Optical resonators with ten million finesse

We report a significant improvement of the finesse and quality factor of a calcium fluoride whispering gallery mode optical resonator achieved with iterative thermal annealing. The initial and final values of the resonator finesse are F-i approximate to 5 x 10(5) and F-f >= 10(7) respectively. The measurements are conducted at room temperature in the laboratory atmosphere. (C) 2007 Optical Society of America.