Matthieu Valla

Pulsed 1.5-

In this paper, we present the development of an axial aircraft wake vortex light detection and ranging (LIDAR) sensor, working in Mie scattering regime, based on pulsed 1.5-mu m high-brightness large-core fiber amplifier. An end-to-end Doppler heterodyne LIDAR simulator is used for the LIDAR design. The simulation includes the observation geometry, the wake vortex velocity image, the scanning pattern, the LIDAR instrument, the wind turbulence outside the vortex, and the signal processing. An innovative high-brightness pulsed 1.5-mum laser source is described, based on a master oscillator power fiber amplifier (MOPFA) architecture with a large-core fiber. The obtained beam quality is excellent (M 2 = 1.3), and achieved pulsed energy is 120 muJ with a pulse repetition frequency of 12 kHz and a pulse duration of 800 ns. A Doppler heterodyne LIDAR is developed based on this laser source with a high-isolation free-space circulator. The LIDAR includes a real-time display of the wind field. Wind dispersion is postprocessed. Field tests carried out at Orly airport in April 2008 are reported. Axial aircraft wake vortex signatures have been successfully observed and acquired at a range of 1.2 km with axial resolution of 75 m for the first time with fiber laser source.

\mu

Abstract In this article, we discuss the designs and performances of narrow-linewidth erbium-ytterbium fiber MOPA for coherent detection lidars. We report on pedestal and multifilament core erbium-ytterbium large mode area fibers specially designed to generate high energy and good beam quality. Pulse energy of 220 μJ with M2 < 1.4 are obtained with all-fiber three amplification stages. An additional amplifier stage generates 600 μJ with M2 < 2.4. We present the first results of pulsed fiber lidar wake vortices detection using a coherent detection lidar. Maturation of this technology leads to compact all-fiber lasers. Several commercial laser sources for lidars are finally described.

m LIDAR for Axial Aircraft Wake Vortex Detection Based on High-Brightness Large-Core Fiber Amplifier

A pulsed fibre lidar based on 1.5 mum fibre technology has been demonstrated for wake vortex monitoring on airport sites. The wake vortex cores position resolution is plusmn2 m, the error on circulation 10%.

High Brightness 1.5 μm Pulsed Fiber Laser for Lidar: From Fibers to Systems

Originally developed for telecommunications, fiber lasers are now becoming new effective sources for coherent lidars allowing new instruments to be designed. The advent of the double clad fiber, along with advances in semiconductor pump diode sources, have allowed rapid power scaling of both pulsed and CW fiber sources. The unique capabilities of fiber sources, coupled with significant commercial and academic progress in implementation, have driven fiber technology to enter active remote sensing markets as signal sources and amplification stages for direct detection lidars and coherent lidars as well. Some interesting fiber lasers benefit from a good transmission in the near infrared spectral band. However, useful wavelengths have to be tuned between absorption H20 and CO2 lines. Eye safety may be an issue for atmospheric lidars. Above 1.4 µm, an eye-safe operation is possible even with multi-watt lasers. Low power fiber lasers using single mode fibers have a good spatial quality. However, higher power lasers and amplifiers need larger fiber cores, to store enough energy and to avoid non linear effects. Trade-off between high power, single mode operation, stable polarization and spectral quality need to be considered for coherent lidars.

1.5 μm all fiber pulsed lidar for wake vortex monitoring

AbstractThe measurement range of a coherent wind Doppler lidar (CWDL) along a laser beam is the maximum distance from the lidar where wind speed data are accurately retrieved. It means that, at this distance, a sufficient number of emitted laser photons are backscattered and received by the lidar. Understanding of the propagation of the laser through the atmosphere, and particularly the backscattering and extinction processes from aerosols, is therefore important to estimate the metrological performances of a CWDL instrument. The range is directly related to specific instrument characteristics and atmospheric content, such as the aerosols type, size, and density distributions. Associated with the measurement range is the notion of data availability, which can be defined, at a given range and over a time period, as the percentage number of data retrieved correctly by the CWDL over the total number of measurement attempts.This paper proposes a new approach to predict the CWDL data availability and range of ...

Fiber lasers: new effective sources for coherent lidars

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.

Simulation of Doppler Lidar Measurement Range and Data Availability

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.

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

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.

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

Abstract. Two important enabling technologies for pulsed coherent detection wind lidar are the laser and real-time signal processing. In particular, fiber laser is limited in peak power by nonlinear effects, such as stimulated Brillouin scattering (SBS). We report on various technologies that have been developed to mitigate SBS and increase peak power in 1.5-μm fiber lasers, such as special large mode area fiber designs or strain management. Range-resolved wind profiles up to a record range of 16 km within 0.1-s averaging time have been obtained thanks to those high-peak power fiber lasers. At long range, the lidar signal gets much weaker than the noise and special care is required to extract the Doppler peak from the spectral noise. To optimize real-time processing for weak carrier-to-noise ratio signal, we have studied various Doppler mean frequency estimators (MFE) and the influence of data accumulation on outliers occurrence. Five real-time MFEs (maximum, centroid, matched filter, maximum likelihood, and polynomial fit) have been compared in terms of error and processing time using lidar experimental data. MFE errors and data accumulation limits are established using a spectral method.

New lidar challenges for gas hazard management in industrial environments

We report the development of a high power single-frequency all-fiber laser for long-range wind speed measurement. The laser source has been integrated in a Lidar architecture and we report wind-speed measurement beyond 10 km.