Agnès Dolfi-Bouteyre

Performance evaluation of a dual fringe-imaging Michelson interferometer for air parameter measurements with a 355 nm Rayleigh-Mie lidar.

A new concept of spectrum analyzer is proposed for short-range lidar measurements in airborne applications. It implements a combination of two fringe-imaging Michelson interferometers to analyze the Rayleigh-Mie spectrum backscattered by molecules and particles at 355 nm. The objective is to perform simultaneous measurements of four variables: the air speed, the air temperature and density, and the particle scattering ratio. The Cramer-Rao bounds are calculated to evaluate the best expectable measurement accuracies. The performance optimization shows that a Michelson interferometer with a path difference of 3 cm is optimal for air speed measurements in clear air. To optimize density, temperature, and scattering ratio measurements, the second interferometer should be set to a path difference of 10 cm at least; 20 cm would be better to be less sensitive to the actual Rayleigh-Brillouin line shape.

Development and assessment of a wake vortex characterization algorithm based on a hybrid lidar signal processing

Since air traffic is in constant expansion, a more efficient optimisation of the airports capacity is expected. In this context, the characterization of aircraft hazardous turbulences known as wake vortices with an operational vortex Lidar is one of the major issues for the dynamic distances separation. A study has been probed to develop a hybrid vortex algorithm , i.e that uses both the velocity envelopes and a parametric estimator in the interest of processing time as short as possible. The aim is to make this algorithm exploitable for operational projects. That is why a methodology has been set up to evaluate its precision and its robustness. The results of tests on simulated scenarios of different aircraft vortices and different weather conditions show that this algorithm is able to localize precisely wake vortices and to estimate accurately their circulation in a short time.

Development of a short-range 355-nm Rayleigh-Mie lidar using a Michelson interferometer for wind speed measurements

We report the development of a 355-nm lidar system for short-range wind speed measurements, using a fringe-imaging Michelson interferometer as a spectral analyzer. The instrument principle is to deduce the wind speed from the phase variations of the two-wave interference pattern provided by the interferometer. A laboratory demonstrator has been realized, which was designed in an original way to minimize the sensitivity to phase fluctuations caused by thermo-mechanical disturbances and laser drifts. An accurate signal processing has been developed, providing with estimates of five fringe parameters: intensity, contrast, periodicity, angular orientation, and phase. It is implemented in two steps: the first step uses a Fourier transform analysis and the second step a maximum-likelihood estimator. To validate the instrument principle, measurement method and signal processing alltogether, a calibrated speed measurement experiment has been performed on hard target, for which the results are shown.

Passively Q-switched Yb:YAG micro-laser for high peak power high repetition rate burst of pulses emission

Engine ignition using a laser requires very high peak power levels, that can be produced by solid-state lasers such as Yb:YAG passively Q-switched lasers. We developed high repetition rate diode pumped Yb:YAG micro-lasers to study the effect of cumulated pulses on the engine ignition process. The Yb:YAG laser oscillator is pumped by a 5-Hz quasi-continuous wave diode laser emitting 3-ms long pump pulses with up to 20 W peak power. It’s passively Q-switched using a Cr:YAG crystal. Various Yb:YAG dopant concentrations and crystal length have been tested and different initial transmittance values for the Cr:YAG crystal have been compared. As a result of quasi-continuous wave pumping and passive Q-switching, bursts of short pulses are emitted at the 5-Hz repetition frequency of the long pump pulses. The control of the intra-burst repetition rate is achieved through tuning the pump power between a few watts and 20 W. The energy per pulse ranges from 250 μJ to 300 μJ, with a lower than 5 ns pulse duration. The intra-burst repetition rate can go up to 20 kHz. An amplifying stage comprised of one single Yb:YAG crystal is added after this laser oscillator.

All-fibered coherent-differential absorption lidar at 1.645 µm for simultaneous methane and wind speed measurements

Here we report on the development of a new coherent-DIAL system as well as first quantitative measurements of simultaneous gas mixing ratio and radial wind-speed with the instrument. Integrated measurement of atmospheric methane (CH4) mixing ratio between the instrument and a hard-target located at 2:25 km has been conducted with a relative precision of nearly 20% in 17 s. The measurement procedure also gives information on integrated water vapor (H2O) mixing ratio.

New lidar challenges for gas hazard management in industrial environments

The capability of Lidars to perform range-resolved gas profiles makes them an appealing choice for many applications. In order to address new remote sensing challenges, arising from industrial contexts, Onera currently develops two lidar systems, one Raman and one DIAL. On the Raman side, a high spatial-resolution multi-channel Raman Lidar is developed in partnership with the French National Radioactive Waste Management Agency (Andra). This development aims at enabling future monitoring of hydrogen gas and water vapor profiles inside disposal cells containing radioactive wastes. We report on the development and first tests of a three-channel Raman Lidar (H2, H2O, N2) designed to address this issue. Simultaneous hydrogen and water vapor profiles have been successfully performed along a 5m-long gas cell with 1m resolution at a distance of 85 m. On the DIAL side, a new instrumental concept is being explored and developed in partnership with Total E and P. The objective is to perform methane plume monitoring and flux assessment in the vicinity of industrials plants or platforms. For flux assessment, both gas concentration and air speed must be profiled by lidar. Therefore, we started developing a bi-function, all-fiber, coherent DIAL/Doppler Lidar. The first challenge was to design and build an appropriate fiber laser source. The achieved demonstrator delivers 200 W peak power, polarized, spectrally narrow (<15 MHz), 110 ns pulses of light out of a monomode fiber at 1645 nm. It fulfills the requirements for a future implementation in a bi-function Dial/Doppler lidar with km-range expectation. We report on the laser and lidar architecture, and on first lidar tests at 1645 nm.

High power pulsed fiber laser development for Co2 space based dial system

High energy fiber lasers emitting around 1579nm is seen as a possible technology for the laser unit of a spaceborn CO2 DIAL system. We are developing an all fiber system with the following expected performances: pulse energy of 260μJ, pulse duration 150ns, beam quality M2 <2, pulse linewidth <60 MHz, laser stability 200 kHz. One of our main concerns has been the radiation induced attenuation mitigation. Various fiber compositions have been investigated.

High Peak Power Fiber Laser for Long Range Lidar Applications

High peak power, narrow linewidth pulsed fiber lasers are key components for the design of long range Lidar. Examples of recent developments for wind monitoring for airport surveillance and gas sensing from space are described.

An Advanced Wind Infrared Doppler Lidar (WIND) for Mesoscale Meteorological Studies

Knowledge of the wind field at various scales is widely recognized at the present as fundamental to advancing our understanding and prediction of the weather development. Wind profiles are measured only at a few locations by radiosondes mainly in the northern hemisphere and there on the continents.