Servizio Comunitario di Informazione in materia di Ricerca e Sviluppo - CORDIS

Acousto-optic Spectrometer for monitoring of plasma parameters during spraying process

A fast and robust diagnostic system was developed for real-time monitoring optical emission of thermal spraying processes. The working principle is an acousto-optic interaction; therefore the AOS has no moving components inside. In contrast to conventional spectrometers, in which diffraction gratings, prisms or Fourier transforming methods performs the spectral dispersion of a broadband optical signal, an acousto-optic tuneable filter (AOTF) is used as a narrow band wavelength filter. That's why it is not necessary to have an entrance slit (one dimensional) and it is possible to use a two dimensional aperture. That means the AOS allows detecting extreme small light appearances.

The wavelength selection is realised by an acousto-optic interaction inside of an anisotropic SiO2-crystal. An ultrasound wave is applied to the crystal, inducing a periodical oscillation of the refractive index. This resonant acousto-optic interaction at such a phase grating results in a refracted light wave that is turned in the polarisation direction.

The AOS supports 3 modes of operation for process control:
Spectral scanning mode (called spectrum mode): The spectrum mode delivers complete intensity information from a wavelength range. The wavelength range is scanned by stepping through with acquisition time of minimum 5 milliseconds per step. Characteristic emission lines can be found in the spectrum using an automatic peak search tool. An advanced emission library tool based on data from the library supports the identification of the lines found in the spectrum.

Chronogram / time tracking mode: The chronogram mode uses the advantage of fast random wavelength access. The AOS spectrometer control software supports the quasi-simultaneous watching of up to 16 spectral emission lines with an acquisition time of down to 5 milliseconds each. The total time resolution depends from the number of selected emission lines. This mode is connected with an analogue output, which can be set to values calculated by a user-defined algorithm from the measured intensities. The chronogram mode can be used to monitor or to control thermal spraying processes.

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