Sub-Hertz frequency stabilization of a commercial diode laser

We report ultra-stable locking of a commercially available extended cavity diode laser to a vibration-insensitive, high finesse Fabry–Perot cavity. A servo bandwidth of 2 MHz is demonstrated. The individual frequency stability of the diode laser after locking is independently measured with a three-cornered-hat method. The resulting Allan deviation reaches a level of 3 × 10 − 15 at 1 s, even without vibration isolation of the reference cavity.