Fraunhofer ILT lidar studies climate at 100 km altitude

01 Mar 2023

Platform employing alexandrite lasers could form part of planned European lidar array.

A lidar platform developed by the Fraunhofer Institute for Laser Technology (ILT) and the Leibniz Institute for Atmospheric Physics (IAP) has been used to calculate wind and temperature data at altitudes of up to 100 km, proving the effectiveness of its laser system.

The results from an installation in Kühlungsborn, Germany, could also be a step towards a proposed European lidar array for climate monitoring, helping to enhance climate models and improve weather forecasts.

Fraunhofer ILT has been studying ways to make atmospheric lidar platforms smaller and easier to transport for some time, focusing on the use of a crystal of alexandrite as the laser medium. Working with IAP it developed a diode-pumped alexandrite laser source intended for both lidar and space applications, unveiled in 2019.

That platform delivered pulses of up to 1.7 mJ at a repetition rate of 500 Hz, providing the spatial and temporal resolution needed for exciting atoms of potassium or iron in the atmosphere in pulsed single-frequency operation. The source has now been optimized further, in particular to ensure that only one spatially narrow mode is pumped in the crystal and the narrow bandwidth needed for lidar measurements is maintained.

"The first prototype of the alexandrite laser delivered 0.15 W, at that time with two crystals but without homogenization of the pump radiation," commented the ILT project. "Since then, the system has been significantly improved over several iterations. The 2-meter beam path is folded several times, and the pump radiation is delivered only via fibers, allowing the pump source to be changed without much effort."

Autonomous long-term lidar coverage

In May and November 2022 the latest generation of the alexandrite laser platform was installed in two ground-based lidar systems at Kühlungsborn, and measurements from 100 kilometers altitude were successfully taken. That work showed that it was possible to explore the atmosphere in vertical and horizontal directions, by directing one laser to four telescopes tilted by 30 degrees.

Fraunhofer ILT believes that using multiple lidar beams in this way could survey tens of thousands of square kilometers of atmosphere. It intends to develop the laser technology further, and produce an amplifier stage for the alexandrite laser independently of its work on lidar applications.

An additional strand of research will see cooperation with the new EULIAA (EUropean LIdar Array for Atmospheric climate monitoring) project, an EU-funded effort that commenced in January 2023 and is scheduled to last four years.

EULIAA wants to develop a lidar array measuring atmospheric wind and temperature at altitudes from 5 to 50 km over extended periods of time and large observation areas. This will require lidar units that are low-priced, compact, efficient, easily transportable, and autonomous.

The developers also intend to modify the lidar source to operate in the UV range, using intra-cavity doubling to convert the output to 386 nanometers. This has only been possible previously with less efficient external frequency doubling, but initial tests using the new source are promising, according to the project.

"We are seeing significant changes in the mesosphere," commented Josef Höffner of IAP. "With the new systems, we want to observe the changes continuously and over wide areas. That will have a significant impact on longer-term climate predictions."