Deliberate misalignment in free electron lasers with a hole coupling

In a conventional laser operation misalignment of resonator mirrors leads usually to undesirable effects and has to be avoided. But in some certain types of cavity configurations deliberate introduction of misalignment makes it possible to improve considerably the characteristics of out-put radiation. The example of such configurations is an optical scheme with hole coupling. Two options are considered: (1) the free electron laser (FEL) with the radiation output through the on-axis hole at the exit mirror; (2) the external resonator (used for pulse stacking) where the exit FEL radiation enters this resonator through the on-axis hole at the input mirror. These configurations are investigated with the continuous wave 3-D code. It is shown that in a FEL with a hole coupling the transverse distribution of intracavity mode is characterised under certain conditions by a on-axis dip. The introduction of deliberate misalignment, characterised by a mirror tilt angle θ, leads to a shift and variation of the spacial structure. It is shown that due to the complicated structure of intracavity field, the dependences of the output power P on θ become nonmonotonic. For optimal value of θ = θopt the output power could be much bigger than for the case θ = 0. Moreover, the introduction of deliberate misalignment into optical cavity provides an opportunity not only to increase the output power but also to smooth the dependences of the output characteristics on the radiation wavelength