Anton Ryabtsev

Vortices in the wake of a femtosecond laser filament.

We report on the experimental observation of fluid flow caused by propagation of femtosecond filaments in dry air. We find that the ionization of the medium deposits a non-negligible amount of heat, which creates vortices in a semi-confined glass cylinder. We confirm the influence of thermal gradients on vortex formation by the use of a heated wire in a similar configuration.

Fluid flow vorticity measurement using laser beams with orbital angular momentum

Vorticity is one of the most important dynamic flow variables and is fundamental to the basic flow physics of many areas of fluid dynamics, including aerodynamics, turbulent flows and chaotic motion. We report on the direct measurements of fluid flow vorticity using a beam with orbital angular momentum that takes advantage of the rotational Doppler shift from microparticles intersecting the beam focus. Experiments are carried out on fluid flows with well-characterized vorticity and the experimental results are found to be in excellent agreement with the expected values. This method allows for localized real-time determination of vorticity in a fluid flow with three-dimensional resolution.

Anomalous laser-induced group velocity dispersion in fused silica

We present 20fs(2) accuracy laser-induced group velocity dispersion (LI-GVD) measurements, resulting from propagation of a femtosecond laser pulse in 1mm of fused silica, as a function of peak intensity. For a 5.5 × 10(11) W/cm(2) peak intensity, LI-GVD values are found to vary from -3 to + 15 times the material GVD. Normal induced dispersion can be explained by the Kerr effect, but anomalous LI-GVD, found when the input pulses have negative pre-chirp, cannot. These findings have significant implications regarding self-compression and the design of femtosecond lasers.

Phase-only synthesis of ultrafast stretched square pulses.

We report the generation of square temporal-shape pulses with no loss of spectral bandwidth, using an analytic expression for the spectral phase modulation dependent only on the input spectrum and stretching factor. We demonstrate numerically and experimentally conversion of 40fs pulses into 150 times longer flat top pulses with sharp on and off fronts. Applications in pulse amplification and free electron lasers are considered.

Real-time single-shot measurement and correction of pulse phase and amplitude for ultrafast lasers

Abstract. The transition of femtosecond lasers from the laboratory to commercial applications requires real-time automated pulse compression, ensuring optimum performance without assistance. Single-shot phase measurements together with closed-loop optimization based on real-time multiphoton intrapulse interference phase scan are demonstrated. On-the-fly correction of amplitude, as well as second- and third-order phase distortions based on the real-time measurements, is accomplished by a pulse shaper.

Eye-safe near-infrared trace explosives detection and imaging

We report the development of a non-contact no-reagents system operating in the eye-safe 1560-1800 nm wavelength range for standoff trace detection of explosives and high-speed imaging. Experimental results are provided for a number of chemicals including explosives on a variety of surfaces at sub-microgram per cm2 concentration. Chemically specific images were collected at 0.06 ms per pixel. Results from this effort indicate that the combination of modern industrial fiber lasers and nonlinear optical spectroscopy can address next generation eye-safe trace detection of chemicals including explosives.

Mitigating self-action processes with chirp or binary phase shaping

We report the use of binary phase shaping to mitigate pulse degradation and self-focusing in fused silica. The results of simulation and estimated mitigation efficiency are supported by experimental results using both chirped and binary phase-shaped pulses. Possible applications are considered.

Characterization of vorticity in fluids by a spatially shaped laser beam

Measuring the speed and direction of vortices is of great importance in fluid dynamics. We report on the use of a CW laser beam with a superposition of Laguerre-Gaussian (LG) modes generated by a phase mask imprinted on a two-dimensional spatial light modulator. The shaped beam is then guided and scattered of a sample which is rotated; the rotational frequency is extracted from spectral analysis of the scattered light. This method allows for virtually real-time determination of vorticity characterization in a fluid.

All-fiber pulse shaper for adaptive dispersion compensation in industrial lasers

We report on industrial-grade femtosecond Yb fiber lasers with >100μJ pulse energy and <300fs pulse duration using a tunable all-fiber pulse shaper. The rugged, compact phase modulator is a lossless addition to the standard chirped-pulseamplification scheme. The automated multichannel phase control across the optical bandwidth enables generation of near transform-limited pulses at the laser output, improves unit-to-unit reproducibility of laser pulse characteristics, and reduces laser build time.

Characterization and adaptive compression of a multi-soliton laser source.

Ultrashort pulse generation in the 1600 nm wavelength region is required for deep-tissue biomedical imaging. We report on the characterization and adaptive compression of a multi-soliton output spanning >300 nm from a large-mode area photonic-crystal fiber rod for two separate laser setups. Sub-30 fs pulses are generated by first compressing of each soliton individually, and then followed by coherently combining all of the pulses in the train, which are separated by hundreds of femtoseconds. Simulations of the source, together with amplitude and phase coherence measurements are provided.