A mid-infrared Free Electron Laser (MIR-FEL) has potentials for medical application research. The MIR-FEL has great two advantages: (1) tunability and (2) double pulse structure. The Osaka MIR-FEL is tunable from 5 to 17 μm, and produces a macropulse, composed of a train of micropulses, each approximately 5 ps long and separated by 44.8 ns. (1) Since there exist main absorbers such as water, protein, and apatite in the tissue, wavelength tunability leads to a choice of the primary absorber. (2) Laser-tissue interactions are mainly determined by the power density of the laser used. Therefore the interactions can be precisely controlled by a combination of the energy density and pulse duration of the macropulse, and the repetition rate of the micropulse. Thus, the MIR-FEL can carefully optimize the laser conditions for a non-invasive treatment without accompanying side and adverse effects by individually controlling the FEL parameters (wavelength, power density, repetition rate).
