Cladding-pumped rare-earth-doped fiber laser technologies are currently among the best sources for high power applications. Theses extremely compact and robust sources appoint them as good candidate for aeronautical and space applications.
The double-clad (DC) fiber converts the poor beamquality of high-power large-area pump diodes from the 1st cladding to laser light at another wavelength guided in an active single-mode core. High-power coherent MOPA (Master Oscillator Power Amplifier) sources (several 10W CW or several 100W in pulsed regime) will soon be achieved. Unfortunately it also brings nonlinear effects which quickly impairs output signal distortions. Stimulated Brillouin scattering (SBS) and optical parametric amplification (OPA) have been shown to be strong limitations.
Based on amplifier modeling and experiments we discuss the performances of these sources.
To fully understand atmospheric dynamics, climate studies, energy transfer, and weather prediction the wind field is one
of the most important atmospheric state variables. Studies indicate that a global determination of the tropospheric wind
field to an accuracy of 0.5 m/s is critical for improved numerical weather forecasting. LEOSPHERE recently developed
a new generation long range compact, eye safe and transportable wind Lidar, named WLS70, capable to fully determine
locally the wind field in real time in the planetary boundary layer (PBL). First results of the measurement campaign put
in evidence both wind velocity vertical profiles and atmosphere structure derived from Lidar data.
We report of a ship borne off-axis laser warning sensor designed and built at Onera. Classical laser warning sensors
detect a laser beam when it is illuminating the warning sensor, meaning that the sensor is located along the laser
propagation axis. Hence, large ships are difficult to protect since they require multiple laser warning sensors installed at
various locations on the ship hull. On the contrary, the off-axis sensor collects the Mie scattered flux out of the beam
propagation axis and looks at the beam sideways. Therefore it can be designed so as to protect the whole ship with only
one sensor covering 360°. This paper describes the system and the instrument performance model in maritime
environment. Field trials off the Mediterranean coast were conducted by Thales Optronique in fall 2007 in order to
validate the concept in operational conditions.
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.
Mid-infrared (IR) lasers are of interest for a variety of applications including environmental sensing, LIDAR and
military counter measures. However, this wavelength range lacks powerful, coherent, robust and compact sources. A
solution can lie in chalcogenide glasses as host materials for rare earth ions. With an extended infrared transparency, low
phonon energy limiting the non radiative multiphonon relaxation rates and suitable rare earth solubility, sulfide glasses
based on Ge-Ga-Sb-S system make available radiative transitions in the mid-IR range. The glasses with nominal
composition of Ge20Ga5Sb10S65 doped with Er3+ (500 to 10000 ppm) were prepared by means of conventional melting
and quenching method. The Er3+, widely studied in glass fibers for near-IR amplification, was initially selected for the
transition 4I9/2 to 4I11/2 emitting at around 4.5 &mgr;m in order to demonstrate the ability of this sulfide composition for midinfrared
fiber lasers application. In these objectives, absorption and emission spectra have been recorded and the
radiative decay lifetime of excited levels (4I9/2, 4I11/2 and 4I13/2) has been determined. These last experimental results were
compared with those obtained by Judd-Ofelt model from absorption cross-sections of all observable transitions.
Therefore, the 4I9/2 radiative quantum efficiency was estimated at 67 %. The emission cross-section was 2.6x10-21 cm2 at
4.6 &mgr;m obtained by Fütchbauer-Ladenburg theory. The product of measured lifetime and emission cross-section for 4I9/2
-> 4I11/2 transition is about 1.87x10-24 cm2.s is comparable with that for GaLaS glasses. The fiber drawing of the Er3+
doped Ge20Ga5Sb10S65 glasses and measurements of optical losses in mid-IR are currently in progress and first results
Vibration measurement using coherent laser radar (LADAR) is a promising way to identify air targets at long range. Laser vibrometers can remotely measure the velocity of micrometric displacements and thus exhibit the target surface vibration frequencies. Some of these frequencies are modal frequencies, which result from the target structure. They define a unique signature and allow target identification to be performed. As vibration amplitudes are not reliable, we choose to consider only frequency positions.
In this article, we explain an "extended identification" method which takes into account cumulative signatures in space and time to improve global system identification performance. Using a nearest neighbor classifier and a suitable metric taking into account a simple off-line processing of measured data, the recognition algorithm leads to good identification rates and very low rejection rates for a nine class problem. We show a strong improvement of the identification rate thanks to the "extended identification" method.
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.
Laser beams can be detected when propagating in the atmosphere, even if they are not pointed straight on the sensors. Mie scattering on aerosols allows to detect off-axis beams and to localize the source thanks to their geometric, spectral and temporal properties.
In the recent years, Onera has conducted both theoretical and experimental studies to detect off-axis non cooperative pulsed laser beams in the lower atmosphere. Some simulations are presented, based on propagation and scattering physics. Experimental results are 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.
We describe a 3D vision system TILT, designed and constructed at ONERA in France. The system was developed for mobile robotics applications and successfully tested on an outdoor vehicle. The system consists of two main units: the optical head and the electronic rack. Original aspects of TILT concern its fast frame rate and its angular resolution. Main parts are described and some results are discussed. Some images are analyzed, giving typical examples of possible performances.
Knowledge of deformation of every point of a wave front over time allows statistical turbulence parameters to be analyzed, and the definition of real time adaptive optics to be designed. An optical instrumentation was built to meet this need. Integrated in a compact enclosure for experiments on outdoor sites, the CASOAR allows the deformations of a wave front to be measured rapidly (100 Hz) and with accuracy (1 deg). The CASOAR is an active system: it includes its own light source (CW CO2 laser), making it self-contained, self-aligned and insensitive to spurious light rays. After being reflected off a mirror located beyond the atmospheric layer to be analyzed (range of several kilometers), the beam is received and detected by coherent mixing. Electronic phase is converted in optical phase and recorded or displayed in real time on a monitor. Experimental results are shown, pointing out the capabilities of this device.