Analysis and observation, on an atomic resonance, of the frequency shift due to the length modulation of an optical cavity

The frequency shift of the light owing to a cavity length modulation has been observed with the spectroscopy of the 1S-3S two-photon transition in hydrogen. The excitation of this transition occurs at a wavelength of 205 nm. One produces this radiation by frequency doubling a 410-nm radiation in an enhancement ring cavity. The length modulation of this cavity induces frequency shifts and a splitting of the observed 1S-3S line. We obtain a good agreement between the experiment and the theoretical analysis.