Seeing higher than ever before: ECE professor Peter Dragic is developing laser technology for upper atmosphere LiDAR

Space weather - how the topmost layers of Earth’s atmosphere in contact with outer space behave - is not only crucial for satellites and crewed missions in orbit, but also has an impact on surface weather and human-made infrastructure. However, we cannot reliably model and predict it without sufficiently accurate measurements of the upper atmosphere.

One promising route is to use metastable helium, a form of the gas more common in the upper atmosphere, as an indicator. It emits and absorbs infrared light, what Peter Dragic, an electrical & computer engineering professor, uses for fiber optics research. He has been collaborating with space scientists Gary Swenson and Lara Waldrop to develop a system to perform LiDAR, a technique that determines temperature and windspeed from how laser light interacts with the metastable helium.

His research group has developed the laser, and he has performed simulations with his collaborators demonstrating that LiDAR experiments will yield signals strong enough to resolve the upper atmosphere from the ground. They are ready to demonstrate the technique at an observatory.

“This would be the first measurement to employ large aperture telescopes and high-power lasers,” Dragic said. “There is a long history of measurements of this kind with visible light in lower parts of the atmosphere, but we’re taking LiDAR further than we’ve ever been able to before.”

LiDAR is a well-established technique in atmospheric science. It has been used to study the mesosphere, the part of the atmosphere 50 to 65 miles above sea level, by tracking residual amounts of sodium from disintegrated meteors. The technique is especially promising for resolving the features of the top layers of the atmosphere that cannot be easily imaged.

Although we have an incomplete picture of the upper atmosphere, we do know that helium rises through all atmospheric layers. We also know that it interacts with the direct sunlight, cosmic rays, and free electrons present in the upper thermosphere to lower exosphere, 190 to 600 miles above sea level. These interactions can knock helium into a long-lived, or metastable state, where it absorbs and emits infrared light. This plays into current technology because fiber optic telecommunication uses infrared lasers.

Dragic’s group has developed a laser with the specific wavelength needed to study metastable helium, 1083 nanometers. It is a rare earth doped fiber laser in which the glass fiber has added ytterbium. As a result, the fiber’s molecular structure amplifies weaker light at 1083 nanometers by a process called optical gain. The result is a 50-watt beam ten-thousand times more powerful than a laser pointer.

They are ready to demonstrate how these devices can be used in practice. Dragic and Swenson performed simulations that were presented to the Advanced Maui Optical and Space Surveillance Technologies Conference indicating that the light which makes its way back to the ground after interacting with the metastable helium is strong enough to make upper atmospheric LiDAR feasible. What remains is to implement the technology at an observatory and test it.

While the simulations have made Dragic and his collaborators optimistic, there is still a significant source of uncertainty: the model of the upper atmosphere they used. Because there are so few direct measurements, its composition is still something of a mystery.

“That’s the whole point of this work,” Dragic said. “We’re at a stage where all we have are scarce measurements and some modeling that tells us what we think is going on. The very first measurement will give us a baseline to help us understand if our models are even correct.”

Once the first measurements with their 50-watt laser are complete, they will be ready to deploy more advanced arrays that can send up to 1000 watts of laser light to truly study the atmospheric patterns at these high altitudes