Abstract

The mode rotation technique has been used in high power chemical lasers to homogenize the density fluctuation of the gain medium and also lessen the cavity misalignment effect due to the mirror jittering. The output beam quality can thus be greatly improved. For free electron laser (FEL) systems, the gain homogenization is not possible because the medium (electron beam) moves with the optical field at nearly the light speed. Also, it is not necessary because initial FEL experiments have shown good beam qualities (~ 1.2). However, high power FEL systems require very long resonators which, combined with the very narrow gain region, become extremely sensitive to the cavity misalignment. The mode rotation technique is reexamined for this particular application to the FEL systems. The critical tilt angles of the mirrors for laser oscillation are studied analytically by observing the change in position of the cavity optical axis with mode rotation. The beam quality of the modes in negative branch confocal unstable resonators is demonstrated numerically in a 3-D resonator mode with a simple FEL gain model. The results show significant improvement in the beam quality of a misaligned FEL cavity. The details are discussed.

© 1985 Optical Society of America