  A spatial light modulator (SLM) is one of the most useful and convenient
device to generate structural light beams such as twisted light and complexed
images used in modern optical science. The unbounded dimension of twisted light
makes it promising in harnessing information carrying ability of a single
photon, which greatly enhances the channel capacity in optical communications.
We perform a detail theoretical study of the birth, evolution and reverse
transformation of twisted light generated from a phase-only SLM based on
diffraction theory, analytical expressions are obtained to show the special
evolution behaviors of the light beam with the propagation distance. Beam
intensity distributions calculated theoretically are in well agreement with
experimental observations. Our findings clearly reveal how twisted light
gradually emerges from a Gaussian spatial shape to a ring shape, and also the
ring shape will gradually evolve to a quasi-Gaussian shape conversely, in the
reverse transformation. These results will provide guidelines for using SLM in
many optical experiments and OAM-mode multiplexing in optical communications.
