  The polarization switch of a free-electron laser (FEL) is of great importance
to the user scientific community. In this paper, we investigate the generation
of controllable polarization FEL from two well-known approaches for Dalian
coherent light source, i.e., crossed planar undulator and elliptical permanent
undulator. In order to perform a fair comparative study, a one-dimensional
time-dependent FEL code has been developed, in which the imperfection effects
of an elliptical permanent undulator are taken into account. Comprehensive
simulation results indicate that the residual beam energy chirp and the
intrinsic FEL gain may contribute to the degradation of the polarization
performance for the crossed planar undulator. And the elliptical permanent
undulator is not very sensitive to the undulator errors and beam imperfections.
Meanwhile, with proper configurations of the main planar undulators and
additional elliptical permanent undulator section, circular polarized FEL with
pulse energy exceeds 100 $\mu$J could be achieved at Dalian coherent light
source.
