Fluence noise dynamics of a laser pulse passing through optical systems with spatio-temporal dispersion

Spatial noise inevitably arising when femtosecond laser pulses are reflected from imperfect surfaces leads to fluctuations of beam fluence (energy density), which are the main cause of optical breakdown in high-power lasers. These fluctuations are evolving significantly when the beam passes through optical systems with spatio-temporal mixing, such as free space or a femtosecond pulse compressor with diffraction gratings. An analytical expression for the spatial spectrum of beam fluence fluctuations is obtained in a general form and verified by numerical simulation. The fluctuation smoothing efficiencies of symmetric and asymmetric compressors are compared. It is shown that the asymmetric compressor is advantageous only if the asymmetry is pronounced, while a slight violation of compressor symmetry worsens beam fluctuation smoothing.