Abstract

The Mark III FEL is a continuously tunable IR laser operating between 1.9 and 9.5 µm.¹ The laser is harmonically mode locked by picosecond electron pulses from a 2.857 GHz pulsed rf linac and contains 39 circulating optical micropulses within the 2.05 m resonator. These micropulses have durations of several picoseconds and energies of several microjoules, and typically yield an output pulse train (or mecropulse) exhibiting a transform-limited optical spectrum and axial modes separated by the 73 MHz round trip frequency (since the 39 independent optical micropulses build up from noise with random phases). In spite of the intrinsically high axial mode resolution, this broad spectral width and narrow line spacing can pose a significant limitation for high resolution spectroscopy. However, by interferometrically coupling the adjacent optical pulses in the resonator, the axial mode spacing can be increased up to the rf frequency of the linac,^2,3 and the modes can then be easily filtered in the laboratory. Typical values of the cavity Q or the duration of the macropulse yield an intrinsic axial mode linewidth of several hundred kilohertz.

© 1994 Optical Society of America