Frequency-to-amplitude modulation in angular filtering based on volume Bragg gratings (Conference Presentation)

Abstract

Issue of frequency-to-amplitude modulation (FM-to-AM) conversion is one of the key scientific problems in the development of high power lasers, especially in fusion laser drivers. In large aperture and high power lasers, the sinusoidal phase modulated pulses are used to avoid stimulated Brillouin scattering and to obtain spatially-averaged focal spot on the target. Propagation through optical components in the laser chains is not optimal and slightly filters the parts of the optical spectrum. Therefore, the frequency modulation (FM) is partly converted into the amplitude modulation (AM), and this AM will lead to higher-order nonlinear effects or even cause damages to optical elements due to instantaneous ultrahigh intensity in laser propagation process. Volume Bragg gratings (VBGs) with programmable angular selectivity from 0.1mrad to 10mrad and high efficiency of 99 % is an effective filtering element to clean up the spatial modulations in laser beams. The FM-to-AM conversion is studied with a sinusoidal phase modulation laser pulse with the bandwidth of 0.30nm and 0.15nm, and is demonstrated with an YLF laser with the wavelength of 1053 nm and pulse width of 3ns. The experimental results show that FM to AM conversion level is increasing with the decrease of the spectrum selectivity bandwidth of the VBGs. At the VBG spectrum selectivity bandwidth of 7.9 nm, the FM to AM conversion level is reduced to about 6% for 0.3 nm and 3.5% for 0.15nm, which can be used in high power laser for controlling the beam spot on the target.