At 6.5 meters in diameter, the James Webb Space Telescope’s primary mirror captures more light than any telescope that’s ever been launched from Earth. But not every astronomer has US $10 billion to spend on a space telescope. So to help bring the cost of space-based astronomy down, researchers at the National Research Council of Canada in Ottawa are working on a way to process starlight on a tiny optical chip. Ross Cheriton, a photonics researcher there, and his students built and tested a CubeSat prototype with a new kind of photonic chip. The goal is to lower the barrier to entry for astronomical science using swarms of lower-cost spacecraft.

“We hope to enable smaller space telescopes to do big science using highly compact instrument-on-chips,” Cheriton says, who is also affiliated with the Quantum and Nanotechnology Research Centre in Ottawa.

Photonics integrated circuits (PICs) use light instead of electricity to process information, and they’re in wide use slinging trillions and trillions of bits around data centers. But only recently have astronomers begun to examine how to use them to push the boundaries of what can be learned about the universe.

Ground-based telescopes are plagued by Earth’s atmosphere, where turbulence blurs incoming light, making it difficult to focus it onto a camera chip. In outer space, telescopes can peer at extremely faint objects in non-visible wavelengths without correcting for the impact of turbulence. That’s where Cheriton aims to boldly go with a PIC filter that detects very subtle gas signatures during an exoplanet “eclipse” called a transit.

The main motivation for putting photonic chips in space is to reduce the size, weight, and cost of components, because it can be produced en masse in a semiconductor foundry. “The dream is a purely fiber and chip-based instrument with no other optics,” says Cheriton. Replacing filters, lenses, and mirrors with a chip also improves stability and scalability compared to ordinary optical parts.

CubeSats—inexpensive, small, and standardized satellites—have proved to be a cost-effective way of deploying small instrument payloads. “The compact nature of PICs is a perfect match for CubeSats to study bright exoplanet systems James Webb doesn’t have time to stare at,” says Cheriton.

For a total mission cost of less than $1 million—compared to the Webb’s $10 billion—an eventual CubeSat mission could stare at a star for days to weeks while it waits for a planet to cross the field of view. Then, it would look for slight changes in the star’s spectrum that are associated with how the planet’s atmosphere absorbs light—telltale evidence of gasses of a biological origin.

Smaller spectroscopy

As a proof-of-concept, Cheriton guided a team of undergraduate students who spent eight months designing and integrating a PIC into a custom 3U CubeSat (10 centimeter x 10 cm x 30 cm) platform. Their silicon nitride photonic circuit sensor proved itself capable of detecting the absorption signatures of CO₂ in incoming light.

In their design, light entering the CubeSat’s collimating lens gets focused into a fiber and then pushed to the photonic chip. It enters an etched set of waveguides that includes a ring resonator. Here, light having a specific set of wavelengths builds in intensity over multiple trips around the ring, and is then output to a detector. Because only a select few wavelengths constructively interfere—those chosen to match a gas’s absorption spectrum—the ring serves as a comb-like filter. After the light goes through the ring resonator, the signal from the waveguide gets passed to an output fiber and onto a camera connected to a Raspberry Pi computer for processing. A single pixel’s intensity therefore serves as a reading for a gas’s presence.

Light travels through a waveguide on a photonic integrated circuit.Teseract

Because it’s built on a chip, the sensor could be multiplexed for observing several objects or sense different gasses simultaneously. Additionally, all the light falling on a single pixel means that the signal is more sensitive than a traditional spectrometer, says Cheriton. Moreover, instead of hunting for peaks in a full spectrum, the technology looks for how well the absorption spectrum matches that of a specific gas, a more efficient process. “If something is in space, you don’t want to send gigabytes of data home if you don’t have to,” he says.

Space travel is still a long way off for the astrophotonic CubeSat. The current design does not use space-qualified components. But Cheriton’s students tested it in the lab for red light (635 nm) and CO₂ in a gas cell. They used a “ground station” computer to transmit all commands and receive all results—and to monitor the photovoltaics and collect data from the flight control sensors onboard their CubeSat.

Next, the team plans to test whether their sensor can detect oxygen with the silicon nitride chip, a material that was chosen for its transparency to the gas’s 760 nm wavelength. Success would leave them well positioned to meet what Cheriton calls the next huge milestone for astronomers: looking for an earth-like planet with oxygen.

The work was presented at the Optica (formerly Optical Society of America) Advanced Photonics conference in July.

Cochlear implants—the neural prosthetic cousins of standard hearing aids—can be a tremendous boon for people with profound hearing loss. But many would-be users are turned off by the device’s cumbersome external hardware, which must be worn to process signals passing through the implant. So researchers have been working to make a cochlear implant that sits entirely inside the ear, to restore speech and sound perception without the lifestyle restrictions imposed by current devices.

