Subcompact hydrogen maser atomic clocks

Recent progress in compact hydrogen maser atomic frequency standards or clocks is reviewed. After a brief description of the principle of a hydrogen maser, the techniques used to realize high-performance compact hydrogen masers are discussed. Two approaches in compact hydrogen maser design are presented. The first approach uses active or atomic resonance sustained maser oscillation. The design uses cavity Q enhancement to overcome the intrinsic higher losses in a compact cavity. The versatility of the cavity design enables masers of various sizes to be realized. The complete package for the smallest oscillating compact maser measures 17.8*30.5*43.2 cm, weighs 19.5 kg, and has a measured stability of 4.14*10/sup -14/ for an averaging time tau =400 s, with a tau /sup -1/2/ dependence for tau <10/sup 5/ s. The second approach uses a passive maser. The design uses the atomic resonance as a narrow-bandpass amplifier. Using a dielectric loaded cavity, a package size of 26.7*66.0*45.5 cm with a weight of 30 kg and a stability of 1.0-3.0*10/sup -12/ tau /sup -1/2/, for 1< tau <10/sup 5/ s, has been attained. Compact masers of both designs have demonstrated frequency drift rates of 1*10/sup -15/ per day or less.<
>