Frequency Stability Improvement of an Active Hydrogen Maser with a Single-State Selection System

A single-state selection system based on the adiabatic rapid passage technique has been designed for active hydrogen masers to improve the frequency stability. This system eliminates more than 96% of undesirable atoms from the beam. State selection effectiveness was determined by measuring generation power as a function of atomic line quality and by the double resonance method for measurement of the population difference of hyperfine sublevels. Frequency stability caused by microwave cavity thermal noises has been improved by a factor of 1.5. Frequency stability of two active hydrogen masers with single- state selection systems was measured. Long-term frequency stability better than 1•10 ⁻¹⁶ at averaging time 10 ⁵ s has been achieved in the experiment.