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PhD : a new generation of atmospheric lidars by coherent combination of semiconductor amplifiers.

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Français - Anglais

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General information

Reference : UMR8501-PATGEO0-013
Workplace : PALAISEAU
Date of publication : Monday, September 14, 2020
Scientific Responsible name : Gaëlle LUCAS-LECLIN
Type of Contract : PhD Student contract / Thesis offer
Contract Period : 36 months
Start date of the thesis : 2 November 2020
Proportion of work : Full time
Remuneration : 2 135,00 € gross monthly

Description of the thesis topic

Current environmental issues require increased monitoring of the atmosphere to know the distribution of aerosols, the proportion of different gases, etc. In particular, the increase in the concentration of water vapour in the troposphere is one of the most significant effects of climate change. It occurs on short time scales (during extreme events such as cyclones, Cevennes episodes, ...) or long time scales (increase in humidity at low altitude in the Arctic conditioning the early onset of ice melting). Only airborne lidars can provide measurements with sufficient spatial and temporal resolution to study these phenomena. The flash-pumped crystal lasers used in these liars so far are obsolete, and need to be replaced by more robust and reliable sources requiring minimal maintenance. In this context, laser diodes are a well-suited solution, due to their electrical-optical efficiency, compactness and robustness. However, the peak power extracted from the components available today remains insufficient for the needs of atmospheric lidars. Therefore, we propose to study a new coherent combination architecture of semiconductor amplifiers in pulse mode and to evaluate the potential of this configuration for Differential Absorption Measurement Lidars (DIAL).
Coherent combining consists of the superposition, by constructive interference, of beams from several laser sources in phase. This technique increases the available laser power without degrading the other properties of the beams, in particular their spectrum and spatial profile. In the field of semiconductor lasers, this is of particular interest to overcome the thermo-optical limitations of individual components.

In the framework of this thesis, we propose to study semiconductor amplifiers with a flared section, which deliver high powers in beams close to the diffraction limit. The amplifiers will be injected by the same spectrally stabilized source that can be rapidly switched between two wavelengths around a water vapor transition.
In a first step, we will evaluate in detail the dynamic operation of these amplifiers in pulse mode (power, phase, spatial profile,...) on a dedicated characterization set-up.
Then we will design and develop an experimental setup for coherent combination of these components in an interferometer architecture. The stabilization of the relative phase of the amplified beams will have to be actively performed, with algorithms and electronics optimized for the pulse regime. We will study the performance of the device realized in relation to the specific needs of airborne atmospheric lidars (energy per pulse, spatial and spectral quality, emission stability...) taking into account the constraints of size, power consumption and mechanical stability inherent to such devices.
Finally, the coherent combination laser module will be integrated into the lidar system, the performances of the complete device will be compared to those of currently available instruments, and the various sources of uncertainty will be quantified. This will allow a first estimation of the gain in resolution and sensitivity of the final instrument.
At the end of this work, the lidar integrating the coherent combination module will be made available to the scientific community; it will contribute to the renewal of the French fleet with high-performance remote sensing equipment for probing the troposphere and enabling the study and understanding of complex convective phenomena. In addition, the work carried out on the coherent combination of semiconductor amplifiers could open up new prospects for applications in different spectral ranges and time regimes.

The Lasers group of the Charles Fabry Laboratory (https://www.lcf.institutoptique.fr/groupes-de-recherche/lasers) has been working for several years around the coherent combination of lasers and amplifiers, in collaboration with European industrial and academic partners specialized in the design, realization and modelling of such components.

This work is being carried out in collaboration with the Laboratoire des Sciences du Climat et de l'Environnement (CEA, Saclay, https://www.lsce.ipsl.fr/).

More information on this subject :
T.Y. Fan, IEEE J. Sel. Top. Quant. Electron. 11(3), 56 (2005)
P. Albrodt et al, Optics Express 27(20), pp 27891-27901 (2019)
S. Spuler et al., Atmos. Meas. Tech., 8, 1073–1087 (2015)

Work Context

The Lasers group of the Charles Fabry Laboratory (https://www.lcf.institutoptique.fr/groupes-de-recherche/lasers) has been working for several years around the coherent combination of lasers and amplifiers, in collaboration with European industrial and academic partners specialized in the design, realization and modelling of such components.

This work is being carried out in collaboration with the Laboratoire des Sciences du Climat et de l'Environnement (CEA, Saclay, https://www.lsce.ipsl.fr/)

Funding provided by the CNRS as part of the 80Prime 2020 call for projects.

Application procedure:
Applications should include a CV, a covering letter, a letter of recommendation and all transcripts and copies of diplomas.
To be sent by Email to Gaëlle Lucas-Leclin (gaelle.lucas-leclin@institutoptique.fr)

Constraints and risks

No particular risk, implementation of laser safety procedures, possibility of in-flight experiments within the framework of the French instrumented aircraft service for environmental research (SAFIRE).

Additional Information

Offer intended for candidates holding an engineering degree in optics, a Research Master 2 with a strong focus on optics or equivalent.

The criteria sought are :
- Solid knowledge in optics, lasers
- Very high scientific rigour
- A definite taste for experimentation
- Autonomy and motivation

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