MODISH · Molecular gas diagnostic with short laser pulses

Final Activity Report Summary - MODISH (Molecular gas diagnostic with short laser pulses)

The research topic of our project was related to molecular diagnostic in gas phase by laser technique. During the last decades, frequency-domain spectroscopy using mainly nanosecond lasers proved its usefulness for temperature and concentration measurements in hostile environments, experienced for instance in combustion processes. These investigations featured major implications related to diagnostic areas of aircraft and rocket engines for which the development of non-invasive measurements is a top priority. Even though frequency-domain spectroscopy was extensively used, several drawbacks, such as the complexity of spectra recorded at high pressure, showed its limitations. Recent development in femtosecond laser technology led to the emergence of alternative time-resolved spectroscopy techniques (TRS). These techniques mainly used a sequence of pulses in a pump-probe scheme. The result of the interaction of the molecular sample with a pump pulse was probed temporally using a time-delayed probe pulse. The comparison of the temporal signal with the theory could provide information with respect to the temperature, concentration and pressure of molecular sample. The TRS featured several advantages with respect to frequency domain. Firstly, femtosecond laser technology offered high repetition rate systems in conjunction with compactness and high stability. Moreover, most of limitations encountered with frequency-domain spectroscopy were insensitive in time-resolved spectroscopy. Despite these different aspects, the TRS techniques remained rather dedicated to fundamental investigations, and therefore relatively unexploited in the framework of diagnostic. The objective of this project was to implement reliable TRS techniques for diagnostic measurements, in terms of temperature and concentration, of molecular species in gas phase. With a view to extend the applicability of diagnostics to turbulent mediums, the undertaking of single-shot detection was planned. Different time resolved spectroscopy techniques could be alternatively used. We started our project with the Raman induced polarisation spectroscopy (RIPS) technique for which we reported the aptness to measure temperature and concentration of molecular gas mixture. Thermometry measurements relied on the fact that the signal shapes could be temperature dependant. As a result, pronounced modifications of the signal with the temperature were required to get reasonable accuracy, particularly with single-shot detection. In order to circumvent this limitation, we proposed to specifically tailor the pump laser pulse for a better sensitivity. Our numerical simulations revealed the possibility of producing signals with an enhanced sensitivity towards the temperature by using pump pulses of specific shape. Our primary investigation focussed on this point by conducting temperature measurement of molecular N2 sample in static cells with tailored pulses. An experimental set-up for such an application was developed. The pump pulses were tailored using a single mask liquid crystal spatial light modulator. The pulse shapes were chosen to improve sensitivity of temperature measurement of N2 molecule in a selected domain of temperature. A set of experiments was recorded to extract temperature. Unexpectedly, a systematic deviation of about 20 K in comparison to the real value was observed. This problem was mainly identified as a residual spatial chirp in our laser system. Nevertheless, the comparison of fits allowed for drawing several conclusions. The main result concerned the expected improvement of sensitivity by tailoring the pump pulse. These first results were very promising and tended to prove that pulse shaping technology could really contribute to significant improvements of the performance of diagnostics from TRS. With the problems related to our laser system being solved by the time of the project completion, new series of measurements were planned in order to finalise the project.

Текст от CORDIS, на английски · Данни: CORDIS, © Европейски съюз