Energy stability of supercontinuum via femtosecond filamentation in sapphire

The energy stability of supercontinuum (SC) significantly impacts its applications. To achieve the most stable SC, we systematically investigated how input pulse energy, numerical aperture (NA), and crystal thickness affect the energy stability of SC generated by femtosecond filamentation in sapphire. Our findings reveal that the SC energy does not always increase monotonically with input energy for different NA and thicknesses. This phenomenon occurs because, when the input pulse energy just exceeds the filamentation threshold, the pulse splitting structure and spectrum are still rapidly evolving. To generate a more stable SC, the numerical aperture and crystal thickness must be carefully coordinated to prevent this rapid evolution from occurring within the crystal.