The IAP metal lidars

The IAP metal lidar system is designed for the observation of layers of metal atoms in the upper mesosphere and lower thermosphere (roughly 75-140 km). The lidar utilizes resonant backscatter of the metal atoms after emission of laser radiation from two separately tunable dye lasers. The dye lasers allow the easy change of the sounded metals. The metal lidar system complements the K lidar system (measuring temperatures resolving the spectral broadening of the K resonance line) by observation of density profiles of different metals like Fe, Ca, Ca+ and Na. Furthermore the dye lasers provide backscatter profiles at additional wavelengths for the examination of NLC.

Tasks

The two dye lasers are simultaneously pumped, each by 50% of the energy of an excimer laser beam. The dye lasers can be combined at identical wavelengths to get largest backscatter from a single metal, or they are tuned to different wavelengths in order to observe two different metals simultaneously in a common volume. The dye lasers are tuned with 0.1 pm resolution, using either the atmospheric backscatter or the signal of a metal vapour cell in the lab as a wavelength reference.

The dye lasers are built in the 1980ies, the excimer laser in 1999. The whole system is very rugged and has also been used during different field campaigns outside a laboratory.

Setup

The scheme below shows the layout of the optical components of the dye lasers. For one laser the replacement of single parts due to wavelength change is illustrated. In the table the technical properties of the laser systems are summarized.

Layout of the lasers of the metal lidar. Replaceable parts for the wavelengths change are shown for one laser.

excimer laser

dye laser

wavelength in air [nm]

308

Ca: 423.6728
Ca+: 393.3663
Fe: 371.9937
Na: 588.9950

spectral width [pm]

0.5

pulse energy [mJ]

700

Ca: 30
Ca+: 20
Fe: 17
Na: 40

pulse length [ns]

25

25

repetition rate [Hz]

30

30

The detection system of the metal lidars uses a single 80-cm-mirror (focal length 3.2 m). The backscattered photons are guided by a glas fiber to a wavelength dependent mirror. Interference filters suppress the largest part of the background light before a photomultiplier detects the transmitted photons. The typical altitude resolution for the detection is 200 m.

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Dye laser

Dye laser room with one dye laser tuned to the resonance wavelength of Ca (left), the other to the Na wavelength (right)