  Independently from electron beam based procedures, photon beam based
diagnostics is an alternative way for alignment and commissioning of the
numerous undulator cells in a high-gain short-wavelength free-electron laser
(FEL). In this paper, using the seed laser modulated electron beam and the
undulator fine tuning technique, a coherent photon beam based diagnostic was
proposed for seeded FEL, and some preliminary experimental results at Shanghai
deep ultraviolet FEL test facility were presented. It demonstrates that the
spatial distribution of the coherent harmonic radiation from one individual or
two consecutive undulator segments can be used to optimize the electron beam
trajectory, to verify the undulator magnetic gap, and to adjust the phase match
between two undulator segments.
