Second harmonic generation of flat-topped Gaussian beams in single and twin BBO crystals

The coupled wave equations for second harmonic generation of flat-topped beam propagating in birefringent uniaxial anisotropic medium are presented. Based on them, specific physical models of single and twin BBO crystals have been established and numerically computations have been carried out. The influences of beam waist, medium structure on second harmonic generation are analyzed. The calculation results indicate that a wider beam waist can make a better phase matching, therefore the second harmonic beam will maintain the flat-topped property better but with a lower intensity and twin crystals can eliminate beam walk-off in some way.