  The free electron laser (FEL), as a next-generation light source, is an
attractive tool in scientific frontier research because of its advantages of
full coherence, ultra-short pulse duration, and controllable polarization.
Owing to the demand of real-time bunch diagnosis during FEL experiments,
precise nondestructive measurements of the polarization and X-ray energy
spectrum using one instrument are preferred. In this paper, such an instrument
based on the electron time-of-flight technique is proposed. By considering the
complexity and nonlinearity, a numerical model in the framework of Geant4 has
been developed for optimization. Taking the Shanghai Soft X-ray FEL user
facility as an example, its measurement performances' dependencies on the
critical parameters were studied systematically, and, finally, an optimal
design was obtained, achieving resolutions of 0.5% for the polarization degree
and 0.3 eV for the X-ray energy spectrum.
