Diagnostic techniques and VUV induced degradation of the mirrors used in the Orsay storage ring free electron laser

Abstract

Due to its low gain, the Orsay storage ring free electron laser necessitated the use of high reflectivity mirrors. Three measurements techniques of the mirror losses are presented. They are all based on a cavity decay time measurement using either an external laser, or the synchrotron radiation stored in the cavity or the free electron laser itself. The high signal over noise ratio allowed measurement of loss variations as low as 10 −7 s − 1 2 . From these diagnostics we were able to identify three distinct processes of UV induced degradation of TiO 2 SiO 2 dielectric mirrors. One of them was a surface absorption of the upper SiO 2-air interface; it was not affected by annealing. The two others corresponded to a volume absorption of the layers which completely recovered after annealing.