Study on the time-varying and propagating characteristics of ultrashort pulse Laguerre-Gaussian beams

This work proposes a simple model of pulse Laguerre-Gaussian Beams (LGBs) by chopping the incident continuous wave LGB into ultrashort pulse. The pulse LGBs are expanded into a series of LGBs with the same angular quantum number (AQN), whose expansion coefficients show the time-varying and propagating characteristics. Bigger radial quantum number (RQN) of incident LGB will cause more serious mode dispersion. This work discusses the case of zero RQN incident LGB intensively, in which the original modes, i.e. the same eigen-modes as the incident beams, can be used to approximate the pulse LGBs in short propagating range, e.g. 2zR. The original modes decrease, expand and delay when propagating, and it's more evident for incident LGBs with larger AQN . These conclusions are important for the optical communication by using the OAM division multiplexing (OAM-DM) technology.