Laser power stabilization using optical ac coupling and its quantum and technical limits.

We demonstrate an active power stabilization of a Nd:YAG laser employing the optical ac-coupling scheme and derive its fundamental quantum limit. This limit is 3 dB better than the one encountered in traditional power stabilization schemes. In our experiment, the optical ac coupling improved the shot-noise-limited sensitivity of the stabilization photodetector by a factor of 11.2. With an independent photodetector, we measured a relative power stability of 3.7x10(-9) Hz(-1/2) at frequencies of around 200 kHz. A detailed investigation of the performance limit of our experiment revealed a novel noise source that disturbed the fundamental mode field in the optical resonator. This effect could be of relevance to many precision experiments using optical resonators.