M. Brunel

Generation of parabolic bound pulses from a Yb-fiber laser

We report the observation of self-similar propagation of bound-state pulses in an ytterbium-doped double-clad fiber laser. A bound state of two positively chirped parabolic pulses with 5.4 ps duration separated by 14.9 ps is obtained, with 1.7 nJ of energy per pulse. These pulses are extra-cavity compressed to 100 fs. For higher pumping power and a different setting of the intra-cavity polarization controllers, the laser generates a bound state of three chirped parabolic pulses with different time separations and more than 1.5 nJ energy per pulse. Perturbation of this bound state by decreasing pump power results in the generation of a single pulse and a two-pulse bound state both structures traveling at the same velocity along the cavity. A possible explanation of the zero relative speed by a particular phase relation of the bound states is discussed.

Evidence of Brillouin scattering in an ytterbium-doped double-clad fiber laser.

We have designed and performed an experiment that permitted direct observation of Brillouin backscattering in an Yb-doped double-clad fiber laser. Fifteen Brillouin-shifted frequencies were observed for the first time to our knowledge. We clearly demonstrate that stimulated Brillouin scattering is directly responsible for both fast transient dynamics of the laser and reduction of the lasers pulse width.

Mode-lock, Q-switch and CW operation of an Yb-doped double-clad fiber ring laser

We investigate the dynamics of a high power unidirectional Yb-doped fiber ring laser with an intracavity polarizer placed between two halfwave plates. We show that, depending on the respective orientation of the phase plates, the laser operates in different regimes: CW, Q-switched, mode locked, or unstable. The passive method exposed here allows us to choose the operating regime of the laser by a simple rotation of a halfwave plate.

Generation of bound states of three ultrashort pulses with a passively mode-locked high-power Yb-doped double-clad fiber laser

We report the generation of high-power ultrashort bound states of three pulses in an ytterbium-doped double-clad fiber laser. The laser is mode-locked through nonlinear polarization rotation technique in a unidirectional cavity configuration. A pair of diffraction grating is incorporated in the cavity to compensate for the normal dispersion of the fiber. The laser generates chirped bound states of three pulses with either equal or different time separations, with more than 500-pJ energy per pulse. These pulses are subsequently compressed to 100 fs with a compression factor of more than 40.

Generation of high energy femtosecond pulses from a side-pumped Yb-doped double-clad fiber laser

We present a passively mode-locked side-pumped Yb-doped double-clad fiber laser, emitting around 1.05 μm. Polarization additive-pulse mode-locking technique is used. Mode locking is self-starting and produces 670 fs pulses after compression with a grating pair inserted in the cavity. The energy per pulse is 24 nJ.

90-fs stretched-pulse ytterbium-doped double-clad fiber laser.

We report ultrashort, high-energy pulse generation from a side-pumped Yb-doped double-clad fiber laser. The laser is operating in the stretched-pulse regime. It directly generates 4.8-ps, 5-nJ pulses that are extracavity compressed to 90 fs.

Influence of cavity losses on stimulated Brillouin scattering in a self-pulsing side-pumped ytterbium-doped double-clad fiber laser

We investigate experimentally the influence of cavity losses on Brillouin backscattering in an Yb-doped double-clad fiber laser. We show that stimulated Brillouin scattering is enhanced around a particular value of the cavity losses.

Real-time global interferometric laser imaging for the droplet sizing (ILIDS) algorithm for airborne research

Our objective was to develop an airborne optical sizemeter for atmospheric water droplet characterization. We chose the ILIDS (interferometric laser imaging for droplet sizing) method. This paper details the acquisition method developed to implement the image processing and the experimental tests performed to validate the method. The purpose is to develop a strategy for real-time data analysis in order to obtain information on droplets size during flight; this work is included in the EUFAR project that is supported by the 7th Framework Program of the European Commission (FP7).

Pump-saturation effects in end-pumped solid-state lasers

Pump-saturation effects in end-pumped solid-state lasers are considered. In particular we investigate the influence of the pump beam width on both the laser threshold and the laser efficiency in four-level laser material. We demonstrate that, in contrast with the results obtained when pump saturation is neglected, a pure on-axis pumping does not ensure the lowest threshold and the highest laser efficiency. Pump-saturation effects lead to an optimum pump beam width for a fixed laser beam radius.

Digital phase contrast with the fractional Fourier transform

A new method of digital phase contrast based on fractional-order Fourier reconstruction is proposed. We show that the diffraction patterns produced by pure phase objects exhibit linear chirp functions that can be advantageously processed using the fractional Fourier transform. The optimal fractional orders lead to the longitudinal location of the phase object, while the analysis of the reconstructed pattern leads to its diameter and to the value of the phase shift. Simulations and experimental results are given. The configuration tested in this paper is a very general Gaussian illumination.