Olivier Vasseur

Kilometer range filamentation

We demonstrate for the first time the possibility to generate long plasma channels up to a distance of 1 km, using the terawatt femtosecond T&T laser facility. The plasma density was optimized by adjusting the chirp, the focusing and beam diameter. The interaction of filaments with transparent and opaque targets was studied.

Coherent beam combining of fiber amplifier arrays and application to laser beam propagation through turbulent atmosphere

Coherent beam combining of fiber amplifier arrays is a promising way to increase power of fiber lasers, and overcome the physical limitations to fiber laser power scaling. We performed the coherent combining of fiber amplifier arrays using active control of the phase of each amplifier. The phase fluctuations in the fiber amplifiers have been measured and their effect on the beam combining process stability evaluated. We extended the coherent beam combining technique to perform wavefront shaping, in order to deliver a high brightness beam after turbulent atmospheric propagation. We present experimental results exhibiting the capability of the modulation multiplexing technique that we implemented to compensate phase fluctuations due to turbulent atmospheric propagation on the laser beam path. Moreover, and for the first time to our knowledge, we demonstrate automatic coherent combining of fiber amplifiers on a diffuse surface, after propagation through turbulent atmosphere, without any external turbulence measurement subsystem.

Assessment of laser-dazzling effects on TV cameras by means of pattern recognition algorithms

Imaging systems are widespread observation tools used to fulfil various functions such as detection, recognition, identification and video-tracking. These devices can be dazzled by using intensive light sources, e.g. lasers. In order to avoid such a disturbance, dazzling effects in TV-cameras must be better understood. In this paper we studied the influence of laser-dazzling on the performance of pattern recognition algorithms. The experiments were performed using a black and white TV-CCD-camera, dazzled by a nanosecond frequency doubled Nd:YAG laser. The camera observed a scene comprising different geometrical forms which had to be recognized by the algorithm. Different dazzling conditions were studied by varying the laser repetition rate, the pulse energy and the position of the geometrical forms relative to the laser spot. The algorithm is based on edge detection and locates areas with forms similar to a reference symbol. As a measure of correspondence it computes the degree of correlation of the different areas. The experiments show that dazzling can highly affect the performance of the used pattern recognition algorithms by generating lots of spurious edges which mimic the reference symbol. As a consequence dazzling results in detrimental effects, since it not only prevents the recognizing of well defined symbols, but it also creates many false alarms.

Study of filamentation threshold in zinc selenide.

Filamentation in different multi-photon absorption regimes was studied using different laser wavelengths in a zinc selenide crystal. The 3-photon ionization/absorption threshold was verified, and the impact of absorption on filament formation was observed.

Quantitative assessment of laser-dazzling effects on a CCD-camera through pattern-recognition-algorithms performance measurements

We used pattern-recognition-algorithms performance as a measurement standard for laser-dazzled images. A black and white CCD-camera observed a scene containing different geometrical patterns, which had to be recognized by the algorithm. The camera was dazzled by a nanosecond frequency doubled Nd:YAG laser. Dazzling conditions were variable in laser repetition rate, pulse energy, geometrical forms size and position relative to the laser spot. We implemented algorithms based on edge detection, which locate areas with similar forms compared with a reference symbol, using either a degree of correlation assessment or a Fourier descriptors quantitative analysis. We also characterized the dazzled area size in the image. Thanks to a cross analysis of both criteria, we succeeded in quantitatively assessing the influence of laser-dazzling on the performances of the algorithms. We point out the key role of the effective distance between a geometrical form and the dazzled area in the image on the computed degree of correlation or Fourier descriptor value of this form. The analysis of these quantitative results contributes to the better understanding of laser-dazzling, which can be useful to design efficient means to protect imaging systems.

Brightness Scaling Based on 1.55 μm Fiber Amplifiers Coherent Combining

Abstract Modulation multiplexing and beam cleanup are efficient techniques to coherently combine multiple fiber laser sources and achieve high power and very good beam quality. Using modulation multiplexing, three fiber amplifiers are coherently combined with a residual phase error of λ/30. Using beam cleanup by stimulated Brillouin scattering in a multimode gradient-index fiber within a ring cavity, coherent combining of two fiber amplifiers with 50% efficiency and 99% slope is achieved. Further potential of these techniques is also discussed.

Laser-dazzling effects on TV cameras: analysis of dazzling effects and experimental parameters weight assessment

Imaging systems are widespread observation tools used to fulfil various functions such as detection, recognition, identification and video-tracking. These devices can be dazzled by using intensive light sources, e.g. lasers. In order to avoid such a disturbance, dazzling effects in TV-cameras must be better understood. In this paper we studied the influence of different parameters on laser-dazzling. The experiments were performed using a black and white TV-CCD-camera, dazzled by a nanosecond frequency doubled Nd:YAG laser. Different dazzling conditions were studied by varying for instance the laser repetition rate, the pulse energy or the settings of the camera. We proceeded in two steps. First the different dazzling effects were analyzed and classified by their mainspring. Pure optical phenomena like multiple reflections, scattering and diffraction were discriminated from electronics effects related to charge transfer processes. Interactions between the laser repetition rate and the camera frequency or the camera exposure time were also observed. In a second step, experiments were carried out for different dazzling conditions. It was then possible to assess the weight of each experimental parameter on dazzling effects. The analysis of these quantitative results contributes to the better understanding of laser-dazzling, useful to design efficient means to protect imaging systems.

Spatial mode cleaning by femtosecond filamentation

We study the characteristic conical emission of a blue femtosecond filament in air. A significant improvement of the beam quality is obtained during the filamentation process.

Coherent combining of low-peak-power pulsed fiber amplifiers with 80-ns pulse duration

We investigate the feasibility of pulsed fiber amplifier coherent combining. Therefore, we characterize phase fluctuations in low-peak-power pulsed fiber amplifiers using two different interferometric techniques. These measurements reveal that for low peak-powers, phase fluctuations remain moderate during the pulses. Noticeable phase fluctuations occurring between the pulses can be perfectly controlled using classical continuous-wave-efficient combining techniques. Results of such realization combining two low-peak-power pulsed fiber amplifiers, using classical frequency-tagging coherent combining techniques, are presented. Phase difference measurement is performed between pulses using a small signal leak from the common master oscillator. For the first time to our knowledge, successful coherent combining of two low-peak- power pulsed fiber amplifiers is thereby demonstrated.

Theoretical analysis and quantitative measurements of fiber amplifier coherent combining on a remote surface through turbulence

Coherent beam combining of fiber amplifier arrays is an efficient way to overcome the physical limitations to fiber laser power scaling. Moreover, coherent combining techniques involving active phase control of the laser emitters offer the largest versatility, as they can also be used for complex purposes such as beam steering, wavefront shaping or atmospheric turbulence compensation. We reported last year the first experimental demonstration of coherent combining of fiber amplifiers on a remote scattering surface, after propagation through turbulent atmosphere, using the backscattered signal. These results were achieved with a frequency-tagging technique, and appropriate spatial filtering to lower sensitivity to backward turbulence, and compensate only for onward turbulent propagation. We present now experimental measurements of turbulence strength and resulting residual phase error. With turbulence compensation using the backscattered signal for phase control, this error is λ/15 rms. We also present the theoretical analysis of this experiment, emphasizing how limiting the aperture and not the field of view of the phase difference measurement subsystem reduces sensitivity to backward turbulence, without decreasing the optical flux on the detector.