FIR optical cavity oscillation is observed with the ATT Bell Laboratories free-electron laser

摘要

Abstract: Laser emission is observed at a wavelength near 225 microns from a microtron-based free electron laser. The microtron accelerator produces macropulses with charge of 1 $mu@C at energies of 19 - 20 MeV and a 30 Hz repetition rate. The 16 $mu@sec macropulse consists of 3 GHz micropulses approximately 6 mm long. The electron beam is steered and focused into a 10 m, 20 cm period, 750 helical undulator whose axis is collinear with that of a 15 m optical cavity formed by two 10 m radius of curvature copper mirrors. The far-infrared radiation is coupled through a 6 mm diameter hole located 5 mm off axis in one mirror and steered 10 m from the free electron laser and detected with a stressed Ge:Ga detector. A positive round trip laser gain is inferred from the temporal profile of the signal. The FIR temporal profile varies periodically with tuning of the rf micropulse frequency with a period of 25 kHz. This evidence of micropulse radiation interference and evidence of threshold behavior indicate optical cavity oscillation in the FIR. The intra-cavity power is estimated to be about 100 Watts. Characterization of the AT&T Bell Laboratories free electron laser is in progress and present status is reported.!8