Spectral parameters of reference-cavity-stabilised lasers

Spectral characteristics of laser systems used for precision two-photon spectroscopy of the 1s — 2s transition in the hydrogen atom are analysed. The spectral properties of radiation from a 972-nm external-cavity semiconductor laser stabilised by external monolithic cavities of different configurations are considered in detail. The classical cavity scheme with a horizontal axis and a new scheme with a vertical axis, which is weakly sensitive to vibrations, are considered. The emission spectrum of a stabilised laser exhibits a narrow central peak and a pedestal caused by the residual phase noise. The semiconductor laser stabilised with respect to the vertical-axis cavity emits the line of width less than 0.45 Hz, which contains ~99% of the total output power. The transformation of the emission spectrum upon frequency doubling in nonlinear crystals is investigated. The laser system developed in the study provides the narrowing of the recorded 1s — 2s transition line in the hydrogen atom down to 2 kHz at 121 nm.