Atmospheric research conducted with LiDAR

Atmospheric research uses pulsed laser beams to measure temperature and wind speed by measuring the Doppler shifted and backscattered light at 100 km height in the atmosphere. The returning light signals are weak and can be blocked by sunlight, but the Leibniz-Institute for Atmospheric Physics (IAP) has solved this problem by developing a portable instrument that can be used during the daytime.

Utilising a novel, diode-pumped laser, the compact system features a Spectrum Instrumentation arbitrary waveform generator (AWG) and two digitisers that provide the speed, sensitivity and real-time capability required. It has already provided new insights into Antarctic atmospheric conditions.

The measuring technique is called Doppler resonance LiDAR, as the light is backscattered by tuning the laser precisely to a resonance transition of a metal atom. The backscattered signals are extremely weak (single photons per laser pulse) and are almost completely drowned out by solar radiation during the day. The difference between night and day measurements is that there is 100 million times more background noise during the day from the sun.

Understanding temperature distributions in the atmosphere at altitudes between 80 and 110 km is crucial for performing numerical simulations of the Earth’s climate. One approach to provide such data is to measure the Doppler broadened line width of metal atoms, eg, of the potassium resonance line at 770 nm and of the iron resonance line at 372 nm or 386 nm, by means of resonance LiDAR systems using pulsed lasers. Until now these lasers have used flashlamp-pumps, but the new system uses an efficient, tuneable, diode-pumped, alexandrite laser that is better able to cope with rough environmental conditions such as on research ships or in polar regions. With this technology, a concentration of about one atom per cubic centimetre is sufficient for temperature and wind measurements at 100 km distance, which is an altitude where the atmospheric pressure is so low than only rockets can access this altitude.

“The aim of the project is to create compact mobile measurement systems that we can take around the world,” said project leader Dr Josef Höffner. “Such systems require automatic operation under extreme environment conditions like Antarctica without access to them for long periods of times. We therefore need components that are very robust and reliable as well as being able to provide the levels of sensitivity, speed and flexibility required under these challenging conditions. Spectrum’s five-year warranty gives us peace of mind that these critical items can be relied on.”

Unloading of the mobile LiDAR onto the fast ice in front of the Australian Antarctic station Davis (69° S) from the ice breaker ‘Aurora Australis’.

There are several aspects to improving the measurements in this project. The first is suppressing the background noise by having a small field of view for high resolution, optical filtering and small fields of view. This means that the laser has to be stabilised in the field of view as well as all filters. The laser itself requires a complex and fast real-time stabilisation system with nanosecond timing. This is controlled by the Spectrum M2i.6012 20 MS/s AWG and fires at 500 pulses a second for 24 hours a day. The signals from the backscattered light are processed by the Spectrum M4i.4421 250 MS/s 16-bit digitiser. The conditions within the laser are measured with a M4i.2221 2.5 GS/s 8-bit digitiser card. The system handles more than 1 GBps for 24 hours a day with a response time of about 1 ms after processing the measured data in real time. In total, 21 signals are managed by a software package developed at the IAP.

“We have achieved a compact, highly integrated solution by combining fast, flexible electronics with real-time capability, which is an impressive improvement compared to the previous, large and difficult-to-handle solutions that were many times bigger,” said Dr Höffner. “Our old system needed a 10-ton, 6-metre container and this has been shrunk to a 1500 kg system using the Spectrum cards and a novel laser. We have nearly finished shrinking this even further to fit into a 1 m³ box of only 250 kg, which will use the same electronics and a more compact and advanced laser.”

The sensitivity and portability of the device is enabling new temperature data to be obtained with high resolution and accuracy in remote places. According to Dr Höffner, “Our measurements have already had a big impact on our understanding of the atmosphere, and what’s really exciting is that we have proven that we can design a system that is reliable, lightweight, compact and efficient enough for future space missions.”

Top image caption: Various lasers being tested at the IAP offices.

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