Free-Space Nonlinear Beam Combining for High Intensity Projection

Abstract

The controlled interaction of two high intensity beams opens new degrees of freedom for manipulating electromagnetic waves in air. The growing number of applications for laser filaments requires fine control of their formation and propagation. We demonstrate, experimentally and theoretically, that the attraction and fusion of two parallel ultrashort beams with initial powers below the critical value (70% P critical), in the regime where the non-linear optical characteristics of the medium become dominant, enable the eventual formation of a filament downstream. Filament formation is delayed to a predetermined distance in space, defined by the initial separation between the centroids, while still enabling filaments with controllable properties as if formed from a single above-critical power beam. This is confirmed by experimental and theoretical evidence of filament formation such as the individual beam profiles and the supercontinuum emission spectra associated with this interaction.

Dynamics of femtosecond synthesized coronary profile laser beam filamentation in air

“…Interestingly, a similar scenario with two subcritical η b < 1 beams separated in space was reported earlier in [28,35] when this twin beam may undergo filamentation as a result of sub-beam diffractive 'fusion', which leads to a net power increase in the common field structure above the critical level (2η b > 1). Even more, two copropagating beams with different wavelengths and subcritical power can also collapse provided that the sum of their powers is above a CB critical power [36].…”

“…Such a synthesized beam in the form of a matrix of four emitters has been experimentally proven to significantly increase the potential of long-range atmospheric systems for laser energy transmission (>3 km) without the use of any additional adaptive optics in the conditions of environmental jamming introduced by atmospheric turbulence and aerosol scattering [27]. It should be emphasized that in these experiments the incoherent sub-beam combining is used without any need for a complicated and expensive control over the partial phases of sub-apertures [28].…”

On parallel laser beam merger in plasmas

“…Thus, the discussed beam merger mechanism may be thought of as a mode selector mechanism, combining pulses of identical phases and repelling pulses with a significant phase shift. In previous experiments on beam combination in air, optical delay lines 8,22 were utilized to control the phase shift between the two beams, at least on a level of the 0.1k. In attosecond physics, there are methods to modify the optical delay lines to compensate for the instability in the optical paths caused by fluctuations, achieving the phase shift control on a level of less than 0:01k.…”

“…For instance, in Ref. 8, it was shown that such beam combination is experimentally feasible for two subcritical laser filaments (P 1 ; P 2 < P cr ) with total power P 1 þ P 2 exceeding P cr , and, thus, triggering mutual focusing and eventual beam merger. Such phenomenon was explained theoretically using nonlinear Schr€ odinger equation (NSE) envelope model.…”

Supercontinuum generation by co-filamentation of two color femtosecond laser pulses

“…Interactions between two filament-forming beams open new perspectives for in-flight control of filament propagation, as demonstrated employing various interaction geometries in gaseous 9–12 and solid-state 13–15 media. Even more possibilities are offered by the so-called two-color filamentation, i.e.…”