Anne Durécu

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.

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.

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.

Coherent combining of fiber-laser-pumped frequency converters using all fiber electro-optic modulator for active phase control

Coherent beam combining (CBC) by active phase control could be useful for power scaling fiber-laser-pumped optical frequency converters like OPOs. However, a phase modulator operating at the frequency-converted wavelength is needed, which is non standard component. Fortunately, nonlinear conversion processes rely on a phase-matching condition correlating, not only the wave vectors of the coupled waves, but also their phases. This paper demonstrates that, using this phase correlation for indirect control of the phase, coherent combining of optical frequency converters is feasible using standard all-fibered electro-optic modulators. For the sake of demonstration, this new technique is experimentally applied twice for continuous wave second-harmonic-generator (SHG) combination: i) combining 2 SHG of 1.55-μm erbium-doped fiber amplifiers in PPLN crystals generating 775-nm beams; ii) combining 2 SHG of 1.064-μm ytterbium-doped fiber amplifiers in LBO crystals generating 532-nm beams. Excellent CBC efficiency is achieved on the harmonic waves in both these experiments, with λ/20 and λ/30 residual phase error respectively. In the second experiment, I/Q phase detection is added on fundamental and harmonic waves to measure their phase variations simultaneously. These measurements confirm the theoretical expectations and formulae of correlation between the phases of the fundamental and harmonic waves. Unexpectedly, in both experiments, when harmonic waves are phase-locked, a residual phase difference remains between the fundamen tal waves. Measurements of the spectrum of these residual phase differences locate them above 50 Hz, revealing that they most probably originate in fast-varying optical path differences induced by turbulence and acoustic-waves on the experimental breadboard.

Coherent combining of fiber lasers

We describe the coherent combining techniques that can be used to scale up fiber laser power far above single fiber laser limitations, and manipulate their wavefronts. The major configurations and realizations of coherent combining are then presented and compared in terms of maximum achievable number of combined lasers.

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

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.

Passively Q-switched Yb:YAG micro-laser for high peak power high repetition rate burst of pulses emission

Engine ignition using a laser requires very high peak power levels, that can be produced by solid-state lasers such as Yb:YAG passively Q-switched lasers. We developed high repetition rate diode pumped Yb:YAG micro-lasers to study the effect of cumulated pulses on the engine ignition process. The Yb:YAG laser oscillator is pumped by a 5-Hz quasi-continuous wave diode laser emitting 3-ms long pump pulses with up to 20 W peak power. It’s passively Q-switched using a Cr:YAG crystal. Various Yb:YAG dopant concentrations and crystal length have been tested and different initial transmittance values for the Cr:YAG crystal have been compared. As a result of quasi-continuous wave pumping and passive Q-switching, bursts of short pulses are emitted at the 5-Hz repetition frequency of the long pump pulses. The control of the intra-burst repetition rate is achieved through tuning the pump power between a few watts and 20 W. The energy per pulse ranges from 250 μJ to 300 μJ, with a lower than 5 ns pulse duration. The intra-burst repetition rate can go up to 20 kHz. An amplifying stage comprised of one single Yb:YAG crystal is added after this laser oscillator.

Coherent combining of fiber-laser-pumped 3.4 μm frequency converters

Coherent beam combining (CBC) by active phase control could be useful for power scaling fiber-laser-pumped optical frequency converters like optical parametric oscillators (OPOs). However, a phase modulator operating at the frequency-converted wavelength would be needed, which is a non-standard component. Fortunately, nonlinear conversion processes rely on a phase-matching condition, correlating not only the wave-vectors of the coupled waves, but also their phases. It is therefore possible to control the phase indirectly, using more standard phase modulators. Feasibility of this technique was previously demonstrated for second harmonic generators (SHG). Controlling the phase of the fundamental wave, excellent harmonic wave combining efficiency was achieved in both cases of phase matching and quasi phase matching, with lower than λ/30 residual phase error. In this paper, coherent combining of difference frequency generators (DFG) is experimentally tested. Even if DFG is more challenging than SHG as it implies handling three waves instead of two, phase control of the sole 1-μm pump waves is sufficient to combine the 3.4-μm waves generated. The mid-infrared DFG crystals are pumped and signal-seeded with standard all-fiber sources at 1 μm and 1.5 μm respectively. Phase control is performed with an electro-optic phase modulator which is a standard all-fiber component operating at 1 μm. CBC of mid-infrared DFG modules is a first step towards combining continuous wave OPOs.

Coherent combining of second-harmonic generators by active phase control of the fundamental waves

Coherent beam combining by active phase control could be useful for power scaling fiber-laser-pumped optical frequency converters. However, a fast phase modulator operating at the frequency-converted wavelength, a non-standard component, would be necessary. Fortunately, nonlinear conversion processes rely on a phase-matching condition allowing for indirect phase control using standard phase modulators. In this Letter, coherent combining of second-harmonic generators is demonstrated in both birefringent and quasi-phase-matching schemes in CW regime. Phase control operates at the fundamental wavelength, using all-fiber electro-optic modulators. An excellent beam combination is achieved with a residual phase error of λ/30 on the second-harmonic wave.

Single-frequency Raman fiber amplifier emitting 11 μj 150 W peak-power at 1645 nm for remote methane sensing applications

Remote methane concentration measurement using a Differential Absorption Lidar system can be performed using a single-frequency pulsed laser source at 1645.55 nm. This wavelength cannot be efficiently amplified in conventional Erbium Doped Fiber Amplifier as the gain band stops around 1620 nm. We report on a single-frequency polarization-maintaining pulsed amplifier at 1645 nm relying on stimulated Raman scattering (SRS) in highly nonlinear silica fibers (HNLF). Considering that SRS converts pump photons to photons frequency-downshifted by 13.2 THz with a gain bandwidth of 2 THz, a 1545 nm pump can efficiently amplify a 1645 nm seed laser. The drawback of using a HNLF is that the single-frequency signal will also experience stimulated Brillouin scattering (SBS) through its amplification. This issue has been partially solved by designing a two-stage amplification setup minimizing SBS. In the first stage, a 20 m piece of HNLF has been used so that the effective length of the amplified signal stays under SBS threshold. In the second stage, we used a 2.5 m piece of HNLF and high pump peak-power to significantly reduce the effective length, allowing more amplification. We report on generation of single-frequency 11 μJ energy pulses at 1645 nm corresponding to 150 W peak-power and 80 ns pulse duration at 20 kHz pulse repetition frequency.