Claudine Besson

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.

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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

Atmospheric gravity waves and turbulence generate small-scale fluctuations of wind, pressure, density, and temperature in the atmosphere. These fluctuations represent a real hazard for commercial aircraft and are known by the generic name of clear-air turbulence (CAT). Numerical weather prediction models do not resolve CAT and therefore provide only a probability of occurrence. A ground-based Rayleigh lidar was designed and implemented to remotely detect and characterize the atmospheric variability induced by turbulence in vertical scales between 40 m and a few hundred meters. Field measurements were performed at Observatoire de Haute-Provence (OHP, France) on 8 December 2008 and 23 June 2009. The estimate of the mean squared amplitude of bidimensional fluctuations of lidar signal showed excess compared to the estimated contribution of the instrumental noise. This excess can be attributed to atmospheric turbulence with a 95% confidence level. During the first night, data from collocated stratosphere-troposphere (ST) radar were available. Altitudes of the turbulent layers detected by the lidar were roughly consistent with those of layers with enhanced radar echo. The derived values of turbulence parameters Cn2 or CT2 were in the range of those published in the literature using ST radar data. However, the detection was at the limit of the instrumental noise and additional measurement campaigns are highly desirable to confirm these initial results. This is to our knowledge the first successful attempt to detect CAT in the free troposphere using an incoherent Rayleigh lidar system. The built lidar device may serve as a test bed for the definition of embarked CAT detection lidar systems aboard airliners.

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

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%.

Tentative detection of clear-air turbulence using a ground-based Rayleigh lidar

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. 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. 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.

1.5 μm all fiber pulsed lidar for wake vortex monitoring

We report on our new test bench dedicated to Supercontinuum Absorption Spectroscopy in the mid-infrared (3.3 µm). It delivers fast (<0.1 s) and wideband spectra (200 nm) at 0.8 cm-1 resolution. Gas concentrations are retrieved using a DOAS-inspired algorithm.

Long range wind lidars based on novel high spectral brilliance all-fibered sources

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.

Fast and wideband supercontinuum absorption spectroscopy in the mid-IR range

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.

Long-range wind monitoring in real time with optimized coherent lidar

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.

High Peak Power Fiber Laser for Long Range Lidar Applications

The objective of the ONERA study in the AIM project “Advanced In-flight Measurement Techniques” is to assess the capability of on board LIDAR technique to investigate in-flight tip vortices behaviour. This paper presents the design of as \(1.5\,\upmu \mathrm{{m}}\) LIDAR sensor dedicated to tip vortex characterization and tests on ground during trials on a DLR helicopter in hover flight. The relevant information resulting from these trials is the tip vortex velocity field as well as the time evolution of the vortex. The technical challenge here is to characterize a very small phenomenon at short range: the core radius varies from typically 10 to 30 mm as the vortex ages. The study results show that LIDAR technique is promising for onboard measurement during flight. The velocity measurement is direct and absolute (no calibration needed) and its accuracy can be up to 0.25 m/s and commonly 1 m/s. However, seeding is necessary to realize a compact and reliable LIDAR system with components ‘off the shelf’: in the framework of flight trials, clouds could provide efficient seeding enabling the use of LIDAR as a powerful technique for tip vortex characterization.