Squeezed-State Enhanced Frequency Modulation Spectroscopy

The recent series of successful experimental observations of optical squeezed states opens up the exciting possibility of improving the sensitivity limits of a number of measurement schemes where the limiting noise is set by quantum mechanically induced fluctuations. One such technique which might benefit from quantum noise reduction is frequency-modulation laser absorption spectroscopy (FMS). For this method, a tunable laser is frequency-modulated and then passed through a medium with weakly absorbing spectroscopic features. Differential absorption or phase shift of the laser sidebands results in conversion of frequency modulation (FM) to amplitude modulation (AM), and the beat note induced on a photodetector is recorded. If the beat note frequency is high enough to exceed the laser technical noise, quantum-limited sensitivity is in principle possible. A simple model designed to understand the potential role of optical squeezing in FMS was analyzed. The essential finding is that the dominant term in the vacuum fluctuations arises from the interaction of the weak FM sidebands and the strong carrier, and that this term may be reduced by squeezing only the FM sidebands and not the strong carrier. With a proper choice of squeezing parameters, the sideband-carrier fluctuations can be made small relative to those induced by losses in the optical detector.