PHASE LOCKING IN AN INFRARED SHORT-PULSE FREE-ELECTRON LASER

In a free-electron laser operating in the infrared and farinfrared spectral regions and using a radio-frequency accelerator for the electron beam, the electron pulse length can be of the same order as the slippage length or even shorter. Such a laser emits short pulses of multimode broad-band radiation. If the repetition time of the electron pulses is short compared to the round-trip time of an optical pulse in the resonator cavity, then phase locking between successive optical pulses can be induced by an intracavity interferometric device. This causes a reduction of the number of active cavity modes, and leads to a significant increase in the power per mode. External selection of a single mode with reasonable power then becomes possible. The feasibility of the phase-locking procedure has been tested by simulation of the optical pulse evolution in a short-pulse free-electron laser, using a model based on the self-consistent solution of the equations of motion for the electrons and the wave equation driven by single-particle currents. The simulations show that a high degree of coherence between successive pulses can be induced by a low-finesse etalon. Saturated operation in a greatly reduced number of modes, but with the same total power, is attained with a slight delay in the growth of the power as compared to the case without phase locking. © 1990 IEEE