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

Abstract

All space-time coupling effects arising in an asymmetric optical compressor consisting of two non-identical pairs of diffraction gratings are described analytically. In each pair, the gratings are identical and parallel to each other, whereas the distance between the gratings, the groove density and the angle of incidence are different in different pairs. It is shown that the compressor asymmetry does not affect the far-field fluence and on-axis focal intensity. The main distinctive feature of the asymmetric compressor is spatial noise lagging behind or overtaking the main pulse in proportion to the transverse wave vector. This results in a degraded contrast but reduces beam fluence fluctuations at the compressor output. Exact expressions are obtained for the spectrum of fluence fluctuations and fluence root mean square that depends only on one parameter characterizing compressor asymmetry. The efficiency of small-scale self-focusing suppression at subsequent pulse post-compression is estimated.

New grating compressor designs for XCELS and SEL-100 PW projects

“…At the same time, they can be easily modified into asymmetric compressors that ensure reduction of fluence fluctuations due to the time delay of high-frequency spatial harmonics [13,15] or spatial dispersion of the output beam [5,12,14]. In asymmetric compressors, grating pairs differ from each other: 𝐿 2 ≠ 𝐿 1 [5,12,14]; 𝑁 2 ≠ 𝑁 1 , 𝛼 2 ≠ 𝛼 1 [13]; 𝛾 2 ≠ 𝛾 1 [15]. Note that 𝛾 2 and 𝛾 1 can have not only different absolute values, but also signs.…”

“…The fill-factor 𝜂 depends on the energy and spectral properties of the spatial noise of the beam, in particular, on rms and effective spatial frequency [11]. Both these parameters can be significantly reduced by using an asymmetric compressor [5,[12][13][14] or a compressor with an out-of-plane geometry [15]. The purpose of this work is to search for the following compressor parameters: angle of incidence on the first grating 𝛼, distance between the gratings along the normal 𝐿, and groove density 𝑁 that allow obtaining the maximum value of 𝑊.…”