We report measurements on a 1.75 m long Fabry-Perot cavity locked to a
Nd:YAG tunable laser using the Pound-Drever technique. The cavity dec
ay time was measured to be about 291 mu S corresponding to a quality f
actor Q of about 5 x 10(11), the highest value ever reported for an op
tical resonant cavity. (C) 1996 American Institute of Physics.