Application of a Fabry-Pérot-Interferometer for multi-point flow measurements

Spectroscopic methods enable flow velocity measurements by analysis of the Doppler frequency shift. Usually the conversion of the frequency shift into an intensity variation is accomplished via a molecular absorption cell. There is the will to solve this function interferometrically by a Fabry-Pérot-Interferometer (FPI), since FPI provide sharper slopes, hence a higher sensitivity. Still there is no limitation to a special wavelength. In common systems for astronomical Doppler frequency measurement the telecentric setup with very low Numerical Aperture (NA) is employed. In contrast, for an imaging measurement of a technical flow a high NA is necessary, which requires a different setup. The article deals with the dimensioning and realization of a multi-point measurement system using a plane-parallel FPI in the collimated setup. Especially the complementarity between detectable stray light power and edge steepness of the transmission characteristic is outlined. Building on that, an estimation for the minimum approachable uncertainty of the frequency determination is presented and compared to measurement results. Consequently a first measurement with FPI on a technical flow is shown.