深圳大学电子科学与技术学院 The radius of curvature r(z) has a variation with distance given analytically by R()=+≈{2f==f The wavefront is flat or planar right at the waist corresponding to an infinite radius of curvallloo or R(O=oo. As the beam propagate toward however, the wavefront gradually becomes curved, and the radius of curvature r(z)drops rather rapidly down to finite values深圳大学电子科学与技术学院 • The radius of curvature R(z) has a variation with distance given analytically by • The wavefront is flat or planar right at the waist, corresponding to an infinite radius of curvature or R(0)=. As the beam propagate toward, however, the wavefront gradually becomes curved, and the radius of curvature R(z) drops rather rapidly down to finite values.      = +  z f z f R(z) z 2 2 z f z f z f  = 