The Effect Of Frequency Doubling In Frequency Doubling DPL On Beam Quality

Based on the frequent-doubling theory of light, Gaussian beam as a beam model, the beam quality affected by the process of frequent-doubling effecting is studied.Assumed that the beam in confocal resonator at initial time is fundamental-mode Gaussian beam, due to the second-order nonlinear effect of frequency doubling crystal, the beam getting through the frequency doubling crystal is not the fundamental-mode Gaussian beam. To describe the process, the modelling method is introduced. It means that make the beam through the frequency doubling crystal expanding in Laguerre-Gauss function space. The weights of modes are the coefficients by normalizing the coefficient of the expansion. The weights of three modes in the events that frequency doubling crystal is outside the resonator, inside the resonator with well-distributed gain, and inside the resonator with nonequilibrium gain distribution are put forward.In the situation of frequency doubling crystal inside the resonator, the fundamental laser beam get though the frequency doubling crystal back and forth, and it would be magnified by laser crystal. In that case, high modes build-up quickly, and the weight of fundamental-mode Gaussian beam can't dominate over the beam.