Iodine-stabilized 633-nm He-Ne laser for long-baseline interferometers

An internal-mirror He-Ne laser (lambda = 633 nm) was frequency stabilized to a hyperfine component of iodine transitions. The absorption signal and its derivative were obtained by the modulation transfer technique with both amplitude-and frequency-modulated pump beams. These modulations were realized using an acousto-optic modulator (AOM), without modulating the laser resonator. Owing to this external frequency modulation, the stabilized output, which is free from frequency modulation, can be used in long-baseline interferometers, such as geophysical laser strainmeters.