As Free Electron Lasers create ultra-short bunch lengths, the longitudinal diagnostic for such femto-second bunches becomes more difficult. We suggest a bunch length method using the spectral analysis of Optical Transition Radiation (OTR) in the visible frequency domain.
The frequency content of OTR is measured by inserting an aluminium coated silicon wafer into the electron beam. The OTR light is collected with mirror optics into an optical fibre, which is coupled to a spectrometer (316 THz to 1500 THz). The resolution of the spectrometer allows us to measure bunch lengths lower than 100 fs rms. The bunch length was varied from 100 fs down to a few fs. The spectral content of OTR showed an increase of the correlation between neighbouring frequencies as bunch length was reduced.
The characterisation of our experimental setup will be presented and the theoretical principle of the measurement explained. Results from bunch length measurements acquired using the OTR spectrometer will then be presented, as well as measurements obtained from a transverse deflecting structure.