  Coherent light with orbital angular momentum (OAM) is of great interest, and
more recently, OAM light generation by coupling a relativistic electron beam
with a Gaussian mode laser pulse at the high harmonics of a helical undulator
has been demonstrated experimentally. In this paper, the possibility of
delivering coherent OAM light at the 3rd harmonic of the Gaussian mode seed
laser was discussed for the Shanghai deep ultraviolet free-electron laser
(SDUV-FEL). The considerations on the experiment setup, the expected
performance and the possible measurement method are given.
