Quantum-limited laser frequency-modulation spectroscopy

Frequency-modulation (FM) spectroscopy permits high-resolution, high-sensitivity, easily calibrated absorption measurements of atomic and molecular species and narrow spectral features in solids. This paper reviews some important developments in laser FM spectroscopy, from its inception as a spectroscopic tool to the demonstration of quantum-limited absorption measurements, emphasizing the sensitivity limitations caused by residual amplitude modulation (RAM). Moreover, a detailed account is presented of a new double-beam, single-detector tech-nique that efficiently suppresses the RAM and permits quantum-limited performance to be achieved in laser FM spectroscopy. We also include some recent results of the first reported FM spectroscopic investigations of the NO2 molecule.