Transit-time effects in saturated absorption resonances

Abstract

The main characteristics of the saturated absorption resonance (its line shape, its amplitude, power and collisional broadening) are studied in detail, experimentally and theoretically, covering both the domain of very small and relatively large saturation levels. In contrast with numerous previous papers, it is shown that the width of the resonance in the free-flight regime is governed only by the typical transit time of molecules across the beam, while the width of the first derivative is equal to the square root of the product of the collisional and transit-time linewidths. The effect of slow molecules on the shape of the resonance and the second-order Doppler shift has been detected and studied experimentally for the first time. The unprecedented accuracy of about 3 parts in 10 13 is reported for the laser frequency standard Etude theorique et experimentale des caracteristiques principales en resonances d'absorption saturee (forme, amplitude, elargissement de puissance et collisionnel) pour des niveaux de saturation tres petits et relativement grands: la largeur de la resonance en region de vol libre n'est determinee que par le temps typique de transit des molecules a travers le faisceau lumineux et la largeur de la derivee premiere est proportionnelle a la racine carree du produit des largeurs de collision et de transit. Detection et etude de l'influence des molecules lentes sur la forme de resonance et le deplacement Doppler du second ordre