Team

TDLAS-based open-path laser hygrometer using reflective scattering targets for water vapour measurements

The aim of the project is measuring horizontal 2D-concentration-profiles in boundary layers. Possible applications are in the field of environmental physics, but also in combustion processes or exhaust gas examination. A recent task in the former is the examination of permafrost that is melting because of the climate change. In this process, greenhouse gases which are stored in the soil are emitted and are expected to lead to a positive feedback to the climate change. For this reason, the exact concentrations of the emitted gases and their spatial distributions have to be studied in detail with adequate field qualified instrumentation.

As a measurement method, TDLAS (Tunable Diode Laser Absorption Spectroscopy) unites several advantages for the field application. It is an absolute and calibration-free in-situ technique that is based on robust, industrially available components such as distributed feedback (DFB) diode lasers and fibre optics. The setup comprises a joint sender/receiver side and a separate low cost reflector as a scattering target. This retro-reflecting target allows the setup of simple mono-static TDLAS instruments which ensure easy installation and maintenance free operation in harsh environment and under varying climate conditions.

By a combination of several scanning laser devices and use of a priori information, near arbitrary 2D concentration fields with low-frequency content shall be reconstructed with minimum expenditure. Spatially resolved 1D water vapour measurements have already taken place in the group^1,2. The lateral distribution of water vapour in the boundary layer of a plant leaf has been measured line-of-sight-averaged. It is now the challenge to add a second dimension to the spectrometer.

Developing a transportable, robust and absolute 2D spectrometer can be considered independent from any application. It may as well be used for field applications in the environment as for industrial research. It is an attractive means to measure 2D concentration profiles for all cases that require quick, robust and transportable devices.

[1] S. Hunsmann, K. Wunderle, S. Wagner, U. Rascher, U. Schurr, V. Ebert: „High Resolution Measurements of Absolute Water Transpiration Rates from Plant Leaves via 1.37 µm Tunable Diode Laser Absorption Spectroscopy (TDLAS)”, Appl. Phys. B 92, 393-401 (2008).

[2] K. Wunderle, S. Wagner, I. Pasti, U. Rascher, U. Schurr, V. Ebert: "Distributed feedback diode laser spectrometer at 2.7 ?m for sensitive, spatially resolved H₂O vapor detection", Appl. Opt. 48, B172-B182 (2009).