ABSTRACT
Filament-induced ablation represents an attractive scheme for long-range material identification via optical spectroscopy. However, the delivery of laser energy to the target can be severely hindered by the stochastic nature of multiple-filamentation, ionization of ambient gas, and atmospheric turbulence. In order to mitigate some of these adverse effects, we examine the utility of beam shaping for femtosecond filament-induced breakdown spectroscopy with Gaussian and structured (Laguerre-Gaussian, Airy, and Bessel-Gaussian) beams in the nonlinear regime. Interaction of filaments with copper, zinc, and brass targets was studied by recording axially-resolved broadband emission from the filament-induced plasma. The laser-solid coupling efficacy was assessed by inferring thermodynamic parameters such as excitation temperature and electron density. While under our experimental conditions the ablation rate with Gaussian- and Laguerre-Gaussian beams is found to be similar, the Airy and Bessel-Gaussian beams offer the advantage of longitudinally extended working zones. These results provide insights into potential benefits of structuring ultrafast laser beams for standoff sensing applications.
ABSTRACT
Self-focusing of ultrashort pulses in air is investigated by means of numerical simulations. The role of the vibrational Raman effect and its dependence on pulse chirp is studied, with results shedding new light on the interpretation of the measurements of the critical self-focusing power. We also discuss computational modeling issues important specifically for few-cycle pulses.
ABSTRACT
Our experiments show that the critical power for self-focusing collapse of femtosecond vortex beams in air is significantly higher than that of a flattop beam and grows approximately linearly with the vortex order. With less than 10% of initial transverse intensity modulation of the beam profiles, the dominant mode of self-focusing collapse is the azimuthal breakup of the vortex rings into individual filaments, the number of which grows with the input beam power. The generated bottlelike distributions of plasma filaments rotate on propagation in the direction determined by the sense of vorticity.
ABSTRACT
Using numerical solutions of Maxwell's equations in conjunction with the Lorentz law of force, we compute the electromagnetic force distribution in and around a dielectric micro-sphere trapped by a focused laser beam. Dependence of the optical trap's stiffness on the polarization state of the incident beam is analyzed for particles suspended in air or immersed in water, under conditions similar to those realized in practical optical tweezers. A comparison of the simulation results with available experimental data reveals the merit of one physical model relative to two competing models; the three models arise from different interpretations of the same physical picture.
ABSTRACT
A new intensity-noise-subtraction scheme for an interferometric fiber-optic gyroscope is demonstrated. Together with the light circulating through the gyro coil, an appropriately attenuated light beam from the source coupler dead end of the gyro is directed to the gyro photodetector. When the gyro is unmodulated, or modulated with a square wave at the proper frequency, the intensity noises of the two beams bear opposite phases and interfere destructively on the detector. For an unmodulated gyro, we demonstrate an intensity noise-reduction factor of ~35, and for the same gyro modulated with a square wave at the proper frequency, we demonstrate a reduction factor of ~15.
