We present a new method of enhancing the efficiency of a Compton free-electron laser. Measurements and a theoretical analysis show that the efficiency of a short pulse free-electron laser can be almost doubled using a two-section step-tapered undulator with the downstream undulator having the higher magnetic field, i.e., reversed with respect to conventional tapering. We also show that the stability of the laser and the spectral purity are improved at the highest output powers because sidebands in the spectrum due to synchrotron oscillations are suppressed. Smooth subpicosecond optical pulses with peak powers of 100 MW have been produced in the midinfrared.

• Received 22 September 1994

DOI:https://doi.org/10.1103/PhysRevLett.74.2224