Variation of 3D vortex lines with the decrease of the waist w₀ of the incident Gaussian beam: (a) w₀ → ∞; (b) w₀ = 8; (c) w₀ = 6; (d) w₀ = 4. The vortex lines are mapped from the connection of phase singularities. w₀ is a normalized waist with respect to the pre-modulated w in Eq. (1).