Clinical beam delivery of the Vanderbilt FEL: design and performance of a hollow-waveguide-based handheld probe for neurosurgery

The design of a handheld hollow waveguide-based probe is presented to deliver the unique output of the Vanderbilt Free Electron Laser (FEL) for medical applications. The neurosurgical hand piece incorporates a hollow waveguide, a focusing microlens, an optical fiber to deliver an aiming beam ((lambda) equals 632 nm) and a channel for purging the probe with nitrogen. The hollow waveguide is preferentially used to transmit a wavelength of lambda equals 6.45 micrometer (vibration mode of the amide-II band) and is able to tolerate the high peak intensity (greater than 10 14 W/m 2 ) of the picosecond micropulses of the FEL. The calcium fluoride lens focuses the beam to a spot diameter of 400 micrometer at a working distance of 20 mm. To maximize the transmission of the probe, the hollow waveguide is purged with nitrogen to prevent atmospheric absorption at 6.45 micrometer. Temporal broadening of the micro pulses that propagate in the hollow wave guide was studied using intensity autocorrelation measurements and beam profile measurements with a pyroelectric camera. Design consequences and application of the probe for FEL neurosurgery are discussed.