BROADLY CURRENT TUNED FAR-INFRARED FREE-ELECTRON LASER

Laser output from a microtron-based free-electron laser is measured to be tunable from 160 to 280-mu-m (approximately 60 to 35 cm-1) by current control of an electromagnet helical undulator field. The output power increases with wavelength; thus, we expect to extend the wavelength tuning range which is now fixed by the limited long-wavelength response of the Ge: Ga photoconductor detector used in the measurements. The radiation linewidth is estimated to be about 0.5 cm-1 from the measurement of the absorption spectra of H2O vapor. The FIR radiation is tuned continuously at a rate of 0.3-mu-m/A by adjusting the helical undulator field from 500 to 750 G corresponding to the normalized helical-field K-parameter value varying from 1 to 1.5. The total spectral range is traversed in less than a minute by computer control of the undulator current and three steering coils. The FIR temporal profile varies periodically with tuning of the rf micropulse frequency with a period near 20 kHz, corresponding to a shift of the accumulative micropulse structure in the optical cavity by half a FIR wavelength. This evidence of micropulse radiation interference, optical threshold behavior, rapid tuning, and other properties of the laser emission of the AT & T Bell Laboratories free-electron laser are reported.