A new biocompatible microphone offers a bridge to such fully internal cochlear implants. About the size of a grain of rice, the microphone is made from a flexible piezoelectric material that directly measures the sound-induced motion of the eardrum. The tiny microphone’s sensitivity matches that of today’s best external hearing aids.

Cochlear implants create a novel pathway for sounds to reach the brain. An external microphone and processor, worn behind the ear or on the scalp, collect and translate incoming sounds into electrical signals, which get transmitted to an electrode that’s surgically implanted in the cochlea, deep within the inner ear. There, the electrical signals directly stimulate the auditory nerve, sending information to the brain to interpret as sound.

But, says Hideko Heidi Nakajima, an associate professor of otolaryngology at Harvard Medical School and Massachusetts Eye and Ear, “people don’t like the external hardware.” They can’t wear it while sleeping, or while swimming or doing many other forms of exercise, and so many potential candidates forgo the device altogether. What’s more, incoming sound goes directly into the microphone and bypasses the outer ear, which would otherwise perform the key functions of amplifying sound and filtering noise. “Now the big idea is instead to get everything—processor, battery, microphone—inside the ear,” says Nakajima. But even in clinical trials of fully internal designs, the microphone’s sensitivity—or lack thereof—has remained a roadblock.

Nakajima, along with colleagues from MIT, Harvard, and Columbia University, fabricated a cantilever microphone that senses the motion of a bone attached behind the eardrum called the umbo. Sound entering the ear canal causes the umbo to vibrate unidirectionally, with a displacement 10 times as great as other nearby bones. The tip of the “UmboMic” touches the umbo, and the umbo’s movements flex the material and produce an electrical charge through the piezoelectric effect. These electrical signals can then be processed and transmitted to the auditory nerve. “We’re using what nature gave us, which is the outer ear,” says Nakajima.

Why a cochlear implant needs low-noise, low-power electronics

Making a biocompatible microphone that can detect the eardrum’s minuscule movements isn’t easy, however. Jeff Lang, a professor of electrical engineering at MIT who jointly led the work, points out that only certain materials are tolerated by the human body. Another challenge is shielding the device from internal electronics to reduce noise. And then there’s long-term reliability. “We’d like an implant to last for decades,” says Lang.

In tests of the implantable microphone prototype, a laser beam measures the umbo’s motion, which gets transferred to the sensor tip. JEFF LANG & HEIDI NAKAJIMA

The researchers settled on a triangular design for the 3-by-3-millimeter sensor made from two layers of polyvinylidene fluoride (PVDF), a biocompatible piezoelectric polymer, sandwiched between layers of flexible, electrode-patterned polymer. When the cantilever tip bends, one PVDF layer produces a positive charge and the other produces a negative charge—taking the difference between the two cancels much of the noise. The triangular shape provides the most uniform stress distribution within the bending cantilever, maximizing the displacement it can undergo before it breaks. “The sensor can detect sounds below a quiet whisper,” says Lang.

Emma Wawrzynek, a graduate student at MIT, says that working with PVDF is tricky because it loses its piezoelectric properties at high temperatures, and most fabrication techniques involve heating the sample. “That’s a challenge especially for encapsulation,” which involves encasing the device in a protective layer so it can remain safely in the body, she says. The group had success by gradually depositing titanium and gold onto the PVDF while using a heat sink to cool it. That approach created a shielding layer that protects the charge-sensing electrodes from electromagnetic interference.

The other tool for improving a microphone’s performance is, of course, amplifying the signal. “On the electronics side, a low-noise amp is not necessarily a huge challenge to build if you’re willing to spend extra power,” says Lang. But, according to MIT graduate student John Zhang, cochlear implant manufacturers try to limit power for the entire device to 5 milliwatts, and just 1 mW for the microphone. “The trade-off between noise and power is hard to hit,” Zhang says. He and fellow student Aaron Yeiser developed a custom low-noise, low-power charge amplifier that outperformed commercially available options.

“Our goal was to perform better than or at least equal the performance of high-end capacitative external microphones,” says Nakajima. For leading external hearing-aid microphones, that means sensitivity down to a sound pressure level of 30 decibels—the equivalent of a whisper. In tests of the UmboMic on human cadavers, the researchers implanted the microphone and amplifier near the umbo, input sound through the ear canal, and measured what got sensed. Their device reached 30 decibels over the frequency range from 100 hertz to 6 kilohertz, which is the standard for cochlear implants and hearing aids and covers the frequencies of human speech. “But adding the outer ear’s filtering effects means we’re doing better [than traditional hearing aids], down to 10 dB, especially in speech frequencies,” says Nakajima.

Plenty of testing lies ahead, at the bench and on sheep before an eventual human trial. But if their UmboMic passes muster, the team hopes that it will help more than 1 million people worldwide go about their lives with a new sense of sound.

The work was published on 27 June in the Journal of Micromechanics and Microengineering.