Measuring Doppler shifted and backscattered light 100km away

December 16, 2018 //By Julien Happich
Measuring Doppler shifted and backscattered light 100km away
Atmospheric research uses pulsed laser beams to measure temperature and wind speed along the beams by measuring the Doppler shifted and backscattered light at 100km height in the atmosphere.

The returning light signals are very weak and can be blocked by sunlight, but The Leibniz Institute for Atmospheric Physics (IAP) has solved this problem: they developed the world’s only portable instrument that can be used during the daytime and it has already provided new insights into Antarctic atmospheric conditions. This new generation of compact and reliable systems uses a novel, diode-pumped laser and, at its heart, are a Spectrum Instrumentation Arbitrary Waveform Generator (AWG) and two Digitizers that provide the extremely high speed, sensitivity and real-time capability required.

Various lasers being tested at the IAP offices.

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 100,000,000 times more background noise during the day, due to the sun.

Understanding temperature distributions in the atmosphere at altitudes between 80 and 110km, called the MLT (Mesosphere and lower Thermosphere), is crucial for performing numerical simulations of the Earth’s climate. One well-established approach to provide such data is to measure the Doppler broadened line width of metal atoms, e.g. of the potassium resonance line at 770nm and of the iron resonance line at 372nm or 386nm, by means of resonance LIDAR systems using pulsed lasers.

Until now, these lasers have used flashlamp-pumps, but this project is the first to develop a novel laser system using a highly efficient, tunable, 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 100km of distance, which is an altitude where the atmospheric pressure is so low that only rockets can access this altitude. 

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