Two-photon spectroscopy: A technique for characterizing diode-laser noise

Using Doppler-free Rb two-photon excitation measurements, we present support for the hypothesis that semiconductor-laser noise is characterized by pure phase-diffusion noise. We measured near-Lorentzian shapes for both the laser and the two-photon excitation spectra and a slope of 3.7±0.3 for the dependence of the two-photon excitation width on the laser width. These represent measurements of the second-order field statistics of a naturally operating laser where the noise is dominated by spontaneous emission. The measured spectral shape and slope are in excellent agreement with the Mollow model, which predicts a Lorentzian spectral shape and a slope of 4 for weak-field, two-photon excitation with a pure phase-diffusion field (Lorentzian spectral density). The dominance of phase-diffusion noise is further corroborated by an analytically solvable microscopic noise model that includes phase and amplitude noise.