Semiconductor laser with the subhertz linewidth

A semiconductor laser emitting at 972 nm is stabilised with respect to a vibrationally and thermally compensated reference Fabry–Perot resonator with the vertical axis. The supporting points lie in the horizontal symmetry plane of the resonator, the influence of vibrations in the vertical direction being substantially suppressed in this case. This configuration provides a low sensitivity of the laser emission frequency to vertical accelerations of the reference resonator. To reduce the influence of temperature fluctuations, the resonator is made of an ULE (ultra-low-expansion) glass and is kept at temperature at which the expansion coefficient of this glass is close to zero. The laser linewidth is smaller than 0.5 Hz and the frequency drift is ~0.1 Hz s^-1. The minimum of the Allan deviation achieved for 3 s is 2 × 10^-15. The laser was used to record the spectra of the 1S — 2S transition in atomic hydrogen.