CARS Probe - ZESS - Center for Sensor Systems

Coherent Anti Stokes Raman Scattering (CARS) Probe

Three different laser beams are focussed inside a probe volume where a fourth beam is generated. Its properties allow measuring the concentration and the temperature of a gas with very high precision.

The coherent anti-Stokes Raman scattering (CARS) technique is a nonlinear four-wave mixing process that is based on the interaction between light and matter. When the three laser beams interact with the gas molecules in the probe volume, a CARS signal is generated, containing information about the thermodynamic state. This well-proven measurement technique is widely used for a spatially and temporally resolved determination of the gas temperature and concentration.

The setup pictured is a dual-pump vibrational CARS probe. This advanced version of a CARS setup utilizes two distinct, narrowband pump beams in combination with a broadband Stokes beam to excite multiple ro-vibrational modes of several molecules simultaneously. A seeded frequency-doubled neodymium-doped yttrium aluminum garnet (Nd:YAG) laser is employed and acts as a pump source for the broadband and the narrowband dye laser, which provide the Stokes and probe beam for the CARS process. A separate part of the Nd:YAG laser beam serves the pump beam for the process. The complicated paths of the different beams are delay lines, implemented to synchronize the temporal overlap of the three laser beams.

A lens focuses the beams inside a folded BOXCARS configuration inside an optically accessible heated gas cell. The folded BOXCARS geometry ensures optimal spatial resolution and enhanced spatial separation of the CARS signal. The center of the gas cell is positioned at the focal point, where the fourth, blue beam – the CARS signal – is generated. Behind the cell, the green, red, and orange beams are blocked by beam dumps. Only the CARS signal is guided to a spectrometer, spectrally dispersed, and detected by a camera with a charge-coupled-device (CCD) sensor (2048 × 2048 pixels).

The next step is to add a fourth beam to supplement the vibrational CARS probe by a rotational CARS setup which can detect additional molecules in a spectral range of 500 wavenumbers and less – targeting CO₂ and H₂.

Contact

Prof Dr Thomas Seeger
Chair for Thechnical Thermodynamics