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 (H<sub>2</sub>, H<sub>2</sub>O, N<sub>2</sub>) 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.
New Lidar applications related to aircraft safety in the area of an airport include mapping wind velocity and monitoring turbulences within a radius longer than 8km in a short acquisition time (360° map in 1 minute). During landing and takeoff, a minimal distance separation between aircrafts is set by referring to wake turbulence categories. However, it was shown that wake vortices can dissipate quicker because of atmospheric turbulence (characterized by eddy dissipation rate - EDR) or can be transported out of the way on oncoming traffic by cross-winds. Long range scanning Lidars provide radial wind data that can be used to calculate EDR. <p> </p>To reach long range within a short acquisition time, coherent wind Lidars require high power (~kW), narrow linewidth (few MHz) pulsed laser sources with nearly TF limited pulse duration (~1μs). Eyesafe, all-fiber laser sources based on MOPFA (master oscillator, power fiber amplifier) architecture offer many advantages over bulk sources such as low sensitivity to vibrations, efficiency and versatility. However, narrow linewidth pulsed fiber lasers and amplifiers are usually limited by nonlinear effects such as stimulated Brillouin scattering (SBS) to 300W with commercial fibers. We investigated various solutions to push this limit further. For example, a source based on a new fiber composition yielded a peak power of 1120W for 650ns pulse duration with excellent beam quality. Based on these innovative solutions we built a Lidar with a record range of 16km in 0.1s averaging time. <p> </p>In this proceeding, we present some recent results obtained with our wind Lidars based on these high power sources with record ranges. EDR measurements using the developed algorithm based on structure function calculation are presented, as well as its validation with simulations and measurements campaign results.
Recent progress in fiber technology has enabled new laser designs along with all fiber lidar architectures. Their asset is
to avoid free-space optics, sparing lengthy alignment procedures and yielding compact setups that are well adapted for
field operations and on board applications thanks to their intrinsic vibration-resistant architectures. We present results in
remote sensing for disaster management recently achieved with fiber laser systems. Field trials of a 3-paths lidar
vibrometer for the remote study of modal parameters of buildings has shown that application-related constraints were
fulfilled and that the obtained results are consistent with simultaneous in situ seismic sensors measurements. Remote
multi-gas detection can be obtained using broadband infrared spectroscopy. Results obtained on methane concentration
measurement using an infrared supercontinuum fiber laser and analysis in the 3-4 μm band are reported. For gas flux
retrieval, air velocity measurement is also required. Long range scanning all-fiber wind lidars are now available thanks to
innovative laser architectures. High peak power highly coherent pulses can be extracted from Er<sup>3+</sup>:Yb<sup>3+</sup> and Tm<sup>3+</sup> active fibers using methods described in the paper. The additional laser power provides increased coherent lidar capability in range and scanning of large areas but also better system resistance to adverse weather conditions. Wind sensing at ranges beyond 10 km have been achieved and on-going tests of a scanning system dedicated to airport safety is reported.
In order to optimize their flight conditions, airborne platforms need to know precisely their true airspeed. In helicopters, measuring low air speeds is a severe issue because of the rotor flow. Optical air data sensors are therefore a good alternative to classical pneumatic probes. ONERA is involved for many years in simulation and design of coherent lidar and focuses its last research on eye-safe solid state lidars. This paper describes the study of performance of a reliable compact airborne system based on a 1.5 μm Erbium fiber laser and architecture. The average heterodyne current power is examined for the case of negligible turbulence and truncation effect. The spatial resolution of the measurement is deduced and its behavior versus transmitter beam parameters discussed.
This paper describes a 1.55 micrometer coherent fiber laser radar designed and developed at ONERA in France. This eye-safe Doppler system uses a 0.5 W codoped Erbium/Ytterbium fiber laser and a compact fiber optical architecture. The system has been tested at distances up to 1 km. Experimental results are presented and performances compared to the theoretical model taking into account atmospheric propagation.