As it passes through the atmosphere, sunlight is absorbed at certain wavelengths by the gases present in the atmosphere. Each gas has its own spectral signature, so measuring the spectrum of light provides information about atmospheric composition. By observing the ground from the air or from space, for example, we can estimate the concentration of greenhouse gases, such as CO2 or CH4. The spectrometers used for this provide spectra containing a few hundred or even thousands of wavelengths, which are then analysed to detect and quantify the spectral signature of the targeted gases. This makes performant, but often bulky, instruments. Another approach is to carry out the analysis optically, replacing the spectrometer with spectral filters that reproduce the signature of the gases of interest (correlation spectroscopy). This greatly reduces the number of measurement points required, and makes the instruments much more compact, making them easier to mechanically and thermally stabilise, and easier to take on board small carriers such as UAVs or mini-satellites. For the many gases with a quasi-periodic spectral signature, this “optically matched filter” can be a Fabry-Perot interferometer, the free spectral range of which corresponds to the periodicity of the gas' absorption lines.

Onera and UGA have been following this path for several years, developing Nanocarb cameras dedicated to CO2 and CH4 monitoring. In these cameras, the Fabry-Perot filter is a coated silicon plate, placed in front of the lens, but this device suffers from a number of limitations, in particular the multi-layer coating of the plate surfaces is difficult to achieve, and the divergence of the beam in the plate shifts the spectral profile with respect to the desired one. These limitations could be overcome with optical waveguides: as the beam is no longer divergent, spectral variations from one point in the field to another are eliminated, and as the interaction distances with the light are longer, we have more degrees of freedom to create a filter that best approximates the gas signature.

The aim of this PhD thesis is therefore to study such waveguide Fabry Perot filters matched to the detection of gas signatures. The work will consist of 3 main parts: 

- a simulation part, to simulate/model the light in single-mode or multi-mode waveguides, in order to find the relevant parameters for reproducing the desired spectral profile;

- an experimental part, consisting of manufacturing and measuring samples, and comparing the results with those predicted by the simulation tools;

- a “performance model” part, to establish the sensitivity of the complete system (imaging objective, coupling, waveguide and detection) to the monitored gas, for various observation scenarii.

This PhD thesis will be done in close collaboration between Onera/Dota (Palaiseau) and UGA/Ipag (Grenoble).

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For more Information about the topics and the co-financial partner (found by the lab!); contact Directeur de thèse - guillermo.martin@univ-grenoble-alpes.fr

Then, prepare a resume, a recent transcript and a reference letter from your M2 supervisor/ engineering school director and you will be ready to apply online before March 13th, 2026 Midnight Paris time!