  Fast polarization switching of light sources is required over a wide spectral
range to investigate the symmetry of matter. In this Letter, we report the
first experimental demonstration of the crossed-planar undulator technique at a
seeded free-electron laser, which holds great promise for the full control and
fast switching of the polarization of short-wavelength radiation. In the
experiment, the polarization state of the coherent radiation at the 2nd
harmonic of the seed laser is switched successfully. The experiment results
confirm the theory, and pave the way for applying the crossed-planar undulator
technique for the seeded X-ray free electron lasers.
