Narrowing effect of focused polychromatic laser with small Fresnel number

Starting from Huygens–Fresnel diffraction integral, the field representation of focused polychromatic laser beams is derived. Intensity distributions of the focused polychromatic laser are investigated with different Fresnel number and bandwidth at focal plane. It is found that the width of transverse intensity distribution of the focused polychromatic laser decreases with increasing the bandwidth at the focal plane and zero local time, and the transverse intensity presents more obvious narrowing effects with smaller Fresnel number. A physical explanation for the narrowing effects is given based on a simplified expression.

Research Highlights
► We investigated intensity distributions of the focused polychromatic laser in small Fresnel number system.
► The width of transverse intensity distribution of the focused polychromatic laser decreases with increasing the bandwidth at the focal plane and zero local time.
► The transverse intensity presents more obvious narrowing effects with smaller Fresnel number.
► The interactional relations of parameters of transverse location, bandwidth, and Fresnel number result in the narrowing effects.