Even in these cases, the performance remained robust. The team also observed that the light exiting the device formed a distinct flower-like shape, confirming that the seven separate cores had successfully merged into a single, coherent guiding region.

This success marks a significant step forward for modular telescope concepts. By proving that light can be combined from multiple large fibers into a single large fiber with minimal loss, the researchers have removed a major barrier to building scalable, cost-effective telescopes. Their work shows that such systems do not require complex, expensive equipment to correct for atmospheric blurring before the light enters the instrument. Instead, the lantern itself handles the combination efficiently, preserving the total amount of light collected. While the team notes that future work will focus on making the manufacturing process more consistent and packaging the devices for the harsh conditions of an observatory, the current results confirm that this technology is ready to help astronomers build the next generation of giant, modular eyes on the sky.