Beam Delivery of the Vanderbilt Free Electron Laser with Hollow Wave Guides: Effect on Temporal and Spatial Pulse Propagation

Hollow Wave Guides were evaluated as a beam delivery system for the Free Electron Laser (FEL) at Vanderbilt in preparation of surgical applications. They can transmit the mid-infrared wavelength range (2µm - 9µm) and tolerate the high peak intensity (>10¹⁴ W/m²) in the micropulse of the FEL. Changes in the temporal and spatial beam characteristics induced by the transmission through 1.5 meter Hollow Wave Guides with bore radii of 250 µm and 530 µm were investigated. Temporal broadening of the micro pulses was studied using intensity autocorrelation measurements and beam profile measurements were performed with a pyroelectric camera. Results demonstrate significant pulse broadening and development of higher order modes induced by sub-optimal coupling of the beam into the Hollow Wave Guide. Bending of the Hollow Wave Guide induced additional losses and reduced propagation of higher modes responsible for broadening the pulse. Calculations with a geometrical ray model support the findings on pulse broadening. Optimal coupling conditions are extremely critical for maximal transmission performance of the Hollow Wave Guide. Design consequences for a FEL delivery system are discussed.