NAIR-APREXIS: Enabling photonics-based instruments for long-baseline interferometry and integral-field spectroscopy

Astronomers have long sought to see the universe with sharper eyes, pushing the limits of what their telescopes can resolve. To do this, they often combine light from multiple telescopes, a technique called interferometry, which acts like a single, giant mirror. However, the traditional way of doing this relies on bulky glass lenses and mirrors that are heavy, difficult to align perfectly, and prone to losing light along the way. In recent years, a new approach has emerged that treats light not as a beam bouncing off large surfaces, but as a signal traveling through tiny, microscopic channels carved directly into glass. This field, known as astrophotonics, promises to shrink complex instruments down to the size of a chip, offering greater stability and precision. The challenge has been proving that these delicate, microscopic structures can survive the harsh conditions of a real observatory and perform as well as their massive, traditional counterparts.

A team of researchers working on the NAIR-APREXIS project has now provided compelling evidence that this microscopic approach is ready for prime time. They have successfully built and tested several new types of instruments using a technique called ultrafast laser inscription, where a powerful laser writes tiny light-guiding paths directly inside blocks of glass. Their work demonstrates that these glass chips can not only combine light from distant stars with extreme precision but also reorganize the light from a telescope to create detailed maps of planets and stars. The team has already tested their devices on actual telescopes in California, and they are preparing to test a new system in Chile in 2027. These tests show that these tiny tools can capture faint starlight with high efficiency and remain stable over long periods, paving the way for a new generation of compact, high-performance astronomical instruments.

The first major success of the project involved a device designed to combine light from two telescopes to measure the brightness and shape of stars. The researchers built this instrument using a special type of glass called Infrasil, which is transparent to the infrared light used to study cool objects in space. They used their laser technique to carve a network of microscopic channels inside the glass that split and recombined the starlight. When they tested this device at the CHARA Array, a collection of six telescopes in California, the results were impressive. The instrument proved to be remarkably stable, maintaining its performance over hours of observation. It was able to measure the interference patterns of stars with a precision of one percent, a level of accuracy required for serious scientific discovery. Even more importantly, the device was efficient, allowing more than forty percent of the starlight to pass through to the detector, and it could detect stars as faint as magnitude five without needing extra equipment to stabilize the light. This success showed that a small, solid block of glass could replace complex, bulky optical systems while delivering superior stability.

Beyond simply combining light, the team is also developing tools to turn the blurry image of a star or planet into a detailed map of light and color, a capability known as integral field spectroscopy. To achieve this, they are creating a new kind of fiber optic cable that contains over one hundred tiny cores, each acting as a separate pixel to capture light from a different part of the sky. A major hurdle in such designs is preventing the light from one core from leaking into its neighbor, which would blur the image. By using a heterogeneous design where the cores are slightly different sizes, the researchers have successfully suppressed this leakage to a level where less than one part in a thousand of the light crosses over. They are currently building a complete system that includes a laser-written device to rearrange these cores into a line, feeding a spectrograph. This system is scheduled to be tested on a telescope in Chile in 2027, aiming to help astronomers detect and study planets orbiting other stars.

The project also explored how to make the instruments themselves even smaller and more integrated. The researchers printed tiny, three-dimensional optical components directly onto the tips of fiber optic cables using a process called two-photon polymerization. These components act as miniature spectrographs, splitting light into its colors to analyze the chemical makeup of celestial objects. Each device is less than one millimeter in size, yet it manages to expand, collimate, and disperse light all within a single, solid piece of material. In laboratory tests, these tiny spectrographs achieved a resolving power of about thirty, meaning they could distinguish between closely spaced colors, and they transmitted more than eighty percent of the light. This level of integration suggests that future instruments could be incredibly lightweight and require no alignment, as the optical parts are permanently fused together.

Another innovative development involves the creation of tiny pyramid-shaped sensors used to measure the quality of the telescope's view. Traditionally, these pyramids are made of glass and require expensive, difficult polishing to achieve perfectly sharp edges. The team used 3D optical printing to create these pyramids, which are only a few millimeters wide. Their measurements showed that these printed pyramids have surface smoothness and edge sharpness that match or even exceed the quality of traditionally made glass pyramids. Because they are so small, they also introduce a unique form of filtering that could improve how astronomers measure atmospheric turbulence. Initial tests on an adaptive optics testbed confirmed that these printed sensors work as predicted, opening the door to cheaper, more versatile wavefront sensors for future telescopes.

Perhaps the most significant validation of this technology came from a long-term study of a sensor installed at the Large Binocular Telescope. This sensor, which uses a ring of tiny printed lenses to help focus light into a fiber, had been operating in the real world for six years. During this time, it was exposed to temperature swings, humidity, vibration, and the constant motion of the telescope. Despite these harsh conditions, the sensor continued to function perfectly, showing no signs of degradation or drift in its performance. This six-year record proves that these 3D-printed optical components are robust enough to survive the demanding environment of an observatory for the long haul. It confirms that the technology is not just a laboratory curiosity but a viable solution for the future of astronomical instrumentation, offering a path to instruments that are smaller, more stable, and capable of revealing the universe with unprecedented clarity.