Under the microscope: Astrophotonic chips—shaping the future of astronomy from the semiconductor foundry

A blog post by Ross Cheriton , Research Officer at the National Research Council of Canada’s Advanced Electronics and Photonics Research Centre

The action starts around 8 p.m. on a cool September night; coffee and cookies are at the ready. The sun has just set. In the midst of a crisp, clear evening in Saanich, just outside of Victoria, BC, a small team of astronomers/scientists are beginning their observing run at the 1.2-metre Dominion Astrophysical Observatory telescope atop a small mountain. Their target for the evening? The light from the brightest, (and ideally) reddest stars in the sky—such as Betelgeuse, the Garnet Star, Vega and Capella—all twinkling and heavily distorted by the atmosphere. The success of their run all depends on the performance of the NRC’s in-house adaptive optics system, named REVOLT , in stabilizing the star’s light.

Opening up the dome and exposing starlight to the telescope, all that can be seen is a bright, messy blob on the main camera, with speckles dancing about a central point at high speed. Looking over to our prototype astrophotonic instrument, the only thing that can be heard is noise. After some calibration and adjustment, the adaptive optics system comes to life, continuously analyzing and correcting for the atmospheric distortions caused by Earth’s ever-changing atmosphere. It’s a clear night. We watch in anticipation as the light from Vega transforms from a shimmering, distorted pattern into a steady, unwavering beam, just as we hoped! We turn to our instrument, ARIES, with a tiny 4mm x 4mm chip at its heart, to see if the starlight has managed to pass through it and onto a camera. Success! With the star’s light coupled through the astrophotonic chip, we are now processing starlight on a chip, guided through channels 50 times narrower than a human hair.

At the NRC, we’re working on developing technology to process light from beyond Earth using chip-scale instruments, whether it’s for communication or astronomy. While extremely large ground-based telescopes (ELTs) are pushing the boundaries of what we can observe, a slow but quiet revolution is happening in astronomy; the shift to photonics, or guided optics, called astrophotonics.

It’s been nearly 2 years since the James Webb Space Telescope was launched and it is already delivering fascinating science to us from over 1.5 million kilometres away. Packing a large gold-coated mirror and a slew of instruments, it’s been used to probe the atmospheres of exoplanets and comets, and to capture images of individual exoplanets beside their host stars.

On the ground, next-generation, 30-metre class observatories are being designed and built that dwarf the existing ones in Chile and Hawaii . It’s not just the telescopes, the instruments have to be larger (and much more expensive) too. If you’re talking about ground-based telescopes, that extra weight and size means that cost is a major problem, but if you’re talking about space-based telescopes, it’s more of a brick wall.

As astronomers are pushing the envelope in terms of the capability, sensitivity and precision of astronomical instruments, it’s becoming increasingly difficult to keep things stable enough when everything is scaled up. Temperature variations, pressure fluctuations and gravity effects are all challenging foes when you’re pushing instruments larger and larger, and sensitivities higher and higher. Enter astrophotonics. It turns out that it’s much easier to keep a small, light single crystal chip stable, than it is to do the same with an instrument the size of a house!

Photonic chips are like electrical chips, but they guide photons instead of electrons. They’re nothing really new. These chips make it so the Internet can handle hundreds of millions of virtual meetings, drive cloud computing and storage, and they are the reason our 5G networks give you high-speed data to your smartphone, even when in crowds of tens of thousands of people.

While the standard for high-speed data processing in data centres and routers, these tiny chips are making inroads into astronomy, helping to shape the future of our understanding of the Universe.

The quest for Earth’s twin

Our journey in astrophotonics started with astronomers from Victoria, BC and photonics experts in Ottawa, ON. The promise of astrophotonics had already been shown. The Very Large Telescope (I know, not so original!) had already revealed stars orbiting an invisible, but immense source of gravity at the centre of our Milky Way using an instrument called (you guessed it) gravity. It was our galaxy’s super massive black hole they were able to pinpoint in space, all thanks to a photonic chip with a pattern circuit of waveguides.

For our efforts, we looked towards 1 of the holy grails of astronomy: finding a planet like Earth, oxygen, water, creatures and all! We settled on a concept of correlation spectroscopy, which is akin to creating a light filter, kind of like a spectral stencil, that matches gas signatures like oxygen, carbon dioxide and methane. By shifting this filter back and forth across the spectrum coming through the telescope, we can try to find a correlation between when the filter is aligned to the gas absorption lines or not. That would mean that gas is present in some amount in the atmosphere of that exoplanet.

