Abstract: A fundamental limit to the sensitivity of optical interferometry is thermal noise that drives fluctuations in the positions of the surfaces of the interferometer's mirrors, and thereby in the phase of the intracavity field. A scheme for substantially reducing this thermally driven phase noise is provided in which the strain-induced phase shift from a mirror's optical coating cancels that due to the concomitant motion of the substrate's surface. As such, although the position of the physical surface may fluctuate, the optical phase upon reflection can be largely insensitive to this motion.

Abstract: A fundamental limit to the sensitivity of optical interferometry is thermal noise that drives fluctuations in the positions of the surfaces of the interferometer's mirrors, and thereby in the phase of the intracavity field. A scheme for substantially reducing this thermally driven phase noise is provided in which the strain-induced phase shift from a mirror's optical coating cancels that due to the concomitant motion of the substrate's surface. As such, although the position of the physical surface may fluctuate, the optical phase upon reflection can be largely insensitive to this motion.

Abstract: An opto-electronic system and technique for comparing laser frequencies with large frequency separations, establishing new frequency standards, and achieving phase-sensitive detection at ultra high frequencies. Light responsive materials with multiple energy levels suitable for multi-photon excitation are preferably used for nonlinear mixing via quantum interference of different excitation paths affecting a common energy level. Demodulation of a carrier with a demodulation frequency up to 100's THZ can be achieved for frequency comparison and phase-sensitive detection. A large number of materials can be used to cover a wide spectral range including the ultra violet, visible and near infrared regions. In particular, absolute frequency measurement in a spectrum from 1.25 .mu.m to 1.66 .mu.m for fiber optics can be accomplished with a nearly continuous frequency coverage.