The invention relates to an optical element (21) which can be used for laser cutting of a material (26), comprising at least one aspherical surface (22) is shaped in order to focus the light rays (30) of the incident beam on a straight segment (25) located on the optical axis (29) of the optical element (21). The optical element (21) is transmissive type (21) or reflecting. For example, the optical element is formed of a lens (21), the aspherical surface (22) is defined by a radius of curvature (24) is continuously variable as a function of the distance to the optical axis (29) of the lens. The laser beam is assisted by an assistance gas containing at least one compound chosen from nitrogen, oxygen, helium, argon and their mixtures.
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