Photonic chip separates three modes of mixed light to reveal hidden details in imaging

Researchers at Stanford University and collaborating institutions have developed a photonic chip that can separate partially coherent light into distinct, mutually incoherent beams, potentially helping cameras, microscopes and telescopes capture information that conventional imaging can miss.

The fingernail-sized device uses a network of light-processing components and an optimisation algorithm to identify and physically separate the underlying coherence modes within a mixed optical field.

The advance, published October 30 in Nature Photonics, demonstrates a capability that researchers had proposed theoretically decades ago but had not previously achieved experimentally.

Most conventional optical systems are designed around coherent light, such as laser beams, whose waves maintain a stable relationship with one another. Ordinary light sources, by contrast, produce light with varying degrees of coherence. When multiple contributions overlap, their combined intensity can conceal information about the underlying optical field.

The new photonic light analyser could help recover some of that information by separating the field into its distinct coherence modes.

Chip learns light patterns

The researchers built the device using a silicon photonic circuit containing a network of interferometers. These components guide and manipulate light on a chip, much like electronic circuits process electrical signals.

The team fed two independent infrared lasers into the chip, creating a partially coherent mixture. The device used a self-configuring network and an optimisation algorithm to identify the dominant coherence modes in the incoming light.

Rather than simply separating the original laser sources, the chip diagonalises the light field’s coherency matrix. This produces orthogonal modes that are mutually incoherent, even when the original beams overlap spatially.

The researchers demonstrated the approach using a nine-input device. It separated the two strongest coherence modes with more than 98 percent fidelity against a reference tomography method. They also verified that the system could operate beyond two modes in a separate three-mode experiment.

Once configured, the chip physically routes the separated modes to different output channels, where they remain available for further optical processing.

“Our chip learns the underlying coherence modes of an unknown light field and physically directs them into separate outputs,” said co-author Shanhui Fan, a professor of electrical engineering at Stanford.

Imaging gains hidden information

The ability to distinguish overlapping optical contributions could improve measurements in situations where conventional cameras record similar-looking intensity patterns.

For example, two fluorescent particles positioned close together may appear as a single feature under a microscope. Their light could nevertheless contain different spatial and phase information. Separating the coherence modes could help reveal that distinction.

The researchers also demonstrated that the chip could measure information contained in phase relationships that intensity-only measurements would miss.

The team is now collaborating with NASAto investigate whether the approach could help analyse light from distant galaxies. A single bright feature in a telescope image, for instance, could contain light from multiple stars or other astronomical sources.

Beyond imaging, the researchers said the technology could supportoptical sensing, communication and computing. The same chip architecture can also operate in reverse to generate partially coherent light with a chosen coherence structure.

The work establishes a programmable approach to measuring, separating and controlling partially coherent light. Further development will be needed to scale the system to larger numbers of optical modes and faster control speeds before it can be used in practical imaging systems.

The research was published inNature Photonics.

Source: https://interestingengineering.com/innovation/stanford-photonic-chip-separates-mixed-light