We report on mid-infrared supercontinuum generation from 4 to 9 µm in orientation-patterned gallium-arsenide waveguides pumped by nanojoule-class ultrafast fiber lasers. The QPM waveguide and the laser source are optimized in tandem to pump the waveguides close to the degeneracy by means of sub-picosecond pulses at 2760 nm. The use of a waveguide geometry drastically reduces the required energy to the nanojoule level, thereby opening supercontinuum generation in GaAs platforms to fiber lasers.
We report on the design of OP-GaAs rib waveguides for frequency conversion in the mid-infrared and explore their performances for parametric generation. The samples used are between 10 and 25 mm long and exhibit quasi-phasematched (QPM) periods from 85 to 100 μm. The waveguides are pumped by a femtosecond erbium-doped fluoride fiber laser combined with a soliton self-frequency shift converter delivering sub-300 fs pulses at a wavelength tunable between 2.8 and 3.3 μm. By adjusting the pump wavelength, our OP-GaAs platform can produce ultrashort pulses widely tunable around 4 and 12 μm for the signal and idler, respectively. These results fit quite well our calculations of QPM curves.
Real-time measurements are now mature enough to cover a wide span of applications from fundamental laser physics and dynamics to applied sciences. In this talk, we focus on the application of time-stretch techniques for the ultrafast imaging of non-repetitive ultrafast events. In particular, we show that amplified time-stretch imaging fills the gap between ultra-high frame rate imaging techniques (burst-mode and/or temporal mapping cameras) and continuous imagers (CCD/CMOS) as it allows MHz frame rates on long - ms - timescales. As an illustration, we demonstrate the real-time tracking - i.e. propagation and reflection - of single laser-induced shockwaves (SWs) with velocities exceeding a few km/s and show that it allows, on the one hand, to monitor its full dynamics, from its deceleration to the observation of the plasma contact wave, and, on the other hand, to easily acquire intensity and velocity statistics on large ensembles of SWs [1]. This study has numerous potential applications in applied physics e.g. in the study of transient phenomena in pulsed laser-material interactions as these dynamics indeed strongly impact many scientific fields such as micromachining, material analysis or high-harmonics generation, to name a few. We also report the use of 1-D amplified time-stretch imaging to capture the rupture of liquid ligaments, which could bring new insights in two-phase flows physics.
[1] Hanzard et al., Appl. Phys. Lett. 112, 161106 (2018)
We use a continuous-wave low power incoherent seed to control spontaneous modulation instability (MI) in a highly-nonlinear
optical fiber. We show both experimentally and numerically that spectral and noise properties of MI can be
accurately controlled provided the spectral characteristics of the seed are chosen carefully. Specifically, we evidence the
strong influence of the seed coherence on the output pulses signal-to-noise ratio and bandwidth. Stochastic nonlinear
Schrödinger equation simulations are in excellent agreement with experiments.
We report the development of a new device able to record an optical wavefront with an improved sensitivity and without resorting to any reconstruction technique such as Zernike polynomial decomposition. A direct comparison with a standard Shack-Hartmann sensor is performed within the field of Z-scan-like experiments. It is also shown using computer-generated holograms that our device is suitable for characterizing phase discontinuities or sharp phase variations.
We use hybrid polymer-chalcogenide optical microwires to realize mid-infrared frequency conversion via the process of
normal dispersion modulation instability (MI). Phase-matching is achieved through a negative fourth-order dispersion
coefficient and leads to the apparition of parametric sidebands located at 2 μm and 3.5 μm, corresponding to a frequency
shift of 30 Thz relative to the pump, which is among the largest reported using normal-dispersion pumped MI in a singlepass
configuration in the mid-IR Stochastic nonlinear Schrödinger equation simulations are in excellent agreement with
experiments.
We explore an interferometric beam shaping technique that considers the coaxial superposition of two Gaussian beams. This technique is traditionally implemented in a Mach-Zehnder interferometer; however, to avoid phase shift drift due to vibrations and thermal effects we employ amplitude and phase modulation with a spatial light modulator (SLM) to achieve the beam shaping. We consider two Gaussian beams of equal but opposite curvature that possess the same phase and width incident on a focusing lens. At the plane of the lens we obtain a multi-ringed beam with a central intensity maximum which develops into a multi-ringed beam with a central null at the focal plane of the lens. The interesting feature of this beam is that it possesses two focal spots on either side of the focal plane of the lens. We investigate obstructing the beam at the focal plane of the lens and by carefully selecting the free parameters we obtain an unobstructed second focus while the equivalent Gaussian beam is sufficiently obstructed.
In this paper we experimentally demonstrate the intra-cavity generation of selected higher-order Laguerre-Gaussian
modes using a simple absorbing ring. First, we show selection of modes of variable radial order, from zero to five, with
zero azimuthal order. Second, we select super-positions of azimuthal modes of zero radial order but high azimuthal
index, up to eleven. In all cases we demonstrate high mode purity and a gain volume proportional to the order of the
mode. Our results suggest a possible route to high-brightness diode-pumped solid-state laser sources.
Many applications of lasers seek nowadays for focal spots whose corresponding volume is getting smaller and smaller in order to ensure high spatial resolution. This problem, studied by many research groups around the world, is the core of this research work which deals with controlling the focal volume of a focused laser beam. Indeed, our objective is to develop a new method based on spatial treatment of laser beams, allowing to solve, in an original and efficient manner, two fundamental issues that have not been treated satisfactorily yet, i.e. :
(i) The generation of a special laser beam, which has the ability to produce a focal volume smaller than the one resulting from a more common Gaussian beam, when focused by an ordinary lens. The expected reduction factor of the focal volume is in the order of several hundreds, when the existing methods do not exceed few tenths.
(ii) The decoupling between transversal and longitudinal resolutions within the focal volume, contrary to Gaussian beams whose depth of field is proportional to the square of its beam-waist radius. The method that it is developed is based on two steps: First, the laser is forced to oscillate on a high-order but single transversal mode TEMp0, which is secondly spatially beam-shaped thanks a proper Diffractive Optical Element (DOE) that allocates the super-resolution
feature².
A variant of the Fox & Li method performing intra-cavity laser beam shaping for resonators containing an arbitrary number of amplitude and phase diffractive optics is presented. As an illustration, the problem of forcing a laser to oscillate on a single high-order transverse mode has been considered. In particular, from numerical simulation, we deduce a simple model for generating such modes with a pi-phase plate inserted into a plano-concave cavity. This model
has been tested experimentally within an active cavity with a diode-pumped Nd:YVO4 laser and an excellent agreement with numerical predictions has been found: a phase aperture located quite close to the concave mirror, and whose normalized radius κ is so that 2κ2 corresponds to a zero of the desired p-order Laguerre polynomial, was sufficient to generate single cylindrical TEMp0 modes (p = 1, 2, 3) as long as its radius is correctly chosen. The laser based on the optimized features was perfectly stable, whatever the order of the generated mode.
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