Toward Defeating Diffraction and Randomness for Laser Beam Propagation in Turbulent Atmosphere

Abstract

A large distance propagation in turbulent atmosphere results in disintegration of laser beam into speckles. We find that the most intense speckle approximately preserves both the Gaussian shape and the diameter of the initial collimated beam while loosing energy during propagation. One per 1000 of atmospheric realizations produces at 7km distance an intense speckle above 20% of the initial power. Such optimal realizations create effective extended lenses focusing the intense speckle beyond the diffraction limit of vacuum propagation. Atmospheric realizations change every several milliseconds. We propose to use intense speckles to greatly increase the time-averaged power delivery to the target plane by triggering the pulsed laser operations only at times of optimal realizations. Resulting power delivery and laser irradiance at the intense speckles well exceeds both intensity of diffraction-limited beam and intensity averaged over typical realizations. I = 0.04 max I = 0.19 max Fig. 1. (Color online) Distribution of laser irradiance I in transverse plane (only 69x69 cm central part of target screen is shown) after L = 7 km propagation of the collimated Gaussian laser beam with the waist w0 = 1.5 cm and the maximal intensity Imax = 1 through the turbulent atmosphere in the strong scintillation regime with σI = 3.3. Left panel: a typical atmospheric turbulence realization with Imax = 0.04 (61% of atmospheric realizations produce higher Imax). Right panel: a rare realization with Imax = 0.19 (0.16% realizations produce higher Imax). Dashed circles show w0 and the waist of diffraction limited beam propagated in vacuum. The initial Gaussian beam disintegrates into several speckles with the width of the most intense speckle being about w0. The intense speckles on left and right panels carry 4% and 19% of the total laser power, respectively.Laser beam propagation though turbulent atmosphere results in disintegration of laser beam into speckles at the distances exceeding several kilometers (strong irradiance fluctuation regime) [1], see Fig. 1 with examples of such propagation. At smaller distances (weak irradi-ance fluctuation regime) classic perturbative approaches well describe modification of laser beam propagation due to turbulence [2,3], while statistically averaged beam propagation in strong scintillation regimes is addressed through semi-heuristic theory [4]. The strength of the fluctuations of the irradiance I (laser beam intensity) at the target plane is characterized by the scintillation index σ I ≡ I 2 / I 2 − 1. Here and below by . . . we denote an the average over the ensemble of atmospheric turbulence realizations. It was shown in Ref.[5] that a significant fraction of deviation between theoretical value of σ I [4] and simulations is due to rare large fluctuations of laser beam intensity. Here we study the structure of large fluctuations and propose to use them for the efficient delivery of laser energy over long distances by triggering the pulse laser operations only during the times of such rare fluctuatio...

Optical vortex trapping and annihilation by means of nonlinear Bessel beams in nonlinear absorbing media

“…We have explored the combination of two vortices into a single vortex, but the same mechanism can provide a N to 1 vortex combiner, which in turn can combine or mix the waves or the particles that the vortices can guide. Also, the vortex annihilation capability could be relevant for cleaning speckled or scintillated beams, a topic that has been extensively discussed [14][15][16][17], and where complex solutions requiring substantial engineering have been described. We have proposed a simple setup able to remove undesired vortices without complicated step-by-step procedures or tight alignment requirements.…”

“…Vortex dipole annihilation in the NBB of the nonlinearly absorbing medium can be used massively to force the annihilation of many vortices of unit opposite charges acquired by speckled or scintillated light beams after propagation through random media such as a rough transparent plate or turbulent gaseous media [15]. Considerable effort has been paid to reduce the speckle of scintillated beams using different techniques [16].…”

“…Stabilizing the vortices and controlling their motion is of utmost importance in diverse applications such as waveguide writing, wave mixing and particle trapping [10][11][12][13]. In other situations, the presence of vortices in a light beam is highly undesirable, such as the vortices appearing spontaneously in speckled fields during propagation in a turbulent atmosphere, since they strongly deteriorate the optical performance of the light beam [14][15][16][17]. Considerable effort has been paid to reduce the speckle of scintillated beams using different techniques such as diffractive optical elements [16] or adaptive optics [17].…”

Statistical properties of a laser beam propagating in a turbulent medium

“…Such giant fluctuations were also observed in numerical experiments [7,8]. The work [9] studied the structure of large fluctuations and proposed to use them for the effi-cient delivery of laser energy over long distances by triggering the pulse laser operations only during the times of such rare fluctuations. It was demonstrated in Ref.…”

“…It was demonstrated in Ref. [9] that after 7 km propagation of the laser beam with the initial beam waist 1.5 cm in typical atmospheric conditions, about 0.1% atmospheric realizations carry 28% of initial power in a single giant fluctuation (a single speckle) of the laser intensity.…”