Pierre Bourdon

Coherent beam combining of two femtosecond fiber chirped-pulse amplifiers

We demonstrate coherent beam combining of two femtosecond fiber chirped-pulse amplifiers seeded by a common oscillator. Using a feedback loop based on an electro-optic phase modulator, an average power of 7.2 W before compression is obtained with a combining efficiency of 90%. The spatial and temporal qualities of the oscillator are retained, with a recombined pulse width of 325 fs. This experiment opens up a way to scale the peak/average power of ultrafast fiber sources.

Beam Shaping of Single-Mode and Multimode Fiber Amplifier Arrays for Propagation Through Atmospheric Turbulence

We report experimental results and theoretical analysis of coherent beam combining with active phase control fiber beam shaping. An original optical configuration for target-in-the-loop single-mode fiber amplifier coherent combining through turbulence is presented, with a lambda/15 residual phase error. The experimental results and theoretical analysis demonstrate that detection subsystem aperture reduction is paramount to lower sensitivity to backward turbulence when using a detector in the laser emitter plane. In this configuration, coherent combining is achieved on a remote scattering surface with sole compensation of the onward turbulence. We also present a numerical model capable of assessing the combining efficiency in the case of high-power multimode large-mode-area (LMA) fiber amplifiers. Preliminary theoretical investigations point out that multiple-transverse-mode combining can result in severe wavefront distortion. In the case of multimode LMA fibers, control of the transverse modes phase relationship has to be achieved to preserve combining efficiency.

Coherent beam combination of narrow-linewidth 1.5 μm fiber amplifiers in a long-pulse regime.

We report what we believe to be the first experimental demonstration of coherent beam combining of two fiber amplifiers in a 100 ns pulse regime using a signal leak between the pulses. Pulses of ∼100 W stimulated-Brillouin-scattering limited peak power are combined with 95% efficiency, a residual phase error of λ/27, and no significant beam quality degradation.

Multifilament-core fibers for high energy pulse amplification at 1.5 μm with excellent beam quality

We report on what we believe to be the first demonstration of an erbium-ytterbium-doped multifilament-core (MFC) fiber for single-mode amplification of narrow linewidth high peak power pulses. A master-oscillator-power-fiber-amplifier laser source has been demonstrated using a 37-filament MFC fiber in the last amplification stage. Pulses with 750 microJ (940 W peak power) and laser linewidth<1 MHz have beam generated with M2 approximately 1.3. This value is close to the theoretical value M2 approximately 1.5.

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.

Characteristics of the Brillouin spectra in Erbium-Ytterbium fibers.

This paper reports the main characteristics of the Stokes spectra for typical pumped and unpumped Erbium-Ytterbium doped fibers. Doped fibers show shorter Brillouin shifts and their spectra are up to 1.6 times broader than undoped fibers. Those spectra are composed of several peaks originating from several longitudinal acoustic modes. The effective Brillouin gain of the secondary modes can be as large as 20% of the main peak gain. They can merge into a more complex structure for the largest cores. Simulations allow to relate these characteristics to the influence of codoping and index profile inhomogeneity. An additional broadening of the Stokes spectrum in pumped fibers is reported and attributed to thermal effects.

Analysis of the multifilament core fiber using the effective index theory

Multifilament core (MFC) fibers are a new type of microstructured fiber recently introduced. We investigate their properties using finite element modeling and show that the equivalent step index fiber based on moments theory does not provide similar properties. We propose an effective index theory based on the fundamental space filling mode which allows to predict the MFC properties using a semi-analytical modeling. Good resistance to bending is thus attributed to increased core effective index due to the high index filaments.

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.

Continuous-wave diode-laser injection-seeded {beta}-barium borate optical parametric oscillator: a reliable source for spectroscopic studies

A beta-barium borate optical parametric oscillator pumped by the third harmonic of a pulsed Nd:YAG laser and seeded by a cw diode laser is described. It operates on a single longitudinal mode of its cavity, with a linewidth lower than 500 MHz (0.017 cm(-1)). The conversion efficiency of the device is as great as 20% on signal output alone. The seeded tuning range is limited by the diodes tunability.