Phase and spectrum control requirements of high intensity laser beam combining

摘要

Because of the limited optical element aperture, damage threshold, gain bandwidth, and so on, the output capability of a single laser beam is limited seriously. The coherent laser beam combining offers an excellent method to improve the peak intensity which could be gotten greatly. Aiming at getting the general requirements of the coherent beam combining for large aperture laser facilities, this work devotes to modeling the influences of the phase factors and spectrum factors on the combine results. The effects of the phase factors, including the piston error and tip/tilt error, are studied analytically and numerically. It is found that the expressions of the intensity in the focal plane can be written as three parts, the scale factor, a point spread function (PSF), and a grid function (GF), for the ideal beam combining and beam combining with piston error. Every part has its special physical meaning, and decides different characteristics of the combined focus. For the beam combining with tip/tilt error, though the expression of focal spot intensity can not be separated like the above situations, every part still has obvious physical meanings. The results show that the beam configuration can not affect the Strehl ratio of the combined beam, but it influences the FWHM of the main peak and the ratio of the main peak and the side peak. The piston error affects the grid function greatly, including its maximum value, transverse translation, and shape. For the two beam combining, a piston error less than 2π/5 rad is suitable. For multi-beam combining, the standard deviation of the piston error should be no more than 2π/10 rad. The tip/tilt error affects the superposition degree of the focal spots of the combined elements directly. A requirement of 0.5~1 μrad for the standard deviation of the tip/tilt error is adequate. The effects of the spectrum factors, including the longitudinal chromatism, high order dispersion, and residual chirp, are studied analyzed. Results show that the above spectrum factors have significant influences on the short pulse coherent beam combining, and must be controlled carefully when the pulse is shorter than lps.