Curved Bragg reflectors for compact waveguide FELs

In this paper a curved Bragg reflector is analysed. Its main characteristic is the capability to confine the optical radiation and because of its transparency to an electron beam, it is particularly suitable for compact free-electron laser devices operating in the far infrared and millimetre wave range. Moreover it can be utilised in any kind of stable or unstable resonator. A mathematical method based on a modal expansion of the waveguide fields is used, in which the Bragg structure is treated as a perturbation in the inhomogeneous wave equation.