Uniform long focal depth with centimeter-scale range produced by an aspherical mirror

Abstract An aspherical mirror used to produce light fields with uniform long focal depth is first proposed by the energy conservation equation and the principle of equal optical path. The focusing characteristics of the novel mirror are studied in detail by numerical simulations based on scalar diffraction theory. Compared with parabolic mirror with similar parameters, the largest difference of surface profile is only at the micron level and the length of focal depth is stretched about 100 times. By using the apodization function, the oscillation of the axial intensity can be eliminated effectively and the axial inhomogeneity can be less than 0.5%. The effect of pulse shape on the focusing properties is also discussed and it is found that the pulse shape must be flat-top to obtain uniform long focal depth for a picosecond pulse. The novel mirror presented in this paper can be used for focusing high power ultrashort pulse laser with pulse width of picoseconds or femtoseconds, and there are many potential applications in physics of strong field.