Spectral line shape analysis and systematic frequency shifts of laser frequency standards

A practical realization of the unit of length often relies on values of the frequency of optical frequency standards in the form of frequency stabilized lasers. A technique of frequency stabilization commonly used for He-Ne lasers at 633 nm stabilized in frequency to hyperfine spectral components in the saturated absorption spectrum of molecular iodine is that based on phase-sensitive detection at the third harmonic of a modulation frequency. A systematic study is made of how frequency shifts are related to observations of offset voltages in the feedback loop which vary across the iodine spectrum. A simple model based on spectral line shape analysis is found to apply to a variety of lasers possessing moderate offsets