FBH lidar diodes promise clearer view of climate change

20 Jul 2020

Franco-German Merlin satellite measuring atmospheric methane to be launched in 2025.

A joint space mission by the German Aerospace Center (DLR) and France's National Center for Space Studies (CNES) is scheduled for launch in 2025 to begin active measurements of atmospheric methane.

Christened Merlin, for MEthane Remote sensing Lidar missioN, it will carry laser diode benches (LDBs) developed and manufactured for the task by the Ferdinand Braun Institute (FBH), six of which have now been integrated into three lidar pump modules and delivered to the project partner ready for use.

"Delivery of LDBs for the Merlin mission has been made possible by the many years of comprehensive know-how accumulated at the FBH in the development of space-qualified diode lasers," commented the Institute.

"FBH technology will thus contribute to the detailed investigation of important and up to now little-known sources of global warming on the climate satellite – a milestone in European climate research."

In use, the modules generate pump energy for the oscillator of a Nd:YAG solid-state laser. This in turn serves as the light source for a tunable optical parametric oscillator generating double pulses at two different wavelengths, both in the infrared range around 1.6 microns.

According to FBH data, each LDB generates a pump power of over 60 watts in double pulses with a repetition rate of 20 hertz and 150 microsecond pulse width. One of these pulses is strongly absorbed by methane and the other is not, allowing methane content to be determined from the ratio of intensities of the backscattered light at the two wavelengths.

Monitoring methane concentrations from space is potentially an important part of climate science and the study of global warming, but matching the accuracy and precision of ground-based measurements has proven to be challenging.

Discussion of the Merlin project in the journal Remote Sensing in 2017 commented that "the magnitude and variability of the absorption signal measurable at the top of the atmosphere for atmospherically relatively long-lived gases such as methane is relatively small in comparison to the changes in reactive gases such as ozone and carbon monoxide."

The presence of other atmospheric species which may have overlapping absorption in the same spectral region is an additional hurdle.

Operating life of four billion pulses

Merlin's dual-wavelength approach relies on differential absorption lidar (DIAL). One wavelength is accurately locked to a spectral feature of the methane molecule, minimizing any small frequency shifts, while the other frequency is selected to have negligible methane absorption and be used as the reference.

"The differential approach, the size of the laser spot (around 120 m at the surface along the track), and the selective sampling guarantees low systematic errors, with almost no contamination by aerosol or water vapor," noted Remote Sensing.

Developing diodes for space-based lidar applications has presented a significantly different set of challenges to those involved in ground-based instruments or the telecoms sector, with constraints around device development and fabrication that have proven to be unique.

Suitable manufacturing, inspection and characterization processes for laser diode arrays intended for satellite use have been critical, with work at FBH on these aspects now playing a part in the Merlin mission's development.

"The extensive life cycle tests performed by European Space Research and Technology Center (ESTEC) in the Netherlands. showed that the power degrades only minimally even after a long operating time of more than four billion pulses," commented FBH. "The scientific team is therefore confident that the Merlin measuring system will function failure-free even under space conditions."