FREE-ELECTRON LASER OSCILLATOR FOR SIMULTANEOUS NARROW SPECTRAL RESOLUTION AND FAST TIME RESOLUTION SPECTROSCOPY

摘要

A technique for increasing the temporal resolution of the optical pulses whi te simultaneously increasing the spectral resolu- tion of the pulse train in a free-electron laser. The increase in temporal resolution is achieved by introducing a quadratic temporal phase variation (i.e. linear frequency chirp) across the optical pulses, and then propagating the outcoupled pulses through a dis- persive delay line such as a grating pair or prism pair in order to compress them to the transform limit of the broadened spec- trum. Essentially, the quadratic time dependence of the optical phase broade ns the spectrum and yields Fourier components whose phase exhibits a quadratic frequency dependence. To operate a free- electron laser in such a configuration, electron micro- pulses with a linear time-dependent energy are produced in the RF linac by positioning the bunches slightly off the peak of the RF wave at phases where the field gradient is large. These electron micropulses are injected into the wiggler and lead to the for- mation of optical pulses with a corresponding linear time-dependent frequenc y.