In practice, we would wait until an exoplanet is passing in front of its star, and then search for that tiny amount of change in spectrum when its atmosphere blocks some tiny amount of starlight. When you have a needle in a haystack, you better be sure how to distinguish the needle from the hay!

When it comes to astrophotonics, simple is best. The more complex the chip becomes, the more difficult it becomes to keep losses in check. After clouds, there’s little astronomers despise more than losing precious light! To this end, we designed tiny resonators in the form of closed racetrack rings to act as the filters, which can be warmed up and cooled down rapidly to shift the spectrum. Simple, but effective!

Armed with a compact astrophotonic chip capable of splitting starlight into this comb of light, we set out to conduct a spectral analysis. As the chip dissects the incoming light into a comb of colours in the infrared, the chemical composition of stars and exoplanet atmospheres to gases in galaxies can be extracted in real-time.

How to build an astrophotonic chip

These minuscule devices, which are poised to push the limits of precision astronomy, are not born overnight. Instead, their fabrication is a complex and precise art form that blends cutting-edge technology with meticulous expertise.

The journey begins with a spark of creativity—a way to guide and process light as a wave on a chip. From a firm idea of the science goals, the targets of interest, and the performance required, scientists and engineers conceptualize the chip’s main function.

Designing the blueprint: the art of precision

With the concept in place, the next step is to translate it into a detailed blueprint. This phase demands precision and expertise, as researchers outline the chip’s architecture, incorporating waveguides, gratings and other (often custom) essential components. Every millimetre matters and the design must account for the chip’s size, materials and intended application. It’s a process where mathematical rigour meets creativity.

For photonic chips, smaller is often better, since that means less loss!

Materials matter: crafting the substrate

One of the critical aspects of astrophotonic chip fabrication is selecting the appropriate materials for the substrate. Glass used to be the typical choice due to its compatibility with light being guided already in glass-based optical fibre. Now, silicon and silicon nitride are taking over due to their unique properties, such as making it possible to create smaller, more functional components. In the end, the choice depends on the specific requirements of the chip and the type of light. The waveguide, components geometry and material properties must align with the chip’s intended purpose, whether it’s high-resolution interferometry, spectroscopy or filtering.

Lithography: etching the cosmic blueprint

Lithography is a pivotal step in the fabrication process. This technique involves writing or projecting a pattern onto the substrate to create the circuit. A mask, featuring the precise design of the chip, guides the process. It’s a delicate dance of light and chemistry, where even the tiniest imperfection can have a significant impact on the chip’s performance.

Etching and deposition: sculpting the universe

Once the lithography step is complete, the chip begins to take shape through etching and deposition processes. Etching removes unwanted material, precisely sculpting the waveguides, gratings and other optical components. Deposition, on the other hand, involves adding thin layers of materials, such as glass, to complete the desired structures. These steps demand extreme precision, as the chip’s functionality hinges on the accuracy of these meticulously crafted components.

The moment of truth: design and fabrication come together

Until now, there is little indication of how the chip will perform. With the chip’s physical structure in place, it’s the moment of truth. Rigorous testing and calibration follow. Each chip undergoes extensive scrutiny to measure its loss and spectral properties, to see if the fabrication and design have come together to hit the mark.

New horizons in astrophotonics

In our research, we are also looking to develop new types of adaptive optics systems that fit onto a single astrophotonic chip—no more deformable mirror or wavefront sensor! We are looking towards developing an astrophotonics concept for ultra-high precision radial velocity to find the faintest wobbles of stars, signatures of hidden Earth-like planets orbiting around and pulling on their stars. Our other work involves designing a multi-telescope beam combiner, which would let us connect multiple large telescopes together, separated by many kilometres, to create a massive synthetic telescope capable of incredible resolution. This would be similar to what was done with the Event Horizon Telescope that produced the first image of a black hole, but using optical technology instead of radio waves.

In conclusion, as researchers continue to push the boundaries of what’s possible, these tiny chips will emerge as key components in the heart of instruments at the forefront of precision astronomy, enabling us to unlock the mysteries of the universe. It turns out that the smallest of chips can have a cosmic impact on our exploration of large celestial bodies in the universe and even in the search of Earth’s twin and extraterrestrial life.

Read more stories about NRC research , and how our work contributes to the success of our clients and partners.