Investigation of the mutual coherence of laser radiation in the Del'fin-1 facility

A method is proposed for investigation of the mutual coherence function Γ (x₁, x₂, τ) of a laser beam on the basis of an analysis of the Fourier transform of a speckle field formed as a result of interaction of laser radiation with a random Gaussian scatterer. The proposed method was used to study the coherence of high-power laser beams at various stages of pulse amplification in the Del'fin-1 facility designed for controlled laser fusion experiments. It was found that when the radiation from a master oscillator in this facility was practically coherent throughout the whole beam, subsequent amplification of a pulse reduced the half-width of the function Γ (x₁, x₂) to 0.1–0.2 of the beam diameter and this was due to nonlinear effects in the active medium at power densities ~ 1 GW/cm². A study was also made of the dependence of the function Γ (x₁, x₂) on the spectral composition of laser radiation.