Developing fiber laser sources for LIDARs at Onera: a story of photons, phonons and scattering

François Gustave

Monitoring atmospheric properties such as wind structures and greenhouse gas concentrations is of great interest for meteorology or global warming survey, but also useful in the aeronautics industry to improve airport security or bring formation flights from birds to planes. Among all the methods employed to measure these atmospheric quantities, LIght Detection And Ranging (LIDAR) instruments offer the possibility to have long distance and spatially resolved measurements. The principle of operation is to emit laser light in the atmosphere and detect the small amount of photons coming back after retro-diffusion on aerosols or molecules. Wind speed measurement is usually achieved with a single frequency pulsed laser, by following the Doppler shift along the line of sight, induced by the motion of aerosols. For gas concentration measurements, one can compare the differential absorption between two wavelengths, one centered on an absorption line of the target molecule, while the other one is slightly detuned to experience a weaker absorption. To fulfil the measurement specifications, we can optimize the laser’s wavelength, polarization state, beam shape, spectral and temporal shape and average power. In most of the situations, increasing the power enables longer distance measurement and higher data availability.
In our group, we design and develop different type of LIDARs that we bring with us to the field, aboard aircraft or even small drones. To do so, we also need to carry on the system’s volume, electrical consumption and mechanical/thermal robustness. This is mainly why we use as much as possible monolithic fibered systems instead of free-space optics. In this presentation, I will detail how physics enters into account when developing such fiber laser sources. Focusing on few specific cases, I will go through the long-term fight against Brillouin scattering, amplification efficiency limitations inducing thermal effects and undesired spontaneous emission.

Dr. François GUSTAVE is currently research engineer in the Optics Department (DOTA) at ONERA, the French Aerospace Lab. His main topic is the development of fiber based laser sources for tele-detection and free space telecommunications. He received his Phd in physics from Université de la Côte d’Azur, Nice in 2016, in which he studied nonlinear dynamics of spatially extended semiconductor lasers. He then had a post-doc position in Université de Lille, working on integrable turbulence in optical fibers before integrating his current position in 2017.