Optical heterodyne measurement of pulsed lasers: Toward high-precision pulsed spectroscopy

In this paper we explore theoretically and experimentally the effect of fluctuations in the instantaneous frequency of a pulsed laser on the shape and position of two-photon transition spectra. The usual procedure of characterizing a pulsed laser by its frequency energy spectrum is insufficient for precision measurements. The nonlinear nature of the two-photon transition produces a systematic shift of the atomic spectrum with respect to the laser frequency spectrum that is dependent on the phase evolution of the laser pulse. In fact, any nonlinear process (e.g., second-harmonic generation) may result in displaced or distorted spectra. We also find that Fabry-Pérot filtering a laser pulse can result in large frequency chirps. We use an optical heterodyne technique to measure the instantaneous frequency of our excimer-pumped dye-laser system to an uncertainty of 1.3 MHz and determine the effect of the inherent frequency chirps of this system on a two-photon transition. We conclude that with this technique, precision nonlinear spectroscopy to the level of 1 MHz may be achieved with pulsed lasers.