Characterization of an ultra-low loss and high finesse Fabry Perot cav
ity at 1064 nm is reported. The characteristics are obtained from meas
urements of cavity decay time, frequency response and transmission eff
iciency. The results show that the mirrors have the loss of 1.5 ppm in
cluding absorption and scattering losses and the reflectance of 99.987
3% which corresponds to a cavity finesse of 2.48X10(4).