[Paper Review] Reducing laser beam fluence and intensity fluctuations in symmetric and asymmetric compressors

Е. А. Хазанов|arXiv (Cornell University)|Sep 3, 2023

Laser-Matter Interactions and ApplicationsPhysics and Astronomy3 citations

TL;DR

This paper analytically models space-time coupling effects in asymmetric optical compressors composed of two non-identical grating pairs, showing that asymmetry suppresses beam fluence fluctuations without degrading on-axis intensity. The key result is an exact expression for fluence root-mean-square (rms) fluctuations dependent solely on a single asymmetry parameter, enabling effective suppression of small-scale self-focusing during post-compression.

ABSTRACT

All space-time coupling effects arising in an asymmetric optical compressor [1] 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 rms that depends only on one parameter characterizing compressor asymmetry. The efficiency of small-scale self-focusing suppression at subsequent pulse post-compression is estimated.

Motivation & Objective

• To analyze space-time coupling effects in asymmetric optical compressors with non-identical grating pairs.
• To determine how compressor asymmetry affects beam fluence and intensity fluctuations.
• To quantify the suppression of small-scale self-focusing through reduced fluence fluctuations.
• To derive exact expressions for fluence fluctuation spectra and rms values dependent only on a single asymmetry parameter.
• To evaluate the impact of asymmetry on far-field fluence and on-axis focal intensity.

Proposed method

• Analytical derivation of space-time coupling effects in asymmetric compressors using two pairs of identical, parallel diffraction gratings with differing groove density, incidence angle, and grating separation.
• Modeling of beam propagation through the compressor to compute fluence and intensity fluctuations in the time and spatial frequency domains.
• Derivation of exact expressions for the spectrum of fluence fluctuations and fluence rms as functions of a single asymmetry parameter.
• Evaluation of the contrast degradation due to spatial noise lagging or leading the main pulse, proportional to the transverse wave vector.
• Assessment of the efficiency of self-focusing suppression in subsequent post-compression stages based on reduced fluence fluctuations.
• Use of exact analytical solutions to show that far-field fluence and on-axis focal intensity remain unaffected by asymmetry.

Experimental results

Research questions

• RQ1 How does asymmetry in grating pair parameters affect fluence and intensity fluctuations in optical compressors?
• RQ2 What is the dependence of fluence fluctuation spectra and rms on the degree of compressor asymmetry?
• RQ3 Does asymmetry degrade on-axis focal intensity or far-field fluence despite introducing spatial noise?
• RQ4 To what extent does asymmetry suppress small-scale self-focusing in post-compression stages?
• RQ5 Can the fluence fluctuation statistics be described by a single universal parameter independent of other system details?

Key findings

• The compressor asymmetry does not alter the far-field fluence or on-axis focal intensity, preserving beam quality.
• Spatial noise is introduced that lags or leads the main pulse, proportional to the transverse wave vector, degrading temporal contrast.
• The fluence root-mean-square (rms) fluctuation is exactly expressible as a function of a single asymmetry parameter, enabling precise control.
• The spectrum of fluence fluctuations is analytically derived and depends only on this asymmetry parameter, simplifying system design and optimization.
• Reduced fluence fluctuations lead to effective suppression of small-scale self-focusing during subsequent pulse post-compression.
• The analytical framework provides a complete description of fluence fluctuation statistics without requiring numerical simulations.