A group of researchers from Stanford University and their colleagues has developed a photonic chip that takes in mixed light from several lasers and turns it into clean, separate beams. This is the first case in which a device does not merely register partially coherent light but sorts its photons into separate beams. The work was published in the journal Nature Photonics.
Partially coherent light is light in which waves oscillate in sync only for a short time and then fall out of sync. This is the kind of light that ordinary sources usually produce, unlike lasers with their strict synchrony.
To grasp the idea, imagine: if a laser beam is directed through a grid, a clear speckled pattern appears on the wall. Light from an ordinary light bulb will produce a soft, blurred picture. The reason is that the photons of a laser oscillate in sync, while those of a light bulb do so out of step. Researchers have long wanted to untangle these different rhythms in order to use light for communications, computing and sensors.
For the experiment, the scientists took a photonic chip the size of a fingernail. Two infrared lasers were directed into it. The mixed light inside the chip automatically separated into individual beams thanks to built-in algorithms. These beams were then mixed again to check whether they would recombine. But they did not synchronize, meaning they had indeed become independent.
The team then added a third laser. The chip handled three sources as well. "If you look at the outputs, they are completely uncorrelated, no correlation remains," explains co-author Paul-Alexis Mor.
According to co-author David Miller, the chip learns the hidden coherence modes, the internal coherent components of an unknown light field, and physically directs them to different outputs. This makes it possible to analyze light while preserving the separated beams for further processing.
The practical benefits could be very diverse. In microscopy, for example, if two luminous particles overlap, an ordinary camera will show one. The new method is in principle capable of distinguishing their light. In astronomy, the chip could be used to parse the light of distant galaxies: what looks like a single spot in a telescope may contain light from different stars and exoplanets. The authors are currently collaborating with NASA to test this idea.
"Most of the information in our world is light," says co-author Annie Krue. "Light is a wonderful medium for measuring the world around us."