General structure of color separating gratings used in ICF

Abstract

It is needed to divert the unwanted harmonic waves out of high power laser system to perform Inertial Confinement Fusion (ICF) and several color separation gratings (CSGs) have been put forward. For a general multilevel grating, the Fraunhofer diffraction patterns of harmonics in ICF laser system have been derived taking no account of optical dispersion. According to the wavelength relationship of the harmonics, the multilevel CSG adaptive for ICF must have special structure features to meet the requests. It is found that a CSG can be used in ICF when its steps number being common multiple of 3 and at the same time the optical path difference for 3ω harmonics between neighboring steps being integral multiple of its wavelength. However, the optical dispersion practically exists for actual laser system and the functions of CSG will be affected. Though by optimization method, the step width and step height of CSG can be changed slightly to compensate the influences, but its performance will be played down. For the CSGs with general structure, the influences of chromatic dispersion have been analyzed when the number of steps and phase delay difference between two neighboring steps varied. By comparison, it is found that the CSG with smaller steps and step height difference is more preferable for color separation in ICF.