  We present a new feasible way to flatten the axial intensity oscillations for
diffraction of a finite-sized Bessel beam, through designing a cardioid-like
hole. The boundary formula of the cardioid-like hole is given analytically.
Numerical results by the complete Rayleigh-Sommerfeld method reveal that the
Bessel beam propagates stably in a considerably long axial range, after passing
through the cardioid-like hole. Compared with the gradually absorbing
apodization technique in previous papers, in this paper a hard truncation of
the incident Bessel beam is employed at the cardioid-like hole edges. The
proposed hard apodization technique takes two advantages in suppressing the
axial intensity oscillations, i.e., easier implementation and higher accuracy.
It is expected to have practical applications in laser machining, light
sectioning, or optical trapping.
