Self-filtering of beam fluence fluctuations at free space propagation

Abstract

It is shown theoretically and experimentally that free space propagation in vacuum provides self-filtering of beam fluence fluctuations (spatial small-scale noise). The linear interference of the noise and the primary smooth beam during the propagation results in temporal filtering invoked by the noise pulse delay with respect to the primary beam pulse and in spatial filtering caused by the noise spatial walk-off from the aperture of the primary beam.

Reducing laser beam fluence and intensity fluctuations in symmetric and asymmetric compressors

“…The suppression of fluence fluctuations at free space propagation was analyzed analytically in Refs. [21,22]. These earlier results may be generalized taking into account Equations ( 10)- (13).…”

“…The transmission coefficient of such a filter can be obtained by generalizing expressions from Refs. [21,22] to the anisotropic case.…”

“…Two terms in the exponent correspond to two effects -lag (overtaking) and GVD. If we neglect the latter, that is, if ω 0 L L , then from Equation ( 26) we obtain a result fully analogous to temporal self-filtering [21,22] : the filter transmittance is equal to the square of the modulus of the autocorrelation function of the field A t with the argument equal to the time lag τ A (Equation ( 21)):…”

“…The detailed theoretical and experimental studies carried out in Refs. [21,22] showed that free space acts as a spectral filter of fluence fluctuations, with the filter transmittance being equal to the intensity autocorrelation function for spatial self-filtering and to the square of the field autocorrelation function for the temporal one.…”

2D-smoothing of laser beam fluctuations in optical compressor

“…To reduce fluctuations in high-power femtosecond lasers, spatial filters [2] before the compressor as well as spatial [3][4][5] and temporal [6,7] self-filtering after the compressor are used. The term 'self-filtering' [8,9] is used in the sense that no dedicated devices are required to observe this effect, just free-space propagation of the pulse. Physically, self-filtering is explained by the fact that spatial noise propagates at an angle to the wave vector of the principal wave.…”